Three-dimensional modeling of dual ion-thruster plumes for spacecraft contamination

Abstract

The plume structure and the backflow of both propellant and sputtered grid material from two 8-cm xenon ion thrusters operating simultaneously were investigated with a three-dimensional model of an ion-thruster plume based on the plasma particle-in-cell technique. Thruster center to center separation distances of 10 and 17 cm were examined. The separation distance was found to play a strong role in determining the potential structure that affects propellant charge-exchange ion transport. Because of the combined potential structure of the two beams, the propellant charge-exchange ion backflow was found to be enhanced along an axis perpendicular to a line joining the thruster centers, whereas this was not the case for the more energetic sputtered molybdenum grid material. The implications of such asymmetry in the structure of the backstreaming flowfield from twin thrusters are important for thruster-spacecraft integration. Nomenclature Ag = sputtered grid area, m2 An = grid neutral flow through area, m2 C = neutral average value speed, m/s e = electron charge, C //, = thruster beam ion current, A jbj = beam ion current density, A/m2 k = Boltzmann's constant, mks M = mass of grid lost due to sputtering over a period of time, kg M = molecular weight, kg/mole m, = ion mass, kg mT = thruster total mass flow rate, kg/s Nccx = charge-exchange (CEX) ion production rate, number/m3/s A/A = Avogadro's number ribi = beam ion density, m~3 m^n = total ion, electron, neutral density, m~3 (a further subscript 0 denotes reference) rh T = beam, thruster radius, m Te = electron temperature, K Tw = thermal wall temperature of neutrals, K VM = beam ion velocity, m/s a = beam divergence angle, rad Fy = sputtered grid material flux, number/m2/s T]P = propellant utilization efficiency acex = CEX cross section, m2 t = period over which grid mass is lost, s 4>/, = beam acceleration voltage, V (/) = electric potential, V