Investigation of Plasma Material Erosion Under Mechanical Stress

Abstract

The operational lifetime of traditional Hall effect thrusters is limited by the erosion of the discharge channel walls. Erosion during long-duration life testing of Hall effect thrusters produces surface features that are unexplained by present models. Anomalous erosion ridges are one such unexplained feature. Hall effect thrusters are also expected to be under moderate (100 kPa to 6 MPa) thermomechanical stresses when operating, due to heat loads from the plasma. Thus, there may be a relationship between mechanical stress and the erosion features observed in Hall effect thrusters. This work investigates the plasma erosion of materials placed under mechanical loads. An experiment is designed to detect any changes to the erosion of material samples due to stress. Amorphous fused-silica samples are investigated in this paper. Fused silica erodes quickly and has no material heterogeneity. Three pairs of fused-silica samples, each pair with one control sample and one sample loaded to 6.0, 14.4, and $25.0\pm 1.1\mathrm{MPa}$ , respectively, are exposed to argon plasma for 11 h, producing erosion depths of $20–40\mu \mathrm{m}$ . Analysis of line scans made on the pre- and postexposure surfaces reveals the presence of a growth process operating on the initially roughened surface of the samples. Microscopy and line scans show the development of $100\mu \mathrm{m}$ parabolic cells bounded by sharp-edged cusps. No difference is detected in the development of surface features between unloaded samples and samples loaded up to 25.0 MPa. A model of the evolution of the surface accurately reproduces the development of the cell pattern, by taking into account the angle dependence of the atomic sputtering yield, yielding surfaces similar to those observed experimentally.