OSP Hall A - Jlab Hall-A

OSP Hall A - Jlab Hall-A
TJNAF
Hall A Experimental Equipment
Operations Manual and Safety Assessment
Info Level 4
Hall A Arms and Beamline Transport
m-drive/martz//graphics/3dart/halla/newfolder/hallacombo.ai jm 7/26/00
The Hall A Collaboration
Editor: E. A. Chudakov1
December 1, 2003
1 Thomas
Jefferson National Accelerator Facility
Contents
I
Hall A OSP Overview
18
1 Introduction
1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Hall A Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2 Hall A Access and Safety
2.1 The Personnel Safety System . . . . . .
2.1.1 Restricted Access . . . . . . . . .
2.1.2 Sweep . . . . . . . . . . . . . . .
2.1.3 Controlled Access . . . . . . . . .
2.1.4 RF Power Permit . . . . . . . . .
2.1.5 Beam Permit . . . . . . . . . . .
2.1.6 Run Safe Boxes . . . . . . . . .
2.2 Hall A Access . . . . . . . . . . . . . . .
2.2.1 Controlled Access Procedure . . .
2.2.2 Restricted Access Procedure . . .
2.2.3 Access Requirements . . . . . . .
2.2.4 The Hall A Safety Walk-through
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II
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Beamline
28
3 General Description
3.1 Introduction . . . . . . . . . . . . . .
3.2 Beam Line Components . . . . . . .
3.2.1 The Beam Entrance Channel
3.2.2 The Beam Optics Channel . .
3.2.3 Beam Diagnostic Elements . .
3.2.4 Beam Exit Channel . . . . . .
3.3 Authorized Personnel . . . . . . . .
3.4 Additional Safety Information . . . .
3.5 Machine/Beamline protection system
3.6 Personnel Safety System . . . . . . .
3.7 Beam Position Monitors . . . . . . .
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38
CONTENTS
2
3.8
Beam Current Measurement . . . . . .
3.8.1 System Layout . . . . . . . . .
3.8.2 Authorized Personnel . . . . .
3.9 Arc Energy Measurement . . . . . . .
3.9.1 Summary of ARC operations .
3.9.2 Summary of field integral . . .
3.9.3 Details on integral run . . . . .
3.9.4 Details on temperatures . . . .
3.9.5 Shed access and safety . . . . .
3.9.6 List of Authorized Personnel for
3.10 Fast Raster . . . . . . . . . . . . . . .
3.11 Bremsstrahlung Radiator . . . . . . . .
3.11.1 Overview . . . . . . . . . . . .
3.11.2 Safety Issues . . . . . . . . . . .
3.11.3 Operations . . . . . . . . . . . .
3.11.4 Special Instructions . . . . . . .
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Access
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41
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53
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70
5 Target Chamber
5.0.1 Target Chamber - Spectrometer Coupling . . . . . . . . . . . . .
5.0.2 Stress Analysis of the Middle Ring . . . . . . . . . . . . . . . . .
5.0.3 Vacuum Pumping System . . . . . . . . . . . . . . . . . . . . . .
72
73
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74
4 eP Beam Energy Measurement
4.1 Purpose and Layout . . . . . . . . .
4.2 Description of Components . . . . . .
4.2.1 High Voltage . . . . . . . . .
4.2.2 MEDM Controls . . . . . . .
4.2.3 Silicon Micro-Strip Detectors
4.2.4 Target . . . . . . . . . . . . .
4.2.5 Cherenkov . . . . . . . . . . .
4.2.6 Data Acquisition . . . . . . .
4.2.7 Data Analysis . . . . . . . . .
4.3 Operating Procedure . . . . . . . . .
4.4 Maintenance . . . . . . . . . . . . . .
4.5 Safety Assessment . . . . . . . . . . .
4.5.1 High Voltage . . . . . . . . .
4.5.2 Silicon Micro-Strip Detectors
4.5.3 Target . . . . . . . . . . . . .
4.5.4 Cherenkov . . . . . . . . . . .
4.6 List of Authorized Personnel . . . . .
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CONTENTS
6 Møller Polarimeter
6.1 Purpose and Layout . . . . . . . . . .
6.2 Principles of Operation . . . . . . . . .
6.3 Description of Components . . . . . . .
6.3.1 MEDM Control . . . . . . . . .
6.3.2 Polarized Electron Target . . .
6.3.3 Spectrometer Description . . .
6.3.4 Detector . . . . . . . . . . . . .
6.3.5 Electronics . . . . . . . . . . . .
6.3.6 DAQ . . . . . . . . . . . . . . .
6.3.7 Slow Control . . . . . . . . . .
6.4 Operating Procedure . . . . . . . . . .
6.4.1 Initialization . . . . . . . . . . .
6.4.2 Initial Beam Tune . . . . . . .
6.4.3 The Magnet Settings . . . . . .
6.4.4 Final Beam Tune . . . . . . . .
6.4.5 Target Motion . . . . . . . . . .
6.4.6 Detector Tuning and Checking
6.4.7 Polarization Measurement . . .
6.5 Safety Assessment . . . . . . . . . . . .
6.5.1 Magnets . . . . . . . . . . . . .
6.5.2 Vacuum System . . . . . . . . .
6.5.3 High Voltage . . . . . . . . . .
6.5.4 Target . . . . . . . . . . . . . .
6.6 List of Authorized Personnel . . . . . .
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7 Compton Polarimeter
7.1 Introduction . . . . . . . . . . . . . . . . . .
7.2 Principle of Operation . . . . . . . . . . . .
7.3 Description of Components . . . . . . . . . .
7.3.1 Optics table . . . . . . . . . . . . . .
7.3.2 Magnetic Chicane . . . . . . . . . . .
7.3.3 Photon Detector . . . . . . . . . . .
7.3.4 Electron detector . . . . . . . . . . .
7.3.5 Fast acquisition system . . . . . . . .
7.4 Operating Procedure . . . . . . . . . . . . .
7.4.1 DAQ Setup . . . . . . . . . . . . . .
7.4.2 Cavity Setup . . . . . . . . . . . . .
7.4.3 Electron Detector Setup . . . . . . .
7.4.4 Vertican Scan . . . . . . . . . . . . .
7.4.5 Taking data . . . . . . . . . . . . . .
7.4.6 Turning off the compton polarimeter
7.5 Safety Assessment . . . . . . . . . . . . . . .
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75
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116
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CONTENTS
4
7.5.1
7.5.2
7.5.3
Magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
High Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
III
Targets
126
8 Overview
127
9 Cryogenic Targets
9.1 Procedure for Normal Running of the Hall A Cryogenic
9.1.1 Introduction . . . . . . . . . . . . . . . . . . .
9.1.1 Alarm Handler . . . . . . . . . . . . . . . . . .
9.1.2 Target Motion and Fast Raster . . . . . . . . .
9.1.3 Cryogenic Consumption . . . . . . . . . . . . .
9.1.4 Checklist . . . . . . . . . . . . . . . . . . . . . .
9.1.5 Target Operators . . . . . . . . . . . . . . . . .
9.2 Safety Assessment . . . . . . . . . . . . . . . . . . . . .
9.2.1 Flammable Gas . . . . . . . . . . . . . . . . . .
9.2.1.1
Electrical Installation . . . . . . . . .
9.2.1.2
Flammable Gas Detectors . . . . . . .
9.2.2 Pressure . . . . . . . . . . . . . . . . . . . . . .
9.2.2.1
Target Cells . . . . . . . . . . . . . .
9.2.2.2
Pressure Relief . . . . . . . . . . . . .
9.2.2.3
Scattering Chamber Vacuum Failure .
9.2.3 Temperature Regulation . . . . . . . . . . . . .
9.2.3.1 Target Freezing . . . . . . . . . . . . .
9.2.4 ODH . . . . . . . . . . . . . . . . . . . . . . . .
9.2.5 Controls . . . . . . . . . . . . . . . . . . . . . .
9.2.6 Authorized Personnel . . . . . . . . . . . . . .
9.3 Cryogenic Target Control System User Manual . . . . .
10 Polarized 3 He Target
10.1 General Description . . . . . . . . .
10.1.1 Physics Principle . . . . . .
10.1.2 Apparatus . . . . . . . . . .
10.1.3 Control System . . . . . . .
10.2 Operation Overview . . . . . . . .
10.3 Laser System . . . . . . . . . . . .
10.3.1 Laser Hut & Beam Path . .
10.3.2 Diode Lasers & Controls . .
10.3.3 Alignment . . . . . . . . . .
10.3.4 Operation I: Local Mode . .
10.3.5 Operation II: Remote Mode
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Targets
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128
128
128
129
129
129
130
130
132
132
133
135
135
135
136
137
139
140
141
141
143
144
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145
145
145
145
147
147
148
149
151
153
156
157
CONTENTS
10.3.6
Target
10.4.1
10.4.2
Target
10.5.1
10.5.2
Target
Target
10.7.1
10.7.2
10.7.3
Half-Wave Plate Motion Control . . . . . . . . . . . . . . . . .
10.4
Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Installation & Replacement . . . . . . . . . . . . . . . . . . .
10.5
Ladder & Motion System . . . . . . . . . . . . . . . . . . . . .
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.6
Enclosure & Windows . . . . . . . . . . . . . . . . . . . . . .
10.7
Cell Heater . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Safety Considerations . . . . . . . . . . . . . . . . . . . . . . .
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.7.3.1 Local Operation . . . . . . . . . . . . . . . . . . . .
10.7.3.2 Remote Operation . . . . . . . . . . . . . . . . . . .
10.8 Helmholtz Coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.9 NMR Polarimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.9.1 NMR polarization measurement . . . . . . . . . . . . . . . . .
10.9.2 Plots to print . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.9.3 Warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.9.4 NMR AFP Safety . . . . . . . . . . . . . . . . . . . . . . . .
10.9.4.1 The DC current . . . . . . . . . . . . . . . . . . . .
10.9.4.2 The AC current . . . . . . . . . . . . . . . . . . . .
10.10EPR Polarimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.10.1 EPR Lineshape Measurement – Frequency Modulation Sweep:
10.10.2 Common Problems . . . . . . . . . . . . . . . . . . . . . . . .
10.10.3 EPR Polarization Measurement – AFP Sweep . . . . . . . . .
10.10.4 Common Problems . . . . . . . . . . . . . . . . . . . . . . . .
10.10.5 EPR Continuous Monitoring with Field Feedback . . . . . . .
10.10.6 Common Problems . . . . . . . . . . . . . . . . . . . . . . . .
10.11Reference Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.11.1 Description of the Reference Cell System . . . . . . . . . . . .
10.11.2 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.11.3 Cautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.11.4 Potential Hazards . . . . . . . . . . . . . . . . . . . . . . . . .
10.11.5 Hazard Mitigation . . . . . . . . . . . . . . . . . . . . . . . .
10.12Hazards and Safety Issues . . . . . . . . . . . . . . . . . . . . . . . .
10.13Laser Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.13.1 Laser Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.13.2 Fire Hazards and Safety . . . . . . . . . . . . . . . . . . . . .
10.13.3 Personnel Safety/ Working in the Hall . . . . . . . . . . . . .
10.14Appendix: Laser Standard Operation Procedure . . . . . . . . . . . .
10.14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.14.2 Personnel . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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158
158
158
159
160
160
161
162
163
163
163
166
166
171
172
173
173
173
175
175
175
178
178
178
181
182
185
186
187
188
188
190
192
192
192
192
193
193
193
194
194
194
194
CONTENTS
6
10.14.3 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.14.4 Optical setup . . . . . . . . . . . . . . . . . . . . . . .
10.14.5 Hazards . . . . . . . . . . . . . . . . . . . . . . . . . .
10.14.6 Laser environment . . . . . . . . . . . . . . . . . . . .
10.14.7 Procedures . . . . . . . . . . . . . . . . . . . . . . . . .
10.14.7.1 Normal procedure . . . . . . . . . . . . . . .
10.14.7.2 Alignment procedure . . . . . . . . . . . . . .
10.14.7.3 Maintenance procedure . . . . . . . . . . . .
10.14.7.4 Off-normal and emergency procedure . . . . .
10.14.8 Controls . . . . . . . . . . . . . . . . . . . . . . . . . .
10.14.9 Laser safety calculations . . . . . . . . . . . . . . . . .
10.14.10List of authorized personnel . . . . . . . . . . . . . . .
10.14.11List of Laser Trained Personnel . . . . . . . . . . . . .
10.14.12List of Authorized People to Change Target Cells . . .
10.14.13List of Authorized People to Perform Laser Alignment
11 The Waterfall Target
11.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2 Description of the System . . . . . . . . . . . . . . . . . . .
11.2.1 The Hydraulic System . . . . . . . . . . . . . . . . .
11.2.2 The Movement System . . . . . . . . . . . . . . . . .
11.2.3 The Slow–Control System . . . . . . . . . . . . . . .
11.3 The Target Cell Windows . . . . . . . . . . . . . . . . . . .
11.4 Target Layouts . . . . . . . . . . . . . . . . . . . . . . . . .
11.4.1 The Target for the Experiment E89-003 and E89-033
11.4.2 The Target for the Experiment E00-102 . . . . . . .
11.4.3 The Target for the Experiment 94-107 . . . . . . . .
11.5 Operating Procedure . . . . . . . . . . . . . . . . . . . . . .
11.5.1 Startup of the system . . . . . . . . . . . . . . . . . .
11.5.1.1 First startup of the System . . . . . . . . .
11.5.2 System shutdown . . . . . . . . . . . . . . . . . . . .
11.5.3 Troubleshooting . . . . . . . . . . . . . . . . . . . . .
11.6 Standard Operations . . . . . . . . . . . . . . . . . . . . . .
11.6.1 Target Selection . . . . . . . . . . . . . . . . . . . . .
11.6.2 Pump Speed Control . . . . . . . . . . . . . . . . . .
11.6.3 Rebooting the IOC . . . . . . . . . . . . . . . . . . .
11.7 Troubleshooting (Experts only) . . . . . . . . . . . . . . . .
11.7.1 Manual motion operation (Experts only) . . . . . . .
11.7.2 Use of the HOME routine . . . . . . . . . . . . . . .
11.7.3 Hardware limit over travel . . . . . . . . . . . . . . .
11.7.4 Manual water pump operation . . . . . . . . . . . . .
11.8 Safety Assessments . . . . . . . . . . . . . . . . . . . . . . .
11.8.1 Radiological hazards . . . . . . . . . . . . . . . . . .
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195
195
197
198
199
199
199
200
200
201
202
202
203
204
204
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205
205
205
208
212
214
220
221
221
223
223
223
224
224
225
225
226
226
227
227
227
227
229
229
229
230
230
CONTENTS
11.8.2 The electrical power and slow control systems
11.8.3 The water system . . . . . . . . . . . . . . . .
11.8.4 Thin windows on the target cell . . . . . . . .
11.8.5 The slow control system . . . . . . . . . . . .
11.8.6 The mechanical system . . . . . . . . . . . . .
11.9 Authorized Personnel . . . . . . . . . . . . . . . . . .
IV
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High Resolution Spectrometers (HRS)
12 High Resolution Spectrometers (HRS)
12.1 Safety with Regards to the Spectrometer . . . . . . . . . . .
12.2 Hall A Vacuum System . . . . . . . . . . . . . . . . . . . . .
12.2.1 Spectrometer Vacuum System . . . . . . . . . . . . .
12.2.2 Target Vacuum System . . . . . . . . . . . . . . . . .
12.2.3 Magnet Vacuum System . . . . . . . . . . . . . . . .
12.2.4 Beam Line Vacuum System . . . . . . . . . . . . . .
12.2.5 Beam Exit Vacuum System . . . . . . . . . . . . . .
12.2.6 Hazards of Vacuum Systems . . . . . . . . . . . . . .
12.3 The High Resolution Spectrometer (HRS) . . . . . . . . . .
12.3.1 Magnets and Power Supplies . . . . . . . . . . . . . .
12.3.2 Personnel . . . . . . . . . . . . . . . . . . . . . . . .
12.3.3 Quadrupole Magnets . . . . . . . . . . . . . . . . . .
12.3.4 Cryogenic Procedures . . . . . . . . . . . . . . . . . .
12.3.5 First Time Startup Check List. . . . . . . . . . . . .
12.3.6 Dipole Magnet . . . . . . . . . . . . . . . . . . . . .
12.4 Operation of the HRS Magnets . . . . . . . . . . . . . . . .
12.5 Field Monitoring . . . . . . . . . . . . . . . . . . . . . . . .
12.5.1 Simple Spectrometer Field Setting (Autopilot Mode)
12.5.2 Dipole Field Monitoring Electron Arm . . . . . . . .
12.5.3 Authorized Personnel . . . . . . . . . . . . . . . . . .
12.5.4 NMR Operating Procedure: . . . . . . . . . . . . . .
12.5.5 Powering Up Dipole Magnets: . . . . . . . . . . . . .
12.5.6 Starting Q1 Power Supply: . . . . . . . . . . . . . . .
12.5.7 Starting Q2/3 Power Supply: . . . . . . . . . . . . .
12.6 Collimators and Sieve Slits . . . . . . . . . . . . . . . . . . .
12.6.1 Authorized Personnel . . . . . . . . . . . . . . . . . .
12.6.2 Safety Assessment . . . . . . . . . . . . . . . . . . .
12.6.3 Operating Procedure . . . . . . . . . . . . . . . . . .
12.7 Spectrometer Alignment . . . . . . . . . . . . . . . . . . . .
12.7.1 Personnel Responsible . . . . . . . . . . . . . . . . .
230
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231
231
231
232
233
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234
239
240
240
242
242
242
242
243
243
244
244
244
245
245
245
246
250
250
251
252
256
259
259
259
261
261
261
264
265
265
CONTENTS
V
8
HRS Detectors
266
13 chapter
267
13.1 Geometry of the Spectrometer Detector Packages . . . . . . . . . . . . . 269
14 Vertical Drift Chambers
14.1 Overview . . . . . . . . .
14.2 Operating Procedure . .
14.3 Handling Considerations
14.4 Other Documentation . .
14.5 Safety Assessment . . . .
14.6 Responsible Personnel .
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271
271
273
276
277
277
277
15 Trigger Scintillator Counters
15.1 Overview . . . . . . . . . . . . . . . . . . . .
15.2 PMT regime and time resolution . . . . . .
15.3 PMT operation monitoring . . . . . . . . . .
15.4 Measures to Protect the PMTs from Helium
15.5 2” PMT Bases for S1 Trigger Counters . . .
15.6 Safety Assessment . . . . . . . . . . . . . . .
15.7 Responsible Personnel . . . . . . . . . . . .
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278
278
279
280
282
282
284
285
16 Gas
16.1
16.2
16.3
16.4
Cherenkov Counters
Concept of the design .
Safety Assessment . . .
Operating Procedure .
Responsible Personnel
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286
286
288
288
288
17 Lead Glass Shower Counters
17.1 Overview . . . . . . . . . . .
17.2 Operating Procedures . . . .
17.3 Handling Considerations . .
17.4 Safety Assessment . . . . . .
17.5 Authorized Personnel . . . .
17.6 Software Algorithms . . . .
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291
291
293
293
298
298
299
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301
301
305
306
306
308
308
308
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18 Aerogel Cherenkov Counter
18.1 Mirror Aerogel Cherenkov Counter
18.2 Safety Assessment . . . . . . . . . .
18.3 Operating Procedure . . . . . . . .
18.4 Handling Considerations . . . . . .
18.5 Diffusion aerogel counters . . . . .
18.6 Responsible Personnel . . . . . . .
18.7 Safety Assessment . . . . . . . . . .
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CONTENTS
19 The
19.1
19.2
19.3
19.4
19.5
19.6
Focal Plane Polarimeter
Overview . . . . . . . . . . .
Operating Procedure . . . .
Carbon Doors . . . . . . . .
Handling Considerations . .
Safety Assessment . . . . . .
Responsible Personnel . . .
9
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313
313
318
324
326
326
327
20 The Hall A Gas System
328
20.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
20.2 Gas Alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
VI
Slow Controls
21 Overview
21.1 System’s Components . . . . .
21.2 Operating Procedures . . . . .
21.3 Alarm Handler . . . . . . . .
21.4 HRS Floor Marks . . . . . . .
21.5 RIGHT HRS and LEFT HRS
21.6 Strip Tool . . . . . . . . . . .
21.7 Snapshot . . . . . . . . . . . .
21.8 Accelerator Menu . . . . . . .
21.9 Cryogenics Menu . . . . . . .
21.10Hall A Menu . . . . . . . . .
21.11Troubleshooting Procedures .
VII
330
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Data Acquisition and Trigger
22 Spectrometer Data Acquisition
22.1 General Computer Information . . . . . . . . . . . . .
22.2 Beginning of Experiment Checkout . . . . . . . . . . .
22.3 Running CODA . . . . . . . . . . . . . . . . . . . . . .
22.3.1 Some Frequently Asked Questions about DAQ
22.3.2 Quick Resets . . . . . . . . . . . . . . . . . . .
22.3.3 Cold Start of CODA . . . . . . . . . . . . . . .
22.3.4 Recovering from a Reboot of Workstation . . .
22.4 Electronic Logbook and Beam Accounting . . . . . . .
22.5 Port Servers . . . . . . . . . . . . . . . . . . . . . . . .
23 Trigger Hardware and Software
331
332
332
333
333
333
336
336
336
336
336
337
339
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340
341
341
341
342
343
343
344
344
345
347
CONTENTS
10
24 Online Analysis, Data Checks
351
24.0.1 Scaler Display and Scaler Events . . . . . . . . . . . . . . . . . . 351
24.1 Analysis using ESPACE or C++ Analyzer . . . . . . . . . . . . . . . . . 351
24.2 Dataspy and Dhist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
VIII
Offline Analysis Software
25 C++ Analyzer
25.1 Running the Analyzer . . . . . . . . . .
25.2 Preparing Analysis of a New Experiment
25.3 Database Files and Directories . . . . . .
25.4 Program Design Overview . . . . . . . .
25.5 Responsible Personnel . . . . . . . . . .
353
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354
356
356
358
359
360
List of Tables
3.1
3.2
3.3
3.4
3.5
Beamline: Hall A Beamline Elements . . . . . .
Beamline: Optics Requirements Target . . . . .
Beamline: Optics Requirements Other . . . . .
Bremsstrahlung Radiator: Raster Radius . . . .
Bremsstrahlung Radiator: Encoder Calibration
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37
37
38
51
51
4.1 eP System: HV Summary . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 eP System: authorized personnel . . . . . . . . . . . . . . . . . . . . . .
55
70
6.1
Moller Polarimeter: authorized personnel . . . . . . . . . . . . . . . . . .
92
7.1
7.2
Compton:vertical scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
compton Polarimeter: authorized personnel . . . . . . . . . . . . . . . . . 125
9.1
9.2
9.3
9.4
9.5
9.6
Cryotarget: Cell Pressure Test Data . . . .
Cryotarget: Relief Device Summary . . . .
Cryotarget: Gas Properties . . . . . . . . .
Cryotarget: Volumes and Geometry . . . .
Cryotarget: Relief Line Information . . . .
Contact Personel for the Cryogenic Targets
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136
137
139
140
141
143
10.1
10.2
10.3
10.4
10.5
10.6
Operational parameters for the 30 W lasers. . . . . . . . . . . . . . . . .
Suggested default parameters for the temperature controller. . . . . . . .
Suggested default parameters for the temperature controller (continued).
KEPCO settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Action of the remote-control switch panel . . . . . . . . . . . . . . . . . .
Laser specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
157
169
170
178
190
196
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and Scattering Chamber
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11.1 Waterfall target: maximum beam currents . . . . . . . . . . . . . . . . . 224
11.2 Summary of thin scattering window hydro-tests . . . . . . . . . . . . . . 232
11.3 List of experts for the Hall A Waterfall Target. . . . . . . . . . . . . . . . 232
12.1
12.2
12.3
12.4
Spectrometers:
Spectrometers:
Spectrometers:
Spectrometers:
Dipole Checklist . . . . . . . . . .
Q1 Checklist . . . . . . . . . . . .
Q2/Q3 Checklist . . . . . . . . . .
Dipole NMR Probe Field Ranges
11
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247
248
248
252
LIST OF TABLES
12
12.5 NMR troubleshhoting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
13.1 Detectors: Right ARM Detector Locations . . . . . . . . . . . . . . . . . 270
13.2 Detectors: Left Arm Detector Locations . . . . . . . . . . . . . . . . . . 270
22.1 Data Acquisition: Port Servers for DAQ . . . . . . . . . . . . . . . . . . 346
25.1 Analysis modules available in version 1.1 of the C++ Analyzer . . . . . . 357
List of Figures
1.1
Hall A schematic cross section . . . . . . . . . . . . . . . . . . . . . . . .
20
2.1 Introduction: Runs/Safe box, Access Keys . . . . . . . . . . . . . . . . .
2.2 Introduction: Location of Hall Safety Items . . . . . . . . . . . . . . . .
2.3 Introduction: Location of Circuit Breakers . . . . . . . . . . . . . . . . .
23
26
27
3.1 Beamline: Hall A Beamline Overview . .
3.2 Beamline: Hall A Beamline Overview . .
3.3 Beamline: Hall A Beamline Overview . .
3.4 Beamline: BPM Readout Electronics . .
3.5 Beam Current Measurement: Schematic
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30
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31
39
42
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
4.12
4.13
4.14
4.15
4.16
4.17
4.18
4.19
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54
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68
69
71
6.1 Møller: layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Møller:MEDM MCC control . . . . . . . . . . . . . . . . . . . . . . . . .
6.3 Møller:target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
78
79
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
eP:
Layout . . . . . . . . . . . . . .
LeCroy HV Screen . . . . . . .
Beamline HV Screen . . . . . .
HV Screen for Slot 1 . . . . . .
HV Screen for Slot 2 . . . . . .
HV Screen for Slot 3 . . . . . .
Slow Controls Screen . . . . . .
Picture Slow Controls . . . . .
SSD Bias Voltages Screen . . .
MX7 Controls Screen . . . . . .
Target Control Screen . . . . .
Picture of Target Control Box .
Cherenkov Controls Screen . .
Layout of CO2 Gas System . .
Picture of CO2 Gas Controller
DAQ VME Crate . . . . . . . .
DAQ NIM Bin . . . . . . . . .
DAQ CAMAC Crate . . . . . .
Trigger Configuration . . . . .
13
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LIST OF FIGURES
14
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
Møller:target rotary dial . .
Møller: FSD crate. . . . . .
Møller:spectrometer . . . . .
Møller: electronics crates. .
Møller:slow control window .
Møller:HV control . . . . . .
Møller: electronics control .
Møller: collimator control .
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80
81
82
84
85
86
87
88
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
7.10
7.11
7.12
7.13
7.14
7.15
7.16
7.17
7.18
7.19
7.20
7.21
7.22
7.23
7.24
7.25
compton:Compton Scattering . . . . . .
compton:Schematic layout . . . . . . . .
compton:Optics Table . . . . . . . . . .
compton:chicane schematic . . . . . . . .
compton:photon detector . . . . . . . . .
compton:electron detector . . . . . . . .
compton:runcontrol connect . . . . . . .
compton:runcontrol configure . . . . . .
compton:runcontrol run type . . . . . . .
compton:runcontrol download . . . . . .
compton:runcontrol start run . . . . . .
compton:runcontrol end run . . . . . . .
compton:spy acq high voltage . . . . . .
compton:epics main control . . . . . . .
compton:epics mini control . . . . . . . .
compton:laser spot . . . . . . . . . . . .
compton:servo control . . . . . . . . . .
compton:vacity lock . . . . . . . . . . . .
compton:laser ramp control . . . . . . .
compton:servo settings . . . . . . . . . .
compton:laser control . . . . . . . . . .
compton:electron detector circuit breaker
compton:Electron detector control . . . .
compton:Electron detector viewer . . . .
compton:vertical scan . . . . . . . . . . .
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94
96
97
98
98
99
101
102
103
104
105
106
108
109
110
111
112
112
113
114
115
116
117
118
120
10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
10.9
Overview of the polarized 3 He target setup . . . . . . . . . .
A topview of the laser hut . . . . . . . . . . . . . . . . . . .
Top view of the optics setup inside the laser hut . . . . . . .
Adjusting back-scattering light . . . . . . . . . . . . . . . . .
Typical 40 cm polarized 3 He target cell . . . . . . . . . . . .
Schematic diagram of the target ladder and target positions.
Schematic diagram of the target cell oven control system . .
Schematic diagram of the oven heater interlock system . . .
Top view of the longitudinal and transverse Helmholtz coils .
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146
150
152
156
159
161
164
165
174
LIST OF FIGURES
15
10.10The electronics is located in the Counting House A. . . . . .
10.11Cabling of the NMR system . . . . . . . . . . . . . . . . . .
10.12Circuit for EPR lineshape measurement . . . . . . . . . . . .
10.13Circuit for EPR measurement with AFP spin flip . . . . . .
10.14Circuit for continuous EPR measurement with field feedback
10.15Schematic of reference cell system . . . . . . . . . . . . . . .
10.16Valve configuration of the reference cell gas system . . . . .
10.17Control panel . . . . . . . . . . . . . . . . . . . . . . . . . .
10.18Polarized 3 He Laser Hut . . . . . . . . . . . . . . . . . . . .
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176
177
179
183
186
189
191
191
198
11.1 Waterfall target system . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2 Scheme of the target system devices . . . . . . . . . . . . . . . . . . .
11.3 Pictures of the target and scattering chamber. . . . . . . . . . . . . .
11.4 The enclosure of the target used for E00-102 and E94-107 . . . . . . .
11.5 Mechanical details of the waterfall target: posts etc. . . . . . . . . . .
11.6 The Waterfall Target Stack . . . . . . . . . . . . . . . . . . . . . . .
11.7 Schematic view of the hydraulic system. . . . . . . . . . . . . . . . .
11.8 Picture of the movement system. . . . . . . . . . . . . . . . . . . . .
11.9 Waterfall target: slow control system layout. . . . . . . . . . . . . . .
11.10Waterfall target: the EPICS Crate. . . . . . . . . . . . . . . . . . . .
11.11Waterfall target control GUI.. . . . . . . . . . . . . . . . . . . . . . .
11.12Waterfall target: control rack. . . . . . . . . . . . . . . . . . . . . . .
11.13Heat behavior of the beam entrance window. . . . . . . . . . . . . . .
11.14Preferential scattering angles for E89-003 and E89-033. . . . . . . . .
11.15A cutaway of the waterfall target cell used in E89-003 and E89-033. .
11.16Preferential scattering angle of the target for the E00-102 experiment.
11.17IOC rack for waterfall target. . . . . . . . . . . . . . . . . . . . . . .
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206
207
208
209
209
210
211
213
216
217
218
219
220
222
222
223
228
12.1 Spectrometers:
12.2 Spectrometers:
12.3 Spectrometers:
12.4 Spectrometers:
12.5 Spectrometers:
12.6 Spectrometers:
12.7 Spectrometers:
12.8 Spectrometers:
12.9 Spectrometers:
12.10Spectrometers:
12.11Spectrometers:
12.12Spectrometers:
12.13Spectrometers:
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235
236
237
238
241
250
251
252
253
254
255
262
263
Elevation View of Hall A HRS . . . . . . . . . .
Plan View of Hall A . . . . . . . . . . . . . . . .
Electron Arm Detectors . . . . . . . . . . . . . .
Hadron Arm Detectors . . . . . . . . . . . . . .
HRS Vacuum System . . . . . . . . . . . . . . .
Magnet Controls Screen . . . . . . . . . . . . . .
NMR System Layout . . . . . . . . . . . . . . .
NMR Gradient Compensation . . . . . . . . . .
Control Voltage Calibration for Electron Dipole
Control Voltage Calibration for Hadron Dipole .
NMR Probe DAC Calibration . . . . . . . . . .
Collimator Box Schematic . . . . . . . . . . . .
Sieve Slit . . . . . . . . . . . . . . . . . . . . . .
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13.1 The side view of the detector stacks . . . . . . . . . . . . . . . . . . . . . 268
LIST OF FIGURES
16
14.1
14.2
14.3
14.4
Detectors:
Detectors:
Detectors:
Detectors:
VDC Geometry . . .
VDC Geometry . . .
Gas Flow Schematic
VDC Overview . . .
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272
272
273
274
15.1
15.2
15.3
15.4
15.5
15.6
15.7
Detectors:
Detectors:
Detectors:
Detectors:
Detectors:
Detectors:
Detectors:
S1 Mounting . . . . . . . .
S2 Layout . . . . . . . . . .
HV HRSR Summary Screen
HV Screen for Single Card .
S2 PMT Housing . . . . . .
2” PMT Base . . . . . . . .
2” PMT Base . . . . . . . .
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279
280
281
281
282
283
283
16.1 Gas Cherenkov counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
16.2 The image from mirror #1 on PMT photo-cathode . . . . . . . . . . . . 289
16.3 The image from mirror #6 on PMT photo-cathode . . . . . . . . . . . . 289
17.1
17.2
17.3
17.4
17.5
Detectors:
Detectors:
Detectors:
Detectors:
Detectors:
Pre-shower Counter HV . .
Pre-shower Counter HV . .
Shower Counter HV . . . .
Map of Pre-shower detector
Map of shower detector . .
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18.1
18.2
18.3
18.4
18.5
18.6
18.7
18.8
Aerogel:layout . . . . . . . . . . . . . . . . . . . . . .
Aerogel:mirrors . . . . . . . . . . . . . . . . . . . . .
Aerogel: amplification chain . . . . . . . . . . . . . .
The diffusion box of A2 detector . . . . . . . . . . . .
Aerogel A1 from inside of the detector . . . . . . . .
The scheme of A1 detector . . . . . . . . . . . . . . .
The scheme of A2 detector . . . . . . . . . . . . . . .
Number of photo-electrons in A1 and A2 vs particle
amplitude spectra . . . . . . . . . . . . . . . . . . . .
19.1
19.2
19.3
19.4
19.5
19.6
19.7
Detectors:
Detectors:
Detectors:
Detectors:
Detectors:
Detectors:
Detectors:
21.1
21.2
21.3
21.4
Right HRS Motion Control . . . . . . .
Right HRS Motion Control - additional
Controls: Hall A Main Control Screen
Controls: Hall A Tools Screen . . . . .
Hadron Arm detector . . . . . . .
FPP HV Termination Board . . .
FPP Readout Board . . . . . . .
FPP Readout Board . . . . . . .
FPP Level Shifter Receiver Board
Hadron Arm Gas Panel . . . . .
FPP Carbon Door GUI . . . . .
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292
294
295
296
297
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momenta
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and the
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304
305
306
310
310
311
311
312
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314
315
316
316
317
320
325
. . . . .
options
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334
335
337
338
LIST OF FIGURES
17
23.1 Data Acquisition: Single Arm Trigger . . . . . . . . . . . . . . . . . . . . 348
23.2 Data Acquisition: Coincidence Trigger . . . . . . . . . . . . . . . . . . . 349
Part I
Hall A OSP Overview
18
Chapter 1
Introduction 1 2
1.1
The Purpose of this Document
This document contains the following information concerning the Hall A “base equipment”:
– safety assessment
– technical overview
– operating procedures
– performance information
A comprehensive description of the equipment performance in given in a published
paper [1].
The operating procedures are intended to provide shift personnel with the information they need to understand, at least at a rudimentary level, the function of the various
subsystems in the end-station. It should also aid in determining if the equipment is
performing properly and provide instructions for what to do in the case of malfunctions.
This document is not intended to be a comprehensive reference but is rather a guide for
the use of on shift personnel. When appropriate, other references are indicated for the
user who requires more information.
It is assumed that the reader of this document has been through all the required
Jefferson Lab safety training. Hence, the material covered in those courses is for the most
part not repeated here. End-station specific safety items are covered in “The Experiment
Safety Assessment Document for the Hall A Base Equipment” which is required reading
for all shift personnel. This document also contains some safety information when deemed
appropriate.
1
2
CV S revision Id: a-intro.tex,v 1.1 2003/06/06 15:26:26 gen Exp
Authors: J.LeRose mailto:[email protected] and E.Chudakov mailto:[email protected]
19
CHAPTER 1. INTRODUCTION
1.2
20
Hall A Overview
The design purpose of Hall A is to study electron scattering on nuclei and nucleons at
high luminosity of up to 5 · 1038 cm−2 s−1 with high momentum resolution. The (e, e0 p)
reaction is often utilized. The spectrometers must have high resolution to be able to
isolate the different reaction channels in nuclei.
The basic lay-out of Hall A is shown in Fig. 1.1, demonstrating the Hall dimensions.
A CAD-drawn 3-dimensional view of the Hall is given on the scalable picture on the
cover page.
Figure 1.1: Schematic cross section of Hall A with one of the HRS spectrometers in the
(fictitious) 0◦ position.
The beam line transports the CEBAF electron beam, in the energy and current
ranges of 0.8 - 6.0 GeV and 0.1 - 120 µA to the target at the Hall center. Various types
of targets have been used, including liquid hydrogen and polarized 3 He gas. Secondary
particles are detected with the two High Resolution Spectrometers (HRS). Both of these
devices provide a momentum resolution of better than 2 × 10−4 and a horizontal angular
resolution of better than 2 mrad at a design maximum central momentum of 4 GeV/c.
The rest of the beam is transported to the high power water cooled beam dump.
The present base instrumentation in Hall A has been used with great success for
experiments which require high luminosity and high resolution in momentum and/or
angle for at least one of the reaction products.
Chapter 2
Hall A Access and Safety 1 2
2.1
The Personnel Safety System
Users and staff working on the accelerator site are protected from the dangers associated
with the prompt ionizing radiation that the accelerator beam produces by the Personnel
Safety System or PSS. The PSS keeps ionizing radiation out of areas where people are
working, and keeps people out of areas where ionizing radiation is present.
There are a total of five states for the Hall A Personnel Safety System: Restricted
Access, Sweep, Controlled Access, RF Power Permit, and Beam Permit.
2.1.1
Restricted Access
Restricted Access is the PSS system state when delivery of beam and/or RF power
is not permitted, and entry to and exit from the hall is not controlled by the Personnel
Safety System. This is the normal state of the hall when the accelerator is off and no
experiments are running. Access is “restricted” only in the sense that the hall is not
open to the general public.
2.1.2
Sweep
Sweep is the state of the PSS when delivery of beam and/or RF power is not permitted and access is limited to the Jefferson Lab personnel conducting the sweep operation.
The hall’s entrance gates are closed from the inside to ensure that no one can enter behind the person conducting the sweep. During the sweep, an Assigned Radiation Monitor
or ARM systematically searches the hall to verify the absence of people and to arm the
run/safe boxes. The ARM posts a guard at the entrance to the hall as another method
of ensuring that no one enters after him.
When the Assigned Radiation Monitor is ready to perform a sweep, the Machine
Control Center or MCC must first place the hall in the Sweep state. The Personnel Safety
System will read “Sweep In Progress.” Once the hall is placed in the sweep state, the
1
2
CV S revision Id:
Authors: J.LeRose mailto:[email protected] and E.Chudakov mailto:[email protected]
21
CHAPTER 2. HALL A ACCESS AND SAFETY
22
sweep monitors enter the first gate to the hall, making sure it locks behind them. The
ARM then notifies the MCC that he is ready to begin the sweep. The MCC communicates
with the sweep monitors via intercom and video camera. Using the video camera, the
MCC makes sure both sweep monitors are wearing the proper dosimetry. At this point
the ARM also indicates that he is in possession of the key needed to arm the Run/Safe
boxes placed throughout the hall. Having confirmed that the dosimetry is adequate, the
MCC will unlock the second entrance gate allowing the sweep monitors to enter the hall.
Once the sweep monitors pass through the second gate, they close the gate and ensure
it is locked. The sweep monitors then proceed to the hall entrance where one sweep
monitor is left to guard the entrance and the other begins the sweep. During the actual
sweep, the ARM walks through every area and secluded workspace in the hall to ensure
that no one could be left inside when the Personnel Safety System moves from the sweep
state to controlled access, power permit, and finally beam permit state. Once he checks
an area, he arms the run/safe box in that area. After all areas of the hall have been
checked and the run/safe boxes armed, the sweep monitors will return to the entrance
where the sweep began. Before arming the last run/safe box, the ARM will contact the
MCC. Upon contact, the MCC will check to see if the sweep has “dropped”; if all is well
he will notify the ARM that it is okay to arm the box. Once the box is armed, the sweep
monitors have 30 seconds to exit both gates or the sweep will drop, and the entire sweep
process will have to be repeated. After exiting, the ARM must contact the MCC to let
them know the Hall can now be moved to the controlled access state.
2.1.3
Controlled Access
Controlled Access is the state of the PSS when delivery of beam and/or RF power
is not permitted but the hall is considered a controlled area. In this state, people are
“counted” both entering and leaving the hall to ensure that no one is left inside when the
Personnel Safety System advances to the RF Permit or Beam Permit states. Hall entry
during the controlled access state is permitted only to people authorized or qualified
by Jefferson Lab . Entry to and exit from the hall is controlled from the MCC. The
Hall cannot be placed in the “controlled access” state without having first been swept.
2.1.4
RF Power Permit
When the PSS is in RF Power Permit the hall is considered an “exclusion area”.
Delivery of RF power is permitted, but beam delivery is not. Reaching this state requires
that the hall has passed through the controlled access state and that no one is left inside
the hall. This is usually a temporary state bridging the transition from the Controlled
Access to the Beam Permit state. Once the Personnel Safety System reads “Power
Permit”, a steady klaxon sounds in the hall. If you are in the hall when this klaxon
sounds, press the emergency safe button on the nearest run/safe box and immediately
exit the hall. The hall entrance gates are locked at this time, but there is an emergency
exit button at each gate which will allow you to exit. A four-minute delay is built in
between the transition from RF Power Permit to Beam Permit.
2.1.5
Beam Permit
CHAPTER 2. HALL A ACCESS AND SAFETY
23
Figure 2.1: Run/Safe box (left) and Access Keys (right)
When delivery of beam and RF power is permitted to the exclusion area the PSS
state is Beam Permit. Reaching this state requires having passed through the RF Power
Permit state.
2.1.6
Run Safe Boxes
The Personnel Safety System includes Run/Safe boxes 2.1 which are located throughout Hall A, and approximately every 100 feet in the linac. A run/safe box has three positions: Safe, Operational, and Unsafe. When the hall is in Restricted Access, the run/safe
box will be in the Safe position. While in this position, the PSS prevents delivery of beam
to the hall. Before beam can be delivered, the hall must be swept to ensure that no one
is left inside. During the Sweep, each run/safe box is moved to the Operational position
in preparation for Beam Permit. After the sweep has been completed and the hall is
placed in the RF Power Permit state, the run/safe box will show Unsafe. Each box has
an emergency stop button. If you see the box in the Unsafe position, you are in danger of
receiving high levels of ionizing radiation. Immediately press the emergency stop button,
exit the hall, and call the Machine Control Center Crew Chief at extension 7050.
2.2
Hall A Access
Access to Hall A is governed by the “Jefferson Lab Beam Containment Policy and
Implementation” document. This document can be found in the Jefferson Lab ES&H
Manual (Section 6310, Appendix T2). Work in designated radiation areas will be governed by the Jefferson Lab RadCon Manual. Access procedures during Research Operations depend on the number of individuals who will be entering the hall and the length
of time they are expected to be there. A controlled access is used when a few individuals
require entry for a short period of time. If the hall must be open for an extended period
and many people will enter, then you should use the restricted access procedure instead
CHAPTER 2. HALL A ACCESS AND SAFETY
24
of the controlled access procedure. Normally, when requesting a controlled access, the
hall will be in either the Beam Permit or RF Permit State - for example, if the beam has
been on or it could be shortly. If the hall is not already in the Controlled Access state
when you wish to access it, you must request a change to that state from the Machine
Control Center at extension 7050 and indicate that you intend to make a Controlled Access. The MCC will then send an Assigned Radiation Monitor to survey the hall. Before
anyone enters the hall, the ARM will carry out a radiation survey and post radiation
areas. Subsequent entry by individuals during the same Controlled Access period does
not require an ARM survey.
2.2.1
Controlled Access Procedure
To make a controlled access when the hall is in the controlled access state, first
contact the MCC. The MCC will unlock the first gate at the entrance to the hall. Once
inside, the MCC will release the master key 2.1. Remove the master key and insert it
into the right-most slot of the row of keys below it. Once the master key is in place,
each person wishing to gain access must remove a key from this row. The MCC will then
verify each person’s name, which key he has, and check that each person is wearing the
proper dosimetry. This key-release procedure allows the MCC to keep a “count” of who
has entered the hall. After the procedure is complete, the MCC will unlock the second
gate at the entrance to the hall. Please note: only one of the entrance gates can be open
at a time while in the controlled access state.
When your work is completed and you are ready to exit, return to the entrance
gates and call MCC (7050) to notify them of your intention to leave. Once you have
entered and closed the first gate, each person must replace his key in the appropriate
slot, otherwise the Personnel Safety System will not allow the master key to be released.
When the master key is released, place it in its slot, and the MCC will unlock the final
gate. When you have exited the final gate, make sure it has closed and locked behind
you. If circumstances dictate, request that the MCC return the hall to the beam permit
state and that beam be restored. It is important to note that if you need to work in the
HRS shield house during the controlled access, you must go to the control room in the
MCC before the access and get a special key which allows you to arm the run/safe box
located in the shield house. The run/safe box inside the shield house will drop from the
operational position to the safe position as soon as the door to the shield house opens.
Unless this box is rearmed with the special key, the beam cannot run.
2.2.2
Restricted Access Procedure
Restricted Access is used when the hall will be open for an extended period of time
or a large group will enter to work. To drop the hall to the Restricted Access state, first
get approval from Run Coodrinator3 (if one is assigned for the given time period) and
Hall A Work Coordinatior4 , then notify the MCC that you wish to open the hall in the
3
The Run Coordination is the immediate on-site manager of the experiment and is appointed for a
period from several days to about two weeks.
4
Ed Folts, pager 584-7857
CHAPTER 2. HALL A ACCESS AND SAFETY
25
Restricted Access state. The MCC will drop the hall status to Controlled Access and
send an ARM to survey the hall. Before anyone can enter the hall, the ARM will carry
out a radiation survey and post radiation areas. The hall is placed in Controlled Access
during the survey to ensure that no one enters before it has been completed. Upon
completion of the survey and posting of radiation areas, the ARM will leave the hall and
notify the MCC that they can drop the hall state to Restricted Access. With the hall in
the Restricted Access state, anyone with the appropriate training may enter and work.
The key- release procedure is not required.
To return the hall to Beam Permit from the Restricted Access state, a full inspection
must be carried out. This is begun by setting all equipment to its operating state (following the Hall A checklist) and then clearing all workers out of the hall. Next, a request
is made to the MCC to arrange a sweep of the hall and to restore the Beam Permit state.
The MCC will send over an ARM and set the hall status to Sweep. The ARM will then
sweep the hall, verifying that everyone is out. Following a successful sweep, the MCC can
move the hall through the Controlled Access and RF Permit states to the Beam Permit
state. While working in the hall you must observe all posted radiation areas. Remember,
work inside a radiation area requires that you obtain an approved radiation permit. You
must also observe the “two-man” rule, and pay attention to the alarms.
2.2.3
Access Requirements
.
Normally only registered experimenters, authorized contractors or sub-contractors
and Jefferson Lab employees may enter experimental areas. In addition, lab policy states
that no one under eighteen years of age is allowed access to the experimental halls.
Lab visitors may work in the halls provided they have completed the full complement
of training courses (EH&S Orientation, ODH, Rad Worker Training and any hall specific
training (the Hall A safety walk through). They must also read/sign any appropriate
documentation (typically the COO and ESAD for the current experiment and the Hall
RWP).
In addition to the above, undergraduate students must undergo a three month trial
period. During this period they may work in the hall provided that:
• Their work in the hall is directly supervised by a hall authorized “buddy” (who
CANNOT be an undergraduate)
• Either a JLab staff member or a fully trained user has supervisory responsibility
for and is fully cognizant of all their work
• The person with supervisory responsibility has approved the “buddy”.
After completion of the trial period undergraduates may be approved for work in
the halls under the standard guidelines.
Physics Division EH&S personnel should be contacted to obtain the current policy
for conducting tours in the experimental areas.
CHAPTER 2. HALL A ACCESS AND SAFETY
26
Figure 2.2: Schematic of the Hall A showing the location of various safety system components. The abbreviations are: Radiation Monitor, RM, Run Safe Box, RS, Fire Alarm
Pull Box, PB.
2.2.4
The Hall A Safety Walk-through
In order to improve user awareness of the systems in the hall, users are required
to complete a self-guided safety walk-through the experimental area. Information about
the walk-through can be found on the web5 . John LeRose is the JLab staff member
responsible for the administration of the Hall A safety walk-through.
Fig. 2.2 shows the location of many of the safety related items in Hall A while Fig. 2.3
shows the location of all the circuit breaker boxes in the hall.
5
http://hallaweb.jlab.org/news/minutes/walkschd.html
CHAPTER 2. HALL A ACCESS AND SAFETY
27
Figure 2.3: Schematic of the Hall A showing the location of the circuit breaker panels.
Part II
Beamline
28
Chapter 3
General Description 1 2
3.1
Introduction
The control and measurement equipment along the Hall A beamline consists of various
elements necessary to transport beam with the required specifications onto the reaction
target and the dump and to simultaneously measure the properties of the beam relevant
to the successful implementation of the physics program in Hall A. The resolution and
accuracy requirements in Hall A are such that special attention is paid to the following:
1. Determination of the incident beam energy;
2. Control of the beam position, direction, emittance and stability;
3. Determination of the beam current;
4. Determination of the beam polarization.
A schematic of the Hall A line starting at the shield wall is shown on Fig. 3.1, 3.2
and 3.3.
3.2
Beam Line Components
The main components of the basic Hall A beamline are described in this section. Table 3.1
gives a listing of all the various elements along the Hall A beamline from the switch yard
to the dump.
3.2.1
The Beam Entrance Channel
The beam entrance channel consists of 63.5 mm inner diameter stainless steel tubing
connected with conflat flanges. Through magnets the inner diameter of the tubing is
1
2
CV S revision Id: beam.tex,v 1.4 2003/11/17 21:10:38 saha Exp
Authors: A.Saha mailto:[email protected]
29
CHAPTER 3. GENERAL DESCRIPTION
30
Figure 3.1: Schematic of the Hall A beamline starting at the shield wall to end of alcove.
Figure 3.2: Schematic of the Hall A beamline from the end of the alcove to the target
chamber.
CHAPTER 3. GENERAL DESCRIPTION
31
Figure 3.3: Schematic of the Hall A beamline from the target chamber to the dump
diffuser.
restricted to 25.4 mm. Sections are isolated by vacuum valves and these are listed in
Table 3.1. Each section has a roughing port and is pumped with an ion pump. The
pressure is about 10−6 Torr. There are several sections along the beamline where users
interface their equipment. Their individual systems are tested leak tight (to ≤ 10−9 Atm
cm3 /sec).
3.2.2
The Beam Optics Channel
These consist of dipoles, quadrupoles, sextupoles, and beam correctors with their standard girders and stands. Starting from the beam switchyard, there are eight dipoles in
the arc section which (along with five other smaller beam deflectors) bend the beam
37.5 degrees into the hall. Each dipole has a quadrupole and a pair of steering magnets
(correctors) associated with it. After the shield wall at the entrance to the tunnel into
the hall the beam is essentially undeflected onto the target and into the dump.
The beamline optics elements are designed to deliver various optical tunes of the
beam on to the physics target as well as simultaneously deliver various optical tunes at
other locations along the beamline. These requirements are listed in Table 3.2. For the
basic beamline we are able to deliver beam onto the hall A target in the achromatic
mode.
3.2.3
Beam Diagnostic Elements
These consist of beam position monitors (BPMs), beam current monitors, wire scanners
(superharps) and beam loss monitors. The wire scanners are fabricated by Saclay (French
CHAPTER 3. GENERAL DESCRIPTION
32
collaboration) and four have been installed along the beamline, two before the arc section
and two after the arc section. They are essential for the beam energy determination by
the arc method. Another two wire scanners are installed on the bench just before the
target to determine the beam position and direction of the beam at the target point with
high precision and also measure the emmitance of the incident beam. They are also used
to absolutely calibrate the two associated beam position monitors located in front of the
target.
3.2.4
Beam Exit Channel
After the target vacuum chamber, which was built by the University of Virginia, there
is an exit beam pipe which transfers the scattered beam onto the dump tunnel under
vacuum. This exit beam pipe is made of a thin walled aluminum spiral corrugated pipe of
welded construction. The largest diameter is 36 inches with a 0.164 inches wall thickness
and the smallest diameter is 6 inches with a 0.042 inches wall thickness. The whole
assembly is rather light (approximately 800 kg) and is supported by H shaped adjustable
stands. To prevent possible linear collapse of the larger diameter sections under vacuum
load, four aluminum channels of total cross-sectional area of 3” are welded to its side. A
vacuum of 10−5 Torr is maintained with a turbomolecular pump. The exit face of this
pipe has a 12” port and is connected to the diffuser with a Beryllium window.
3.3
Authorized Personnel
All magnets (dipoles, quadrupoles, sextupoles, beam correctors) and beam diagnostic
devices (BPMs, scanners, Beam Loss Monitor, viewers) necessary for the transport of
the beam are controlled by Machine Control Center (MCC) through EPICS [2], except
for special elements which are addressed in the subsequent sections. The detailed safety
operational procedures for the Hall A beamline should be essentially the same as the one
for the CEBAF machine and beamline. The Hall A staff liaison with the MCC are:
Primary Contact A. Saha -x7605
Secondary Contact J.P. Chen -x7413
The Liaison between Accelerator Division and Physics Division is:
Liaison H. Areti -x7187
3.4
Additional Safety Information
Additional safety information is available in the following documents:
– EH&S Manual [3];
CHAPTER 3. GENERAL DESCRIPTION
33
– PSS Description Document [4]
– Accelerator Operations Directive [5];
– OSP - CEBAF Beam Operations (updated annually)
3.5
Machine/Beamline protection system
The MPS system is composed of the fast shutdown system (FSD), beam loss monitor
(BLM), and gun control system.
The FSD system is a network of permissive signals which terminate at the electron
gun and chopper 1. The permissive to the gun and chopper 1 may be inhibited by any
device connected to an FSD mode. Devices connected to the FSD system include vacuum valves, RF systems, Beam loss systems, beam current monitors, beam dumps, and
particular to Hall A, the target motion mechanism and the raster (value and derivative).
The gun control system includes software program which monitors beam operating
conditions and the state of the FSD and BLM systems. the program will warn the
operators if a potential for beam damage exists. Potential for damage exists when running
high average current beam, when FSD nodes are masked and when the beam power
approaches the operating envelope limits for a specific beam dump.
3.6
Personnel Safety System
Personnel safety system (PSS) console equipment is located in the main control room. It
includes:
• Access control
• Safety interlock
• ODH monitoring
• Radiation monitoring
• Public address system.
Girder
#
Diagnostic
Elements
Optics
Valves
Elements
& Pumps
Beam Switch Yard
TV Viewer
1CB2
1CB3
Element
#
ITV2C00
MLA1C02
Dipole (2.3m +
0.13m shim)
BD
Valve
MBD1C00V
VBV1C00A
Distance (m)
from target
148.12862
146.60000
144.18700
∼142.78686
CHAPTER 3. GENERAL DESCRIPTION
1C01
Current
Beam Stopper
Beam Stopper
BPM
BC
1C02
BPM
BC
1C03
BPM
BC
1CB4
1C04
Dipole
BPM
Quad
BC
1C05
BPM
Quad
BC
1C06
BPM
Quad
BC
1C07
BPM
BC
1C08
BPM
Quad
BC
Sext
TV View
1CB5
1C09
Dipole
Quad
SBC1C00
SSS1C00
SSS1C00A
IPM1C01
MQA1C01
Ion Pump
VIP1C01
IPM1C02
MQA1C02
MBC1C02V
IPM1C03
MQA1C03
Ion Pump
VIP1C03
Rough Pump VRV1C03
Convectron
VTC1C03
(1m)
MBN1C04
IPM1C04
MQA1C04
MBC1C04H
Ion Pump
VIP1C04
IPM1C05
MQA1C05
MBC1C05V
Valve
VBV1C05A
Ion Pump
VIP1C05
Rough Pump VRV1C05
Convectron
VTC1C05
Valve
VBV1C06
IPM1C06
MQA1C06
MBC1C06H
Ion Pump
VIP1C06
IPM1C07
MBC1C07V
Ion Pump
VIP1C07
Arc Section ↓
IPM1C08
MQA1C08
MBC1C08H
MSA1C08
ITV1C08
Ion Pump
VIP1C08
(3m)
MBA1C05
MQA1C09
34
∼136.99564
∼136.53844
∼136.08124
134.32465
133.95000
133.04195
132.02465
131.65000
131.30685
129.02465
129.35000
128.44195
116.70000
115.42465
115.05000
114.70685
114.14195
112.12465
111.75000
111.40685
110.84195
104.82465
104.45000
104.10685
103.54195
99.52465
98.61076
98.24195
93.42465
93.05000
92.70685
92.40500
92.21180
90.30000
87.85000
CHAPTER 3. GENERAL DESCRIPTION
1CB6
1C10
BC
Sext
Dipole (3m)
BPM
Quad
BC
Sext
1CB7
1C11
Dipole (3m)
Quad
BC
Sext
1CB8
1C12
Dipole (3m)
BPM
Quad
BC
Sext
1CB9
1C13
1CB10
1C14
BPM
Dipole (3m)
Quad
BC
Sext
Dipole (3m)
Quad
BC
Sext
1CB11
1C15
Dipole (3m)
Quad
BC
Sect
Dipole (3m)
1CB12
1C16
BPM
Quad
BC
Shield
MBC1C09H
MSA1C09
MBA1C06
IPM1C10
MQA1C10
MBC1C10H
MSA1C10
Ion Pump
VIP1C10
MBA1C07
MQA1C11
MBC1C11V
MSA1C11
Covectron
VTC1C11
MBA1C08
IPM1C12
MQA1C12
MBC1C12H
MSA1C12
Ion Pump
VIP1C12
MBA1C09
MQA1C13
MBC1C13V
MSA1C13
MBA1C10
IPM1C14
MQA1C14
MBC1C14H
MSA1C14
Ion Pump
VIP1C14
MBA1C11
Valve
VBV1C15
Rough Pump VRV1C15
MQA1C15
MBC1C15V
MSA1C15
MBA1C12
Ion Pump
VIP1C16
Convectron
VTC1C16
IPM1C16
MQA1C16
MBC1C16H
Valve
VBV1C16
Wall → Hall A
35
87.50685
87.20500
85.10000
83.02465
82.65000
82.30685
82.00500
81.81180
79.90000
77.45000
77.10685
76.80500
76.61180
74.70000
72.62465
72.25000
71.90685
71.60500
71.41180
69.50000
67.05000
66.70685
66.40500
64.30000
62.22465
61.85000
61.50685
61.20500
61.01180
59.10000
56.65000
56.30685
56.00500
53.90000
51.82465
51.45000
51.10685
CHAPTER 3. GENERAL DESCRIPTION
36
SHIELD WALL(entrance surface)
SHIELD WALL (exit surface)
1C17
TV Viewer
Quad
1C18
BPM
Quad
BC
BC
French Scanner
Bench Scanner
1C19
Quad
1C20
BPM
Quad
BC
BC
Ion Pump
ITV1C17
MQA1C17
IPM1C18
MQA1C18
MBC1C18H
MBC1C18V
IHA1C18A
IHA1C18B
MQA1C19
IPM1C120
MQA1C20
MBC1C20H
MBC1C20V
VIP1C20
COMPTON Polarimeter Region
Ion Pump
Ion Pump
Current
Fast Raster
Valve
eP Energy Target
1H01
TV View
BPM
Valve
Quad
BC
Moller target
1H02
1H03
1H03
Moller
1H04
1H04
Bench
BLM
BPM
Quad
Quad
Quad
Dipole
Quad
Quad
BD
VIP1C20A
VIP1C20B
IBC1H00
IUN1H00
IBC1H00A
MRA1H00H
MRA1H00V
VBV1H00B
VTP1H00A
VBV1H01
ITV1H01
IPM1H01
MQA1H01
MBC1H01H
MQM1H02
MQM1H03
MQO1H03A
MMA1H03
MQA1H04
MQA1H04A
MBD1H04H
IBC1H04A
IPM1H04A
50.70700
49.651
49.411
49.100
48.650
48.300
47.957
47.761
47.381
43.673
43.000
42.550
42.200
41.857
41.661
41.450
41.000
25.500
34.500
29.500
24.501
23.000
22.053
19.999
19.018
18.938
18.650
18.300
17.957
17.500
16.500
15.415
14.758
13.272
9.362
8.676
8.133
7.906
7.517
CHAPTER 3. GENERAL DESCRIPTION
37
Scanner
BPM
BPM
Current
Ion Pump
BPM
BPM
Current
OTR
BPM
Scanner
Valve
Radiator
IHA1H04A
IPM1H04BH
IPM1H04BV
IBC1H04B
VTP1H04
VTC1H04A
IPM1H04CH
IPM1H04CV
IBC1H04C
IOR1H04
IPM1H04B
IHA1H04B
VBV1H04B
ERR1H
ITV1H03A
TARGET TV Viewer
DUMP Face
Table 3.1: Hall A beamline elements from switchyard to
Hall A beam dump (revised - 11/17/03)
All distances are from the center of each element to the target (in meters).
Mode
Spot Size
4*σx,y
Achromat
140µm
Dispersive
∝ ηδ
Defocussed 0 to 3mm
Dispersion
η
0
4m to 12m
0
Position
Stability
50µm
50µm
±10%
Size
Stability
50µm
50µm
±10%
Table 3.2: Line A Optics and Beam Requirements at Target
7.354
6.829
6.533
6.256
4.493
3.784
3.488
3.211
2.673
2.378
2.215
2.046
0.726
0.000
-50.000
CHAPTER 3. GENERAL DESCRIPTION
Mode
Spot Size
4*σx,y
Energy-eP(d)
>100µm
Moller Pol. (d) >250µm
Compton Pol.
>80µm
*Energy-arc(d)
Dispersion
η
0
0
0
15m
38
Position
Stability
50µm
50µm
50µm
50µm
Size
Stability
50µm
50µm
50µm
50µm
Table 3.3: Line A Optics and Beam Requirements at Other Locations.
Notes:
* Build dispersion in arc section with all magnetic elements except dipoles turned off.
(d) Destructive measurements.
3.7
Beam Position Monitors
3 4
To determine the position and the direction of the beam on the experimental target point,
two Beam Position Monitors (BPMs) are located at distances 7.524 m (IPM1H03A) and
1.286 m (IPM1H03B) upstream of the target position. The BPMs consist of a 4-wire
antenna array of open ended thin wire striplines tuned to the fundamental RF frequency
of 1.497 GHz of the beam [6]. The standard difference-over-sum technique is then used
[7] to determine the relative position of the beam to within 100 microns for currents above
1 µA. The absolute position of the BPMs can be calibrated with respect to the scanners
(superharps) which are located adjacent to each of the BPMs (IHA1H03A at 7.353 m and
IHA1H03B at 1.122 m upstream of the target). The schematic of the readout electronics
is shown in Figure 3.4. The position information from the BPMs can be recorded in
three different ways:
1. The averaged position over 0.3 seconds is logged into the EPICS [2] database (1 Hz
updating frequency) and injected into the datastream every 3-4 seconds, unsynchronized
but with an orientative timestamp. From these values we can consider that we know the
average position of the beam calculated in the EPICS coordinate system which is left
handed.
2. Approximately once a shift (or more often if requested by the experimenters) a
B-scope procedure [8] can be performed using the same EPICS electronics which then
gives the peak-to-peak variation of the beam.
3. Event-by-event information from the BPMs are recorded in the CODA datastream
3
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CHAPTER 3. GENERAL DESCRIPTION
39
[email protected] ESRUT
A .2=
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56_$s)r Z[;?(*&)Z[[email protected]=
\ 0RvPwxwWy{z l%A 10RUT t)u R
O |~}59v l%A 10RUT
+ q f:a^(*3hbdcegjb X ikb g
\ €RvPwxwWy{z l%A 10RUT t)u R
^lol nHDJ#p m: "C;
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XY([Z[;[email protected]=
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XY([Z[;[email protected]=
EF([email protected]=
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\-] A^D A^D 56_;>_`_;?.2=
‚;?.ƒyxw c | t
Figure 3.4: Schematic of the BPM readout electronics
†q
CHAPTER 3. GENERAL DESCRIPTION
40
from each of the 8 BPM antennas (2x4) from which the position of the beam can be
reconstructed. However, these raw values belong to a parallel electronics chain whose
constants have to be retrieved by calibrations to the EPICS or scanner data.
CHAPTER 3. GENERAL DESCRIPTION
3.8
Beam Current Measurement
41
5 6
The Beam Current Monitor (BCM) is designed for stable, low noise, non-intercepting
beam current measurements. It consists of an Unser monitor, two rf cavities, the electronics and a data acquisition system. The cavities and the Unser monitor are enclosed
in a box to improve magnetic shielding and temperature stabilization. The box is located
25 m upstream of the target. You can recognize it as a grey object on the stands, about
2 m downstream from where the beam enters the hall.
The DC 200 down-converters and the Unser front end electronics are located in Hall
A. The temperature controller, the Unser back end electronics and its calibration current
source, cavity’s RF unit (housing the RMS-to-DC converter board) and all multi-meters,
VME crate and computers are located in Hall A control room.
3.8.1
System Layout
The schematic diagram of the BCM system is presented in Fig. 3.5.
The Unser monitor is a Parametric Current Transformer designed for non-destructive
beam current measurement and providing an absolute reference. The monitor is calibrated by passing a known current through a wire inside the beam pipe and has a
nominal output of 4 mV/µA. It requires extensive magnetic shielding and temperature
stabilization to reduce noise and zero drift. As the Unser monitor’s output signal drifts
significantly on a time scale of several minutes, it cannot be used to continuously monitor the beam current. However, this drift is measured during the calibration runs (by
taking a zero current reading) and removed in calibrating the cavities. The more stable
cavities are then used to determine the beam current and charge for each run. We also
use the OLO2 Cavity Monitor and the Faraday Cup 2 at the Injector section to provide
an absolute reference during calibration runs.
The two resonant rf cavity monitors on either side of the Unser Monitor are stainless
steel cylindrical high Q (∼ 3000) waveguides which are tuned to the frequency of the
beam (1.497 GHz) resulting in voltage levels at their outputs which are proportional to
the beam current. Each of the rf output signals from the two cavities are split into two
parts. One part of the signal is converted to 10 kHz signals (by the “downconverters”)
and fed into an RMS-to-DC converter board consisting of a 50 kHz bandpass filter to
eliminate noise, amplified and split to two sets of outputs, which after further processing
are recorded in the data stream. These two paths to the data stream (leading to the
sampled and integrated data ) will now be described. (The other part of the split signal
is downconverted to 1 MHz signals and represents the old system (pre Jan 99). Only the
HAPPEX collaboration presently uses these signals.)
For the sampled (or EPICS [2] or Slow) data, one of the amplifier outputs is sent
to a high precision digital AC voltmeter (HP 3458A). Each second this device provides a
digital output which represents the RMS average of the input signal during that second.
5
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CHAPTER 3. GENERAL DESCRIPTION
42
KEITHLY
I-Source
10 kHz
D.C.
PCT
front
10 kHz
D.C.
Downstream
BCM
UNSER
Upstream
BCM
1 MHz
D.C.
e- Beam
Hall A BCM System
1 MHz
D.C.
Gigatronics
Power Meter
Exp. Hall
PCT
back
DVM
10 kHz IF Out
DVM
DOWNSTREAM
+
RMS-DC
DCx1 to V2F
RMS-DC
DCx3 to V2F
RMS-DC
DCx10 to V2F
RMS-DC
DCx1 to V2F
RMS-DC
DCx3 to V2F
RMS-DC
DCx10 to V2F
f = 10 kHz
BW = 50 kHz VSB
UPSTREAM
+
f = 10 kHz
BW = 50 kHz VSB
10 kHz IF Out
DVM
10 kHz RMS-DC
DOWNSTREAM
+
RMS-DC
DCx1 to V2F
RMS-DC
DCx3 to V2F
f = 1 MHz
BW = 100 kHz DSB
UPSTREAM
RMS-DC
1 MHz IF
DCx10 to V2F
Out
DVM
+
RMS-DC
DCx1 to V2F
f = 1 MHz
BW = 100 kHz DSB
1 MHz IF
Out
1 MHz RMS-DC
Counting House
Figure 3.5: Schematic of the Hall A beam current measurement system.
DVM
CHAPTER 3. GENERAL DESCRIPTION
43
The resulting number is proportional to the beam charge accumulated during the corresponding second (or, equivalently, the average beam current for that second). Signals
from both cavity’s multi-meters, as well as from the multi-meter connected to the Unser,
are transported through GPIB ports to the HAC computer where they are recorded every
1 to 2 seconds via the data-logging process which is described in the calibration procedure. They are also sent through EPICS to CODA and the data stream where they are
recorded at quasi-regular intervals, typically every two to five seconds.
For the integrated (or VTOF or Fast) data, the other amplifier output is sent to an
RMS-to-DC converter which produces an analog DC voltage level. This level drives a
Voltage-To-Frequency (VTOF) converter whose output frequency is proportional to the
input DC voltage level. These signals are then fed to Fastbus scalers and are finally
injected into the data stream along with the other scaler information. These scalers
simply accumulate during the run, resulting in a number which is proportional to the
time integrated voltage level and therefore more accurately represents the true integral
of the current and hence the total beam charge. The regular RMS to DC output is linear
for currents from about 5 µA to somewhere well above 200 µA. Since it is non-linear at
the lower currents, we have introduced a set of amplifiers with differing gains (x3 and
x10) allowing the non-linear region to be extended to lower currents at the expense of
saturation at the very high currents. Hence there are 3 signals coming from each BCM
(Upx1, Upx3, Upx10, Dnx1, Dnx3, Dnx10). All 6 signals are fed to scaler inputs of each
spectrometer (E-arm and H-arm) . Hence we have a redundancy of 12 scaler outputs for
determining the charge during a run. During calibration runs we calibrate each of these
scaler outputs.
3.8.2
Authorized Personnel
All Hall A members are authorized to take BCM calibration data using the Standard
Non-Invasive Hall A BCM Calibration Procedure. The extended calibration procedures
involving the Faraday Cup 2 and the OLO2 monitor at the Injector are presently performed by A. Saha.
The Accelerator AES group performs the maintenance of the BCM monitors. These
include:
1. The Unser calibration.
Every 3 months
2. Resonant Cavities Tuning.
Every Downtime
3. Multi-meters Autocalibration.
Every Downtime
4. Connectors Cleaning.
Every year
5. Unser Keithley Current Source.
Calibration Yearly
6. Digital Multi-meters HP3458A and HP 34401A. Calibration Yearly
System Contacts:
Arun Saha -x 7605
Jean-Claude Denard -x 7555
CHAPTER 3. GENERAL DESCRIPTION
3.9
Arc Energy Measurement
44
7 8
The ARC energy measurement is under EPICS [2] control through a MEDM [9] display.
Two independent control systems are used: the beam bend angle measurement through
the arc (”scanners”) and the field integral of the arc (”integral”). To measure the energy:
• perform several angle measurements
• perform an integral measurement
• analyze the integral measurement and note the value of the arc field integral
• analyze the angle measurements, average the results (proposed by the software),
then ask for the energy calculation, enter the above arc field integral and you will
get the beam energy computed from the average angle.
3.9.1
Summary of ARC operations
Six scanners of the same type, called “ARC scanner” and labelled from scanner #1 to
#6, are installed on the Hall-A beamline. Scanners #1 to #4 are used for the ARC
energy measurement and they are located on the Hall-A arc: #1 [1HA1C07A] and
#2 [1HA1C07B] just upstream of the arc, in the BSY, and #3 [1HA1C18A] and #4
[1HA1C18B] in the Hall-A tunnel, just upstream the Compton polarimeter. Scanners
#5 [1HA1H03A] and #6 [1HA1H03B] are located between the Moller and the target to
control the beam geometry on the target and their use will not be discussed here.
Procedure for running a harp scan is described in:
scanrun.
Each scanner has a motor/ball-screw/shaft-encoder/vacuum-penetrator system moving accurately a set of 3 tungsten wires through the beam. Each time a wire crosses the
beam a PMT located a few meters downstream records a signal due to the electromagnetic shower induced by the beam in the wire. Both forward and backward passes are
recorded. The motion is a horizontal translation and, for a forward pass:
-the translation is from beam left to beam right,
-the two first wire crossing the beam are at 45deg from the vertical,
-the third wire, which is the only important for the ARC energy measurement, is
vertical.
Recording, during the scan, the scanner position and the PMT output voltage allows
us to determine the beam position at each scanner location. Then, using calibration
data not detailed here, we deduce the net beam bend angle through the arc. This result
measured in dispersive arc tuning, along with the field integral of the arc dipoles, provides
an accurate determination of the beam energy.
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CHAPTER 3. GENERAL DESCRIPTION
3.9.2
45
Summary of field integral
The purpose is to measure absolutely the straight field integral of a ”BA” 3m long dipole,
called the ”9th dipole” and located in the ”Dipole Shed”. It is of the same type as the
8 arc dipoles and is powered in series with them.
The ARC integral setup is basically made of a 3m long plate (the ”probe”) which
is able to move inside the 9th dipole gap along the beam axis and carrying two field
measurement devices: a pair of pick-up coils connected in series and a set of NMR
probes. The coils are on both ends of the probe and the NMRs close to the center.
-at the ”upstream” probe position, the ”downstream” coil is close to the dipole
center, the ”upstream” is outside the dipole and the NMRs at one end of the dipole:
Door< −– ....................< −——-DIPOLE—–− − − >
.............< −——-PROBE——− − − >
-at the ”central” probe position, each coil is at one end of the 3m long dipole and
the NMRs close to the dipole center:
Door< −– ...................< −——-DIPOLE—–− − − >
..................................< −——-PROBE——− − − >
-at the ”downstream” probe position, the ”upstream” coil is close to the dipole
center, the ”downstream” is outside the dipole and the NMRs at one end of the dipole:
Door< −– ...................< −——-DIPOLE—–− − − >
....................................................< −——-PROBE——− − − >
We call upstream the position where the probe is the closest to the shed access door.
Among the 3 above positions, the only one where the NMR can lock on the dipole field
is the central one as in the extreme position of the probe, the field homogeneity is not
sufficient. The probe position is controlled by a linear encoder. The Z axis refers to the
”beam” direction, increasing from upstream to downstream. We use three kinds of ”Z”:
-Zm to locate a point inside the magnet. The dipole center is at Zm=0 and the yoke
ends at +-1500.mm
-Zp to locate a point inside the probe. The probe center is at Zp=0. Each of the 4
NMR probes has a Zp given in the file ”magnet.dir”. At a temperature of 21C, the coils
are at Zp=+-1519.815mm (from magnet.dir)
-Zd to refer to a displacement of the probe w.r.t. the dipole. Zd=0 refers to the
upstream (home) position of the probe. The integral measurement is performed from
Zd=0.000mm (1st PDI trigger) to Zd=3199.000mm (last PDI trigger), for forward pass.
Zd is given by the display (at the top of the rack) or by the master screen (”OUT”).
The relationship between Zm, Zp and Zd is:
Zd-Zm+Zp=C
where C is a constant given in magnet.dir (C=1604.000 nomin.). Example of use:
to have the probe center at the dipole center, one must set Zd=1604.000mm (set Zm=0
and Zp=0 in the above formula, and solve for Zd)
The integral measurement sequence is the following:
-from the current position (a priori arbitrary) move the probe upstream, up to a
limit (optic) switch.
CHAPTER 3. GENERAL DESCRIPTION
46
-move downstream by a few mm to cross the encoder index (encoder initialization)
-move to the central position to measure the central field by NMR, the system checks
if the NMR locks and if the reading is stable, it will be the ”before” field
-move back to upstream position
-move to downstream position while integrating the flux through the coil system,
this measurement will be called the ”forward” integral (duration ∼ 7s)
-move back to upstream position while integrating the flux through the coil system,
this measurement will be called the ”backward” integral (duration ∼7s)
-move to the central position to measure the central field by NMR, the system checks
if the NMR locks and if the reading is stable, it will be the ”after” field.
In addition to the central field, 4 probe temperatures, a local excitation current
measurement, the setting of the dipoles P.S, the readback of the dipoles P.S and the
probe position at NMR measurement time are recorded ”before” and ”after”.
To perform an integral field measurement:
1-check if the system works (see ”details on integral system check” below)
2-run the above integral sequence (see ”details on integral run” below)
3-fix the error(s) if any (see ”details on integral errors” below)
4-save the data in a file (see ”details on integral data save” below)
5-analyze the data (see Arun Saha).
3.9.3
Details on integral run
To run the integral measurement sequence, call the arc integral.adl medm screen, then:
-push ”start” to start the full sequence
-look at the results displayed:
-after the ”before” NMR measurement: the ”before” data set
-after the ”forward” integral pass: the forward velocity profile and the forward
voltage-after-gain profile
-after the ”backward” integral pass: the backward velocity profile and the backward
voltage-after-gain profile
-after the ”after” NMR measurement: the ”after” data set
-if ”BAD NMR” or ”PDI saturation” flags are set, or if something is obviously wrong
in the data or plots, call expert.
-data are ready to be saved (see ”Details on integral data save” below)
3.9.4
Details on temperatures
The AC system of the shed is made of two cooling units, a heating unit and a controller
connected to two temperature sensors : one located in the shed and one located in the
BSY. This system is programmed in such a way that the temperature of the shed follows the BSY temperature within +-2C. The BSY temperature can be anywhere in the
[18C,35C] range, regardless of the season. The BSY temperature and the shed temperature are given (in F) by a display panel located close to the workstation, on the wall.
CHAPTER 3. GENERAL DESCRIPTION
47
The AC system can be set in manual control by turning from ”auto” to ”manual” a set
of switches controlling the cooling units and the heater unit. These switch boxes are
located on the shed wall. If the shed temperature is above 34.4C (94F), call Arun Saha
(the electronics can be damaged) and cool down the shed in manual AC mode. The 4
temperature sensors of the probe are labelled Tx+z+, Tx+z-, Tx-z+, Tx-z- depending
on their position w.r.t. the following (x,z) frame:
Door< −--
x
^
|
|
< −------------D I P|O L E ----------− >
|
< −----Tx+z- -------|------Tx+z+-----− >
|
|
|
|
|---------− >z
|
|
|
|
P R O B E
|
< −----Tx-z- --------------Tx-z+-----− >
Both ”x+” sensors are on the probe edge which is inside the dipole gap and both
”x-” sensors on the opposite edge which is outside the dipole gap. Both ”z-” sensors
are at 1/4 of the long dimension of the probe and both z+ at 3/4 of this length. The
average of the 4 temperatures is used by the analysis program to correct the coil distance
from the thermal expansion of the probe, so it is important to make sure that the 4
sensors are working well. The user can just make sure that the temperatures displayed in
arc-master.adl or recorded in arc-integral.adl are realistic. In arc-integral.adl they are given
in the order: Tx+z-, Tx+z+, Tx-z-, Tx-z+ Tx-z- and Tx-z+ should be close to the shed
temperature. Tx+z- and Tx+z+ depend on the probe position, as the gap (iron yoke)
is warmer than the shed and the dipole coil (at both ends of the dipole) is warmer than
the iron yoke. For a probe in a central position for more than about one hour, the Tx+zand Tx+z+ sensors should give the yoke temperature, i.e the shed temperature plus 0.
to 5.C, depending on the current, LCW temperature and the magnet/shed temperature
history. The 4 temperatures are also displayed inside the shed, on the electronics rack.
These values are digitized by separate ADCs, so they may differ from the remote values
by ∼0.1C.
3.9.5
Shed access and safety
For safety reasons, the access to the shed is limited to authorized persons which are
listed in the ESAD and listed below. To be added to the list, ask the Hall-A leader. The
standard operation mode of the integral measurement setup is the remote mode, through
the network, from the counting house. In case of problem needing an access in the shed,
unauthorized users must contact Arun Saha.
CHAPTER 3. GENERAL DESCRIPTION
3.9.6
List of Authorized Personnel for Shed Access
Pascal Vernin
Jacques Marroncle
Christian Veyssiere
Francois Gougnaud
Arunava Saha
Mike Tiefenback
Yves Roblin
Rick Gonzales
Bill Merz
Mark Augustine
Hari Areti
Pete Francis
Douglas Higinbotham
Scott Higgins
David Seidman
Ron Lauze
Tony Day
48
CHAPTER 3. GENERAL DESCRIPTION
3.10
Fast Raster
49
9 10
The beam is rastered on target with an amplitude of several millimeters to prevent
overheating. The raster is a pair of horizontal (X) and vertical (Y) air-core dipoles
located 23 m upstream of the target. The raster has been used in two different modes,
sinusoidal and amplitude modulated. In the sinusoidal pattern both the X and Y magnet
pairs are driven with pure sine waves with 90◦ relative phase, and frequencies which do
not produce a closed Lissajous pattern. In the amplitude modulated (or square root of
time) mode both the X and Y magnets are driven at 18 kHz with a 90◦ phase between X
and Y producing a circular pattern. The radius of this pattern is changed by amplitude
modulation at 1 kHz. The radius modulation is controlled by a function generator whose
function creates a uniform distribution of the area swept out by the beam motion. It is
not possible to switch on the fly between the two modes of operation as hardware changes
are required.
One can view the status of the raster in the EPICS overview screen called “General
Accelerator Parameters” where the set-point for the radius amplitude and the readback
of the peak-current in the raster are displayed.
Control of the raster is done by first asking the MCC operators to set up the raster
for a particular radius, typically 2.5 mm. The control software assumes a field-free region
between the raster and the target, so it is only approximately correct because there are
four quadrupoles in this region. It is important to check the raster spot size and make
adjustments if necessary. The main adjustment is made by asking MCC to change the
radius. Relatively small independent adjustments to the gains on the X and the Y raster
coils are available in the middle room of the hall A counting room using the “PGA
Controller” knobs. Near these knobs is also located an oscilloscope X-Y trace of the
current in the raster. A fast shutdown (FSD) shuts the beam down within 0.1 msec if
the raster fails, thus affording some protection of the target.
NOTE: If you are unsure of the status of the raster, measure the spot size with very
low current (≤ 2µA) or with the target out of the beam. An unfortunately common
and potentially fatal error is to check the beam spot size with high current on target;
by the time you check it, the target might already be destroyed. The rastered beam
spot on target can be checked with plots in ESPACE or by using the stand alone code
called “spot”. Spot is probably already running somewhere; it runs on the Linux or Sun
computers in the counting room; if it’s not running, type “spot”. When a new CODA
run is started, spot automatically clears its histograms and displays in a window the X-Y
beam position monitor coordinates from the first few thousand events from the start of
the run. For more details on usage, type “spot -h” (help).
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CHAPTER 3. GENERAL DESCRIPTION
3.11
Bremsstrahlung Radiator
3.11.1
Overview
50
11 12
The Bremsstrahlung radiator is the last element in the Hall A beam line before the
scattering chamber, and is about 72.6 cm from the center of the physics targets. Its
design is based on the Hall C radiator system built by David Meekins, and documented
in the Hall C operations manual.
The central component of the system is a U-shaped, oxygen-free copper target ladder,
with six positions for differing thicknesses of oxygen-free Cu foils. The ladder is designed
so that it never intersects the beam. The 3.175-cm wide gap in the ladder is spanned only
by the target foils, which are 6.35 cm wide, 3.175 cm high, and 3.332 cm apart (center to
center). A stepper motor moves the target ladder with foils up and down, into and out
of the beam. Hard stops prevent motion of the ladder beyond the limit switches. Water
cooling of the radiator ladder cools the foils, preventing damage from overheating by the
beam.
The interaction of the beam with the foils produces background radiation in the Hall.
At 3 GeV, ion chamber trip levels do not need to be adjusted, and increases in detector
background rates are minimal; further tests are planned for 0.8 GeV. No local shielding
is installed, as calculations indicate that this will not significantly affect dose at the site
boundary. Any installation and/or subsequent modifications must be coordinated with
RadCon.
3.11.2
Safety Issues
The only safety issue concerning the Bremsstrahlung radiator is that of induced radioactivity in the Cu targets and in the water used for cooling the targets. The water cooling
system is a closed loop, using a portable welding-torch water cooler, located under the
beam line just upstream of the target. The cooler is kept in a tray which is intended
to provide secondary containment in case of a leak. The cooling system must not be
breached or drained without concurrence from the RCG. Accidental breach or spill constitutes a radiation contamination hazard. A spill control kit, capable of containing a
system leak or spill, is staged by the door to the hall. In the event of a spill notify the
RCG.
3.11.3
Operations
Although the radiator foils are water cooled, a high current electron beam may melt the
foils. Beam currents with the radiator will be limited to 30 micro-amperes. Including a
safety factor, the raster radii given in Table 1 will limit the temperature rise to 100 ◦ C.
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51
Table 3.4: Raster radius as a function of beam current.
Current (µA) Minimum raster radius (mm)
10
15
20
25
30
(not needed)
0.2
0.7
1.3
2.1
Table 3.5: Encoder voltage calibration. See text.
Position Voltage ratio Vencoder for Vsupply = 5 V
out limit
foil 1
foil 2
foil 3
foil 4
foil 5
foil 6
in limit
0.030
0.102
0.269
0.436
0.603
0.769
0.936
0.966
0.15
0.511
1.346
2.179
3.013
3.847
4.681
4.831
The only operational control consists of moving the ladder in and out. Radiator
position is determined by the ratio of the readback voltage from a linear encoder to the
voltage applied to it. Table 2 gives the radiator position as a function of this ratio. The
foil thicknesses are set so that the thickness, in percent of a radiation length, equals the
foil number, except that no foils are mounted in position 1.
Software controls of the ladder position are under development; radiator position is
changed by calling MCC and requesting that the radiator be set to some foil position, or
to the out limit. The position may be changed with beam on. A manual-control backup
system also exists.
Both software and manual backup systems control an Oregon Micro Systems MH10DX
step motor driver, which drives a Slo-Syn M063-LS09 stepper motor. The MH10 driver,
power supplies, and other control circuitry, are in a custom-built box located in the hall
in rack 1H75B10. The linear encoder voltage ADC is in slot 5 of the CAMAC crate
in rack 1H75B02; radiator inputs use channels 15 and 16, and are connected through a
patch panel to block 30 in rack 1H75B08.
When the radiator is not being used, the system should be set to the out-limit
CHAPTER 3. GENERAL DESCRIPTION
52
position, so that it is clear of the beam. Power to the control box in the hall may be
turned off with a front-panel switch if the radiator will not be used for a long time - and
should be turned off if work is to be done on the radiator. This deactivates the limit
switches and the linear encoder, but does not affect positioning. Additional hard stops
should be installed as a safety measure. The Hall A technical staff checklist, done as part
of preparations for closing the Hall for beam, includes checking the radiator position, the
status of the control box, and the installation of hard stops.
3.11.4
Special Instructions
Care must be taken in case any removal or disassembly of the radiator system is needed.
Disconnecting the stepper motor from the motor driver while power is on can damage
the motor, motor driver, and VME44 board.
The Cu targets will certainly be activated in the course of an experiment. Therefore,
only remove the Cu target, the target ladder, and/or the whole radiator system in the
presence of a Radcon officer.
Ron Gilman should be informed in case of any problems with the radiator. Except
for normal operations of the radiator, any work on the system hardware requires that
RadCon has concurred in the work and either Ron Gilman or David Meekins is present.
Chapter 4
eP Beam Energy Measurement 1 2
4.1
Purpose and Layout
The Hall A eP system is a stand-alone device to measure the energy of the electron beam.
It is located along the beamline 17 m upstream of the target. The beam energy E is
determined by measuring the scattered electron angle Θe and the recoil proton angle Θp
in the 1 H(e, e0 p) elastic reaction according to the kinematic formula:
E = Mp
cos(Θe ) + sin(Θe )/ tan(Θp ) − 1
+ O(m2e /E 2 ),
1 − cos(Θp )
(4.1)
in which Mp denotes the mass of the proton and me the mass of the electron. The
schematic diagram of the eP system is presented in Fig. 4.1. Two identical arms, each
consisting of an electron and a corresponding proton detector system, made up of a set
of 2 x 8 silicon micro-strip detectors in the reaction plane, are placed symmetrically with
respect to the beam along the vertical plane. The target consists of a rotating CH2 tape.
Simultaneous measurements of the beam energy with both arms result in cancellation, to
first order, of uncertainties in the knowledge of the position and direction of the beam.
4.2
4.2.1
Description of Components
High Voltage
The eP system is equipped with two gas Cherenkov detectors and altogether 18 scintillators. The high voltage for the photomultiplier tubes of these detectors are provided by a
LeCroy 1450 HV power supply, located in the electronics racks along the beamline. The
channel assignment and HV voltages (as of summer 2003) are given in Tab. 4.1.
The standard way to control the high voltage is the use of the Hall A MEDM [9]
graphical user interface (EPICS [2]), which is running on the hacsbc2 computer. This
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54
Figure 4.1: Schematic layout of the eP energy measurement system, showing the arrangement of its components, the polyethylene (CH2 ) target, the Cherenkov detectors,
the silicon micro-strip detectors (SSD) for protons and electrons, and the scintillator
detectors.
computer is located in the counting house, but can also be accessed from other terminals.
Usually at least one terminal in Hall A itself has a MEDM screen running, as well. If it
is not running, log into hacsbc2 as user hacuser, and start the GUI with the command
hlamain. A screen labeled “Hall A Main Menu” will appear (Fig. 21.3). Chose LeCroy
HV, and select Beamline in the screen which will pop up (Fig. 4.2).
Figure 4.2: Epics Menu for the LeCroy High Voltage supplies in Hall A. All slots related
to the eP system can be accessed from the Beamline button.
For a measurement, all HV channels defined in Tab. 4.1 should be turned on. The
demand voltages in these slots (Slot 1, Slot 2 “(e,p) & ARC” and Slot 3 “Moller”) should
have the correct preset values. To turn the HV on (or off), or to change the preset values,
press the button below the title of the slot. Another screen will pop-up, where status
and preset values can be adjusted. (See Fig. 4.3 - 4.6)
During a measurement, the alarm handler should be running, so that the operator
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
Channel HV (Volts)
1.2
2201
1.3
2200
1.4
1963
1.5
1963
1.8
1039
1.9
1027
2.0
2250
2.1
2250
3.0
1004
3.1
1113
3.2
1097
3.3
1144
3.4
1126
3.5
1119
3.6
1006
3.7
1112
3.8
1104
3.9
1071
3.10
1061
3.11
1051
55
Detector
S1 (bottom)
S2 (bottom)
S1 (top)
S2 (top)
S3
S3
Cherenkov
Cherenkov
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
S3
Table 4.1: HV connections and HV values.
will be informed, should one of the detectors trip. This can also be done manually, by
watching the beamline screen Fig. 4.3. All fields should be green and showing a voltage
close to the values given in Tab. 4.1. If the EPICS screens are not working, there is an
alternative way to control the HV, by connecting via telnet directly to the LeCroy 1450.
This can be done from nearly any Linux PC in the counting house with the command:
> telnet hatsv5 2011.
4.2.2
MEDM Controls
The target, the silicon micro-strip detectors, and the setting of the Cherenkov detector
are controlled by an EPICS GUI (Fig. 4.7). It can be started from the “Hall A Main
Menu” (Fig. 21.3) running on hacsbc2 by pressing the EP Energy Measure button. (see
previous chapter, to learn how to start the “Hall A Main Menu” in case it is not already
running) The controls are actually running on a VME computer hallasc6 (Bob calls this
e-p 2). It is located in the eP electronics racks along the beamline in Hall A (Fig. 4.8).
This computer sometimes requires rebooting. The computer is reached through the
portserver hatsv5 at port 12. To reboot:
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
56
Figure 4.3: Overview screen for the high voltage status of devices belonging to the
beamline instrumentation.
Figure 4.4: Control screen for all high voltage channels from Slot 1.
> telnet hatsv5 2012
user: adaq
password: *******
if you do not see a prompt, press Ctrl C.
-> reboot
wait for it to finish and then load EPICS:
-> < epics
...
-> Ctrl ]
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
57
Figure 4.5: Control screen for all high voltage channels from Slot 2.
Figure 4.6: Control screen for all high voltage channels from Slot 3.
telnet> q
>
4.2.3
Silicon Micro-Strip Detectors
There are three GUI’s associated with the silicon micro-strip detectors. Two of them are
important for everyday operations. They are labeled MicroStrip Polarization and MX7RH
Power Supply and Currents. To operate the SSDs, pull up the micro-strip polarization
display and turn on all the bias voltages (see Fig. 4.9). Make sure that the bias voltages
are set to a reasonable value (30 Volts). Pop up both current strip charts so that you
can see when the currents have stabilized. Pull up the MX7RH display and turn on all
the supply’s (see Fig. 4.10). Pop up the power supply strip charts. It takes at least 30
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
58
Figure 4.7: EPICS main screen for the controls of the various devices in the eP system.
minutes for the strips to stabilize.
4.2.4
Target
The target of the eP system is made of a thin polyethylene (CH2 ) tape, which is moving
while it is in the electron beam. To operate the target one has to pull up the target GUI
(Fig. 4.11). There are two controls, one to start the target moving labeled Motor Control
and another labeled Target Motion to place the target in the beam. The CH2 tape must
always be moving before it is placed in the beam. There are two monitors of the tape
motion: an output that shows the motor is powered and a diode-pin combination that
triggers on a reflective strip. The diodes are often damaged.
Always make sure, that the target is moving while it is in the beam !!!
The target movement and motion can also be controlled locally. The control box is
located under the beamline next to the eP system (see Fig. 4.12.)
If you operate the target manually, make sure that the system is set back to remote
control afterwards.
The CH2 -tape has only a limited life time. Therefore it should be exchanged on a
regular basis (twice per year, or before a long beam time). This work has to be done by
the Hall A technical staff.
4.2.5
Cherenkov
The detectors for the protons (the scintillators S1 and S2, and a silicon micro-strip
detector) are installed at a fixed angle of 60o . Therefore the scattering angle of the
electron varies between 9o and 40o depending on the beam energy. There are seven
mirrors in each arm, covering the full angular range, but only one photomultiplier tube
per arm, which only looks at one mirror at a time. Depending on the beam energy the
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
59
Figure 4.8: VME crate containing modules for the slow controls of the eP system.
PMT has to be rotated to see the corresponding mirror. This movement is controlled by
the Cherenkov GUI (see Fig. 4.13). To change the setting, pull up the Cherenkov GUI
and enter the desired energy in MeV into the widget. One arm at a time will move. After
the first PMT is in position you must re-enter an energy that is 1 or 2 MeV different
in order to move the second PMT. This is a rather slow process, and can take several
minutes.
The Cherenkov detector is filled with pure CO2 -gas. The schematic of the gas system
is shown in Fig. 4.14, a picture of the gas-controller in Fig. 4.15. The gas-controller is
located in the same rack as the DAQ system. This rack is located in Hall A next to the
beamline. When performing an eP measurement, the gas system should be in Pressuremode. Therefore the left rotary switch should be at PRESSION and the right one at
FERME. The two digital displays should both indicate a pressure of roughly 10.0 mbar,
and the two flow-meters should be at zero. However the flow regulator under the left
flow meter needs to be open. In this mode the system is pressurized, if the pressure falls
below 10 mbar the automated valve on the gas inlet side opens, until the pressure is
restored. On the other hand, if the pressure rises above 15 mbar, the automated valve in
the exit pipe opens, to release pressure.
If the gas Cherenkov detector needs to be opened, one should turn down the gas
flow on the regulator beneath the left flow meter and open the exit valve (right switch,
OUVERT). After the work on the detector is finished, and the volume is closed again,
the detector needs to be set in Flow Mode. The left rotary switch needs to be in the
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
60
Figure 4.9: EPICS screen to control the bias voltages for the silicon micro-strip detectors.
DEBIT and the right one in the OUVERT position, the gas flow regulator needs to be
opened. After the detector is purged for a sufficient time, one should switch back to the
Pressure-mode, and verify that a pressure of 10 mbar is restored. The CO2 is supplied by
the Hall A gas system, which also supplies the Cherenkov detectors in the HRS with CO2 .
The cylinders and the main vallve (operated manually) are located in the gas-shack.
4.2.6
Data Acquisition
The data acquisition (DAQ) is running on adaqep in the epmeas user account. It is a
standard CODA 2.2 system. The DAQ system also downloads and initializes logic modules, and thresholds of discriminators. Since these settings depend on the beam energy,
they have to be configured individually for each measurement. The DAQ hardware itself
is located in two racks along the beamline in Hall A (see Fig. 4.16 - 4.18 ).
Trigger-configuration
Before data taking can start, a trigger file appropriate for the nominal beam energy
must be created. This file (settings.conf) insures that the trigger MLU is programmed
correctly. You have to be logged into adaqep as user epmeas. There you have to change
to the correct directory (use goconf) and run a short program (trigger) to generate the
trigger file. An example is shown in Fig. 4.19. Make sure that you give the beam energy
in MeV. The file is read in by CODA during the PRESTART.
Rebooting Acquisition-VME
The DAQ system utilizes a VME computer as its Readout Controller (ROC). This
computer is designated hallasc15 and can be accessed from the portserver hatsv5 at
port 2. To reboot it, use the following procedure:
[email protected]> telnet hatsv5 2002
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
61
Figure 4.10: EPICS screen for the MX7 power supplies.
user: adaq
password: ********
if you do not see a prompt, press: Ctrl C
-> reboot
-> Ctrl ]
telnet> q
[email protected]>
If the reboot fails, or if CODA afterwards still does not work, check that the ROC is
configured for CODA 2.2. Therefore one has to interrupt the reboot by pressing the
any-key. Press p to show the present setting, it should look the following way:
boot device : ei
processor number : 0
host name : adaqs3-ep.jlab.org
file name : /home/epmeas/vxworks/vx162lc-8MB
inet on ethernet (e) : 129.57.188.14:ffffff00
inet on backplane (b):
host inet (h) : 129.57.164.45
gateway inet (g) : 129.57.188.1
user (u) : epmeas
ftp password (pw) (blank = use rsh):
flags (f) : 0x20
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
62
Figure 4.11: EPICS screen for the MX7 power supplies.
target name (tn) : hallasc15
startup script (s) : /home/epmeas/vxworks/epmeas 22.boot
other (o) :
Press c to change these settings and reboot the ROC by pressing @ afterwards.
Running CODA
To run CODA, you have to be logged into adaqep as user epmeas. From the prompt
CODA can be started with the command runcontrol. Withing CODA you have to click
on Configure and choose configuration epm1, then click on Download, and finally on
Prestart. At this point the information in the settings.conf file, that controls the acquisition (thresholds, discriminator widths, and trigger MLU logic) is downloaded to the
hardware and spooled to the diagnostics window. This provides an opportunity to check
this information.
The actual data taking starts after pressing Go. The rate is usually rather low, below
one per second. However if after a few minutes the number of events is not increasing,
one has to verify if:
• the trigger is programmed correctly,
• all components of the DAQ are running,
• the Cherenkov is at the correct position,
• the target is in the beam and moving.
After collecting enough data, the End button should be used to end data-taking, and to
ensure that all data is written into the datafile.
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
63
Figure 4.12: Control box for the eP target system.
4.2.7
Data Analysis
The data analysis is currently done in two steps, using two different programs. Both run
on adaqep in the epmeas account.
In the first step, the CODA raw file is converted into an ASCII file. For this part
of the analysis one has to change to the epcoda directory, which can be done by typing
goep, and start the program eplong:
[email protected]> goep
[email protected]> eplong
How many events (-1= lots) ?
-1
What file name ?
epmeas02 ###.dat
What output filename ?
###
opening/adaqep/data1/epraw/epmeas02 ###.dat
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
64
Figure 4.13: EPICS control screen for the Cherenkov detector. User input is only
possible for the beam energy. Be aware that only one detector at a time is moved.
Have opened epmeas02 ###.dat
bank length is wrong
bank length is wrong
Finished; events read = 234
[email protected]>
In this example ### is the three-digit CODA run number. eplong can be started, while
CODA is still taking data for that run.
The second step of the analysis utilizes a stand-alone analysis code, which asks for
nominal beam energy, beam position, beam intensity and duration and uses the output
of eplong. One has to change into the ep directory and start the code:
[email protected]> cd
[email protected]> cd ep
[email protected]> ep
Make sure, that the nominal beam energy is given in GeV. The program prints the
result for the energy, together with the path and name for log-files and ntuple files. It is
recommended to repeat the analysis with a slightly changed nominal energy value or with
slightly changed cuts, to verify that the automatic fitting procedure does really find the
eP events, and does not trigger on noise. One also has to be aware, that one needs elastic
events in both arms to get a reliable results. Furthermore, for beam energies between
2.7 GeV and 3.4 GeV, where micro-strip detector E3 is used, the obtained values are
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
Figure 4.14:
detectors.
65
Scheme of the gas system for the two carbon dioxide gas Cherenkov
systematically shifted as compared to the results from the ARC energy measurements,
probably due to a misalignment of this detector.
4.3
Operating Procedure
In preparation of an eP measurement, the mirrors of the Cherenkov should be driven to
the appropriate position (see Sec. 4.2.5), and the silicon micro-strip detectors should be
turned on (see Sec. 4.2.3). These two measures should be started several hours before
the actual eP measurement is scheduled.
Shortly before the measurement, the high voltages for the scintillator photomultiplier
tubes and for the Cherenkov photomultiplier tubes need to be turned on (see Sec. 4.2.1).
Finally the DAQ should be prepared (see Sec. 4.2.6).
For the eP measurement, the following requirements need to be communicated to
MCC:
• 3-4 µA CW beam
• Raster OFF
• OTR target 1C12 OUT
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
66
Figure 4.15: Picture of the gas controller of the eP gas Cherenkov detectors.
• Physics target empty ( or be able to stand unrastered, uncentered beam )
• Centered on BPM 1H01 absolute
• Fast Feedback must be ON
To check the beam position (recommended!), you can use the Monticello screen from
MCC, which is usually also available on one monitor in the Hall A counting house.
On the Monticello main menu select BPM, and there click on BPM Spikes and Position
Summary. This will pop up a new screen, go to the top row of this screen (“Injector, BSY,
Hall A, B and C Transport”) and select Pos Sum. From here select Hall A Transport. A
screen will show up, which summarizes beam positions at various locations. For the eP
system the numbers in BPM 1H01 absolute are the only ones relevant.
When MCC has established those conditions, the high voltages and the micro-strip
detectors should be checked one more time. Next the eP target tape motion should
be turned on (Motor Control) and then the target can be moved into the beam (Target
Motion, see Sec.4.2.4.)
Now the actual data-taking can start, by pressing Prestart/Go in the CODA runcontrol screen. The rate should be a few tenth of a Hz. If the BPM position changes, the
fast feedback system fails, or a lot of beamtrips accrue, consider stopping the run and
starting a new one.
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
67
Figure 4.16: VME crate for the eP data acquisition.
One should analyze the data, while CODA is still running. With a hundred events
one can already check the quality of the data, and estimate how much more statistics are
needed. Typically one needs 40-50 minutes of stable beam or a few hundred events.
After data taking is finished, and it is verified, that there is a sufficient number of
events to extract a number for the beam energy, the following steps should be taken:
• eP target: should be moved out of the beam
• eP target: motor should be turned off (after it is moved out)
• MCC can restore the beam needed for the experiment:
– restore beam position at target
– restore raster
– insert OTR 1C12, if needed for the experiment
– restore beam current
• Shift workers can go back to physics target
• high voltages for eP scintillators and eP Cherenkov should be turned off
• MX7 power supplies and micro-strip bias voltages should be turned off
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
68
Figure 4.17: NIM bin for the eP data acquisition.
• CODA windows should be closed
• remaining windows from the epmeas account should be closed
Before posting the result of the eP measurement, one should make sure, that the full
statistics of the run is analyzed, that the result is independent of the chosen cuts, and
that there are events on both arms of the eP system.
4.4
Maintenance
The CH2 tape of the eP target should be exchanged on a regular basis (twice per year,
or before a long beam time). This work involves opening the eP scattering chamber
and therefore breaking the vacuum in this section of the beamline. This work has to
be coordinated by the Hall A work coordinator, and can only be done by the Hall A
technical staff personnel.
4.5
4.5.1
Safety Assessment
High Voltage
The LeCroy 1450 HV crate equipped with LeCroy 1461N high voltage cards provides up
to 3 kV of low current power. RG-59/U HV cables, certified for up to 5 kV, with standard
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
69
Figure 4.18: CAMAC crate for the eP data acquisition.
SHV connectors are used to connect the power supply to the photomultipliers. The PMTs
for S1,S2 and for the Cherenkov detector are usually operated at 1900 - 2300 V and draw
up to 1.5 mA currents. The PMTs for the S3 scintillators are operated at 1000 - 1150 V,
drawing 0.9 mA current. The high voltage MUST be turned off during all work on the
detector.
4.5.2
Silicon Micro-Strip Detectors
The SSD are prone to radiation damage, regardless if they are turned on or off. Ion
chambers next to the eP measure radiation levels in this part of the beamline and interrupt beam delivery via the fast shutdown system (FSD), in case the levels are not
acceptable. Therefore these ion chambers should never be masked.
4.5.3
Target
The target is controlled by the experimenters, not by MCC. Therefore it is the responsibility of the eP operator to ensure that it is properly operated. To avoid damage to the
eP target, the following instructions have to be followed:
• The target should only be in the beam during an eP measurement
• Before inserting the target into the beam, the tape motion has to be turned on.
The target should not be in the beam when the tape is not moving.
CHAPTER 4. EP BEAM ENERGY MEASUREMENT
70
• The target should not be in the beam if the beam current is greater than 5 µA.
• After finishing the eP measurement, the target should be moved out of the beam,
and then the tape motion stopped.
• The tape should not run, and the target should not be in the beam without an eP
operator being present.
4.5.4
Cherenkov
If for work on the Cherenkov detector the detector needs to be opened, the CO2 gas flow
needs to be stopped. After the work is finished the detector needs to be purged and later
the operating mode needs to be restored. (see Sec. 4.2.5)
4.6
List of Authorized Personnel
3 4
The list of the presently authorized personnel is given in Tab. 4.2. Other individuals
must notify and receive permission from the contact person (see Tab. 4.2) before adding
their names to the above list.
Name
Bodo
Pierre
Ed
Jack
Reitz
Bertin
Folts
Segal
Dept.
JLab
JLab
JLab
Telephone
JLab Pager
5064 5064
7544 5035
7857 7857
7242 7242
e-mail
Comment
[email protected]
Contact
[email protected]
[email protected]
eP-Target
[email protected]
Gas System
Table 4.2: eP System: authorized personnel. The primary contact person’s name is
marked with a slanted font.
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CHAPTER 4. EP BEAM ENERGY MEASUREMENT
Figure 4.19: Example for the generation of a trigger configuration file.
71
Chapter 5
Target Chamber 1 2
The Hall A target chamber is a large evacuated multistaged can that contains the target
struck by the CEBAF electron beam. The chamber was designed to isolate the beam
line vacuum from each HRS so that each HRS could rotate around the target without
vacuum coupling and without jeapordizing certain desired kinematic and acceptance
specifications of both high resolution spectrometers needed for approved experiments. It
was also designed to simultaneously contain a liquid or gas target and an array of water
cooled thin metallic foils, both remotely controlled and also be adaptable for the waterfall
target. The desired kinematic specifications that were considered included momentum
and energy resolution in both arms, angular range of spectrometers, angular acceptance,
and luminosity. The chamber vacuum is isolated from the HRS by using thin aluminum
foils.
The target chamber is designed so that each spectrometer will have continuous coverage in the standard tune from θmin =12.54 ◦ to θmax = 165 ◦ .
The target chamber is supported by a 24 in diameter pivot post secured in concrete,
rising about 93.6 in above the Hall A cement floor. The Hall A target chamber consists of
an aluminum middle ring, a stainless steel base ring, each with a 41.0 in inner diameter,
and a stainless steel cylindrical top hat with 40 in inner diameter to enclose the cryotarget
and secure the cryogenic connections.
The aluminum ring with an outer diameter of 45.0 in and wall thickness 2.0 in is
necessary for a sturdy support structure and to permit machining of the outside surface
to accommodate the flanges for fixed and sliding seals mounted on opposite sides of the
ring that vacuum connect the chamber to each HRS. The height of the aluminum ring
shown is 36.0 in, which is designed to accommodate the mounting flanges. The stainless
steel base ring is 11.50 in in height with one pump-out 6 in diameter port and with
seven 4 in viewing and electrical feed-through ports. The base ring will also contain
support mechanisms for the solid target ladder assembly, a rotisserie for collimating slits,
radiators, and magnetic fingers for removing the solid target vacuum-lock can. The total
height of the top ring, middle ring, and base ring is 93.81 in. This length is partly
1
2
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CHAPTER 5. TARGET CHAMBER
73
determined by our desire to include with the cryogenic extended target a solid target
vertical ladder secured in an inverted hat through a hole in the base of the chamber.
The base ring includes an end plate through which the inverted hat will be adapted
to fit into the large vertical pipe serving as the pivot post for the Hall A spectrometers.
The stainless steel cylindrical top hat has 40.0 in inner diameter, and is 0.375 in
thick and 46.31 in high , which is necessary to permit the cryotarget to be withdrawn
and to make space available to expose the solid targets to the electron beam.
The 200 µA electron beam, presumably focussed to a 0.1 mm × 0.1 mm spot and
rastered ±5 mm horizontally or vertically on the target, enters through a oval hole in the
middle ring which is 2.06 in wide and exits through a 1.81 in hole connected to the exit
pipe.
5.0.1
Target Chamber - Spectrometer Coupling
The aluminum middle ring will support a flange on each side for each high resolution
spectrometer. Four flanges will be available: Two flanges will contain a 6 in window
opening which will be covered with a thin foil (e.g., 10 mil aluminum) . These two flanges
will be used for experiments utilizing extended targets that do not require optimum
momentum resolution. The other two flanges will have two fixed ports (with a 8 in × 6
in opening) which will be mainly used for calibration of the spectrometers . Fixed ports
are centered at 16.11 ◦ and 45 ◦ for one flange and at 16.11 ◦ and 90 ◦ for the second
flange.
For a point beam on target a vertical opening in the walls of the chamber of height
57.15 cm x 0.065 x 2 = 7.43 cm is required so that the scattered beam is within full
acceptance of the spectrometer. If the beam is rastered on target ±0.5 cm in the vertical
direction, then the opening in the outer side of the chamber must be at least 8.5 cm for
full acceptance.
From consideration of the angular range of the spectrometers in the standard tune,
the scattered beam acceptance envelope, the effects of an extended gas target on acceptance, and the effects of a rastered beam ± 5 mm on acceptance, the target chamber
requires a window of at least 8.5 cm high in the aluminum ring extending from 6.33 ◦
(2.48 in) from the beam exit point to 8.83 ◦ (3.47 in) from the beam entrance point on
one side and a similar window on the other side of the beam. For future considerations
(e.g., using a third arm or sliding seal) the width of the window on the middle ring was
actually constructed to be 17.78 cm (7 in).
5.0.2
Stress Analysis of the Middle Ring
Since the middle ring has an extensive cut across the midplane on both sides as well as
entrance and exit holes and loaded with about 25,000 lbs, calculations of the stresses
and deformation of the midplane support area of the middle ring and deflection of the
window opening were made using the finite element analysis code ANSYS . The work
was conducted by a graduate student in the Department of Civil Engineering at the
CHAPTER 5. TARGET CHAMBER
74
University of Virginia and a REU student. A scaled down model of the middle ring
was constructed and then tested by applying forces to it using the Materials Testing
Service of the Department of Transportation at the University. ANSYS was first checked
by comparing calculations of the test model deflections to the actual data. Agreement
was within ±10%. Results of ANSYS for the target chamber showed that the maximum
deflection of the opening of the window in the middle ring varied from 0.007 in to 0.015
in depending on how the middle ring was loaded. This was decided to be a safe limit.
In the final design, several movable 7 in long, 2 in diameter aluminum support rods
are placed in the window for added support. In addition, flanges defining the ports and
coupling to the spectrometers can be added, giving additional support to the middle ring.
Compressional stresses, calculated using ANSYS assuming the middle ring was attached
to the top hat and loaded with 25,000 lbs, were less than 3000 psi almost everywhere.
However, stresses over small areas rose to levels 6000 psi near the entrance and exit
holes. These calculations indicated that we did not exceed the safety limit of 15,000 psi
for aluminum. A simple model calculation shown in Appendix A gives the result 1434
psi, which represents some average value over the midplane contact area.
5.0.3
Vacuum Pumping System
The vacuum in the target chamber is maintained by an Alcatel ( 880 l/s) turbomolecular
vacuum pump. The pump is connected to a 6 in port in the stainless steel ring between
130 ◦ ≤ θp ≤ 180◦ . The vacuum pump is fastened to a horizontal pipe connected to the
chamber. The vacuum pressure in the chamber is about 10−5 mm. An additional Alcatel
pump connected to an 8 in port should be added to obtain lower vacuum. Both pumps
may be isolated from the target chamber using gate valves which are remotely operated
from the vacuum control rack and interlocked to the FSD system.
A 2 in all metal gate valve is located between the entrance flange to the chamber
and the beam profile monitor. An additional gate valve is located 2 m downstream of
the target chamber to isolate the chamber from the exit beam pipe.
Chapter 6
Møller Polarimeter 1 2
6.1
Purpose and Layout
The Hall A beam line is equipped with a Møller polarimeter whose purpose is to measure
the polarization of the electron beam delivered to the hall.
Figure 6.1: Layout of Møller polarimeter. The origin of the coordinate frame is at the
center of the polarimeter target, which is 17.5 m upstream of the Hall A target.
1
2
CV S revision Id: moller.tex,v 1.6 2003/06/06 21:41:39 gen Exp
Authors: E.Chudakov mailto:[email protected]
75
CHAPTER 6. MØLLER POLARIMETER
76
The Møller polarimeter consists of (see Fig.6.1):
• a magnetized ferromagnetic foil used as a polarized electron target, placed 17.5 m
upstream of the central pivot point of the Hall A High Resolution Spectrometers;
• a spectrometer consisting of three quadrupole magnets and a dipole magnet, used
to deflect the electrons scattered in a certain kinematic range towards the Møller
detector;
• a detector and its associated shielding house;
• a stand alone data acquisition system;
• off-line analysis software which helps to extract the beam polarization from the
data immediately after the data are taken.
The beam polarization is measured by measuring the difference in the counting rates
for two beam helicity samples.
There are also external resources of information3 .
6.2
Principles of Operation
The cross-section of the Møller scattering e~− + e~− → e− + e− depends on the beam and
target polarizations P beam and P target as:
σ ∝ (1 +
X
(Aii · Pitarg · Pibeam )),
(6.1)
i=X,Y,Z
where i = X, Y, Z defines the projections of the polarizations. The analyzing power A
depends on the scattering angle in the CM frame θCM . Assuming that the beam direction
is along the Z-axis and that the scattering happens in the ZX plane:
AZZ = −
sin2 θCM · (7 + cos2 θCM )
sin4 θCM
,
A
=
−
, AY Y = −AXX
XX
(3 + cos2 θCM )2
(3 + cos2 θCM )2
(6.2)
The analyzing power does not depend on the beam energy. At θCM = 90o the
analyzing power has its maximum Amax
ZZ = 7/9. A transverse polarization also leads to
max
an asymmetry, though the analyzing power is lower: Amax
XX = AZZ /7. The main purpose
of the polarimeter is to measure the longitudinal component of the beam polarization.
The Møller polarimeter of Hall A detects pairs of scattered electrons in a range of
75o < θCM < 105o . The average analyzing power is about < AZZ >= 0.76.
The target consists of a thin magnetically saturated ferromagnetic foil. In such a
material about 2 electrons per atom can be polarized. An average electron polarization of
about 8% can be obtained. In Hall A Møller polarimeter the foil is magnetized along its
3
(Home page:
http://www.jlab.org/~moller/)
CHAPTER 6. MØLLER POLARIMETER
77
plane and can be tilted at angles 20 − 160o to the beam. The effective target polarization
is P target = P f oil · cos θtarget .
The secondary electron pairs pass through a magnetic spectrometer which selects
particles in a certain kinematic region. Two electrons are detected with a two-arm
detector and the coincidence counting rate of the two arms is measured.
The beam longitudinal polarization is measured as:
PZbeam =
N + − N−
1
· f oil
,
target
N + + N− P
· cos θ
· < AZZ >
(6.3)
where N+ and N− are the measured counting rates with two opposite mutual orientation
of the beam and target polarizations, while < AZZ > is obtained using Monte-Carlo
calculation of the Møller spectrometer acceptance, P f oil is derived from special magnetization measurements of the foil samples and θtarget is measured using a scale, engraved on
the target holder and seen with a TV camera, and also using the counting rates measured
at different target angles.
The target is rotated in the horizontal plane. The beam polarization may have a
horizontal transverse component, which would interact with the horizontal transverse
component of the target polarization. The way to cancel the influence of the transverse
component is to take an average of the asymmetries measured at 2 complimentary target
angles, say 25 and 155o .
6.3
6.3.1
Description of Components
MEDM Control
Several components of the polarimeter, namely the target position and the current in
the magnets can be checked using the regular MEDM [9] program of Machine Control
Center (MCC). The appropriate window 6.2 can be called from the Hall A MEDM
menues. Only the MCC can change the values in this window.
6.3.2
Polarized Electron Target
The Møller Polarized Electron Target is placed on the beamline 17.5 m upstream of the
main Hall A physics target. The photograph on Fig. 6.3 shows the target chamber, the
beam pipe, Helmholtz coils and other elements.
Two target slots exist, called “bottom” and “top”. For the Møller target at the
moment a supermendur foil 12 µm thick is used, positioned in the “bottom” slot. The
target block can be moved vertically from the center, which contains the hole for the
beam, to either “bottom” or “top” position. The foil can be tilted to the beam at an
angle required, in a range from 20o to 160o .
Both vertical movement of the target and its rotation is controlled by the Machine
Control Center (MCC) operators. The vertical movement is controlled by 3 buttons on
the Møller MEDM display, named “center”, “bottom” and “top”. Rotation is controlled
CHAPTER 6. MØLLER POLARIMETER
Figure 6.2: The Møller MEDM MCC control.
78
CHAPTER 6. MØLLER POLARIMETER
Figure 6.3: The Møller target.
79
CHAPTER 6. MØLLER POLARIMETER
80
by typing an angle, measured in certain units, in a “rotary” window and pressing RETURN. The size of the unit is defined by the end switches which stop the target rotation
at low and high end points. The distance between these 2 switches is about 140o , and
this distance is divided into 80 units. Therefore one unit is about 1.75o . In order to turn
the target to 90o one should set a value of about 38 units. The default rotary position is
at 0 units and the target cannot be moved vertically being at a different position.
Before the target is moved vertically the target movement should be “masked” by
the MCC operators, since the vertical movement may bring thick construction elements
into the beam area and therefore is connected to the Fast Shutdown (FSD). The target
rotation is safer and can be performed without “masking” the target movement and
is normally performed without turning off the beam. A potentially dangerous target
rotation to angles lower than 15o is prevented mechanically.
he target motion is supervised using 2 TV cameras, displayed in Hall A counting
house. One camera is looking from the side. A glass window in the target vacuum box
allows one to see the beam area in the place where a target can be moved in. The central
position (empty) of the target block is seen as an empty round hole. A target moved in
is clearly visible. The second camera looks from the top at the target holder. A scale
(Fig. 6.4) engraved on the holder shows its angle in degrees. This scale gives correct
Figure 6.4: The Møller target rotary dial.
relative angles of the target. The absolute angle of the target to the beam is measured
using the event rates, measured at a given target angle and at about 90o . At the moment
the scale has a shift, that: θtarget ≈ θscale − 2.5o .
The target is magnetically saturated using 2 external Helmholtz coils, providing a
field of about 240 Gs along the beam axis at the target center. The coils are turned on
by the Møller CODA task, during the data taking. Its polarity is reversed for each new
run of data taking (one run typically takes 2-3 min).
CHAPTER 6. MØLLER POLARIMETER
81
The beam of a few µA may heat up the target locally by 20-40K, which may change
slightly the target polarization. Therefore the beam current is limited to about 2µA4
because of heating, while the dead time problems actually limit the current to about
0.5µA.
The temperature of the target holder is measured using cernox resistors.
The target magnetization has been measured before the installation5 .
The Møller target positions are connected to the Fast ShutDown (FSD) system of the
accelerator. The photograph on Fig. 6.5 shows the crate which controls the signals.
Figure 6.5: The Møller target motion may cause the Fast ShutDown (FSD) of the accelerator. The LEDs in the top right corner show the appropriate signals from the target.
The top LED lit indicates no FSD signal.
4
A system for cooling the target with liquid nitrogen has been built. However, it has not been used
and there are no plans to use it.
5
The magnetization can be also measured in situ, by changing the field in the Helmholtz coils and
measuring the voltage at pick-up coils, wound around the target foils, although this method is less
accurate than the lab method and has not been used.
CHAPTER 6. MØLLER POLARIMETER
82
Figure 6.6: The Møller spectrometer. The target is located at the right side of the
photograph, the blue dipole magnet is close to the center.
6.3.3
Spectrometer Description
The Møller polarimeter spectrometer consists of three quadrupole magnets and one dipole
magnet (see Fig.6.1 and also Fig.6.2 ):
•
•
•
•
quadrupole MQM1H02 (on the yoke labeled P AT SY );
quadrupole MQO1H03 (on the yoke labeled T ESSA);
quadrupole MQO1H03A (on the yoke labeled F ELICIA)
dipole MMA1H01 (on the yoke marked as U niversity of Kentucky).
The photograph on Fig. 6.6 shows the side view of the spectrometer.
These magnetic elements are controlled by MCC operators.
The spectrometer accepts electrons scattered close to the horizontal plane (see
Fig.6.1). The acceptance in the asymuthal angle is limited by a collimator in front
of the dipole magnet, while the detector vertical size and the magnetic field in the dipole
magnet limit the acceptance in the scattering angle θCM .
The electrons have to pass through the beam pipe in the region of the quads, through
the collimator in front of the dipole magnet, with a slit of 0.3-4 cm high, through two
vertical slits in the dipole, about 2 cm wide, positioned at ±4 cm from the beam. These
slits are ended with vacuum tight windows at the end of the dipole. The dipole deflects
the scattered electrons down, towards the detector. The detector, consisting of 2 arms 2 vertical columns - is positioned such that electrons, scattered at θCM = 90o pass close
CHAPTER 6. MØLLER POLARIMETER
83
to its center. This acceptance is about 76 < θCM < 104o . At beam energies below 1 GeV
the vertical slits in the dipole limit the acceptance to about 83 < θCM < 97o .
For a given beam energy there is an optimal setting of the currents in these 4 magnets
(see section 6.4.3). At energies higher than 2.5 GeV it is possible to optimize the beam
line for both regular running and for Møller measurements. Typically, the dipole magnet
should be turned on only for the Møller measurements.
6.3.4
Detector
The Møller polarimeter detector is located in the shielding box downstream of the dipole
and consists of two identical modules placed symmetrically about a vertical plane containing the beam axis, thus enabling coincidence measurements. Each part of the detector
includes:
• An aperture detector consisting of a 28×4cm×1cm scintillator with Plexiglas light
guide and a Hamamatsu R1307 (3 inch) photomultiplier tube.
• A “spaghetti” lead - scintillating fiber calorimeter6 , consisting of 2 blocks 9×15×30 cm3 ,
each separated into 2 channels equipped with Photonis XP2282B (2 inch) photomultiplier tubes. Thus, The vertical aperture is segmented into 4 calorimeter channels.
The HV crate is located in the Hall A rack 15 and is connected to a portserver hatsv5,
port 11. HV for the lead glass detectors is tuned in order to align the Møller peak position
at a ADC channel 300 for each module, which means that the gain of the the bottom
modules is about 50% higher than the gain of the top modules.
6.3.5
Electronics
The electronics, used for Møller polarimetry, is located in several crates in the Hall:
1.
2.
3.
4.
5.
VME, board computer halladaq14 - for Helmholtz coils control;
VME, board computer hallavme5 - for DAQ;
CAMAC - for the trigger and data handling;
NIM - for the trigger and data handling;
LeCroy 1450 - HV crate, slot 4.
The photograph on Fig. 6.7 shows the crates 2-4. One can connect to the CPU boards
and the HV crate via a portserver:
1. halladaq14 - hatsv5 port 3;
2. hallavme5 - hatsv5 port 4;
5. LeCroy 1450 - hatsv5 port 11.
6
before summer 2002 a lead glass calorimeter consisting of 4 8×8×30 cm3 was used. It lost a big
fraction of the amplitude due to the radiation damage and deterioration of the optical contact.
CHAPTER 6. MØLLER POLARIMETER
84
Figure 6.7: The Møller electronics, located in the Hall, at the right side of the beam
line. The top crate is the VME DAQ crate, the middle one is the CAMAC crate and the
bottom one is the NIM crate. The first VME crate, used to control the Helmholtz coils,
is above these three.
CHAPTER 6. MØLLER POLARIMETER
6.3.6
85
DAQ
The DAQ7 is based on CODA [10] and runs at adaql2, connecting to hallavme5. The
database server for CODA is located on adaqs2.
6.3.7
Slow Control
The Helmholtz coils are controlled via a script starting automatically at the beginning
of each CODA run. The polarity of the current in the coils is reversed at every new run.
The HV, the electronics settings and the collimator position are controlled from a
Java program, equipped with a GUI.
Start the slow control task:
– Login to adaql1 as moller;
– adaql1> cd Java/msetting/
– adaql1> ./mpc - start the slow control task.
It may take about a minute to start all the components and read out the proper data
from the electronic crates.
The slow control console is presented on Fig. 6.8. The
Figure 6.8: The slow control console (Java).
components are:
–
–
–
–
–
EPICS [2] Monitor: these EPISC variables are stored for every DAQ run
Detector Settings is used to set up the thresholds, delays etc.
High Voltage Control for the photomultiplier tubes
Motor Control to move the collimator
Target Monitor information on the target position, magnets etc.
High voltage can be changed or turne on/off using the HV console (Fig. 6.9), where
the first 8 channels belong to the calorimeter and the other 2 channels belong to the
aperture counters. The settings of the CAMAC electronics used to make the trigger and
control DAQ are controlled using the Detector Setting window (Fig. 6.10):
7
(More details in:
http://www.jlab.org/~moller/guide1_linux.html)
CHAPTER 6. MØLLER POLARIMETER
86
Figure 6.9: HV control console.
– Delay line - the delays for the calorimeter and counter signals;
– LedDiscriminator - discriminator thresholds for the calorimeter and the counters
– PLU Module - settings of the logical unit
The collimator width can be changed using Motor Control window (Fig. 6.11),
6.4
Operating Procedure
The procedure includes general steps as follows:
• “Non-invasive” preparations - start the appropriate computer processes, turn on
the HV and learn the magnet settings needed;
• “Invasive” preparation: beam tuning with the regular magnet settings, loading the
Møller settings, beam tuning, if neccessary, installing the Møller target;
• Detector check/tuning;
• Measurements;
• Restoring the regular settings.
The “non-invasive” preparations can be done without disturbing the running program
in the Hall. It is reasonable to perform these preparations before starting the “invasive”
part.
In more details, the “invasive” procedure looks as follows:
CHAPTER 6. MØLLER POLARIMETER
Figure 6.10: Detector setting console.
–
–
–
–
–
–
–
–
–
Remove the main target;
Tune the beam position with any convenient beam current;
Load the Møller settings in the magnets;
Check the beam position;
Tune the beam to ∼ 0.3 µA for Møller measurements;
Pull in the Møller target;
Make measurements at the forward target angle (∼ 23◦ );
Make 2 short runs at the normal target angle (∼ 90◦ );
Make measurements at the backward target angle (∼ 163◦ );
87
CHAPTER 6. MØLLER POLARIMETER
88
Figure 6.11: Control console for the collimator (and also the slide, which is not relevant
here).
6.4.1
Initialization
In order to control the operations several sessions of moller account must be opened at
computers adaql1,.... The data analysis and some initial calculations are done using a
PAW [11] session on adaql1:
– Login to adaql1 as moller;
– adaql1> cd paw/analysis, start PAW (type paw), select Workstation type 3.
CODA runs on adaql2:
–
–
–
–
Login to adaql2 as moller, make two sessions;
adaql2> kcoda - clean up the old coda;
adaql2> et start moller & - start ET if it is not running;
Reset hallavme5 and halladaq14 by pressing two top left green reset buttons in the
middle room of the counting house;
– adaql2> runcontrol - start CODA;
– Click Connect and select the configuration beam pol;
– Click Download to download the program into the VME board.
Slow control:
– Login to adaql1 as moller;
– Start the slow control (see section 6.3.7);
– Load the regular settings and the appropriate HV.
CHAPTER 6. MØLLER POLARIMETER
6.4.2
89
Initial Beam Tune
Typically, the Møller measurements are taken during the regular Hall A running, when
the beam has been tuned for this running. However, the Møller measurements require a
different magnetic setting. At least the dipole magnet has to be turned on. This magnet
slightly deflects the beam downward. The deflection at the main target could be 2-8 mm,
depending on the beam energy. It is, therefore, useful to tune the beam position before
the dipole is turned on. It can be done before the magnets are set to th Møller mode.
The requirements are:
– On BPM IPM1H01 (in front of the Møller target) |X| < 0.2 mm, |Y | < 0.2 mm.
– On BPM IPM1H04A/B |X| < 2 mm, |Y | < 2 mm.
The request should be given to MCC.
6.4.3
The Magnet Settings
In order to find the proper settings for the given beam energy, say 3.25 GeV, type on the
PAW session:
PAW> exec sett magp e0=3.25 nq=3 for 3-quad configuration
PAW> exec sett magp e0=3.25 nq=2 for 2-quad configuration
The printed values for GL and BdL should be checked with the current values, displayed
on the MEDM window 6.3.1. The MCC should be asked to load the Møller settings in
the magnets - they have a tool to load the proper settings for the Møller magnets and
a few other magnets on the beam line. The set values should be compared with the
calculated values8 . The beam must be turned off when the magnets are tuned.
6.4.4
Final Beam Tune
The beam parameters for Møller measurements are:
– Remove the main target;
– the beam current ∼ 0.3 µA and < 2 µA;
– the beam current should be reduced mainly by closing the “slit” in the injector
(not by the laser attenuator), in order to reduce the effect of current leak-through
from the other halls.
6.4.5
Target Motion
MCC should be asked to put in one of the targets (typically, the bottom (see Fig. 6.2)
target is used). The requirements for the target motion are:
– Vertical motion: beam OFF, target motion MASKED.
– Rotation: no constraint, the beam can be ON.
8
The reasonable acuracy in the magnets settings is about 1-2%
CHAPTER 6. MØLLER POLARIMETER
90
Ask the MCC to resume the same beam and check the Ion Chamber SLD1H03
reading. It should not exceed 1000. At beam energies above 1.6 GeV it should not
exceed 300.
6.4.6
Detector Tuning and Checking
The goal is to check that the detector is working, that the counting rates are normal
and that the Møller peaks are located at about ADC channel 300 for all the calorimeter
blocks.
A. Data taking with CODA
1. Take a RUN for about 20k events. Let us assume the run number is 9911.
B. Data analysis with PAW
1. PAW> exec run run=9911: build an NTUPLE and attach it to the PAW session;
2. PAW> exec lg spectra icut=60 run=9911: look at the ADC distributions. The
peaks should be at about ADC channel 300 for all 8 modules. If the peaks are
off - try to adjust the HV (do not go beyond 1990V).
C. Check of the background
1. Raise the thresholds to 240 mV of the channels 1 and 2 of the discriminator,
using the slow control window (see section 6.3.7);
2. Take a run of about 20k events, say run=9915;
3. PAW> exec lg spectra icut=60 cut=11 run=9915: look at the ADC distributions. The peaks should be at about ADC channel 300 for all 8 modules. The
histograms 9 and 10 present the sums of the left and right arms. The histogram 11 (sum of both arms) should contain a clean peak at about channel
600;
4. PAW> exec asyms angl=23.0 run=9915: polarization analysis should provide
a reasonable number. Check the scaler rates per second. The counting rates
in each arm should not exceed 600kHz. If they are higher ask the MCC to
reduce the beam current.
6.4.7
Polarization Measurement
1. Make a note in the logbook of the target angle on the scale, seen with the TV
camera.
2. Take 4 runs of data with the given angle, each run of about 20-30k events (30k at
Ebeam < 2 GeV).
CHAPTER 6. MØLLER POLARIMETER
91
3. Ask the MCC to turn the target to 38 units and make a note of the angle on the
scale.
4. Take 2 runs of data with the given angle, each run of about 15k events.
5. Ask the MCC to turn the target to 80 units and make a note of the angle on the
scale.
6. Take 6 runs of data with the given angle, each run of about 20-30k events (30k at
Ebeam < 2 GeV).
7. Analyze the data
1. PAW> exec run run=???? and
2. PAW> exec asyms run=???? angl=tang, for each RUN, tang is the target angle
observed on the scale.
3. PAW> call prunpri.f(9000,20000), print a table with the results for a given
range of runs.
6.5
6.5.1
Safety Assessment
Magnets
Particular care must be taken in working in the vicinity of the magnetic elements of
the polarimeter as they can have large currents running in them. Only members of the
Møller polarimeter group are authorized to work in their immediate vicinity, and only
when they are not energized. The quadrupole magnets and the leads for the dipole magnet are protected with Plexiglas shields. As with all elements of the polarimeter which
can affect the beamline, the magnets are controlled by MCC. There are four red lights
which indicate the status of the magnets. The dipole has two lights which are activated
via a magnetic field sensitive switch placed on the coils of the dipole. One light is placed
on the floor on beam left, and the other is placed on the raised walkway on beam right.
The quadrupoles have similarly placed lights (one on the floor on beam left and one
on the walkway), and are lit up when any one of the Møller quads is energized. The
status of the quadrupole power supplies is on the checklist for closing up Hall A. Lock
and tag training is required of all personnel working in the vicinity of the Møller magnets.
The power supply for the dipole is located in the Beam Switch yard Building (Building 98). The maximum current for the dipole is 450A. The quadrupole power supplies are
located in Hall A electronics rack 13, 2 supplies connected in parallel per one quadrupole.
The maximum current per one power supply is 60A at about 20V.
CHAPTER 6. MØLLER POLARIMETER
6.5.2
92
Vacuum System
One must be careful in working near the downstream side of the dipole magnet, as
there are two 2 by 16 cm, 4 mil thick titanium windows. Only members of the Møller
polarimeter group should work in this area.
6.5.3
High Voltage
There are 38 photomultiplier tubes within the detector shielding hut, with a maximum
voltage of 3000 V. The detector is serviced by sliding it back on movable rails. The high
voltage must be turned off during any detector movement. Only members of the Møller
group should move the detector.
6.5.4
Target
To avoid damage to the Møller target, the target should not be in the beam if the beam
current is greater than 5 µA. Only MCC can move the target, but the experimenters are
responsible for ensuring that it is properly positioned.
6.6
List of Authorized Personnel
9 10
The list of the presently authorized personnel is given in Tab. 6.1. Other individuals
must notify and receive permission from the contact person (see Tab. 6.1) before adding
their names to the above list.
Name
Eugene
Alexander
Viktor
Roman
Dept.
Telephone
JLab Pager
Chudakov
JLab
6959 6959
Glamazdin
Kharkov 6378
Gorbenko
Kharkov 6378
Pomatsalyuk Kharkov 6378
e-mail
[email protected]
[email protected]
[email protected]
[email protected]
Comment
Contact
Table 6.1: Moller Polarimeter: authorized personnel. The primary contact person’s
name is marked with a slanted font.
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Chapter 7
Compton Polarimeter 1 2
7.1
Introduction
In order to measure the longitudinal polarization of the 3-6 GeV high intensity TJNAF
electron beam, a Compton Polarimeter was built by CEA Saclay, LPC Clermont-Ferrand,
and Jefferson Laboratory. The Compton polarimeter has been running since february
1999 and has been used by severals HALL A experiments.
7.2
Principle of Operation
The Compton effect, light scattering off electrons, discovered by Arthur Holly Compton
(1892-1962), Nobel prize in Physics, 1927, is one of the cornerstone of the wave-particle
duality. Compton scattering is a basic process of Quantum Electro-Dynamic (QED), the
theory of electromagnetic (EM) interactions. During 50’s and 60’s, the QED theoretical
developments allow Klein and Nishina to compute accuratly the so-called Compton interaction cross section. Experimental physicists performed serveral experiments which are
in good agerement with the predictions. This is now a well established theory, and is thus
natural to use the EM interaction, such as Compton scattering, to measure experimental
quantities such as polarization of an electron beam .
Many of the Hall A experiments of Jefferson Laboratory using a polarized electrons
beam require a measurement of this polarization as fast and accurate as possible. Unfortunately the standard polarimeters, like Møller or Mott, require the installation of a
target in the beam. Therefore, the polarization measurement can not to be performed
at the same time than the data taking because the beam, after the interaction with the
target, is misdefined in terms of polarization, momentum and position. Another physical
solution has to be found in order to permit a non-invasive polarization measurement of
the beam. This is the primary motivation for Compton Polarimetry.
This physical process, schematically illustrated in Fig.7.1, is well described by QED.
1
2
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Authors: S.Nanda mailto:[email protected]
93
CHAPTER 7. COMPTON POLARIMETER
94
Figure 7.1: Schematic view of compton scattering
The cross sections of the polarized electrons scatterred from polarized photons as a function of their energies and scattering angle can be precisiely calculated. The cross sections
are not equal for parallel and anti-parallel orientations of the electron helicity and photon
polarization. The theoretical asymmetry Ath defined as the ratio of the difference over
the sum of these two cross sections is then the analyzing power of the process. With the
kinematical parameters used at JLab, the mean value of this analyzing power is of the
order of few percent.
The polarization of the Jefferson Lab electron beam is flipped 30 times per second.
Upon interation with a laser beam of known circular polarization, an asymmetry, Aexp =
N + −N −
, in the Compton scattering events N ± detected at opposite helicity. In the
N + +N −
following, the events are defined as count rates normalized to the electron beam intensity
within the polarization window. The electron beam polarization is extracted from this
asymmetry via [12]
Pe =
Aexp
,
Pγ Ath
(7.1)
where Pγ denotes the polarization of the photon beam. The measured raw asymmetry Araw has to be corrected for dilution due to the background-over-signal ratio BS , for
the background asymmetry AB and for any helicity-correlated luminosity asymmetries
AF , so that Aexp can be written to first order as
B
B
Aexp = 1 +
Araw − AB + AF .
(7.2)
S
S
The polarization of the photon beam can be reversed with a rotatable quarter-wave
plate, allowing asymmetry measurements for both photon states, A(R,L)
raw . The average
asymmetry is calculated as
L
ωR AR
raw − ωL Araw
,
(7.3)
ωR + ωL
where ωR,L denote the statistical weights of the raw asymmetry for each photon beam
polarization. Assuming that the beam parameters remain constant over the polarization
reversal and that ωR ' ωL , false asymmetries cancel out such that
Aexp =
CHAPTER 7. COMPTON POLARIMETER
Aexp '
L
AR
B
raw − Araw
(1 + ).
2
S
95
(7.4)
Using a specific setup, the number of Compton interactions can be measured for each
incident electrons helicity state (aligned or antialigned with the propagation direction).
These numbers are dependant of process cross sections, luminosity at the interaction
point and time of the experiment. At first order, assuming the time and luminosity
are equal for the both electron helicity states, the counting rates asymmetry is directly
proportionnal to the theoretical cross section asymmetry. From one to the other The
proportionnality factor is equal to the values of the photon circular polarization Pphoton
multiplied by the electron polarization Pelectron, so that :
Measuring the photons polarization and experimental asymmetry, calculating theoretical asymmetry, one can deduce the electron beam polarization. One electron over
a billion is interacting with the photon beam which means 100000 interactions per second. So as only few incident electrons are interacting, these polarization measurements
are completly non-invasive for the electron beam in term of positions, the orientations
and the physical characterictics of the beam at the exit of the polarimeter. Compton
polarimeter principles at JLab The backward scattering angle of the Compton photons
being very small, the first priority is to separate these particles from the beam using a
magnetic chicane. The energy of the backward photons will be measured by an electromagnetic calorimeter, the so-called PbWO4 coming from the LHC’s R & D. The third
dipole of the chicane, coupled to the electrons detector, will be used as a spectrometer
in order to measure the scattered electron momentum. To perform a quick polarization
measurement, the photon flux has to be as high as possible. A Fabry-Prot Cavity, made
of 2 multi-layers concave mirrors with very high reflectivity, will amplify this flux to a
factor greater than 7000. The 15 meters long Compton Polarimeter has been installed in
the last linear section of the arc tunnel, at the entrance of the Hall A at spring 98. The
complete setup, including the optical cavity was installed in February 99 and is running
successfully since then.
7.3
Description of Components
As shown in Fig.7.2, the Compton polarimeter consists of four major subsystems and
associated data acquisition system as described below:
7.3.1
Optics table
A high-finesse Fabry-Perot cavity housed on a optics table serves the role of the photon
target. The optical setup consists of four parts:
1. a 240 mW infra-red Laser operating at 1064 nm wavelength,
CHAPTER 7. COMPTON POLARIMETER
96
Electrons detector
Electron Beam
E’
E
Magnetic Chicane
Photons detector
k’
λ =1064 nm, k=1.65 eV
P=1kW
Figure 7.2: Schematic layout of the Compton polarimeter
2. input optical transport form the laser beam to the cavity to optimize laser beam
size and polarization,
3. the resonant Fabry-Perot cavity that delivers more than 1kW of circularly polarized
infra-red light
4. optical devices to measure the circularly polarization of the photons at the exit of
the cavity
The layout of the optical setup is shown in Fig.7.2. Details of the resonant FabryPerot cavity for Compton polarimetry can be found in Nuclear Instruments And Methods In Physics Research Section A412 1 (1998) pp. 1-18 http://hallaweb.jlab.org/
compton/Documentation/Papers/nima4592001.pdf
7.3.2
Magnetic Chicane
The Compton magnetic chicane, illustrated in Fig.7.4, consists of 4 dipoles (1.5 T maximum field,
1 meter magnetic length) here after called D1,2,3,4. (D1,D2) deflect the electrons
vertically down to steer the beam through the Compton interaction point (CIP) located
at the center of the optical cavity. After the CIP, the electron are vertically up deflected
(D3,D4) to reach the Hall A target. The scattered electron are momentum analyzed by
the third dipole and detected thanks to 4 planes of silicon strips. The magnetic field is
scaled with the beam energy, insuring the same vertical deflection at the CIP, up to 8
GeV electrons for 1.5 T field. The parameters of the Chicane are as follows:
• The distance between the geometrical axis of the dipoles (D1,MMC1P01) and
(D2,MMC1P02) in the longitudinal plane is 5400 mm
• The distance between the beam entry axis in (D1,MMC1P01) anfd the beam exit
axis in (D2,MMC1P02) in the bending plane (vertical axis) is 304 mm
CHAPTER 7. COMPTON POLARIMETER
N. Falletto et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 412}425
97
417
Fig. 5. Layout of the optics table in the hall A tunnel (top and side views).
Figure 7.3: Optics setup of the Compton polarimeter
for the Servo-loop control used for mode-locking
[12]. The JLab Compton polarimeter uses the
same primary laser (Lightwave series 126) and the
same opto-electronics mode-locking system which
width of the high-"nesse cavity and the accelerator
environment.
3.1. The high-xnesse monolithic Fabry}PeH rot cavity
CHAPTER 7. COMPTON POLARIMETER
SLD1C20
MBC1C18V+H
MVS1P04
MBT1P01H
~7m ~1m
0
MVS1P02
IPM1H01
MVS1P03
MCP1P01
IPM1P03A
IPM1P02A
MAT1H01H
MAT1H01V
Target
MBT1P03H
ILM01P02
IPM1C20
~18m
ILM01P03
MVS1P01
2m
~24m
ILM01P04
ILM01P01
MBC1C20V+H
98
IPM1H03A
IPM1H03B
MCP1P04
IPM1P02B
MCP1P02
MCP1P03
Figure 7.4: Schematic layout of the beamline elements along the compton chicane area.
• The longitudinal magnetic length on the axis of (D1,MMC1P01) and (D2,MMC1P02)
is 1000 mm.
Under these conditions :
• the bending angle is 3.2226◦
• The radius of curvature is r=17.7887 m
• B.r (T.m) = 3.33564 p (GeV/c)
• At the centre of (D2,MMC1P02) : B(T) = 0.1875145 p (GeV/c).
7.3.3
Photon Detector
To detect Compton backscattered photons, an electromagnetic calorimeter is used. It
consists of 25 PbWO4 crystals (2cmx2cmx23cm) read by XP1911 Philips photomultiplier
tubes and is located in the line of sight of the optical cavity, just behind the third dipole
Figure 7.5: View of the Compton photon detector
of the chicane. A photographic view of the calorimeter as installed in the beamline is
shown in Fig.7.5. Details on this calorimeter can be found in Nuclear Instruments And
Methods In Physics Research Section A443 2-3 (2000) pp. 231-237 http://hallaweb.
jlab.org/compton/Documentation/Papers/nima4432000.pdf.
CHAPTER 7. COMPTON POLARIMETER
7.3.4
99
Electron detector
The electron detector is made of 4 planes of silicon strips composed of 48 strips each
of width 650 (600 + 50) microns and 500 microns thick. The planes are staggered by
200 microns to allow for better resolution and the first strip of the first plane is about 8
mm away from the beam. Illustrated in Fig.7.6 is a view of the actual electron detector.
Figure 7.6: View of the Compton electron detector
Distance between the CIP and the first strip is 5750 mm. We recall that between
the CIP and the end of the Dipole 3 is 2150 mm. For a beam of 3.362 GeV the Compton
edge is at 3.170 GeV. This corresponds to a deviation of 17 mm. Thus at this energy,
only one half of the Compton spectrum is covered and it extends to the 13th strip of the
first plane. The trigger logic looks for a coincidence between a given number of plane in
a ”road” of 2 strips. For each trigger it outputs a signal check by the Polarimeter DAQ.
7.3.5
Fast acquisition system
The goal of this system is to acquire for each electron helicity state the energie of the
scattered photons at a rate up to 100 kHz. The energy of each Compton event can be
reconstructed from the signals of the 25 PMT of the photon calorimeter with front-end
electronics and ADCs. Each helicity state, given by the accelerator, is also numbered.
Further information is given for each event (type of event, status of the polarimeter at
event’s time) and for each polarization period (duration, dead time, counting rate,...).
A specific tool, the so-called spy acq, has been developped in Tcl/Tk to manage all
acquisition system parameters. Finally, a web-based logbook is available on this site at
http://hallaweb.jlab.org/compton/Logbook/index.php.
CHAPTER 7. COMPTON POLARIMETER
7.4
100
Operating Procedure
The main operations computer for the compton polarimeter is compton.jlab.org located
in the central isle of the Hall A counting house. A dedicated console labelled as Compton is in the backroom. This machine, running RedHat Linux 7.3, runs the compton
data acquisition, analysis, and the EPICS [2] slow control system. To begin compton
polarimeter activity log on to:
machine: compton.jlab.org
username: compton
password: *******(contact Sirish Nanda (7176))
All necessary environment variables are automatically defined on logon. Follow the
steps below paying careful attention to ensuring that you have checked the result of each
step:
7.4.1
DAQ Setup
• Go to the CODA desktop and open a new terminal window. Type
$ coda start
The general syntax is coda start—stop—restart. If CODA is in a bad state, do
coda restart. All relevant CODA processes are started and you whould get the
runcontrol panel as shown in Fig.7.7.
• Click on the ”CONNECT” button. You will get the window shown in Fig.7.8.
• Click on the Configure button and you will get the sub-panel shown in Fig.7.9.
Click on the ”Run type” button and choose of of the following configurations:
• Fastacq This is the standard run type to take data in a nominal situation.
• Beamtune This run type could be use during beam tuning phases. It does not
record good data, but by connecting to ROC1 (telnet cptaq1), you could monitor
the trigger counting rate more quickly than with spy acq.
• Scanacq This run type is used during the electrons detector harp scan procedure.
It stores in particular the electrons detector ruler values at high counting rates.
• Linscan This is the procedure to determine the non-linearity of the ADC. This
run type is used to scan the response of the photon detector electronic with test
pulses of different amplitude. The acquisition system set the electronics to send
50ns long test pulses to the integration and ADC devices, and increase progressively
the amplitude of these pulses.
CHAPTER 7. COMPTON POLARIMETER
Figure 7.7: Compton DAQ setup: connect
101
CHAPTER 7. COMPTON POLARIMETER
Figure 7.8: Compton DAQ setup: configure
102
CHAPTER 7. COMPTON POLARIMETER
103
Figure 7.9: Compton DAQ setup: run type
• Others The other run types are essentially used for debug purposes. If you are
not an expert of the acquisition do not use them.
• Confirm via the ”OK” button You should see in the window below the following
message ”transition configure succeeded”
• Click on the ”Download” Button to get the messages shown in Fig.7.8.
• Start the run clicking on the ”Start Run” button
You should see the run control display as in Fig.7.8.
Check that the following happens:
transition Go succeeded
the counting rates distribution
the number of events in this run is updating
the run status active
the run number updated
• To end a run, click on End Run (Fig.7.12) button to stop the acquisition.
CHAPTER 7. COMPTON POLARIMETER
Figure 7.10: Compton DAQ setup: download
104
CHAPTER 7. COMPTON POLARIMETER
Figure 7.11: Compton DAQ setup: start
105
CHAPTER 7. COMPTON POLARIMETER
Figure 7.12: Compton DAQ setup: end
106
CHAPTER 7. COMPTON POLARIMETER
107
• Start the acquisition control panel
In a fresh terminal window, excecute the command to start the spy acqusition control panels
$ spy acq
Don’t be surprised. 7 windows will be open but regroup in only one within few
seconds. From time to time, it may happen that one window does not go inside
spy acq window. Click on the corresponding widget.
• Check the High Voltages applied on the Photon detector PMT’s. Go to the ”Acq”
panel where spy acq is currently runnning, Go in the Logbook panel and select
photon detector. You will get the the panel shown in Fig.7.13) with the high
voltages values:
• If the HV are off, switch them on.
The cards of the COMPTON Polarimeter PMT HV are located in crate #2 telnet
hatsv5 2011, then 1450, vt100 and usual display.
High Voltage channel for the Compton polarimeter are in cards # 12, 13, 14 and
15. Typical HV for Beam Diag and Cristal PMTs is 1500 V. The voltage of the
monitoring LED (channel 15.10) should stay in the range 110-130 V.
NB: Only one user can connect on hatsv5 at the same time!! If you can’t connect
check if others are logged in. See also procedure posted in rack #CH01B05.
CHAPTER 7. COMPTON POLARIMETER
Figure 7.13: spy acq high voltage status panel
108
CHAPTER 7. COMPTON POLARIMETER
7.4.2
109
Cavity Setup
Choose the EPICS desktop and in a fresh terminal window and start the MEDM [9]
EPICS panel by executing the command:
$ epicshall
This will open the main EPICS menu for the Compton as shown in Fig.7.14.
Figure 7.14: Compton polarimeter main EPICS control panel
• Switch on the laser
On the EPICS control panel, pull the ”OPTICS” menu down. Click on ”Mini
Optic” (see Fig.7.15).
To turn the Laser On .... Click on the Laser On button.
Check LASER STATUS and INCIDENT POWER.
A Laser spot may blink on the CCD control TV screen
CHAPTER 7. COMPTON POLARIMETER
Figure 7.15: Compton polarimeter mini optics control panel
110
CHAPTER 7. COMPTON POLARIMETER
111
(second from left among the 4 screens)
and you should see a bright spot on the miror control TV screen labelled ”laser.”
(see Fig.7.16).
Figure 7.16: TV viewer images of laser spot
If Laser doesn’t turns ON most problable problem is an interlock fault. You need
an access in Hall A and check the different parts of the laser interlock around the
optic table:
Two crash buttons on the left wall, inside and outside the green tent.
The optic table is protected by a metallic cover. Four magnetic switches connect
the top and the walls of this cover. Some of them might be opened. CAUTION!!:
you are standing close to the optic table. Do not try to open the cover. Just move
the top carefully sideways to try to close the switches.
• Lock the cavity
To lock the cavity click on the Servo On button shown in Fig.7.17. You should
see the cavity locking on the CCD control TV as in Fig.7.18, and you should have
more than 1000 W stored in the optical cavity.
If it isn’t the case, turn on the Slow Ramp and then Click on the Slow On button
shown in Fig.7.19.
If successfull you can turn OFF the Slow Ramp button.
Photons are now ready to meet electrons and give some Compton photons children.
If the cavity still doesn’t lock after few minutes with SERVO and Slow Ramp ON:
Check the Yokogawa generator in the Compton rack (CH01B00). Frequency should
be 928 kHz, Amplitude 80 mVpp and phase -4 deg. Pull down the OPTICS menu
in the main epics window. Click on ”Optic table” and then on ”Servo”. The laser
servo control panel appears as shown in Fig.7.20 Gain should be close to 167. A
CHAPTER 7. COMPTON POLARIMETER
Figure 7.17: Laser servo control
Figure 7.18: Compton cavity lock indicator
112
CHAPTER 7. COMPTON POLARIMETER
Figure 7.19: Compton Laser slow ramp control
113
CHAPTER 7. COMPTON POLARIMETER
114
too high traking level in the feedback can prevent the cavity from locking. Bring
the ”tracking Level” cursor down to low values (0.20 - 0.40) and try to lock again
with Servo and Slow Ramp on.
Figure 7.20: Compton Laser servo control
• Unlock the Cavity
On the EPICS control panel, pull the ”OPTICS” menu down. Click on ”Mini
Optic”
To unlock the cavity, click on the Servo off button as shown in Fig.7.21.
CHAPTER 7. COMPTON POLARIMETER
Figure 7.21: Compton Laser control setup to unlock the cavity
115
CHAPTER 7. COMPTON POLARIMETER
7.4.3
116
Electron Detector Setup
• Turn on the electron detector
The detector system needs to be powered. In hall A there is an electrical box called
A-UH-B1 left of the stairs going to the tunnel (see Fig.7.22) In this box, the main
power switch for the electron detector is number 21 (it says electron detector on
it). It must be on turned ON. In the tunnel, there is a crate attached to the wall
above the electron detector, it also needs to be turned ON. When it is ON a red
LED is lit (at the right end of the crate). Below this crate there is a black electrical
box controlling the displacement system. On the left side of this box it should say
”Idle”.
Figure 7.22: Location of the electron detector circuit breaker
• Slow control of the electron detector
To perform operations on the electron detector. Open the MISC... screen from
the main Polarimeter EPICS screen and then choose ”Electron Detector”. On
this screen, ashown in Fig.7.23, active buttons appear in blue and readback values
appear on a yellow background. To use the electron detector a high voltage (80
V) must be applied to polarize the silicon microstrips and a low voltage must be
provided to the preamplifier circuit board and some threshold must be set for each
plane for the detection of the signals. To do this execute the following operations :
Turn the low voltage ON
Turn the high voltage ON. The return value should increase gently to 80.
Set thresholds to 35. The return value should read 35.
Turn calibration OFF.
CHAPTER 7. COMPTON POLARIMETER
117
The electron detector can be put in data taking position remotely. When the
detector is inserted the chicane must be ON, when it is being moved the beam
must be OFF too. If it is not the case the detector will eventually be destroyed.
Click on either ”garage” or ”beam” depending on where you want to put the detector.
To make sure the detector is where you want watch the detector move on the TV
screen (there is one in tha Hall A counting house and one in the back room). The
switches readback must oscillate a little bit if the system is running properly.
Figure 7.23: Compton electron detector control panel
The electron detector must be on the garage position.
Check the status of the electron detector on
the video screen shown in Fig.7.24. The arrow must be exactly in front
of OUT (outside) nominal position.
• Switch on the the Compton chicane
CHAPTER 7. COMPTON POLARIMETER
118
Figure 7.24: Compton electron detector TV viewer
This procedure is only performed by MCC operators.
First of all, the Hall A Run Coordinator must request that MCC tune the beam
through the Compton chicane. MCC operators have to apply the section 2 of the
procedure MCC-PR-04-001. If necessary (after a long shutdown for exemple), let’s
remind to the operator to open valves located on the Compton line. The complete
procedure is available on the MCC web site
at this URL
• Lock once again the cavity
7.4.4
Vertican Scan
Perform a vertical scanning of the electron beam inside the magnetic chicane in order to
maximize the counting rates in the Photon detector.
In the case the crossing of the electron and Laser beams has been lost, or is not
optimal, a ”Vertical scan” has to be performed. By stepping the magnetic field of the
chicane dipoles, the beam is moved vertically. Step size should be small with respect to
the laser spot size (1̃00 micro m). Here are some step sizes corresponding to a 25 or
100µm vertical displacements versus typical beam energies, MCC operator are used to
Gauss.cm unit:
Although the procedure is non-invasive for Hall A, let the shift leader know when
you start and finish the scan.
The scan is done in contact with MCC (7047) by checking the online evolution of
the counting rate using ”spy acq”. The optimal Y-position is at the upper part of the
bell appearing on the middle of the spy acq picture (see Fig.7.25) (”Counting Rates
CHAPTER 7. COMPTON POLARIMETER
step 25µm
10 G.cm
20 G.cm
30 G.cm
40 G.cm
50 G.cm
60 G.cm
70 G.cm
step 100µm
40 G.cm
80 G.cm
120 G.cm
160 G.cm
200 G.cm
240 G.cm
280 G.cm
119
Energy
0.8 GeV
1.6 GeV
2.4 GeV
3.2 GeV
4.0 GeV
4.8 GeV
5.6 GeV
Table 7.1: Chicane vertical scan step values for various energies.
versus DAQ Y at vertex”). As a first pass, one can use bigger step size to locate the
maximum and then go back to small steps to fine tune the position.
When this procedure is over, come back to the right Y-position and ask to the
machine operator to lock the Y-position of the beam. Then an automatic magnetic feedback will run to keep the electron beam Y-position within 50 micro m of this optimal
position. Click on Set X and Set Y buttons in the frame ”Beam drift alarm on Epics
pos at vertex”. Click on Alarm ON to set it green. It will bip when the 50 micons
limit is reached. This is an important task to avoid sensitivity to beam position false
asymmetry.
• Ask to the MCC operators to switch the beam off.
• Insert the electron detector in the beam line.
First of all, the electron beam must be off (see Hall A run coordinator and call
MCC operator) If it is not the case the detector will eventually be destroyed. To
perform operations on the electron detector. Open the MISC... screen from the
main Polarimeter EPICS screen and then choose ”Electron Detector” to get the
panel shown in Fig.7.23.
Click the ”beam” button.
To make sure the detector is where you want watch the detector move on the TV
screen (there is one in tha Hall A counting house and one in the back room). The
switches readback must oscillate a little bit if the system is running properly.
Finally, Ask to the MCC operators to switch the beam on.
7.4.5
Taking data
This is a list of check points to run Compton. It assumes the polarimeter has already
been started up as described in the previous sections and that a run has just ended and
you want to take a new one.
Bring up the following three screens to control the data taking:
CHAPTER 7. COMPTON POLARIMETER
Figure 7.25: Vertical scan trace in spy acq panel
120
CHAPTER 7. COMPTON POLARIMETER
121
• EPICS screen: slow control for the optic table + cavity, the photon and electron
detectors, beam parameters.
• Acq screen: runcontrol and spy acq display.
• Netscape screen: Electronic logbook of the Compton polarimeter, check histograms.
Follow the following procedure:
• Go to the EPICS screen, check the cavity is loocked with 1̃200 Watts or more.
• Go to the Acq screen and click on the Trigger window in spy acq.
– Check Random, Mouly and central cristal are activated.
– Check Raw data rates. Assuming a trigger rate of 1kHz/muA, prescaler
factors should keep the read data rates at the few kHz level.
– Check the trigger condition in General Daq setup (Photon only, e only,
coinc, ...).
• Check the state of the acquisition in the Acquisition system window of spy acq.
After an ”End run succeded” each module should be in ”downloaded” state. If not,
follow error messages displayed in the bottom window. Most of the problems are
fixed by clicking on Reset or Reboot + Download buttons. Display needs some delay
to refresh after these actions. Don’t click like crazy on every enabled button. If
everything is stuck try ”restart this window” in the spy acq menu to refresh the
display.
• Click on Start Run in the Runcontrol window. Check that each module of the
acquisition goes from downloaded to paused and then active state.
• Click on the Online Counting Rate window in spy acq. Check the rate in the
central cristal (red curve) is close to the optimal value from the last vertical scan
(it should be around 1kHz/muA). If counting rates are low and Beam Drift Alarm
keeps ringing, the crossing of the electron and Laser beams is not optimal. Stop the
run and perform a vertical scan.
• Start the photon polarization reversal by clicking on Procedure ON in the LeftRight procedure frame. Periods of cavity ON and OFF will alternate, starting
with OFF (bkg measurement).
• Take a 1̃ hour run.
• Before ending a run, fill up the LogBook window in spy acq (name, run type,
title). Ensure the Logbook and Checklist buttons are not inhibited if you want the
run to be analysed and stored in the electronic logbook.
CHAPTER 7. COMPTON POLARIMETER
122
• Click on End Run in the runcontrol window. Look for the ”End of run succeeded” message in the bottom frame. If End of run failed, go to Acquisition system
in spy acq and follow error messages.
• A yellow window should pop up for few second after the end of run indicating
that the run is saved and the online analysis (checklist) is runnig.
• Go to Netscape screen and reload the Compton logbook web page. Last run
should appear on the first line.
• By clicking on more you access to detailed informations about the running conditions as well as to control histograms generated by the checklist script. This
script may take few minutes to run. It is important task to check the control
histograms after each end of run. Quality off the data depends on it. See
section ”Control Histograms”.
• Go to first point, start a new run.
Two kinds of alarm can turn ON during data taking:
• Y Position: Go to On Line Counting Rate window in spy acq and check the
”Beam drift alarm” frame. If the ”Average Y” differs to ”Y settings” by more than
50 microns, Alarm is ringing and stop bell button is red. Click on stop bell and
wait few seconds to see if the position feedback brings Y back to its nominal value.
If it doesn’t, call MCC (7047) and ask them to check if the position feedback on
Y in the Compton chicane is still running. If necessary stop the run, perform a
vertical scan and re-lock the vertical position at the new optimal value.
• DAQ system: If something goes wrong in the DAQ system during data taking
you should see an effect on the ”photon read” counting rate. Go to Acquisition
system window in spy acq, click on stop bell button if alarm is ringing. End Run in
runcontrol window. Follow error messages displayed in spy acq to fix the problem.
ADC spectrum of the photon detector should show the pedestal peak (pink), the
diode signal (green), Compton + background spectrum (blue). Gain has to be adjusted
so that the Compton edge is between 1/2 and 2/3 of the ADC range.An automatic fit
procedure substracts the background and calibrate the threshold value using the Compton
edge. If the fit doesn’t succeed it won’t affect the quality of the data but prevents
further online analysis. Call a Compton expert to fix it.Typical experimental Compton
asymmetries are of the order a 1%. Check the electron beam current asymmetry stays
below few 100 ppm.Vertical position of the electron beam is the most important
parameter. It drives our luminosity (electron and Laser beam crossing) as well as our
sensitivity to beam position differences correlated with the helicity. Check we spend
most of the running time at the optimal Y position, which is at the summit of the
bottom left curve (counting rate % Ybpm). If the most probable position is off by
more than 50 micron, perform a vertical scan.
CHAPTER 7. COMPTON POLARIMETER
123
Any comment about the running conditions, shift summary, ... are welcome to help
the offline analysis. You can insert them in the Compton electronic logbook by filling up
the LogEntry window in spy acq. Click on Submit to dowload your comments in the
logbook.
7.4.6
Turning off the compton polarimeter
• Stop the magnetic chicane
This procedure is only performed by MCC operators.
First of all, the Hall A Run Coordinator must request that MCC tune the beam
through the Compton chicane. For a foreseen shutdown or maintenance days, you
do not need this step.
MCC operators have to apply the section 3 of the procedure MCC-PR-04-001.
Let’s remind to the operator to close valves located on the Compton line. It is very
important to keep the best vacuum in the Compton line and avoid dust deposit on
the high reflectivity mirrors of the cavity The complete procedure is available on
our web site at :
http://hallaweb.jlab.org/compton/Documentation/Procedures/compton_off.
frm.ps
• Set the electron detector on the GARAGE position
First of all, the electron beam must be off (see Hall A run coordinator and call
MCC operator) If it is not the case the detector will eventually be destroyed. Slow
control of the electron detector. To perform operations on the electron detector.
Open the MISC... screen from the main Polarimeter EPICS screen and then choose
”Electron Detector”. On this screen, active buttons appear in blue and readback
values appear on a yellow background. This screen is also reachable, by loading
the file ComptonElectron.adl located under the home directory of hacuser on the
hac computer.
Click on ”garage” button.
To make sure the detector is where you want watch the detector move on the TV
screen (there is one in tha Hall A counting house and one in the back room). The
switches readback must oscillate a little bit if the system is running properly.
• Switch off the PMT High Voltage of the photon detector
The cards of the COMPTON Polarimeter PMT HV are located in crate #2 telnet hatsv5 2011, login:adaq, paswd:ask people on shiftthen 1450, vt100 and usual
display.
CHAPTER 7. COMPTON POLARIMETER
124
High Voltage channel for the Compton polarimeter are in cards # 12, 13, 14 and
15. Switch off all the channels.
NB: Only one user can connect on hatsv5 at the same time!! If you can’t connect
check if others are logged in. See also procedure posted in rack #CH01B05.
• Unlock the cavity
On the EPICS control panel, pull the ”OPTICS” menu down.
Click on ”Mini Optic”.
To unlock the cavity, click on the Servo off button.
• Switch off the laser
On the EPICS control panel, pull the ”OPTICS” menu down.
Click on ”Mini Optic”.
To turn the Laser Off .... Click on the Laser Off button. Check LASER STATUS
and INCIDENT POWER.
The Laser spots would switch off on the CCD control TV screen
7.5
7.5.1
Safety Assessment
Magnets
Particular care must be taken in working in the vicinity of the magnetic chicane dipoles of
the compton polarimeter as they can have large currents running in them. Only members
of the Compton polarimeter group are authorized to work in their immediate vicinity,
and only when they are not energized. As with all elements of the polarimeter which can
affect the beamline, the magnets are controlled by MCC. All four dipoles are powered in
series from a common power supply. The power supply for the dipoles is located in the
Beam Switch yard Building (Building 98). The maximum current for the dipole is 600A.
There is a red light which indicate the status of the dipoles. The warning red light is
activated via a magnetic field sensitive switch placed on the coils of one of the dipole.
Lock and tag training is required of all personnel working in the vicinity of the Compton
magnets.
7.5.2
Laser
The primary hazzard in the optical table of the compton polarimeter is the Class IIIB,
240 mw CW infra-red laser. It is housed in the tunnel in a laser safety enclosure interlocked with the laser power. Welding curtains are provided on all sides to isolate the
laser enclosure from other pathways. A flashing yellow beacon installed in the tunnel
indicates laser on status. Three crash buttons are provided in the tunnel for emergency
shutdown of the laser.
CHAPTER 7. COMPTON POLARIMETER
125
All functions of the laser are remotely controlled and personnel access to the laser
”hut” is not necessary during routine operation of the compton polarimeter. However,
in case of repair or mainetance work, access to the laser enclosure may be necessary.
The safe operating procedure for this laser is described in Jeffeson Lab Laser Standard
Operating Procedure (LSOP) 101-2-99-1-4. A copy of the LSOP is available in the tunnel
wall next to the laser hut. Only personnel aouthorized in the LSOP are permitted to
access the laser hut.
7.5.3
High Voltage
There are 25 photomultiplier tubes within the compton photon detector module. Each
tube is connected to a high voltage power supply located in the beamline instrumentation
area with SHV cables. The maximum voltage is 3000 Volts. The high voltage supply
must be turned off prior to accessing any of the photon detector elements for servicing
purposes. Only members of the Compton group are authorized to access the detector.
The list of the presently authorized personnel is given in Tab. 7.2. Other individuals
must notify and receive permission from the contact person (see Tab. 7.2) before adding
their names to the above list.
Name
Nanda
Segal
Zhang
Authier
Colombel
Deck
Delbart
Lhuillier
Lussignol
Neyret
Tarte
Veyssire
Sirish
Jack
Joseph
Martial
Nathalie
Pascale
Alain
David
Yves
Damien
Grard
Christian
Dept.
JLab
Jlab
Jlab
CEA
CEA
CEA
CEA
CEA
CEA
CEA
CEA
CEA
Telephone
JLab Pager
7176 7176
5849 5849
5849 5849
4324*
8350*
2426*
3454*
9497*
2828*
7552*
8464*
9704*
e-mail
Comment
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
Contact
Technical
Optics
Engineering
Mechanical
Electronics
Optics
Analysis
EPICS
DAQ
Electronics
Electronics
Table 7.2: Compton Polarimeter: authorized personnel. The primary contact person’s
name is marked with a slanted font. *For CEA extensions, dial 9-011-33-1-6908-XXXX.
Part III
Targets
126
Chapter 8
Overview 1 2
The physics program in Hall A utilizes a number of different target systems of varying
complexity. There is a set of cryogenic targets which currently operates with liquid
hydrogen, liquid deuterium and gasous helium 3 or helium 4 as target materials.
A variety of solid targets is also provided; Be0, Carbon or Kapton are typical but
other self supporting materials are available if need arises.
The combination of cryogenic targets and a few solid targets is the standard configuration. In addition, there is a large program based on polarized helium 3. This is a
special installation and hence is not available at the same time as the cryogenic target
system.
Finally, a waterfall target was used during the commissioning of the hall spectrometers and is also available for experiments. This system also requires a special installation.
Each of these systems is discussed in following chapters.
1
2
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127
Chapter 9
Cryogenic Targets 1 2
9.1
Procedure for Normal Running of the Hall A
Cryogenic Targets
This procedure provides guidelines for the everyday running of the Hall A cryogenic
Hydrogen and Deuterium targets.
9.1.1
Introduction
The Hall A cryotarget system contains three target loops. The top loop (loop 1) has a
single 10 cm “tuna can” helium cell, which will be filled with either 3 He or 4 He gas with
pressure up to 15 atm (about 220 PSIA). This loop is usually not been used for normal
Hydrogen and Deuterium running, in whihc case it will be filled with allitle over one atm
helium gas. Both the middle loop and the bottom loop contain one 15 cm and one 4 cm
“beer can” cells. The middle loop (loop 2) is usually filled with liquid Hydrogen during
normal operation. The bottom loop (loop 3) is usually filled with liquid Deuterium
during normal operation. If only the Hydrogen target is used, such as the case for the
HAPPEX running, loop 3 (the Deuterium loop) will be filled with 17 PSIA helium gas
to save cooling power.
During the normal operation, the Hydrogen and/or Deuterium target should have
already been liquefied and are in a stable state of about 2 degree sub-cooled. The
normal operating conditions of the targets are given in Table 1. Also listed in Table
1 are the freezing and boiling temperatures. These parameters should be reasonably
stable provided that the End Station Refrigerator (ESR) is stable. The temperature
is controlled by a software PID loop with a high power heater (up to 600 Watts) and a
hardware PID loop with a low power heater (up to about 60 Watts). Both PID loops read
the output of one of the Cernox resistors and adjust the power in the high or low power
heater appropriately. The control loops function extremely well and the temperature
1
2
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128
CHAPTER 9. CRYOGENIC TARGETS
129
fluctuations with steady beam are typically measured in hundredths of degrees. During
beam off- beam on transitions high power fluctuations of a few tenths of a degree are not
uncommon.
Target Temperature (◦ K) Pressure (PSIA)
H2
19
25
D2
22
22
Freezing T (◦ K) Boiling T (◦ K)
13.86
22.24
18.73
25.13
Graphical User Interface
The principal interface with the target is through the Graphical User Interface (GUI),
of the control system. Every target operator should be familiar with the short version of
the GUI manual. The long version can be used to find more details or for reference.
9.1.1
Alarm Handler
It is mandatory to have an alarm handler,ALH, running at all times. Further, it is
mandatory that the alarm handler be visible in all work spaces on the target control
computer. Even though the target safety is ultimately insured by mechanical measures,
the alarm handler can save you lots of time, grief and potentially prevent problems with
data. The ALH will alarm if any of its parameters goes out of normal range. Servicing
the alarm is the responsibility of the target operator. At high beam current, the ALH will
usually alarm when the beam goes from on to off or from off to on, since the temperature
change is out of normal range. The ALH can also repeated alarm if there are noisy analog
channels. If the AH alarms repeatedly or the cause of the alarm is not clear, the target
operator should contact the on-call target expert.
9.1.2
Target Motion and Fast Raster
The target motions are interlocked with the machine Fast Shut Down (FSD) system.
Therefore, it is mandatory that you call MCC so that they can remove beam from the
Hall and mask our FSD node before using any target motion mechanism.
When full power beam with tiny beam spot hit the cryotarget, there is a danger that
the beam can melt the target cell. The fast raster is used to prevent this from happening.
Every time when moving the cryotarget into beam position, the target operator must
check to make sure that the faster raster is on and has a reasonable size.
9.1.3
Cryogenic Consumption
The ESR is not a bottomless reservoir of helium coolant. Every effort should be made
to keep our consumption within reasonable bounds. This means that heater overheads
CHAPTER 9. CRYOGENIC TARGETS
130
should be tens and not hundreds of Watts and that loops which will be dormant for
extended periods should be powered down as much as possible. If you feel that the
cryogenic consumption is too high (or have received complaints from another ESR user)
and are uncertain about the appropriate action contact the on-call target expert.
9.1.4
Checklist
The Hall A target experts and the JLab target group like to track the state of the target.
To help them in this task the target checklist must be filled out once per shift and store
in the target checklist binder.
9.1.5
Target Operators
One individual on each shift is responsible for target operations. This individual is the
designated target operator. To become a target operator, one must be trained by one of
the target experts and to sit at least one shift with an already certified target operator.
The training usually takes place in the Hall A counting house and consists of a guided
walk through of the control system.
The target operator must read this document, the Safety Assessment Document
for the Hall A Cryogenic Targets, and the short version of the GUI manual. The target
operator should be familiar with the GUI system and be able to handle the normal target
loop operation, the cryostat operation and the target motion. He/she should also be able
to deal with the GUI crash, the IOC crash and the usual alarms.
After the target operator’s training, if he/she feels comfortable with the normal
operation of the cryotargets, he/she should sign his/her name on the target operator
authorization list, indicating that he/she has read this procedure and has been trained.
The target expert who trained him/her should inform the Hall A staff who is responsible
for the cryotarget system (J. P. Chen), who will then sign off to authorize him/her to be
a certified target operator.
The table below lists the qualified target operators and provides space for additional
entries. The names of all operators must appear in this table.
CHAPTER 9. CRYOGENIC TARGETS
Operator Name
J. P. Chen
K. McCormick
G. Rutledge
R. Suleiman
e-mail
jpchen
mccormic
grutledg
suleiman
Date
Signature Authorization
3/21/99
–
–
3/21/99
–
–
3/21/99
–
–
3/21/99
–
–
131
CHAPTER 9. CRYOGENIC TARGETS
132
Target Experts
The following table contains the names of the currently recognized target experts (who
have worked on the Hall A cryotarget system and have extensive knowledge of the system)
and their pager and/or home numbers
Expert Name
Affiliation
J. P. Chen
JLab Hall A
K. McCormick
Rutgers
D. Meekins
JLab Target Group
M. Seely
JLab Target Group
C. Keith
JLab Target Group
Pager
Office
Home
584-7413
584-5433
584-5434
584-5036
584-5878
7413
7011
5434
5036
5878
867-7380
640-8062
974-4750
833-7890
596-3002
A cryotarget expert will be on call all the time when a cryotarget is in cooled state.
An on-call cryotarget-expert list will be posted in the Hall A Couting House.
9.2
Safety Assessment
The cryogenic hydrogen and deuterium targets present a number of potential hazards,
such as the fire/explosion hazard of the flammable gas as well as the hazards connected
with the vacuum vessel and the of handling cryogenic liquids (ODH and high pressure).
In this document the hydrogen target will be referred to, but the deuterium target is
essentially identical and almost all comments apply to both targets.
9.2.1
Flammable Gas
Hydrogen and deuterium are colorless, odorless gases and hence not easily detected by
human senses. Hydrogen air mixtures are flammable over a large range of relative concentrations from 4 % to 75 % H2 by volume. Detonation can occur with very low energy
1
input, less than 10
that required by mixtures of air and gasoline. At temperatures above
-250 C hydrogen gas is lighter than (STP) air and hence will rise. At atmospheric pressure, the ignition temperature is approximately 1000 ◦ F but air H2 mixtures at pressures
of 0.2 to 0.5 Atm can be ignited at temperatures as low as 650 ◦ F. Hydrogen mixtures
burn with a colorless flame [13].
The total volume of liquid hydrogen in the heat exchanger is about 2 l. The target
cells and their associated plumbing hold an additional 3.4 l. Thus the total volume of
hydrogen in the target is approximately 5.4 l. The volume changes between the liquid
state and gas at STP by a factor of about 800. Thus filling the target would require
about 4,300 STP l of hydrogen. The hydrogen target is connected to a 1,000 Gallon
(about 3,800 l) recovery tank. The normal running condition for hydrogen is 26 PSIA.
So the total amount needed to fill the target and the tank is about 10,900 STP l. For
deuterium, the target is about 4,300 STP l. The normal running condition for deuterium
CHAPTER 9. CRYOGENIC TARGETS
133
is 22 PSIA. So the total volume needed to fill the tank is about 5,600 l. The total to fill
both the target and the tank is about 9,900 STP l.
The Hall A inventory of hydrogen and deuterium gas is stored outside the Hall A
gas shed, adjacent to the counting house. The current inventory is two A size cylinders
of hydrogen (≈ 6,800 STP l each) and four A size cylinders of deuterium (≈ 5,000 STP l
each). One bottle of hydrogen and one deuterium bottle will be kept in the Hall in order
to fill the targets. These bottles will be placed in a gas rack behind the gas panels.
The basic idea behind safe handling of any flammable or explosive gas is to eliminate
oxygen (required for burning) and to prevent exposure to any energy source that could
cause ignition. In the Hall A environment, the most likely source of oxygen is of course
the atmosphere and the most likely ignition sources are from electrical equipment.
9.2.1.1
Electrical Installation
Hall A contains a lot of electrical equipment and almost all of it could serve as an ignition
source in the presence of an explosive oxygen and hydrogen mixture. We have made an
effort to minimize the dangers from the equipment that is most likely to come into contact
with hydrogen gas.
There are a number of electrically powered devices associated with the target gas
handling system. All the pressure transducers in the system are approved for use in a
hydrogen atmosphere. The solenoid valves on the gas panels are explosion-proof. The
AC power for the solenoids is carried by wires which are contained in either hard or
flexible conduit. There are also LEDs on the gas panels that provide an indication as to
the status of the valve solenoids. These are powered by a 24 V DC supply. The readouts
for the pressure transducers are mounted on the gas panels and the AC power for these
readout units is in conduit. All the pressure transducers have 4-20 mA outputs.
In addition to the electrical devices in the gas handling system, there are a number
of devices inside of or mounted on the scattering chamber.
All the devices which are in the scattering chamber must have their power delivered
to them by wires in vacuum. The insulation of these wires should be radiation resistant,
so Kapton has been used where available.
The following electrical items are in close proximity to or are actually in the hydrogen
system.
Axial Circulation Fan The fans which circulate the hydrogen in the target are AC
induction motors and therefore contain no brushes and are practically immune to
sparking. The three phase power for these fans is delivered to them by 18 gauge
stranded copper wire with Kapton insulation. The maximum current that the fans
draw is 5 A for a maximum power consumption of 200 W when pumping liquid
hydrogen/deuterium. The current and voltage drawn by the fans is monitored by
the control system.
Fan Motor Tachometer The fans have a tachometer which consist of a coil that views
the flux change caused by a permanent magnet attached to the motor rotor. The
CHAPTER 9. CRYOGENIC TARGETS
134
tachometer signals are carried on 22 gauge stranded wire with Kapton insulation.
This is a low power signal. The control system monitors the frequency of the fans.
Low Power Heater This is a “hair dryer” style heater ( it resembles the heater elements
found in hair dryers and heat guns) that is immersed in the hydrogen. The heater
is made of 0.0179 in diameter Nichrome wire with a resistance of 1.993 Ω per foot
wrapped on a G10 carrier board. The maximum power available to this heater
is 80 W . The power for the low power heater is supplied by a Oxford ITC-502
temperature controller. The heater lead wire is 18 gauge Kapton insulated copper
stranded wire. The heaters have a DC resistance of 20 Ω and hence will draw a
maximum of 2 A. The power supplied to this heater is monitored by the control
system.
High Power Heater There are two kapton enclosed incoloy heater foils wrapped on the
inside wall of each heat exchanger. The maximum power available to each heater
is 500 W . The heater has a DC resistance of 26 Ω and two heaters in parallel are
driven by a 150 V , 7 A power supply. The current and voltage supplied to this
heater are monitored by the control system and there is a software power maximum
enforced on the power setting of this heater. The heater is connected to the outside
world by 18 gauge stranded wire with Kapton insulation.
Resistors There are two Allen Bradley and four Cernox resistors immersed in each
target loop. These resistors provide temperature measurements of the target fluid.
The temperature controllers that read them use a current of less than 30 µA to
excite them ( they are excited with a constant voltage which for our resistors is on
the order of 30 mV). The Cernox resistors are connected to the outside world with
quad strand 36 gauge phosphor bronze wire with Formvar insulation. The Allen
Bradley resistors are wired with 30 gauge Kapton insulated copper stranded wire.
Target Lifter There are two AC servo motors which provide the power to lift the target
ladder. These motors are powered by three phase 208 V power and are equipped
with fail safe brakes (the brakes are released by a 24 V DC control voltage) and
50 to 1 gear reducers. On power up, there is a delay relay that insures that the
motors are always energized before the brakes are released.
Vacuum Pumps The scattering chamber is evacuated by two Leybold 1000 l/s turbo
pumps that are backed by a Leybold 65 cf m mechanical pump. The turbo pumps
are powered by 120 V AC power while the backing pump requires three phase 208
V AC power. The motor on the backing pump is explosion proof and approved
for use in NEC Class 1, Division 1, Group D (hydrocarbons but not hydrogen)
environments. An identical mechanical pump is used in the pump and purge system
of the gas panels. Both the scattering chamber backing pump and the pump and
purge system’s mechanical pump exhaust to the vent line.
Vacuum Gauges The chamber vacuum is monitored by an HP cold cathode gauge.
This gauge has a maximum operating voltage of 4000 V and a maximum current
CHAPTER 9. CRYOGENIC TARGETS
135
of 133 µA. The pressure at the entrance to the roughing pump is measured by a
convectron gauge.
9.2.1.2
Flammable Gas Detectors
There are four flammable gas detectors installed (one on top of the target, one each on top
of the hydrogen and deuterium gas panels, one on top of the gas tanks) to provide early
detection of hydrogen/deuterium leaks. These detectors are sensitive (and calibrated)
over the range from 0 to 50 % Lower Explosive Limit (LEL) of hydrogen. The electrochemical sensors were manufactured by Crowcon Detection Instruments LTD and the
readout (four channels) was purchased from CEA Instruments, Inc. (The Gas Master
Four System). The readout unit provides two alarm levels per channel. The low level
alarm is tripped at 20 % LEL while 40 % LEL activates the high level alarm. Each
channel has a relay output for both low and high level alarm states and there is also a
set of common relays for both alarm levels (these common relays respond to the ”logical
or” of the sensor inputs). The common relays will be connected to the Fast Shut Down
System, FSD, which removes the beam from the hall by disabling a grid bias at the
injector.
9.2.2
Pressure
The most important aspect of hydrogen safety is to minimize the possibility of explosive
mixtures of hydrogen and oxygen occurring. Therefore the gas handling system has been
made of stainless steel components (wherever possible) and as many junctions as possible
have been welded.
The pressure in the gas handling system is monitored in numerous places. Most
importantly, the absolute pressure of the target is viewed by two pressure transducers,
one on the fill line, PT127 for H2 and PT136 for D2 , and one on the return line, PT131 for
H2 and PT140 for D2 . These pressures are also measured by manual gauges. The fill line
gauges are PI126 for H2 and PI135 for D2 . The return line gauges are designated PI130,
H2 and PI139, D2 . The gas tanks are viewed with both pressure transducers (PT133 for
hydrogen and PT142 for deuterium) and pressure gauges (PI123 for hydrogen and PI112
for deuterium).
9.2.2.1
Target Cells
The target cells themselves represent the most likely failure point in the hydrogen system.
The outer walls and downstream window of the cells are made of ≈ 0.03 to 0.045 in thick
3004 aluminum (Coors beer cans in a former incarnation) (all the final ones are above
0.035 in). There are two cells soldered to each cell block, one 15 cm long and one 4
cm long. Both cells have an outer diameter of approximately 2.5 inches. The upstream
windows of the cells are made from 0.0028 in thick 5052 aluminum. These windows are
soldered to 1.75 in diameter (0.065 in wall) upstream window tubes which are in turn
soldered to the cell block.
CHAPTER 9. CRYOGENIC TARGETS
136
Since all the components are made of aluminum it is necessary to plate them before
soldering. The final components were copper plated before assembly.
The cell block components have been pressure tested hydrostatically at Jefferson
Lab. We chose the thinnest beer cans for the pressure burst test. Results are listed in
the summary table. Upstream windows have been tested to similar pressures. Finally,
the entire completed cell block assemblies were pressurized to 85 PSID with helium gas.
A summary of the testing program to date is presented in Table 9.1.
Object
Can
Can
Can
Can
Can
Window
Window
Window
Window
Window
Window
Complete Blocks
P (PSIG) thickness (in)
55
0.003
80
0.0035
80
0.0035
60
0.0038
85
0.0039
110
0.028
125
0.028
125
0.028
115
0.028
147
0.028
150
0.028
85
size
test method
short test (J)ig, (D)estructive
short
(J), (D)
long
(J), (D)
long
(V)acuum, (D)
long
(V), (D)
(J),(D)
(J),(D)
(J),(D)
(J),(D)
(J),(D)
(J),(D)
(V), Non-Destructive
Table 9.1: A summary of the early cell block pressure test data.
9.2.2.2
Pressure Relief
The gas handling and controls systems have been designed to prevent excessive pressure
build up in the system in order to protect the target cells from rupture.
In the event that the pressure in the system begins to rise there are multiple vent
paths to release it. The first line of defense is the recovery tank. The second line of
defense is a small orifice solenoid valve which is slaved to a pressure transducer. This
valve, CSV28 for H2 and CSV57 for D2 , is normally controlled by the limit output of
the computer (via a VME based relay) readout of the pressure transducer that views the
target relief line, PT131 for H2 and PT140 for D2 . The valve itself is mounted in the fill
line relief assembly. The separation of the valve from its controlling pressure gauge should
provide some dampening of the response and the small orifice of the valve also ensures
that it will be able to make pressure adjustments gently if need be. There is a separate
relief valve on the fill side of the target, CRV30 for H2 and CRV59 for D2 . This relief is
mounted in parallel with the small orifice solenoid valve. Right on top of the cryocan, on
the return side of the target, there is a large size (one in) relief valve. All target pressure
reliefs are connected to the nitrogen vent line of the Hall A superconducting magnets.
This is a 3.5 in diameter copper pipe which is filled with nitrogen gas at atmospheric
CHAPTER 9. CRYOGENIC TARGETS
137
pressure. Thus any vented target gas is placed in an inert environment until it is released
outside of Hall A. Each gas tank has one relief valve and one rupture disk (CRV43 and
CRD44 for hydrogen, and CRV72 and CRD143 for deuterium).
In addition to the reliefs on the gas handling system described above, the scattering
chamber itself has a four-in one PSIG relief, VRV01. This is the path that the hydrogen
will take in the event of a cell failure.
The target pressure reliefs are summarized in Table 9.2.
Name
Target Location Diameter (in) Pressure (PSIG)
CSV28
H2
FRA
0.125
40
CRV30
H2
FRA
0.5
40
CRV82
H2
RL
1
40
CRV43
H2
TANK
1
55
CRD44
H2
TANK
1
55
CSV57
D2
FRA
0.125
40
CRV59
D2
FRA
0.5
40
CRV64
D2
RL
1
40
CRV72
D2
TANK
1
55
CRD143
D2
TANK
1
55
CRV35
He
RL
1
40
CRV01
SC
4
2
Table 9.2: A summary of the pressure relieving devices on the hydrogen/deuterium
targets and the scattering chamber. FRA is an abbreviation for Fill Line Relief Assembly.
and RL is an abbreviation for Relief Line. SC stands for Scattering Chamber.
9.2.2.3
Scattering Chamber Vacuum Failure
The scattering chamber will be leak checked before service but obviously the possibility
of vacuum loss cannot be eliminated. The most likely sources of vacuum failure are:
Spectrometer Windows Initially the scattering chamber will have two aluminum windows, one for each side of the beam line.
Target Cell Failure This is a multiple loop system. If a target cell fails, the remaining
targets will have their insulating vacuum spoiled.
The two spectrometer windows are both made from aluminum. Each window is
seven in high and subtends 170 ◦ on the 43 in outer diameter of the scattering chamber.
This window is made of 0.016 in thick 5052 H34 aluminum foil.
The scattering chamber was evacuated (and cycled several times) with both windows
covered by the same 0.016 in material. The foil forms regularly spaced vertical ridges
when placed under load. The window had an inter-ridge spacing of 3 inches. If the
CHAPTER 9. CRYOGENIC TARGETS
138
window is treated as a collection of smaller rectangular windows which have the full
vertical height of 7 inches and the inter-ridge spacing as a width, then stress formulas
predict that the 0.016 in material would reach ultimate stress at a pressure higher than 35
PSI. There is a gate valve between the scattering chamber and the beam entrance (exit)
pipe. Both valves will be closed automatically in the event that the chamber vacuum
begins to rise and an FSD will be caused ( this is done via a relay output of the scattering
chamber vacuum gauge). If either valve is closed an FSD will result.
In the unlikely event of a catastrophic vacuum failure, it is important that the relief
line of the targets be sized such that it can handle the mass flow caused by the sudden
expansion of its cryogenic contents due to exposure to the heat load. A calculation has
been performed which models the response of the system to sudden vacuum failure. That
calculation indicates that the relief plumbing is sized such that the flow remains subsonic
at all times and that the maximum pressure in the cells remains well below their bursting
point.
The calculation was performed by following methods in an internal report from the
MIT Bates laboratory [14]. The formulas and algorithm in the report were incorporated
in two computer codes and those codes were able to reproduce results in the report (hence
they represent an accurate implementation of the Bates calculation).
The calculation can be logically broken into two parts. First, the mass evolution rate
is calculated from geometric information and the properties of both the target material
and vacuum spoiling gas. The principal results of this first stage are the heat transferred
per unit area, q, the boil off time, tb , and the mass evolution rate, w. Second, the
capability of the plumbing to handle the mass flow is checked. The principle result of
this second step is the maximum pressure in the target cell during the discharge, P1 .
The formula involved will not be repeated (readers are referred to the Bates report
for detail). The information that was used as input to the calculation is given in tables
9.3, 9.4 and 9.5.
For the calculation of the boil off rate the target was split into two pieces: the cells
plus cell block, both aluminum; and the heat exchangers plus the connecting plumbing,
all steel. The mass evolution rates for the two pieces were then added in order to find
the total mass flow rate.
The calculation of the pressure drop includes all the plumbing up to the large relief
valve. The calculation assumes that all the mass flow is carried out the relief side of the
target gas handling system (no flow out of the fill line reliefs). The friction factor for each
diameter was taken from a Moody plot. A typical value was f = 0.017. The effective
K values, Kef f , were adjusted to the average tube inner diameter which was taken to be
0.71 in. The final Kef f value was 40. The minor losses are from bends, expansions and
contractions in piping.
The final result shows the cells subjected to 58 PSIA during the boil off, which is
comparable to the 75 PSIA pressure that the assembled cell blocks were tested at, and
is significantly below the tested pressure of the cell components.
The scattering chamber has a volume of about 2,100 l with perhaps an additional
200 l of volume in the bellows and the cryo can. If one target cell were to rupture and
CHAPTER 9. CRYOGENIC TARGETS
Fluid and Phase
Hydrogen/Liquid
Hydrogen/Vapor
Air
Property
Temperature
Density
Specific Heat
Enthalpy of Vaporization
Temperature
Density
Viscosity
Specific Heat
Thermal Conductivity
Volume Expansivity
Temperature
Pressure
Density
Viscosity
Specific Heat
Thermal Conductivity
Volume Expansivity
139
Symbol
T(K)
ρ (kg/m3 )
Cp (J/(kg K))
Hv J/kg
T(K)
ρ (kg/m3 )
µ (kg/(s m))
Cp (J/(kg K))
k (W/(K m))
β K−1
T(K)
P (Torr)
ρ (kg/m3 )
µ (kg/(s m))
Cp (J/(kg K))
k (W/(K m))
β K−1
Value
22
67.67
11520
428,500
22
2.4991
1.29∗10−6
13,550
0.02
0.00366
273
760
1.224
1.8∗10−5
1005
0.0244
0.00367
Table 9.3: The properties of the gases used to calculate the heat transferred to the
target during a catastrophic vacuum failure.
the chamber were unrelieved, the chamber pressure would rise to about 2 Atm. It takes
approximately 150 seconds to bring 5 l of 22 ◦ K hydrogen to room temperature by
conductive heat transfer with the scattering chamber walls. In order that the maximum
pressure in the chamber stay near one atmosphere, it is necessary to vent one half of the
target mass in approximately one half of the total expansion time. Therefore the relief
valve for the scattering chamber should be capable of venting about three grams per
second at a low pressure difference (say two PSIG). If one considers the case where all
three targets fail at once, the vent must be capable of handling three times that amount.
A four in diameter relief valve placed near the top of the scattering chamber should be
capable of handling this rate. A rise in the chamber vacuum will stop the beam, FSD,
and cause the gate valves on either side of the scattering chamber to close.
In the unlikely event that a line which carries helium coolant were to rupture the
four in chamber relief valve is capable of handling the full coolant flow rate.
9.2.3
Temperature Regulation
This is really more an issue of target stability than one of safety. However, a target
with a carefully regulated temperature will presumably not undergo worrisome pressure
changes.
Each target contains four quality temperature measurements, two Cernox resistors
CHAPTER 9. CRYOGENIC TARGETS
Quantity
D
k
A
V
x
q
tb
w
Cell
Piping
Heat
Block
Exchanger
2.5 in (0.063 m)
1.5 in (0.038 m)
7 in (0.1778 m)
55 W/(K m)
6.5 W/(K m)
6.5 W/(K m)
0.146 m2
0.185 (m2 )
0.216 m2
0.001 m3
0.0019 (m3 )
0.002 m3
0.004 in (0.0001 m) 0.065 in (0.00165 m) 0.12 in (0.003 m)
14903 W/m2
10526 W/m2
11235 W/m2
26.78 s
28.29 s
23.89 s
0.0038 kg/s
0.0045 kg/s
0.0056 kg/s
140
Total
0.510 m2
0.0054 m3
26.3 s
0.014 kg/s
(0.03 lbs/s)
Table 9.4: The geometric quantities needed for and the results of calculations of the
mass evolution rate after a catastrophic vacuum failure.
and two hydrogen vapor pressure thermometers. The primary temperature regulation is
done with a dedicated temperature controller (an Oxford ITC-502) which slaves a heater
(the ”low power heater”) to the temperature read by one of the Cernox resistors. This is
a three parameter control loop (Proportional, Integral and Differential Control or PID).
In addition, the return temperature of the target systems coolant gas is used to
regulate the supply of coolant from ESR.
Finally, the heat load from the beam will be compensated in the ”active” target
loop by use of the high power heater. This is not a true regulation but rather a one for
one replacement of the beam load should the beam disappear for whatever reason. The
beam load is calculated from the target length, the beam current as read from a current
monitor and the target material.
Excursions of the target temperature outside acceptable limits will cause the control
system to take action. Finally the redundancy of temperature measurements can be used
by the control system to pick up the failure of a sensor or its readout channel. A more
complete discussion of target temperature regulation is available in Reference [15].
9.2.3.1
Target Freezing
Solid hydrogen is more dense than the liquid phase, so freezing does not endanger the
mechanical integrity of a closed system. The chief hazard is that relief routes out of
the system will become clogged with hydrogen ice, making the behavior of the system
during a warmup unpredictable. When the hydrogen and deuterium targets are in use,
we are using only 15 K coolant. While the hydrogen freezing point is about 13.8 K,
the hydrogen target should not get frozen. The freezing point of deuterium is higher
than that of hydrogen and higher than the temperature of the gas used for cooling (15
K).There is a chance that the deuterium target can freeze.
The coolant flow through the three target heat exchangers is connected in parallel
CHAPTER 9. CRYOGENIC TARGETS
Inner Diameter Length
0.44 in tube
10 ft
0.88 in tube
10 ft
Quantity
Minor Losses
Ktotal
ef f
Average Diameter
xmax
wsonic
m
x
P2
P1
P1
141
K (Kef f )
4.64 (31.5)
2.32 (0.98)
Value
7.4
40
0.71 in
0.890
0.065 lbs/s
0.323
0.748
14.7 PSIA
58.3 PSIA
43.6 PSIG
Table 9.5: Tubing sizes, and other information needed to analyze relief line response.
The mass flow rate was 0.03 lbs/s.
for the three target loops. The entire target system will be run so that it represents a
constant heat load on the ESR. For instance, the ESR will deliver a constant mass flow of
helium cryogen at a constant temperature, about 15 K, and the coolant will be returned
at an approximately constant but higher temperature, usually about 20 K.
The targets are always temperature regulated by temperature controllers. Also a
high power heater will be in the PID loop to compensate any large temperature fluctuations to keep the temperature constant. In the unlikely event that the target temperature
drops too low, an alarm will sound and the target operator will turn down the corresponding J-T valve(s).
9.2.4
ODH
The total volume of the targets is relatively small, with the entire scattering chamber
containing only 9,000 STP l of target fluid when all three targets are full. As the scattering chamber is located in the middle of Hall A (i.e. not in a confined area) and the
total Hall A volume is 40,000 m3 , the ODH hazard is minimal.
9.2.5
Controls
The target controls have been implemented with the EPICS [2] control system and with
hardware very similar to that employed by the accelerator. The basic control functions
reside on a VME based single board computer. The graphical interfaces to the control
system use a PC, and also require the Hall A Hewlett Packard, HP, computer for control
(HAC) to be present as well.
CHAPTER 9. CRYOGENIC TARGETS
142
All of the instrumentation for the target is downstairs in Hall A. Most of the equipment (in fact all of the 120 V AC equipment) is on an Uninterruptable Power Supply,
UPS. The items whose power is not on UPS are:
• The scattering chamber vacuum pumps and the gas panel backing pump
• The target lifting mechanism
• The target circulation fans.
This is a 7 kVA zero switching time UPS which is dedicated to the target. The PC, HAC
and the counting house target X-terminal are on Uninterruptable Power as well. The
targets dedicated UPS provides 18 minutes of power at full load (or 50 min at one half
load). The status of the UPS, online or offline, is read by the control system and after
ten minutes the control system will initiate an orderly shut down of the targets.
The principal functions that the control system performs are:
Pressure Monitoring The pressure at various places in the system is monitored and
alarm states are generated if a transducer returns a value that is outside user defined
limits. High pressures will cause the small orifice solenoid valve to open and cause
an FSD.
Temperature Monitoring The temperature of the target is read from resistors and
vapor pressure bulbs and alarm states are activated when any temperature sensor
returns a value outside the user defined limits. High temperatures will cause an
FSD to occur.
Temperature Regulation The control system allows the target temperature to be
regulated. In the default operating scenario this regulation is performed by a stand
alone temperature controller.
Solenoid Valve Control The gas systems have a number of solenoid valves that must
be switched.
J-T Valve Control The flow of coolant through the heat exchangers is controlled by a
set of J-T valves. These valves control the coolant helium flow through the three
loop heat exchangers and the precool heat exchanger.
Circulation Fan Monitoring and Control The fans which circulate the target fluid
are monitored (current, voltage, frequency). The voltage supplied to the fans is
adjustable and alarm states can be set on out of range frequency, voltage or current
values.
Vacuum Monitoring The scattering chamber vacuum is monitored by the control system. Unacceptable values will generate an FSD and close the upstream and downstream scattering chamber valves.
Target Lifter The target lifting mechanism is controlled by the computer. This allows
one to place the desired target in the beam.
CHAPTER 9. CRYOGENIC TARGETS
Component
Chamber Vacuum
Cryogenic Target
A Staff
Ed Folts
#
Outside Group
7857
143
#
J. P. Chen 7413
ESR
ESR
K. McCormick
D. Meekins
M. Seely
C. Keith
CHL
Cryo on Call
7011
5434
5036
5878
7405
7048
Remark
or Technician
On Call
or Cryo-target
On Call
Rutgers
Target Group
Target Group
Target Group
Cryo Group
MCC
Table 9.6: Contact Personel for the Cryogenic Targets and Scattering Chamber
9.2.6
Authorized Personnel
The principle contacts for the cryogenic targets are listed in table 9.2.6. Every shift
must have a trained target operator whenever the cryogenic targets contain liquid. These
operators are trained by one of the “experts” listed in the table and certified by J.P. Chen.
CHAPTER 9. CRYOGENIC TARGETS
9.3
144
Cryogenic Target Control System User Manual
A short version of the cryotarget target control system user manual, written by Kathy McCormick, is avialble at http://hallaweb.jlab.org/equipment/targets/cryotargets/
Halla_tgt.html. An updated User’s Guide to the Hall A Cryotarget, written by Chris
Keith, is available at https://polweb/guides/atarg/ATARG_MAN.html. Othe useful
information for cryotarget operators is also available at the above web sites.
Chapter 10
Polarized 3He Target 1
10.1
General Description
10.1.1
Physics Principle
This target system provides a high-density (≈ 2.5 × 1020 nuclei/cm3 ) polarized 3 He gas
target for spin physics experiments.
The target employs the so-called spin-exchange technique. In this technique 3 He is
polarized in a two-step process. First, rubidium vapor is polarized by optical pumping
with circularly polarized 795 nm laser light. Second, the electronic polarization of the
Rb is transferred to the 3 He nucleus in spin-exchange collisions, in which 3 He nuclei are
polarized via the hyperfine interaction. The target cell contains high pressure 3 He gas
and a small amount of rubidium vapor. In addition, it also contains a small amount of
nitrogen to increase the pumping efficiency.
10.1.2
Apparatus
High power diode lasers (about 100 W) provide an intense monochromatic light beam for
optical pumping. The lasers are housed in a large shielding house, known as the “laser
hut”, which is located about 5m away from the target on one side of the beam line.
An overview of a typical arrangement of the target components in the Hall is shown in
Fig. 10.1.
The polarized target comprises several components beyond its target cell (see Fig. 10.1)
related to its operation, they are in subsection:
1. A double sets of Helmholtz coils which provide the target spins with a holding magnetic field of few tens of Gauss as well as define the orientation of the polarization
in any required direction.
1
Authors: Revised by: J. P. Chen, P. Chevtsov, K. Kramer, N. Liyanange, K. McCormick, X. Zheng;
Original: T. Black, J. P. Chen, H. Gao, O. Hansen, S. Incerti, S. Jensen, M. G. Jones, M. Liang, Z.-E.
Meziani. mailto:[email protected]
145
CHAPTER 10. POLARIZED 3 HE TARGET
146
Laser Hut
Optics
Target
Longitudinal light tube
Lasers
Lasers
Transverse light tube
Optical
Table
target
pivot
Rack for
Laser
Controllers
Figure 10.1: Overview of the target setup. Shown are the laser hut, the beam pipes
covering the laser beam line extending to the target area, and the Helmholtz coils with
the RF assembly and support sub-assembly.
CHAPTER 10. POLARIZED 3 HE TARGET
147
2. A pair of RF coils which allow for the measurement of the target polarization
using the Adiabatic Fast Passage (AFP) technique and the Electron Paramagnetic
Resonance (EPR) technique.
3. A multi-purpose RF enclosure which holds the RF coils but more importantly is
crucial for the containment of the target cell in case of explosion. It also provides
a containment volume for the 4 He gas to cool the target windows with cooling jets,
minimizing the radiation length crossed by the electron beam.
4. An oven with all its related components for providing the necessary temperature
to the pumping cell in order to bring the rubidium in its vapor phase and control
its number density.
5. A target ladder subassembly which supports a target cell, a reference cell, (which
can be filled with 3 He, Nitrogen or 4 He gas with pressure up to 10 atmospheres
to study dilution, density or background), a 7-foil 12 C optics target, a BeO beam
viewer and an oven. It also includes a full mechanism for positioning the targets
and bears the target cell viewing mirrors and the optical beam line mirrors.
6. A laser shielding house known as “laser hut” where two laser beam lines are formed
by seven 30-Watt diode lasers. These laser beam lines are used for optical pumping
of the rubidium alkali atoms in the direction either along the electron beam (longitudinal, 4 lasers) or perpendicular to the electron beam line (transverse, 3 lasers).
The nominally longitudinal laser beam can also be directed at a different (specific)
direction.
10.1.3
Control System
The control (including monitoring and measurement) system for the target Helmholtz
coil magnet power supplies, the NMR polarimetry and the EPR polarimetry is based
on the LabView system on a PC. The control system for the target vertical motion,
the half-wave plate motion, the lasers, the oven heater, and temperature and pressure
monitoring runs under the EPICS [2] environment utilizing an IOC in a VME crate.
The LabView system records data on disk and communicates with the EPICS system
through the network. Information from the EPICS IOC is logged on disk and selected
information passed to the event data stream.
10.2
Operation Overview
In normal operation the target will be under one of the following situations described
below.
1. The target is in “beam position”, the fast raster set to a nominal 4 mm × 4 mm area
coverage and the experiment is running to collect physics data. In this configuration
CHAPTER 10. POLARIZED 3 HE TARGET
148
the monitoring of the target consists of reading a set of temperatures of the pumping
cell, target cell and reference cell. The temperature in the pumping cell provides
feedback for the temperature controller. A spectral-analyzer is monitoring the
laser wavelength and the relative intensity. All interlocks of electron beam and
laser beam should be on.
2. The beam is turned off and the target is moved to “Pickup Coil Position” (polarized 3 He target in a position between the NMR pickup coils) and a polarization
measurement is performed using the NMR (AFP) method. The laser beam will
be stopped first. If the polarization is in the transverse mode, a rotation of the
polarization to the longitudinal direction is performed (procedure described in Section 10.9). Once the measurement is completed the target cell will be moved back
to the “beam position”. If the next running is in transverse mode, a rotation of
the polarization to the transverse direction is performed. The laser beam will be
turned on. The beam interlock is reset and the fast raster enabled before turning
the beam back on target. The physics data taking should resume.
3. The “Polarized 3 He Target” is either in “beam position” or in “pick-up coil position”. The data taking is stopped and the beam is turned off. An EPR measurement is performed (procedure described in Section 10.10). After the measurement
is completed and the fast raster enabled, the beam is put back on and data taking
resumes.
4. The “BeO Target” is moved to “beam position”. The electron beam current lowered
(to < 5 µA with raster-off) as not to damage the BeO target. The monitoring of
the beam position is performed by looking at the TV monitor which displays the
view of the target.
5. The multi-foil 12 C “Optics Target” is in “beam position”. Data on the optics target
are taken for spectrometer optics study or detector calibration. Since the BeO and
the optics target are in beam at the same time, the above two measurements are
often performed at the same time.
6. The target ladder “Empty Target” frame is moved to “beam position” whenever
beam tuning is performed, and when checking for possible beam halo background.
7. The “Reference Cell Target” is in “beam position” with the beam fast raster on
and data on the reference cell are taken for calibration.
10.3
Laser System
The laser system provides 795 nm circularly polarized light for optical pumping of the
target. The light is generated by seven solid-state diode lasers, which are located in a
shielding hut several meters away from the target, and polarized by passing through a
CHAPTER 10. POLARIZED 3 HE TARGET
149
polarizer (beam splitter) followed by standard λ/4 waveplates. Each laser emits roughly
30W of power. Because of the invisibility and high intensity of the laser beam, the lasers
present a significant safety hazard. The system generates two independent laser beams
for optical pumping of two different target spin directions: parallel and perpendicular to
the electron beam. In the standard configuration, four lasers are used for the parallel
and three for the perpendicular direction. Further, it is possible to reverse each of these
polarization directions by 180◦ (“flipping” the target spin). This requires the rotation of
the target holding field (see Section 10.8) and reversal of the pumping light helicity. The
light helicity can be inverted by inserting a λ/2 waveplate into the beam path of each
laser.
10.3.1
Laser Hut & Beam Path
To protect the lasers from radiation damage due to the electron beam and shield personnel
from accidental exposure to hazardous laser light, all laser systems are located in a
shielding house (”laser hut”). It is positioned in Hall A on the right-hand side of the
target at a distance of approximately 5 meters as shown in Fig. 10.1 and 10.2. This
placement limits the maximum angle for the hadron spectrometer to about 45◦ , which is
sufficient for the planned 3 He experiments.
Inside the laser hut, an optical table with an anodized aluminum top supports all
optics. The setup of the polarizing optics is shown in Fig. 10.3.
Seven infrared diode lasers (four for longitudinal pumping and three for transverse
pumping) and the related interlock control box are located on a 19” rack. Light is guided
out of each laser and goes to an optical table via an optical fiber. Each beam is focused
by a 2” diameter convex lens with a focal length of 88.3mm, then divided by a polarizing
beam splitter cube into two linearly polarized rays. Because the laser light is initially
unpolarized, both the direct and the split beam carry approximately half the power. To
utilize the full laser power it is necessary to combine both beams and focus them onto
the optical pumping cell. This is accomplished as follows:
The direct beam is reflected by a 3” diameter dielectric mirror which can be adjusted
to steer it towards the pumping cell. The split beam passes through a λ/4 waveplate, is
reflected by a 2” diameter dielectric mirror, and passes through the λ/4 waveplate again.
The fast and slow axes of this λ/4 waveplate should be oriented at an angle of 45◦ to
the horizontal or vertical direction. The linear polarization of this beam is thus rotated
by 90◦ , and is able to pass through the beamsplitter, essentially without reflection. The
second passing through the splitter is necessary to achieve a very high degree of linear
polarization for the split beam since the splitter only gives high polarization for direct
beam (TP > 95%,RS > 99.8%).
Now both beams from each laser have identical linear polarizations. Each passes
through a λ/4 waveplate that transforms its polarization from linear to circular. The
orientation of each λ/4 waveplate is shown in Fig. 10.3, note that all eight (or six for
transverse pumping) λ/4 waveplates should have the same orientation.
The eight (or six for transverse pumping) resulting beams are carefully aligned to
CHAPTER 10. POLARIZED 3 HE TARGET
45
de
g
150
HRS
Detectors
Laser hut
target pivot
light tubes
lasers
Optics
Rack for
laser
controllers
Target
Optics table
Beam Line
Figure 10.2: A topview of the laser hut shows the location of the interlocked door as
well as the position of the laser table where all lasers and optics will be set up.
CHAPTER 10. POLARIZED 3 HE TARGET
151
coincide on the pumping chamber of the cell. They enter the target chamber via transparent windows in the target enclosure and oven walls. There is one pair of windows for
each polarization direction.
The total path length from the lasers to the pumping cell is about 5 meters. The
beam path for transverse pumping is a straight line from the polarizing optics to the
pumping cell (via the transparent windows). However, for any other pumping direction,
e.g. longitudinal, the beam must be reflected twice such that it goes first down and then
horizontally in the desired direction. To this end, a pair of adjustable 4” diameter mirrors
is mounted on the outside of the target enclosure. For longitudinal pumping, these
mirrors can be standard dielectric ones since they will be in a polarization-preserving
compensating configuration (one mirror rotated by 90◦ with respect to the other).
Under normal conditions, laser light will be completely absorbed by the rubidium
in pumping cell. However, there is small amount of the laser light being reflected by
the pumping cell wall or deflected by the pumping cell itself. There is a 4” diameter
mirror on the left side (viewing towards downstream direction) to collect the light for
the EPR measurement. The light is focused by two lenses (mounted on the oven) first,
then guided by a pipe with mirror and additional focusing lenses in it, finally reaches the
EPR photodiode which is located a few meters away from the target. At the meantime
this light will also be viewed by a fiber and enters the spectral-analyzer, which is used to
monitor the spectrum and intensity of the lasers.
The laser beam line is protected by beam pipes that extend from the laser hut to the
target top cover. It is important to point out that the laser beam lines are fully enclosed
from the rest of the hall.
The entire laser hut is a laser controlled area that requires special safety precautions
(see Section 13 and Appendix A).
10.3.2
Diode Lasers & Controls
The laser light for this experiment is provided by seven 30 Watt, 795 nm solid-state diode
lasers. The seven primary lasers used during the experiment are made by Coherent Semiconductor Inc. There will also be two diode lasers made by the Opto-Power Corporation
that will be used as spares. Though there are some minor differences, both companies’
laser systems perform the same function. Only the Ceherent lasers and control will be
described here. If the Opto-Power lasers are used, readers are refered to the original
version of the polarized 3 He target OSP manual.
The diode lasers consist of a main enclosure which contains the diode, power supplies,
fans and control systems for the laser. The laser light from the diode is sent into a flexible
optical fiber which extends out of the main enclosure.The fiber is 5m long in a stainless
steel jacket and is removable.
The lasers operate at diode temperature from 15◦ C to 26.5◦ C depending on the
system. They are run at output power of 30 Watts which uses an operating current of
35-40 Amps. The output laser light has a central wavelength of 795 nm with a spectral
width of less than 2 nm. The fiber diamter is 800 microns. The beam divergence is
P
laser fiber
focusing lens
FL=8.83cm
Holding posts
TP > 95%, R S> 99.8%
S
S
P
Beam splitter: performance:
p
p
2" mirror
beam
splitter s
λ/2
λ/4
waveplate
3" mirror
λ/2
waveplate
Table Top Outline
laser
helicity
control
λ/4 waveplate
λ/4 waveplate (back)
p
p
circularly
polarized
To target
down
f
right
either
f
s
or
s
f
f
s
for left−handed laser
λ/4 waveplate (back)
λ/2 waveplate
left
s
up
for right−handed laser
λ/4 waveplate (left and right path)
waveplate orientation
(facing target)
CHAPTER 10. POLARIZED 3 HE TARGET
152
Figure 10.3: Top view of the optics setup inside the laser hut. All items are on a laser
table which is sitting on a mezzanine inside the laser hut
CHAPTER 10. POLARIZED 3 HE TARGET
153
< 0.20 N.A.
The lasers have a local control system, which is integrated into the main enclosure.
The local control allows adjustments in operating current (and therefore the laser power)
and diode temperature (and therefore the wavelength within a small range).
During the experiment the lasers will be controlled remotely through EPICS. The
lasers have a serial IO port on the back of the main enclosure which is connected to a
VME RS-232 port. There is an MEDM [9] GUI interface for each of the lasers, which
allows remote control and monitoring of current and temperature as well as being able
to turn each laser on and off.
10.3.3
Alignment
Basic procedure for laser alignment
1. Set the height of the diode laser, the center of the polarizing cube (beam splitter),
the two-inch mirror, the quarter wave plates, the lens, and the three-inch mirror to
be at the same height.
2. Viewing along the direction of the cube, the mirror and the wave plates, you should
be able to see the target. Here target refers to the 4” mirror on top of the oven (for
longitudinal laser beam path), or the side window on target (for transverse laser
beam path). You might need to adjust the position of each post to compromise
between transverse and longitudinal lasers.
3. Place diode alignment laser such that it passes through the center of the polarizing
cube and the center of the target.
4. Rotate the cube and change the angle such that the back reflection hits the alignment laser and such that the reflection hits the diode laser simultaneously.
5. Place the focusing lens in its mount with the flat side away from the diode laser.
Adjust it so that the back reflection goes back on itself.
6. Turn the diode laser on at LOW POWER.
7. Adjust the head of the diode laser such that the light goes through the center of
the lens.
8. Rotate the cube mount such that the light passes through the center. Be sure
that the cube is mounted so that the light enters the marked side and is reflected
towards the two-inch mirror.
9. Turn off the diode laser.
10. Place the two-inch mirror into its mount and move such that the alignment laser
hits the center.
11. Place a quarter wave plate between the cube and the two-inch mirror.
12. Turn the laser on at LOW POWER.
CHAPTER 10. POLARIZED 3 HE TARGET
154
13. Adjust the position of quarter wave plate so that the laser passes through the
center.
14. Adjust the two-inch mirror so that the reflected light hits the target.
15. If you know the axises of quarter wave plate, rotate it according to Fig. 10.3. If
not,
• Rotate the quarter wave plate so that the light which passes through the cube
from the two-inch mirror is a minimum.
• Rotate the quarter wave plate forty-five degrees.
16. Adjust the two-inch mirror so that the light hits the target.
17. Check to see where the back reflection of the diode is hitting. It should be near the
head of the diode laser but not directly on top of it. If it is then rotate the cube
slightly and realign the two-inch mirror. Repeat if necessary. It is important to
keep the back reflection away from the fiber since a small amount of back reflected
laser could reach the diode through fiber and will damage the laser.
18. Place the three-inch mirror in its mount.
19. Check to see that the diode light is hitting the mirror.
20. Adjust the three-inch mirror so that the light is hitting the target.
21. Center a quarter wave plate in the path of each beam.
22. If you know the axises of quarter wave plate, rotate it according to Fig. 10.3. If
not, you need to do bench test first to find the axises of each quarter wave plate.
23. Repeat for all beams heading towards the target.
24. Be sure that the helicity of each beam line is the same.
25. Place the half wave plates in mounts, adjust the position of the post and the
mounts such that all either (or six) laser beams are able to hit the center of the
corresponding half wave plates simutanuously.
26. If you know the axises of half wave plate, rotate each one according to Fig. 10.3.
If not, you need to do bench test first to find the axises of each half wave plate.
Testing quarter (half ) wave plates
• Find the direction of the axes:
1. Use the laser beam which passes through the polarizing cube (beam splitter).
2. Place the unknown wave plate in the beam.
3. Place a polarizing cube after the unknown wave plate.
4. Place a power meter perpendicular to the polarizing cube such that it measures
the power of reflected light.
CHAPTER 10. POLARIZED 3 HE TARGET
155
5. Rotate the unknown wave plate such that the reflected light coming from the
second cube is at a minimum.
6. Now one of the axes is in horizontal direction and the other is in vertical
direction.
• Make sure the axes of all quarter wave plates are identical (either fast or slow):
1. Use the laser beam which passes through the polarizing cube (beam splitter).
2. Measure the laser beam power P.
3. Place the 1st quarter wave plate in the beam, rotate it such that one of the
axes (marked axis) is in vertical direction.
4. Rotate the 1st quarter wave plate by 45◦ clockwise.
5. Place a 2nd quarter wave plate after the 1st one, rotate it such that one of
the axes (marked axis) is in vertical direction.
6. Rotate the 2nd quarter wave plate by 45◦ clockwise.
7. Place a polarizing cube after the 2nd quarter wave plate.
8. If the reflected light coming from the cube is at a minimum, then the marked
axes of these two quarter wave plates are opposite (one is fast and the other
is slow);
9. If the reflected light coming from the cube is at a maximum and equals roughtly
the full power P, then the marked axes of these two quarter wave plates are
identical (both are fast or both are slow);
10. If the reflected light coming from the cube is at a maximum and equals roughtly
the half power P/2, one of the two wave plates is a half wave plate.
Some technical details
• One needs to be careful about backscattering light since it may damage the laser
FAP (diode) if it is right on top of the fiber. However, if the backscattering light is
far away from the fiber, it indicates that the system is not optimized and will affect
the quality (intensity and polarization) of laser light to the target. The major
part of backscattering light comes from the P light reflected from the 2” mirror
and the beam-splitter. To check position of backscattering light, one should use a
backscattering test plate. Never use paper or any flammable material around fiber.
Always turn off the laser when putting on or taking off the test plate.
• Adjust position of back-scattering spot vertically: Probablly the reflecting surfact
of beam-splitter is not exactly perpendicular to the laser beam. Make sure the
fiber, focuing lens and cube(beam-splitter) are at the same level, then rotate cube,
at the same time adjust the 2” mirror if necessary.
CHAPTER 10. POLARIZED 3 HE TARGET
156
Table Top Outline
Holding posts
focusing lens
FL=8.83cm
cube holder
laser fiber
1
2
beam
splitter
2
2
1
1
λ/4
Back−scattering
test plate
2" mirror
Figure 10.4: Adjusting back-scattering light
• Adjust position of back-scattering spot horizontally:
1. Moving spot to the left (viewing towards fiber): Rotate cube holder clockwisely
(viewing from the top), then rotate cube (relative to its holder) anticlockwisely.
Keep the spot at the center of 2” mirror. See Fig. 10.4
2. Moving spot to the right (viewing towards fiber): Rotate cube holder anticlockwisely, then rotate cube anticlockwisely. Keep the spot at the center of
2” mirror.
10.3.4
Operation I: Local Mode
To turn a laser on:
1. Be sure to connect the laser control box to the interlock and to the correct laser.
2. Turn on the switch on the back panel of the laser.
3. Turn the key on the front panel to turn on the power.
4. Set the current through the keypad.
5. Press the On/Off button. The screen should show ’laser enabled’.
6. Press the On/Off button again. The screen should show ’laser on’. The laser should
be on now.
CHAPTER 10. POLARIZED 3 HE TARGET
157
To turn a laser off:
• Press the On/Off button. The screen should show ’laser disabled’.
Table 10.1 lists the optimum parameter determined for the 30 W lasers used in
experiments E99-117 and E97-103 in the summer of 2001.
Laser
Coherent
Coherent
Coherent
Coherent
Coherent
Coherent
Coherent
1
2
3
4
5
6
7
PS # FAP-I # Set Current Set Temperature
3115
V7645
38500 mA
15 ◦ C
4151
V7250
37000 mA
24 ◦ C
4157
V7251
40000 mA
26.3 ◦ C
4206
V8491
40000 mA
15.5 ◦ C
4209
V8511
36000 mA
19.7 ◦ C
4208
V8492
40000 mA
14.7 ◦ C
Maximum Current ?
45000 mA
44600 mA
48000 mA
48000 mA
43200 mA
48000 mA
? Factory set absolute maximum current
Table 10.1: Operational parameters for the 30 W lasers.
10.3.5
Operation II: Remote Mode
For the remote mode to work, the lasers must be connected to the interlock box, the
interlock system must be engaged and the laser key turned on as described above. The
lasers can then be controlled through the MEDM GUIs:
To turn a laser on:
1. Set the desired temperature and current in the “Set Temp” and “Set Current” enter
boxes.
2. Press the “Enable” button. If you have a camera pointing at the laser box in the
Hall then you should see ’laser enabled’.
3. Press the “Start” button. If you have a camera pointing at the laser box in the
Hall then you should see ’laser on’. The laser should now be on.
To turn a laser off:
• Press the “Stop” button. If you have a camera pointing at the laser box in the Hall
then you should see ’laser disabled’.
In addition to cameras pointing at the lasers boxes in the Laser Hut to monitor
lasers on or off, there is a spectral-analyzer which monitors both the relative power and
the waveform of the lasers. The spectral-analyzer has remote readback. Alarm will be
set when the readback deviate from the regular running condition by more than 10%.
This will enable us to know when there is one laser goes off or when the cell ruptured.
CHAPTER 10. POLARIZED 3 HE TARGET
10.3.6
158
Half-Wave Plate Motion Control
The target polarization can be reversed by insertion or removal of a set of half-wave
plates. Before moving the half-wave plates, all lasers must be turned off first.
The half-wave plate positions are controlled through the MEDM GUI running on
one of the computers in the Hall A Counting House. The half-wave plates have only two
positions: IN (in the laser beam path) and OUT (pulled out of the laser path). The
speed, acceleration, deceleration and number of steps can be defined for the half-wave
plate motor, but the defaults should be fine except in special circumstances. The GUI
allows insertion and removal of the half-wave plates:
To move the half-wave plates IN, click the “Go” button under “Move Half-Wave
Plates IN”.
To move the half-wave plates OUT, click the “Go” button under “Move Half-Wave
Plates OUT”.
After the motion is finished, you can get the position of the motor by clicking on
the “Get Position” button. This button shouldn’t be pressed while the motor is moving
because the motor likes to perform one operation at a time.
The motor that drives the half-wave plates isn’t 100% reliable, so the plates should
be checked on the camera monitor to make sure they are in the correct position after a
move. If you see a problem with the motion of the motor (i.e. it’s driving too far forward
or about to drive off the table) then press the Emergency Stop “HALT” button.
10.4
Target Cell
10.4.1
Description
The target cell is either a 25 cm or 40 cm long aluminosilicate glass (GE180) high pressure
(up to 15 atm) double chamber cell. Typical dimensions for a 40 cm cell are shown in
Fig. 10.5. A 25 cm cell is identical except the target chamber is 25 cm long instead of 40
cm. It is a closed system filled with a gas mixture which consists of 3 He, rubidium and
nitrogen.
The cell volumes range from 160 ml for a 25 cm cell to 200 ml for a 40 cm cell. The
interior pressure of the cell at room temperature is between 8 and 10 times atmospheric
pressure. The cells contain approximately 70 torr of nitrogen to help with the spinexchange process. There is usually 0.1-0.3 g of Rb in the pumping chamber. The tritium
contamination of the 3 He gas used to fill the cell is less than 10−11 % according to the
specifications from the manufacturer, Spectra Gases.
The glass walls of the cell vary in thickness from cell to cell and from chamber to
chamber. The end windows of the target chamber are 120-150 microns thick and are
therefore the thinnest part of the cell. The walls of the target chamber and transfer tube
are over a millimeter thick and the pumping chamber walls are up to 2 mm thick.
The cell is installed on a target ladder and then this ladder is mounted to the bottom
of the oven. The cell is attached to the target ladder with a high-temperature elastomer,
CHAPTER 10. POLARIZED 3 HE TARGET
159
64 mm
12 mm
62 mm
19 mm
0.7 mm
400 mm
Figure 10.5: Typical 40 cm polarized 3 He target cell used in this experiment.
GE RTV106. The ladder is then bolted onto the oven.
10.4.2
Installation & Replacement
The target installation is a delicate procedure and should be performed only by qualified
personnel. A minimum requirement is Radiation Worker II training. Only the first time
installation does not require the monitoring of radiation around the pivot area. After
the cell has been exposed to the electron beam, any replacement of the cell must be done
under strict observation of possible radiation and contamination hazards.
The cells are fragile and under high pressure so they can cause damage not only
to exposed skin and eyes, but also hearing. It is therefore important to wear hearing
protection in the vicinity of an exposed cell and also to wear a face shield and a safety
glasses when handling or in the vicinity of someone handling a cell.
In the following we describe the procedure for cell replacement. The procedure
applies both to a routine replacement and a replacement after a cell explosion.
• If the replacement of the cell follows the explosion of a previous target, a new Q
curve of the pickup coils should be measured. If the pickup coils have deteriorated,
they should be replaced as well.
• Prepare the replacement target and, if necessary, a new set of pickup coils. Request Hall access from MCC, and wait for a member from the RADCON team to
accompany the group of qualified people authorized to change a target cell.
CHAPTER 10. POLARIZED 3 HE TARGET
160
• The member of the RADCON group will evaluate the radiation level around the target area. If safe levels of radiations are observed, the work can proceed. Otherwise
a Radiation Work Permit will have to be written. The member of the RADCON
group will either clean up the glass pieces or, after determining that there is no
contamination, informs us that we can proceed to clean up the glass pieces.
• All personnel within the target area platform must wear ear protection, a face shield
and a pair of safety glasses during any access to the inside of the target enclosure.
• Raise the target top assembly support and if necessary remove the scattering windows of the RF enclosure to provide sufficient access to the target. Monitor the
radiation level close to the target ladder and oven. Determine what conditions are
required to comply with a radiation safe work condition.
• Qualified personnel should proceed to replace the target and align it according to
well defined reference lines of the target ladder. The motion of the target and the
clearance between the pickup coils should be tested. When finished, the scattering
windows of the RF enclosure should be put back in place.
• If an intact target cell was removed, it should be stored in a wooden box with
a screwed lid or a metal box specially designed for store target cell. It must be
left in the hall with “Pressurized Glass Cell, Open By Authorized Personnel Only”
warning sign on the box. It can only be removed from the Hall after approval from
RADCON.
10.5
Target Ladder & Motion System
10.5.1
Description
The polarized 3 He target system has four target positions: the actual polarized target
cell (which can be replaced by a water cell for NMR calibration (see Section 10.9), a solid
BeO target inline with a seven-foil Carbon target for alignment and optics calibration, an
empty target position that contains no target and allows the beam to pass undisturbed
through the apparatus and a reference cell position (see Section 10.11). These targets
are mounted on a target ladder as shown in Fig. 10.6.
The target ladder can be positioned vertically using a stepper-motor-driven motion
control system. The controller can drive the ladder to five different fixed positions,
three of which correspond to the three targets, the fourth to the empty (no target)
configuration, and the fifth to the position used for NMR measurements where the target
cell is surrounded by the pick-up coils. The total required vertical motion range is
approximately 15 cm.
The motion controller employed was developed at JLab and will be first used in experiment 99-117 (Precision Measurement of the Neutron Asymmetry An1 at Large x using
CHAPTER 10. POLARIZED 3 HE TARGET
Target Support Frame
161
Target Oven
Pol
3
He Cell
BEAM
BeO + 7 C Foils
Empty Target Position
Reference Cell
Pick-up
Coils
Ref. Cell piping
Figure 10.6: Schematic diagram of the target ladder and target positions.
CEBAF at 6 GeV). Vertical target motion is provided by a screw-driven cylinder (Industrial Devices Corporation model N2P22V1205A12MF1MT1L). The cylinder incorporates
a 200 step/rev stepper motor with a 1:12 gear, resulting in a position repeatability of
about ±13 µm at a maximum load of 600 lbs (272 kg). The maximum range of motion is
31.5 cm. Mechanical position sensors are mounted on the target support frame to indicate motion limits. These switches are wired normally closed and fail open. A magnetic
position sensor (Reed Sensor, PSR-1Q) mounted on the motor cylinder serves as the
home switch. This switch is wired normally open, using only two of its three wires (12 V
is not connected for this type of switch). Additionally, a linear potentiometer provides
independent readback of the cylinder position.
10.5.2
Operation
The motor can be controlled locally and remotely. Local control is provided through the
SmartStep 23 keypad controller. The keypad allows the target to be jogged into position.
It also allows for small codes to be saved which will move the target ladder into preset
target positions. Remote control is provided through the EPICS environment. During
norml operation, target motion is controlled through the lifter MEDM GUI running on
one of the computers in the Hall A counting house. The final GUI is under developemnt
at this time, but it will have buttons for preset target positions, as well as lights indicating
CHAPTER 10. POLARIZED 3 HE TARGET
162
the status of the controller (i.e. any faults, at a limit, etc.). The velocity, acceleration
time, deceleration time and position can also be manually set from the GUI, but the
defaults should be correct for all but special situations. The position of the motor is
readback on the GUI. This position is displayed in revolutions from the home position
and can be compared with preset target positions.
The electronics provide a Fast Shutdown (FSD) interlock signal to the accelerator
that is triggered whenever the target is moved with the beam on. This prevents damage
to the target cell and ladder due to the electron beam. The FSD signal is generated by
the SmartStep controller and sent to an FSD node in the Hall. Target changes require
Hall A personnel to telephone MCC and request temporary masking of this FSD channel.
10.6
Target Enclosure & Windows
The target is surrounded by an enclosure similar to a scattering chamber. The enclosure
serves three purposes: First, it provides a containment volume in case of explosion of
the target cell and so limits the potential hazard working area. Second, it holds the RF
coils in a stable position and provides a strong frame for the support assembly. Third,
it allows the region around the target to be filled with helium gas, minimizing ionization
and energy loss of the electron beam crossing the target area.
The enclosure is made of high density polyvinyl (HDPV) material and has several
removable windows. The windows are of three types: The incident electron beam entrance and exit windows are 0.127 mm thick aluminum. The windows on the side of
the chamber through which the scattered particles pass are 2.54 mm thick aluminum.
Five-mm thick optical-perfect windows allow the laser beam to enter the enclosure.
If any windows are open, people working anywhere on the target platform must wear
ear protection and a face shield. This region is designated as the “target area”. Signs
will be posted indicating that the windows are open and that work is in progress. If the
windows are closed the target area is no longer restricted except for possible reasons of
radiation safety.
The electron beam crosses two windows before reaching the target, the electron
beam pipe window and the target enclosure beam inlet window. Similarly, it crosses two
similar windows as it leaves the target area. The electron beam line is under vacuum
and separate from the target enclosure for both upstream and down-stream of the target
enclosure. The gap between the upstream beam line and the enclosure inlet window is
very narrow. Both upstream and downstream beam lines are terminated with a 0.254mm
thick beryllium window. The beam inlet and outlet windows of the target enclosure are
made of 0.127mm thick aluminum and are strong enough to stop any flying glass shreds
in case of a target cell explosion.
CHAPTER 10. POLARIZED 3 HE TARGET
10.7
163
Target Cell Heater
The target pumping cell must be kept at a temperature of about 170◦ C (340◦ F) for
optical pumping of Rb to be effective. This is accomplished by flowing hot air through a
special temperature-resistant enclosure (“oven”) around the pumping cell and regulating
the temperature of the air using a process controller. The system is described in detail
in the following.
10.7.1
Description
Fig. 10.7 shows a schematic diagram of the heater system and the associated controls.
Pressurized dry and filtered air at room temperature is provided by a dedicated compressor in the Hall. The air enters the system through a shut-off valve and a pressure
regulator, which is typically set to an output pressure of 15 psi. The flow rate is measured
with a gas velocity sensor (Omega model FMA-905) connected to a display unit (Omega
DP41-E-S2R) that provides an alarm to indicate insufficient flow. The air then passes
through a resistive heater element (120 VAC, 700 W) and continues through insulated
copper tubing into the target oven. The oven material is Torlon and Vespel plastic which
can withstand a temperature of at least 260◦ C continuously. The air finally exits the
system through a several meters long exhaust pipe where it can cool down.
A 100Ω Pt RTD (Omega model F3105) measures the temperature inside the oven.
A process controller (Omega CN77540-C2) operating in PID mode drives the heater
via a solid-state relay (SSR) regulating the heater power dependent on the temperature
detected by the RTD. The SSR (Omega model SSR240DC10) accepts a low-voltage (3
− 32 VDC) control signal of about 30 mA. A mechanical relay between the SSR and
the heater allows interruption of the 120 VAC heater power in case of a malfunction (see
subsection 10.7.2).
A thermocouple (Omega 5SC-GG-K-30-36) is mounted on the tubing right after the
heater to allow monitoring of the temperature of the air exiting the heater. Another
display unit (Omega DP41-TC-S2R) reads the thermocouple and generates an alarm if
the temperature exceeds a preset threshold.
The PID controller as well as the two display units are installed in a 19” chassis in the
electronics racks on the second floor of the counting house where they can be manually
operated if necessary. All other components are located in the Hall in the vicinity of the
target. The instruments can be monitored and programmed remotely via serial RS-232
communications, which allows convenient control via an EPICS/MEDM graphical user
interface (GUI) in the counting house.
10.7.2
Safety Considerations
The heater system represents a significant fire hazard and therefore requires good failsafe
protection. Possible failure modes include:
CHAPTER 10. POLARIZED 3 HE TARGET
164
Heater
Thermocouple
Flow
sensor
SSR &
Interlock
Hot air
exhaust
Pressure
regulator
Target Oven
RTD
compressed
air input
Flow Readout
PID
Controller
Temperature
Readout
Figure 10.7: Schematic diagram of the target cell oven control system. The instruments
in the dashed box are located upstairs, while the remaining components are located in
the Hall in the vicinity of the target. For clarity, the interlock circuitry is not shown (see
Fig. 10.8).
CHAPTER 10. POLARIZED 3 HE TARGET
120 VAC
PID Controller
SSR
DC out
165
SSR
Heater
Heater
Interlock
Relay
Overtemp
Alarm
12 VDC
Laser
interlock
Air Flow Readout
Temperature Readout
Alarm 1
Alarm 1
Alarm 2
Alarm 2
Figure 10.8: Schematic diagram of the oven heater interlock system. The switches represent relays. In normal operation, the relays are energized (contacts closed). An alarm
condition (or instrument power failure) will de-energize (open) the corresponding relays,
interrupting the interlock circuit. The instruments are programmed such that their dual
alarm outputs operate simultaneously.
CHAPTER 10. POLARIZED 3 HE TARGET
166
1. Insufficient air flow. Possible causes: Compressor failure; obstruction in filter;
operator error. Possible harzards: Overheating of heater element, resulting in
equipment damage and/or fire. Protection: Air flow is monitored; insufficient flow
disables heater (and laser) via hardware interlock.
2. Heater overtemperature. Possible causes: Insufficient airflow; temperature controller failure; RTD failure; operator error. Possible hazards: Damage to heater
element and/or tubing; fire. Protection: Heater temperature is monitored; excessive temperature disables heater (and laser) via hardware interlock.
3. Oven overtemperature. Possible causes: Temperature controller failure; RTD failure; insufficient air flow; excessive laser power; operator error. Possible hazards:
Explosion of target cell; damage to oven enclosure and/or optical elements; fire.
Protection: Temperature controller will generate alarm if RTD indicates excessive
temperature, disabling heater via internal logic and and laser via hardware interlock.
A schematic diagram of the interlock system is shown in Fig. 10.8. The instruments
are programmed such that their dual alarm relays operate simultaneously if a value is out
of range. An interlock condition disables the heater as well as puts the lasers in standby
mode. Interlocking the lasers is important as the laser light contributes significantly to
the heating of the target oven.
10.7.3
Operation
The oven can be controlled from a GUI running on one of the Hall A counting house
computers, or manually using the front panel controls.
10.7.3.1
Local Operation
A brief description of the manual operating procedure is given here:
1. The oven controller and temperature and flow displays are located in a 19” chassis
in one of the racks upstairs. Make sure power to this chassis is on. The green light
on the front panel should be lit.
2. Verify the alarm set points for the flow meter and temperature indicator. Press
the SETPTS button until the display shows SP3. After about 1 second, the setpoint
value appears. It should be 250 for the air flow display and 220 for the temperature
display. Press SETPTS again and check setpoint SP4. It should be identical to SP3.
To change any of the values use the MIN and MAX buttons. The MIN button selects
the digit to be changed whereas the MAX button changes the value of the currently
selected digit. Press SETPTS again to store the new value. When finished, press
SETPTS until RUN appears in the display.
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167
3. Verify the current temperature of the oven. It is shown in the upper (red) display
of the temperature controller and is labeled PV for “Process Value”. The units are
◦
C. The value should be reasonable, e.g. around room temperature if the oven
has been off for several hours or more. If the value does not make sense, either
the controller is misconfigured (see below) or the RTD in the oven is broken or
incorrectly connected. Do not proceed before you have a sensible reading.
4. (Optional) Verify the correct configuration of the controller. Use the MENU button
on the controller to scroll through the various configuration menus. To inspect
parameters within a menu, press ENTER followed by MENU again. The suggested
default parameters are listed in Tables 10.2 and 10.3. This step is time-consuming
and can be skipped if you are relatively certain that the configuration is ok. A
detailed description of the controller parameters is given in the controller manual.
5. Verify that air is flowing. The air flow readout should indicate a value of 350–450.
These are arbitrary units. As long as the heater is off, the reading should not
fluctuate by more than about ±10 units.
6. Verify the current temperature of the heater. It should be close to room temperature. If the value is unreasonable, either the readout is misconfigured or the
thermocouple is broken. Do not proceed before the problem is corrected.
7. Verify that alarms are reset. Underneath the main display there are four LEDs,
one for each alarm 1–4. If either LED 3 or 4 is on on either instrument, it indicates
that an alarm has been triggered and that the system is interlocked. You must
reset the alarms before you can continue. To do so, first correct the problem (e.g.
turn the air flow on) then press RESET once on the affected instrument(s). If the
LEDs stay on despite correct setpoints and readings, the instrument is probably
misconfigured.
8. Begin heating. To avoid damage to the oven, the temperature must be increased
to the final value slowly. A good final operating temperature is 170◦ C, and a good
ramping rate is 60◦ C/h, i.e. heating of the oven will take about three hours to
complete.
In manual mode, you must enter a new temperature setpoint by hand at fixed time
intervals. (“Ramp and Soak” does not seem to work reliably with this controller.)
You should increase the value by 10◦ C every 10 minutes. For example, if the current
oven temperature is 35◦ C, start with a setpoint of 45◦ C and increase this value by
10◦ C in approximately 10 minute intervals.
To enter a new setpoint, do the following
(a) Press MENU on the temperature controller. A little green light marked SP1 in
the upper left corner of the display will start to blink. Also, the first digit of
the green numerical display, labeled SV for “Setpoint Value”, will blink.
CHAPTER 10. POLARIZED 3 HE TARGET
168
(b) Use MIN to select the digit you wish to change and MAX to modify the value.
(c) When done, press ENTER. The display will briefly show run when the controller
enters normal operating mode. This starts the heating process.
9. Check correct operation of the controller. A little green light marked SP1 in the
upper left corner of the temperature controller display indicates that the heater is
active. This light should blink slowly, being mostly on while the oven is heating
up and being mostly off (or even completely off for periods of up to a few minutes)
when the oven temperature has reached the setpoint.
The heater temperature should increase proportionally to the fraction of time that
the SP1 indicator is on. Note that the temperature reading is not directly related
to the oven temperature. In particular, the heater may become significantly hotter
than the oven, and its temperature might fluctuate from almost room temperature
to high values over short periods of time as the heater power is automatically
cycled on and off by the controller. As long as the temperature stays below the
alarm threshold (220◦ C) there is no reason for concern.
10. Check stability of the final temperature. The temperature might overshoot slightly.
If the overshoot is less than about 5◦ C then this is normal. If the stability is poor it
is probably due to incorrectly set PID parameters in the controller. Changing these
parameters is best done by an expert since this requires in-depth understanding of
the system.
The laser contributes significantly to the heating of the oven. Therefore, you will
notice sudden temperature instabilities when the laser is turned off or on. It will
take several minutes for the controller to compensate for such changes.
11. The air flow rate is slightly dependent on the heater power applied (conductance
varies with temperature). Therefore, the flow rate will fluctuate by some 10-20%.
This is normal.
12. At any time you can place the controller in standby mode by pressing ENTER twice.
The display will show a blinking text STBY. This will turn the heater off completely
and can be used when the system appears to malfunction. However, exercise some
caution if the oven is at an elevated temperature since it will quickly cool down if
heater power is disabled and you will lose time bringing it back up to operating
temperature.
13. In an emergency, simply turn the power to the chassis off completely. This will
open the interlock loops, thereby cutting power to the heater and placing the laser
in standby mode.
CHAPTER 10. POLARIZED 3 HE TARGET
Menu
Output Redirection
Input Type
RDG Configuration
Submenu
RTD Type
RTD Value
Decimal Point
Temperature Units
Filter Constant
Alarm 1
Type
Latched
Contact
Setup
Power On
Low Value
Hi Value
Alarm 2
Loop Break
Output 1
Output 2
Self
% Low
% High
Control Type
Action Type
Auto PID
Adaptive Control
Anti Integral
Start PID
Proportional Band
Reset Setup
Rate Setup
Cycle Time
Damping Factor
(any)
169
Setting
S1.o1
RTD
385.3
100
FFF.F
◦
C
0004
Enabled
Absolute
Latched
n.c.
Above
Enabled
(anything)
210.0
Not Installed
Disabled
Disabled
0000
0095
PID
Reverse
Disabled
Disabled
Enabled
Disabled
0038
0050
0000
0001
0001
(anything)
Table 10.2: Suggested default parameters for the temperature controller.
CHAPTER 10. POLARIZED 3 HE TARGET
Menu
Ramp & Soak
Submenu
Ramp
Soak
Analog Output
Communication Option Baud
Parity
Data Bits
Stop Bits
Bus Format
Checksum
Line Feed
Echo
Standard
Mode
Separator
Data Format
Status
Reading
Peak
Valley
Unit
ID
Address Setup
(any)
Transmit Time
(any)
Remote Setpoint
170
Setting
Disabled
Disabled
Not installed
9600
Odd
7bit
1bit
no
no
no
232C
Command
Space
yes
yes
no
no
yes
no
(anything)
(anything)
Not installed
Table 10.3: Suggested default parameters for the temperature controller (continued).
CHAPTER 10. POLARIZED 3 HE TARGET
10.7.3.2
171
Remote Operation
1. Make sure the power is on to the Oven Heater Controller Chassis as described above.
Also verify the alarm set points for the oven air flow and heater temperature as
above.
2. Verify the current temperature of the oven. It is shown on the GUI on the blue
HacOMEGA RTD readback. It’s shown on the meter and also in the readback
box. Also, a plot of oven temperature vs time is shown on the stripchart in the
bottom-left of the GUI (labelled oven temperature). The units are ◦ C. The value
should be reasonable, e.g. around room temperature if the oven has been off for
several hours or more. If the value does not make sense, either the controller is
misconfigured (see directions for manual control above) or the RTD in the oven is
broken or incorrectly connected. Do not proceed before you have a sensible reading.
3. Verify that air is flowing. The air flow readout should indicate a value of 350–450.
These are arbitrary units. As long as the heater is off, the reading should not
fluctuate by more than about ±10 units.
4. Verify the current temperature of the heater. It should be close to room temperature. If the value is unreasonable, either the readout is misconfigured or the
thermocouple is broken. Do not proceed before the problem is corrected.
5. Verify that alarms are reset. The alarms are reset by pressing the “Reset” button
in the upper-left of the GUI.
6. Begin heating. To avoid damage to the oven, the temperature must be increased
to the final value slowly. A good final operating temperature is 170◦ C, and a good
ramping rate is 60◦ C/h, i.e. heating of the oven will take about three hours to
complete.
In controlling the Oven Temperature from the GUI, you must enter a new temperature setpoint by hand at fixed time intervals. Enter the desired setpoint in the
“SP1” enter box in the lower right of the GUI. You should increase the value by
10◦ C every 10 minutes. For example, if the current oven temperature is 35◦ C, start
with a setpoint of 45◦ C and increase this value by 10◦ C in approximately 10 minute
intervals. The heater is controlled in PID (proportional, integral, derivative) mode.
It approaches the setpoint according to the PID parameters defined in the “Prop.
Band”, “Reset” and “Rate” boxes in the lower-right of the GUI. These values can
be changed, but the defaults should be fine except for special circumstances.
7. Check stability of the final temperature. The temperature might overshoot slightly.
If the overshoot is less than about 5◦ C then this is normal. If the stability is poor
it is probably due to incorrectly set PID parameters on the GUI. Changing these
parameters is best done by an expert since this requires in-depth understanding of
the system.
CHAPTER 10. POLARIZED 3 HE TARGET
172
The laser contributes significantly to the heating of the oven. Therefore, you will
notice sudden temperature instabilities when the laser is turned off or on. It will
take several minutes for the controller to compensate for such changes.
8. The air flow rate is slightly dependent on the heater power applied (conductance
varies with temperature). Therefore, the flow rate will fluctuate by some 10-20%.
This is normal.
9. At any time you can place the controller in standby mode by pressing the “Standby”
button in the upper-left of the GUI. This will turn the heater off completely and
can be used when the system appears to malfunction. However, exercise some
caution if the oven is at an elevated temperature since it will quickly cool down if
heater power is disabled and you will lose time bringing it back up to operating
temperature.
10. In an emergency, simply turn the power to the chassis off completely. This will
open the interlock loops, thereby cutting power to the heater and placing the laser
in standby mode.
10.8
Helmholtz Coils
The Helmholtz coils are four large interlocking rings of coils that provide a large constant
magnetic field. Two sets are necessary so that the magnetic field can point in any direction
in the horizontal plane. The larger two coils have an internal diameter of 1.45 m and
consist of 272 turns of coil. The smaller set of coils each have a internal diameter of 1.27
m and are made of 256 turns of coil.
The normal holding field for the target is 25 Gauss which corresponds to approximately 7.2 amps of current in the coils. However, when doing an NMR measurement the
field gets as high as 32 Gauss, which corresponds to 9.2 amps of current.
The coils are controlled by a DC voltage from two Stanford Research System DS345
Function Generators. These are in turn controlled by a LabView Vi running on a PC.
There is a small glitch in the DS345 function generator : there sometimes will be a short
drop in voltage before going to the correct voltage. This is a problem when rotating the
field, since it causes sudden magnitude shifts which in turn destroy a little of the target
polarization. This glitch is compensated for by a specially designed rotation box to which
a Wavetek 80 function generator is attached. This function generator compensates for
the small decrease in voltage so that the rotation can be performed smoothly.
Attached to the small set of Helmholtz coils are a smaller set of coils. These consist
of 20 loops of 16 AWG wire attached to a 150 W power supply. This target occasionally
has a problem with a phenomena known as masing. This is a loss of target polarization
due to coupling of the helium nuclei in their higher energy states (such as during an
NMR measurement) and a coil of wire (the pick-up coils, a Rb ring, or some other
source). Masing can be easily identified with EPR, and introducing a small field gradient
generally makes it disappear. These coils are used to introduce that field gradient.
CHAPTER 10. POLARIZED 3 HE TARGET
10.9
173
NMR Polarimetry
This guide explains briefly how to perform safely an NMR AFP sweep on the polarized
3
He target. Please, refer to Fig. 10.9 showing the target setup and explaining the angle
conventions. Longitudinal or parallel field configuration is named if the holding field is
pointing towards 0◦ , i.e. along the incident electron momentum direction.
10.9.1
NMR polarization measurement
• Turn off the lasers (3 or 4), rotate the magnetic holding field to the parallel configuration (if the field is in transverse mode).
• Make sure the target is in the Helium 3 (beam) position.
• Make sure you are in the parallel field configuration.
• Run the NMR Record Settings panel. Make sure you get the print out.
• Run the Current Monitoring panel. Print it at the end.
• Call MCC and tell them you are going to move the target down to the pick-up coils
to measure its polarization. When the target is masked, move the target down to
the pick-up coils.
• Print the RTD screen from the HAC X terminnal.
• Run the NMR Measurement panel and print it when completed.
• Move the target back to its beam position.
• Call MCC, tell them you have measured the polarization and that the target is back
to its beam position. Be sure to follow the beam back procedure before sending the
beam back to the target (beam position, raster on with a radius setting r = 2 mm,
current less than 15 µA)
• Run the NMR Extract Polarization panel and print it.
• Staple the plots together and store them in the target binder.
10.9.2
Plots to print
For every NMR measurement, print :
• NMR Record settings panel (will be printed by the Vi)
• NMR Measurement panel (print it at the end of the sweep)
CHAPTER 10. POLARIZED 3 HE TARGET
Coil Orientation
(Top View)
174
270 deg
Helmholtz Coil Large
RF Coil
Main Holding Field
Electron Beam
180 deg
Laser Beam
(longitudinal pumping)
Mag03
(Z)
0 deg
Helmholtz Coil
Small
Laser Beam
(Transverse
Pumping)
(X)
90 deg
Figure 10.9: Top view of the longitudinal and transverse Helmholtz coils used in the
polarized 3 He target setup. The RF coils are needed to perform NMR as well as EPR
measurements. The 0◦ angle gives the incident electron beam direction. The dashed lines
indicate the coils main axis, the long bold arrow indicates the holding field direction which
can be rotated from 0◦ to 360◦ . We also show the laser beam longitudinal and transverse
directions.
CHAPTER 10. POLARIZED 3 HE TARGET
175
• NMR Extract Polarization panel (print it when completed)
• RTD temperatures screen (print it from the X terminal)
Staple the four plots together and store them in the target binder.
10.9.3
Warnings
• Never stop a running LabVIEW Vi
• Never put CW beam on the target without raster
• Every target operator must read and sign the Target Operation and Safety Procedure
• If the target ruptures, turn OFF the lasers immediately
• The lasers must be off before rotating the holding field
• We indicate on Table 10.4 the voltage and current values for each KEPCO.
10.9.4
NMR AFP Safety
The NMR AFP system provides the DC current in the holding coils (up to 10 A) and the
AC current in the RF coils (usually 1 A rms) of the target setup. Any human contact
with the wires could be fatal and should therefore be avoided. All the electronic devices
and the PC used to sweep are located in the Counting House A. Refer to Fig. 10.9.3 for
a description of the electronics.
10.9.4.1
The DC current
It is produced by the two Kepco power supplies. One drives the large coils, and the other
the small ones, as stamped on their front panel. The output of each Kepco consists of
four wires, two to lead the current, and the two others used for sensing. The four output
plugs are located on the back panel of each power supply which can only be reached
through the back panel of the rack. The connections are protected by a plastic strip
and can not be touched by hands. In the hall, simple copper screws are used to connect
those wires to the coils. They should never be touched. The grounding of the system is
achieved in the hall.
How to turn the Kepco Power Supplies off in an emergency ?
1. Turn the Function Generators SRS DS345 off (one for each Kepco) by pressing the
on/off switch.
2. Turn the Kepco power supplies off by setting the manual on/off switch to the off
position. The digital displays should turn off.
CHAPTER 10. POLARIZED 3 HE TARGET
176
Laser
#1
Laser
#2
Oven Control
reference cells
DS345
SCOPE
SR620 Countr
PC
RF Capacitor
EPR PI FB Box
Field FB Box
PC
UPS
Rotation Box
DS345
DS345
2100L RF
Amplifier
RF Generator
Field FB PI Box
Documentation
Wavtk80
Wavtk80
EPR Lock-In
NMR Lock-In
RF
Current Monitr
Monitr
Figure 10.10: The electronics is located in the Counting House A.
CHAPTER 10. POLARIZED 3 HE TARGET
177
Cabling of the NMR System
In the Counting House
Monitor
PC
Trigger Input
Sync TTL
Printer
Trigger
Input
HP33120A
RF FG
Q−Coil
SRS844 Lock−In
Amplifier
Trigger
Output
Wavetek 80
FG
DS345 FG
DS345 FG
ATT
ENI Power
Amplifier
Capacitor Box
Rotation Box
Oscilloscope
Field Feedback
P&I Box
RF
Current Monitor
Field Rotation
Chassis
Electronics Rack in Hall
PC
Monitor
Fluke 45
Multimeter
Kepco Power
Supply for Small Coils
Fluke 45
Multimeter
Kepco Power
Supply for Large Coils
Ethernet
Target System
Large Coils
Small Coils
Coaxial BNC
Large Coils
RF Coils
10 AWG Wire
GPIB Cable
Small Coils
Shielded Wire
Q−coil
Pick−Up
Coils
Pre−Amp
Ground
RF Coils
Figure 10.11: Description of the cabling of the NMR system. 10 cables go from the
counting house to the target location in the hall : 8 (main+sensing) ESSEX AWG 10 for
the Helmholtz coils, 1 RG58A/U for the RF coils and 1 RG58A/U for the pick-up coils.
CHAPTER 10. POLARIZED 3 HE TARGET
10.9.4.2
178
The AC current
It is produced by the EIN RF Power Amplifier located at the bottom of the same rack.
When it is running, the manual switch is set to the ON position, the red LED is steady
and the needle on the display is above zero. The AC current comes out from the BNC
OUTPUT plug, goes through a current monitor (looks like a small toroid) and finally
goes to the RF coils. In the hall, the cable is connected to the RF coils along the RF
mounting using simple connectors with screws. Those are protected by electrical black
tape. They should never be touched when the RF Power Amplifier is running. The
grounding of the system is provided by the RF Power Amplifier.
How to turn the RF Power Amplifier off in an emergency ?
1. Turn the Function Generator HP 33120A connected to the RF Power Amplifier off.
2. Turn the RF Power Amplifier off by setting the manual ON/OFF switch to the
OFF position. The red LED should turn off and the needle should go back to the
zero position.
Orientation
0 deg
90 deg
Table 10.4: KEPCO settings
KEPCO
KEPCO
KEPCO
3 Voltage 3 Current 4 Voltage
38.0 V
7.1 A
0.0 V
0.0 V
0.0 A
38.2 V
KEPCO
4 Current
0.0 A
7.2 A
Detailed description of the NMR system is contained in the following technical note
by S. Incerti: The NMR system of the Hall A polarized Helium 3 target, JLab-TN-98-049.
A copy of this note is included in the target binder in the Hall A counting house.
10.10
EPR Polarimetry
In what follows we describe two methods to setup and perform an EPR measurement of
the target polarization.
10.10.1
EPR Lineshape Measurement – Frequency Modulation
Sweep:
Use this configuration when you wish only to find the Electron Paramagnetic Resonance
(EPR) frequency, not actually track its position. This method allows the user to observe
the lineshape of the EPR transition (Rb D2 line) light emitted from the cell as a function
of applied frequency to the EPR coil.
First, construct the circuit described in Fig. 10.10.1 by following these steps:
CHAPTER 10. POLARIZED 3 HE TARGET
Frequency
Modulation
Setup
179
Photodiode
D2 filter
PC
GPIB
EPR RF Coil
sync
Modulation
Source
DS 345
out
Counter
Lock−In out
A
SR 620
ref in Amplifier
in
EPR
PI Feedback
(integration off)
out
out
A
sync
Wavetek 80
VCO
vco
in
Figure 10.12: Circuit for EPR lineshape measurement
1. Position the PIN diode so that it looks directly at the light beam coming from the
cell. If pipe is used for guiding the light from the cell to where the PIN diode is
located, you may need to adjust the focusing lenses within the pipe such that most
of the light is collected and reaches the PIN diode.
2. Measure PIN diode output, it should be a DC signal with an amplitude of -100 mV
to -150 mV. If the signal is less than -100mV, check the light and PIN diode again.
3. Set Lock-in amplifier parameters:
AC Gain: 50 dB;
Sensitivity: 1 mV;
Time Constant: 100 ms;
Input Limit: 10 mV;
DR 16;
Osc: 0.000 Hz;
4. Connect PIN diode to input A of the Lock-in amplifier.
5. Set Modulation source DS345 parameters:
Function: sine wave;
Sweep/modulate: LIN SWP.
6. Connect DS345 ’output’ to PI circuit ’mod in’, DS345 ’sync output’ to Lock-in
Amplifier ’ref in’.
7. Set PI circuit Integration off, make sure the input is disconnected.
8. Set Wavetek 80 parameters:
Modulation Mode:
VCO;
Output:
sine wave;
CHAPTER 10. POLARIZED 3 HE TARGET
Operating mode:
180
FUNC.
Be sure that the VCO indicator is lit. (The VCO is used because it is very sensitive
to small changes in voltage. At a carrier frequency of 10MHz, a 1mV input will
produce a 400Hz shift in the output frequency.)
9. Connect PI circuit output to Wavetek 80 VCO IN through an 100:1 attenuator.
Set the 100:1 attenuator switch to “FMS” (no attenuation).
10. Connect Wavetek 80 sync out to SR 620 Counter. Connect Wavetek 80 output to
the cable leading to the EPR coils. Make sure that the Wavetek 80 output is not
in standby mode.
11. Bypass the RF capacitor by setting the switch to ’EPR’ on RF capacitor box front
panel.
Now that the circuit is in place, open the LabView program called “FMSweep.vi”
(in windows-diagram).
1. Input the start frequency for your sweep in the upper left of the screen. At the
start of pumping, the resonance frequency should be around the number given by
this formula:
EPR base resonance = (466000 * holding field in Gauss)
Thus, your sweep should begin below and end above this number. However the resonance frequency is different for different Rb polarization directions in the pumping
cell. Typically with 25 Gauss holding field the starting sweep frequency is 11.1 MHz
for right-handed light, or 11.5 MHz for left-handed light.
2. Input the voltage at which you will run the EPR coils. Typically, this is 8.0 Vpp,
but it can go as high as 16.0 Vpp.
3. Next, set the step to be 1KHz and the number of steps to be 500. Keep in mind
that the lineshape you are looking for is about 500kHz in width.
4. Set the number of points per sweep, as well as the sweep duration. Typically we
take 2 points per sweep and 500ms for each point. The maximum speed should not
exceed two points per second.
5. The program will automatically generate the data file to store the data, with a
filename related to date and time the program is running. However, you can change
the filename and path through the dialog bar located at the right bottom corner.
6. After all of the necessary data has been entered, the sweep may begin. To start
the program, simply click on the white arrow in the upper left corner of the screen.
You should be able to read the current Lock-in signal amplitude and the EPR coil
frequency from the screen.
CHAPTER 10. POLARIZED 3 HE TARGET
181
7. Under normal conditions, in the plot of Lock-in signal vs. EPR coil frequency
showed by the program, you should find a dip followed by a peak, or vice versa.
8. You might need to adjust the Lock-in Amplifier phase. You can use “auto-phase”
in reference channel menu or you can adjust it manually as follows:
When the program goes at the maximum of the peak (or mininum of the dip),
check the Lock-in Amplifier phase by reading the X and Y signals from Lock-in
Amplifier front panel. Since the LabView program only reads X channel, the phase
should be adjusted such that X channel has a big signal while Y channel is roughly
zero (or only consists noise). If not, adjust the reference channel phase through the
front panel menu of the Lock-in amplifier. You don’t need to stop the program to
do it, it is easier to adjust phase by checking the output of the running program
simutaneously. After the phase is optimized, run the program again.
Manual adjustment is useful when the signal is small compared to the background
(cable resonance, noise etc.), where “auto-phase” will track the phase of the background.
9. An EPR signal of amplitude higher than 25 µV extracted by FM Sweep will be
good enough to do the EPR polarization measurement.
10. Once the measurement is done, disconnect the output of Wavetek 80 to the cable
leading to RF coil.
10.10.2
Common Problems
1. The plot shows no peak/dip.
• Check the PIN diode and light, make sure there is enough light.
• If the PIN diode signal amplitude is above 100 mV already, try changing the
Lock-in Amplifier phase by 90◦ .
• If still no peak is detected, try changing the start sweep frequency and the
number of steps such that the program will cover larger region.
2. The plot shows multiple peaks. It might be the signal is mixed with background
(cable resonance, noise etc.). The solution might be complicated and will not be
discussed here. Contact target experts on call.
3. How do I use the output data? The data is saved in a text file consisting of two
columns. The first column contains the frequency of the EPR coil, while the second
column holds the corresponding signal from the Lock-in Amplifier. Its format can
be read by programs such as SigmaPlot, Excel, and Paw.
CHAPTER 10. POLARIZED 3 HE TARGET
10.10.3
182
EPR Polarization Measurement – AFP Sweep
This configuration uses the lock-in amplifier with a PI feedback box to lock into the EPR
resonance frequency and then track its behavior. With AFP spin-flip, a shift in EPR resonance frequency will be measured, which is proportional to the pumping cell polarization.
Construct the circuit described in Fig. 10.10.3:
1. First construct FM Sweep circuit described above.
2. Connect Lock-in amplifier channel 1 output (from rear panel) to PI circuit ’in’.
3. Switch the 100:1 attenuator from “FMS (no attenuation)” to “AFP (100:1)”.
4. Set Modulation source DS345 parameters:
Frequency: 200 Hz (This frequency is arbitrary. 200Hz is good since it is far away
from most noise sources.)
Waveform: sine wave
5. Set Wavetek 80 parameters:
Amplitude: 8.0 V, higher amplitude will be needed if the signal extracted by FM
Sweep is less than 25 µV;
6. Bypass the RF capacitor by setting the switch to ’EPR’ on RF capacitor box front
panel.
Now that the circuit is built, you can begin the initialization of the setup.
1. Set the modulation source DS345 amplitude to approximately 0.6 Vpp. This number can be optimized for different conditions such as different target polarization
and EPR D2 signal amplitude. At an EPR D2 signal amplitude of 25 µV the optimized DS345 amplitude is roughly 0.05 Vpp for P< 20%, between 0.05 and 0.6
Vpp for 20% <P< 40%, and 0.6 Vpp or higher for P> 40%.
2. Find the EPR resonance frequency:
If you have done a FM Sweep measurement, then you already know the EPR resonance frequency, continue to the next step.
If you don’t know where the resonance frequency is, you may want to do a FM
sweep measurement, or you can find the current resonance frequency by hand, To
do this,
CHAPTER 10. POLARIZED 3 HE TARGET
183
EPR AFP Setup
Photodiode
D2 Filter
Func. Generator
EPR RF
Coil
Lasers
HP 3324A
RF Amplifier
AFP RF Coils
GPIB
PC
out
A
Lock−In
Amplifier
ref in
DS345
100:1
in
out
sync
Mod. Source
Attenuator
in
out
EPR PI Feedback
mod in
Mixer
Wavetek 80
VCO
sync
GPIB
PC
Counter
SR620
Figure 10.13: Circuit for EPR measurement with AFP spin flip
• Temporarily remove the connection to Vin on the PI feedback box, turn off the
integration function, set the 100:1 attenuator switch to “FMS (short)”. Now
you are back to the FM Sweep circuit setup. Manually adjust the Wavetek 80
frequency with the smallest step 0.01 MHz, while observing the lock-in. Look
in the area given by the formula:
EPR base resonance = (466000 * holding field in Gauss)
This is typically about 11MHz.
• You should be able to observe the lock-in signal changing when adjusting the
frequency. It should show a maximum (>0, peak) followed by a minimum
(<0, dip), or vice versa, and the resonance is between them when the lock-in
signal is small. Keep in mind that the lock-in will read zero when you are
far from resonance as well, so be sure to look for this characteristic resonance
behavior.
• Reconnect the Vin input, and set the 100:1 attenuator switch from “FMS
(short)” back to “AFP (100:1)”.
3. Set the Wavetek 80 frequency to be 0.1 MHz lower than the current resonance
frequency if the light is right-handed polarized, or 0.15 MHz lower than the current
resonance frequency if the light is left-handed polarized. The lock-in signal should
be large since you are not at the resonance frequency.
CHAPTER 10. POLARIZED 3 HE TARGET
184
4. Turn on the integration on the PI feedback box.
5. Observe the Vout of PI circuit on an oscilloscope. If it is “jumping around” erratically, or looks as if the Opamps have been saturated, you must adjust the gains.
The best advice is to begin with the relative gain, and if this does not rectify the
problem, move on to the absolute gain. Basically, adjust these parameters such
that the feedback signal is not too strong, neither too weak.
6. Once this is done, the circuit should track the resonance frequency and the counter
reading should change towards it. The lock-in signal should be stablized to minimum.
7. Wait until the counter reading is stablized, change Wavetek frequency manually by
0.01 MHz. You can observe a jump of 0.01 MHz in counter reading, but then it
should change back to the resonance frequency, which means the circuit is following
the resonance. If not, then the frequency it was stablized before is not the true
resonance frequency, check the circuit and try again.
8. Once this is done, the circuit should take care of itself. Left alone, it will track the
movement of the resonance peak.
To measure the shift in resonance frequency due to AFP spin-flip, open the LabView
program AFPSweep.vi. Enable the program by clicking on the large white arrow in the
upper left corner of the panel.
1. Set the sweeping time to be 6 seconds and the waiting time to be 20 seconds. Set
the number of sweeps to be 2.
2. Run the program.
3. Download parameters by clicking the corresponding button, you should read “downloading parameters” on the program status dialog.
4. Wait until the program status shows “waiting for data taking to start”.
5. Start data taking by clicking the “pause :: run” button. Now the program should
read resonance frequency from counter, and the lock-in amplifier signal. The program status should show “waiting for sweep trigger”.
6. Wait for roughly 20 seconds, start sweeping by clicking on the “start sweeping”
button. The program status should show “sweeping...”.
7. You should then be able to see the jump in resonance frequency. When the light is
right-handed polarized, this should be a positive jump, while for left-handed light
it is negative.
CHAPTER 10. POLARIZED 3 HE TARGET
185
8. After the sweeping is completed, wait until the program status shows “waiting for
sweep trigger”, now you can stop the program.
9. Disconnect the output of Wavetek 80 to the cable leading to RF coil.
10. Never stop the program during sweeping, this will cause the 3 He to stay at the wrong
state and hence a big loss in target polarization.
10.10.4
Common Problems
1. The lock-in does not remain stable. Or the lock-in but does not seem to track the
resonance.
You may see a big fluctuation in either lock-in signal or resonance frequency. This
effect can be caused by several things.
First, it is possible that the frequency to which you are locked is not the true
resonance, or you are completely out of resonance region. The solution is again to
find manually the resonance, looking for the most pronounced signal.
The problem could also be caused by a wrong lock-in amplifier phase. Please refer
to Section 10.10.1.
Finally, it could be that the PI Feedback box is improperly set. Adjust the gains
until the lock-in becomes more stable. Admittedly this method is not very quantitative. Based on experience, at high polarization (>40%), both gains should be
adjusted anti-clockwisely nearly to the end. At lower polarization (∼20%), both
gains should be adjusted by about 4∼5 turns clockwisely. These numbers may also
vary at a higher or lower EPR D2 signal amplitude.
You may also optimize the PI circuit gains by observing how fast the circuit follows the resonance. When counter reading is stablized, change Wavetek frequency
manually by 0.04 MHz (which is close to the real jump during sweeping, at a target
polarization ∼ 40to resonance in roughly 3 5 seconds, then the circuit is working
well. If less than 2 seconds and counter reading is not stable, decrease both the
relative and absolute gain. If longer than 6 seconds or lose the signal, increase both
the relative and absolute gain.
2. The lock-in does not track the resonance when doing AFP flips. This is common,
since the frequency shift during the spin flip can be quite large, on the order of
20-40kHz.
First, it is possible that the PI feedback is not strong enough for the circuit to
follow the resonance shift. Try increasing the absolute gain of PI circuit if possible.
Second, it could be that the modulation DS 345 amplitude is too small. The size
of this amplitude determines how far from the central frequency the circuit looks
for the resonance. Try increasing the modulation amplitude (but do not increase
too much, usually it is less than 0.8 Vpp). This will cause the counter reading less
CHAPTER 10. POLARIZED 3 HE TARGET
Main Holding Field B
Continuous EPR and Field Feedback Setup
Field Feedback
Chassiss
X in
186
Photodiode
D2 Filter
Y in
Func. Generator
PI Feedback
X
Z
HP 3324A
EPR RF
Coil
Lasers
RF Amplifier
Magnetometer
Power Supply
DC
Power
Supply
GPIB
HP 3458A
Multimeter
Lock−In
Amplifier
Compensation Coil
100:1
in
out
sync
DS345
Attenuator
ref in
in
out
EPR PI Feedback
mod in
Mixer
Wavetek 80
VCO
sync
PC
GPIB Magnetometer
Sensor
out
A
Mod. Source
GPIB
AFP RF Coils
PC
Counter
SR620
Figure 10.14: Circuit for continuous EPR measurement with field feedback
stable and you need to compromise between stablizing the circuit and following the
frequency shift.
10.10.5
EPR Continuous Monitoring with Field Feedback
This configuration uses the lock-in amplifier with PI feedback circuit to lock into the
EPR resonance frequency and then track its behavior. At the same time a field feedback
system is used to stablize the main holding field to a level of 10−5 . The EPR resonance
frequency is the central frequency proportional to the main holding field plus (or minus)
a field originated from the 3 He polarization. By measuring the EPR resonance frequency
and the main holding field, one is able to monitor the pumping cell polarization continuously.
The circuit and labview program are the same to that are used for AFP Sweep
measurement described in Section 10.10.3, except that field feedback system is needed,
as shown in Fig. 10.10.5
1. Construct circuit and set parameters. To track the resonance frequency, the only
difference between EPR AFP measuremen and EPR continous monitoring is that
the modulation source DS 345 amplitude does not need to be large. Typically 0.05
Vpp is enough.
CHAPTER 10. POLARIZED 3 HE TARGET
187
2. Make sure the circuit is tracking the resonance frequency.
3. Use a scope or multimeter to measure the two field feedback signals, the amplitude
should be less than 500 mV. If either of them is larger than 500 mV, you must
adjust the position of magnetometer sensor such that the signal perpendicular to
the main holding field is back to 200 mV or lower. Then adjust the power supply
for the compensation coil such that the signal parallel to the main holding field is
back to 200 mV or lower.
4. Enable the field feedback system by setting the switch ’bypass/through’ on field
feedback chassiss to ’bypass’. You should observe that both field feedback signals
are stablized to zero.
5. Run LabView program HP3458A read.vi on computer downstairs to record the
current of compensation coil.
6. Open LabView program AFPSweep.vi, enable the program by clicking the white
arrow in the upper left corner.
7. Set the paramters, download parameters and start data taking. Do not do sweeping
(AFP spin flips).
8. After enough data is collected, stop the program.
9. Disconnect the output of Wavetek 80 to the cable leading to RF coil.
10.10.6
Common Problems
1. See the problems in the previous section.
2. The field feedback signals jump to +10V or -10V when enabling the field feedback
system, and stay there afterwards.
This is usually caused by a wrong state of the field feedback system. For example,
wrong sign of the feedback settings (XCOS, YCOS) can cause a positive feedback
such that the signals are stablized to the satuated values.
3. The field feedback signals oscillate between +10V or -10V when enabling the field
feedback system.
This is also caused by a wrong state of the field feedback system. Because we are
using two dimensional field feedback, wrong sign of the feedback settings (XSIN,
YSIN) will cause the longitudinal and transverse components of the field interfering
with each other through a positive feedback so both are not stablized.
4. The field feedback signals are stablized at a small but non-zero value.
This means the field feedback system is working properly and in principle should
not affect the measurement. However, a non-zero offset of either of the signals
CHAPTER 10. POLARIZED 3 HE TARGET
188
may cause a non-linearity between holding field and the compensation coil current,
which should be avoided in high precision polarization measurements. With the
field feedback system in ‘Bypass’ mode, you can adjust the screws on field feedback
chassiss to adjust the ‘XOFFSET’ and ’YOFFSET’, meanwhile use a scope to
measure the two field feedback signals from magnetometer sensor until they are
stablized to zero.
10.11
Reference Cell
10.11.1
Description of the Reference Cell System
The reference cell system is comprised of three subsystems—the reference target cell, the
gas handling system, and the control electronics. The reference target cell is mounted
inside the target enclosure, which is located in the beam path immediately upstream of
the spectrometers. The gas handling system consists of a gas manifold, which is located
in the electronics rack behind the laser hut, and a series of gas lines that connect the
manifold to the reference cell. A control box consisting of electronics that control the
solenoid valves of the gas system is also located in the electronics rack. A remote control
box, identical to the one in the hall, is located in the counting house. A schematic of
the entire system is shown in Fig. 10.15. The individual subsystems are depicted in Fig.
10.16–10.17.
Under normal operating conditions, the manifold, the reference cell, and the intervening gas lines will be filled with either 3 He, N2 , or 4 He gas at high pressure (typically
≈ 10 atm). The system may be operated either locally (at the panel) or remotely (via
switches mounted in the counting house). A switch on the gas panel itself allows one to
toggle between local and remote control. All switches, local and remote, are equipped
with LED displays so that one can readily determine the status of any switch.
The reference cell consists of a very thin-walled glass flask. An outlet at the opening
is joined to a Copper tube by means of a glass-to-metal seal. This Copper tube is
connected to a 1/8” dia. Copper tube with quick-plug-on gas fittings. High pressure 3 He
gas bottle is mounted near the target chamber and is connected to the other end of the
1/8” Copper tube through solenoid valve V3. The rest of the gas system is connected to
the 1/8” copper tube by a long 1/2” Copper tube. The reference cell is mounted at the gas
fitting. A special coupling tool has been designed to facilitate the coupling/de-coupling
the reference cell at the quick-plug-on gas joint.
The gas handling system consists of 8 solenoid-controlled, air-actuated valves, one
hand-set needle valve, two baratron pressure gauges (0–1000 torr and 0–1000 psia), two
pressure relief valves, a bottle of high-pressure 3 He gas, a bottle of high pressure N2 gas,
a bottle of high pressure 4 He gas, an oil-free turbo pump backed up by a pre-pump, and
sufficient tubing and pipe fittings to connect them all together. There are 5 basic actions
that the gas handling system must accomplish; pump-out, vent, fill with N2 or 4 He, and
fill with 3 He. The selection between N2 and 4 He is done by a special directional solenoid
valve (V8).
CHAPTER 10. POLARIZED 3 HE TARGET
Target Area
189
Electronics Rack
Counting house
Vacuum
Ref. Cell
V
Valve
Box
4He
V
N2
Control
Box
2 MKS
Displays
3He Bottle
Control
Box
25−pin D−sub
cable
2 MKS
Displays
To VME break−out box
(2−10 Volt signal)
2−conductor cable
Gas lines
V
Solenoid Valve
Figure 10.15: Schematic of reference cell system
CHAPTER 10. POLARIZED 3 HE TARGET
Condition
C1
C2
C3
C4
C5
Action
Evacuate
Vent
3
He fill
N2 fill
4
He fill
190
Valves activated (open)
V1+V4+V5+V6
V4+V5+V6+V7
V3+V4+V6
V2+V4+V6
V2+V4+V6+V8
Table 10.5: Action of the remote-control switch panel
The reference cell control panel (shown in Fig. 10.17 ) is comprised of 5 pushbutton
switches, which activate the 5 possible valve setting conditions, a baratron high-pressure
gauge which reads between 0-1000 psia, a baratron low-pressure gauge which reads between 0-1000 torr, two toggle switches to turn on and off the vacuum pump and a toggle
switch to select local or remote operation. The valve configurations for the five actions
are described in Table 10.5.
10.11.2
Operation
The following operational sequences will allow the filling of the reference cell with the
three gases:
• Fill with N2
– Step 1: Vent to 16 psia.
– Step 2: Evacuate until vacuum reaches 1 mtorr.
– Step 3: Hold the N2 button down while checking the high-pressure gauge,
until the desired pressure is established.
– Repeat steps 2,and 3 three times.
• Fill with 4 He
– Step 1: Vent to 16 psia.
– Step 2: Evacuate until vacuum reaches < 10 mtorr.
– Step 4: Hold the 4 He button down while checking the high-pressure gauge,
until the desired pressure is established.
– Repeat steps 2, and 3 three times.
• Fill with 3 He
– Step 1: Vent to 16 psia.
– Step 2: Evacuate until vacuum reaches < 1 mtorr.
CHAPTER 10. POLARIZED 3 HE TARGET
Target Area
191
Electronics Rack
Ref. Cell
V5
V4
V1
relief valve
175 psi
G1
V3
G2
Needle
valve
3He Bottle
V6
V7
Vacuum
Pump
relief valve
175 psi
vent
V2
4He
V8
N2
Vi = solenoid valves
G1 = low pressure Baratron gauge
G2 = high pressure Baratron gauge
Figure 10.16: Valve configuration of the reference cell gas system
Vacuum Pump
On
Evac.
Off
Local/Remote
toggle
Vacuum
gauge
3He N 2
4He
Figure 10.17: Control panel
Pressure
gauge
Vent.
CHAPTER 10. POLARIZED 3 HE TARGET
192
– Step 4: Hold the 3 He button down while checking the high-pressure gauge,
until the desired pressure is established.
The pressure at the location of the gas manifold is measured by a baratron gauge.
The baratron pressure is read into the data stream and is also output to a remote sensing
unit located in the counting house.
10.11.3
Cautions
1. Be careful with the 3 He, we have a limited supply.
2. If the system is above atmospheric pressure, always vent it before pumping.
3. Always monitor the high-pressure gauge when filling. Do not exceed 150 psia.
10.11.4
Potential Hazards
The principal hazards associated with the reference cell are flying material and loud noise
if the gas panel or reference cell fails catastrophically under high pressure. Of the two
subsystems, the reference cell subsystem presents the greater hazard, both because it
is more likely to fail, and because it is likely to be more dangerous if it does. The RF
enclosure is designed to contain all the debris in case of a cell explosion.
10.11.5
Hazard Mitigation
After testing the reference cell system to 200 psia and to protect the cell from rupturing
during normal operation, the relief valves have been set to 175 psia.
All personnel shall wear hearing protection when accessing the platform area while
the cell is under high pressure and no windows are on the target enclosure.
Special care must be taken when accessing the region inside the target enclosure.
Full face shielding and safety glasses should be worn under these conditions. Before
working in the vicinity of any of the reference cell components, personnel shall check the
pressure in the manifold from the baratron readout in the remote control area.
10.12
Hazards and Safety Issues
The main potential hazards encountered in the overall operation of the target are listed
below. As we address the operation of each subsystem, a description on how to alleviate
the potential hazards is reported.
• Personnel eye sight damage due to a laser light exposure;
• Fire due to the operation of the high power lasers;
CHAPTER 10. POLARIZED 3 HE TARGET
193
• Fire due to the operation of the target oven;
• Explosion of the high pressure target cell;
• Explosion of the reference cell;
• Irradiation of the target by the electron beam.
For personnel safety to be effective all personnel authorized to operate any subsystem
of the target will be required be familiar with that specific subsystem as well as read the
target OSP.
10.13
Laser Safety
10.13.1
Laser Safety
1. Always have your safety goggle on when the laser is on!
2. When the yellow beacon is flashing, have goggle on when you enter the hut.
3. Alignment should be done at low power.
4. Be sure that the beam is hitting the target.
5. Do not turn the beam up to full power unless the oven temperature is at least 150
degrees Celsius.
6. Do not look directly into the beam even with safety goggle on.
7. Do not stand in the way of a beam that is at full power.
8. Understand where the beam is and where the reflections are.
10.13.2
Fire Hazards and Safety
The fire and safety in the laser hut is covered in the LOSP for the laser hut area, however,
in the target area where the laser beam is directed, there is a case where a potential fire
hazard exists.
In case the target cell explodes during optical pumping, the temperature sensors
mounted on the target and pumping cells will respond immediately and an alarm will
be triggered. The alarm will be triggered whenever a temperature reading of any sensor
is 10% out of its norminal range. The target operator should shut off all the lasers
immediately. Based on the tests performed, the target oven will sustain on the order
of 10 minutes with full laser power incident and no rubidium atoms to absorb the laser
power.
CHAPTER 10. POLARIZED 3 HE TARGET
10.13.3
194
Personnel Safety/ Working in the Hall
When the installation of the full target setup is finished, working in the hall shall be safe
from laser light hazards or target explosion hazards, because laser light as well as the
target cell will be safely enclosed. Therefore when considering the overall aspects of the
safety of personnel working in the Hall two distinct periods are to be considered.
1. One period is during the laser beam alignment because the laser beam pipes from
the laser hut to the target need to be removed. During this time period we will
ensure that no other person except those people who are laser trained and qualified
for laser beam alignment are in the hall. This will be arranged by using a controlled
access to the Hall provided by the CANS system. This alignment will be performed
only during the night time and weekend. Clear warning signs will be posted at the
entrance of the Hall when the alignment is under progress.
2. One period is during the setup of the high pressure target cell in its final position.
In this case the “target platform” which is a natural perimeter around the target
area will be marked and signs posted requiring the wearing of ear protection and
faceshield. Beyond that defined perimeter all personnel working in the hall will
not be affected in case of explosion of the cell if they are not wearing a faceshield.
Nevertheless, it is strongly recommended to have ear protection when working
anywhere in the Hall.
10.14
10.14.1
Appendix: Laser Standard Operation Procedure
Introduction
A polarized 3 He target system was built and used for several JLab experiments. A
number of new experiments will continue use the polarized 3 He target system in the
future. The polarized 3 He target is based on the principle of spin exchange between
optically pumped Rubidium vapor and 3 He gas. Several high power (30 Watts) 795 nm
diode lasers will be used for the optical pumping. A laser hut has been built and installed
in Hall A. This LSOP describes the setup of the laser system in the laser hut and at the
target area, details the potential hazards associated with the operation of this setup and
provides instructions for the safe and effective use of the equipment. In addition, this
manual provides information about the functioning of the various safety systems installed
to protect personnel and equipment.
10.14.2
Personnel
The 30 watts infrared diode lasers (Coherent FAP-System) may only be operated by
personnel who have :
CHAPTER 10. POLARIZED 3 HE TARGET
195
• completed a Laser Safety course administrated by the laser safety officers at Jefferson Laboratory (Patty Hunt).
• read the Laser Safety section of the EH&S Manual(6410);
• completed and passed an opthalmological exam;
• had a safety walkthrough by the Laser Safety Supervisor of the Polarized 3 He Target
System (Jian-ping Chen);
• read this document;
• been added to the authorized list of Laser Personnel, included as the last page of
this LSOP.
Jefferson Lab personnel or outside visitors, who have not completed all of above training,
are only allowed to enter the laser control area under the following conditions :
• have permission of the Laser Safety Supervisor of the Polarized 3 He target system
• be accompanied by a laser authorized personnel
• if the laser is operational, with required safety goggles
• if any equipment, including the laser, is operational, no touching of equipment due
to electrical hazards.
10.14.3
Laser
The main Laser specifications are outlined in Table 1. For more specific information, we
refer to the Coherent FAP-system diode laser users manual, which will be available in
the lab.
10.14.4
Optical setup
The optical setup is shown in Fig. 10.3, and is made of:
• A optical table (stainless steel) supporting one breadboard (anodized aluminum);
• Seven infra-red diode lasers (four for longitudinal pumping and three for transverse
pumping) located on one side of the table, with optical fiber bundles.
• Seven lens to have each laser beam focused at the pumping cell;
• Seven beam splitters to split each laser beam to two beams;
• Fourteen λ/4 waveplates to transform each beam from linear polarization to right
or left circular polarization;
CHAPTER 10. POLARIZED 3 HE TARGET
Specifications
COHERENT FAP-System
Operational Specifications
Output power
30 W
Mechanical Specifications
Weight
Cooling Requirements
Delivery Optical Fiber Bundle
Delivery Fiber Length
Delivery Fiber Termination
60 pounds
None required
0.8 mm diameter
5.0 meter nom.
SMA 905 conn.
Operational Specifications
Typical Operating Temperature
Typical Storage Temperature
Humidity (non-condensing)
0◦ C to 35◦ C
−20◦ C to 65◦ C
5% to 95%
Electrical Specifications
Input Power
115 Vac 60 Hz, < 1200 W (500 W typical)
Optical Specifications
Beam Characteristic
Beam Divergence
Diode Laser Center Wavelength
Wavelength Temp Coefficient
Emission Bandwidth (FWHM)
Semiconductor, multimode
< 0.20 N. A.
780 to 840 nm
0.27 to 0.30 nm/◦ C
±2 nm
Table 10.6: Laser specifications
196
CHAPTER 10. POLARIZED 3 HE TARGET
197
• Fourteen dielectric mirrors to reflect each split beam back into the pumping cell;
• One transparent window on the RF enclosure and one transparent window on the
oven to allow the combined transverse laser beam to pass through;
• Two dielectric mirrors to reflect the combined longitudinal laser beam into the
pumping cell;
• One pumping cell to absorb all the laser beam power;
• Three mirrors for monitoring the pumping cell;
• One spectral-analyzer for monitoring the pumping cell.
The optical table will be about 2 meters from the platform. The laser beams will
be from about 15 cm to 105 cm above the table Each of the 795 nm diode laser light,
after passing through the lens, will be split into two beams. Each one will go through
a λ/4 waveplate to transform linear polarization to circularly right or left polarization.
All beams, after passing some windows and being reflected by some mirrors, will shoot
into a glass pumping cell filled with mixture of Rb vapor and 3 He gas. The laser beams
will be completely absorbed by the pumping cell. The total path length from the lasers
to the pumping cell is about 5 meters. The pumping cell is inside an oven and connected
to a target cell. The whole target assembly is inside two pairs of Helmholtz coils, which
provide a magnetic field for the polarization of the target. A NMR system with a set of
RF drive coils and a set of separate pickup coils, and an EPR system are used to measure
the polarization of the target.
10.14.5
Hazards
The primary beam hazards associated with Class IV lasers consists of eye and skin
injuries. The most severe eye injuries are caused by viewing the beam either directly
or through specular reflection. At an infra-red wavelength of 795nm most of the laser
light entering the eye is absorbed in the retina. The primary adverse effects from direct
or specular viewing are blindness and severe retinal burns. The primary adverse effects
from accidental viewing are retinal burns. The retina is most sensitive to radiation of
this wavelength, and if the laser energy incident to the eye is too high, it can cause an
irreversible retinal burn.
Laser radiation of the intensity associated with Class IV diode lasers can also cause
irreversible damage to the skin. The damage caused is either associated with temperature
rise of the skin tissue following the absorption of laser energy (skin burns) or with surface
reactions resulting from photon interactions at the molecular level (photochemical effect),
disrupting the normal functionality of the skin tissue.
The normal Hazard Zone for the 30 Watts FAP system is 2.76 × 103 meters.
CHAPTER 10. POLARIZED 3 HE TARGET
198
To Target Pivot
Laser Hut Topview
Laser Hut Topview
upstairs
downstairs
Window
Hyper-filter screen
lasers
Emergency
Light
Optics Table
Run-Safe
Box
Crash
Button
Phone
To interlock
Crash
Button
Yellow
Beacon
Keypad
Rack
Laser Controller
Rack
Laser Controller
Interlock controller
Door
Interlock controller
Emergency
Light
Interlock
Reset Box
Figure 10.18: The Polarized 3 He Target Laser Hut Setup
10.14.6
Laser environment
The diode lasers and the associated devices will be located in the polarized 3 He target
laser hut in Hall A. The entire hut is a laser controlled area. Its organization is shown
in Fig.10.18. The setup will be on the second floor and the direct laser light will not be
visible from the entrance of the room.
The laser beams are horizontal and are pointing away from the entrance door. After
passing through the optical setup, the laser beams are directed into the pumping cell and
terminated there.
Outside the laser hut, the laser beam path is completely enclosed with beam pipe
between the laser hut and the target pivot and an enclosure on top of the the target.
Since the beam path is not enclosed inside the laser hut, any partially reflective surface
may cause deflection of the beam path. When operating the diode laser, safety goggles
are required at all times.
Apart from direct beam hazards to eyes and skin, since the diode lasers are Class IV
lasers, there exists a potential fire hazard. Flammable material should not be brought
into the laser area.
CHAPTER 10. POLARIZED 3 HE TARGET
10.14.7
199
Procedures
In this section, we review the various procedures that are required to operate the laser
and optical devices. Hazards are least likely to occur during normal operation when laser
beams are switched on. During tests, maintenance, upgrades and/or alignment, beam
hazards are more likely.
At all times, when operating the diode lasers in lasing mode, safety goggles are
required.
10.14.7.1
Normal procedure
In the operation mode, each diode laser is in lasing mode rendering an output power of
approximately 30 W. The laser beam is already aligned, properly focused and directed
into the pumping cell. The lasers are interlocked with the entrance door of the laser
hut. All the laser beam pipes and the laser enclosure on top of the target are securely
installed.
Working with the lasers in normal operational mode will require protective eye wear
with a minimum optical density (OD) of 4.7 at wavelength of 795 nm. Before starting
the laser in its normal operation mode personnel have to enable the laser safety interlock
box. This will cause access to the laser hut to be in a controlled mode. Authorized
personnel with an access code can bypass the interlock for 45 seconds when entering the
laser room. Unauthorized personnel entering the room will cause the lasers switching to
stand-by mode when the door is opened. If the laser is to be unattended for a long time,
the power should be switched off.
Thus the general procedures for normal operation of the diode lasers in the target
lab are:
• Enable laser safety interlock box;;
• Wear protective eyewear;
• Switch on AC power;
• Turn on the control box with a key;
• Turn laser to Ready and then On.
10.14.7.2
Alignment procedure
All the mechanical stands supporting the optical components have been designed and
surveyed in order to achieve a preliminary safe alignment of the entire setup (laser off).
Initial setup of laser and optical system will be done while the laser hut window is closed
(such that no laser beam will come out the laser hut). Initial alignment will be done
with a standard class 3 HeNe laser (class 3A after attenuation, 650 nm) or a laser pointer
(class 3A). Laser safety goggles are mandatory for all procedures except for alignment
using class 3A HeNe laser. Use precaution for class 3A laser when using the HeNe laser:
CHAPTER 10. POLARIZED 3 HE TARGET
200
Do not look directly into the beam or use collecting optics. The final alignment will be
done with the diode laser but at a reduced laser power (less than 10 amps, when the laser
spot on a card can be clearly seen with IR viewer). The alignment is performed with one
laser beam line at a time. The beam can be tracked by the use of either an IR viewing
card or an IR viewer. The photosensitive card can be displaced along the beam, and the
IR viewer allows the tracking by the light slightly diffused on the optical components.
During this final alignment, the laser hut window will be opened and the laser beam will
be directed to the target, while the laser beam pipes and the laser enclosure on top of
the target will be taken off. Therefore the whole hall will be classified as laser area. We
will use the CANS system to have controlled access. Before the laser alignment starts, a
sweep will be performed to clear the Hall. Then the CANS system will be programmed
to only allow authorized laser trained persons to have access to the hall. The door to the
BSY tunnel will be magnetically locked with a toggle push-button switch. The “laser
danger” warning sign will be posted at all the entrances. The doors will be checked by
pulling the door handle. In addition, a red flashing light will be put at the BSY side
of the door. Most of the final laser alignment will be performed during night time to
minimize interference with other work going on in the hall. When the alignment stops,
all signs will be take off and the CANS system will be disabled and the door to BSY will
be unlocked.
10.14.7.3
Maintenance procedure
Replacement of used or damaged optical components of the setup will be made with the
laser off (power switched off and unplugged). The positions and orientations of the new
components will be mechanically surveyed and extensively checked before turning to any
procedure needing the laser on.
When the target enclosure windows need to be opened (such as to inspect the target
or to replace a target cell) and the hall is not in laser controlled access, the lasers must
be turned off before opening the target enclosure. The keys for all of the laser power
supplies will be secured in a lock box. All staff working in the vicinity of the target
enclosure must apply personal locks to this box in accordance with JLab lockout/tagout
procedures. The vicinity area will be determined from the power measurement and with
clear danger sign posted.
In case of the failure of any electromechanical or electrical or electronical device, the
lasers and the other power supplies will be turned off and the out of order devices will
be fixed by Coherent service personnel. The Laser power can simply be unplugged.
10.14.7.4
Off-normal and emergency procedure
In case of an emergency, power to the laser should be shut off. This can be performed in
three ways.
• Push the Crash button;
• Turn off the control key on the laser power supply;
CHAPTER 10. POLARIZED 3 HE TARGET
201
• Pull the plug from the power outlet.
In the event of a fire, the users should leave the laser room and pull the nearest fire
alarm. Then leave the hall.
In case anybody is accidentally exposed to the laser beam (direct or indirect) without
eye protection, he or she should immediately contact the Jefferson Lab medical center
(Mary Gibson, phone: 7539, page: 584-7539). If it is off business hours, please contact
local hospital emergency. At the mean time, please inform the laser system superviser
(Jian-ping Chen, phone: 7413, page: 584-7413) and/or laser safety officer (Patty Hunt,
phone: 7039, page: 584-7039).
10.14.8
Controls
Several controls have been added as preventive measures to the laser room and to the
direct laser area. We will enumerate these controls here.
1. The laser control area will have danger signs posted and will have a yellow beacon
indicating the presence of Class IV diode lasers. Danger sign will also be posted
near the target enclosure area.
2. A controlled access interlock system will limit the entrance to the laser hut with a
coded number pad. The code is given only to the authorized laser users listed in
section 10 with currently valid training.
3. The laser switches are interlocked to allow an opening of the door to turn off the
laser (to stand-by).
4. The main power plug to the laser can be easily pulled. It is plugged into a power
strip with a on/off switch which can be easily switched off.
5. Protective safety goggles (minimum OD 4.7 at 795 nm) have to be worn when the
laser is operational.
6. All laser beam paths outside the laser hut are enclosed with beam pipe or other
enclosure (except during laser alignment process).
7. All personnel need to fulfill the training requirements as indicated in Section 1 of
this document.
8. the LSOP will be posted on the outside door of the laser hut to inform personnel
about the hazards associated with the setup and the proper procedures.
All controls will be inspected every six months and the inspection will be documented.
CHAPTER 10. POLARIZED 3 HE TARGET
10.14.9
202
Laser safety calculations
1. Maximum Permissible exposure The Coherent FAP-System emit nominal continuous beams of 30 W at a wavelength of 795nm. With a limiting aperture size of 7
mm and exposure time of 10 seconds, the calculated MPE is 1.51mW/cm2 .
2. Optical Density The minimum Optical Density is calculated for the beam diameter
of 0.64 mm with maximum CW power of 30 watts (the worst case) to be 4.70. OD
of 5 safety goggles for the wavelength of 795 nm were selected to be used in the lab.
With all 4 lasers, when the beams overlap, the combined beam will have a size
larger than 2.75 inches. The power density will be lower than the maximum power
density with one laser.
3. Nominal Hazard Zone
The nominal hazard zone is calculated for 3 conditions:
(a) intrabeam: 78.5 meters
(b) after lens: 5 meters
(c) fiber-optic output: 6.7 meters
The condition of use will always be fiber-optic output with nominal hazard zone of
6.7 meters.
The laser hazard zone is nimized by confining operation to an interlocked laboratory
or interlocked enclosure.
10.14.10
List of authorized personnel
The following personnel are authorized to operate the Coherent FAP-system diode lasers
and the associated polarized 3 He target facility, under the assumptions, they have completed the training requirements defined in Section 1:
CHAPTER 10. POLARIZED 3 HE TARGET
203
Gordon Cates
University of Virginia
Jian-Ping Chen
Jefferson Lab (Laser system Supervisor)
Alexandre Deur
Jefferson Lab
Gary Dezern
Jefferson Lab
Ed Folts
Jefferson Lab
Haiyan Gao
MIT
Ole Hansen
Jefferson Lab
Wolfgang Korsch
University of Kentucky
Kevin Kramer
William and Mary
Nilanga Liyanage
Jefferson Lab
Zein-Eddine Meziani Temple University
Karl Slifer
Temple University
Patricia Solvignon
Temple University
Scot Spiegel
Jefferson Lab
Mark Stevens
Jefferson Lab
Vince Sulkosky
William and Mary
Xiaochao Zheng
MIT
Names can be added to this list by the Laser System Supervisor, Jian-ping Chen,
phone 269-7413, pager 584-7413, email [email protected]
10.14.11
List of Laser Trained Personnel
Todd Averett
William and Mary
Gordon Cates
Princeton University
Jian-ping Chen
Jefferson Lab (Laser system Supervisor)
Alexandre Deur
Jefferson Lab
Haiyan Gao
MIT
Ole Hansen
Jefferson Lab
Wolfgang Korsch
University of Kentucky
Kevin Kramer
William and Mary
Nilanga Liyanage
Jefferson Lab
Kathy McCormick
Kent State University
Zein-eddine Meziani Temple University
Karl Slifer
Temple University
Patricia Solvignon
Temple University
Vince Sulkosky
William and Mary
Xiaochao Zheng
MIT
CHAPTER 10. POLARIZED 3 HE TARGET
10.14.12
204
List of Authorized People to Change Target Cells
Jian-ping Chen
Jefferson Lab
Alexandre Deur
University of Virginia
Kevin Kramer
College of William and Mary
Nilanga Liyanage Jefferson Lab
Karl Slifer
Temple University
Patricia Solvignon Temple University
Vince Sulkosky
William and Mary
Xiaochao Zheng
MIT
10.14.13
List of Authorized People to Perform Laser Alignment
Jian-ping Chen
Kevin Kramer
Wolfgang Korsch
Patricia Solvignon
Vince Sulkosky
Xiaochao Zheng
Jefferson Lab
College of William and Mary
University of Kentucky
Temple University
William and Mary
MIT
Names can be added to the above lists after proper training and authorized by
Jian-ping Chen, pager 584-7413, phone 7413 and email:[email protected]
Chapter 11
The Waterfall Target 1 2
11.1
Overview
The waterfall target system provides a target for experiments on 16 O. Using a waterfall
for oxygen experiments has many advantages. Pure oxygen is difficult to handle, as it is
highly reactive. The use of other oxygen compounds requires additional measurements
to subtract the non-oxygen background, whereas the hydrogen in water can be used for
calibration purposes. The technique of using continuously flowing water as an electron
scattering target was first developed in 1982 in Mainz [16]. The conceptual design of the
waterfall target system for Hall A developed by INFN Roma, is very similar to one used
at Saclay [17], with the parameters as follows: ∼ 120 mg/cm2 one-foil thickness, target
thickness stable in time within 1%, and insensitive to beam current up to at least 20 µA.
The target may be configured for one or multiple waterfall “foils”.
11.2
Description of the System
The main components of the target system (see Fig. 11.1 and Fig. 11.2 ) are:
• a) the waterfall target cell, the target, the solid target ladder;
• b) the hydraulic system;
• c) the movement system;
1
CV S revision Id: waterfall-target.tex,v 1.4 2003/11/21 18:13:11 gen Exp
Authors: David Meekins mailto:[email protected] and Maurizio Lucentini mailto:[email protected]
jlab.org.
This file is a combination of two files taken from:
http://www.jlab.org/~meekins/h20_target/safety_doc/,
Operation manual.lyx and waterfall safety.lyx. The original documents are available:
http://www.jlab.org/~meekins/h20_target/safety_doc/Operation_manual.ps and
http://www.jlab.org/~meekins/h20_target/safety_doc/waterfall_safety.ps
The file has been formatted by J.LeRose mailto:[email protected] and E.Chudakov mailto:[email protected]
org
2
205
CHAPTER 11. THE WATERFALL TARGET
206
• d) the slow-control system;
Figure 11.1: Schematic overview of the target system with the hydraulic system on
the left side, scattering chamber, movement system and target cell in the middle. The
hydrogen system on the right side is not implemented in the present setup.
The waterfall foil(s) is (are) produced in a cell mounted in the standard scattering
chamber (Fig. 11.3 and 11.4) of Hall A. The water, continuously pumped from a
reservoir (S), goes into the target cell and then back into the reservoir. The water passes
through a system of slits and holes to form one or more flat rectangular films, which are
stable due to the surface tension and to the adherence to stainless steel poles (Fig. 11.5).
Under the cell, a target holder allows one to put up to 5 solid targets cooled by the water
(Fig. 11.6).
The waterfall target can consist of a single foil or multiple foils, according to the
needs of the particular experiment. Notice that it is possible to modulate, slightly, the
thickness of the waterfall target by changing the pump speed. This adds flexibility to the
system and allows the user to choose the best value according to the desired resolution
and luminosity.
Elastic scattering from hydrogen in the target is used to measure the target thickness.
For continuous monitoring of the target thickness, one ‘calibrates’ the raw counting rate
CHAPTER 11. THE WATERFALL TARGET
Figure 11.2: Scheme of the target system devices
207
CHAPTER 11. THE WATERFALL TARGET
208
of either spectrometer by the elastic scattering measurement. It is then possible to
convert the electron or hadron rate observed during the measurement to an average
target thickness.
Figure 11.3: Pictures of the target and scattering chamber.
11.2.1
The Hydraulic System
The hydraulic system comprises of a closed circuit (Fig. 11.7) containing a pump which
forces the water from a stainless steel container up to the target manifold and back to
the container by means of stainless steel tubes. A gear pump, magnetically coupled to a
dc motor, is used to produce a stable film.
There is a provision to place a cooler along the circuit to keep the water at constant
temperature. Moreover, hydrogen can be introduced into the target container to reduce
the background. In the present setup, neither of these provisions have been implemented.
A tachometer on the pump axis measures the pump speed (RPM) and a flowmeter,
just before the entrance of the scattering chamber, measures the flow rate. The command
voltage value is also read. By continuously measuring these parameters, and comparing
them, one can monitor the target thickness stability.
Basically, the water flows in a closed loop; there is a magnetic gear pump, capable
of 40 liters/min, 0-90 V DC voltage controlled, which pulls the water in a stainless steel
tube, from the rack up to the target cell, at a distance of about 20 metres. Before arriving
in the cell, the water flows through a flowmeter. A display shows the flux in front of the
rack.
CHAPTER 11. THE WATERFALL TARGET
209
Figure 11.4: The enclosure of the target used for E00-102 and E94-107 is shown on the
left. The waterfall target within the enclosur is shown on the right.
Figure 11.5: Mechanical details of the waterfall target: posts etc.
CHAPTER 11. THE WATERFALL TARGET
210
Figure 11.6: The Waterfall Target Stack, the waterfall cell with solid targets mounted
beneath.
CHAPTER 11. THE WATERFALL TARGET
Figure 11.7: Schematic view of the hydraulic system.
211
CHAPTER 11. THE WATERFALL TARGET
212
When the water arrives at the target cell, it passes through a small compression
chamber, which has three rectangular holes at its bottom. Three waterfalls are formed.
Finally, the water falls down the cell where an output stainless steel tube goes back to
the tank3 . When the pump is off, the water nearly fills the tank. When the pump is on,
the water level must be sufficient to let the pump pull water, and not air. The pump
speed is read by a tachometer, which consists of an optical counter-timer which counts
the turns of the pump axis, and sends this value to a display in front of the rack and to
the computer by a 4-20 mA standard loop.
The pump has a driver located on top front of the rack, which allows the user to
drive it manually, or from a remote position using the computer. A manual switch on
the front of the driver selects the operating mode. If manual mode is set, the knob close
to the switch can be turned in order to increase/decrease the pump speed. The value of
the pump speed represents the percentage of maximum reachable pump speed (see Operating Procedure). The pump speed regulator is also available in the Counting Room.
The pump cannot be switched off by computer.
A short list of the instruments used in this systems is listed below:
Device
Model
Flowmeter probe
Flowmeter transducer and transmitter NA
Flowmeter display
ITECO trading mod. 9210001 out 4-20 mA
Tachometer
ITECO trading mod. 9210001 out 4-20 mA
Water pump
Pacific Scientific mod. 6324-452
Pump driver
Dart Controls mod.253G 200E + PCM23
Tubes
Swagelock tubes (various sizes)
Tube Connectors
Swagelock tube connectors
11.2.2
The Movement System
The movement system allows the vertical translation of the waterfall target and the solid
target ladder. The total mass of the system is about 50 kg. A picture of it can be seen
in Fig. 11.8. The movement is done using a stepping motor connected to a driver module.
The driver module is put in a separate chassis and it can operate locally in manual mode,
by pressing the UP/DOWN preset switch and the STOP/GO switch, both located on
the front panel of this chassis, or in remote mode, from the counting room by computer
(see Waterfall target EPICS MEDM display)4 (see Fig. 11.11). Microswitches serves two
purposes:
• 1) switch off the beam while the system is moving;
3
The tank is a parallelepipedal volume of about 20 liters made of stainless steel with a front perspex
window, which can be seen in front of the rack, to see the inner water level.
4
For EPICS and MEDM see the references [2, 9]
CHAPTER 11. THE WATERFALL TARGET
Figure 11.8: Picture of the movement system.
213
CHAPTER 11. THE WATERFALL TARGET
214
• 2) stop the motor if either the top or bottom position is reached.
There are two low voltage heavy duty electro-mechanical microswitches at the extreme
positions (high and low limit). If the target approaches one of these extreme positions
then the corresponding microswitch changes its status and the logical signal is used to
stop the movement.
The movement system has been carefully shielded and grounded to reduce noise from
the driver and its power supply. The motor driver and the power supplies are placed in
a separate chassis, close to the EPICS crate, containing the ADC, DAC and serial driver
module. A magnetic positional encoder monitors the vertical position of the targets. The
encoder output is a 4 – 20 mA signal and it is converted into a pure number proportional
to the target position and is displayed on the PC panel. The restore procedure has
to be performed manually, in Hall A, due to the serious risk of damaging the system
(see Operating Procedure). The level signal on the microswitches is a 24 V DC voltage,
supplied from the Hall A system. A short list of the instruments used in this systems is
given below:
Device
Stepping Motor
Encoders
Chassis
Power Supply unit
Motor Driver
Controller
11.2.3
Model
Phytron ZSS100-200
ASM SENSORS
custom made
custom made
SHS - APS 3/A
Serial RS232, via EPICS
The Slow–Control System
All controls and monitoring are handled by the slow control system (see Fig. 11.9). The
data acquisition is performed using EPICS [2], the standard control system at JLab. The
EPICS crate (Fig. 11.10) contains ADC, DAC modules, and a serial line to interface
with the motor driver. The control equipment is mounted in a rack (see Fig. 11.12). The
user interface (GUI) (Fig. 11.11) is based on MEDM [9]56
The components of the system are given below:
• Slow controls
List of components:
5
The software is maintained by David Wetherholt mailto:[email protected] of the accelerator
controls group.
6
Software location: mccops/medm/hla (monticello), MEDM application ha wt stepmot.adl;
Epics: r3132j0; Application: ha waterfall (Current version: 1-1);
IOC: iochawt1;
ha waterfallhawt1 : Contains 32 ADC channel readbacks, motor records and D/A pump controls;
adcchan - Standard template for VMI3122 ADC Channels.
hawf ll : Low-level code for RS232 communication and high-level subroutines calls for sequencer code to
communicate with the Star2000 stepped motor.
seq hawf: Sequencer code for stepped motor movement.
CHAPTER 11. THE WATERFALL TARGET
215
• CONTROLS:
– pump + driver + analog interface to EPICS
– stepping motor + driver + serial interface to EPICS
• SENSORS:
– optical tachometer
– flowmeter
– position magnetic encoder
• OTHER SIGNALS:
– microswitches
• CONTROL LINES:
– 4-20 mA for tachometer and flowmeter, encoder
– 0-5 V DC for I/V converting board to ADC in EPICS crate
– 0-5 V DC for pump command, from DAC in EPICS crate
– 12 V DC voltage for microswitches
– serial RS232 for motor driver to EPICS interface
All of the analog signals coming from the transducers are to be read by the ADC
module, in the EPICS crate. For this reason, they are grouped in a single cable, coming
out of the rack, and connected to the the ADC patch panel. These are slow signals:
tachometer [arbitrary units], flowmeter[l/min], position encoder [arbitrary unit], and do
not need a fast acquisition rate. The transducers, as indicated above, all have 4-20 mA
current output, for better noise immunity. The transducers are located in the rack and
send their signal to a current-to-voltage converting board, located on the back side of
the rack. This is called 5B Backplane, and contains 5B32 modules, which are insulated
current input and 4-20 mA to 0-5 V converters. Conversion to voltage is necessary, to
let the signals be acquired by the ADC of the MEDM system. The pump command is a
0-5 V signal coming from the DAC module in the EPICS crate.
The software control of the target is written in EPICS and run on the IOC iochawt1.
The IOC uses serial communication, DAC and ADC to control the target. The pump
motor tachometer, flowmeter, position encoder, and home switch are read through the
ADC card. It is common to have noise on the flowmeter, tachometer, and encoder. The
user should expect this noise and not be alarmed. The user should check the camera on
the control rack to make sure that the postion encoder does indicate the proper target
(there is some acceptable slop on this number).
CHAPTER 11. THE WATERFALL TARGET
Figure 11.9: Waterfall target: slow control system layout.
216
CHAPTER 11. THE WATERFALL TARGET
217
Figure 11.10: Waterfall target: the EPICS Crate.
The motion control is achieved via a SHS - APS 3/A stepper motor controller.
Communication with this controller is achieved through a RS232 serial connection. This
controller counts absolute steps from the home postion. The home postion is defined
using the HOME routine on the expert page of the GUI. Non-Experts are forbidden to
use this function as serious down time may result. If the target motion mechanism does
not seem to be moving to the selected target, execution of the HOME routine might be
needed and an expert must be called.
The pump speed control is achieved through a -5 - 5V DAC. The DAC is 12 bit and
negative voltages are not used in the application. Therefore the minimum set postion
for the pump speed is 2048. This corresponds to 0V and 0 pump speed (pump is off). A
setting of 4095 corresponds to 5 volts and maximum pump speed. It is not possible to
set the pump speed outside these limits in software.
A counter readout (ITECO trading mod. 9210001) provides the tachometer signal.
The signal output from the module is 4-20 mA which is converted to 0-5V and connected
to the ADC. There is significant noise on the ADC channel which seems to be a feature
of the ADC. The user should expect this noise and not be alarmed at oscillating values
for the tachometer signal.
The same counter readout is used for the flowmeter. The device is read using the
same ADC and thus, the signal has a similar level of noise. The user should expect this
noise and not be alarmed at oscillating values for the flowmeter signal.
The postion encoder is the same type used on the collimators for both arms. The
encoder is displayed using a simple digital display on the control rack. This display unit
CHAPTER 11. THE WATERFALL TARGET
Figure 11.11: Waterfall target control GUI..
218
CHAPTER 11. THE WATERFALL TARGET
Figure 11.12: Waterfall target: control rack.
219
CHAPTER 11. THE WATERFALL TARGET
220
Figure 11.13: Heat behavior of the beam entrance window.
has a 4-20 mA output that is converted to 0-5V and connected to the ADC. Again, the
ADC value has significant noise. The user should expect this noise and not be alarmed
at oscillating values for the encoder signal.
11.3
The Target Cell Windows
A crucial component of the target is the cell window. Because it is intended to employ
beam current exceeding 50 µA, care must be taken in choosing the window material,
which otherwise could melt from overheating. Consider the general case of a circular
beam spot and a circular target window, as illustrated in Fig. 11.13. There is a continuous
heat sink surrounding the window which is maintained at temperature T0 and located
at a distance r0 from the center of the beam spot. The beam spot radius is r1 . The
temperature pattern which develops is characterized by two distinct regions. In the
logarithmic one the temperature is given by
r0
iρ dE
ln( )
2πκ dx
r1
while, in the linear region, the temperature is given by
∆T1 = T1 − T0 =
(11.1)
iρ dE
(11.2)
4πκ dx
In both cases, i is the beam current (in mA, ρ is the window material density, κ is
is the differential energy loss). The total temperature
the thermal conductivity and dE
dx
rise above the surrounding heat sink temperature T0 is thus
∆T2 = T2 − T0 =
CHAPTER 11. THE WATERFALL TARGET
∆T = ∆T1 + ∆T2 =
221
iρ dE
r0
[1 + 2ln( )]
4πκ dx
r1
(11.3)
If a 100 µA beam of radius 25 µm and a window of radius 3 cm are considered (very
extreme situation), these are the results for various materials:
ρ
Material
Al
Fe
Ti
Cu
Be
W
Ta
( cmg 2 ) (
2.7
7.9
4.5
8.96
1.85
19.3
16.65
dE
dx
M eV
g
cm2
1.62
1.48
1.51
1.44
1.61
1.16
1.2
κ
) (
M eV 0 C
)
µA
2.37
0.8
0.22
4.0
2.1
1.75
0.58
Tmelt
(◦ C)
∆T
(◦ C)
660
1530
1660
1083
1278
3410
2996
223
1765
3731
390
171
1545
4161
It is clear from the table that Be is the most suitable choice for the entrance and
exit windows. The window thicknesses vary for the different targets used.
11.4
Target Layouts
Three targets have been built:
• a) the first one for the experiments E89-003 and E89-033 has got three foils;
• b) the second one for the experiment E00-102 also has got three foils but with some
modifications;
• c) The third one for the experiment E94-107 has a single foil;
11.4.1
The Target for the Experiment E89-003 and E89-033
Care was taken to optimize the foil configuration with respect to the spectrometer acceptance and ejectile trajectory. A kinematical overview of the target can be seen in
Fig. 11.14, a cutaway of the target in Fig.11.15. The three foils are identical, 12 mm
wide, and guided by poles which are 2 mm x 2 mm (Fig. 11.5). In the direction normal
to the target, the foils are separated by 22 mm. Along the target, the first foil is shifted
down the page by 13 mm and second up the page by 13 mm. The foils are parallel, and
the angle between the beam direction and the direction normal to the target is 300 . The
machining tolerance is less than 0.15 mm.
CHAPTER 11. THE WATERFALL TARGET
222
Figure 11.14: Preferential scattering angles for E89-003 and E89-033.
Figure 11.15: A cutaway of the waterfall target cell used in E89-003 and E89-033.
CHAPTER 11. THE WATERFALL TARGET
223
Figure 11.16: Preferential scattering angle of the target for the E00-102 experiment.
11.4.2
The Target for the Experiment E00-102
This target has been modified for the experiment 00-102 to accommodate the new kinematics (Fig. 11.16). The concept is the same, the geometry is slightly different, both for
the target and for the container.The Be window thickness is 200 µm.
11.4.3
The Target for the Experiment 94-107
The E94-107 collaboration, in consultation with Hall A, has decided to use a single foil
one. The choice is driven by ease of construction, minimization of software reconstruction
for extended targets and improved PID which is realized with the RICH. The missing
mass resolution is not significantly affected by this choice. The thickness of the Be
window has been increased (150 µm) because of the bigger diameter of the downstream
window.
11.5
Operating Procedure
There are two main points of operation, when using the system: the first, to position the
appropriate target in front of the beam, by moving the target up or down, and the second
one is, to control the water pump. Both these operations can be done by using the GUI
(see Sec.11.2.3) (see Fig.11.11)accessable from the standard Hall A controls screen. User
operation of the target is achieved through this GUI only. Any problems with the target
must be addressed by an expert. A list of experts is give in Table 11.3. The experts may
perform some operations manually.
The target consits of a positioning system that places one of 5 targets (plus empty)
on beam line and a water pump which circulates the target fluid for the waterfall target.
Under normal operations, the pump should circulate ∼ 3 l/min. A list of targets and
maximum allowed currents is given in Table 11.1.
CHAPTER 11. THE WATERFALL TARGET
Target
H2 O
Al alignment
12
C (thin)
BeO
12
C (thick)
Empty
224
Max Current Max Current
Unrastered
Rastered
140 µA
0
0
0
10 µA
50 µA
10 µA
50 µA
10 µA
50 µA
-
Table 11.1: Target positions and maximum beam currents for the Hall A Waterfall Target
stack. For increases in these current limits contact the Hall A run coordinator.
As it has been mentioned in Sec.11.2.3, it is common to have noise on the ADC reading out the flowmeter, tachometer, and encoder. The user should expect this noise and
not be alarmed. The user operator should compare the displays on the GUI (Fig.11.11)
with the displays visible in the camera focused on the control rack. In particular, one
should make sure that the postion encoder does indicate the proper target (there is some
acceptable slop on this number).
11.5.1
Startup of the system
The startup and shutdown procedures require access to the equipment inside Hall A.
11.5.1.1
First startup of the System
Before remote operation of the system can be used, the system must be configured
correctly. To do this, one has to perform the following procedure with the waterfall
control hardware (inside Hall A):
• Set the pump manual/remote switch to “manual” position; also make sure the
pump speed knob, in front of the rack, is set to zero.
• Go to the motor chassis module and set B1 microswitch, there indicated, to the
“manual” position (that is the “off” position for the switch).
• Go to the slow control rack, and turn the main 110 V switch to “ON”.
• Turn the pump control to on, and gently turn the pump speed knob clockwise.
Check the water circuit operation:
– If they are off, switch on the displays ( FLOWMETER, TACHOMETER),
and check for proper operation.
– If everything working correctly the pump control should be set to the ”remote”
position.
CHAPTER 11. THE WATERFALL TARGET
225
• Check the positioning system:
– Switch the motor driver to “manual” mode.
– Go to the motor driver chassis and move the motor up and down by pressing
the switches on the front panel.
– Check that the motor is moving and make sure that the encoder display is on,
and working during the motor movement.
• If all components are operating correctly the system may be configured for remote
operation:
– Turn the main 110 V switch to “OFF”, in front of the slow control rack.
– Settle the microswitch, on the back of the motor module, to “ON” (that is
remote control position.)
– Switch the 110V power supply to “ON”.
– Check the remote operation of the system in the counting room. The procedure for remote operation is given in Sec 11.6.
11.5.2
System shutdown
To shutdown the system one must take the following steps:
• In Counting Room, set the pump speed to zero, and be sure the flowmeter and
tachometer indicate zero on their displays
• Then go down to Hall A, turn the pump knob to zero and switch it off.
• Finally, switch off the whole system by turning off the main 110VAC switch, in
front of the Rack.
11.5.3
Troubleshooting
• Emergency: Use of the system when the motor is broken
First of all, there is a spare motor in the shelf, ask the Hall A maintenance technicians for details.
If for any reason, the motor can not be used, it is possible that movement can be
performed by using the crank, which is on top of the target. To do this the crank
must be put on the motor seat, and the movement can be performed manually. If
the encoder is working, one can use the above table to positionsfor the targets, if
the encoder is unavailable, the microswitches put on top of the chamber will give
the correct alignment.
This procedure can be performed ONLY BY AUTHORIZED PERSONNEL, due
to the large vacuum load on the system and risk of possible free fall when the motor
is removed.
CHAPTER 11. THE WATERFALL TARGET
226
• Emergency: Use of the restore system, when motor has reached some extreme
position microswitch.
Due to the possibility of permanent damage to the system, this can be done ONLY
BY AUTHORIZED PERSONNEL.
When a limit position is reached, an automatic control turns off the power supply
for the whole slow control system. In this case, a switch is put in Hall A, near the
slow control rack, which bypasses this emergency stop and lets the operator move
the motor in the correct direction, bringing the system back to a more central
position. This has to be done manually, and visually checking the motor behavior,
because of the risk of damaging the target motion mechanism. Finally, when done,
the moving system has to be put back to the remote position, and it has to be
checked if this works well, or not. Fix the trouble ( a spare motor driver is in the
shelf, if needed), and normal work can begin again.
11.6
Standard Operations
These operations include moving the target ladder in order to install the target needed
and the water-pump control. They should be performed using the GUI (Fig. 11.11).
11.6.1
Target Selection
The target postion is selected on the left side of the GUI under Position Select. There
are six options. The desired target is selected by clicking on the particular target button.
Basically, this takes a present position and places it in the “set position” input box. A
specific position can also be entered directly into this box. If the target is not already
on the desired postion, the user should see the “moving” LED turn yellow in the Status
display. The graphic display to the right of the selection boxes should update; the blue
diamond indicating the current postion of the target. In addition, the “Encoder Postion”
and “StepMot Postion” displays to the right of the GUI should update indicating the
target is in motion. If the motor hits a limit switch, the motion will stop and the “Limits”
status will indicate which limit is active.
NOTE: the motor can be stopped at anytime by pushing the purple button labeled
“STOP”.
If the motion does not appear to be operating correctly, call an expert. At the end
of the motion sequence the encoder postion should be close to the value in text next to
the target button. The value on the camera should be within ±0.0030 on the encoder
readout. If this is not the case call an expert.
Note that any target motion will trip and FSD. The MCC should be informed of all
target motion. If for some reason during the target motion there is a need to stop, click
on the stop button. Motion can be restarted by selecting another target.
There is an access from the “Status” section to an expert screen. Only experts
should use the functions on this screen there are no user features here.
CHAPTER 11. THE WATERFALL TARGET
227
In addition to the camera on the control rack, there is a camera on top of the pivot
focused on the motion mechanism. The target postions and names are labled in a visible
fashion. The user should double check this camera as well to make sure the motion
mechanism is behaving as expected.
Finally, there are micro-switches that provide an FSD signal when not on a target.
If for some reason an FSD is still tripped after a move to a new target, an expert should
be called.
11.6.2
Pump Speed Control
The pump speed control is set using the slider bar or input field in the “Pump Speed”
section. This input may range from 2048 to 4096 corresponding to a 12 bit DAC 0-5V
output. The output of the DAC is converted to 4-20 mA and used to control the pump
speed. An input of 2048 (0V) will stop the pump. An output of 4095 (5V) sets the pump
speed at ∼ 1800 on the display which is roughly 6 l/min. This should be more than
adequate for normal operations. If the pump does not function properly or more flow is
desired, an expert must be called. The tachometer and flowmeter displays should always
be checked with the displays on the camera.
11.6.3
Rebooting the IOC
The name of the IOC is iochawt1. The IOC may be rebooted by logging into the IOC
via telnet and typing reboot. If this is not possible, (i.e. the IOC is not responding to
remote login) call an expert.
11.7
Troubleshooting (Experts only)
This section is intended as a reference for experts only. Figure 11.12 shows the control
rack. Figure 11.17 shows the IOC rack.
11.7.1
Manual motion operation (Experts only)
Ensure that the motor controller power is off. The power may be turned off at the front
of the main control rack under the beam line. In the motor control rack located next to
the IOC, there is a small chassis mounted box. At the back of the box, there is a DIP
switch clearly marked with black pen labled B1. This switch must be set to OFF to put
the controller in manual mode. Restore power from the control rack under the beam
line. Return to the motor control box. If the green LED at the right of the box turns
on, all is well and the target can be moved up or down by setting the direction switch
and toggling the move switch. To restore the remote operation of the system cut power
at the main control rack under the beam line. Return the B1 DIP switch to ON and
restore power to the system. At this point the controller does not “know” where it is.
CHAPTER 11. THE WATERFALL TARGET
Figure 11.17: IOC rack for waterfall target.
228
CHAPTER 11. THE WATERFALL TARGET
229
The HOME routine must now be run. See Section 11.7.2 for proper use of the HOME
routine.
11.7.2
Use of the HOME routine
Use of the HOME routine can be effected from them the main control GUI. In the
“Status” section of the GUI, there is a small button that brings up the expert page.
Be sure that the physical position of the target is a few centimeters above the H2O
target. This can be seen at the top of the scattering chamber. The HOME routine will
not properly execute from the H2O target postion. The target might have to be moved
manually above the H2O target postion (see Section 11.7.1). Press the HOME button.
The target should move in the direction of the HOME switch which is located near the
empty target position. If the target does not move in the correct direction, move the
target manually to some position close the the BeO target position. This may take some
time. The motor controller will look for the HOME switch by moving the system up.
When the HOME switch is activated, the target will start moving down; when it finds
the HOME switch again it will stop. This ensures that the HOME position is not affected
by the hysteresis in the switch.
11.7.3
Hardware limit over travel
If a limit switch (either upper or lower) is tripped, the target motion control system
will lose power. This is designed so that the lifting mechanism will not be damaged. In
this case, the limit must be bypassed and the target moved off of the limit manually.
First, the controller must be set to manual mode. See Section 11.7.1. At the front of
the box, there is a switch labled bypass power. Turn this switch down; the yellow LED
above should turn on. Also on the front of the box is limit switch bypass button. Press
this button and move the target manually. BE SURE TO MOVE IN THE CORRECT
DIRECTION; severe damage to the system may result from improper use of this feature.
If the motor does not seem to be moving, page Dave Meekins or Scot Spiegel. As the
power has been cycled to this module, it is now necessary to run the home routine (see
Section 11.7.2). Disable bypass power when finnished with motor reset.
11.7.4
Manual water pump operation
Operation of the water pump in the manual mode is simple. There is a switch at the front
of the main control rack (located under the beam line) that can be toggled to remote
or manual operation. The pump power switch must be on for the pump to work. The
pump speed is controlled via the knob which can be rotated clockwise for more speed.
Please do not run the pump faster than 2000 as shown on the tachometer display. This
causes undue strain on the system.
CHAPTER 11. THE WATERFALL TARGET
11.8
230
Safety Assessments
The water fall target system is much more simple than the standard target configuration
for Hall A. There are therfore fewer hazards which need to be addressed. The target
does use the standard Hall A scattering chamber for experiment E00-102. The safety
hazards for the scatting chamber have been addressed in the standard Hall A safety
documentation (see Sec.5 and Sec.9.2.2.3). The following subsections outline the potential
hazards and how each must be addressed.
11.8.1
Radiological hazards
Due to the large beam currents required for the referenced experiments, the potential
for radiological contamination of the target fluid and scattering chamber area exists.
Therefore, all personnel entering the target area after beam has been impinging on the
target must follow standard radiological control procedures. Prior to entry into the area
the Radiation Control Group must be consulted. The system must only be accessed by
authorized personnel
11.8.2
The electrical power and slow control systems
The system is supplied by 110 V AC voltage. All the lines at these voltages are kept
inside the rack, and the plugs satisfy the European CE rules for these voltages. The
supplies for the single diagnostic and control devices are inside the rack are all at 5 V
DC, 12 V DC, 24 V DC. The pump requires 110 V DC. The Motor Driver needs a special
80 V DC voltage. A custom power supply unit has been built for this purpose, and this
unit is kept in the so called “Power Supply Unit” at the top of the rack, together with
the other supply units.
The signal lines are low voltage (no higher than 12 V DC the digital lines, 0-5 V or
4-20 mA loops) and kept separated from the supply lines by using different connectors
and separated cables. Isolated input lines have been used for the analog lines. A potential hazard outside of the rack, are the motor cables, which may be handled only by
AUTHORIZED PERSONNEL and with the motor power supply switched off. An 80 V
DC/2A (square wave, duty cycle 0,5) current flows in these cables when the motors are
in standby mode. This current reaches up to 7 A (square wave, duty cycle 0,5) when
motors are moving. These high power lines are kept separated from the low voltage lines
in all cases. The pump power supply line is kept inside the rack, and in a position that
cannot be reached unless the side panel of the rack is opened.
Access to the rack is controlled by appropriate signage indicating the hazard and
list of personnel authorized access.
11.8.3
The water system
There are about 17 liters of pure water in the circuit. The water tank, the tubes, the
target and the cell containing the target fluid are made of stainless steel. Connections
CHAPTER 11. THE WATERFALL TARGET
231
between individual parts of the system have been made with ”swagelok” connectors.
The system has been leak checked with gas up to 2 bars. No water leakage is expected
except in case of breaking of the windows of the target cell. In this case, some water
carrying some radioactivity may leak into the scattering chamber (which is a contained
environment). The calculations performed by Geoff Stapleton show that the radiological
precautions necessary are rather low. The water should not be released into the hall
drains. Measurements at convenient intervals will be made by the radiation control
group on samples to permit determinations of the radionuclide yields. In the case of
water loss or the need to drain the water, the radiation control group has to be informed
to permit precautionary measurements and controls. Appropriate sinage on the water
system indicates this.
11.8.4
Thin windows on the target cell
The beam passes through two windows, which are made of Be (203 µm thickness).
It has been calculated (using the FNL-1380 Thin Window Vacuum Vessel Guide from
Fermilab) that the Be windows will have a safety factor of 3 beyond the 1/2 yield requirement listed in the Fermilab document. It has therefore been concluded that the
windows are adequate for use on this target cell.
The two scattering windows are made of kapton except for experiment E00-102 where
the scattering windows are made from 25 µm thick stainless steel shim stock (SST 302;
Hardness: 40-45 RC; Tensile Strength: 185000 psi). Three of these scattering windows
have been hydro-tested and in each case the window did not fail until slightly above 3
times over pressure (50 psig or higher). A summary of the test results is shown in Table
11.2. It is important to note that the windows only have a 1 atm load on them when the
scattering chamber is under vacuum. In this case, the windows are not accessable and
present no hazard to personnel.
11.8.5
The slow control system
Due to the low voltages and little currents, there are no significant hazards in working
on the small signals; however, one must switch the power off to the whole system before
beginning to operate on it. ONLY AUTHORIZED PERSONNEL may touch the signal
lines using appropriate lockout tagout procedures.
11.8.6
The mechanical system
There are no hazards in handling the system when it is not in motion. When the
system is in motion one has to beware of the cog-wheels movements, and keep hands and
loose clothing away from the moving parts. For this reason, it is FORBIDDEN to touch
the system when it is in motion. Additionally, the potential exists for remote operation
of the motion mechanism while it is being serviced. Therefore, to service the system
while it is installed, the appropriate lockout tagout procedures are necessary. Signage
stating this will be posted on the motion mechanism upon installation in the hall.
CHAPTER 11. THE WATERFALL TARGET
Pressure
(psig)
0
5
10
15
20
25
30
35
40
45
50
55
232
Deflection Deflection Deflection
Window 1 Widow 2 Window 3
0
0
0
53
69
55
71
85
72
82
93
82
94
111
93
104
117
103
113
128
113
124
136
122
135
141
128
143
150
Failed
160
Failed
Failed
Table 11.2: Summary of thin scattering window hydro-tests. Deflections are given in
(in×1000). The scattering windows are made from 25 µm thick stainless steel shim stock
(SST 302; Hardness: 40-45 RC; Tensile Strength: 185000 psi).
11.9
Authorized Personnel
Name
Maurizio Lucentini
Dave Meekins
Ed Folts
Rusty Salmons
Scot Spiegel
Mark Stevens
Gary Dezern
Phone
5434
7857
6914
5900
6383
7119
Pager
872-2442
989-4580
820-9205
820-9013
820-9480
820-5864
Table 11.3: List of experts for the Hall A Waterfall Target.
Part IV
High Resolution Spectrometers
(HRS)
233
Chapter 12
High Resolution Spectrometers
(HRS) 1 2
The Hall A spectrometers and associated instrumentation are designed to perform high
resolution and high accuracy experiments. The goal is to achieve a missing mass resolution of ∼ 200-500 keV to clearly identify the nuclear final state. An absolute accuracy
of ∼ 1% is also required by the physics program planned in the Hall, which implies ∼
10−4 accuracy in the determination of particle momenta and ∼ 0.1 mr in the knowledge
of the scattering angle.
The instruments needed are a high resolution electron spectrometer (HRES) and a
high resolution hadron spectrometer (HRHS), both with a maximum momentum capability matching the TJNAF beam energy, and large angular and momentum acceptance.
A layout of the 4 GeV/c High Resolution Electron Spectrometer is shown on Figures 12.2 and 12.1. Its main design characteristics are given in the attached table.
The spectrometer has a vertical bending plane and 45◦ bending angle. The QQDQ design includes four independent superconducting magnets, three current-dominated cos2θ
quadrupoles and one iron-dominated dipole with superconducting racetrack coils. The
second and third quadrupoles of each spectrometer have sufficiently similar field requirements that they are of identical design and construction. The overall optical length, from
target to focal plane, is 23.4 m. Optically, the HRHS is essentially identical to HRES.
In fact the two spectrometers can be used interchangeably to detect either positively or
negatively charged particles as needed by any particular experiment.
The support structure includes all system elements which bear the weight of the
various spectrometer components and preserve their spatial relationship as required for
45◦ vertical bending optics.
The alignment and positioning system includes all the elements which measure and
adjust the spatial relationship. The support structure consists of the fabricated steel
components which support the magnets, detector, shield house and associated equipment.
It is composed of the box beam, which supports the outer elements in fixed relative
1
2
CV S revision Id: hrs-1999.tex,v 1.1 2003/06/06 15:44:08 gen Exp
Authors: J.LeRose mailto:[email protected]
234
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
Figure 12.1: A side view of the Hall A HRS spectrometer.
235
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
236
Figure 12.2: A bird’s eye view of the Hall A end-station at TJNAF.
position atop the dipole; the dipole support bracket, upon which the dipole rests on the
jacks; the cradle, upon which the dipole rests through the vertical positioning system,
VPS; and a portion of the shield house load through the inboard legs of the gantry; the
gantry, which supports the shield house and the magnet power supplies; and the bogies,
which support the cradle-gantry assembly and slide on the floor plates and provide the
driving power to move the two spectrometer arms.
The detector package is supported on the box beam and is surrounded by the shield
house. It must perform two functions, tracking and particle identification, PID. The most
important capability of focusing spectrometers is measuring precisely the momenta and
entrance orientations of the tracks. Momenta resolution of 10−4 is obtainable, consistent
with the resolution of the incident beam.
A particle traversing the detector stack (Figure 12.3) encounters two sets of horizontal drift chambers (x,y) with two planes of 368 wires in each chamber. The track
resolution is ∼ 100 µm. From the chamber information both positions and angles in
the dispersive and transverse directions can be determined. The information from these
chambers is the principal input of the tracking algorithms.
The chambers are followed by a scintillator hodoscope plane designated S1. This
plastic scintillator array provides the timing reference for the drift chambers, and is also
used in trigger formation and in combination with a second hodoscope pair it can provide
time of flight particle identification. These scintillators can also be used to perform crude
tracking.
The next element encountered by a particle is a gas threshold Cherenkov detector.
This is used for particle identification. In the hadron spectrometer this gas threshold
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
237
Figure 12.3: The electron spectrometer detector stack.
Cherenkov detector can be swapped against an Aerogel detector, with a similar function.
The second hodoscope plane, S2, is located directly behind the gas Cherenkov. Its
function is essentially the same as that of S1. In the hadron spectrometer an option
exists to have this hodoscope pair be preceded by a third chamber, to improve tracking.
Each of the two spectrometers have gas and Aerogel Cherenkov detectors which can be
used when they are in electron detection mode.
The final elements in the detector stack on HRSE are the pre-shower and the lead
glass shower calorimeter. This is used for energy determination and PID.
The hadron detector is shown schematically in Figure 12.4. It consists of two sets
of (x,y) vertical drift chambers identical to those of the electron arm. The remaining
part of the detection system is used to define the level 1 trigger, as well as for particle
identification and timing. It consists of three minimally segmented planes of scintillation
counters equipped with photomultipliers at both ends, and it includes Cherenkov counters
(gas CO2 and Aerogel).
In addition, a proton polarimeter is installed in the back of the detector package
to measure the polarization of the proton using a segmented carbon analyzer up to 60
cm in thickness to allow measurements over a wide range of proton energies. A pair
of front and a pair of rear straw chambers determine the incident and scattered angles,
respectively. The third scintillation counter, located at the rear end, provides the trigger
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
Figure 12.4: The hadron spectrometer detector stack.
238
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
239
for the polarimeter. The polarimeter detectors are dimensioned to accept a 20◦ cone of
scattered protons.
Several support systems are necessary in addition to the basic components mentioned
above. They include gas supply systems for the wire chambers, high voltage supplies,
readout electronics, a second level trigger, software for data analysis and testing, and a
remotely controllable mechanical system.
As for the electron spectrometer, all detectors are mounted on a single rigid carriage
along with their associated electronics. The FPP components are mounted on an FPP
subframe for installation and removal as a unit. The trigger electronics are located next
to the detectors, as for the electron arm.
To reduce the resolution degrading effects of multiple scattering, the entire interior
of the spectrometer from the pivot to the detector hut is a vacuum vessel. The ends of
this evacuated volume are capped by relatively thin vacuum windows.
As mentioned, subsystems will be discussed in more detail in the next three sections. The remainder of this section will describe some features common to the two
spectrometers, then the following major sections will be devoted to the specifics that are
not common.
12.1
Safety with Regards to the Spectrometer
The principle concern with the spectrometers is that they are large, and have associated vacuum, hydraulic, cryogenic and magnet systems all of which can be potentially
dangerous.
The bogies which move the massive 1200 ton spectrometers must be carefully operated. Inspection of the wheels to ensure there is no debris which the wheels could
ride over is mandatory. Similarly personnel need to be aware that the spectrometers are
moving so that no one inadvertently gets trapped.
The vacuum systems associated with the spectrometers are essentially pressure vessels. Care should be exercised so as not to puncture the windows.
The magnets themselves are installed inside cryostats. These vessels are exposed to
high pressures and are therefore equipped with safety relief valves and burst discs.
The hydraulic system that operates the vertical positioning system VPS and the
horizontal positioning system HPS operates at high pressure, 3000 - 5000 psi. Therefore
one should be careful when operating those systems.
The cryogenic system operates at elevated pressure at 4K. One must guard against
cold burns and take the normal precautions with pressure vessels when operating this
system. Only WBS7 are permitted to install and take out U tubes.
The magnets have a great deal of stored energy as they are large inductors. Always
make sure people are clear of them and that the dump resistor is attached to the magnet.
There are several major safety concerns with regards to the detectors, namely 1)
flammable gas located in the VDC and FPP, 2) ODH hazard due to CO2 in the Cherenkov
counter, 3) high voltage due to the photo multipliers on the various detectors and 4)
a thin vacuum window separating the detector array from the vacuum system in the
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
240
spectrometers. The clean agent fire suppression system, while installed to suppress fires,
can also be a safety hazard. It is possible for an individual to drop down alongside the
box beam to the gantry roof inside the shield house. This area, although technically not
a confined space, could conceivably become one in the event that the clean agent system
was attached. Personnel should have a 5 minute air pack with them in the event they
must enter the area alongside the box beam to the gantry roof inside the shield house.
12.2
Hall A Vacuum System
The Hall A vacuum system consists of 5 separate but interconnected subsystems. The
largest is designed to supply the Hall A HRS with a self contained 5 × 10−6 Torr vacuum
that enables both spectrometers to be pumped down from atm. in a few hours. The
target vacuum system is designed to maintain a 1 × 10−6 Torr in order to minimize
contamination and provide an insulating vacuum for the cryo target. Rough insulating
vacuum for the 4 superconducting magnets is provided by a 360 cf m Roots type blower
that can be connected to each magnet. The beam line vacuum is maintained by 1 `/s
ion pump system used in the accelerator ring and a small turbo pump located near the
target. The final subsystem is a differential pumping station located near the target exit
port.
12.2.1
Spectrometer Vacuum System
The spectrometer vacuum system is shown in Figure 12.5. Vacuum for the HRS is
supplied by an Alcatel 880 `/s Turbo pump backed by a Balzers 360 cf m Roots type
Blower. This Blower, via a special manifold, also supplies the roughing vacuum to the
HRS at the Dipole Inlet Transition. The first Turbo is mounted on the lower side of the
Dipole entrance transition. The roughing port is also located on this transition, on the
top side. The upper turbo is located on the lower side of the window transition.
Vacuum readouts and interlock outputs are supplied by five (5) HPS series 421 Cold
Cathode gauges and seven (7) series 275 Mini-Convectron gauges. In addition to these
there will also be a FIsons Micromass 386 RGA head installed in the system for diagnostic
purposes. Most of this instrumentation will be located on the Turbo pump manifold (for
detailed information see Figure 12.5).
Powered valves, instrumentation and pumps will be controlled and powered at the
Vacuum System equipment rack located on each respective spectrometer on the gantry
platform. Selective equipment will also be controllable from the Hall A counting house.
Chamber
The HRS vacuum chamber consists of an associated vacuum window, a sieve slit
and Q1 transition, Q1 to Q2 transition, Spool section, Dipole transition, Dipole to Q3
transition, and the Q3 to exit window assembly. The spectrometer vacuum is contained
by a .007 kapton window at the entrance and a .004 titanium window at the exit.
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
Figure 12.5: HRS vacuum system.
241
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
12.2.2
242
Target Vacuum System
Vacuum for the target chamber is supplied by an Alcatell 880 `/s Turbo pump backed by
an Alcatell 21 cf m 2 stage vane pump. The Turbo is mounted on the lower ring of the
Target Chamber to one side so as not to interfere with the Target Chamber windows.
The same instrumentation is used here as on the spectrometer.
Powered valves, instrumentation and pumps will be controlled and powered at the
Vacuum System equipment rack located on the access Balcony. Selective equipment will
also be controllable from the Hall A counting house.
12.2.3
Magnet Vacuum System
Vacuum for the magnet insulating vacuum is provided by the Cryo pumping effects of
each individual magnet.
All controls for the Magnets are manual as we expect no problem after initial pump
down.
The insulating vacuum for each magnet is self contained within the magnet.
12.2.4
Beam Line Vacuum System
Vacuum for the entrance beam line is supplied by 65 `/s Balzers turbo pumps, the first
of which is located on the E P chamber, and the second located 3 m upstream of the
target chamber. Both turbos are equipped with a HPS 7 Series 275 mini Convectron
gauge and a HPS series 421 Cold Cathode gauge located near the balcony.
Vacuum readouts and relay outputs for interlocks are supplied by HPS series 421
Cold Cathode gauges. In addition to these there will also be Convectron gauges. Most
of this instrumentation will be located on the Turbo pump manifold.
Powered valves, instrumentation and pumps will be controlled and powered at the
Vacuum System equipment rack located on the Balcony. Selective equipment will also
be monitored from the Hall A counting house. All control is by Accelerator in the MCC.
12.2.5
Beam Exit Vacuum System
Vacuum for the target chamber is supplied by an Alcatell 880 `/s Turbo pump backed
by an Alcatell 21 cf m 2 stage vane pump which maintains a 1x10−4 vacuum on the exit
beam pipe.
Between the target chamber and the exit beam pipe there is a .007 in kapton window
that has a .0375 in hole in it at the beam spot. This window acts as a differential pumping
station.
Also between the target chamber and the exit beam pipe is an 8 in air actuated gate
valve that is operated from the MCC.
Vacuum readouts and interlocks outputs are supplied by an HPS 7 Series 275 mini
Convectron and an HPS series 421 Cold Cathode gauge which are located near the
balcony.
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
243
Controls are interlocked to the beam.
The chamber is made of a low mass aluminum corrugated vacuum tube of 1 m
diameter.
At the exit point of the exit beam pipe is a beam diffuser that consists of 2 .025
in beryllium windows with a water filled cavity between them for cooling. The water is
circulated through the cavity by a water cooling system located on the Hall floor, and is
interlocked through the FSD system with 2 flow switches, one on the supply and one on
the return line.
Due to high radiation levels at the exit beam pipe all seals in this area are metal.
12.2.6
Hazards of Vacuum Systems
Hazards associated with the vacuum system are due to rapid decompression in case of a
window failure. Loud noise can cause hearing loss. To mitigate the hazard, all personnel
in the vicinity of the large chamber with a window are required to wear ear protection
when the chamber is under vacuum. Warning signs must be posted at the area.
The scattering chamber is equipped with a large 10 mil aluminum window that
allows the spectrometers to swing from 12.5◦ to 165◦ on the EA and 12.5◦ to 140◦ on
the HA. In order to protect this window when the Hall is open, lexan window guards are
installed.
At the inlet of the sieve slit a Møller 8” diameter 7 mil kapton window is provided
to separate the target chamber from the spectrometers.
Finally, under the detectors, a 4 mil titanium window is provided. Eventually this
will be replaced with a low mass mylar/kevlar window.
The 1 `/s vac ion and the cold cathode gauges operate at several KV; consequently
there is also a shock hazard.
Additionally, all vacuum vessels and piping are designed as pressure vessels.
12.3
The High Resolution Spectrometer (HRS)
The HRS is composed of three superconducting quadrupole magnets, Q1, Q2, and Q3,
and one superconducting dipole magnet. The large quadrupoles were manufactured for
TJNAF by SIEMENS, the small quadrupole by SACLAY, while the dipole was built for
TJNAF by WANG NMR. The quadrupole magnets are referred to as Q1, Q2, and Q3,
where a particle first traverses Q1, then Q2 and the dipole magnet and finally traverses
Q3.
The magnet system is followed by a large steel and concrete detector hut, in which
all detector elements reside. Most of the detector elements have been built by universities
involved in the Hall A physics program.
The HRS magnet system is the cornerstone of the Hall A activities. Many of the
experiments approved in Hall A center on physics at high resolution and other short-range
phenomena, and rely on a spectrometer able to momentum analyze charged particles up
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
244
to very high momenta. The design value for the maximum momentum accessible to the
HRS magnet system is 4 GeV/c.
12.3.1
Magnets and Power Supplies
The HRS magnet’s are all superconducting and hence their coils must be maintained at
cryogenic temperatures during operations. The LHe required by the magnets is supplied
by the End Station Refrigerator, ESR.
All the HRS magnets cryogenic services are supplied through the overhead cryogenic
lines. The distribution network begins at the distribution box over the pivot. This box
is connected to the rest of the network via the flexible transfer lines over the pivot. The
network is adjacent to the upstairs catwalk of the HRS.
Cryogenic information about each magnet is available on the control screens in the
counting house, one for each magnet. Normally during run periods the control screens
are sent upstairs to the Hall A counting house and information on all the HRS magnets
is available on the HRS control screen located in the center of the main console. The
control of all magnets is described in a following Section.
The power supplies for the magnets are located on the gantry balcony adjacent to
the magnets. The supplies are all cooled with LCW.
The front panels of the power supplies are interlocked. Under no circumstances
should the front panel of any supply be opened by anyone other than authorized personnel. There is a keyed electrical interlock located in the Hall A counting house main
console to prevent the power supplies from being energized at inappropriate times. There
are also signs posted listing the dangers of high magnetic fields.
The control interface for the power supplies is available through the HRS control
screen in the Hall A counting house.
12.3.2
Personnel
In the event that problems arise during operation of the magnets, qualified personnel
should be notified. This includes any prolonged or serious problem with the source of
magnet cryogens (the ESR). On weekends and after hours there will be a designated
individual on call for magnet services. Any member of the Hall A engineering group is
qualified to deal with unusual magnet situations but in the event of serious problems the
technician on call should be contacted.
12.3.3
Quadrupole Magnets
The quadrupoles provide some of the focusing properties of the spectrometer and to a
large extent its acceptance. Operating limits imposed on the quads are as follows: 1850A
for Q2 and Q3 and 3250A for Q1.
All three quadrupoles for the HRS spectrometer are warm iron superconducting
magnets. The soft iron around the superconducting coil enhances the field at the coil
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
245
center and reduces stray fields. The basic parameters for the first quadrupole, Q1, are
an effective length of 0.9 m, useful aperture of 0.3 m and a field gradient of 9.5 T/m. To
achieve the lowest possible angle setting of the HRS spectrometer (with respect to the
beam line) the incident electron beam passes through a notch in the outer yoke of Q1
when the spectrometer is at its smallest angle of 12.5◦ . The other two quadrupoles Q2
and Q3, are essentially identical with an effective (magnetic) length of about 1.8 meter,
a useful aperture of 0.6 m and a field gradient of 3.5 T/m.
The maximum operating currents (assuming a 4 GeV/c momentum particle) for the
quadrupoles are about 3000 A, 1700 A, and 1600 A, for Q1, Q2, and Q3, respectively.
This will render pole field values of 1.2, 1.0, and 1.0 T, respectively. The energy stored
in the quadrupole fields is sufficient to cause an unrecoverable quench if all the energy
stored is dumped into the magnets. Therefore a quench protection circuit is incorporated.
However, a quench can only happen if the cryomagnets have a helium level below the
coil 60% during operation.
The operating current to the Q1 quadrupole coils is provided by Danfysik System
8000 power supplies, which can operate up to 3500 A current and 5 V. The power supplies
will be cooled with a combined maximum water flow of 45 liters per minute.
In addition to the main quadrupole windings, all quadrupoles have multipole windings. To further optimize focusing properties of the HRS magnet system, it was intended
to operate including some of these multipole trim coils in order to reduce higher order
aberrations. The operating current for these multipole corrections is small, only (the multipole corrections are typically less than 2% of the main quadrupole field), of order 50 A,
and will be provided by thirty two Lakeshore power supplies. These power supplies can
operate up to 100 A current and 30 V voltage. Since the sextupoles were inadvertently
installed rotated 90 ◦ from their correct orientation, these trim coils are now considered
useless and there are at present no plans to use them.
12.3.4
Cryogenic Procedures
All cryogenics control is handled by WBS7. The cryo control coordinator can be reached
at the CHL (x7405) or by calling the MCC.
12.3.5
First Time Startup Check List.
See attached check lists for all quadrupole and dipole magnets (Tables 12.1, 12.2, and
12.3).
12.3.6
Dipole Magnet
The dipole, by virtue of its field index, provides both dispersion and focusing. The
present operations envelope states that the supply for the electron dipole may not be
operated at a current above 1800 A (4.4 GeV/c). The supply for the hadron dipole may
not be operated above 1200 A (3.2 GeV/c).
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
246
The dipole for the HRS spectrometer is a superconducting, cryostable magnet. Its
basic parameters are an effective length of 6.6 m, a bend radius of 8.4 m, and a gap width
of 25 cm. It is configured to achieve a 45 degree bending angle for 4 GeV/c momentum
particles at a central field excitation of 1.6 T. For the HRS dipole to reach 1.6 T an
operating current of about 1500 A is required.
The dipole has been designed to achieve cryostability up to a field of 2 T, and
this property has been extensively tested up to a field of 1.6 T. The cryostable coils
are equipped with an energy removal circuit to cover the possibility of an unrecoverable
quench. However, this can only happen if the helium level drops below the coil during
operation. The current to the coils will be provided by a Dynapower System power
supply, which can operate up to 2000 A and 10 V. This power supply is located on the
gantry beside the dipole, and will be cooled with a maximum water flow of 35 liters per
minute. The flow of the magnet cooling water will be regulated by flow meters installed
on the floor of Hall A. The total water flow needed to cool the 4 power supplies for the
HRS magnet system (dipole and quadrupoles) amounts to 80 liters per minute, with a
supply pressure of cooling water for Hall A of 100 psi.
12.4
Operation of the HRS Magnets
Introduction
This is an abbreviated operating manual for the HRS superconducting magnets
specifically designed for Hall A experimenters. It provides instructions for setting currents, invoking NMR field regulation and general system monitoring. Curious readers
are directed to the references for more in-depth operating instructions and other technical manuals. Copies of the following supporting documents are available in the Hall A
Control Room.
References
WANG NMR Dipole Operation Manual Power Supply
Dynapower
User Manual
Appendix
NMR Tesla meter
Appendix
NMR Field REgulation
Siemens/Fug
Q2/Q3 Power Supply Manual
Saclay/Danfusik
Q1 Powersupply Manual
TOSP
HRS Dipole
TOSP
HRS Quadrupole Q1
TOSP
HRS Quadrupole Q2, Q3
HRS
SC Dipole Magnet Safety Review Vol. 2
HRS
SC Quad Safety Review Vol. 1
Starting Hall A Controls The following is an abbreviated operational manual for the
magnets supplied by Javier Gomez.
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
247
Circuit Parameters:
Imax = 1800 A, L = 2.52H, Tau(slow) = 420 s, 640 MITS
Rdish = dump resistor + cable resistance = total resistance
Rtotal = .0045 + .0015 = .006 Ω
Rdump .134 Ω , L= 2.55 H, Tau(fast) = 19 s, 29 MITS
Magnet Dipole
signature, date
Arm (Circle one)-Electron Arm, Hadron Arm
Megger check of coil @ 250 V DC
Visual inspection walk through
Set water inlet pressure to 100 psi
Coil A Trip Voltage (1.2V+) value
Coil B Trip Voltage (1.2V-) value
Magnet Lead A Trip (1.2V+)
Magnet Lead B Trip (70mV) value
Magnet Leads are not bipolar and only work in the PS forward polarity.
Magnet lead A must be connected to the PS+
Level Trip (70%) value
Magnet Flow A Trip (60 SLPM) value
Magnet Flow B Trip (60 SLPM) value
Operational Test of trips
PS overcurrent trip (2000A)
See manual for voltage setting and gain (4V=800A)
Magnet Ready for Operation
Table 12.1: Hall A Dipole Magnet Check List (15 August 1996)
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
Circuit Parameters:
Rext = 0.075 Ω, L = 25 mH, Tau = 0.3 s, Vthreshold = .1V Imax = 3250A
Magnet (Circle one) - Q1,
Arm (Circle one) - Electron Arm, Hadron Arm
Visual inspection walk through
Set water pressure at 100 psi inlet
Coil A Trip Voltage + value 25mV
Coil B Trip Voltage (-) value 25 mV
Magnet Lead A Trip (+) Voltage 80 mV
Magnet Lead B Trip (-) Voltage 80 mV
Level Trip percent 980%)
Magnet Flow A Trip (30 SLPM+) setting
Magnet Flow B Trip (30 SLPM-) setting
Operational test of trips
Magnet Ready for Operation
Table 12.2: Hall A Q1 Quadrupole Magnet Check List (15 August 1996)
Circuit Parameters:
Rext = .125 Ω, Tau = 3 s, Quench Vthreshold = .1V, Imax = 1850A
Magnet (Circle one) - Q2, Q3
Arm (Circle one)-Electron Arm, Hadron Arm
Visual inspection walk through
Meggelr Magnet @ 250 V DC
Set water pressure at 100 psi inlet
Coil A Trip Voltage (+) value
Coil B Trip Voltage (-) value
Magnet Lead A Trip (+) Voltage
Magnet Lead B Trip (-) Voltage
Magnet Lead Trip (trim) Voltage
Level Trip percent (80%)
Magnet Flow A Trip (50 SLPM+) setting
Magnet Flow B Trip (50 SLPM-) setting
Magnet Flow trim Trip (3.6 SLPM) setting
Operational test of trips
Magnet Ready for Operation
Table 12.3: Hall A Q2/Q3 Quadrupole Magnet Check List (15 August 1996)
248
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
249
On hac x-terminal
Account: hacuser
Password: hacuser
Type: HAC xt or HAC hp (as per instructions when you log-on). A Hall-A Main
Control Window pops up, and all subsystem control windows can be accessed via pull
down menus from there.
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
250
Figure 12.6: Magnet Portion of Main Hall A Control Screen.
3 4
12.5
Field Monitoring
12.5.1
Simple Spectrometer Field Setting (Autopilot Mode)
(All you need when everything is working and power supplies are turned on and ready
to go.)
On the Hall A main control screen there is a rectangular box for each spectrometer that
looks similar to the illustration (see Figure 12.6).
This box displays a brief summary of the status of the spectrometer magnets and
their cryogenic systems. The blue fields (with white numbers) give readbacks of the
magnetic fields and currents in each magnet. The black fields also give readbacks, however
in this case if the text appears green those parameters are OK while if they are red then
that parameter is out of tolerance and may indicate a fault condition. For example if the
helium level goes below a certain point the magnet will be automatically turned off. In
some cases it may be desirable to monitor certain critical quantities on a strip chart (e.g.
Magnet settings). A strip chart tool is available for this purpose from the bottom of the
main control screen.
To set the spectrometers for a given value of central momentum (P0) type the
desired P0 value into the yellow P0 SET box and hit return. The magnets will be
automatically set to the correct values. All green numbers in the P0 column indicates that
the desired field or current settings have been reached. Caution: Re dipoles, in general
3
4
CV S revision Id: nmr-1999.tex,v 1.1 2003/06/06 15:44:08 gen Exp
Authors: J.LeRose mailto:[email protected]
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
Teslameter
Slow Controls
MUX 2031
MUX 2032
251
Purcell Gap Probes
(Group 1)
Spares
Spare Channels
Spares
MUX 2031
Gap Probes
Vacuum
Box
(Group 0)
Figure 12.7: Basic layout of NMR system
it’s a bad idea to assume that at the first instant that the P0 display turns green that the
desired field has been reached and you can start taking data. Stable field is in general not
achieved for from 15 to 30 minutes after reaching the nominal desired field. This settling
time depends on the magnet (Hadron is slower than Electron) and the magnitude of the
field change (small changes settle faster than big changes). Experimenters are advised to
observe both the field reading and current reading on the magnet in question and verify
that things are stable to their satisfaction before proceeding.
12.5.2
Dipole Field Monitoring Electron Arm
(see special instructions for running the Hadron Dipole in field regulation mode)
Basic Setup
Each spectrometer dipole magnet is equipped with a Metrolab PT 4025 NMR Teslameter, several field probes, and multiplexers (to allow switching between the probes).
Details of the operation and theory of operation for the Teslameter can be found in its
user manual, a copy of which is available in the the counting house. The basic layout is
shown in Figure 12.7
The ”Gap Probes” (Group 0 in the controls) are located in two groups of three; one
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
C o il P a c k
C o il P a c k
(3 c o ils)
(3 c o ils)
252
3 .2 5 Ω
V a ria b le P /S
1 0 V D C M a x C o n tro l S ig n a l
Figure 12.8: Gradient Compensating Circuit.
Table 12.4: Dipole NMR probe field ranges
Probe Type Field Range (T)
3
0.17 - 0.52
4
0.35 - 1.05
5
0.70 - 2.10
group on the low field side of the gap and the other on the high field side of the gap.
The groups of three are made up of one each of the manufacturer’s type 3, 4 & 5 probes,
designed to cover different field ranges (see Table 1). The six “Purcell Gap Probes”
(Group 1 in the controls) are located in the Purcell gap of the magnet and consists of
two each of the above types. Note: Since the fall of 1998 the multiplexer-multiplexer in
the electron arm, MUX 2032, has been bypassed and hence the “Purcell Gap Probes” are
currently unavailable. There are no plans to fix this multiplexer in the immediate future.
The ”Gap Probes” are equipped with coils which provide a field gradient that cancels out the field gradient of the magnet in the vicinity of the probe. These gradient
compensating coils are part of a simple circuit that is completely independent of the
Teslameter. The basic circuit for the compensating coils is shown in Figure 12.8
The following graphs (see Figures 12.9, and 12.10can be used to determine optimum
values for the compensating coil control voltage. It should be noted that the setting of
the compensating coil current is not very critical in most cases. In general if you’re within
10% of the correct value everything should work fine.
12.5.3
Authorized Personnel
The following individuals are responsible for NMR operation problems.
J. Gomez - x7498
J. LeRose -x7624
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
Figure 12.9: Control Voltage calibration for Electron Dipole.
253
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
Figure 12.10: Control Voltage calibration for Hadron Dipole.
254
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
4500
4000
3500
3000
DAC #
2500
2000
Type 3
1500
Fit 3
Type 4
Fit 4
1000
Type 5
Fit 5
500
0
0
0.5
1
1.5
2
B (Tesla)
DAC = C 0 + C1 ⋅ B + C 2 ⋅ B 2 + C 3 ⋅ B 3
Probe type
3
4
5
C0
-5157.04
-5157.39
-5156.74
C1
45026.48
22514.30
11255.70
C2
-95678.57
-23920.73
-5979.12
C3
82559.35
10320.28
1289.79
Group 0 Probes Group 1 Probes
0,3
1,4
2,5
Figure 12.11: DAC Calibration for manual operation of NMR probes.
255
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
12.5.4
256
NMR Operating Procedure:
When running in Autopilot mode (see: Simple Spectrometer Field Setting) the compensating coil voltage is set automatically and the probe appropriate for the field desired
is selected. The gaussmeter is placed in SEARCH Mode and the dipole power supply
regulator is turned on. In this case the dipole current is adjusted to achieve the desired
field. If the NMR gaussmeter is not ”locked” a backup Hall Probe is used until the NMR
”locks”. The user should just stand back and let it work. What follows are instructions
for using the NMR gaussmeter in situations where Autopilot doesn’t work or some special
supplemental measurements are required.
In principle it is possible to make the field measurements using the SEARCH mode
in the Teslameter. In this mode you select a probe and the meter explores the whole
field range of the probe until it finds and ”locks” on the resonant signal indicating that
it has a field measurement. A ”lock” is indicated on the controls display by positive field
values. This has the advantage of simplicity but in practice can be time consuming and
doesn’t always work. The problem being, in situations where there is a lot of noise mixed
in with the signal, the circuitry has problems distinguishing the signal from the noise
and gets lost before it ever finds a lock. The problem is exacerbated when the field being
measured is at the high end of the probe’s range. In this case the search starts at the low
end and keeps getting hung up on the noise and never gets to the field range of interest.
The solution to this problem is to tell the device approximately what field it’s looking
for and use the AUTO mode to find the lock. In the procedure below that is what we
will be doing.
In any case, for ”gap probes” (group 0) you must energize and adjust the gradient
compensating coils for the field ranges to be measured before trying to make measurement.
For studies involving 10% changes in the field settings the compensating coil current
can be set once and left alone.
Recommended Procedure:(turn the REGULATOR OFF for all non-autopilot field
measurements)
For group 0 probes set compensating coils appropriately (see figures).
Put meter in MANUAL mode with SEARCH OFF
Select a probe and polarity (Group 0: Probes 0, 1, 2 negative; Probes 3, 4, 5 positive)
Type in DAC number for the field range being measured (see below)
Select AUTO and wait for a lock (positive field reading)
Verify that you have a good lock by checking the oscilloscope for a clear resonant signal.
Go back to 2. for the next probe
If you have problems see the table listing problems and possible solutions.
Selecting DAC #’s
In selecting the DAC # to use for the field of interest use either the graph in Figure 12.11 or the polynomial below that.
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
257
Problems and Solutions
Symptom
Weird numbers on displays, controls for
all magnets fouled up
NMR Teslameter does not respond to
commands and display shows all zeros.
Will not lock
Still will not lock
You find resonance manually
but still can’t get a lock
Can’t find resonance manually
Diagnosis and Cure
Need to reboot.
See instructions below.
Meter’s communications are
somehow hung up.
Push RESET.
Very high noise level
makes resonance hard to find.
Very high noise level makes
resonance hard to find. Search
for the resonance manually by
adjusting the DAC in manual
mode until you see the resonant
signal. (It helps if you know
what field you expect so you’ll
know where to look).
Check probe polarity.
Try decreasing and
increasing DAC number by 1.
Optimize signal by adjusting
compensating coils.
Try a different probe. Use
readings from other probes to
tell you where to look for
the resonance with the probe
that’s giving you trouble.
Make sure compensating coils are
energized properly.
Make sure magnet is on.
Hadron Dipole Field Setting Instructions: Turn on power supply (contact Mark
Stevens, John LeRose, or Javier Gomez for assistance).
Turn on field regulation mode. Type desired field value into FieldSet (yellow field).
Wait for field to reach the desired value (read from the blue field). After reaching the
desired field wait 15 minutes to assure stable field. Pay attention to the current and field
readings. If the NMR is locked, (the NMR is locked if the field reading in the blue field
has a positive number and is updating itself), don’t worry it will get there.
If the NMR is not locked (negative but updating number in the blue field) but the
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
258
current appears to be going in the right direction, don’t worry it will get there and the
NMR should lock when you get into the vicinity of the desired field. If you’re worried
anyway and want to watch the field change, push SEARCH. (The NMR will search over
the full range of the probe rather than just in the vicinity of the desired field.) The NMR
should lock in a minute or two.
If the NMR is not locked and the current is going in the wrong direction (This
can happen if you ask for a new field before you reach a stable setting), you must push
SEARCH. (The NMR will search over the full range of the probe rather than just in
the vicinity of the desired field.) The NMR should lock in a minute or two and the
software will correct the current setting appropriately but slowly. If the NMR doesn’t
lock you may have to ask for a field value appropriate for the present current (∼ 1.1 ×
10−3 T/Amp). The NMR will only search in the limited region of the requested field
and should find a lock more easily. After getting the lock let things settle in for a few
minutes and then ask for the field you want again. In extreme cases you may have to
nurse it through the transition by asking for multiple small increments. If you ask for a
change of less than 5% the NMR should not lose its lock.
To shut down: Set FieldSet to .18. Let current run down to just over 100 Amps. Then
push stop on Dipole control screen
Problems
NMR not locked but
current is changing in
the right direction
Explanation
Normal operation for
large field
changes
Action
Wait. (see above)
NMR locked but current
going in the wrong
direction.
NMR locked but field not
correct and current not
changing
Normal operation.
Wait.
Field regulation is
disabled or software
is confused.
Check that field regulation
is enabled. Enter desired
field value or one
very near the
desired value again.
NMR field display freezes. NMR Gaussmeter is not Push RESET.
(Usually but not always
communicating with
shows -#.0000000)
software.
Table 12.5: NMR troubleshhoting
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
12.5.5
259
Powering Up Dipole Magnets:
Use these instructions to recover from loss of a magnet due to a fault (e.g. He level or
lead flow fault). The order of actions matters.
(Contact Tech on call if anything behaves funny or things don’t respond as expected.
Sometimes after a trip an access to the Hall is required to reset things)
1. Wait for Iout=0 (you can’t and don’t want to do anything while the magnet is in
emergency fast dump mode.)
2. While waiting, make a log entry re the fault. Give details such as time, coincident
activities, and nature of the fault.
3. Make sure the fault is cleared. (e.g. He level and flow rates returned to normal values
and stable)
4. In the HRS Hadron (Electron) Dipole Systems’ control panel:
a) Press RESET (verify that all faults are cleared in the middle column)
b) Press START (Display will indicate Power Supply ON and magnet ENGAGED)
Power supply and magnet are ready to go. From here you can return to ”Autopilot
Mode” (type in desired P0 on control screen and wait) or proceed as described below.
For Electron dipole type in desired current in I Set and the power supply will respond.
For Hadron dipole go to HRS HADRON DIPOLE NMR control panel.
1. Press RESET
2. Press Reg. Enable YES
3. Press Search ON (should already be on but it doesn’t hurt to check)
4. Type desired field into Field Set (T) field.
5. Wait for field to reach the desired value. (In general it’s a good idea to wait about
15 minutes after first reaching the desired field before taking data.) See Hadron Dipole
Field Setting for more details and help with trouble shooting.
12.5.6
Starting Q1 Power Supply:
Do this when a fault causes the power supply to shut off.
Wait for fault to clear (watch He levels).
1. Push RESET (check all faults cleared)
2. Select desired polarity
3. Push ON
4. Type in ISET (yellow field) or re-enter P0 in Autopilot Mode).
12.5.7
Starting Q2/3 Power Supply:
Do this when a fault causes the power supply to shut off.
1. Wait for cause of fault to clear (e.g. low Helium level)
2. Press RESET
3. Select polarity
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
4. Press ON
5. Type in ISET (yellow field) or re-enter P0 in Autopilot Mode.
260
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
12.6
Collimators and Sieve Slits
261
5 6
Both spectrometers have front-end devices for calibrating the optical properties of the
spectrometers. These are known as the collimator boxes. These boxes are positioned
between the scattering chamber and the first quadrupoles (Q1). Each box is carefully
aligned and rigidly attached to the entrance flange of the Q1 of the respective spectrometer. The boxes are part of the vacuum system of the spectrometer.
Inside each box a ladder is mounted which is guided by a linear bearing and moved
up and down by a ball screw. On this ladder 3 positions are available to insert collimators. Below this ladder a special valve is mounted that can isolate the vacuum in the
spectrometer from the target system. This valve should be activated when it is moved in
front of the holes connecting the box with spectrometer and target chamber. A schematic
view of the collimator box is shown in Fig. 12.12.
Vacuum requirement is 10−6 Torr. The material for the box is aluminum. It is possible to open one side of the box so that collimators can be exchanged. The reproducibility
of collimator positions after moving the ladder and/or after replacing a collimator is better than 0.1 mm in horizontal and vertical direction. The dimensions of the box are
roughly height=175 cm , width=35 cm and depth=15 cm. The tolerance in the dimension of the 7 msr collimator hole is ±0.5 mm in each direction. The tolerance in the
position of each of the sieve-slit holes is ±0.1 mm in each direction.
A typical sieve slit collimator is shown in Fig. 12.13. It consists of a plate of roughly
14 cm x 20 cm containing 49 holes positioned in a regular 7x7 pattern. This slit is made
out of 5 mm thick tungsten. The holes have a diameter of 2 mm except for the central
one and one positioned off-diagonal which have a diameter of 4 mm. The horizontal
distance between the holes is 12.5 mm while the vertical distance is 25.0 mm.
To get the latest information on the dimensions and locations of the collimators see
the Hall A homepage on the web7 .
12.6.1
Authorized Personnel
E. Folts - x7857 (mechanical and vacuum systems).
J. Gomez - x7498 (computer controls and electrical systems).
12.6.2
Safety Assessment
The collimator boxes form part of the vacuum system for each spectrometer. All hazards
identified in section spectrometer vacuum section applies to the collimator box as well.
In addition, safe access to the top of the collimator boxes is needed during manual
operation of the box as outlined below. Due to the proximity of the collimator boxes
5
CV S revision Id: slit.tex,v 1.3 2003/06/06 16:13:37 gen Exp
Authors: J.LeRose mailto:[email protected]
7
http://hallaweb.jlab.org/
6
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
Figure 12.12: Schematic layout of the collimator box.
262
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
Figure 12.13: Sieve slit collimator for optics calibration.
263
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
264
to the scattering chamber, and Q1 quadrupoles, all necessary safety precautions with
regards to vacuum windows, electrical power cables, cryogenic transfer lines, and high
magnetic field should be taken.
12.6.3
Operating Procedure
Slit position is changed remotely from the standard Hall A control screen.
CHAPTER 12. HIGH RESOLUTION SPECTROMETERS (HRS)
12.7
Spectrometer Alignment
265
8 9
At present, the systems implemented to determine the alignment of each spectrometer
(roll, vertical angle/pointing and horizontal angle/pointing) without the help of the Accelerator Division Survey group are limited to roll, vertical angle and horizontal angle. All
alignment information is displayed in the “ALIGNMENT” mosaic of the Tools MEDM
screen (see Fig. 21.4) (“Hall A Menu −− > “Tools”).
A bi-axial inclinometer is used to determine the roll and vertical angle (also known
as pitch) of each spectrometer. These inclinometers are attached to the back of the
dipoles at the power supply platform level. The raw inclinometer measurements, in Volts,
are displayed as “Tilt X” and “Tilt Y”. The inclinometer temperature is also given (“
Tilt T”), in degree Celsius. From these values, the “ROLL” and “PITCH” values are
calculated Agreement between the inclinometer readings and survey measurements are
better than ± 0.1 mrad over all presently available history.
The horizontal spectrometer angle is determined from floor marks set in place by
the survey group. Floor marks have been placed every 0.5 ◦ covering the useful range
of both spectrometers. There are two concentric rings of floor marks in the hall. We
will concentrate in the inner ring which covers the angular range of both spectrometers.
The outer-ring covers only small angular sections but these floor marks are made on
metal plates which allow to read them with higher resolution. The inner-ring floor marks
are located at a distance of ∼10 m from the target center. A ruler attached to each
spectrometer dipole runs over the floor marks and it acts as a vernier to interpolate
between marks. The location of a given floor mark on the ruler can be viewed from
the Hall A Counting House through a TV camera (labeled “Front Camera”) . The
camera is able to move along the length of the ruler so that any parallax effect can
be eliminated. The camera motion is controlled from the “Tools” screen through two
push buttons (“FRONT CAMERA” - “MOVE +” and “MOVE –”). Two fields in the
“ALIGNMENT” mosaic (“Flr Mrk” and “Vernier”) allow to input the values read from
the TV monitor. The effective spectrometer angle is then calculated and displayed as
“Angle”. The application “HRS Floor Marks” calculates the floor mark and vernier value
to which the spectrometer should be set to obtain a given angle. Spectrometer horizontal
angle surveys and floor mark determinations agree to ± 0.2 mrad.
12.7.1
Personnel Responsible
J. Gomez (pager: 849-7498).
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HRS Detectors
266
Chapter 13
Overview 1 2
The detector packages of the two spectrometers are designed to perform various
functions in the characterization of charged particles passing through the spectrometer.
These include: providing a trigger to activate the data-acquisition electronics, collecting
tracking information (position and direction), precise timing for time-of-flight measurements and coincidence determination, and identification of the scattered particles. The
timing information is provided from scintillators, as well as the main trigger. The particle
identification is obtained from a variety of Čerenkov type detectors (aerogel and gas) and
lead-glass shower counters. A pair of VDCs provides tracking information. The main
part of the detector package in the two spectrometers (trigger scintillators and VDCs) is
identical; the arrangement of particle-identification detectors differs slightly. The HRS-L
can be equipped with a focal-plane polarimeter to determine the polarization of detected
protons. The focal-plane-polarimeter operates with proton momenta up to 3 GeV/c with
figure-of-merit of 0.03. The side view of the detector stacks are shown in Fig. 13.1.
The optics of the HRS spectrometers, results in a narrow distribution of particle
trajectories in the transverse direction, leading to an aspect ratio of the beam envelope
of about 20:1 at the beginning of the detector package and 4:1 at the end.
The detector package and all data-acquisition (DAQ) electronics are located inside
a Shield Hut (SH) to protect the detector against radiation background. The SH is also
equipped with air conditioning and fire suppression systems. The individual detectors
are installed on a retractable frame, so that they can be moved out of the SH for repair
or reconfiguration. The DAQ electronics are mounted on the same frame.
The concept of VDCs fits well into the scheme of a spectrometer with a small acceptance, allowing a simple analysis algorithm and high efficiency, because multiple tracks
are rare. The VDCs are bolted to an aluminum frame, which slides on Thomson rails
attached to the box beam. Each VDC can be removed from its SH for repair using these
Thomson rails. The position of each VDC relative to the box beam can be reproduced
to within 100 µm.
There are two primary trigger scintillator planes (S1 and S2), separated by a distance
1
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CHAPTER 13. CHAPTER
Figure 13.1: The side view of the detector stacks.
268
CHAPTER 13. CHAPTER
269
of about 2 m. The long path from the target to the HRS focal plane (25 m) allows
accurate time-of-flight identification in coincidence experiments if the accidental rate is
low. After correcting for differences in trajectory lengths, a TOF resolution of ∼ 0.5 ns
(σ) is obtained. The time-of-flight between the S1 and S2 planes is also used to measure
the speed of particles β, with a resolution of 7% (σ).
A gas Čerenkov detector filled with CO2 at atmospheric pressure is mounted between
the trigger scintillator planes S1 and S2. The detector allows an electron identification
with 99% efficiency and has a threshold for pions at 4.8 GeV/c. Čerenkov in the HRS-R,
leading to an average of about twelve photoelectrons. In the HRS-L, the gas Čerenkov
detector in its standard configuration has a pathlength of 80 cm, yielding seven photoelectrons on average. The total amount of material in the particle path is about 1.4%
X0 .
Two layers of shower detectors are installed in each HRS. The blocks in both layers
in HRS-L and in the first layer in HRS-R are oriented perpendicular to the particle tracks.
In the second layer of HRS-R, the blocks are parallel to the tracks. The front layer in
HRS-R is composed of 48 lead glass blocks, 10 cm by 10 cm by 35 cm. The second layer
is composed of 80 lead glass blocks, 15 cm by 15 cm by 35 cm each. The front layer
in HRS-L is composed of 34 lead glass blocks, of dimensions 15 cm by 15 cm by 30(35)
cm. The second layer is composed of 34 similar blocks. Because of its reduced thickness,
the resolution in HRS-L is not as good as that of the shower detector in HRS-R. The
combination of the gas Čerenkov and shower detectors provides a pion suppression above
2 GeV/c of a factor of 2 · 105 , with a 98% efficiency for electron selection in the HRS-R.
There are three aerogel Čerenkov counters available with various indices of refraction,
which can be installed in either spectrometer and allow a clean separation of pions, kaons
and protons over the full momentum range of the HRS spectrometers. The first counter
(AM) contains hygroscopic aerogel with a refraction index of 1.03 and a thickness of
9 cm. The aerogel is continuously flushed with dry CO2 gas. It is viewed by 26 PMTs
(Burle 8854). For high-energy electrons the average number of photo-electrons is about
7.3.
The next two counters (A1 and A2) are diffusion-type aerogel counters. A1 has 24
PMTs (Burle 8854). The 9 cm thick aerogel radiator used in A1 has a refraction index of
1.015, giving a threshold of 2.84 (0.803) GeV/c for kaons (pions). The average number of
photo-electrons for GeV electrons in A1 is ' 8. The A2 counter has 26 PMTs XP4572B1
made by Photonis. The aerogel in A2 has a refraction index of 1.055, giving a threshold
of 2.84 (0.415) GeV/c for protons (pions). The thickness of the aerogel radiator in A2 is
5 cm, producing an average number of about 30 photo-electrons for GeV electrons.
13.1
Geometry of the Spectrometer Detector Packages
Tables 13.1 and 13.2 give geometry information for the Left arm and Right arm detector
packages. The values in the tables indicate the position of the central point of the
CHAPTER 13. CHAPTER
270
detector. The origin of coordinate system (0,0,0) is located at the intersection of the
mid plane of the spectrometer and the nominal focal plane ( ∼ middle of the Bottom
VDC ). The configurations can be modified to meet experiment needs, such as short gas
Cherenkov counter can be made long to increase pion rejection or two aerogel counters
can be installed on one spectrometer or and additional CH2 analyzer for FPP and so
on. The locations are fixed for the VDC, the S1, the shower detectors, but some other
detectors can be moved.
detector
VDC1*
VDC2*
S1
AERO
GAS
S2
preSHOW
SHOW2
location
actual
0
572
1311
1646
2535
3358
3502
3780
location
width
IDEAS model
X
1321
3378
3546
3912
1942
1942
1718
199
2200
2197
2400
2400
width BEAM
Y
X(+)
271
271
356
414
650
540
700
900
843
932
696
709
886
897
925
964
X(−)
ENVELOPE
Y
- 824
- 911
-1022
- 888
-1110
-1124
-1158
-1207
+/- 57
+/- 85
+/- 163
+/- 182
+/- 279
+/- 285
+/- 301
+/- 322
Table 13.1: Locations of the detectors on Right Arm in mm.
detector
VDC1*
VDC2*
S1
AERO
SC1
GAS
SC2
S2
Analyzer
SC3
SC4
location
location
actual IDEAS model
0
500
1287
1617
1837
2409
2952
3141
3495
3907
4264
width
X
width
Y
BEAM
X(+)
X(−)
ENVELOPE
Y
1942
1942
1760
1872
1780
2200
2080
2220
2190
2540
3170
271
271
360
414
480
650
640
640
680
1000
1500
843
932
675
709
738
857
865
877
916
1099
1382
- 824
- 911
- 845
- 888
- 924
-1073
-1083
-1099
-1147
-1343
-1645
+/- 57
+/- 85
+/- 163
+/- 182
+/- 198
+/- 263
+/- 268
+/- 274
+/- 296
+/- 457
+/- 705
Table 13.2: Locations of the detectors on Left Arm in mm.
Chapter 14
Vertical Drift Chambers 1 2
14.1
Overview
The High Resolution Spectrometer Vertical Drift Chambers provide a precise (±125 µm)
measurement of the position and angle of incidence of both recoil electrons (in the HRSe)
and knockout protons (in the HRSh) at the respective spectrometer focal planes. This
information may be combined with the knowledge of the spectrometer optics to determine
the position and angle of the particles in the target.
Each Hall A spectrometer boasts its own VDC detector package. These packages
are located on permanent rails mounted on the spectrometer decks in the shielding huts
above the outrun windows but beneath the space frames. The packages consist of two
VDCs, and are identical in all aspects. The VDCs have been constructed without guard
wires. Each VDC is composed of two wire planes in a standard UV configuration - the
wires of each plane are oriented at 90◦ to one another, and each plane is oriented at 45◦
with respect to the nominal particle trajectories (see Figures 14.1,14.2).
Operation of the VDCs requires the application of both High Voltage (HV) across
the chambers themselves and Low Voltage (LV) across the preamp/disc cards, which
are mounted on the sides of the VDCs, within the confines of the protective aluminum
Faraday cage. The chamber gas is a combination of argon (Ar) and flammable ethane
(C2 H6 ) which is bubbled through alcohol. Gas is routed from bottles located in the Hall
A gas supply shed to gas supply control panels located on the main level of the space
frames in the detector huts.
As charged particles pass through the chamber gas in the VDCs, they produce
ionization. This ionization drifts along the electric field lines defined by the high voltage
planes and the signal wires. Ionization is collected in the form of analog pulses on the
signal wires. The pulses are then amplified, discriminated and used to start multihit
TDCs, which are subsequently stopped by the overall event trigger. The TDCs are read
out by the CODA acquisition software. The data are histogrammed online by the DHIST
software. In-depth offline data analysis requires the ESPACE software.
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CHAPTER 14. VERTICAL DRIFT CHAMBERS
272
Upper Chamber
50
0.
U2
m
V2
nominal 45o particle trajectory
Lower Chamber
V1
U1
SIDE VIEW
TOP VIEW
nominal 45o particle trajectory
0.288 m
2.118 m
Figure 14.1: Relative VDC geometry
45o
nominal 45o particle trajectory
45o
45o
Figure 14.2: Relative VDC geometry
CHAPTER 14. VERTICAL DRIFT CHAMBERS
273
regulator
valve
upper chamber
P
P
lower chamber
Q
input
bubbler
valve
Q
exhaust
bubbler
rotameter
Figure 14.3: Gas flow schematic
14.2
Operating Procedure
Gas Flow Operating Procedures Chamber gas is delivered to a given VDC detector
package via HAGS, the Hall A Gas System. Complete details of this system are presented
elsewhere in this manual.
Each VDC detector package consists of two VDCs connected in parallel (see Figure
14.3). All gas connections are made using PolyfloTM tubing and Jefferson Lab specified
connectors. Gas enters the chamber assembly after bypassing an overpressure bubbler
containing 15 mm of (edible) mineral oil. Gas is exhausted from the VDC package
through a second bubbler containing 5 mm of mineral oil. Each chamber has a volume
of approximately 30 ` and is operated slightly above atmospheric pressure. Standard
flow rate set points are clearly labeled next to the control panel flow meters. The gas
flow through the chambers may be independently varied and is typically set to 7 `/hr.
A typical chamber leakage rate measured against the 5-mm mineral oil load is ≤ 3 `/hr.
The flow rate of 7 `/hr when combined with the leak rate of ≤ 3 `/hr ensures a complete
exchange of gas in the chambers roughly every 8 hours. When a bottle is nearly empty
(say 90%), it should be changed since the quality of the gas at the bottom tends to be
low. Gas bottles may only be changed by authorized personnel.
CHAPTER 14. VERTICAL DRIFT CHAMBERS
274
6 kV
HV plane 1
8.2 MΩ
field lines
10 kΩ
V
signal wires
-4.00 kV
6 kV
HV plane 2
8.2 MΩ
field lines
10 kΩ
U
signal wires
6 kV
HV plane 3
8.2 MΩ
10 kΩ
Figure 14.4: VDC overview.
The status of the gas handling system should be monitored carefully every shift.
Manual logging is not required as the system status is constantly logged via EPICS [2].
Any substantial deviation from the median parameters can result in a change in the
operational parameters of the VDCs and should be immediately investigated. If at all
possible, gas flow should be continuously maintained, even in no-beam time periods. This
avoids time loss to reconditioning and maintains the desirable steady-state operating
condition. Further, it is critical that gas flow has been maintained for 24 hours prior to
any power up.
Power Supplies and Electronics Procedures The power supplies and readout electronics associated with the HRS VDCs are all commercially designed. The reader is
directed towards the manuals made available by the manufacturer for the detailed information not provided here.
A Bertan 377N HV power supply, modified to allow a remote reset, provides -4.00
kV nominal to each of three HV planes in a given VDC detector package via a 10 MΩ
CHAPTER 14. VERTICAL DRIFT CHAMBERS
275
Hammond splitter box (see Figure 14.4). The power supply is located in the detector hut
in a NIM bin on the upper level of the space frame. This unit may be controlled either
manually or remotely via the EPICS control software, and also provides a monitor of the
current drawn (nominally 70 nA) by the VDCs to which it is attached. Connections from
the power supply to the Hammond splitter box, as well as from the Hammond splitter
box to the VDCs are made using standard SHV connectors on red RG-59/U cable good
to 5 kV.
A Kepco ATE 15-3m discriminator power supply provides +3.0 V (92 cards draw
≤ 2 A) and the Kepco ATE 6-100m pre-amp power supply provides ±5 V nominal to
the LeCroy 2735DC pre-amp/discriminator cards used to instrument the chambers via a
heavy-duty fuse panel. The precise voltages provided are +5.0 V (92 cards draw 22 A)
and -5.2 V (92 cards draw 58 A). These LV supplies are located in the detector hut on
the main level of the space frame for the HRSe and on the upper level of the space frame
for the HRSh . Complete connection schematics and instructions for making or breaking
the connections are located on the aluminum Faraday cage protective plates covering the
respective interface nodes between the power supplies and the VDCs.
Each VDC wire plane consists of 400 20 µm φ, Au-plated tungsten wires. The first
16 wires on each end of the wire plane are connected to ground for field-shaping purposes.
There are 368 wires per wire plane which act as signal wires. Thus, each spectrometer is
instrumented with 1472 channels of LeCroy 1877 multihit Fastbus TDCs. These TDCs
are located in a Kinetic Systems F050 Fastbus crate with a BiRa FB8189-4 power supply
located on the main level of the spectrometer space frame in the detector hut. The
connections between the pre-amp/discriminator cards mounted on the VDCs and the
TDCs are made with 34-conductor twisted-pair cables. Clip-on ferrites are used to filter
noise. A connection schematic is posted on the side of the rack holding the Fastbus crate
on the space frame in the detector hut.
Power-up Procedure
1. ensure gas flow has been established in the chambers as previously outlined. If it
has not, STOP RIGHT HERE! Gas flow must be well-established and steady-state
BEFORE the HV may be enabled.
2. Ensure that all power supplies as well as the Fastbus crate are off and then connect
the LV, HV, and TDC cables.
3. enable the LV. Set points are clearly labeled on the face of the power supplies. Note
that they have overcurrent setpoints, and some fine adjustments over the first 30
minutes after a cold start power-up may be required. Appropriate LEDs should all
be active on both the power supplies and the pre-amp/discriminator cards.
4. slowly (steps of no more than -300 V) ramp the HV to its nominal set point of
-4.00 kV using either the manual or the remote controls. While the trip current
is set to 10 µA, do not allow the chambers to draw more than 1 µA during the
ramping procedure or serious damage may result. If the power supply trips during
CHAPTER 14. VERTICAL DRIFT CHAMBERS
276
the ramping procedure, you are moving too fast. Rezero things and begin the
procedure again. NEVER USE THE AUTO-RESET FUNCTION. If the power
supply trips again, STOP IMMEDIATELY AND INVESTIGATE.
5. enable the Fastbus crate. Appropriate LEDs should all be active.
6. check for poor signal connections evidenced by hot wires (wires counting extremely
fast) or dead wires (wires with no counts) using the histograming software and
cosmic rays. Remake any connections as necessary by first powering down the
Fastbus crate.
If at all possible, the HV and LV power supplies should be left on continuously if
and only if gas is available to the chamber. This avoids time loss to reconditioning and
maintains the desirable steady-state operating condition.
14.3
Handling Considerations
The VDCs are very delicate devices which are absolutely essential to the instrumentation
of the Hall A spectrometers. Thus, extreme care must be exercised whenever they are
moved or used.
• Before moving a VDC detector package, ensure that the protective plates are in
position. Plates include tapped aluminum sheets to be bolted over the entrance
and exit aperatures, as well as aluminum sheets which slide in between the two
chambers.
• Disconnect and reconnect all TDC cables with extreme care. The conductor pins
are relatively fragile, and should one be broken off, repair will be extremely difficult.
• When initiating gas flow, pay strict attention to the feedback parameters. Overpressure may damage the chambers.
• Never attempt to apply HV to the chambers until gas flow conditions have reached
steady-state.
• As the amount of heat generated by the pre-amp/discriminator cards it substantial,
always make sure adequate cooling is provided before attempting to run. This
cooling takes the form of four 12VDC fans mounted in the aluminum Faraday
cage.
• When ramping the HV, never allow the chambers to draw more than 1 µA instantaneously. If they do, something is wrong!
CHAPTER 14. VERTICAL DRIFT CHAMBERS
14.4
277
Other Documentation
See the URL3 .
14.5
Safety Assessment
The following potential hazards have been clearly identified.
The High Voltage System The Bertan 377N HV low current power supply provides
a nominal -4.00 kV. Red HV RG-59/U cable good to 5 kV with standard SHV
connectors is used to connect the power supply to a Hammond splitter box, and
then to connect the splitter box to each of the three high voltage planes in a given
VDC. A given chamber draws a current from 50-100 nA.
The Low Voltage System Kepco LV power supplies are used for the the LeCroy
2735DC pre-amp/discriminator cards. Each card (23 per chamber) requires +5.0
V (92 cards draw 22 A), -5.2 V (92 cards draw 58 A) and +3.0 V (92 cards draw
≤ 2 A).
Explosive Gas The Ar C2 H6 chamber gas is explosive and must be handled accordingly.
Further, gas flow should be maintained for at least 24 hours prior to the enabling
of HV.
High Pressure Gas Bottles The gas used in the chambers is supplied in high pressure
(≥ 2000 psi) gas bottles. This confined high pressure gas represents a tremendous
(potentially lethal) amount of stored energy.
14.6
Responsible Personnel
The following individuals are responsible for chamber problems.
Segal, Jack - x7242, pager 584-7242
Wojtsekhowski, Bogdan - x7191, pager 584-7191
3
http://www.jlab.org/~fissum/vdcs.html
Chapter 15
Trigger Scintillator Counters 1 2
15.1
Overview
In the standard detector configuration each HRS has two trigger scintillator planes, S1
and S2. The paddles in each plane are arranged to provide segmentation along the
detector-x direction. An additional un-segmented scintillator plane, S0, can optionally
be inserted into the detector stack for experiments that require a high hadron trigger
efficiency. Fast signals from these planes are used to form the trigger, as well as providing
timing information useful for particle identification. Typically a coincidence between twoor-more scintillator planes is used to form the trigger, and through different combinations
the triggering effiency of each plane can be measured.
The S1 scintillator plane consists of six paddles, each with an active area of 29.5 cm
by 35.5 cm. The counters are made of 5 mm thick BICRON 408 plastic scintillator and
use multi-strip adiabatic light guides which end in a long cylindrical spool. There is an
inlet for optical fiber mounted on the side of the cylindrical light guide. Each paddle
is viewed by a 2” photo multiplier tube (Burle 8575) on each end. The S1 paddles are
installed at a small angle to the S1-plane and overlap by 10 mm. The detectors are
clamped to the detector frame through an additional A1 channel, and supported from
the PMT housings. Figure 15.1 shows the mounting scheme for S1. Signals from the
PMTs are sent to Camac modules on the second level of the shielding hut for processing.
The S2 plane (also called S2m) consists of sixteen bars mounted on a steel frame,
as shown in Figure 15.2. The bars are made a fast plastic scintillator (EJ-230) with
dimensions of 17 in by 5.5 in by 2 in thick. Since the S2 detector is located after the
tracking and PID detectors in the HRS, the extra material does not compromise the
particle detection while providing a greater photon yield for an improved timing resolution
as compared to S1. The bars are individually wrapped with 25 µm of mylar and 50 µm
of black tedlar. The bars do not overlap, but are pressed together by a force 60 lbs to
minimize the dead area between adjacent bars. Trapezoidal lucite light guides on both
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CHAPTER 15. TRIGGER SCINTILLATOR COUNTERS
279
Figure 15.1: S1 mounting
ends couple the bar to 2” photo multiplier tubes (Photonis XP2282B). S2 is assembled
on a sub-frame mounted on rails in the detector frame. The bars are supported by two
thin aluminimum honeycomb panels placed over the scintillators, leaving the PMTs and
bases accessible for servicing. On the frame are mounted analog splitters and threshold
discriminators for the initial signal processing.
The optional S0 plane is made of 10 mm thick BICRON 408 plastic scintillator with
an active area 170 cm long by 25 cm wide. This area is covered by a single paddle, viewed
from each end by 3” PMTs (XP4312B). The signals from these PMTs are sent to Camac
modules on the second level of the shielding hut for processing.
15.2
PMT regime and time resolution
High energy electrons passing perpendicular to the S1 detector plane yield about 400-500
photons at the photo cathode of each PMT. In a fresh PMT this leads to 80-100 photo
electrons. On the HRS the discriminators have a threshold of 45 mV and a typical PMT
has gain 3∗106 . The HV for a fresh PMT should be in the range -1800 to -2000 V. Based
on PMT pulse rise time ( 2.8 ns ) and photo electron statistics the time resolution for
the counter is about σt ≈ 0.2 ns. The propagation time of the light inside the detector is
about 10 ns, which needs to be corrected by using track position information.
Due to its thicker cross-section, the initial photon yield in S2 is larger than in S1.
With cosmic rays around 900 photo electrons per PMT were observed. To match the
gains, the HV on the PMTs were adjusted, and set between -1700 and -2000 V. The signals
from each PMT are sent to a passive 90/10% splitter, with the greater and lesser portions
CHAPTER 15. TRIGGER SCINTILLATOR COUNTERS
Discriminator
280
Splitters
17 in
etc.
F = 60 lbs
88 in
Figure 15.2: The layout on the frame of the S2 paddles and electronics.
sent to the on-frame discriminator and Fastbus ADCs,respectively. The discriminator is
a Phillips-Scientific model 706 with the threshold set at 10mV. Both NIM outputs are
used on each channel, with one line as input for the trigger-logic and the other going
to a TDC after passing through a NIM-ECL converter and a delay of some 880ns. The
average resulting timing resolution for a single PMT was measured to be better than
σpmt < 150 ps.
The geometry of S0 counter limits its timing resolution. In 1999 the resolution was
measured to be σt ≈ .2 ns.
LeCroy HV 1460 modules are used to supply HV power for the trigger counters.
The HV can be controlled from a VT100 terminal connected through a terminal server
or through the EPICS [2] system based on the HAC computer. Current HV settings for
the trigger counters should be found from a printout of the EPICS control in the last
experimental logbook.
Figures 15.3 and 15.4 give examples which are included for guidance only. The
settings used in the plots may be not correct.
15.3
PMT operation monitoring
There are two ways to monitor PMT/detector performance. The first is based on a scaler
display program which provides information about PMT counting rates and coincidence
counting rates. A large variation of the rates between paddles is an indication of a possible
problem. The second technique is to track the average amplitudes of each PMT for good
CHAPTER 15. TRIGGER SCINTILLATOR COUNTERS
Figure 15.3: EPICS HV HRSR summary screen.
Figure 15.4: HV screen for a single card.
281
CHAPTER 15. TRIGGER SCINTILLATOR COUNTERS
282
track events after a complete event reconstruction. For high efficiency of the trigger it is
important to keep the average amplitude for the S1 PMTs above 600 channels. Due to
the passive splitting, the S2 amplitudes should be expected to be only about 50 channels
above the pedestal.
15.4
Measures to Protect the PMTs from Helium
Clamp
Air input
Black shinking tube
XP2282B
Light guide
Rubber seal
Al housing
Figure 15.5: Details of the PMT Housing for S2.
There has been found in the past large He concentrations in Hall A, which can lead
to a dramatic reduction in the PMT lifetime. To mitigate this problem, each PMT for
S0, S1 and S2 is enclosed in a hermetic housing. Air from outdoors is supplied to the
housing at a slight over-pressure. Figure 15.5 shows a schematic of the housings used for
the S2 PMTs.
15.5
2” PMT Bases for S1 Trigger Counters
A schematic diagram of the 2” PMT Base is shown in Figure 15.6. The Base consists
of three main components. These are the front tubular housing (06), which encloses the
PMT, part of the scintillator counter’s light guide (01), and the mu-metal shield (10).
The actual base with the socket and the dynode chain is a separate part, actually an
assembly of parts (09-19). The rear tubular housing (07) completes the assembly and
encloses the dynode chain and wiring. The three main sections join at the coupling
nut (14), which threads partly inside the front tubular housing, while the rear tubular
housing threads on the remaining part.
The PMT and the electronic amplification components are mounted on a P.C. board
(15) which is enclosed in an aluminum Faraday cage. This assures rigidity and protection
from stray RF fields. The mu-metal shield is at cathode potential to minimize the dark
current due to capacitive discharge in the photo cathode glass window.
The Electronic Amplification Chain The arrangement of the resistor dynode chain
is shown in Figure 15.7. The cathode is connected to the mu-metal shield through a 10
CHAPTER 15. TRIGGER SCINTILLATOR COUNTERS
2 inch PMT Base Assembly
Figure 15.6: The 2” PMT base used in S1 trigger scintillators.
Figure 15.7: The 2” PMT base used in the S1 trigger scintillators.
283
CHAPTER 15. TRIGGER SCINTILLATOR COUNTERS
284
MΩ resistor, in addition to the 1 MΩ resistor between the cathode and the negative HV.
The dynode chain incorporates an adjustable potentiometer (0-500 Ω) to allow a match
between the PMT and the external load, in order to eliminate after-pulse ringing. This
potentiometer should be adjusted at first to 250 Ω and then make fine adjustments as
needed by observing the anode pulses on the oscilloscope for critical matching. It is
not advisable to do the adjustments with HV on. Instead, the process should be done
with HV off; remove the rear tubular housing, adjust the potentiometer, replace the rear
housing, and then turn the HV on again. Iterate until the matching is accomplished. In
addition to the obvious safety concerns, one does not want to remove the light sealing
rear housing from an active PMT and induce a large light leak which could destroy the
PMT.
The bases have been extensively tested under beam conditions. They have several
safety related features but these cannot protect anyone who is bent on violating operating
procedures and common sense. They allow the removal of the PMT/Base assembly, for
repairs of the electronics or replacement of a PMT, without decoupling the housing and
collets from the light guide. Thus, replacement of PMTs can be done in minutes without
the need to remove the scintillator counters from their subframes.
15.6
Safety Assessment
WARNING: The bases are high voltage devices: the high voltage should be turned off
before handling.
The maximum (negative) voltage for both the PMTs and dynode chain is 3 kV.
In actual use, however, there should be no need to exceed the 1.8-2.1 kV operating
parameters, since both PMTs and dynode chain have high gain. Nevertheless, the bases
are high voltage devices and care should be exercised during handling and setup. The
external aluminum parts, the front and rear housing, and the back plate (17), are all
grounded via the ground of the BNC (18) and SHV (19) connectors. Since the back
plate is connected to the coupling nut via the three steel posts, the front plate is also
grounded via the coupling nut and the back plate. Common sense, however, dictates that
the bases are not to be handled while under high voltage, even when multiple grounding
connections are provided.
The mu-metal shield is also under high voltage, since it is connected to the cathode.
Electrical isolation between the mu-metal shield and the front tubular housing is assured
by the high dielectric retainer ring (12) and the plastic insulator (09) at the free end of
the mu-metal shield. The air gap between the mu-metal shield and the front tubular
housing is 6 mm, thus the breakdown value (18 kV) far exceeds the maximum 3.0 kV of
the PMT.
In the event that the mu-metal shield is inserted without the plastic insulator ring,
or some oaf decides to operate the base without the outside housings, the 11 MΩ resistors
between the -HV and the mu-metal shield will restrict the current flow through the mumetal shield (and the oaf’s hands) to less than 0.2 mA with 2.1 kV on the base.
CHAPTER 15. TRIGGER SCINTILLATOR COUNTERS
15.7
Responsible Personnel
The following individuals are responsible for operation of the trigger counters.
Segal, Jack - x7242
Wojtsekhowski, Bogdan - x7191
285
Chapter 16
Gas Cherenkov Counters 1 2
A gas Cherenkov detector filled with CO2 at atmospheric pressure is mounted between
the trigger scintillator planes S1 and S2. The detector allows an electron identification
with 99% efficiency and has a threshold for pions at 4.8 GeV/c. The detector has ten
spherical mirrors with 80 cm focal length, each viewed by a PMT (Burle 8854); the
light-weight mirrors were developed at INFN. The focusing of the Cherenkov ring onto
a small area of the PMT photo-cathode leads to a high current density near the anode.
To prevent a non-linear PMT response even in the case of few photoelectrons requires a
progressive HV divider. The length of the particle path in the gas radiator is 130 cm for
the gas Cherenkov in the HRS-R, leading to an average of about twelve photoelectrons.
In the HRS-L, the gas Cherenkov detector in its standard configuration has a path length
of 80 cm, yielding seven photoelectrons on average. The total amount of material in the
particle path is about 1.4% X0 .
16.1
Concept of the design
Two similar threshold gas Cherenkov counters have been constructed as a part of the
particle identification equipment to be included in the focal plane detectors of the High
Resolution Spectrometers (HRS) of the TJNAF experimental Hall A (see Fig. 16.1).
Each counter housing is made in steel with thin entry and exit windows made of tedlar.
Light weight spherical mirrors have also been built resulting in a very thin total thickness
traversed by particles.
These two counters have identical sections but different length of the gas radiator,
80 cm for the left arm and 130 cm for the right arm. There is an additional section
50 cm long which can be attached to the short counter if needed. Each Cherenkov is
made of 10 tubes (PMT) and 10 mirrors. Each mirror has a rectangular profile built
in a empty sphere of interior radius (reflective face) of 80 cm and thickness of 1 cm.
The very light structure of the mirror is built like honeycomb and is constituted as the
1
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CHAPTER 16. GAS CHERENKOV COUNTERS
Figure 16.1: Gas Cherenkov counter.
287
CHAPTER 16. GAS CHERENKOV COUNTERS
288
following manner: the MgF2 layer, which protect the aluminum; the aluminum which
assure the reflectivity; the plexiglas, which assure a good surface; a sandwich (carbonepoxy, phenolic honey comb, carbon epoxy), which assure the rigidity of the mirror.
The 10 mirrors are placed just before the output window and are grouped in two
columns of 5 mirrors. Each mirror reflect the light on a PMT placed at the side of the
box. The mirrors of the same column are identical and the two columns are almost
symmetrical. Positions and angles of the PMTs are not placed regularly like for the
mirrors but were adjusted by a optical study in order to maximize the collection of light
coming from the particular envelope of particle which have to be detected with the High
Resolution Spectrometer (HRS) of the Hall A. PMT are fixed and mirrors orientation
can be adjusted by hand.
The alignment procedure use the small light source located about 820 cm from the
mirror plane in the axis of symmetry of the counter. The picture on figs 16.2 and 16.3
shows the image of the small light source on the PMT photo-cathode during mirror
alignment procedure.
Five photomultiplier tubes (PMT) are fixed of the two side walls. Each one is
surrounded by a high magnetic permeability shielding (mu-metal). The fixing provides
high voltage insulation between the PMT and the steel vessel. A set of optic fibers
provides light pulses to each PMT for their calibration.
16.2
Safety Assessment
The PMTs are under high voltage and care is required when handling any components
of the counter. The body of the Cherenkov counter must be grounded.
16.3
Operating Procedure
Operating Voltage The operating voltage on the PMTs is about -2,500 V. The voltage
must be set to zero before HV cable will be connected or disconnected from HV divider.
The HV cables must be disconnected from all HV dividers before replacement of any
PMT on the gas Cherenkov counter.
The high voltage has to be adjusted in order to have for each PMT the position of the
photoelectron peak at the same position which is around 100 channels above the pedestal.
For good PMT the noise counting rate should not exceed 10 kHz. Past experience show
that PMT need to be replaced in average every three years due to aging. Such short life
time about 3-4 times less than normal is due to He content in Hall A, which led to loss
of the PMT quantum efficiency.
16.4
Responsible Personnel
The following individuals are responsible for the operation of the gas Cherenkov counters.
CHAPTER 16. GAS CHERENKOV COUNTERS
Figure 16.2: The image from the mirror #1 on the PMT photo-cathode.
Figure 16.3: The image from the mirror #6 on the PMT photo-cathode.
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CHAPTER 16. GAS CHERENKOV COUNTERS
Segal, John - x7242
Wojtsekhowski, Bogdan - x7191
290
Chapter 17
Lead Glass Shower Counters 1 2
17.1
Overview
Electromagnetic shower counters offer a useful means of particle identification (PID)
separating electrons from hadrons or muons [18], [19]. Shower counters complement
other means of PID such as time-of-flight (TOF) or threshold Cherenkov counters, due to
the independent physical processes involved which result in different detector limitations
[20]. Independent PID allows multiple detectors (i.e., a Cherenkov counter followed by a
shower counter) to obtain excellent rejection ratios that are the product of the individual
rejection ratios.
Shower counters measure the energy deposited by the incoming particle. The detected light output is linearly proportional to the energy lost by the incoming particle.
Electromagnetic showers are stopped in the counters, whereas hadronic showers, due to
the longer hadronic mean free path, are not. Looking at the longitudinal distribution
of the energy deposited in the calorimeter differentiates between electromagnetic and
hadronic showers and therefore identifies the incident particle.
Typical pion rejection with a lead glass counter is of the order of 100-1000:1 in
the 1 to 10 GeV region [21]. The Hall A electromagnetic shower counter is meant to
offer rejection ratios better than 100:1 [22]. The limitation in using a shower counter
comes from separating the tails of the distributions, and is therefore dependent on energy
resolution. At higher energy the relative resolution of a shower counter improves, leading
to better separation between distributions. Conversely, other techniques perform worse
at higher energy. The TOF separation for a given path length decreases, and above
4 GeV/c pions can trigger a threshold CO2 Cherenkov counter operated at standard
temperature and pressure (STP) [21]. [The threshold for a CO2 Cherenkov counter at
STP is γ = 34.1, meaning that for an electron the threshold is just over 17 MeV, while
for a pion the threshold is just over 4 GeV/c momentum].
A Cherenkov counter is routinely capable of pion rejection of the order of 1000:1
at CEBAF energies [21]. A combination of successive Cherenkov counters might achieve
1
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CHAPTER 17. LEAD GLASS SHOWER COUNTERS
292
Figure 17.1: Typical values of the preshower counter high voltages.
higher rejection ratios. However this only works if the backgrounds in the two devices
are uncorrelated. A knock-on electron which triggers the first Cherenkov counter and
travels forward through both detectors will also trigger the second. Independent PID
provided by a measurement of the particle energy in a shower counter offers a solution to
this problem of correlated backgrounds. Used in conjunction with a threshold Cherenkov
counter, the combination can achieve rejection ratios of 5 × 105 [22].
The Hall A electron spectrometer is equipped with a 2-layer, segmented shower
counter. The first layer, the so-called “pre-shower” counter is made of 48 blocks of TF1
lead glass. Each block is nominally 10 cm by 10 cm by 35 cm long. The second layer, the
so-called “total absorber” counter is nominally 15 cm by 15 cm by 35 cm viewed head-on
by the beam.
Operation of the shower counter requires the application of High Voltage (HV) across
the photomultiplier tubes and bases, which are mounted on the back of the shower counter
blocks for the total absorber and on the sides of the shower counter in the case of the
pre-shower, within the confines of the protective aluminum support frame.
As charged particles pass through the lead glass of the shower counter, they produce electron-positron particle-antiparticle pairs. These particles in turn both produce
additional particles and Cherenkov light which is collected in the phototubes. The pulses
are then amplified, delayed and sent to ADCs, which are gated by the overall event
trigger. The ADCs are read out by the CODA acquisition software. The data are histogrammed online by the DHIST software. In-depth offline data analysis requires the
ESPACE software.
CHAPTER 17. LEAD GLASS SHOWER COUNTERS
17.2
293
Operating Procedures
Power Supplies and Electronics Procedures The HV power supplies and readout
electronics associated with the HRS VDCs are all commercially designed. The reader
is directed towards the manuals made available by the manufacturer for the detailed
information not provided here.
The LeCroy HV power supply provides -1500 V nominal to the total absorber and
1200 V to the preshower. The power supply is located in the detector hut in a NIM bin
on the upper level of the space frame. This unit may be controlled either manually or
remotely via the EPICS [2] control software, and also provides a monitor of the current
drawn by each phototube to which it is attached. Connections from the power supply to
the base are made using standard SHV connectors mounted on red RG-59/U HV cable
good to 5 kV. Typical values of the high voltages are shown in the tables for both the
pre-radiator and the total absorber.
If at all possible, the HV and LV power supplies should be left on continuously. This
allows the tube noise to quiet down after high voltage is applied and the temperature of
the base components stabilizes.
Signal Handling, Summing, Amplification and the Multiplexer The upstairs
electronics racks hold two NIM bins containing the summing modules of the multiplexer.
The output of these is sent to the ADCs. Both the individual channels and the sum of
every six channels is sent into an ADC where it can later be analyzed by the software. The
operations manual for the multiplexer was written by H. Breuer (UMd) and is available
from him upon request.
The detector signals are physically plugged into the ADC channels and connected to
the signal wires labelled as shown in the diagrams below, for both the preshower counter
and the total absorber counter.
17.3
Handling Considerations
The shower counters are very delicate devices which are easily damaged. Thus, care must
be exercised whenever they are moved or used.
• Before turning on the high voltage for the shower counters (HAC5 for the preshower
and HAC6 for the total absorber), check the shower counter log book located in
the Hall A counting house for the latest values of the high voltage.
• Never disconnect or connect all the high voltage to the bases with the high voltage
power turned on. Doing so will damage the bases (the zener diodes are destroyed).
• Never service the lead glass with the high voltage on. The high voltage poses a
safety hazard.
CHAPTER 17. LEAD GLASS SHOWER COUNTERS
Figure 17.2: Typical values of the preshower counter high voltages.
294
CHAPTER 17. LEAD GLASS SHOWER COUNTERS
Figure 17.3: Typical values of the total absorption counterhigh voltages.
295
CHAPTER 17. LEAD GLASS SHOWER COUNTERS
Figure 17.4: Map of the Pre-shower counter detectors.
296
CHAPTER 17. LEAD GLASS SHOWER COUNTERS
Figure 17.5: Map of the shower counter detectors.
297
CHAPTER 17. LEAD GLASS SHOWER COUNTERS
298
• Never drop anything onto the bases and tubes; they are extremely fragile. They
should not be used as support or to hold any weight. Also, do not drop the lead
glass blocks. They will be damaged or destroyed.
• When ramping the HV, keep an eye on the current drawn by the individual bases.
A light leak in the wrapping will produce a large current ( > 1µA at low voltages
(1 kV). If the tubes draw an excess amount of current turn them off and check for
leaks in the wrapping.
17.4
Safety Assessment
The following potential hazards have been clearly identified.
The High Voltage System The LeCroy 1443 HV crate equipped with LeCroy 1461N
negative high voltage cards supplies provides up to 3.3 kV of low current power.
Red HV RG-59/U cable good to 5 kV with standard SHV connectors is used to
connect the power supply to the photomultiplier tube voltage divider bases. A
given base on the TA draws typically 500-600 µA of current with the high voltage
on at between 1400 and 1500 V. The PS bases typically draw 900 µA with the high
voltage on at between 1100 and 1200 V.
The Lead Glass Support Structure The lead glass shower counters are mounted on
top of the space frame for the detectors. Access for servicing the shower counters
requires climbing on top of the support frame. Only the responsible personnel
identified below should attempt to service the shower counter; such work requires
proper safety precautions and prior training/experience.
The Lead Glass blocks The lead glass shower blocks and tubes weigh approximately
70 – 80 pounds apiece. Lifting, replacing, or moving such blocks should be done
properly to avoid muscle problems and damage to the blocks.
17.5
Authorized Personnel
The following individuals are authorized to work on shower counters.
Breuer, Herbert - 301-405-6108
Markowitz, Pete - x7237, 305-348-1710
Segal, Jack - x7242
Voskanyan, Hakob - x5105
Wojtsekhowski, Bogdan - x7191
CHAPTER 17. LEAD GLASS SHOWER COUNTERS
17.6
299
Software Algorithms
The purpose of the shower cluster reconstruction in the Preshower and the Shower detector is to:
• Define all clusters of fired blocks, which belong to the showers, registered in the
detector;
• Calculate parameters of showers in the detector: energy deposition of showers, X
and Y coordinates of the shower center;
• Set parameters and identifier of the so-called “main” cluster.
Cluster in the shower detector is determined as follows:
— Cluster is a group of continuous blocks;
— Cluster can occupy a maximum of 6 (2 × 3) blocks in the case of Preshower and 9
(3 × 3) blocks in the case of Shower;
— Central block is defined as the block that has maximum energy deposition.
The “main” cluster in Preshower/Shower is the cluster with the biggest energy, which
is coincident with the “golden track”, or coincident with some Shower/Preshower cluster.
Coincidence of the cluster with the “golden track” means that the distance between the
shower cluster center and crossing point of “golden track” with the detector plane is less
than a certain magnitude. Coincidence of the Preshower and Shower clusters means,
that distance between the clusters centers is less than a certain magnitude (it is assumed
that both of these points are on the same Z–plane).
The shower clusters reconstruction in Preshower and Shower is performed by the
following steps:
1. Sort fired blocks in order of decreasing deposited energy;
2. Pick out the block with maximum energy deposition and all fired blocks in its
arrangement (2 × 3 blocks for Preshower, 3 × 3 blocks for Shower), as belonging to
one cluster;
3. Remove all blocks associated with the found cluster from further consideration;
4. Repeat steps 2 and 3, until all of the fired blocks are associated with a cluster;
5. Calculate energy deposition, X and Y coordinates of each cluster and sort clusters
by decreasing energy deposit;
6. Define coordinates of crossing point of “golden track” with detector plane in detector local coordinate system;
CHAPTER 17. LEAD GLASS SHOWER COUNTERS
300
7. Analyze geometrical position of the “golden track” point on the detector plane and
cluster centers in order to determine the “main” cluster and to set its parameters
and identifier.
Energy deposition E, X and Y coordinates of the shower center are calculated by
the formulas:
E=
X
i∈M
ei ,
X=
X
ei · xi /E ,
Y =
i∈M
X
ei · yi /E ,
i∈M
where: i — number of detector block, included in the cluster; M — set of blocks numbers,
included in the cluster; ei — energy deposition in block i of detector; xi , yi — X and Y
coordinates of center of block i of detector.
The shower cluster reconstruction algorithm described above has been implemented
by the ESPACE analysis subroutine tot shower. A complete description of the program
as well as information about the ESPACE routines and kumac files used to perform the
analysis, photographs of the detectors and more information can be found at the URL 3
3
http://www.jlab.org/~armen/sh_web_page/welcome.html
Chapter 18
Aerogel Cherenkov Counters 1 2
There are three aerogel Cherenkov counters available with various indices of refraction,
which can be installed in either spectrometer and allow a clean separation of pions, kaons
and protons over the full momentum range of the HRS spectrometers. The first counter
(AM) contains hygroscopic aerogel 3 . with a refraction index of 1.03 and a thickness of
9 cm. The aerogel is continuously flushed with dry CO2 gas. It is viewed by 26 PMTs
(Burle 8854 [23]). For high-energy electrons the average number of photo-electrons is
about 7.3 [24].
The next two counters (A1 and A2) are diffusion-type aerogel counters. A1 has 24
PMTs (Burle 8854). The 9 cm thick aerogel radiator used in A1 4 . has a refraction index
of 1.015, giving a threshold of 2.84 (0.803) GeV/c for kaons (pions). The average number
of photo-electrons for GeV electrons in A1 is ' 8 (see Fig. 18.8). The A2 counter has
26 PMTs (XP4572B1 from Photonis [25]). The aerogel in A2 also hygrophobic has a
refraction index of 1.055, giving a threshold of 2.8 (0.415) GeV/c for protons (pions).
The thickness of the aerogel radiator in A2 is 5 cm, producing an average number of
about 30 photo-electrons for GeV electrons.
18.1
Mirror Aerogel Cherenkov Counter
5 6
It is a silica aerogel Cherenkov counter of the compact reflection mirror design, which
was dictated by the available space (36.3 cm along the incident particle direction). In
addition, the high singles rates expected in Hall A are better handled with segmented
detectors covering the focal plane, which requires short pulse decay times. Even though
the diffusion length in silica aerogel can be quite short for low λ light generated in
the SiO2 radiator [26], enough directionality remains in the visible λ region, where the
1
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3
Airglass AB, BOX 150, 245 22 Staffanstorp, Sweden.
4
Matsushita Electric Works, www.mew.co.jp.
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302
selected PMTs have good quantum efficiency, to make light collection with mirrors an
attractive and practical alternative.
An effective segmentation of the aerogel Cherenkov counter, matching the segmentation of the trigger scintillators, can be used to separate multiple tracks through the
focal plane and will allow an additional element of selectivity and track sensitivity in
the focal plane instrumentation. This means that specific sections of the focal plane can
be physically disabled from the trigger, if the experimental conditions require it. It will
also provide the capability of identifying and separating pions and protons traversing
the focal plane trigger scintillators and the vertical drift chambers (VDCs) within the
resolving time of the system (double hits). For example, in the off-line analysis, the
aerogel counter PMT with the highest number of photoelectrons can be matched with
the trigger counter and VDC information to identify the actual path of a pion, thus
separating it from a simultaneously detected proton, which has no Cherenkov signature.
Such a capability of double hit resolution is not possible with diffusion Cherenkov counter
designs, because the photon collection efficiency does not have a strong correlation with
the incident particle track within the aerogel material.
The requirement for segmentation, in addition to supplementing the information on
the individual particle position along the focal plane, also couples well with the desirability of increasing the active solid angle viewed by the PMTs in the counter. Although
the photon detection probability is not as directly proportional to the solid angle covered
by PMTs as in the case of a diffusion box, clearly, the larger the effective coverage, the
higher the probability will be that a photon will end up on a PMT. Given the divergence
of the beam envelope incident on the aerogel, and the diffusion of the light in the low
λ region by the aerogel material, an increase in the area covered by PMTs results in
an increase in the number of photons detected. As a result, a total of 26 PMTs are
used in the counter, as shown in Fig. 18.1, with minimal spacing between their µ-metal
shields (2.8 mm). The total area covered by the PMT photo-cathode windows comprises
72% of the area of the counter opposite the planar parabolic mirrors. A cross sectional
schematic of the detector is shown in Fig. 18.2, clearly illustrating the planar parabolic
design of the mirror surfaces and their relative orientation with respect to the PMTs,
and the orientation of the counter relative to the central axis of the spectrometers.
The close spacing of the µ-metal shields creates dielectric breakdown problems. The
µ-metal shields are at cathode potential (-2950 V ) to avoid the capacitive discharge from
a grounded µ-metal shield to the glass of the photo-cathode, which would contribute to
the noise level in the PMT, and adversely affect their performance at high operating
voltages. This necessitates extra precautions, in order to avoid dielectric breakdown
between adjacent shields, and between the shields and the aluminum structure of the
counter, which is at ground potential. The solution was to wrap the outer surfaces of the
µ-metal shields with a high dielectric value (12,000 V /mm), thin (0.254 mm) Teflon film7 .
In addition, the PMT housings consist of fiberglass-epoxy composites, with added inner
and outer skins of 0.0254 mm thick Tedlar1 , with a further combined insulating value of
3,000 V . Such a combination of insulating materials eliminates any breakdown or small
7
DuPont Canada Inc., Box 2200, Streetsville, Mississauga, ON L5M 2H3, Canada.
CHAPTER 18. AEROGEL CHERENKOV COUNTER
303
leakage current induced noise and, at the same time, satisfies all safety requirements.
The final construction of the counter, described in this report, is built around the
two sides of the main (PMT) section, each consisting of two pieces of aircraft quality
aluminum alloy, with stiffening aluminum rods formed integrally on the top and bottom.
The openings for the PMT housings were machined on these structures using CNC milling
machines to keep tolerances to fine levels. The double walled structure, on both sides
of the enclosure, further increases the rigidity of the exoskeleton by forming a second
“outer” wall on each side, very similar in configuration to the inner one, and attached
to the latter with cross-bolt braces. Each end plate is made out of the same aluminum
alloy as the side walls, and also incorporates stiffening lips folded integrally to each plate,
one at the top and one at the bottom. Each end plate has been provided with inlet and
outlet gas line connections, which will be used to fill the counter enclosure with dry CO2
gas to protect the silica aerogel from water vapour absorption.
All internal surfaces of the detector, except the planar parabolic mirrors, themselves,
are lined with aluminized mylar8 to increase the overall reflectivity of the counter. The
mirrors are made in 45 × 20.5cm2 moulded surfaces, formed in one rigid structure. The
rigidity is provided by two layers of carbon fiber epoxy composite backing, with a combined thickness of 0.28 mm, and a single sheet of mylar with thickness 0.127 mm. The
special mylar material was obtained from exposed negative film used in the cartographic
industry, and is of high smoothness and uniformity. One side was aluminized at CERN,
while the other side remains in its exposed negative (black) state, further adding to the
successive light penetration barriers into the enclosure.
The upper section of the counter containing the mirrors is mounted on its own
aluminum sub-frame, which is bolted to the main frame that houses the PMTs. The
light and gas sealing action is provided by continuous twin parallel rubber strips along
the joint area, and by Tedlar film of 0.025 mm thickness covering the top of the outer
planar parabolic area.
The third major component of the counter consists of a removable tray where the
silica aerogel is placed. The tray occupies the bottom part of the counter and has inside
dimensions of 195 × 41cm2 where the SiO2 silica aerogel is placed. It is formed by a
frame with twin aluminum panels, which, in turn, secure the removable frame strung
with fishing line in a criss-cross pattern to hold the aerogel panels in place. This “fishnet” frame is secured by screws and is easily removed without disturbing the aerogel
panels or requiring re-stringing. The bottom of the tray is formed out of a single layer
of carbon fiber epoxy skin (0.127 mm thick) and a layer of aluminized mylar of equal
thickness. Externally, it is covered by a single layer of Tedlar film to assure integrity
from light penetration; further environmental isolation is provided by two parallel strips
of rubber gasket seals enclosing the circumference of the tray and containing the feedthrough spacers for the retaining bolts. The tray is equipped with SMA-type fiber optic
feed through connectors for the gain and timing monitor system, which utilizes fiber
optic cables. Each fiber illuminates two adjacent PMTs, except the last PMT on either
side (13T and 13B in Fig. 18.1), which have their own dedicated fiber. The light is
8
National Metalizing, P.O. Box 5202, Princeton, NJ 08540, USA.
CHAPTER 18. AEROGEL CHERENKOV COUNTER
304
Figure 18.1: Schematic diagram of the aerogel Cherenkov counter as viewed by the
incoming particles. The numbers indicate the sections, 1 to 13, in the counter. Each
section is viewed by two PMTs, one on the top (T) and one in the bottom (B). The
labeling carries no significance other than identifying the PMTs during the testing phase,
as described in the text.
generated in a gas plasma discharge unit9 and duplicates the spectrum expected from
Cherenkov radiation. In addition, the fibers terminate beneath the silica aerogel, thus,
the light reaching the PMTs will have the absorption characteristics of real Cherenkov
light produced in the aerogel radiator.
Due to the nature of Cherenkov detectors, where few photoelectrons (PEs) are emitted by the photo-cathodes in the PMTs, any extraneous light entering the enclosure is
very troublesome. As a result of the small number of PEs expected, the PMTs operate
either near to, or at, maximum high voltage, and, thus, at maximum gain. As such, they
can suffer damage if a sudden light leak develops. In testing, we verified the extreme
sensitivity to minute light leaks, even across the whole length of the structure, because
of the mirrored surfaces inside the enclosure. With 26 PMTs operating at maximum
gain, and viewing, effectively, a giant mirror, sealing the enclosure against single photon
penetration requires extra care during initial testing and operations.
The PMTs chosen for the counter were Burle model number 8854, 127 mm photocathode diameter 10 . The PMT amplification electronics have been described in Refs. [27,
28]. The dynode chain incorporated a 600 kΩ resistance between the cathode and first
dynode, instead of the nominal 300 kΩ. This generates a Vdyn = 885V across the cathode
to dynode gap, thus, increasing the photo-electron collection efficiency and peak to valley
(P/V) ratio. This modification has been proven successful in increasing the PE collection
9
10
Optitron Inc. 23206 S. Normandie Ave. #8, Torrance, CA 90502, USA.
Burle Industries Inc., 1000 New Holland Ave., Lancaster, PA 17601, USA.
CHAPTER 18. AEROGEL CHERENKOV COUNTER
305
Figure 18.2: Cross sectional drawing of the counter, along the particle direction, showing
the planar parabolic nature of the mirrors and the geometry of the PMTs, as well as the
final dimensions. The joint of the two mirror surfaces in the middle of the counter defines
the mirror “ridge”.
efficiency and the single PE resolution. The dynode amplification chain also incorporates
a 11 M Ω resistor in series with the µ-metal shield to eliminate the possibility of electric
shock through careless handling; this high impedance also limits the current drawn, in the
unlikely event of a complete dielectric breakdown between the shields and the aluminum
parts of the detector. A schematic diagram of the electronic amplification chain is shown
in Fig. 18.3.
The operation of the aerogel detector is discussed in Ref. [24].
18.2
Safety Assessment
The PMTs are under high voltage and care is required when handling any components
of the counter. As stated earlier on in this report, the insulating material between the µmetal shield and the aluminum exoskeleton far exceeds the operating voltage. In addition,
the 11 M Ω resistor between the µ-metal shield and the HV source, restricts the current
flow below the critical 1 mA level. The combination of Tedlar film, plexiglas composites,
and injection moulded bases, are all safe to handle but care should be exercised when
handling the aluminum parts of the counter or touching the metal back plate of the
base. It is strongly recommended to ground the aluminum exoskeleton of the counter,
at several spots, to a common ground with the HV and signal cable ground. This will
further enhance safety and eliminate potential ground loops in the unlikely event of a
slow, and otherwise difficult to diagnose, dielectric breakdown from the µ-metal shield
to the aluminum structure or the aluminized mylar in the interior.
CHAPTER 18. AEROGEL CHERENKOV COUNTER
306
Figure 18.3: Schematic diagram of the electronic amplification chain. The total resistance of 600 kΩ between the cathode and the first dynode is shown as three 200 kΩ
resistors for sake of clarity. In the actual PC boards, the arrangement is of six resistors
of 100 kΩ each, in order to keep the voltage across each resistor low and avoid surface
discharge between the closely packed resistors.
18.3
Operating Procedure
Operating Voltage The operating voltage on the PMTs is -2,950 V. This is a near
maximum rated voltage and it has been shown to combine high efficiency, good P/V
ratio, and long PMT life. The overall gain of the PMT is not maximum, as is measured
by BURLE, since the dynode chain of the 13 dynodes (2nd dynode to 14th dynode) is
kept at -2,600 V equivalent with the original 300 kΩ resistor value between Cathode and
1st dynode. However, the gain is more than sufficient to separate the one PE from the
pedestal of all ADCs we have used so far. It should not be necessary to increase the
voltage above the recommended one.
18.4
Handling Considerations
It is generally not advised to open up the counter if the persons involved are not thoroughly familiar with the assembly and specific component function. Routine operation
does not require any hands on modifications to the detector, as along as the following
CHAPTER 18. AEROGEL CHERENKOV COUNTER
307
operating principles are followed:
Installation and Removal of PMTs A replacement of a PMT or repairs of the
electronic amplification chain can be accomplished by removal of that specific PMT-Base
combination. Turn the HV off on all PMTs and remove the rubber hood covering the
base and housing interface region. Now remove the three small screws attaching the base
to the integral housing. Note that the base can only be secured to the housing in one
specific orientation.
Carefully slide out the base with the PMT and µ-metal shield mounted as one unit.
Remove the elastomeric ring positioned between the PMT and the µ-metal shield. Loosen
the nut securing the µ-metal shield to the base and carefully apply upward force on the
shield while someone else is holding onto the base. This will remove the PMT and the
µ-metal shield from the socket and base, respectively.
Replacement of the PMT requires experience because it has to be done with the µmetal shield installed in, but not secured to, the base. The PMT pins need to be aligned
with the socket pins in a specific geometry, thus, the insertion has to be done by feel and
experience. Once the PMT is inserted in the socket, the µ-metal shield is secured the
base with the nut. Make sure the shield protrudes past the photo-cathode as much as
the tapered design allows. Carefully insert the elastomeric ring between the PMT rim
and the µ-metal shield. This ring supports the PMT and prevents it from sliding out of
the pins during movement; it also helps seal the interior of the counter from the outside
environment and reduces CO2 leakage rate. Reverse the process for installation.
Installation and Removal of the SiO2 Tray PLEASE NOTE: The SiO2 aerogel
panels are extremely fragile and sensitive to water and chemical vapour. Do not handle
with bare hands, use clean cotton, or other fabric type, gloves instead. Surgical gloves
often are contaminated with lubricants and are not suitable for this purpose.
The tray is secured to the main section by hex bolts. Removal of the bolts results in
the straightforward removal of the tray. There is minimum clearance between the tray
walls and the main section; as a result, the tray has to be removed and installed in a
uniform translation with respect to the main body. The frame supporting the fish net (or
tennis racket) can be removed from the tray proper by removal of the two small screws in
the middle of the tray walls and a tool (hook) is provided for this operation. The SiO2
aerogel panels can now be removed or replaced. Reverse the procedure for installation.
The securing bolts do not need to be tightened very much and, although spacers are
inserted between the rubber strips to prevent damage, care and common sense should be
exercised. Light and gas sealing is provided by the rubber strips NOT by brute force.
WARNING: After each removal of any components of the counter, check for light
leaks before turning the HV on at operating values. Even a small light leak can destroy
the PMTs if they are at -2,950 V! Check for light leaks with lights out, using a small
portable light, and reduced voltage around -2,000 V.
CHAPTER 18. AEROGEL CHERENKOV COUNTER
18.5
Diffusion aerogel counters
308
11
For reliable PID of the kaon with momentum up to 2.84 GEV/c the aerogel detector
with low refraction index of 1.015 (A1) was constructed. Because with low index the
light yield suppose to be less a new design of the counter was evaluated and optimized.
It resulted in achievement of the average number of 8 photo-electrons.
For reliable positive identification of the kaon and rejection of the protons large
number of photo-electrons is very important. The second diffusion aerogel counter (A2)
was constructed with the aerogel refraction 1.055. With only 5 cm thickness of aerogel
almost 30 photo-electrons were collected. Large collection efficiency was achieved by
several design considerations and use of different type of PMT - XP4572B.
Each detector consists of a tray for the aerogel radiator and a diffusion box which
holds PMTs. The surface of the each box covered with millipore paper. The hydrophobic
aerogel was used for both detector, however the boxes are gas tight, so hydro-scopic
aerogel also can be used. The positive HV used in the detector allow to increase the
solid angle viewed by each PMT and as result the light collection efficiency. The PMTs
don’t have µ-metal shields, because the magnetic field at the location of these detectors
doesn’t effect the light collection efficiency. The schematics of A1 and A2 are shown in
figs 18.6 and 18.7. The structure of the diffusion box is shown in Fig. 18.4. The picture
was made before installation of the millipore paper. The Fig. 18.8 shows performance
characteristics of the A1 and A2 counters. The Fig. 18.5 shows the view of A1 counter
from inside. The are semi-spherical photo-cathode of 8854 on left and right side. The
white wires on the bottom are installed to avoid aerogel blocks motion during detector
transportation.
18.6
Responsible Personnel
The following individuals are responsible for the operation of the aerogel Cherenkov
counters.
Segal, John - x7242
Wojtsekhowski, Bogdan - x7191
18.7
Safety Assessment
The PMTs are under high voltage and care is required when handling any components
of the counter. The body of the counter must be grounded. Positive polarity of the HV
made operation much more simple, but in some situation the HV can be on the body of
counter, for example in case of the PMT vacuum failure. The important requirement is
to switch HV off before disconnection or connection HV cable to the HV divider. If some
11
Author: B. B. Wojtsekhowski mailto:[email protected]
CHAPTER 18. AEROGEL CHERENKOV COUNTER
309
of the PMT need replacement the HV must be off for all of them and HV supply must
be disconnected from all PMT.
CHAPTER 18. AEROGEL CHERENKOV COUNTER
Figure 18.4: The diffusion box of A2 detector.
Figure 18.5: Aerogel A1 detector from inside of the diffusion box.
310
CHAPTER 18. AEROGEL CHERENKOV COUNTER
PMTs assembly box
Millipore
RCA 8854
Honeycomb
RCA 8854
Aerogel assembly box
Aerogel n= 1.015, 9 cm
Figure 18.6: The scheme of A1 detector.
PMTs assembly box
Millipore
XP4572
Honeycomb
XP4572
Aerogel assembly box
Aerogel n = 1.055, 5 cm
Figure 18.7: The scheme of A2 detector.
311
CHAPTER 18. AEROGEL CHERENKOV COUNTER
photon collection vs particle momentum
312
amplitude spectra for 2 GeV electrons
Electrons
8
Aerogel index n = 1.015
4
Pions
0
0.5
1.0
1.5
2.0
GeV/c
0
4
8
12
photo−electrons
Electrons
30
Aerogel index n = 1.055
20
Pions
10
0
0.5
1.0
1.5
2.0
GeV/c
0
10
20
30
photo−electrons
Figure 18.8: Number of photo-electrons in A1 and A2 vs particle momenta and the
amplitude spectra.
Chapter 19
The Focal Plane Polarimeter 1 2
19.1
Overview
The focal plane polarimeter measures the polarization of protons in the hadron spectrometer detector stack. When the protons pass through a carbon analyzer, the nuclear
spin-orbit force leads to an azimuthal asymmetry in scattering from carbon nuclei, if the
protons are polarized. The particle trajectories, in particular the scattering angles in
the carbon, are determined by pairs of front and rear straw chambers, a type of drift
chamber.
As shown in Figure 19.1, the front straw chambers are separated by about 114 cm,
and are located before and after the gas Cherenkov detector. The second chamber is
followed by scintillator 2, which is in turn followed by the polarimeter carbon analyzer.
The rear chambers, chambers 3 and 4, are separated by 38 cm and are immediately
behind the carbon analyzer.
The carbon analyzer consists of 5 carbon blocks. Each block is split in the middle
so that it may be moved into or out of the proton paths, so that the total thickness
of scattering carbon may be adjusted. The block thicknesses, from front to rear, are 9”
(22.9cm), 6” (15.2cm), 3” (7.6cm) , 1.5” (3.8cm) , and 0.75” (1.9cm). The block positions
are controlled through EPICS [2]; the controls may be reached through the Hall A /
hadron spectrometer / detectors menus (see Fig. 21.4). Particles passing through the
carbon analyzer can be absorbed in it.
The straw chamber planes are designated as X, U, and V planes. The central ray
defines the z axis. X wires measure position along the dispersive direction. The UV
coordinate system is created by a 45 degree rotation in the transverse plane of the XY
coordinate system, with +U between the +X and +Y axes, and +V between the +Y
and -X axes.
The straw chamber operation is described in the following paragraphs.
When a charged particle passes through the chamber in typical Jefferson Lab operating conditions, there will be about 30 primary ionizations of gas molecules. Positive high
1
2
CV S revision Id: fpp.tex,v 1.4 2003/11/14 21:57:36 nanda Exp
Authors: S.Nanda mailto:[email protected]
313
CHAPTER 19. THE FOCAL PLANE POLARIMETER
Gas
Cerenkov
S2
314
Rear Straw
Chambers
Aerogel
S1
VDC
Carbon Analyser
max. 51 cm
Front Straw
Chambers
Figure 19.1: Schematic of the hadron detector stack.
voltage of about 1.8 - 1.9 kV is applied to the wire in the center of each straw. Electrons
from the ionizations drift towards the wire. When the electrons get within about 100
µ of the wire, the gain in energy between collisions with gas molecules is sufficient that
gas molecules are further ionized in collisions. This leads to an avalanche, and a gain of
about 105 per primary ionization under the conditions in which the FPP is run.
The movement of the positive and negative ions leads to a voltage drop on the wire,
or equivalently to a negative analog signal. The analog signal is about 20 ns long, with a
(negative) peak current of about 40 µA, and propagates towards each end of the straw. At
one end of each straw is a board that supplies high voltage (see Figure 19.2); impedance
matching on this board, with a 1500 pF capacitor and a 370 Ω resistor, reduces reflection
of the signal.
The other end of each straw is connected to a readout board, that amplifies, discriminates, and multiplexes the input signals – (see Figures 19.3 and 19.4 ). At the
readout end, the signal is “coupled to ground” through a 1500 pF capacitor followed by
310 + 50 Ω resistors. In parallel with the 50 Ω resistor are diodes to limit the signal size,
preventing damage to the readout board circuitry. An amplifier samples the signal over
the 50 Ω resistor. The amp gain is about -10 mV/µA, resulting in a +400 mV signal to a
comparator. A threshold voltage input to the readout board is put over a voltage divider
CHAPTER 19. THE FOCAL PLANE POLARIMETER
4
3
315
2
1
REVISIONS
ZONE
LTR
DESCRIPTION
DATE
APPROVED
D
D
Pin Numbers for J1 and J2
C
B
Channel #
J1- Sig
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
15
13
11
9
7
5
3
1
15
13
11
9
7
5
3
1
+HV
J1-RET
J2-STRW
16
14
12
10
8
6
4
2
16
14
12
10
8
6
4
2
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
1M
R1-X
C
J2-STRW
1500p
3KV
C1-X
30.1K
J1-SIG
R3-X
357
R2-X
x=channel #
DRAWING NAME
PROJECT NAME
Straw Termination
Gilman
APPROVALS
DATE
RUTGERS UNIVERSITY
DRAWN
A
8/24/94
EHB
Department of Physics
and Astronomy
CHECKED
SIZE
ENGINEER
Edward Bartz
4
3
B
J1-RET
2
A
REV.
A
SHEET
A-0
1 OF 1
1
Figure 19.2: Circuit diagram for the high voltage / termination board.
consisting of 1500 + 10 Ω resistors. For the typical 4 V threshold applied to the board,
the comparator puts out a logical pulse when the 400 mV (peak) signal rises above the 4
V / 151 = 26 mV threshold. One-shots are then used to fix the width of the logical pulse
for each channel – the one-shot width is fine tuned by the use of high precision resistors
in an RC circuit; these resistors are mounted in sockets so as to be easily replaced if the
need arises. An OR circuit then combines eight individual straw outputs into a single
electronics channel.
Internally, within the Faraday cages, the high voltage is distributed to stacks of high
voltage / test pulser boards, through which it is connected to each straw via a 1 MΩ 1/4
watt resistor.
The readout cards require a high-current low-voltage power supply and a low-current
low-voltage power supply for a threshold level. The readout electronics are mounted on
the chamber, shielded within Faraday cages. The high-current power supplies were built
by the Rutgers University Department of Physics & Astronomy Electronics Shop. These
supplies are set to provide sufficient current at ±5 V for the boards to which they are
hooked up. No adjustments, except for turning the supplies on / off, should be needed in
normal operation. There are voltage setting, current limiting, and overvoltage protection
potentiometers within the boxes; adjustment information is given in the FPP logbooks.
CHAPTER 19. THE FOCAL PLANE POLARIMETER
316
+5va
-5
C65
+5va
0.1
0.1
C1-1
R1-1
0.01
2
C3-1
6
1
1
R51
510
C66
C2-1
1n4150
1n4150
49.9
1500p-3Kv
D1-1
D2-1
R2-1
309
2
R3-1
191
U1
220p
VTH
1
C4-1
+
U17
MVL407
3
16
11
R50
100
5
3
8
4
2
7 Nec1663
8
1
4
19
4
U22
5
20
10102
0.1
C69
7
0.01
-5va
C70
10102
0.1
-5
Figure 19.3: Circuit diagram for the amplifier / discriminator section of the readout
board.
+5va
-5
C65
0.1
C96 -5
1
R51
510
C66
0.01
-1
1
R64
510
0.1
2
8
1
4
2
0p
R50
100
1
VTH
+
U17
MVL407
3
16
11
2
19
4
20
U22
12
2
U22
13
5
15
10
9
10102
C94
470p
10102
R61
732
7
U22
3
6
C70
10102
0.1
14
3
10102
510
R64
1
R59
10.0
OUT1
C69
0.01
U22
11
-5
7
-5
-5
R64
510
C102
1
R69
510
-5
-5
0.1
OUT1
OUT5
OUT6
OUT7
OUT8
7
5
6
4
U25
3
OUT4
OUT3
OUT2
2
10
12
9
11
13
14
C103
0.1
R72
15
10109
453
10109
R71
510
R68
510
R70
453
J2
U25
4
C105
0.1
J2
3
C104
C101
0.1
0.1
U22
6
-5
-5
Figure 19.4: Circuit diagram for the logical / multiplexing section of the readout board.
CHAPTER 19. THE FOCAL PLANE POLARIMETER
317
0.1
+5v
-5v
-5v
1
56
0.1
390
R17
C17
R2-1
31
J1
C1-1
6
1
2
4
2
R3-1
1.00k
U1
0.1
8
R5-1
3
R1-1
2
7
U17
11
C2-1
0.1
56
10
9
5
J2
10116
1.00k
7 Nec1663
32
J1
2.74k
R6-1
6
J2
Vbb
1
3
1.56k
R4-1
390
R17
C3-1
1
-5v
-5v
Figure 19.5: Circuit diagram for the level shifter / receiver board.
The low current supplies are Hewlett-Packard 6111A supplies. The 6111As can provide
up to 1 A for voltage from 0 to 20 V. The supplies are currently hooked up through the
rear panel to a DAC in the data acquisition panel; front panel controls on the supplies
are disabled, except for the on/off switch. The voltage is controlled through an EPICS
FPP threshold window, that is accessed through the Hall A / hadron spectrometer /
detectors menus. The high-current supplies are not computer controlled. All supplies
are mounted in the detector stack.
The multiplexed logical signals from the chambers have amplitudes smaller than
ECL levels, to prevent noise at the chamber. These signals are fed to level shifter boards
(see Figure 19.5), located in the FPP rack on the lower electronics level of the detector
stack, on the beam right side. A high-current ±5 V power supply for the level shifter
boards is located at the bottom of the same rack. The boards convert the signals to
ECL standard levels. The level shifter outputs are connected to the starts of LeCroy
Model 1877 FASTBUS TDCs, located in the lower electronics level on the beam left side.
The TDCs measure both leading and trailing edge times to allow demultiplexing. The
TDCs are subsequently stopped by the overall event trigger, and are read out by the
CODA acquisition software. The data are histogrammed online by the DHIST software.
In-depth offline data analysis requires the ESPACE software.
The chamber gas is presently a combination of argon and ethane, about 63% and
37% by weight. The Hall A gas shed is outside next to the entrance of the Hall A truck
ramp. Gas is routed from the Hall A gas shed mixing system to the gas panel located on
the lower electronics level of the space frame, and subsequently to the FPP chambers.
The gas system is shared with the VDCs. A detailed description of the system has been
written by Howard Fenker3 .
In addition, the chambers are outfitted with a test pulser capability. A pulse is
3
http://www.jlab.org/Hall-A/document/HAWGS/HAWGS_OpMan.html
CHAPTER 19. THE FOCAL PLANE POLARIMETER
318
introduced into an 8 channel (16 wire) twisted pair cable on each chamber, which connects
to the high voltage boards, at the opposite ends of each straw from the readout boards.
The pulse is resistively coupled through a 20 kΩ resistor to the ground leg of a 1500 pF
capacitor, and thence into the straws. After propagating through the straw, the pulse
enters the readout board. A pulse of about 1 V amplitude in the twisted pair cable is
sufficient to provide a few mV signal into the readout boards, resulting in a logical output
signal. The system may be used to test the functionality of each readout channel and /
or the continuity of the high voltage wire in each straw. The system currently is only
implemented for manual operation, except that data may be read out through CODA.
This procedure requires some familiarity with trigger logic and setup, should only be
done by experts, and is not documented here.
19.2
Operating Procedure
Gas Flow Operating Procedures The chamber gas is mixed 63%-37% (by weight)
Ar ethane. The gas is mixed in the Hall A gas shed which is located next to the entrance
to the Hall A truck ramp. One needs key #8, which is located in a key box in the Hall A
counting house, to get inside the shed where the gas mixing is done. The argon and
ethane bottles which feed the gas mixing system are located outside the shed and can be
exchanged when they are empty. The mixed gas is sent down into Hall A and to each of
the detector huts. There are two each of argon and ethane bottles connected to the gas
system and a Matheson 8590 controller switches between the two bottles when the gas
pressure in the bottle drops below a certain level. At this point the one bottle can be
replaced while the other is being used. The procedure for changing gas bottles is outlined
below:
1. Warning: High pressure gas bottles contain significant stored energy and are potentially hazardous. Handling of gas bottles should be done only by qualified, trained
personnel.
2. For smoothest operation, used gas bottles should be replaced before their internal
pressure drops below the desired regulator output pressure.
3. Two possible cases exist in which a gas bottle needs to be replaced: only one empty
gas bottle on a system or both bottles empty on a gas system.
4. For case 1 the sequence of steps is as follows:
(a) Check in the Hall A Gas Shed. If all bottles have sufficient pressure each of the
Matheson 8590 controllers will have one green ”RUN” LED lit and one yellow
”READY” LED lit. A red ”EMPTY” LED lit indicates a bottle with low
pressure, the corresponding bottle needs to be replaced. If a red ”EMPTY”
LED is lit the central ”ALARM” LED should also show red. Nothing further
needs to be done here; go outside to the Gas Bottle Pad.
CHAPTER 19. THE FOCAL PLANE POLARIMETER
319
(b) Visually verify that the corresponding pressure gauge on the flex line is showing
a low pressure. A low pressure is not necessarily zero. Close the bottle valve
for the empty bottle.
(c) Disconnect the empty bottle from the high-pressure flex-line. The in-line
check-valves will prevent gas escaping from the manifold. Replace the bottle’s
cap, and move the empty bottle to the EMPTIES storage rack. Note that
ethane bottle fittings, type CGA-350, have left-handed threads.
(d) Place a full bottle of gas in the on-line rack, remove the bottle cap, and connect
the bottle to the flex-line.
(e) Open the new bottle’s valve, check for leaks at the bottle fitting. The corresponding pressure gauge should now read full bottle pressure.
(f) The ALARM state of the Matheson 8590 controller should have automatically
reset. Check inside the Hall A Gas Shed. Each controller should show a green
”RUN” and yellow ”READY” LED lit. If not, re-check the installation of the
gas bottle.
5. For case 2 the sequence of steps is as follows:
(a) Check in the Hall A Gas Shed. If all bottles have sufficient pressure each
of the Matheson 8590 controllers will have one green ”RUN” LED lit and
one yellow ”READY” LED lit. If a Matheson 8590 controller shows two red
”EMPTY” LEDs lit and the central red ”ALARM” LED lit, both bottles of
the corresponding manifold need to be replaced. Nothing further needs to be
done here, go outside to the Gas Bottle Pad.
(b) Follow steps 2. through 5., as detailed immediately above, for both bottles.
(c) The ALARM state of the Matheson 8590 controller should have automatically
reset. Check inside the Hall A Gas Shed. Each controller should show two
yellow ”READY” LEDs lit. If not, re-check the installation of the gas bottle.
Press either of the two buttons labeled ”LEFT BANK” and ”RIGHT BANK”.
The lit LED above the button you pressed will change from yellow ”READY”
to green ”RUN”. You will most likely need to reset the Low Supply Pressure
shutdown at this point.
The four FPP straw chambers are connected in parallel to the gas system. (see
Figure 19.6).
(The FPP chambers are also in parallel with the VDC chambers.) All gas connections
are made using POLYFLOT M tubing and TJNAF-specified connectors. The chamber
volumes range from approximately 120 to 220 `. Gas pressure in the chambers is typically
a few Torr above atmospheric pressure. The gas flow through the chambers may be
independently varied and is typically set to 7 `/hr, leading to a replacement of the
chamber volumes about every 15 - 30 hours. Gas is exhausted from the FPP chambers
through a bubbler containing < 1 mm of mineral oil. A typical chamber leakage rate at
CHAPTER 19. THE FOCAL PLANE POLARIMETER
vdc
115 fpp 2
vdc
40
fpp 3
35
fpp 4
115 fpp 1
320
Digital output flowmeters
I
B
vdc
vdc
I
B
vdc
NU B
fpp1
I
B
fpp2
I
B I
I
fpp3x fpp1
vdc fpp1 fpp2 fpp3 fpp4
manual flowmeters
13-15psi
vdc
small
flowmeter
at side of
gas panel
for fpp ch3
x planes.
fpp
INput pressure gauges
Figure 19.6: Drawing of the gas panel on the hadron detector stack.
CHAPTER 19. THE FOCAL PLANE POLARIMETER
321
this flow rate is 25 - 50 %. The flow rate of 7 `/hr when combined with the leak rate
of ≤ 3 `/hr results in a complete exchange of gas in the chambers roughly every 1 - 2
days. At this level of consumption, a full gas bottle connected to the FPP system lasts
approximately 10 days. When a bottle is nearing empty (≈ 90%), it should be changed
since there may be heavy contaminants in the gas. Gas bottles may only be changed by
authorized personnel.
Gas-handling Procedures
1. Typically gas is continually flowing though the chambers. If at all possible, gas flow
should be continuously maintained, even in no-beam time periods. This avoids time
loss to reconditioning and maintains the desirable steady-state operating condition.
If the chambers are not being used in an experiment, the flowmeters for the front
chambers are set to 20 and the flowmeters for the rear chambers are set to 60.
When the chambers are used in an experiment the standard setting for the front
chambers is 40 and for the rear chambers it is 105.
2. Gas pressure at the gas panel on the detector stack should be in the range 13 - 15
psi. With the large leakage rate of the FPP chambers, we typically run at near
the limit of the capacity of the gas mixer to supply the gas flow demanded by the
FPP and VDC chambers. Therefore it is possible to demand too much flow rate
from the mixer. If the gas pressure drops below 13 psi drop the flow to the FPP
chambers and contact Jack Segal or Howard Fenker to determine the cause and
remedy for the situation.
The status of the gas handling system should be monitored carefully as well as logged
at least once per 8-hour shift. Any substantial deviation from the median parameters
indicates a change in the operational parameters of the FPP and should be immediately
investigated.
Power Supplies and Electronics Procedures The power supplies and readout electronics associated with the FPP are a mixture of commercially purchased equipment
and equipment designed and/or assembled with the Rutgers University Department of
Physics & Astronomy Electronics Shop. The reader is directed towards the manuals
made available by the manufacturer for the detailed information not provided here for
the commercial equipment. For the Rutgers constructed equipment, further documentation is available on the web page4 .
and through FPP notebooks (try for example contacting R. Gilman for notebooks maintained by Rutgers, CEBAF Center, phone 757.269.7011).
The LeCroy 1458 HV control crate houses the Lecroy 1469P modules which control
the HV for the FPP chambers. The 1469P has 3 master HV channels and each master
HV channel controls eight slave channels. In slot 7 of the 1458 is the 1469P module
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which controls chamber 1 and chamber 2. In slot 8 of the 1458 is the 1469P module
which controls chamber 3 and chamber 4. The individual slave channels can trip from
high current faults or other trip faults, but all eight slave channels must be raised and
lowered together by setting the master high voltage. The HV provides +1.8 - 1.9 kV
nominal to each of the ≈ 5100 wires in the four FPP straw chambers. The power supply
is located in the detector stack at the top of crate 6 in the upper electronics level. This
unit is controlled through HAC13. Connections from the power supply to the chambers
are made using standard SHV connectors mounted on red RG-59/U HV cable good to 5
kV.
The high-current low-voltage supply boxes were assembled by Rutgers University.
They are designed to provide a maximum current of about 1.6 / 0.6 A at -5 / +5 V to
each of the 318 readout cards on the four chambers. There are 63 / 63 / 90 / 102 cards on
chambers 1 / 2 / 3 / 4. Typical operating currents are about two-thirds of this nominal
maximum value. The +/-5 V power lines are independently fused to each card. Each of
the eight supply boxes contains two or three power supplies, each rated for either 35 or
50 A. There are two power boxes for each chamber. Six boxes are located at the lower
rear end of the detector stack. The second boxes for chambers 3 and 4 are located at
the top of the detector stack, on an aluminum plate just off the upper electronics level.
These power boxes are monitored through EPICS, but turned on/off though front panel
switches.
Hewlett-Packard 6111A power supplies are used to provide typically 2 - 3 mA current
per readout card. Each of the front and rear chambers have their own power supply. The
front chambers thresholds are fused, to limit current drawn in case of a short on the
board. The rear chamber cards use a 1.5 kΩ resistor external to the board to limit
current drawn, in case of a short on the board. Board threshold circuitry also has a 1.5
kΩ to ground which with the external 1.5 kΩ makes a voltage divider. Therefore, the rear
threshold supplies are typically set to a voltage which is a factor of two larger than the
front threshold supplies to give the same threshold voltage at the readout board. Initial
tests indicate that at least a 1.5 V threshold must be applied to the cards to prevent
oscillations - this level will stop oscillations that arise when the voltage applied is reduced
to about 1.0 V. In practice it has been found that the front chambers should be operated
at 4 V and the rear at 7 V. Efficiency studies show that the chamber threshold could be
raised by 50% with minor loss in efficiency. The HP supplies are also mounted in the
hadron arm detector stack, on an aluminum panel located beneath the two upper high
current supplies.
Each straw wire contains a 25µm φ, Au-plated tungsten-rhenium wire. The number of wires per plane varies from 176 to 272. Wires are multiplexed 8 wires into one
electronics channel, leading to a required 636 TDC channels. In practice a few extra
channels are used, so that each 34 wire (16 differential signal channels plus one ground
pair) twisted pair cable contains only signals from one of the four chambers. LeCroy
1877 multihit FASTBUS TDCs are used to measure the leading edge time and width of
the pulses, to demultiplex the wire hit. Within each group of eight wires, the widths are
set to about 25, 45, 35, 55, 90, 65, 105, and 75 ns. The TDCs are located in the upper
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FASTBUS crate located on the lower electronics level of the spectrometer space frame
in the detector hut. The FPP rack, containing level shifter cards, is located opposite the
FASTBUS crates on the lower electronics level. It shifts signals sizes from the reduced
±50 mV readout card output levels to ECL standard levels, for input to the TDCs. The
connections between the readout cards and the level shifter cards, as well as between
the level shifter cards and the TDCs, are made with 16-conductor twisted-pair cables. A
wiremap, detailing the cabling, is posted on the side of the FPP rack.
Power-up Procedure
1. Ensure that gas flow has been established in the chambers as outlined in the previous section. If it has not, STOP RIGHT HERE! Gas flow must be well-established
and steady-state BEFORE the HV may be enabled.
2. Ensure that all power supplies as well as the FASTBUS crate are off and the LV,
HV, and TDC cables are connected.
3. Turn on the threshold and LV power supplies. Use EPICS to turn the threshold
voltages up to correct values, about 4.0 V for front chambers 1 and 2, and 7 V for
rear chambers 3 and 4.
4. Use HAC13 to turn up the chamber voltages. Standard values are 1875 V for front
and rear chambers. It is probably best to raise the HV in 300V steps. After each
step wait for the current to settle below 1 µA, then go up to the next level until
1875V is reached. Peak currents during turn-on should not exceed about 40 µA. A
10 V/s ramp rate leads to a leakage current of several µA. Trip levels should be set
to 110 µA both for turning on HV and for normal operation, so that bad spills do not
trip the chambers. Current should settle to about a µA or less within a few minutes.
If the power supply trips during the ramping procedure, it is possible that you are
moving too fast, or that some problem has developed with a chamber. Rezero things
and begin the procedure again. NEVER USE THE AUTO-RESET FUNCTION. If
the power supply trips again, STOP IMMEDIATELY AND INVESTIGATE. There
is probably a problem and expert advice may be needed. Some detailed information,
intended for experts debugging hardware problems, is available in the Rutgers web
pages.
5. Check for poor signal connections evidenced by hot wires (wires counting extremely
fast) or dead wires (wires with no counts) using the histogramming software and
cosmic rays. Be careful: apparent problems may result from bad demultiplexing
rather than from poor signal connections. Remake any connections as necessary by
first powering down the FASTBUS crate.
If at all possible, the HV and LV power supplies should be left on continuously if
and only if gas is available to the chamber. This avoids time loss to reconditioning and
maintains the desirable steady-state operating condition.
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19.3
324
Carbon Doors
Four of the five doors operate remotely, the fifth needing further testing before it is
certified reliable. The doors use the EPICS control system to activate and read back the
various components.
Each layer of carbon doors has one relay board. Each board is identical in operation
and there is one spare in the event one of them should fail. The global purpose of the
relay board is as follows:
1. Turn on the 12V to power to rest of circuit board.
2. Set the polarity on the 90V used to power the motors.
3. Turn the 90V on.
4. Cut off the 90V to a motor if the appropriate limit switch is hit.
5. Read back the status of the limit switches.
The 12V used to power the circuit board runs through this relay and it is activated
via an EPICS relay in VME crate 4 (hallasc4). Relay #1 turns on the 90V and it too is
activated by an EPICS relay in VME crate 4. Relay #2 switches the polarity of the 90V
being fed to the driving motors. When activated it reverses the polarity to the motors
and it is controlled by a relay in VME crate 4. Relays #3 and #5 are activated by the
inner limit switches of the carbon doors. When these switches are depressed the relay
activates and the 90V is cut off. Relays #4 and #6 are activated by the outer limit
switches of the carbon doors and like relays #3 and #5 cut off the 90V when activated.
Relays #4 and #6 activate when opened rather than when depressed. It would be nice
in the future to have relays #3 and #5 also activated by an open limit switch condition
and deactivated when the switch is closed. This way the 12V could be off to one of the
switches and the doors would stop moving. As it is now, a broken wire/short while the
doors are closing could cause the doors to continue moving risking possible damage.
The status of the limit switches is readout via an ADC in VME crate 4. If the
switches are closed a -4V is seen at the ADC input. This is effected via a voltage splitter
of 3 kΩ - 6 kΩ resistors. The readouts are plugged in via telephone jacks (PJ4, PJ5,
PJ6, and PJ7). A temporary fix has been put in place which sends the signals through
a capacitor first to block voltage spikes going into the ADC. These voltage spikes caused
the ADC to trip off-line which can only be fixed by resetting the VME crate.
The operation of the carbon doors is done via a GUI style control panel . This panel
is located under the detector screen of the hadron arm (FPP Carbon Doors). The 3/4”
carbon door has been disconnected at the 90V power supply and is not implemented in
the software GUI. This door had what may have been some sliding problems. Since it
may take a great deal of force to remove this door if it should jam, it will need to be
tested so it can be removed easily if it should jam. The normal operating procedure with
the GUI is to first make sure all the 90V power is off to each door (Blue switches), then
CHAPTER 19. THE FOCAL PLANE POLARIMETER
Figure 19.7: EPICS GUI for the carbon doors.
325
CHAPTER 19. THE FOCAL PLANE POLARIMETER
326
to turn on the 12V power to each door to see where it is located in the stack (in vs.
out). If you wish to change the status of a door (in/out) then simply toggle the IN/OUT
switch appropriately and turn on the 90V. It takes some time for the doors to move the
entire range, so be patient. When the limit switches have been reached the appropriate
indicators will light up. You should then turn the 90V off. The important aspect of this
procedure is to make sure that you do not change the polarity of the 90V while the doors
are moving. This place undue stress on the motors and the power supply as well.
19.4
Handling Considerations
The FPP straw chambers are very delicate devices which are absolutely essential to many
Hall A physics experiments. Thus, extreme care must be taken whenever they are moved
or used. Also, extreme care must be taken that other objects are not moved into them.
• Before moving a straw chamber, ensure that any protective plates are in position.
• Disconnect and reconnect all TDC, HV, and LV cables with care.
• When initiating gas flow, pay strict attention to the feedback parameters. Straw
chambers are not very sensitive to overpressure of perhaps 50 - 100 Torr, but the
straw chambers can be easily destroyed by a few Torr underpressure.
• Never attempt to apply HV to the chambers until gas flow conditions have reached
steady-state.
• As the amount of heat generated by the pre-amp/discriminator cards is substantial,
always make sure adequate cooling is provided before attempting to run. This
is mostly ensured by making certain that the various cooling holes through the
Faraday shields are not covered. The chambers have internally mounted fans where
needed, which are powered up along with the readout cards.
• If the leakage current on the high voltage rises linearly with voltage, then a wire
has broken and is shorted to ground!
19.5
Safety Assessment
The following potential hazards have been clearly identified.
The High Voltage System The LeCroy 1458 HV low current power supply provides
a nominal +1.80 kV. Red HV RG-59/U cable good to 5 kV with standard SHV
connectors is used to connect the power supply to the chambers. Each HV channel,
of the 6 per chamber, typically will draw a few hundred nA.
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The Low Voltage System LV power supplies are used for the pre-amp/discriminator/multiplexer
cards. Each card requires up to 1.6 A at -5 V and 0.6 A at +5 V, plus a few mA
threshold at 4 - 8 V.
High Pressure Gas Bottles The gas used in the chambers is supplied in high pressure
(≥ 2000 psi) gas bottles. This confined high pressure gas represents a tremendous
(potentially lethal) amount of stored energy.
19.6
Responsible Personnel
The following individuals are responsible for chamber problems. Generally, the non
Jefferson Lab people are responsible for FPP detector problems, whereas the Jefferson
Lab people are responsible for more general data acquisition problems or, e.g., gas /
voltage supplies shared with other systems.
Sirish Nanda, Jefferson Lab - x7176, pager 7176
Ronald Gilman, Rutgers - x7011, pager 0161
Charles Perdrisat, William & Mary - 221.3572 or 3522
Vina Punjabi, Norfolk State - x5304
Xiangdong Jiang, Rutgers, x7011, pager 849-6664
Jack Segal, Jefferson Lab - x7242
Chapter 20
The Hall A Gas System 1 2
20.1
Overview
The Hall A detector gas systems are located in the Hall A Gas Shed alongside of the
truck ramp for Hall A. The gas cylinders in use are along the outside of the Gas Shed
in a fenced area. There are racks next to the Gas Shed for storage of full gas cylinders.
On the other side of the truck ramp there are racks for storage of both full and empty
cylinders. Hall A currently uses ethane, argon, ethanol, carbon dioxide, methane, and
nitrogen. Details of these systems can be found in the Hall A Gas Systems (HAGS)
manual. A copy of the current manual is in Counting Room A and on the Hall A web
page.
Four systems are supplied from two cylinders of Coleman grade CO2. One system
is for the gas Cherenkov counters in the HRS detector arrays. One system is for flushing
the mirror aerogel Cherenkov counter in the HRS detector arrays. One system is for
the gas Cherenkov counters in the (e,p) setup in the beamline. One system is for the
FPP straw tube wire chambers. Argon and carbon dioxide for the FPP straw tube wire
chambers are mixed inside the Gas Shed.
Three systems are supplied from two cylinders of UHP grade argon. One system is
for the VDC wire chambers of both arms. Argon and ethane for the VDC wire chambers
are mixed inside the Gas Shed and bubbled through ethyl alcohol. One system is for
the FPP straw tube wire chambers. Argon and carbon dioxide for the FPP straw tube
wire chambers are mixed inside the Gas Shed. One system is for flushing clean, inert gas
through the RICH detector wire chamber.
One system is supplied from two cylinders of Chemically Pure grade ethane. This is
for the VDC wire chambers of both arms. Argon and ethane for the VDC wire chambers
are mixed inside the Gas Shed and bubbled through ethyl alcohol.
Two systems are supplied from two cylinders of UHP grade nitrogen. One system is
used to provide pressurized gas for the automatic cylinder switch-overs in the systems.
One system is used to flush impurities from the RICH detector freon resevoir.
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One system is supplied from two cylinders of UHP grade methane. The system is
for the wire chamber of the RICH detector.
Maintenance of the gas systems is routinely performed by the Hall A technical staff.
Shift personnel are not expected to be responsible for maintaining the detector gas systems. Unexpected maintenance requirements should be handled by contacting
Jack(John) Segal - pager and phone are both extension 7242
Hall A Technician on call
20.2
Gas Alarms
In Counting Room A there are two alarm panels associated with the gas systems for
the detectors. They are located on the far left end of the control console, mounted one
above the other. The upper panel is a Gas Master flammable gas monitoring system.
The lower panel is a gas systems status indicator. The Gas Master system will go into
alarm if elevated levels of flammable gas are present in either of the Detector Shielding
Huts or the Gas Shed. The gas systems status will alarm if any of a number of faults are
detected in the Hall A Wire-chamber Gas System. The LED for the specific fault will
turn red to indicate which fault caused the alarm.
Response to an alarm should be to contact either of the personnel listed above.
Part VI
Slow Controls
330
Chapter 21
Overview 1 2
A distributed computer system based on the Experimental Physics and Industrial Control
System (EPICS) [2] architecture monitors and commands the various Hall A systems.
The basic components of the system are:
• Input/Output Controllers (IOCs) - VME systems containing single board computers (SBCs) and I/O modules (i.e analog-to-digital converters (ADCs), digital I/O
and RS-232C interfaces). Each SBC executes the real-time operating system VxWorks and the corresponding EPICS application (signal database and sequencers).
• Operator Interfaces (OPI) - Computers capable of executing EPICS tools to interact
with the IOCs. The four most used tools in Hall A are (a) a Web-enabled version
of the Motif-based Display Editor/Manager (MEDM) [9], (b) StripTool and, (c) a
signal archiver. MEDM is the main interface used for monitoring and controlling
both the hall and accelerator equipment. StripTool allows to monitor the behavior
of one or more signals as a function of time. The signal archiver keeps a record of
a selected set of signals.
• Boot Servers - IOCs load the various software components needed to perform their
functions from these machines (i.e. operating system, signal database and controls
algorithms).
• MEDM Servers - OPI computers obtain the framework of each MEDM screen from
these machines.
• Local Area Network (LAN) - the communication path joining the IOCs, OPIs and
various servers.
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CHAPTER 21. OVERVIEW
21.1
332
System’s Components
Four Linux based computers are used as OPIs: hacsbc2 (Hall A counting house), hacweb1
(101B), hacweb2 (hall) and hacweb3 (101B). Two computers act as boot servers: hacsbc2
and hlasrv (2nd-floor of counting house). Hlasrv also acts as MEDM server. The signal
archiver resides in hacweb1. The tasks assigned to the various IOCs are,
hallasc7 Right HRS motion control.
hallasc6 e-p energy measurement system.
hallasc18 Left HRS motion control.
iocha1 Arc energy measurement system - beam position and profile wire
scanners.
R
iocha2 Arc energy measurement system - 9th magnet Bdl measurement.
iocha3 RICH counter.
iocha4 Electron detector stack - VDCs high voltage and discriminator
thresholds, reset lines to various DAQ crates.
iocha5 Beam current monitors.
iocha11 Hadron detector stack - VDCs high volatge and discriminator
thresholds, reset lines to various DAQ crates.
iocha14 Left HRS - Q2, Q3 and Dipole power supplies and cryogenics, magnetic field probes and, collimator.
iocha16 Right HRS - Q2, Q3 and Dipole power supplies and cryogenics,
magnetic field proves and, collimator.
iocha17 Monitors supply of various gasses to tracking chambers.
iocha22 Electron detector stack - LeCroy high voltage supplies located at
various points in the hall (i.e. beam-line and both electron and
hadron detector stacks).
iocha26
iocha48 Left and Right HRS Q1 power supplies. BigBox power supply.
iocha49 Septum magnets
21.2
Operating Procedures
Log into the Hall A control system through one of the computers hacsbc2, hacweb1,
hacweb2 or hacweb3. The task bar has a “tool box” icon with a small arrow on top.
Clicking on the arrow brings up a menu. In the case of hacsbc2, the menu contains the
Alarm Handler application and three further sub-menus: Spectrometer Motion, StripTool
and WebMedm. The Alarm Handler application is not available in hacweb1 - hacweb3.
Three applications are located under Spectrometer Motion (HRS Floor Marks, LEFT
HRS and RIGHT HRS), two under SripTool (Strip Tool and Snapshot) and, three under
WebMedm (Accelerator Menu, Cryogenics Menu and Hall A Menu). To start any of
CHAPTER 21. OVERVIEW
333
these applications, use the left mouse button to click on the application name. These
applications, as described below, can be started from a terminal. In that case, type the
commands given without the quotes.
21.3
Alarm Handler
The Alarm Handler notifies the user when either a signal being monitored is outside some
pre-defined limits or communication with the IOC in which the signal resides has been
lost. The Alarm Handler will only detect an abnormal signal condition if the signal is
included in the Alarm Handler configuration file and, the corresponding IOC database
record is set to produce an alarm condition.
The Alarm Handler can be started from a terminal with the command start alh.
The default configuration file is /̃alh/ALH-default.alhConfig. A detail description of the
operation and configuration of this application can be found in the Alarm Handler Users
Guide.3
21.4
HRS Floor Marks
This application determines the floor marks and vernier readings required to set each
spectrometer to a given angle. The application can be started from a terminal with the
command setspec.
21.5
RIGHT HRS and LEFT HRS
The “RIGHT HRS” (“LEFT HRS”) application is used to set the right (left) spectrometer
angle. To start the “RIGHT HRS” (“LEFT HRS”) application from a terminal type the
command “bogies RIGHT” (“bogies LEFT”). The “RIGHT HRS” and “LEFT HRS”
applications are very similar so, we will use “RIGHT HRS” as an example.
Upon starting the “RIGHT HRS” application, a screen labeled “RIGHT-HRS Bogies” will open as shown in Fig. 21.1.
Pressing the button labeled “Graph” in the top-left corner of “RIGHT-HRS Bogies”
will open two more screens: one labeled “Strip Chart” and an associated, column like,
signal selection screen (see Fig. 21.2). The signal selection screen allows to select the
signals to be plotted in the Strip Chart screen. All signals are plotted with the same
color. To highlight a given signal, use the plot legend located towards the right of the
Strip Chart screen: clicking on the line next to the signal name will change its color in
the main plot. The plot screens are likely to be more useful to the Hall A technical staff
than to the shift personnel.
The application screens show the Motor Current Output (MCO) and Motor Velocity
Output (MVO) for each of the four middle-ring (M1-M4) and four outer-ring (O1-O4)
3
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CHAPTER 21. OVERVIEW
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Figure 21.1: Right HRS motion control.
motors. Also shown are the status and request buttons for the Power Supply Module
(PSM), Drive Modules (DM), brakes and, clamps. The clamp request button (“CLAMP
RELEAS”) actually releases two interlock circuits (the Forward Amplifier Clamp or FAC
and the Reverse Amplifier Clamp or RAC) so that the spectrometer is able to move in
any direction (i.e. clockwise or counter-clockwise). It is worth to stress that the PSM,
DM, brakes and clamps request buttons represent requests that the hardware interlock
circuits may negate. This can be clearly seen in Fig. 21.1 which was taken with the
electrical power to the PSM and DMs disabled for septum magnet installation in the
central pivot area. Note that PSM and DM request buttons are selected (red color) yet
the corresponding status fields show interlock incomplete status (“NO”).
To move the spectrometer, select the request buttons in descending order, starting
with “PSM ENABLE” and ending with “CLAMP RELEAS”. After selecting a button,
wait until the corresponding status changes to “YES”. If the status does not change,
reboot the IOC using the green buttons located in the middle room of the counting
house. If the failure persists, contact the Hall A on-call tech. After the clamps have
been successfully released, enter a value in the “VELOCITY SET” field (see Fig. 21.1
- “RIGHT-HRS Bogies” screen). The sign of the velocity will determine the sense of
spectrometer rotation. The sense of rotation is displayed by the field “DIRECTION”.
Safe operation of the spectrometer motion systems requires,
• Find out from the shift leader the administrative constraints imposed on spectrometer motion. These constraints are communicated by the Hall A technical staff to
the run-coordinator. Moving the spectrometers while no experiment is taking place
(for example, a maintenance period), must first be approved by the head of the Hall
A technical staff (E. Folts) or the person designated by him.
CHAPTER 21. OVERVIEW
335
Figure 21.2: Right HRS motion control - additional options.
• If the administrative constraints allow to move the spectrometers remotely, use the
Hall A cameras to ensure there are no objects in the path of the spectrometers.
• Check that the floor marks are seen in the TV monitors.
• Bring up the spectrometer motion application and go through the required steps
to get the spectrometer moving. Look at the floor marks to ensure that the spectrometer is moving in the desired direction.
• While the spectrometer is moving, use the Hall A cameras to check that everything
looks normal (for example, the cryogenic lines around the pivot). If something does
not look right, de-selecting ANY of the interlocks (“PSM ENABLE”,..,“CLAMP
RELEAS”) will stop the spectrometer immediately.
• As the spectrometer approaches the desired floor mark, reduce the spectrometer
speed. De-select the “CLAMP RELEAS” button to stop the spectrometer at the
desired floor mark.
• De-select the remaining interlocks: “BRAKE RELEAS”, “DM ENABLE” and
“PSM ENABLE”.
• Press the button labeled “Disconnect” to close the spectrometer motion application.
CHAPTER 21. OVERVIEW
21.6
336
Strip Tool
Strip Tool plots a real-time strip chart of the values of one or more signals. It is useful
to monitor data trends.
A detail description of the options and operation of this application can be found
in the Strip Tool Users Guide4 with one difference; the version used by Hall A does not
have a “print” function. To print a strip chart use the application “Snapshot” described
below. To start Strip Tool from a terminal type the command StripTool.
21.7
Snapshot
Snapshot refers to a KDE desktop application (ksnapshot) which allows to grab an image
of either the whole screen or an individual window. The image can then be sent to a
printer or stored on disk. The application can be started from a terminal with the
command ksnapshot.
21.8
Accelerator Menu
The “Accelerator Menu” application brings up a web-version of Monticello, the root
MEDM screen giving access to the various accelerator systems. Access to those systems
is read-only mode except for some Hall A applications which are described elsewhere
in this OSP. Not all the menus shown in this web-version of Monticello are operational
because they still are linked to directory structures residing in specific Machine Control
Center (MCC) computers. The application can be started from a terminal with the
command accmain.
21.9
Cryogenics Menu
The “Cryogenics Menu” application brings up the End Station Refrigerator (ESR) menu.
Access to all ESR systems is read-only mode. This application is typically used by the
Hall A technical staff to monitor the hall cryogenics. The application can be started from
the terminal with the command esrmenu.
21.10
Hall A Menu
The “Hall A Menu” application brings up a menu giving access to all the EPICS based
control systems in Hall A. The application can also be started on, say hacsbc2 from a
terminal with the command hlamain, opening a control window (see Fig. 21.3). Using
this window one can open the “Tools” window (see Fig. 21.4) containing many available
functions for slow control of Hall A equipment.
4
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CHAPTER 21. OVERVIEW
337
Figure 21.3: Hall A Main Control Screen.
21.11
Troubleshooting Procedures
The status of most IOCs can be seen by opening the ‘Hall A Menu” −− > “IOCs”.
White entries means that the IOC is not responding which can be due to either the IOC
not being used by the present experiment or it has failed. Rebooting of the IOCs is
accomplished in several ways depending on the specific IOC. If the specific IOC can be
rebooted through the Web, the url address is given next to it. The required user and
password are posted in the Hall A Counting House. The remaining IOCs are rebooted
through either the green buttons located in the middle room of the counting house or
the crate resets screen “Hall A Menu” −− > “Tools” −− > “Crate Resets”.
If an IOC fails to reset and its name is “iocha..”, call MCC and request that the
software on-call person be notified. If the name is “hallasc..” call J. Gomez.
CHAPTER 21. OVERVIEW
Figure 21.4: Hall A Tools Screen.
338
Part VII
Data Acquisition and Trigger
339
Chapter 22
Spectrometer Data Acquisition 1 2
The Hall A data acquisition uses CODA [10] (CEBAF Online Data Acquisition), a toolkit
developed at Jefferson Lab by the Data Acquisition Group. For general information about
CODA, see the CODA site3 . Up to date information about the Hall A DAQ is kept at 4 .
We typically run with two fastbus crates in each spectrometer, plus VME crates for
scalers. The fastbus modules are of the following types: (1) LeCroy model 1877 TDCs
operating in common–stop with 0.5 nsec resolution for our drift chambers and straw
chambers; (2) model 1875 TDCs operating in common–start with 0.1 nsec resolution or
0.05 nsec resolution depending on the setup, for our scintillators and trigger timing; and
(3) model 1881M ADCs for analog signals from scintillators, Cherenkov, and leadglass
detectors. In some run periods the beam position monitors and raster current were
available in a VME system, but presently they are read out in fastbus.
The trigger supervisor is a custom–made module built by the data acquisition group.
Its functions are to synchronize the readout crates, to administer the deadtime logic of the
entire system, and to prescale various trigger inputs. We have two trigger supervisors, one
in each spectrometer. This allows us to run the spectrometers independently if needed.
The public account a-onl is normally used for running DAQ and adaq is used for
running other online software including ESPACE or the C++ analyzer. On “adaq” the
directory tree of an experiment is adaq/$EXPERIMENT which is organized in subdirectories of various tasks, such as scaler display, ESPACE, and other online codes, all of
which will be described in sections below. The trigger management software is run from
the atrig account and is described in the Trigger chapter.
22.1
General Computer Information
In the counting room we have various computers for DAQ, analysis, and controls. The
controls subnet is the responsibility of J. Gomez and is documented in another chapter.
1
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3
http://coda.jlab.org/
4
http://hallaweb.jlab.org/equipment/daq/daq_trig.html
2
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CHAPTER 22. SPECTROMETER DATA ACQUISITION
341
The DAQ computer’s names are denoted by adaqXN, where X = s is SunOS and X = l
is for Linux PC. adaqs1 is the Compton DAQ computer and is normally reserved by the
Compton group. adaqs2 and s3 are for running the spectrometer DAQ or doing online
analysis; we are gradually phasing out the Suns. adaql1 and l2 are Linux computers
for running DAQ while adaql3,4... and higher are for analysis. The Linux PCs are
administered mostly by Ole Hansen.
To reboot the Suns, login as adaq and type reboot. To reboot the Linux machines,
first hit Ctrl-Alt-F1 to switch to a text console, then hit Ctrl-Alt-Del to reboot. If power
fails for a prolonged time, you must shutdown before the UPS fails.
22.2
Beginning of Experiment Checkout
This section describes the checkout of DAQ and trigger needed before an experiment can
start.
1. First ensure that all the fastbus, VME, CAMAC, and NIM crates are powered
on. They should boot up in a functional state, except for heavily loaded fastbus crates that sometimes lose their NVRAM. (If that happens, see notes in
adaq/doc/vmeram.doc).
2. You may download a default trigger, following the directions in the trigger chapter.
If the hadron momentum changes you may need to set a new delay. A trigger
expert should do the start-of-experiment trigger checklist.
3. Make sure the HV is on for all detectors and that the values are normal.
4. Start the xscaler display following the instructions below and check that the rates
from detectors are normal.
5. Startup runcontrol (CODA) using the directions below and start a run. With the
trigger downloaded and the HV on, you are taking cosmics data, typically at a rate
of 3 Hz per spectrometer. Examine the data using ESPACE or the C++ analyzer.
Compare the plots and printouts to normal values.
22.3
Running CODA
This section describes how to run CODA for the spectrometer DAQ. There are two modes:
(1) The most common is the “1-Trigger-Supervisor (1-TS)” mode which uses one trigger
supervisor and is used for coincidence experiments; and (2) The “2-Trigger-Supervisor
(2-TS)” mode which is used for running the two spectrometers independently.
The 1-TS mode can also handle single–arm triggers but is about 1/2 the aggregate
speed of the 2-TS mode. When running the 2-TS mode, one uses the a-onl account
on adaql2 for one spectrometer and the adev account on adaql1 or adaqs2 for the other
CHAPTER 22. SPECTROMETER DATA ACQUISITION
342
spectrometer. The 1-TS mode normally uses the a-onl account on adaql2 only. The
information that follows refers to the a-onl account, but the other account is quite similar.
Here is how to start and stop a run. Normally, when you come on shift, runcontrol
will be running. If not, see the section on “Cold Start” below. To start and stop
runs, push the buttons “Start Run” and “End Run” in the runcontrol GUI. To change
configurations use the “Run Type” button. If you have been running you will first
have to push the “Abort” button before you can change the run type. Typically the
configurations you want are the following.
TWOSPECT – For running the two spectrometers in 1-TS mode.
PEDRUN – To do a pedestal run in 1-TS mode
RIGHTHRS – For R-arm in 2-TS mode
LEFTHRS – For L-arm in 2-TS mode
PEDRUNR – To do a pulser run for R-arm in 2-TS mode
PEDRUNL – To do a pulser run for L-arm in 2-TS mode
A note about pedestal runs. They have the exclusive purpose of obtaining pedestals
used for pedestal suppression. For details about what is done and hints for getting
pedestals for ESPACE (which does not want the PEDRUN result), see /ped/README.
22.3.1
Some Frequently Asked Questions about DAQ
• Q: Where is the data ? Use a command “find run 1745” to find where run
1745 has been written on disk and MSS. The data are first written to disks like
/adaql2/dataN, N=1,2,3...etc. Files are automatically split, with suffixes .0,.1,.2...etc.
Splitting occurs at 2 Gbytes to avoid problem of system file size limit. Files are
archived automatically to tape in the MSS tape silo. Two tape copies are made.
Data are purged from disk automatically. Users should never attempt to copy,
move, or erase data.
• Q: How to adjust prescale factors ? Edit the file /̃prescale/prescale.dat. One common problem is putting typographical errors here which then leads to no triggers
getting accepted.
• Q: What is the deadtime ? The deadtime is displayed in the datamon window,
which normally is running next to the runcontrol window, but if this window is not
up, type datamon to bring it up. This window also shows the full-path-name of the
file being written by CODA for the present run.
CHAPTER 22. SPECTROMETER DATA ACQUISITION
343
• Q: Why is the deadtime so high ? (and related) Search for answers among the
following. The standard lore is that 30% deadtime is tolerable, but you should
ask your analysis team to decide. Sometimes people seeing large deadtimes have
forgotten to observe that the beam is in pulsed mode. Another possibility is that
the workstation is overloaded. The computer used for CODA should not be used
for much else. Do not attempt to read or write rapidly to the same physical disk
to which CODA is writing. Sometimes it is observed that the workstation itself
is very sluggish. This could be due to a foreign mounted disk having gone away,
and there are other possible reasons. If a Cold Start of CODA doesn’t solve the
problem, you may try rebooting the workstation (see computer section). Also, if
the event size changes substantially, e.g. due to VDC thresholds being turned off
(a common mistake), the deadtime as a function of rate will change, especially in
the regime of high rates.
22.3.2
Quick Resets
Problems with CODA can usually be solved with a simple reset. If not, try a Cold Start
(see next section). Do not waste an hour of beam time on resets; if they fail, call an
expert. The expert claims he can restart CODA 90% of the time within 10 minutes.
If a ROC (ReadOut Controller, or crate) is hung up, reboot by going the workspace
“Components” and typing reboot. If this doesn’t work, try pressing the reset button
which is on the “Crate Resets” section of the Hall A General Tools EPICS [2] Gui.
Telnet back into the ROC to verify its alive. Then press “Reset” in runcontrol, download
and start a new run.
22.3.3
Cold Start of CODA
If CODA is not running, or if it gets hung up, you can do a cold start. Frequently a
subset of these steps is sufficient to recover from a hangup, but it takes some experience
to realize the minimum of steps that are necessary, so the simplest thing is to do them
all, which takes a few minutes.
• Kill off all CODA processes on the workstation by typing kcoda. This stops runcontrol, the event builder, and other processes, and allows for a clean start. The
kcoda script will then tell you exactly what to do next.
• Make sure the fastbus and VME crates are running. The crates are usually known
by “ROCnumber-computer-(portserver-port)” where ROCnumber is the unique
number for that ROC (ReadOut Controller, or crate), computer is the internet
name and the portserver-port is the portserver IP and port# where to login. An
example might be ROC4-hallasfi4-(hatsv4,port3) which is ROC4, a fastbus crate
with IP hallasfi4 attached to the portserver IP hatsv4 at port 3. You can check if
the ROCs are up by looking on the Components work space at the telnet session
(if it’s not logged, try to telnet in). If the ROCs don’t talk to runcontrol, you can
CHAPTER 22. SPECTROMETER DATA ACQUISITION
344
type reboot at the arrow prompt (→). If you don’t get this arrow prompt, or if you
can’t telnet in, the computer is hung up, so press the reset button in the “Crate
Resets” GUI available from the EPICS screen for Hall A General Tools. After the
ROC comes back (2 minutes), telnet back in to verify it’s up. On rare occassions
it is necessary to power cycle the crate, which requires access.
• Start runcontrol interactively by typing runcontrol. Actually there may be more
steps prior to starting runcontrol, depending on the experimental setup. See the
printout from kcoda for instructions.
• In runcontrol, press the “Connect” button. After “connect” wait 10 seconds and
press “Run Types”. After configure and before download, press the “Reset” button
in the upper left corner. Choose the run type from the dialog box (see section on
Running CODA for descriptions of run types).
• After you configure and download the Run Type, you can “Start Run” to start a
new run.
22.3.4
Recovering from a Reboot of Workstation
If the workstation from which you are running CODA was rebooted, here is how to
recover DAQ. Login as the relevant account, which is usually a-onl for 1-DAQ operation.
Passwords for the online accounts should be available on a paper on the wall in the
counting room, or ask the run coordinator. In the workspace for “Components” telnet
into all the ROCs. If the x-terms windows are not available, type setupxterms. Start
emacs for the prescale factors: emacs /prescale/prescale.dat. Make sure msqld is running
in the process list; it is supposed to start when the computer boots. Then do a Cold
Start (see section above).
22.4
Electronic Logbook and Beam Accounting
Two tools are available for logging information by the shift workers: (1) The Electronic
Logbook “halog”, and (2) The Hall Beam–Time Accounting Table.
The electronic logbook is a web-based repository of logbook data. There are two
ways to make entries: One can use the halog GUI (type halog and make your entry), or
one may use a script to insert a file. Some data from EPICS and scalers, among other
things, are inserted automatically into halog on each start-of-run and each end-of-run.
These data also get written into files with the run number in their name in /epics/runfiles.
Data appear on the web at a certain URL 5 . It is recommended that one software expert
from the experiment be assigned to modify the logging scripts as he or she sees fit.
The Hall Beam–Time Accounting Table is the mechanism to summarize and record
how the beam time in a shift was spent. The shift leader is responsible for submitting
5
http://www.jlab.org/~adaq/halog/html/logdir.html
CHAPTER 22. SPECTROMETER DATA ACQUISITION
345
this table at the end of the shift. When submitted, the data are logged in a database and
a summary is e-mailed to various people like the run coordinators and the hall leader.
When you come on shift, the GUI is probably already running. If not, you may start it
by logging onto adaql1 as the adaq account and type “bta”. It is a fairly obvious GUI,
but there is also some online help.
22.5
Port Servers
Portservers are devices on the network that allow access to RS232 ports. Here is how to
connect from a computer: telnet hatsv5 2011 will connect to the portserver at IP hatsv5
and port 11. Note, the offset of 2000 is needed. For dealing with HV, it is best to use a
Linux PC for which the keymap is F1 = PF1 and F2 = PF2.
If another person is connected to a certain port, you cannot connect. To bump off
another user, login as root with password available from the paper posted on the wall of
the counting room (or ask run coordinator) as follows telnet hatsv5 as user = root. At
the prompt, type kill tty=4 to clear port 4, then exit. Now you can telnet hatsv5 2004.
CHAPTER 22. SPECTROMETER DATA ACQUISITION
Table 22.1: Port Servers for DAQ
server IP Port Device
hatsv3
1
vt100 Dumb Terminal
hatsv3
2
ROC1 Lower Fastbus Crate
hatsv3
3
TS0 Trig. Super. VME Crate
hatsv3
4
R-arm Upper HV Crate
hatsv3
5
R-arm Lower HV Crate
hatsv3
8
ROC2 Upper Fastbus Crate
hatsv4
1
vt100 Dumb Terminal
hatsv4
2
ROC3 Lower Fastbus Crate
hatsv4
3
ROC4 Upper Fastbus Crate
hatsv4
5
HV Crate
hatsv4
6
RICH HV Crate
hatsv4
7
RICH VME Crate
hatsv4
14
TS1 Trig. Super. VME Crate
hatsv5
1
vt100 Dumb Terminal
hatsv5
2
e-P Crate 1
hatsv5
3
Moller 1
hatsv5
4
Moller 2
hatsv5
8
Compton ROC3
hatsv5
9
Compton ROC4
hatsv5
10
Compton ROC5
hatsv5
11
Beamline HV
hatsv5
12
e-P Crate 2
hatsv5
13
ARC Energy
hatsv5
14
ROC14 VME Crate
hatsv5
15
ROC15 VME Crate
hatsv12
5
Compton ROC1
hatsv12
6
Compton ROC2
hatsv15
2nd Floor Counting Room
hatsv9
4
Parity DAQ Crate
346
Chapter 23
Trigger Hardware and Software 1 2
The Hall A trigger was designed by the University of New Hampshire. Here we give a
brief overview of the hardware arrangement, the logic of the trigger, and the usage of the
software control. Diagrams of the hardware layout are shown in accompanying figures.
Scintillators make the main trigger in each spectrometer arm. For coincidence experiments a coincidence is formed between the spectrometer arms. The main trigger is
formed by requiring that scintillator planes S1 and S2 both fired (and both phototubes
of the paddles that got a hit) in a simple overlap. To repeat, the trigger requires that
one paddle in S1 and one in S2 both got a hit in both of their PMTs (4 PMTs total).
The coincidence between spectrometers is formed in an overlap AND circuit. The Right
Spectrometer singles triggers are called T1, the Left Spectrometer triggers are called T3,
and the coincidence triggers are T5. Other triggers might be formed which require other
detectors to measure the efficiency of the main trigger. The most important is T2 on
R-arm and T4 on L-arm, whose definition has changed over time but typically require 2
out of 3 from among the S1, S2, and Cherenkov detectors (i.e. the ”or” of S1 is used,
etc).
The Hall A HRS trigger system is remotely configured by CAMAC modules. The
main change that can occur during an experiment is in the delays required to adjust the
timings of triggers which change with momentum and particle ID relevant for coincidence
setup only. Of course for single arm running one may just use the defaults, but it may
still be a wise investment in 2 minutes time to download in order to make sure of the
state of the modules. If the power is turned off, the CAMAC modules certainly must be
reprogrammed. Instructions to download the trigger are given below.
The trigger design is quite flexible and it is relatively easy to add detectors to define
new trigger types or to modify existing ones, so long as the detector is fast enough. The
trigger supervisor also allows for the possibility of 2nd level triggers which could be used
for a later decision.
Here we describe the software control of the CAMAC modules involved in the trigger.
The software control was written by Tim Smith and Jeff Vieregg of MIT with some
1
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347
DISCR.
DISCR.
RIGHT
DISCR.
LEFT
DISCR.
RIGHT
LEFT
S! PMT Signals
S2m PMT Signals
(16)
(16)
(6)
(6)
outputs.
“AND” T1
Logic
MLU strobe
S2m
Strobe
and
Retiming
(S2m leads)
S2m
Figure 23.1: Single Arm Trigger Circuit.
NIM to
TDC via
NIM/ECL
880 nsec
analog delay
to ADC
NIM to
Trigger
(1)
S2m
Signal
TM
L1A
Gates for
ADCs, TDCs
Gate
Retiming
RT
T1..and.
Cher.
2/3 Trig.
T1
Trigger
Supervisor
Notes: S2m defines timing for T1, strobe, and RT.
MLU defines 2 out of 3 trigger. EDTM signal added
to S1 with EDTM modules, and added to S2m via pulser
input to discriminator.
(16 )
“AND” Logic
16 in “OR”d to 1 out
P/S 758 Left and Right PMTs P/S 757
CAMAC
are AND’d. Have 16
delay
Cher
Cher.
S1
Logic
Fan In
“OR”
Main
Scint.
Trigger
Coinc. Trig
(see other diagram)
R. Michaels (Aug 2003)
“AND” Logic
On back of 4516 is output corresp. LeCroy 4516
to the Logic result (L.and.R) “OR”d.
S1
So, this is S1.
Cherenkov
or Other Detector
(1)
“AND” Logic
LeCroy 4516
Single Arm Triggers in Each Spectrometer
CHAPTER 23. TRIGGER HARDWARE AND SOFTWARE
348
R-arm
Singles
Trigger
Logic
Fan In/Out
“AND” Logic
LeCroy 4516
“AND Logic”
LeCroy 4516
Retiming Signal
(local scintillators)
Delay
Cable
Length
ca. 120 nsec
L-arm
T1.and.Aerogel
L-arm
Singles
Trigger
Variable Delay
(ca 120 nsec)
Coincidence Trigger
Electronics on Left Arm
TM
Gates for
ADCs, TDCs
Retiming
L1A
Trigger
Supervisor
To R-arm for
Local Retiming
R. Michaels (Aug 2003)
CHAPTER 23. TRIGGER HARDWARE AND SOFTWARE
Figure 23.2: Coincidence Trigger Circuit.
349
CHAPTER 23. TRIGGER HARDWARE AND SOFTWARE
350
input from Bob Michaels. There are four types of modules that are controlled: (1)
Discriminators; (2) Delay Units; (3) Memory Lookup Unit (4) AND/OR Modules
Here are the instructions to download the trigger. First login to the ADAQ Linux
box adaql1 or adaql2 (and no others) as atrig account. (E.g. ssh adaql1 -l atrig). The Run
Coordinator should know the password. Type trigsetup. A self-explanatory graphical user
interface pops up, where if you are in a coincidence experiment setup you must enter the
momenta and particle ID’s and then press ”Download” and WAIT for it to finish and do
not press Ctrl-C. However, for single arm running like Spin Duality or GDH, just press
”Download” with the defaults, and WAIT for it to finish and do not press Ctrl-C. The
user should look for suspicious error messages in the window from which trigsetup was
launched, e.g. to check if connection to the crate is ok.
If individual modules need to be modified for test purposes etc. (e.g. to change
thresholds), one may use the expert mode. Login to an ADAQ linux box as explained
above, then type trigsetup mapfile where mapfile is the name of the trigger map file. Some
examples of map files are in /home/atrig/trigger, see trigger left.map and trigger right.map
for the left and right spectrometers respectively. These are default databases. One can
modify each module on the fly, save the database, etc.
After you download, a record of what was sent is put into a file
/home/atrig/trigger/trigger.setup which gets put automatically into the electronics logbook “halog”. Also, whenever a CODA run is started, this file is inserted as a special
event type 136 at the start of run. This will be the setup IF the download was successful.
It is also interesting to know what is actually in CAMAC, but that can only be done in
expert mode, and the delays cannot be read from CAMAC. The simplest way to be sure
about what is in the trigger is to download again.
Chapter 24
Online Analysis, Data Checks 1 2
The following tools are available for checking data online.
24.0.1
Scaler Display and Scaler Events
Scaler rates and values are displayed using a MOTIF based display called xscaler written by C. Howell of Duke University.
Normally this is already running on adaqs2
or s3. If it is not running, login as adaq and go to the appropriate directory, which is
/home/adaq/$EXPERIMENT/electron/scaler and /home/adaq/$EXPERIMENT/hadron/scaler
for the E-arm and H-arm respectively where $EXPERIMENT is an environment variable
like e95001. Then type xscaler there. Remember to push the button “Start”. The first
several pages are the scaler rates and the next half of the pages are the absolute scaler
counts. The scalers are cleared at the beginning of each CODA run. Scalers are read out
at approximately 0.5 Hz and injected into the CODA data-stream as event type 140. A
file scaler history.dat is maintained which is a complete history of scaler readings at the
end of each run that ended normally. For 1-TS mode, this file is in /home/adev/scaler.
24.1
Analysis using ESPACE or C++ Analyzer
ESPACE is the main offline software package for analyzing Hall A experiments. It is
being replaced by a ROOT/C++ analyzer. Both may be used for rapid near-online
analysis in the counting room. These codes are documented in separate chapters, but it
is worth mentioning here in a list of essential tools for checking data.
24.2
Dataspy and Dhist
Dataspy and Dhist are somewhat obsolete online diagnostic programs. It remains to be
seen if we replace these by an offshoot of the C++ analyzer. The purposes of dataspy and
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CHAPTER 24. ONLINE ANALYSIS, DATA CHECKS
352
dhist are: (1) To print out randomly sampled detector data; and (2) To automatically
plot to the screen histograms of online data. Dhist is actually a shell script which runs
the executable dplot. For some online help, simply type dataspy or dplot. These codes
analyze a random sample of of raw, uncut data in real time from the CODA computer.
The data are distributed on the network by a server which obtains data from shared
memory.
This paragraph assumes you are on shift and wish to run dhist. While dplot can run
in several ways, let’s be definite: Log in to adaqh2 (or s3) as adaq account. If dhist isn’t
running, type dhist. Now you will see a reminder of what directory to go to and that you
should type ./dhist there.
There are optional interfaces dopte and dopth for turning on/off the histogram pages.
The dopte(h) interfaces also show an alarm status for the histograms by statistical comparison to a set of reference histograms. To start these interfaces, type dopte or dopth
for the E-arm and H-arm respectively.
dhist makes about 20 pages of plots which pop up on the workstation screen and
remain for a few seconds in succession. Each page also results in a postscript file for
printing, and there is a histogram file dplot.his which one can view in PAW.
Part VIII
Offline Analysis Software
353
Chapter 25
C++ Analyzer 1 2
The standard offline analysis software for Hall A data is the “C++ Analyzer”, an objectoriented C++ class package developed at Jefferson Lab by Hall A staff. The Analyzer is
built onto the ROOT3 programming framework. All of ROOT’s analysis and visualization
tools are available, as well as specialized code for Hall A physics analysis. The current
version of the Analyzer is 1.1. Detailed information about the software (downloads,
documentation, etc.) can be found at
http://hallaweb.jlab.org/root/.
Due to its object-oriented design, the Analyzer is modular and extensible. Individual
analysis components are designed as plug-in modules that can be added as needed. As
a result, the scope of the data analysis is to a large extent user-configurable. Only data
from those spectrometers and detectors is analyzed, and only those physics calculations
are carried out that the user specifies. Reconfiguration can be done at run time without
any need for recompilation of the program.
Currently supported are the analysis of the Hall A HRS spectrometers, the beamline
instrumentation, scaler and EPICS [2] slow control data, and helicity information. The
event decoder is compatible with the CODA event data format described in the section
on Data Acquisition. Decoding of basic helicity information as well as a sophisticated algorithm for decoding and prediction of the G0 helicity sequence is possible. The following
detectors can be used in either HRS spectrometer:
• Vertical Drift Chambers (VDCs)
• Scintillators (one or more paddles with up to two PMTs each)
• Cherenkov counters (arbitrary number of PMTs/mirrors, usable for both gas Cherenkovs
and aerogels in each HRS)
• Shower counters (shower, preshower, pion rejectors with arbitrary organization in
terms of rows and columns of blocks)
1
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3
http://root.cern.ch
2
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CHAPTER 25. C++ ANALYZER
355
• Total shower counter (combination of a preshower and shower)
The VDC code performs tracking in the focal plane and reconstruction to the target.
The tracking algorithm has been shown to be accurate for events with one cluster per
plane. Noisy events with higher cluster multiplicity and events with more than one good
track in the focal plane may not be reconstructed correctly by the present version of the
code, but work is in progress to make this type of analysis also reliable.
The scintillator, Cherenkov, and shower counter classes perform basic decoding, calibration (offset/pedestal subtraction, gain multiplication), and summing (for Cherenkovs)
or cluster-finding (for showers) of hits. The cluster-finding algorithm of the shower class
is basic and currently only capable of finding a single cluster per event. These classes are
largely generic and should be able to accommodate most new detectors of the respective
type, even with a different geometry and number of channels.
Several beamline apparatuses are available: a dummy beam (always at zero position
and angle), an “unrastered” beam, and a “rastered” beam. BPM and raster detectors
are implemented and can be analyzed to obtain the beam position on an event-by-event
basis. The BPM code currently only supports standard ADCs (e.g. LeCroy), not the
older Struck readout.
To carry out standard post-reconstruction calculations, the following Physics Modules are available:
• Single-arm electron kinematics (Q2 , ω etc.)
• Coincidence kinematics (missing energy etc.)
• Coincidence time
• Reaction point (vertex position) reconstruction
• Extended target tracking corrections
As many physics modules as needed can be added to the analyzer before starting replay.
Identical modules can be added multiple times with different parameters. For example,
on can calculate “electron kinematics” using both uncorrected and energy-loss corrected
tracks in one analysis pass without modifying or recompiling any code. For experiments
requiring specialized calculations of kinematics or any other quantities, writing a new
physics module is the preferred approach.
The results of calculations performed by the various analysis modules (spectrometers,
detectors, physics modules) are made available via so-called global variables. Global
variables provide access to data via names (text strings). Scalars as well as fixed and
variable-size arrays are supported. The global variable names are used in the definition
of the analysis output and of logicals.
To control the analysis flow, to collect statistics, and to select interesting data, tests
and cuts (“logicals”) can be defined dynamically at run time. If certain tests fail for a
given event, further analysis of that event is skipped, and the event is not included in
the output. Such tests can be put at the end of all the major stages of the analysis. This
CHAPTER 25. C++ ANALYZER
356
allows making decisions about an event early in the analysis, improving performance. A
summary of all test results is written to a file at the end of the analysis.
Data of interest can be histogrammed and/or written to a ROOT Tree in the output
file. The contents of the output is defined dynamically at the beginning of the analysis.
Both 1- and 2-dimensional histograms are supported. Histograms can be filled selectively
using logical expressions (cuts).
Table 25.1 lists the analysis modules available in version 1.1 of the C++ Analyzer.
25.1
Running the Analyzer
Precompiled binaries of the latest version of the Analyzer are installed on the Hall A
counting house analysis machines (adaql3–5, adaqs2–3). To run the Analyzer, log into
any standard account on these computers, for example as adaq, and type
analyzer
As installed, the Analyzer uses the default database in the location pointed to by
the environment variable DB DIR. The Hall A staff makes an effort to keep this database
reasonably up-to-date for completed experiments. If you wish to analyze older data, the
default database might work for you. However, if you wish to use a customized set of
database files specific to your experiment (usually the case for the current experiment),
you will need to re-define DB DIR to point to the location of that database before starting
the Analyzer. For details on the database, see Section 25.3.
The pre-installed analyzer may not work in certain accounts if the PATH and/or
LD LIBRARY PATH variables have been changed from the system defaults. If this is the
case, you should correct the login script(s) of the problematic account. To restore the
system defaults, you may execute one of the following commands:
For csh/tcsh shells:
For bash shells:
source /adaqfs/apps/env/login.adaq
source /adaqfs/apps/env/profile.adaq
If the Analyzer is to be used outside of the Hall A counting house environment, it is
currently necessary to build the program from source. A tar archive of the sources can
be obtained from the following location
http://hallaweb.jlab.org/root/download.
25.2
Preparing Analysis of a New Experiment
Setting up offline analysis for a new experiment typically involves the following steps:
1. Determine the experimental configuration (spectrometers, detectors, beamline) to
be analyzed and identify the corresponding analysis modules.
CHAPTER 25. C++ ANALYZER
Class name
Apparatuses
THaHRS
THaIdealBeam
THaRasteredBeam
THaUnrasteredBeam
Detectors
THaVDC
THaScintillator
THaCherenkov
THaShower
THaTotalShower
THaBPM
THaRaster
Physics Modules
THaReactionPoint
THaTwoarmVertex
THaAvgVertex
THaElectronKine
THaPrimaryKine
THaSecondaryKine
THaExtTarCor
THaCoincidenceTime
THaGoldenTrack
THaDebugModule
Description
HRS spectrometer (left or right) with VDC (“vdc”)
and two scintillator planes (“s1”, “s2”).
Dummy beam with zero position and angle.
Beam with raster
Beam without raster (for calibration)
VDC package for HRS
generic scintillator
generic Cherenkov (gas or aerogel)
generic shower counter
combination of preshower and shower
beam position monitor with standard ADCs
beam raster system
vertex position (intersection of spectrometer track
with beam)
vertex position (intersection of two spectrometer tracks)
vertex position (average of reaction points from two
spectrometers)
single-arm electron kinematics
single-arm kinematics for particle with arbitrary mass
coincidence kinematics
extended target corrections
coincidence time calculation
selects Golden Track from multiple reconstructed tracks
prints values of global variables for each event and waits
Table 25.1: Analysis modules available in version 1.1 of the C++ Analyzer
357
CHAPTER 25. C++ ANALYZER
358
2. Create a database for the new experiment, using the start date of the data taking as the time-stamp for new new entries (see Section 25.3). At the minimum
the database should contain up-to-date detector map entries for every detector
and rough starting values for the spectrometer reconstruction matrix elements and
VDC timing offsets. Often this information can be carried over from a previous
experiment with only minor modifications. Also, enter any other calibrations and
geometry data that are available, even if approximate.
3. In the database, create initial run database values. These are typically the starting
beam energy and spectrometer momentum and angle settings. If the experiment is
already completed, extract the history of these settings from logs and enter them
into the database. These values affect the kinematics calculations; they are not
important for detector checkout.
4. Determine which physics calculations are needed for the offline analysis and identify
corresponding Physics Modules.
5. Identify desired output histograms and tree variables. Create an output definition file. The file $ANALYZER/examples/output example.def contains most of the
necessary documentation.
6. If desired, create a definition file for logicals. An example generating detailed VDC
statistics is given in $ANALYZER/examples/cuts example.def.
7. Write a CINT4 script that sets up the configuration identified in Step 1 and the
physics analysis decided on in Step 4. Often, a script from a previous experiment,
or one of the examples in the directory $ANALYZER/examples, can serve as a guide.
The script usually also locates raw data files, creates one or more THaRun objects,
configures various options of the event loop object THaAnalyzer, and starts the
replay. In particular, the names of the output file, the output definition file, and
the logicals definition file must be given to THaAnalyzer.
8. Identify the plots that you wish to generate from the analysis results and write
a script to create them. This may be part of the script created in the previous
step. Note that there is no need to quit the Analyzer and start a new session or
another program after completion of the analysis; all of ROOT’s visualization tools
are available from within the Analyzer.
25.3
Database Files and Directories
Version 1.1 of the C++ Analyzer uses simple ASCII text files to store database information. There is one file for each analysis module. The name of each file is composed as
4
CINT is a C/C++ interpreter that acts as the interactive interface to ROOT and the C++ Analyzer.
CHAPTER 25. C++ ANALYZER
359
follows:
db apparatusname.detectorname.dat
For example, a Cherenkov detector named “a1” which is part of the Left HRS
spectrometer, named “L”, would be associated with a database file named db L.a1.dat.
The “run database”, which contains global run-specific parameters such as beam
energy and spectrometer momentum and angle settings, is stored in a special file named
db run.dat.
All of the above database files should be stored in a location that can be modified by
the user, for instance in˜/DB. The environment variable DB DIR must be defined
to point to this top-level database directory. Since database parameters change
with time, database files are organized in time-dependent subdirectories within $DB DIR.
The name of each subdirectory has the form YYYYMMDD, where YYYY, MM and DD
represent the year, month, and day, resp., of the date that is the start of the validity
of the entries. Upon initialization, the Analyzer locates the most appropriate timedependent subdirectory based on the contents of $DB DIR and the time-stamp of the
run to be replayed. Often there is only one time-dependent subdirectory per experiment,
but if significant changes occur during an experiment, it may be appropriate to create
several directories. A finer division of time-dependent information can be provided by
timestamps within each database file. This is especially true for the run database file
which frequently will have many time-stamped sections.
For example, an experiment running in April and May of 2004 would create a
database subdirectory ˜/DB/20040401 and set DB DIR= ˜/DB. Other files supporting
the replay of this experiment would reside in an experiment-specific directory, usually
$EXPERIMENT.
25.4
Program Design Overview
Spectrometers (and similar major installations) are abstracted in an Apparatus class hierarchy, while individual detectors belong to a Detector class hierarchy. Apparatuses are
collections of detectors that are analyzed in a particular way. Specialized physics analysis, such as kinematics calculations, vertex determination, and energy loss corrections,
can be done in Physics Modules. All three types of objects, Apparatuses, Detectors, and
Physics Modules, are kept in lists that are processed during replay. In setting up the
replay, it is up to the user which objects to place in the lists.
Both the Apparatus and the Detector class hierarchies, as well as the Physics Modules, inherit from a common base class, THaAnalysisObject. Physics Modules currently do
not use a particular class hierarchy; all physics modules inherit from THaPhysicsModule,
which in turn inherits from THaAnalysisObject.
The behavior of existing analysis modules can be modified or extended easily by
using class inheritance. In such a case, the only code that needs to be written is the
implementation of the new feature. For example, the standard Cherenkov detector class
CHAPTER 25. C++ ANALYZER
360
currently only calculates the total sum of ADC amplitudes. For a new type of Cherenkov
counter, or to do a more sophisticated analysis of the standard Cherenkov detectors,
one might want to calculate separate ADC sums for certain groups of PMTs. To do
so, one would write a new class inheriting from the standard Cherenkov class, which
could contain as little as one function, performing the additional calculations, and the
corresponding data members. New types of detectors and even entire spectrometers, as
well as new types of physics calculations, can be added similarly easily, again using class
inheritance. No change to and no rebuilding of the core program is necessary to support
such new modules.
25.5
Responsible Personnel
For all questions and suggestions regarding the C++ Analyzer, please contact Ole Hansen
at ext. 7627, email [email protected] Bob Michaels (ext. 7410, [email protected]) and Robert
Feuerbach (ext. 7254, [email protected]) are also available for help.
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