Electrical facility effects on Hall effect thrusters

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Walker, Jonathan A.
Walker, Mitchell L. R.
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The Hall effect thruster (HET) is a type of spacecraft propulsion that is used for satellite orbit raising and station keeping. HET development and lifetime qualification tests are performed in ground-based vacuum facilities. To ensure predictable flight operation of HETs, the ground-based testing environment must be representative of the on-orbit environment, or there must be a clear path to correlate the ground test results to expected on-orbit HET behavior. Much of the previous work related to understanding how to correlate HET ground-testing behavior and HET on-orbit behavior is focused on understanding the impact on HET operation of the elevated neutral gas pressures that HETs experience in ground-based test facilities. Flight data, from satellite missions using HETs, shows that HETs have variations in behavior that cannot be explained through neutral pressure considerations. As experienced during the SMART-1 mission, certain characteristics of the on-orbit HET electrical circuit can be influenced by external electrical factors. For ground-based testing, the vacuum facility walls represent an artificial electrical boundary that is not present during in-flight operation. The electrical impact that the walls and other conductive surfaces in the vacuum chamber have on the behavior of HETs is unknown. The results of the SMART-1 mission demonstrate that there is a gap in the knowledge of HET-vacuum facility interactions. The goal of this work is to better understand how HET thruster operation is influenced by electrical interactions with the conductive walls of the vacuum chamber and other conductive surfaces that are only present within the ground-based testing environment. To examine these electrical interactions, this work varies the electrical boundary conditions of key electrical surfaces in the vacuum facility environment and measures how those electrical boundary conditions influence a testbed 3 kW HET. This work identifies two key conductive surface systems in the vacuum testing facility that influence the operation of HETs: the walls of the vacuum facility and the HET body. The walls of vacuum facility influence the plume of the HET by mediating the charge-loss rate to the walls of the vacuum facility. The results of this work suggest that the walls of the vacuum facility artificially bound the plasma properties of the HET plume. This augmentation of the plasma plume indicates that there can be variations in the on-orbit plume characteristics that are not measured during ground testing. For the HET body, the conductive metal structure of the thruster is found to be an active component of the HET electrical circuit. While on-orbit, the HET body is directly connected to the satellite electrical common, and the HET electrical circuit voltage relative to the HET body has been measured to change significantly on-orbit. The results of this work indicate that the thruster body of the HET can play a critical role in the oscillations of the discharge current. Due to the complex plasma conditions present in the near-field of the thruster body, it is difficult to pinpoint the exact physical mechanisms governing the electrical interaction between the thruster body and the HET. The results of this work strongly indicate that the electrical boundary condition of the thruster body is an important consideration for the ground testing of HETs. Collectively, this work provides valuable insight into the field of HET ground testing. Understanding HET-facility interactions are integral to both HET development and HET-satellite integration.
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