Influence of immersed probes and SEE on the charge neutralization process in a gridded ion thruster plume

Gridded ion thrusters typically employ the emission of electrons through externally mounted cathodes to guarantee the neutralization of the positive space charge and current emitted by the thruster. However, recent studies [Guaita et al, PSST, 2025] have shown that this process of charge neutralization heavily depends on inelastic collisions between electrons and neutrals, that promote the reduction of the electrons' total energy and their trapping in the ion beam's potential well. In the case of low background pressures, where the inelastic collision frequencies are relatively small, the equilibrium that the neutralized plume would reach is extremely fragile, and may therefore be easily altered by any perturbation providing a source or a sink of trapped electrons. In this study we consider, through a series of axis-symmetric PIC simulations of the plume of the RIT10-EVO, two such perturbations that may result particularly important during experimental investigations in ground facilities. The first is the immersion of probes in the plume bulk, required to experimentally measure the plasma properties, and which depending on their surface area and potential are expected to represent an important sink of trapped electrons. Indeed, we observe that for probe potentials close to or above the local plasma potential, the immersion of the probe in the plume relevantly alters the general response of the plasma. The second perturbing mechanism modeled and studied is the secondary electron emission by ion bombardment of the beam dump at the end of the experimental facility. In this case, the secondary electrons are observed to populate the regions of phase-space of trapped electrons, promoting the neutralization of the plume and reducing the potential values in front of the thruster grids.