Defense Intelligence Reference Document Pulsed High-Power Microwave Source Technology

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HIGH-SPEED LIQUID SWITCHING
Liquid switching has also been used in UWB HPM sources with great success. The same
phases of breakdown exist for liquid switches as do for gas switches. The electrode
spacing is typically smaller for liquid switches, and electrodes can be made smaller for
the same level of energy transfer owing to greater thermal diffusion to the liquid as
compared with a gas. Liquid switching does not have the extreme safety concerns
associated with gas switching; however, for repetitively pulsed operation, flow of the
liquid insulating media is required. Filtering, evacuation, and processing may also be
required. Liquid switches have achieved rise times of less than 100 ps and PRRs of
1,500 pulses per second.
SOLID-STATE SWITCHING
Solid-state switches have seen some improvement in voltage holdoff capability but
generally still do not have the capability of operating at tens of kilovolts required of
HPM sources. The current technology in lateral gallium arsenide (GaAs) switches is
greatly improved compared with the old bulk avalanche semiconductor switch
technology of the last decade. The power handling capabilities of this technology are
impressive; however, it still suffers from short lifetimes because of heat dissipation
problems. Source designs using GaAs switches typically involve an array of horns with
one switch per horn. The array can then be phased in time to allow steering of the
beam. GaAs switches operate at about 10 kV and, therefore, in the large arrays
required, several switches fail during any burst mode operation. The most promising
new developments in semiconductor switches today are based on physics pioneered by
I. V. Grekhov and colleagues at the Ioffe Physical-Technical Institute in St. Petersburg.
The AFRL is currently collaborating with Dr. Grekhov and the University of New Mexico
in studies of delayed breakdown devices, silicon avalanche shapers, and drift step
recovery diodes in efforts to improve the performance of these devices. It is also
investigating the use of silicon carbide as an alternative to silicon and GaAs. State-of-
the art pulse generators using these devices currently are capable of 6-8 kV output with
100-ps rise times and 20-ps switching jitter. Conventional solid-state devices such as
junction gate field-effect transistors (JFETs) have not seen substantial improvement
and operate at about a kilovolt with rise times of a few nanoseconds, making them
useful for trigger supplies but not in HPM sources.
Photoconductive solid-state (PCSS) switching is still of great interest because of the
inherent advantages it could provide. PCSS switches have very low jitter, have fast rise
times, and are compact. This technology, sufficiently developed, could allow design of
HPM sources with fewer compression stages, allow greater frequency agility and pulse
width adjustment, and be used in arrays by phasing many lower power sources
together. The technology's main limitations at present are power handling and a limited
lifetime. PCSS switches have three modes of operation. In the linear mode, one
electron-hole pair is generated by each photon absorbed, and so the conductivity is
linearly proportional to the incident photon flux. Linear mode PCSS switches are made
from silicon, doped GaAs, and indium phosphide. The electrical pulse output follows the
amplitude of the optical trigger pulse. Switching in this mode requires about 1 mJ/cm 2
of optical energy and thus requires a larger laser trigger than do other operating
modes. PCSS switches also operate in a lock-on mode in which once the optical trigger
causes conduction, carriers remain as long as current still flows, even if the optical
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