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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.
“BEAMS”14 pages
UNCLASSIFIED/ /P9Pl 8ffllil.t.k WE'lii ODIi Y These mechanisms include: • Thermionic emission (apply heat - 1,000 "Celsius) • Secondary emission (electron bombardment; > 100 eV) • Field emission (apply a very strong electric field; 107 V/cm) • Explosive emission (form a plasma on the surface; c:,.,= 0) The emission mechanisms of major importance for HPM at present are thermionic emission and explosive electron emission; however, field emission shows some hope with the advancements in nanostructures. Explosive emission, creating a dense plasma at the cathode surface, is of primary importance at this point. A review of pure metals reveals a direct correlation between the work function (r.,.,) and melting temperatures. When cathodes are made from metals with low work functions, there are problems with metal deposition onto other components. Most cathodes of use in HPM tubes depend on a surface flashover at a dielectric-metal interface. The surface flashover generates plasma, typically at tens-of-kilovolts-per-centimeter electric fields. The threshold and nature of the plasma depend greatly on the cathode materials. Therefore, the choice of cathode material is of critical importance in the design and operation of any HPM tube. No discussion of HPM diodes could be complete without mentioning space charge limited current flow. This stems from the fact that at some magnitude of current density, the density of electrons in the anode-cathode gap begins to shield the cathode from further emission owing to their cumulative effect on the electric field at the cathode surface. The current density at which this happens is given by the Child- Langmuir law: lsc(kA/cm 2 ) = 2.33 x 10- 5 (V(MV) 3 i 2 /d(cm) 2 ) and is dependent on the diode voltage and the anode-cathode spacing. So, if we could have the ideal cathode material, what would its characteristics be? The response has not changed much in more than 60 years, as can be seen in the following extraction from a textbook on the subject. Primary Characteristics of an Ideal Cathode (J. R. Pierce, 1946): • Emits electrons freely, without any form of persuasion such as heating or bombardment (electrons would leak off from it into vacuum as easily as they pass from one metal to another). • Emits copiously, supplying an unlimited current density. • Lasts forever, its electron emission continuing unimpaired as long as it is needed. • Emits electrons uniformly, traveling at practically zero velocity. Efforts are still under way to increase the output power, pulsed emission duration, repetition rate, and emission uniformity by investigating new and existing cathode materials in an effort to draw closer to the ideal cathode. Some of the materials currently being investigated are ceramic cloth and felt, carbon structures including nanotubes and microfibers, and carbon structures coated with cesium iodide. 8 UNCLASSIFIED//liiAR AFFICIOI l!SF ONI X
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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.