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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.
“The Times”17 pages
UNCLASSIFIED//F8R 8PPl!ltllt USE Uiltf VELVET Velvet has been an explosive electron emission standard for more than 20 years. This material works by means of the dielectric-metal surface flashover mechanism mentioned earlier. Five steps are involved in the explosive emission process for dielectric fibers: • Surface flashover generates a cold, dense plasma/gas column. • The applied electric field extracts a space-charge-limited current flow. • The flow of current resistively heats the gas column. • The gas columns expand at a rate determined by the gas temperature. • The gas continues to expand into the anode-cathode gap. Velvet has several desirable properties that have endeared it to the pulsed power community and kept it a useful material for all this time. First, it emits at relatively low field strengths (~10 kV/cm), allowing a wider range of use than do many other materials. Second, it has a fast turn-on time. Third, the insulating nature of the velvet fibers provides a sort of built-in ballast during operation. Velvet also has a wide range of vacuum compatibility (pressures from 10- 3 to 10-s Torr), easing the expense of vacuum hardware. Finally, velvet is inexpensive and readily available. All these factors combined have made velvet cathodes common for the past two decades. However, velvet cathodes also have drawbacks. First, velvet outgases heavily, especially during and after explosive emission. Significant amounts of material are released from the velvet during this process. The increased pressure inside the HPM device then leads to gap closure (conductive bridging of the anode-cathode region) and early termination of the RF output from the device. The closure rate can be estimated from: Velocity of closure (m/s) = 100 (d*/d) 213 Vci 1/ 2 where: dis the diode gap, d* is the velvet tuft density, and Vci is the diode voltage. Second, velvet has a very limited lifetime, partly owing to the material lost during each shot. Some material lasts for only about 100 shots in single-shot mode. Third, because of the increase in pressure after each shot, the repetition rate is very limited. Finally, lack of control over the manufacturing process results in a wide variation in performance. Results are not reproducible, even between one roll and the next from the same manufacturer. CARBON Carbon cathodes have been used in diodes for more than three decades and have some appealing characteristics. The outgassing characteristics of carbon cathodes are much better than those of velvet, although the threshold voltage for emission is generally much higher. The primary material given off during outgassing from carbon cathodes 9 UNCLASSIFIED/ /FOR AFFJGJ11J.k WSI!! 8HE I
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Report, from the dia collection. The PDF is mirrored here; the original link is under it. 37 pages are in the text index: search them above, or from the library's search.