Optical Studies of Sputtering in Magnetically Enhanced Helium Discharges

A cold-cathode gas-discharge switch for the electric power grid must operate at the highest possible current density to be competitive. Magnetic enhancement, similar to that of a magnetron sputtering discharge, achieves current densities far above the classic ``normal'' cold-cathode fall current density. One of two physical mechanisms, power dissipation or sputtering, is likely to limit the ultimate current density of a magnetically enhanced device. Using forced cooling a power dissipation density of about 1 kW/cm$^{2}$ should be achievable. This corresponds to a current density of 5 A/cm$^{2}$ assuming a 200 V cathode fall. Sputtering can be much reduced using a light buffer gas such as hydrogen or helium. We are studying the transition to `metal mode' operation in such discharges. Metal mode is often described as a current density at which lines of sputtered metal dominate buffer gas lines in the emission spectrum. Preliminary results in a magnetically enhanced discharge operating in the A/cm$^{2}$ range with helium buffer gas over some cathode materials are presented.