Development of a Magnetically Shielded Hall Thruster without Pole Erosion

Hall effect thrusters (HET) are widely employed electric propulsion (EP) devices that have received significant attention and investment from NASA. HETs offer high specific impulse (> 1500 s) at relatively high thruster density (> 10 N/m^2) making them well-suited for long-duration, deep-space missions. While recent advances in this technology have greatly extended their lifetime, there remain life-limiting processes related to the erosion of Hall thruster magnetic poles that still pose a risk to deep-space applications. This erosion stems from the presence of high energy ions that impact and gradually erode the magnetic poles. Current HET pole erosion mitigation strategies are based on increasing pole thickness or installing covers constructed from sputter-resistant material to delay the onset of pole erosion-induced failure. While these types of margin-based approaches may be effective, pole erosion still could represent a potential risk to thruster lifetime. This stems from the fact that the mechanisms driving energetic ion production remain unknown. As such, pole erosion rates may evolve over in time in such a way that increased margin may not be sufficient. With this challenge in mind, recent work has focused on understanding and modeling the source of energetic ion production. This has led to an emerging consensus that plasma instabilities may be the dominant driver for this outstanding erosion process. Ideally, mitigation strategies would leverage this insight to eliminate erosion by interrupting the instabilities. For example , drawing a parallel from an adjacent field, a fundamental understanding of instabilities in rocket engines has led to prescriptive mitigation strategies such as the installation of acoustic resonators in combustion chambers to dampen frequencies associated with combustion instabilities. With a similar aim in mind, my goal is to develop novel Hall thruster designs that actively or passively damp erosion-inducing instabilities and thus greatly reduce, or eliminate entirely, the primary erosion mechanism in state-of-the-art Hall thrusters.

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