Wear trends of the 12.5 kW HERMeS Hall thruster

Abstract

This work presents the results of over 6500 h of wear testing completed during the maturation of the NASA 12.5 kW Hall Effect Rocket with Magnetic Shielding. Erosion of the thruster front pole covers was found to be the primary life-limiting mechanism and exhibits strong dependencies on thruster operating condition and material properties. Specifically, average pole cover erosion rates increased by 76% as discharge voltage decreased from 600 to 300 V and 42%-96% as the magnetic field increased from 0.75 to 1.25 times the nominal value. The cause of both trends is hypothesized to be ion heating from a modified two-stream instability that becomes dominant for 300 V operation and grows with magnetic field strength. Rougher pole covers were observed to have 33% lower average erosion rates than those that were polished due to local surface features that lower the effective angle of attack of eroding ions and the concomitant sputter yields. Alumina pole covers were shown to erode over 250% faster due to the higher sputter yield of alumina relative to graphite. Shifting the cathode upstream of the pole covers reduced average keeper erosion rates by 84% by reducing the view factor to high-energy beam ions. Cathode keeper erosion was also shown to exhibit azimuthal nonuniformities, which resemble the azimuthal oscillations observed in the cathode region. Taken together, these results provide in-depth insights into the life-limiting mechanisms impacting magnetically shielded Hall thrusters.