Electron-wall interaction in Hall thrusters

TLDR

Hall thruster models predict temperature saturation as a transition to a space‑charge saturated near‑wall sheath. The study measured plasma response to variations in channel width and discharge voltage and proposes that an enhanced anomalous crossed‑field mobility near the wall explains the reduced electric field in narrow channels. A discharge voltage threshold separates two regimes: below it electron energy gain is constant and secondary electron emission is insufficient, while above it the electron temperature saturates at a value nearly three times higher than predicted, channel narrowing alters the plasma potential more than temperature, and no coupling between temperature and the accelerating voltage drop is observed.

Abstract

Electron-wall interaction effects in Hall thrusters are studied through measurements of the plasma response to variations of the thruster channel width and the discharge voltage. The discharge voltage threshold is shown to separate two thruster regimes. Below this threshold, the electron energy gain is constant in the acceleration region and therefore, secondary electron emission (SEE) from the channel walls is insufficient to enhance electron energy losses at the channel walls. Above this voltage threshold, the maximum electron temperature saturates. This result seemingly agrees with predictions of the temperature saturation, which recent Hall thruster models explain as a transition to space-charge saturated regime of the near-wall sheath. However, in the experiment, the maximum saturation temperature exceeds by almost three times the critical value estimated under the assumption of a Maxwellian electron energy distribution function. The channel narrowing, which should also enhance electron-wall collisions, causes unexpectedly larger changes of the plasma potential distribution than does the increase of the electron temperature with the discharge voltage. An enhanced anomalous crossed-field mobility (near wall or Bohm-type) is suggested by a hydrodynamic model as an explanation to the reduced electric field measured inside a narrow channel. We found, however, no experimental evidence of a coupling between the maximum electron temperature and the location of the accelerating voltage drop, which might have been expected due to the SEE-induced near-wall conductivity.