Electron-wall Interaction in Hall Thrusters
Authors: Y. Raitses, D. Staack, M. Keidar, and N.J. Fisch
Date of PPPL Report: February 2005
Published in: Physics of Plasmas 12:5 (May 2005) Article No. 057104 (9 pages). Invited paper at the 46th Annual Meeting of the APS Division of Plasma Physics (15-19 November 2004, Savannah, Georgia).
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 electron temperature and the location of the accelerating voltage drop, which might have been expected due to the SEE-induced near-wall conductivity.