Numerical Investigation of Carbon Sputtering and Transport for the Ion Thruster Testing in the Ground Facility

Carbon contamination due to backsputtering in an electric propulsion (EP) facility is a well-known concern in EP testing. Numerical simulation is a strong tool to isolate the facility effects from on-orbit performance and increase in wear characteristics as EP systems evolve toward larger power levels. This study uses particle-in-cell (PIC) and direct simulation Monte Carlo (DSMC) simulation results to study carbon backsputtering in a simplified vacuum chamber using a thruster modeled in our previous studies with new PIC and ion-surface interaction models. We investigate how the ion plume changes in a fully kinetic PIC simulation in addition to the quasi-neutral case that is commonly used. Ions, including those that have experienced MEX and CEX collisions, are sampled when they hit the vacuum chamber wall to understand the dynamics of backsputtering. In addition, we evaluate the impact of the ion-surface interaction model on the backsputtering rate on the thruster face by comparing theoretical semi-empirical expressions that have been used to fit past measurements with the classical cosine distribution. As a result, while the backsputtering rate is largely independent of differences in the PIC model, differences in the sputtering model have a significant impact.