Efficient, monochromatic, high-power microwave generator Page: 3 of 7
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AN EFFICIENT, MONOCHROMATIC, HIGH-POWER MICROWAVE GENERATOR
Thomas J. T. Kwan and Charles M. Snell
Los Alamos National Laboratory
P.0. Box 1663, MS B255
Los Alamos, New Mexico 87546 UbA
ABSTRACT
Microwave generation by electron beams In
virtual cathode configurations can achieve sig-
nificant power levels. However, most designs
Inherently have two competing mechanisms gener-
ating microwaves: namely the oscillating virtual
cathode and the reflexlng electrons. These two
mechanisms tend to Interfere destructively with
each other. Specifically, the reflexlng
electrons subject the electron beam to two-stream
Instability, causing considerable heating of the
electron beam. In addition, the space-charge of
the reflexing electrons can cause the diode
Impedance to fluctuate, resulting In oscillations
of the electron beam energy. We have Inves-
tigated a novel Idea to remove these undesirable
effects and we found that hlgh-power, narrow-
band, and monochromatic microwaves could be
generated with efficiency of 10 to 20>.
INTRODUCTION
Microwave generation resulting from the
formation of a virtual cathode of a relativistic
electron beam has been demonstrated In a number
of experiments [1-6]. With the possible excep-
tion of the Didenko ct ul. experiments ( 3 I, this
new class of microwave tube generates radiation
with multiple frequencies and modes at low
crriclency. Also, recent work shows that, tin-
output. occurs In random subnanosecond bursts [71.
Although most virtual cathode tubes have a simple
geometry, they exhibit complex nonlinear behavior
that has resulted in i hr slow development of a
narrow-bandwl dtli virtual e-iltiode tubr.
The complex, nonlinear character of the
virtual cathode dev lee necessitates part 1 c 11 ri-
ce 11 plasma simulation techniques, which have
hern used extensively to understand the microwave
generation m-r-w--, Tnese Investigations I M 1 () ]
1 mil cat c two sources of the rail 1 at 1 on: (I) l hr
trapped electrons relli-xlng between t lie real and
virtual cathiides, anil (II) the osi-l llatlnn i f t hr
virtual cathode. In I lie ei.n'-ent innal design, I he
two mechanisms coexist; therefore, the efficiency
of microwave generation suffers. These lwo
mechanisms interfere with each other destruc-
tively, so it is essential to select a dominant
mechanism by design of the device. We hive
Investigated a novel conf 1 gui-at 1 on whir*-, can
effectively eliminate the reflexlng electrons.
We have confirmed via two dimensional particle -
ln-cell simulations that th.s configuration
exploits the oscillations of the virtual cathode
exclusively and It generates nonbursting, single-
mode, narrow-bandwi dth , hlgh-power electro-
magnetic radiation. The efficiency of microwavi
production can be as high as 38X In the most
optimized case. The virtual cathode microwave
generator has a large operation frequency range,
about 100 MHz to 40 GHz. Further, It can gener-
ate Significant power (tens of g l g a w.i < t s .-
concentrated in a single waveguide mode. This
configuration is therefore, ideal for developri-r.t
of single modules of the extremely hlgh-power
phase-array microwave generator.
A NARROW-BANDWIDTH, EFFICIENT MICROWAVE GENERATOR
Microwave generation by the oscillating
virtual cathode of a relativistic electron bean
suffers from rapidly decreasing efficiency as tin-
energy spread or angular scattering Increases
111], In the configurations considered to date,
the quality of the Incoming electron beam can In-
adversely affected by the electrons reflexlng
between the virtual cathode and the real cat hod.
of the diode. lipec 1 f 1 ea 1 1 y , the reflexlng
electrons subject the electron beam to twi>-Str«-,i.*i:
Instability, causing conslderab.i heating of tin-
electron beam. Further, the spaer-rhargi- of tin-
reflexlng electrons can cause the diode Imprd.mci-
to fluctuate, resulting In oscillations of tin-
electron beam energy. These effects which have
been observed In our computer simulation gi rally
degrade the cIT I e 1 eiiey of microwave product Ion by
oscillating virtual catlunli-n. A typical micro
wave spri-l mini gi-ni-ratml by a virtual catholic
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Kwan, T. J. T. & Snell, C. M. Efficient, monochromatic, high-power microwave generator, article, January 1, 1986; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1092848/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.