Radio frequency plasma processing effects on the emission characteristics of a MeV electron beam cathode

Experiments have proven that surface contaminants on the cathode of an elec tron beam diode influence electron emission current and impedance collapse. This letter reports on an investigation to reduce parasitic cathode curren t loss and to increase high voltage hold off capabilities by reactive sputt er cleaning of contaminants. Experiments have characterized effective radio frequency (rf) plasma processing protocols for high voltage anode-cathode (A-K) gaps using a two-stage argon/oxygen and argon rf plasma discharge. Ti me-resolved optical emission spectroscopy measures contaminant (hydrogen) a nd bulk cathode (aluminum) plasma emission versus transported axial electro n beam current turn on. Experiments were performed at accelerator parameter s: V = -0.7 to -1.1 MV, I(diode) = 3-30 kA, and pulse length = 0.4-1.0 mu s . 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 incr ease in cathode turn-on voltage required for axial current emission from 66 2 +/- 174 kV to 981 +/- 97 kV. The turn-on time of axial current was increa sed from 100 +/- 22 to 175 +/- 42 ns. (C) 1999 American Institute of Physic s. [S0003-6951(99)01827-6].