Demonstration of 25,000 ignitions on a proto-flight compact heaterless lanthanum hexaboride hollow cathode

Abstract

Heaterless hollow cathodes are increasingly attractive for low-power electric propulsion systems due to their compact size and lack of a heater and associated power supply which are well-known failure mechanisms. A low-current, ultra-compact, heaterless, lanthanum hexaboride (LaB6) hollow cathode has been developed for the Magnetically Shielded Miniature (MaSMi) Hall thruster, which is part of the Jet Propulsion Laboratory's Ascendant Sub-kW Transcelestial Electric Propulsion System (ASTRAEUS) aimed at enabling interplanetary missions with high-Δv SmallSats. In the framework of that program, there is an increasing focus on the reliability and life cababilities of every system component. MaSMi's low-current center-mounted heaterless hollow cathode, specifically designed to reduce thermal losses and maximize life, is required to demonstrate a minimum of 5700 h of continuous operation at its peak discharge current of 4 A with a minimum of 1200 ignitions. One of the most critical aspects of the cathode operation is understanding the life implications of repeated heaterless ignitions and any impacts on cathode performance. To understand this issue, an ignition cycling test was performed at JPL. The test successfully demonstrated more than 25,000 heaterless ignitions of the cathode. Measurements of cathode parameters such as discharge and keeper voltage showed that the cathode steady-state operation was not degraded by the repeated ignitions. Photos after cycle testing of the cathode orifice plate, keeper orifice plate, and a cross section of the entire cathode assembly, show surface roughening and some graphite deposition on cathode electrodes from microarcing during the ignitions, but no significant damage inhibiting continued cathode operation.