Extension of HPM Pulse Duration by Cesium Iodide Cathodes in Crossed Field Devices
Final rept. May 97-May 98
MICROWAVE SCIENCES LAFAYETTE CA
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When explosive emission cathodes are used in higher power 100 MW devices, microwave pulse shortening occurs because of motion of the cathode plasma at speeds 1 to 5 x 10exp 6 cmsec. This limited duration in turn can limit present-day high power microwave HPM sources to a few hundred joules. The introduction of cathodes made from Cesium Iodide-coated CsI carbon fiber has shown plasma speeds reduced by factors of a few from uncoated carbon fiber, but previous work was at low diode fields of a few 10s of kVcm. We have demonstrated reduced CsI plasma speed for macroscopic electric fields of 265 kVcm, sufficient for the diodes of GW microwave sources. Carbon fiber coated with CsI in saturated solution gives plasma speeds at these high electric fields of -0.6 x 10exp 6 cmsec, about three times less than the bare carbon fiber. The apparatus had oil-free high vacuum conditions and the cathode was baked both before assembly at atmospheric pressure and in vacuum after assembly, to temperatures of 600 deg C. A residual gas analyzer showed burnout of the water base pressure was approx. 10 to the minus 6th power Torr. The diode current and voltage traces are substantially more reproducible than with bare carbon fiber. This may be because CsI emits copious UV, lighting up the surface much more uniformly. With this reduced plasma velocity, CsI cathodes should produce an extension of the HPM pulse length and an increase in pulse energy in sources now limited by low-Z contaminant, cathode plasma motion.
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