High Current Cathodes Without A-K Gap Closure

Development of high power microwave (HPM) sources for Air Force applications requires cathodes with long usable lifetimes, negligible anode-cathode (A-K) gap closure, reproducible functioning under high repetition pulsing, uniform current density, and negligible plasma/current production from nearby surfaces. Cathodes must maintain such stable electron beam output characteristics for > 1 nsec. We propose to meet these requirements in an extremely low cost, robust carbon cathode with a matrix of carbon fiber tufts as the emitting surface. Innovations to improve performance include: cesium iodide coating to reduce plasma gap closure and improve current density uniformity; an intermediate electrode to reduce unwanted plasma production from nearby surfaces and increase total current; increased carbon tuft surface density to produce more uniform current density and reduce explosive plasma production; and a ballast resistor array behind the fibers to prevent hot spots of current/plasma production. Cathodes will be produced utilizing these novel features and tested on a crowbarred Marx for 1-2 nsec pulses. Using a corona bushing to eliminate edge effects, 5-cm radius cathodes will be tested with 4-cm A-K gaps up to the 150-200 kV range. Gap closure rates, current density uniformity, and field stress applied to cathode will be determined, and configuration optimized for meeting target performance goals.