A Kinetic Study of Electron-Wall Collisions in a Hall Thruster

The role of electron-wall collisions in Hall thruster operation is studied through simulation of the electron energy distribution function (EEDF) inside the thruster channel. The electron Boltzmann equation is solved using the “local-field” approximation, using experimental results as input data. The equation takes into account inelastic losses due to ionizing collisions and wall-collisions, as well as secondary electrons from ionization and from wall-collisions. Electron balances are used to calculate the sheath potential at the insulator walls. Results show an EEDF cut off at high energy due to electron loss to the walls. The calculated EEDFs agree well with experimental electron temperature data when an experimentally-determined effective collision frequency is used for electron momentum transport. Predicted values for the wall-sheath potential agree with results of a Maxwellian flux-balance, but only when secondary electron generation is neglected. The electron wall-loss and wall-return frequencies are extremely low compared to those predicted by a Maxwellian of equal average energy, a difference due solely to the shape of the simulated EEDF. The very low wall-collision frequency suggests that for this particular laboratory Hall thruster, secondary electrons do not contribute to cross-field transport.