Self-Consistent Model of a High-Power Hall Thruster Plume

Abstract

A new model of the plasma plume from Hall effect thrusters (HETs) has been developed for the purpose of more accurately predicting the interactions between future high-power thrusters and large high-voltage solar arrays, such as those being developed under NASA’s Game Changing Technology awards by ATK and Deployable Space Systems. The HET plume mainly consists of two types of ions. The first are the energetic main beam ions produced upstream of the thruster acceleration zone. These are the dominant ion species along the thrust axis. The other group of ions have lower kinetic energy and are generated downstream of the acceleration zone from neutral xenon gas atoms by charge exchange (CEX) with main beam ions and by electron impact ionization. The neutral gas is due to neutral propellant atoms leaving the thruster and the hollow cathode without being ionized, and, in the case of laboratory testing, background neutrals present in the vacuum chamber. The new model uses the 2-D axisymmetric Hall thruster code, Hall2De, to self-consistently calculate the three major components of the plume in the vicinity of the thruster, which are the neutral gas atoms, high-energy beam ions, and low-energy ions. From the boundary computational region in the near plume, the Hall2De results are propagated to distances of tens of meters using a continuum hydrodynamic algorithm. This approach offers important advantages with respect to prior models of Hall thruster plumes, such as the NASA Electric Propulsion Interactions Code (EPIC), which uses an analytical fit to laboratory data from a single thruster for the main beam velocity boundary conditions at the channel exit. EPIC assumes that the neutral gas density emanates uniformly and isotropically from the channel exit. Low-energy ions are generated only by CEX; low-energy ions generated by electron impact are not included. The results for the far-field plume of a conceptual high-power thruster (H6) show important differences between EPIC and simulations using the new plume model. High-energy ions undergo less expansion in the azimuthal direction than in EPIC. This can be attributed to magnetic focusing of the beam. We also observe that the peak in low-energy ion density at approximately 90° from the thrust axis predicted by EPIC is moved downstream to angles from 70° to 80° when the new plume model is employed.