Electron Energy Distribution Function in a Hall Discharge Plasma

The role of inelastic 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 Lorentz approximation (two-term expansion) and the "local-field" approximation. The resultant zero-dimensional Boltzmann equation takes into account inelastic losses due to ionizing collisions and wall-collisions. Secondary electrons from ionization and wall-collisions are also included in the model. Electron continuity is 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. Secondary and scattered electrons from ionization provide a large population of low-energy electrons. 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 from a charge-balance model, except where said model predicts sheath collapse.