Pulsing, Capacitive, H.V. Cable

4.7 BEAM PULSING OPTIONS cont.
CAPACITIVE BEAM PULSING with GRID PULSING CABLE: INSTALLATION
A H.V. Multiconductor Source Cable with a Grid Pulse Box attached (Grid Pulsing Cable) and a
separate, user-supplied, pulse generator capable of producing appropriate voltages (equal to the Grid
cut off value) are required for installation of the Fast Beam Pulsing Option.
HIGH VOLTAGE WARNING: DISCHARGE THE CABLE BEFORE HANDLING. If the cable is stored
separately, the Power Supply connector end must be covered with the attached shorting plug.
The Grid Pulse Box on the Source Cable contains a capacitor which can charge up when the cable is
disconnected after being energized. A H.V. potential can exist between the exterior ground shield and the
internal conductors. Do not touch the pins at the end of the cable and the metal connectors or box
at the same time. Before touching the cable ends, discharge the pins to ground, for example with
a discharging rod. When the Grid Pulsing Cable is connected to the grounded Power Supply which is
turned off, there will be no H.V danger, as the cable will be grounded. However, if the Power Supply has
just been turned off, allow time for the Power Supply to bleed off the charge.
2.
Refer to the installation procedures outlined in Section 2.2 and 2.3. Connect the optional Grid Pulsing
Cable instead of the standard H.V. Multiconductor Source cable to the EGPS Power Supply connector
labeled SOURCE and to the central feedthrough on the Electron Gun. Avoid touching the insulators on
the cable ends, and do not twist the cable. Also, connect the interlock cable attached to the Grid
Pulsing Cable to the lemo connectors on the EGPS and the Electron Gun.
3.
Connect the user-supplied pulse generator to the input MHV on the Grid Pulse Box attached to the Grid
Pulsing Cable. The input MHV on the Pulse Box should not be grounded or else the grid in the
gun will be grounded.
Grid Pulse Box
attached to
Source cable
Source
connection
to EGPS
Power
Supply
Interlock
cable with
lemo
connectors
Input MHV for
connection to
pulse generator
Source
connection to
Electron Gun
Figure 4.7.3 Grid Pulsing Cable (H.V. Multiconductor
Source Cable with Grid Pulse Box)
1-1
Shorting Plug for
Grid Pulsing Cable,
attach to power supply
connector end
4 OPERATION
1.
4.7 BEAM PULSING OPTIONS cont.
CAPACITIVE BEAM PULSING with GRID PULSING CABLE: OPERATION
1.
Start up the Electron Gun in normal Source mode according to Section 4.2. CAUTION: Do not use
ECC mode when pulsing.
2.
On the EGPS, adjust the Grid voltage with the GRID potentiometer to just completely cut off the
electron beam current. (The way the voltages control the beam is described below.)
3.
On the user-supplied pulse generator, set the input pulse:
a. Turn on the generator, and set the desired pulse rate.
b. Adjust the positive voltage input to the Grid Pulse Box so that the desired pulsed beam current is
achieved.
c. Using an oscilloscope, the pulse amplitude and the grid voltage can be fine-tuned to reduce ringing
and improve beam output. Note: When monitoring the beam pulse, the input impedance of the
oscilloscope may need to be changed by use of a terminating resistor.
4.
When not using the pulsing option: Disconnect the user-supplied pulse generator. The input MHV on
the Pulse Box should NOT be grounded or else the grid in the gun will be grounded.
CAPACITIVE BEAM PULSING with GRID PULSING CABLE:
DESCRIPTION OF GRID PULSING VOLTAGES AND BEAM RESPONSE
Pulsing of the electron beam current is accomplished by applying a pulse through the Grid Pulse Box
to the electron gun control grid. The grid voltage is negative with respect to the cathode. To pulse the gun
on, positive voltage pulses are required. The grid voltage on the EGPS Power Supply should be turned up
so that the electron beam is cut off; data is supplied in the Data Section showing grid cut off values for the
electron gun. By sending a pulse of the appropriate amplitude through the Grid Pulse Box, the gun will
turn on for the duration of the pulse.
A capacitor located in the Grid Pulse Box isolates the high voltage of the energy supply from the pulse
generator. The pulse is transmitted from a ground-referenced pulse generator, through this capacitor to the
control grid which is floating at high voltage.
Figure 4.7.4 (below) illustrates the beam current response in fast beam pulsing. The grid power supply
and pulse generator outputs superimpose to produce the voltage at the grid aperture; for example, a grid
cut off voltage of -150 V plus a positive pulse of +100 V yields a pulse of -50 V applied to the grid inside the
gun. This grid voltage then controls the beam current. The values shown for illustration purposes are for a
standard EMG-12 Electron Gun at mid Energy range; the actual values of grid cut off and beam current will
vary with the gun model and operating parameters (see Data Section).
1-2
4.7 BEAM PULSING OPTIONS cont.
CAPACITIVE BEAM PULSING with GRID PULSING CABLE
DESCRIPTION OF GRID PULSING VOLTAGES AND BEAM RESPONSE
ON 100 µA
BEAM CURRENT
RESPONSE
OFF 0 µA
0V
GRID VOLTAGE -50 V
IN GUN
-150 V
+100 V
0V
0V
GRID POWER
SUPPLY OUTPUT
CUT OFF -150 V
TIME
(b)
(a)
Figure 4.7.4 Capacitive Fast Beam Pulsing diagram: At time (a) the grid power supply control is set to
the cut off voltage, and at time (b) the pulse generator is turned on.
This completes the Capacitive Beam Pulsing Instructions.
1-3
4 OPERATION
PULSE
GENERATOR
OUTPUT
Download PDF