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

Abstract

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. The issues under investigation were to reduce parasitic current loss by reactive sputter cleaning of contaminants and to increase high voltage holdoff capabilities for large-scale transmission lines. 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. MELBA was used to study thermal and stimulated desorption of contaminants from cathode surfaces due to electron impact, and breakdown of contaminants from cathode surfaces during the high voltage pulse. 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 (100W) argon/oxygen RF discharge followed by a higher power (200W) pure argon RF discharge yielded an increase in turn-on voltage required for axial current emission from 662 +/- 174 kV to 981 +/- 97 kV. After two-stage RF plasma processing axial current emission turn-on time was increased from 100 +/- 22 nanoseconds to 175 +/- 42 nanoseconds. Aluminum optical emission was delayed >150 nanoseconds after the overshoot in voltage after two-stage RF plasma processing. Removal of hydrogen contamination on the cathode surface was observed by optical spectroscopy during the MELBA pulse. Axial and diode current were reduced 40--100% after RF plasma processing. SEM analysis suggests the aluminum cathode surface is being modified by the RF plasma discharge.