Temperature Degradation of Magnesium Oxide Cathode Heaters

The Advanced Electric Propulsion System (AEPS) consists of a 12 kW Hall thruster with a centrally mounted hollow cathode. The thermionic insert in this hollow cathode is externally heated to approximately 1000°C. Swaged coaxial cathode heaters used for this purpose were tested to characterize failures due to time-at-temperature and to measure the leakage current through the magnesium oxide insulation separating the center conductor from the heater sheath as a function of voltage and temperature. The heaters subjected to time-at-temperature tests demonstrated lifetimes exceeding 500 h at temperatures over 100°C higher than their expected operating temperature. The demonstrated lifetimes are greater than the time-at-temperature that will be accumulated by AEPS heaters in the required number of ignition cycles. The leakage current calculated using the measured bulk conductivity of the magnesium oxide insulator and predicted AEPS heater temperatures was less than 5% of the nominal heater current, a level which will have negligible impact on heater performance.