Fundamental Ion–surface Interactions in Plasma Thrusters

Ion thrusters offer the potential to enable many future interplanetary robotic missions presently under consideration by NASA. To realize the benefits offered by these low thrust devices, the sputtering mechanisms that are responsible for the degradation of thruster components over time must be well understood. Predictions of thruster life depend directly on the material removal rates from thruster electrodes such as the ion optics and hollow cathodes. To better understand the conditions encountered at these surfaces, this study includes an investigation of low energy sputtering at glancing incidence. Relevant ion–target combinations that were considered included Xe incident on Mo, C, and Cu, as well as Ar incident on W, C, and Cu. To characterize the sputtering yield angular dependence experimentally, an ion beam was used to etch a coated quartz crystal microbalance. This required the development of techniques to accurately measure the incident low energy ion flux to the target and the use of surface diagnostics to investigate the properties of target materials. Measurements of C and Mo sputtering yields were obtained for Xe incidence angles up to 80o from the surface normal and for energies ranging from 80 eV–1 keV. In addition, existing transport theory models were used to examine projectile scattering within the different target media. The models also indicate that the sputtering behavior as a function of angle of incidence is not a strong function of energy, a conclusion that is supported by the experimental results. The surface roughness of the targets was investigated using atomic force microscopy to obtain local incidence angle distributions. A surface layer activation technique served as an alternate method of evaluating the sputtering rates of thruster components for situations where the ion bombardment conditions are not well known. In this study, a radioactive tracer was produced in the surfaces of a number of laboratory model ion thruster cathode assemblies by high energy proton bombardment. The cathodes were tested in a 30 cm diameter xenon ion thruster to provide insight into the relevant wear mechanisms at