A Numerical Study of Wall Erosion and Material Transport in Hall Thrusters

Abstract

This dissertation describes the development and application of numerical models for the analysis of channel wall erosion and the transport of erosion products in Hall thrusters. The models build on previous work to characterize the sputtering process that underlies wall erosion using the molecular dynamics (MD) technique, and then implements the results of the sputtering model into a hybrid fluid--particle-in-cell (hybrid-PIC) model of a Hall thruster to predict the motion of the erosion products inside the thruster and in the near-field plume.

The MD model is applied to characterize the sputtering of hexagonal boron nitride (h-BN) under xenon ion bombardment. It is demonstrated that h-BN tends to sputter in the form of atomic boron and diatomic nitrogen, with heavy molecules becoming somewhat more common at higher incident ion energies and angles of incidence. The calculated sputter yields are shown to agree well with experimental measurements over the range of ion energies (20 eV{300 eV) and incidence angles (0{75) investigated. The behavior of sputtered boron atoms is also analyzed, and these atoms are shown to obey the Sigmund-Thompson velocity distribution function (VDF) predicted by sputtering theory in the direction normal to the h-BN surface. The calculated surface binding energy for boron is shown to agree to within 6% of experimental measurements for ion energies of 100 eV and greater.

The calculated sputter yields and boron VDFs are implemented within the established hybrid-PIC model HPHall, and the model is then applied to simulate the NASA HiVHAc Hall thruster at operating conditions that were investigated experimentally using cavity ring-down spectroscopy (CRDS). The effects of ionization and excitation of boron are assessed, and it is found that the majority of sputtered boron atoms do not undergo ionization or excitation in the bulk plasma. These results provide partial validation of CRDS as an in situ diagnostic for measurement of erosion in Hall thrusters. The computed ground-state boron number density in the near-field plume is found to be more uniform than measured using CRDS, but otherwise shows reasonably good agreement with the measurements given the numerous assumptions and sources of uncertainty associated with the models.