Conditioning of Carbon Fiber Cathodes in UHV-Sealed Tubes at 200 A/cm2

This paper presents a study on outgassing and electrical conditioning for three carbon fiber cathode types in a vacuum-sealed, high-power microwave virtual-cathode-oscillator (vircator) that operates in the low 10^-9 torr pressure regime. The three cathode types consist of a bare bimodal fiber structure, a bare unimodal fiber structure, and a cesium-iodide coated bimodal fiber structure with identical fiber coverage of 2% by area with a surface area of ~20 cm². The electrodes are cleaned by a 1.2 kW, argon/oxygen microwave plasma prior to complete vircator assembly, followed by a 300 °C bake-out for 72 h. Each cathode was pulsed in a single pulse operation by an 80 J, low inductance Marx generator with an approximate pulsewidth of 100 ns full-width at half-maximum for 10000 current pulses. The data presented includes individual gas constituents, high-speed intensified charge coupled device (ICCD) imaging, and voltage and current waveforms. The conditioning process resulted in a gas load reduction of ~80% overall, with the indication that the bare bimodal fiber structure performed the best as diode power increased throughout the experiment, while the power decreased for the other tested cathode types.