Results from numerical modelling of RF ion thrusters

Abstract

An entirely self-consistent numerical model of RF ion thrusters was developed. The model includes the RF discharge with singly and doubly charged ions, a calculation of the ion impingement current on the accelerator grid and a model of the electrodynamics in the thruster. The thrust is calculated while taking into account the effect of doubly charged ions. Since the model is very efficient, entire fields of characteristics with several hundred operating points can be calculated. Verification Results of this model are shown as well as performance predictions for thrusters which were optimised on the basis of the model. Comparisons with the RIT10 Artemis thruster showed that the required RF power for given beam current is determined within a maximum error of 14 %. Thrusters optimised with help of this model are predicted to require up to 25 % less RF power. The model requires only little computational resources and runs on a regular PC. Introduction RF ion thrusters represent a companion principle to the electron bombardment, or Kaufman, thruster. Both devices hold a plasma—usually xenon—inside a discharge vessel as an ion source. Two (sometimes three) grids on the downstream side of the vessel extract the ions by means of a strong electric field applied between them, accelerating the ions to several tens of kilometres per second. The difference between RF and Kaufman thrusters lies in the type of gas discharge used to generate the plasma ions which are ejected with the beam. While Kauf°Copyright © 2000 by Astrium GmbH. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. man devices employ a DC discharge supported by a magnetic field, RF thrusters use a high-frequency, electrode-less discharge: A coil is wound over the discharge vessel and driven at 1 MHz frequency (Figure 1). The azimuthal electric field induced in the discharge vessel by the coil heats the electrons in the plasma to temperatures between 4 and 10 eV. The hottest electrons carry sufficient energy to ionise xenon by inelastic collisions, therefore generating a continuous flux of ions and secondary electrons. The neutraliser injects electrons into the ion beam in order to balance the electric charge on the spacecraft. Several ion thrusters have been designed and tested at DaimlerChrysler Aerospace AG: • the RIT10 Artemis, for a thrust of 15 mN, which was successfully qualified and acceptance tested in 1997 and is currently under(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.