Computational Modeling of Hall Thruster Channel Wall Erosion.

Abstract

Hall thrusters, a type of space electric propulsion, offer high specific impulses attractive for a variety of space missions. As lifetime requirements desired for Hall thruster operations increase, there is a greater need to be able to predict and analyze the wear of the thruster over time. The main mechanism of Hall thruster failure is the erosion of the acceleration channel walls to the point where the magnetic circuit is exposed to the plasma flow. Experimental testing to determine the rate and extent of wall erosion is time consuming and expensive. Thus, capturing the erosion process through computational simulations is a useful means of predicting lifetime for design and analysis purposes. Two models are employed to simulate the erosion process. The first is a hydrodynamic model that is used to describe the plasma flow within the thruster and to calculate the ion fluxes to the thruster channel walls. The second method is a molecular dynamics model that is used to calculate the sputter yields for the wall material based on the incoming ion fluxes. The results of these two methods are used together to simulate the erosion of the channel walls and their evolution over time. Three test cases are analyzed. The hydrodynamic model is used to compare the differences between krypton and xenon propellants in the NASA-173Mv1 Hall thruster. The molecular dynamics model is used to calculate the sputter yields of hexagonal boron nitride due to low-energy xenon ions. Both methods are used to model the erosion of the channel walls of an SPT-100 Hall thruster over a 4000-hour life test. The computational results are compared to available experimental results for all three cases along with additional analysis. This work represents the only known use of a fluid-based plasma model for Hall thruster erosion simulations. It also contains the only use of a molecular dynamics model without additional surface binding energy assumptions for boron nitride sputtering simulations. Results of boron nitride sputter yields for xenon ion energies below 100 eV are presented.