Influence of residual gases on witness plate measurements during Hall-effect thruster testing

Abstract

It is known that the presence of residual gases can significantly suppress the sputter erosion rate of surfaces. Concerns about the validity of witness plate measurements are raised when this happens in ground-based tests of plasma thrusters. Here we quantify the influence residual gases impose upon measurements made with witness samples placed within the energetic ion plume of a Hall-effect plasma thruster. Specifically, sputter rates of thin films composed of iron by xenon ions are presented as functions of ion energy and partial pressure of molecular oxygen and nitrogen. When gas contaminates are present, the sputter rate is determined as a function of the poisoning ratio, Rp, defined as the rate of arrival of contamination sticking to a surface divided by the removal rate of sputtered material due to ion bombardment. The iron sputter rate was measured versus poisoning ratio from 0.001 to 10. At low poisoning ratios, ion bombardment maintains a dynamically clean surface, sputtering is not suppressed and sputtering rates can be estimated from the literature or from experiments. At poisoning ratios of Rp > 0.1 however, the net sputtering rate can be attenuated. The sputter rate of iron in the presence of oxygen at Rp > 1 was found to be only 5.4% (0.054) and 11.5% (0.115) of the sputter rate at Rp < 0.01 by 300 eV and 600 eV xenon ions, respectively. Although oxygen reduced the iron sputter rate, nitrogen was not found to affect the sputter rate of iron.