Decoupling the Thermal and Plasma Effects on the Operation of a Xenon Hollow Cathode With Oxygen Poisoning Gas

Hollow cathodes are used as the electron source for generating the plasma discharge in electric thrusters. These cathodes contain porous tungsten emitters impregnated with BaO material to achieve a lower surface work function and are operated with xenon propellant. Reactive contaminants such as oxygen in the xenon gas can modify the surface chemistry and morphology of BaO dispenser cathodes and degrade the electron emission properties. Hollow cathodes that operate with reactive impurities in the propellant will experience higher operating temperatures, which increase the evaporation of the emission materials and reduce cathode life. A significant amount of work has been previously done to understand the effects of oxygen poisoning on vacuum cathodes; however, the xenon plasma adds complexity and its role during cathode poisoning is not completely understood. A deeper understanding of the mechanisms initiating cathode failure will improve the thruster operation, increase the lifetime, and ultimately reduce the cost. In this paper, we present the results of experiments in which the cathode was operated with 100 ppm of oxygen gas in the xenon plasma at two different discharge currents. Poisoning was indicated by an increase in the emitter temperature that was measured using a two-color pyrometer and thermocouples. The emitter temperature rose monotonically during poisoning and decayed to its original value when the oxygen flow was turned OFF, indicating that short exposures to oxygen do not result in permanent cathode damage. The experiments show that removal of oxygen adatoms from the cathode surface following poisoning is dominated by thermal desorption rather than xenon ion sputtering. Operation at higher discharge currents improves the resistance to poisoning as a result of the higher operating temperatures at these conditions.