Optical diagnostics of sputtering in magnetically enhanced high-current discharges

We have investigated a gallium-based liquid cathode for use in a high-voltage, high-power gas switch for grid-scale electric power conversion. The cathode requirements include conduction of high current density (1-10 A cm$^{\mathrm{-2}})$, preferably at low voltage, along with minimal loss by evaporation and/or sputtering. The approach to satisfy these criteria has been to operate with a modified commercial magnetron system at high pressure where the choice of working comprises the light elements, such as hydrogen or helium. A separate anode is used to form a plane-parallel geometry. We have demonstrated pulsed operation with current densities exceeding 2 A cm$^{\mathrm{-2}}$ and voltages below 200 V, over a pressure range of 50-800 mTorr. The sputtering rate on gallium and other cathode materials has been estimated for various plasma conditions using a line ratio emission spectroscopy diagnostic based on analysis of the radiation trapping.