Wall material effects in stationary plasma thrusters. II. Near-wall and in-wall conductivity

TLDR

The study compares simulations and experiments of a stationary plasma thruster with four wall materials to investigate near‑wall and in‑wall conductivity. Simulations and experimental characterizations are used to assess the impact of dielectric and conducting walls on discharge behavior. The results show that electron‑wall backscattering sustains high electron temperatures and drives large wall currents, with near‑wall conductivity arising from secondary electron emission and backscattering; space‑charge saturation correlates with discharge‑current jumps when varying voltage or magnetic field, and graphite walls exhibit anomalously high currents due to plasma short‑circuiting from in‑wall currents.

Abstract

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.