AAWSAP DIRD, Pulsed High-Power Microwave Source Technology, January 2010

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breakdown. One solution to these difficulties has been transformer-coupling the load by
having the FCG drive the primary of a transformer. The load for HPM production is
typically some type of loop antenna with very small inductance and driven directly from
the generator. Much research has been done on the effectiveness of this type of
antenna, and one method of improving the operation is to fuse the loop along its length
such that at peak current, the fuses open and radiate large voltage dl/dt spikes.
Pulsed High-Power Microwave Sources
Pulsed HPM sources can be divided into two types:
•
Pulsed electron beam sources - these are typically narrow-band HPM sources such
as relativistic klystron amplifiers, backward wave oscillators, traveling wave tubes,
split-cavity oscillators, reltron and super reltron, virtual cathode oscillators,
magnetrons, gyrotrons, and the magnetically insulated line oscillator.
•
Impulse HPM sources - these are typically wideband or ultrawideband sources such
as SNIPER, EMBL, Phoenix, Jolt, Thor, GEM II, and the H series.
PULSED ELECTRON BEAM SOURCES
In pulsed electron beam sources, the RF source includes an electron beam generator, a
beam transport, and a wave structure. These sources work by converting the kinetic
energy of an electron beam into EM energy.
BWOs, TWTs, AND RKAs
HPM tubes, such as backward wave oscillators (BWOs), traveling wave tubes (TWTs),
and relativistic klystron amplifiers (RKAs), are also very similar in concept to their
conventional counterparts. The main differences lie in the techniques of beam
formation; the use of pulsed, large magnitude axial magnetic fields for beam transport;
and the application of relativistic voltages and very high beam currents. BWO
efficiencies as high as 35 percent have been obtained at moderate power levels, but as
power levels are increased, the output levels tend to saturate, and the radiated spectra
tends to broaden. These effects are due to beam breakup and turbulent transport. To
maintain high beam quality, large magnetic fields are required for high-power
operation. An approximately 25- to 50-kG applied axial field implies that mechanically
strong solenoidal magnets are required with pulsed capacitor banks to drive them.
These requirements in turn dictate a much larger and heavier HPM source. TWTs can be
made using many of the same techniques as BWOs. The difference is that the beam­
wave interaction is with a forward wave. Thus, TWTs have many of the same issues and
limitations as BWO sources. RKAs use cavities for beam bunching and power extraction
rather than continuous slow wave structures as in BWOs and TWTs. Some RKA designs
use extended structures for power extraction to reduce the power densities and to
increase efficiency. They use high beam currents where space charge forces become
dominant in the bunching process. Tubes have been developed at the 10-GW power
level. They use 0.5 to 1 MeV beams guided by axial magnetic fields of about 10 kG.
Efficiencies are on the order of 40-50 percent. Some of the issues with these tubes are
beam transport, beam loading of the cavities, base pressure of the vacuum system, x-
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