The importance of anomalous ion effects in pole erosion simulations of a magnetically shielded hall thruster

Magnetic shielding in Hall thrusters has been shown to protect the discharge channel walls from ion sputtering thereby eliminating this process as a failure mode. Some low yet measurable erosion has been observed however on surfaces along the side of such thrusters facing the near-plume plasma. With measured rates below 100 μm/kh, NASA’s Hall Effect Rocket with Magnetic Shielding (HERMeS) has featured a low sputtering yield cover of a few millimeters in thickness to protect the front magnetic poles from erosion. In recent work, we argued that anomalous ion heating due to lower hybrid instabilities can play a major role in the erosion of these surfaces. Here we show that such heating indeed contributes to the erosion both directly and indirectly, through various processes in the plume region, and present models that capture them. Comparisons between numerical simulations and wear tests performed at NASA on a Technology Demonstration Unit (TDU) of HERMeS show very good agreement when all these processes are accounted for. We also provide pole erosion predictions for a Qualification Model of the thruster currently under development by NASA’s industry partner Aerojet-Rocketdyne which, from the perspective of front pole wear, is nearly identical to HERMeS. The predictions are made at discharge voltage and power of 600 V and 9-12 kW, respectively and include the recession of the front pole covers as a function of time. We find that the adopted design will protect the front magnetic pole for more than 35,000 hours, which is current throughput requirement for this thruster with 50 % margin.