Electron populations and neutralization process in the plume of a gridded ion thruster

Abstract In order to assess the importance of collisional processes in the neutralization of a gridded ion thruster plume this study presents a series of simulations carried out with a planar, full-PIC code provided of a library of electron, ion and neutral collisions. In particular, we find that the inclusion of electron inelastic collisions, such as ionization and excitation, results fundamental in the generation of a population of electrons trapped in the plume’s potential well, which play a major role in the neutralization of the plume and in the formation of the electric potential map. A further investigation is therefore carried out on the properties of the electron populations present in the plume, suggesting that the trapped population results mostly insensitive to the emission properties of the cathode, but displays a strong dependence on the inclusion of inelastic collisions and a slow approach to stationary conditions dictated by the timescales of these collisions. Given the symmetry of the plume bulk even in presence of an externally mounted cathode, we further extend the study to an axisymmetric simulation case with an annular cathode, that allows the evaluation of a three-dimensional expansion process. The build up of the trapped electron population in this case is even slower, because of the smaller neutral density observed when expanding the plume in three dimensions, and an acceleration strategy that speeds-up the approach to steady state without altering it is therefore proposed. Nevertheless, the main governing physical processes observed in the planar case remain prominent also in this latter case.