Three-Dimensional Kinetic Simulation of an Ion Thruster Plume with Carbon Backsputtering in a Vacuum Chamber

TL;DR

This paper uses a fully kinetic simulation to analyze how chamber effects, including backsputtering and carbon deposition, influence ion thruster plume behavior and performance in vacuum chamber tests compared to space conditions.

Contribution

It introduces a comprehensive kinetic simulation approach to accurately model facility effects on ion thruster plumes, highlighting the importance of electron sheath and angular sputtering effects.

Findings

Chamber conditions increase neutral particle density by 100 times compared to space.

Electron sheath significantly alters charge-exchange ion flux, energy, and angles.

Carbon deposition rate is highly sensitive to angular sputtering dependence.

Abstract

Gridded ion thrusters are tested in ground vacuum chambers to verify their performance when deployed in space. However, the presence of high background pressure and conductive walls in the chamber leads to facility effects that increase uncertainty in the performance of the thruster in space. To address this issue, this study utilizes a fully kinetic simulation to investigate the facility effects on the thruster plume. The in-chamber condition shows a downstream neutral particle density 100 times larger than the in-space case due to ion neutralization at the wall and limited vacuum pump capability, resulting in a significant difference in the density and distribution of charge-exchange ions. The flux, energy, and angle of charge-exchange ions incident on the chamber wall are found to be altered by the electron sheath, which can only be simulated by the fully kinetic approach, as opposed to the conventionally used quasi-neutral Boltzmann approach. We also examine the effect of backsputtering, another important facility effect, and find that it does not necessarily require a fully kinetic simulation as the incident flux and energy of the sampled charge-exchange ion are negligibly small. Finally, we demonstrate that the carbon deposition rate on the thruster is significantly influenced by the angular dependence of the sputtered carbon, with a nearly 50% effect.