Radio-frequency biasing of ion acceleration grids with different propellants

Abstract

Abstract In order to ensure the space charge compensation of the plume, conventional ion thrusters need an additional neutralizer to release electrons. When a radio-frequency (RF) voltage is applied across the grid system instead of a direct-current voltage, the simultaneous extraction of ions and electrons is achieved, thereby a neutralizer is not required. In this paper, based on the non-uniform distribution of neutral gas density calculated using the angular coefficient method, the particle-in-cell Monte Carlo collision method is used to thoroughly investigate the spatial and temporal dynamics of particles and the grid system performance for different propellants (argon, krypton and xenon) in such an RF grid system. RPA and E × B probe are employed to measure the ion flux distribution functions (IFDFs) of RF ion thruster with RF biasing, which are used to compare with the simulations. The simulated linear relationship between the self-bias voltage and the RF voltage amplitude and the multi-peak behavior of IFDF under low RF frequency conditions are comparable with the experimental data. The simulated IFDFs compare well with the experiments with the deviation of energy peak position less than 7% and 10% from those by RPA and E × B probe respectively, indicating the effectiveness of the used model. Simulations show the RF grid system is able to realize the extraction of electrons for all three propellants, so as to achieve the plume neutralization without an external neutralizer through the spatial and temporal oscillations of the beams. Electrons pass through the grid twice (extracted from the upstream, and backflow from the downstream), bringing two peaks of electron current to the accelerator grid in one period. The thrust-RF voltage curves for all three propellants show obvious slope transition, when the perveance limit is reached. The low-energy ions in the plume are mainly generated by the electron impact ionization processes for Xe while by CEX collisions for Ar. A larger ion current density of Xe on the downstream surface of the accelerator grid, which may lead to possibly more serious erosions of grids, is found compared with those of Kr and Ar. This is mainly contributed by the larger density of electron impact ionization generated ions of Xe in the downstream because Xe propellant has a larger electron density and ionization cross-section.