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.

A critical review and meta-analysis of xenon-on-carbon sputter yield data

“…In long duration tests of a high power Hall thruster, Frieman showed that high density graphite pole covers that were initially polished developed μm-scale roughness after about 5-6 μm were removed. 2 In this case, the ions are believed to have much larger global incidence angles. In contrast, pyrolytic graphite appears to remain unchanged or get smoother due to exposure.…”

“…Graphite is used in many electric propulsion applications because of its thermal and mechanical characteristics and its sputter resistance. For example, high density graphite is used for sputter-resistant keeper electrodes in ion and Hall thrusters 1,2 and as front pole covers in magnetically shielded Hall thrusters to protect more sensitive magnetic components. 2,3 Graphite and carbon-carbon composite grids are used in ion thrusters to extend engine life.…”

“…For example, high density graphite is used for sputter-resistant keeper electrodes in ion and Hall thrusters 1,2 and as front pole covers in magnetically shielded Hall thrusters to protect more sensitive magnetic components. 2,3 Graphite and carbon-carbon composite grids are used in ion thrusters to extend engine life. [4][5][6][7][8] Electric thruster test facilities are often lined with graphite panels to reduce the amount of materials backsputtered onto thrusters from the walls.…”

“…This provided a good initial recommendation for the best fit parameters, but a critical review of the data is the next step in refining the regression inferences. The objectives of this paper are to (1) critically review the experiments to determine if systematic errors or different methods and test conditions could impact the reliability of the data or their applicability to particular uses, (2) define recommended values of yield as a function of energy, (3) quantify the uncertainty to use in wear analyses, (4) determine if apparent differences in yield for the three types of graphite are statistically significant, and (5) assess the need for additional data and modeling. The data sets plotted in Fig.…”

The application of coherent microwave scattering and multiphoton ionization for diagnostics of electric propulsion systems

“…E.g. deriving factors which foster differences between in-space and facility performance [20], minimizing erosion rates [21][22][23][24], studying anomalous electron diffusion, microturbulence, and axial ion-ion streaming instabilities [25][26][27][28][29], etc. The resolution of this phenomena is particularly important in the improvement of numerical simulations-seeking to partially supplant timely, expensive test and qualification campaigns.…”