Simulations and experimental characterizations of a stationary plasma thruster are compared for four different wall materials to investigate near-wall conductivity (dielectric materials) and in-wall conductivity (conducting materials) in such a discharge. Using a one-dimensional transient fluid model that takes into account a possible electron temperature anisotropy, it is shown that electron-wall backscattering plays a crucial role by maintaining a relatively high electron temperature along the magnetic field lines which in turn drives large electron currents toward the walls. The large differences in discharge current observed experimentally for the dielectric materials are qualitatively recovered, confirming that near-wall conductivity results from the combined effects of secondary electron emission and electron backscattering. A clear correlation is found between the appearance of space charge saturation at the walls and a jump of the discharge current observed in experiments when varying the discharge voltage or the magnetic field. The anomalously high values of discharge current observed experimentally with graphite are also correctly recovered in simulations, which highlight a plasma short-circuiting effect resulting from in-wall currents.

The operation of a laboratory version of the flight-qualified SPT-100 stationary plasma thruster is compared for four different discharge chamber wall materials: a boron nitride-silica mixture ͑borosil͒, alumina, silicon carbide, and graphite. The discharge is found to be significantly affected by the nature of the walls: changes in operating regimes, up to 25% variations of the mean discharge current, and over 100% variations of the discharge current fluctuation amplitude are observed between materials. Thrust, however, is only moderately affected. Borosil is the only material tested that allows operating the thruster at a low mean current, low fluctuation level and high thrust efficiency regime. It is suggested that secondary electron emission under electron bombardment is the main cause of the observed differences in discharge operation, except for graphite, where the short-circuit current inside the walls is believed to play a major role. It is also suggested that the photoelectric effect, which has apparently not been given attention before in the Hall thruster literature, could increase the cross-field electron current.

A discussion is presented on the results and predictive capabilities of a two-dimensional (2D) hybrid Hall effect thruster (HET) model. It is well known that classical (collision-induced) cross-field electron transport and energy losses are not sufficient to explain the observed HET characteristics. The 2D, quasineutral, hybrid discharge model uses empirical parameters to describe additional, anomalous electron transport and energy loss phenomena. It is shown that, for properly adjusted empirical parameters, the model can qualitatively reproduce the observed thruster behavior over a large range of operating conditions. The ionization and transit-time oscillations predicted by the model are described, and their consequences on the time-averaged thruster properties are discussed. Finally, the influence of the empirical parameters on the model results is shown, especially on quantities that can be measured experimentally.

A two-dimensional hybrid particle-in-cell numerical model has been constructed in the radial-axial plane with the intent of examining the physics governing Hall thruster operation. The electrons are treated as a magnetized quasi-one-dimensional fluid and the ions are treated as collisionless, unmagnetized discrete particles. The anomalously high electron conductivity experimentally observed in Hall thrusters is accounted for using experimental measurements of electron mobility in the Stanford Hall Thruster. While an experimental mobility results in improved simulation of electron temperature and electric potential relative to a Bohm-type model, results suggest that energy losses due to electron wall interactions may also be an important factor in accurately simulating plasma properties. Using a simplified electron wall damping model modified to produce general agreement with experimental measurements, an evaluation is made of differing treatments of electron mobility, background gas, neutral wall interactions, and charge exchange collisions. Although background gas results in two populations of neutrals, the increased neutral density has little effect on other plasma properties. Diffuse neutral wall interactions are in better agreement with experimental measurements than specular scattering. Also, charge exchange collisions result in an increase in average neutral velocity of 11% and a decrease in average ion velocity of 4% near the exit plane. The momentum exchange that occurs during charge exchange collisions is found to be negligible.