Breakdown and ignition limits in LaB6 hollow cathode discharges

Summary form only given. Conventional BaO hollow cathodes used for satellite electric propulsion suffer from two major failure modes: emitter poisoning and heater failure. Lanthanum hexaboride (LaB₆ ) hollow cathodes are two orders of magnitude less sensitive to emitter poisoning, but require higher operating temperatures to attain similar emission current densities. As a result, higher heater currents are required to start a self-sustaining discharge, leading to thermal degradation of the heater insulation. Heaterless designs exist for BaO hollow cathodes, but not for LaB₆. As a first step toward development of a heaterless LaB₆ hollow cathode, we present results from a series of breakdown and ignition experiments with a LaB ₆ hollow cathode. This Moscow Aviation Institute (MAI) cathode design can maintain a self sustaining discharge at currents above 6 A. Disappearing wire pyrometry permits measurement of the pre-ignition LaB₆ insert temperature as a function of the heater filament current and voltage. We plot breakdown voltage and discharge current growth (or decay) rates as a function of heater power and xenon mass flow rate. Heaterless breakdown voltage measurements as a function of xenon mass flow rate provide Paschen curves for the MAI electrode configuration. These results permit initial modeling of the insert thermal balance and the cathode breakdown characteristics, permitting semi-empirical design of a followon prototype heaterless LaB ₆ cathode