Gas evolution, conditioning, and electron emission properties of carbon fiber cathodes in a sealed reflex-triode vircator
Parson, Jonathan M.
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The impact of electrode material and conditioning on the performance of a virtual-cathode-oscillator (vircator) with 200 A/cm2 current density is investigated at ultra-high-vacuum (UHV). Cold-cathode type high power microwave (HPM) sources suffer from plasma expansion due to the outgassing of contaminants contained within electrode materials. Two cathode materials and three anode materials were examined in this study. The anode materials tested were nickel 201L (Ni201L), stainless steel 316L (SS316L) and grade-1 titanium (TiG1). The cathode materials examined were aluminum, and bimodal and unimodal carbon fiber. Of the two bimodal cathodes used, one cathode contained a 1 μm thick cesium-iodide (CsI) coating applied by chemical vapor deposition (CVP). Titanium and carbon fiber were the materials that outgassed the least, with evidence pinpointing the cathode as the main outgassing culprit. Hydrogen was found to be the most abundant outgassing species with smaller quantities of nitrogen, carbon-monoxide, methane (and other hydrocarbons) and argon having a noticeable presence. Also presented, are conditioning outgassing curves for 10,000 shots on the three different carbon fiber cathodes. Of the three carbon fiber cathodes, the unimodal cathode exposed to a 15 minute argon plasma dc discharged performed the best. Plasma cleaning methods, both microwave and in-situ dc discharges have shown significant improvements in outgassing characteristics and overall vacuum cleanliness. Scanning electron microscope (SEM) and high-speed ICCD imaging in conjunction with energy dispersive x-ray spectroscopy (EDX) and carbon fiber thermal modeling have indicated the primary electron emission mechanism for carbon fibers to be explosive electron emission (EEE).