Impact of Neutral Density on the Operation of High-Power Magnetically Shielded Hall Thrusters

Abstract

High-power Hall thrusters are enabling for a variety of space missions. Developments in the field have led to an increase in the achievable power levels and the lifetime of these devices. Magnetic shielding increases the lifetime of Hall thrusters by three orders of magnitude, while nested Hall thrusters allow these devices to be scaled to the high powers necessary for Mars missions. Despite these advancements, there remain open questions about the exact implications on Hall thruster operation that pose large risks for transitioning the thrusters to flight. One of the most critical is how neutral gas particles emanating from various sources influence the overall performance and lifetime of these thrusters. There exist several sources of neutrals such as the background facility pressure, the cathode, and adjacent channels on nested Hall thrusters.

This work examines the role neutrals can play on the performance and lifetime of high-power Hall thrusters. We start with a theoretical framework for how neutrals should impact these parameters. In particular, we detail how neutrals will shift the main acceleration region of the Hall thrusters resulting in a reduced divergence angle and increased thrust. We discuss how this shift could lead to changes in the erosion profile. Previous results on nested Hall thrusters indicated that the performance in multi-channel operation was higher than the sum of each individual channel. We postulate that these difference are due to the increased neutral density when moving from single-channel to multi-channel operation. Again, the neutrals drive the acceleration region inward which improves the thrust. Additionally, neutrals from an adjacent channel can be ingested resulting in higher performance. We provide a framework to calculate the increased thrust due to both of these parameters.

In order to experimentally characterize these impacts, we use three Hall thrusters: the H9, a 9-kW Hall thruster, the X2, a two-channel 10-kW nested Hall thruster, and the N30, a two-channel 33-kW nested Hall thruster. We employ a variety of diagnostics including a thrust stand, a neutral pressure probe, a Faraday probe, and laser-induced fluorescence to measure the thruster properties in addition to the standard telemetry. Our results show that the acceleration region of Hall thrusters does shift with a change in neutral density. We show this shift is independent of the neutral source and the critical parameter is the neutral density at the exit plane. Crucially, we also show that this shift can lead to changes in performance. For nested Hall thrusters, we show that the thrust improves 5% when moving from single channel to dual channel operation on the X2. This 5% increase is explainable by our theoretical framework which shows that this improvement is due to neutral effects. Finally, we show that the near-wall ion trajectories (for both single-channel and nested-channel magnetically shielded thrusters) are not substantially changed with varying local neutral density - indicating that the erosion profile of the thruster does not depend on the outside pressure environment substantially.

Ultimately, we have improved our understanding of the mechanisms behind neutral impacts on the performance and lifetime of magnetically shielded and nested Hall thrusters. This improves of fundamental understand of Hall thruster operation and decreases the risk for transitioning to flight. For nested Hall thrusters, we have expanded our understanding into the differences between them and traditional Hall thruster and increased the technology readiness level.