Computational Modelling of the Plasma in the Charge Exchange Thruster

Abstract

The Charge Exchange Thruster (CXT) is a novel plasma thruster that produces thrust via energetic neutrals leaving the device. This is done by accelerating ions with an electrostatic potential inside of a so-called hollow cathode device: some of these ions then undergo charge exchange and leave the thruster as energetic neutrals whilst others contribute to the formation of an electrical discharge inside the thruster. In this thesis a new 2D-3V particle-in-cell-Monte Carlo (PIC-MCC) simulation code is developed and tested with the purpose of modelling the plasma inside the CXT. This code includes multiple chemical processes involving hydrogen ions, atoms, and molecular species, an external circuit simulation, probabilistic reflections from boundaries, and the generation of secondary electrons, all of which are found to be vital in the plasma initiation and development. The new PIC-MCC code is benchmarked successfully against the theoretical plasma sheath width as well as experimental Paschen curves for a parallel plate discharge, being found to reproduce these results well. Similarly, by modelling hollow cathode discharge systems it is found that the code can reproduce two effects specific to hollow cathode devices, a higher plasma density inside the cathode than outside, as well as beams of energetic neutrals. The code was able to reproduce the experimental results of the original CXT paper reasonably well but suffered from numerical instabilities that prevented the simulation from modelling the discharge in a steady state. It was found that simulating the thruster in a lower pressure discharge mode did not produce as much thrust but was able to reach a steady state and was numerically stable over timescales of tens of microseconds. Future work on investigating the cause of the numerical instabilities at high pressures, as well as a more complex model for the background gas dynamics in the thruster are recommended.