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

• Published in: IEEE transactions on plasma science 2014-08, Vol.42 (8), p.2007-2014
• Main Authors: Parson, J.M., Lynn, C.F., Mankowski, J.J., Kristiansen, M., Neuber, A.A., Dickens, J.C.
• Format: Article
• Language: English
• Subjects: Anodes; carbon fiber; Cathodes; electron beam; electron emission; electron sources; Electron tubes; high power microwave generation; Plasmas; Surface treatment; virtual-cathode-oscillator (Vircator)
• ISSN: 0093-3813; 1939-9375
• DOI: 10.1109/TPS.2014.2329878

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 cm 2 . 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.