Wall material effects in stationary plasma thrusters. I. Parametric studies of an SPT-100

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 (borosil), 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.