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This Defense Intelligence Reference Document from the Defense Intelligence Agency, dated 28 January 2010, surveys pulsed high-power microwave (HPM) source technology. It was produced under the Advanced Aerospace Weapon System Applications (AAWSA) program. It covers insulation, cathode materials, high-voltage switching, pulse generators, microwave sources such as magnetrons, gyrotrons and the Phoenix and Jolt sources, and antennas. The document concludes that progress depends on advances in cathodes, switching and insulation. It says compact ultrawideband antennas will remain difficult to build.
UNCLASSIFIED/ ;Sf&lil: 8FF11il.t.k WE&i a••k>/ rise time is the time required to heat the gas channel to full conductivity, and the inductive rise time is the delay caused by the addition of the switch into the circuit. There are two contributions to the inductive rise time, with Le being the spark channel inductance and Lh the housing inductance. A shorter switch gap reduces the inductive time by lowering the channel inductance but also increases the electric field in the gap, reducing the resistive time and resulting in a faster rise time. Even though the switch electrodes are usually designed for minimal cross-sectional area at a given current, the very short electrode separation required can still result in high interelectrode switch capacitance. As mentioned earlier, it is also preferable to charge the switch very quickly to achieve an overvolted switching condition, and, therefore, very fast switches always have some level of pre-pulse. Because the PRR is also of great importance, hydrogen has been chosen most often for high-speed gas switching in UWB HPM sources. Switches of this type have achieved rise times of just over 100 picoseconds (ps) and PRRs of 1,500 pulses per second. Another type of gas switch meriting mention for its utility and indispensability in the HPM pulsed-power driver circuits is the hydrogen thyratron. The thyratron is a partial vacuum switch. Figure 1 shows what is known as the Paschen curve for air; however, all gases exhibit the same curve characteristics. At some product of pressure and electrode spacing, a minimum value of breakdown voltage is reached. While high- pressure gas switches operate in the region on the right side of the Paschen minimum, the hydrogen thyratron operates on the left side, beyond the Paschen minimum. The physics of voltage breakdown in this region results in smaller electrode spacing holding off higher voltages and reduced pressure at the same spacing enabling greater voltage holdoff. The single-stage thyratron operates at only tens of kilovolts, while high- pressure gas switches may operate at several hundreds of kilovolts. When coupled with a good pulse transformer, a properly chosen thyratron forms the heart of an excellent driver for HPM sources. The thyratron also has the capability to initiate breakdown using modest trigger levels (-1 kilovolt) and with nanosecond timing, allowing the use of multiple switches to share current. "'-0 > .,C, "'-0 > C: ~ 0 "C -"' "'., -"' 1 00CI0IJ 10000 ICIIJ0 100 Breakdown Voltage vs. Pressure x Gap (Air) "" 1 OOE-[12 1 OOE-01 1 OOE+OO I 01JE+01 1 IJOE+02 1 OOE+03 1 IJOE+04 Pr-essure x Gap -Torr Inches. Figure 1. Paschen Curve for Air 13 UNCLASSIFIED/;CF8lil: 8FFI1il.«1k 1!181! &••LY
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 37 pages are in the text index: search them above, or from the library's search.