An axisymmetric hybrid model of the plasma in a radio-frequency ion thruster chamber

Abstract An axisymmetric hybrid particle/fluid code has been applied to simulate the plasma behavior in the discharge chamber of a radio-frequency ion thruster (RIT). Suitable assumptions have been made to include the coupling with the three other elements of the thruster: the time-averaged deposited power from the radio-frequency (RF) source, the effective transparencies for ion and neutral extraction from the grid system, and the electrical connection with the external neutralizer. A detailed parametric analysis has been carried out in terms of deposited RF power and injected mass flow rate of xenon. As the RF power increases: high propellant utilization and ion beam current are obtained, the maximum plasma density moves away from the chamber center and close to the flow injector, and the beam-to-total energy efficiency decreases. As a result, for a given mass flow rate, there is an optimal RF power that maximizes the thrust efficiency. At these conditions, about 60% of the RF deposited power is lost into the walls due to the lack of magnetic confinement and multiple re-ionizations of wall-recombined neutrals penalize the inelastic losses too. Additional studies have analyzed the influence of: the electrical connections to the neutralizer, which just change the electron current paths; the effective transparency of the grids, where a large sensitivity is found to the ion transparency; the reduction of the energy accommodation of heavy species after wall impact, which penalizes much the maximum thrust efficiency; the presence of metastable atoms and stepwise ionization, which although present, have marginal effects on performances as a result of the multiple wall interactions. A global model, derived from the 2D model and constituted of algebraic expressions, can yield the main performance figures and averaged plasma magnitudes in the chamber with a relative error of up to ± 15%. Finally, 2D simulations are compared indirectly with existing data for a RIT-4.