Effects of RF plasma processing on the impedance and electron emission characteristics of a MV beam diode

Summary form only given. Experiments have proven that both the surface contaminants and microstructure topography on the cathode of an electron beam diode influence impedance collapse and electron emission current. Experiments have characterized effective RF plasma processing protocols for high voltage A-K gaps using argon and argon/oxygen gas mixtures. RF processing time, feed gas pressure, and RF power were adjusted. Time resolved optical emission spectroscopy measured contaminant (hydrogen) and bulk cathode (aluminum) plasma emission versus transported axial electron beam current. Experiments utilize the Michigan Electron Long Beam Accelerator (MELBA) at parameters: V=-0.7 to -1.0 MV, I(diode)=3-30 kA, and pulselength=0.4 to 1.0 microseconds. Microscopic and macroscopic E-fields on the cathode were varied to characterize the scaling of breakdown conditions for contaminants versus the bulk material of the cathode after plasma processing. Electron emission was suppressed for an aluminum cathode in a high voltage A-K gap after RF plasma processing. Experiments using a two-stage low power (100 W) argon/oxygen RF discharge followed by a higher power (200 W) pure argon RF discharge yielded an increase in turn-on voltage required for axial current emission from 662/spl plusmn/174 kV to 981/spl plusmn/97 kV. After two-stage RF plasma processing axial current emission turn-on time was increased from 100/spl plusmn/22 nanoseconds to 175/spl plusmn/42 nanoseconds. Aluminum optical emission was delayed >150 nanoseconds after the overshoot in voltage after two-stage RF plasma processing.