Axial extraction of high-power microwaves from relativistic traveling

Axial extraction of high-power microwaves from relativistic traveling
wave amplifiers
S. A. Naqvi, G. S. Kerslick, and J. A. Nation
Laboratory of Plasma Studies and School of Electrical Engineering, Cornell University, Ithaca,
New York 14853
L. Schächter
Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
~Received 10 June 1996; accepted for publication 3 July 1996!
We report theoretical and experimental results from research into coaxial extraction of high-power
microwaves from X-band traveling wave tube amplifiers. Power levels exceeding 60 MW have been
measured at 9.1 GHz. The output level is relatively constant for the full 70 ns duration of the 700
kV, 500 A electron beam pulse. Results indicate that this coaxial geometry is broadband when
compared to traditional, highly tuned radial extraction and may thus have applications in a range of
high-power microwave devices. © 1996 American Institute of Physics. @S0003-6951~96!04637-2#
A number of high-power microwave sources are based
on the interaction between a relativistic electron beam and
axial electric fields. Within this category the klystron1,2 and
the traveling wave tube amplifier ~TWT! are perhaps the
most common devices when control of phase and frequency
are essential requirements. In both devices a beam is
bunched as a result beam-wave interactions in either discrete
cavities or in a periodic traveling wave structure. The rf output is typically extracted via a rectangular waveguide side
arm mounted in the last cell of the output structure. In
klystrons the output structure is used for both rf conversion
and extraction and the rf efficiency is very sensitive to the
cavity dimensions, whereas in the TWT the output is only
used to extract the rf power developed in the amplifier.
We present results from theoretical and experimental
studies describing the coupling from a high-power traveling
wave tube amplifier, operating in the TM01 circular mode, to
a transmission electron microscope ~TEM! mode in a coaxial
waveguide while the beam is dumped in a hollow inner conductor. It is found that this low reflection configuration exhibits advantages over that used in existing devices, namely:
~i! The mode conversion efficiency of the extractor is insensitive to the dimensions of the inner conductor and is relatively broadband. ~ii! The inner and outer conductors are
parallel to the confining magnetic field and hence the output
section is magnetically insulated. Although the work described used a TWT amplifier the results may be applicable
to a klystron with a traveling wave output section.
The coaxial extraction section of the TWT used in this
work is shown in Fig. 1. The amplifier consists of a twostage TWT with a phase advance of p/2 per cell designed to
work with an 800 kV, 500 A pencil electron beam. The
periodic structure has an inner ~iris! radius of 0.9 cm, an
outer radius of 1.56 cm, a periodic length of 0.77 cm, and
has a 12-period taper to the outer tube radius at each end.
The output of the amplifier is fed to a coaxial section in
which the radius of the inner conductor has been varied between 0.7 and 1.1 cm, and power extracted with various
penetration lengths of the inner conductor into the tapered
section. The taper is sufficiently gradual that the output microwave signal level is constant to within a few percent
whether an inner conductor is present or not. This is shown
in Fig. 2 for the case with the inner conductor present. Previous experimental measurements and simulation data2 have
shown that reflections from the ends of the amplifier lead to
the development of side bands and result in severe fluctuations in the power envelope of the output rf. The relatively
flat output signal is therefore a sensitive indicator of a low
reflection coefficient at the output of the amplifier. The narrow bandwidth of traditional side-arm output couplers places
a stringent requirement on the beam quality and pulse width
in order to prevent excitation of unwanted frequencies. These
requirements can be relaxed if reflections from each end of
the amplifier are made small enough, not only at the input
frequency, but also for the range of frequencies for which the
amplifier gain is substantial. This may be accomplished using the coaxial extractor concept described in this letter.
The axial converter design arose from noting that the
TM01 circular mode can be very effectively coupled into a
TEM coaxial waveguide mode in a guide with the same
outer radius if the inner radius is selected correctly. Analytic
calculations, supported by simulation data indicate that for a
tube radius of 1.56 cm a range of inner conductor radii from
1.0 to 1.1 cm have power reflection coefficients below 0.5%.
Simulation data indicate that similar results may be obtained
within the tapered section of the TWT provided that the end
of the inner coaxial tube is located midway between two
irises. In Fig. 2 we show simulation data for the energy reflected when an incident 300-MHz-wide Gaussian pulse enters the coaxial mode converter. Data are presented with the
mode converter at three axial locations, and is given as a
function of the radius of the inner conductor. The data points
~d! are obtained from a modal analysis of the junction between a uniform circular waveguide and a coaxial waveguide
at 9 GHz. Note that the reflected energy is less than 0.5%
approximately 1.5 periods from the end of the uniform amplifier for an inner conductor radius of 0.7 cm.
An important feature of the axial converter is the shielding of the electron beam from the periodic structure by the
inner conductor. Figure 3 shows the total Poynting flux, including both rf and beam component as a function of distance, for both a uniform cylindrical output @trace
Appl. Phys. Lett. 69 (11), 9 September 1996
© 1996 American Institute of Physics
FIG. 1. Schematic of the coaxial extraction section of the TWT amplifier.
~b!# and for the coaxial output case @trace ~a!#. It can be seen
that the microwave signal grows in amplitude throughout the
amplifier and decays as the wave enters the tapered section.
This reduction in power level occurs as energy is extracted
by bunches rapidly slipping out of the decelerating phase and
entering the accelerating phase of the wave ~in the absence of
the inner conductor the wave phase-velocity gradually increases from 0.9c at the start of the tapers to 1.7c at the end!.
The reduction can be as much as 80% of the peak power
level developed in the amplifier. In the case with the inner
conductor the rf power level is maintained at the value set by
the location of the mode converter. The dc level shifts at
z50, 19, and 33 cm correspond to changes in the Poynting
flux associated with the beam as the return conductor geometry is changed. The rf power output in the two cases shown
is 25 MW in the absence of the inner conductor and 90 MW
with the inner conductor. In this figure the mode converter is
located at z519 cm and the beam is dumped in the collector
at 33 cm. The inner conductor decouples the wave and the
beam when the beam enters the inner conductor.
The optimal design for coaxial extraction is a compro-
FIG. 2. rf pulse energy reflected from the junction of a tapered slow wave
structure and a coaxial waveguide as a function of the inner conductor
radius ~outer radius is 15.6 mm!. The three axial locations are: ~a! between
first and second iris of tapers; ~b! between sixth and seventh iris; and ~c! 5
mm beyond the tapered section. Data points ~d! are from modal analysis of
the junction between a circular waveguide and a coaxial waveguide at
9 GHz.
FIG. 3. MAGIC code output showing the time-averaged Poynting flux as a
function of axial distance. Results are shown for the coaxial extraction geometry ~a! and for a cylinder without a center conductor present ~b!.
mise between ~a! maximizing power extracted relative to
saturation level, ~b! minimizing reflections from the extractor
and, ~c! minimizing surface fields. The power extracted will
be maximized when the inner conductor is close to the start
of the tapers, but this would also reduce the radial gap between the coaxial conductors and thus increases the probability of rf breakdown. A smaller inner conductor radius may
slightly increase the reflections ~Fig. 2! but will increase the
gap and hence reduce the breakdown probability. For example, the radial excursion of a nonrelativistic electron emitted close to the coaxial extractor is about 5% of the radial
gap. The excursion scales as the square root of the radiated
power for fixed magnetic field strength so breakdown thresholds may be increased by an increase in the guide magnetic
We have tested the axial extraction system experimentally with a 100 ns, 700 kV, 500 A beam generated using a
field emission diode. It should be noted that the beam energy
is 100 kV less than the design figure, but based on earlier
results3 we expect good amplifier gain even if the beam and
slow wave structure parameters do not satisfy the classical
resonance condition exactly. The beam is 0.6 cm in diameter
FIG. 4. Waveforms showing, from the top: Output signal heterodyned with
local oscillator signal; output power, measured by Er in coaxial extractor;
beam current; and diode voltage.
Appl. Phys. Lett., Vol. 69, No. 11, 9 September 1996
Naqvi et al.
and is guided by a 10 kG magnetic field. After passing
through the amplifier the beam is dumped into the hollow
center conductor. Preliminary data are shown in Fig. 4 which
presents results for a 7.1 mm radius inner conductor located
3.5 periods into the taper from the end of the uniform section
of the amplifier. The microwave envelope, as detected by a
calibrated radial electric field probe mounted in the coaxial
section, and a mixer output signal are shown. The microwave
output is relatively constant for approximately 70 ns matching the stable portion of the beam profile, and the signal has
the correct frequency as indicated from a FFT of the mixed
signal. Peak output signal levels exceed 60 MW, and are 2.5
times that measured when the signal is detected in the uniform pipe at the end of the taper. The smoothness of the
output pulse indicates that the reflection from the mode converter is small. The fluctuations observed are believed to be
largely due to the relatively poor beam quality associated
with the field emission diode used.
We have presented preliminary data, backed by simulation that demonstrate the efficient electromagnetic conversion from a TM01 mode in a circular cross section traveling
wave periodic structure to a TEM mode in a coaxial guide.
The performance of the mode converter is broadband when
compared to radial extraction geometries and is relatively
insensitive to the dimensions of the inner conductor. For
small enough inner conductors the conversion may occur
close to the end of the uniform section of the amplifier enabling the saturated power of the amplifier to be extracted.
Except for the location of the inner conductor relative to
saturation point, the extractor has no effect on the efficiency
of the beam wave interaction. The overall efficiency of the
TWT is limited by the degree of beam bunching and the
design of cells in the output section prior to the extraction
point. This has not been optimized in the experiments reported here.
This work was supported by the Department of Energy
and also, in part, by the AFOSR under the MURI program.
The MAGIC code was provided by MRC.
G. Caryotakis, IEEE Trans. Plasma Sci.22, 683 ~1994!.
M. Friedman, J. Krall, Y. Y. Lau, and V. Serlin, Rev. Sci. Instrum. 61,
171 ~1990!.
S. Naqvi, G. S. Kerslick, J. A. Nation, and L. Schächter, Phys. Rev. E53,
4229 ~1996!.
Appl. Phys. Lett., Vol. 69, No. 11, 9 September 1996
Naqvi et al.
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