Characterization of background neutral flows in vacuum test facilities and impacts on hall effect thruster operation

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Frieman, Jason David
Walker, Mitchell L. R.
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Hall effect thrusters (HETs) are a type of electrostatic electric propulsion device characterized by high specific impulses, thrust efficiencies, and thrust densities. These performance attributes make HETs an appealing choice for use as the primary propulsion system onboard a number of Earth-orbiting and interplanetary satellite missions. However, extensive ground testing of HETs has revealed that HET operation, performance, and plume properties are impacted by facility-dependent parameters such as pumping capacity. Specifically, it has been shown that increases in facility pressure result in artificial increases in device thrust and efficiency due to the ingestion of ambient background neutrals present in the vacuum facility. Although several analytical and semi-empirical models of HET neutral ingestion exist, none have been shown to be able to accurately predict empirical observations across a range of HETs and test facilities. This work focuses on investigating the hypothesis that a bulk background flow of neutrals exists inside vacuum test facilities, which varies as a function of facility-specific design and operating parameters (i.e., pump placement and pressure modulation technique) and contributes to HET neutral ingestion and the concomitant impacts on performance and plume characteristics. The first portion of this work determines if a bulk background flow exists inside ground test facilities, and characterizes how this flow field changes as a function of facility-specific parameters including pump placement. To do this, a general analytic model of the organized flow of background neutrals inside ground test facilities is created and validated using existing empirical measurements taken using several different facilities and HETs. This model is then used to analytically determine the sensitivity of the background flow field to facility variables including pump placement and pressure modulation technique. These studies are repeated empirically and confirm the accuracy of the model as well as the existence of the bulk background flow, its relationship to HET ingestion, and its sensitivity to facility operating parameters. The second portion of this work seeks to quantify the impact of the background flow field (and concomitant neutral ingestion) on HET operation. Empirical measurements of the time-resolved discharge current, ion energy distribution, thrust, plume plasma properties, and ion current density profile of three HETs are performed and confirm the hypothesis that a full description of the background flow field can better explain the observed sensitivity of certain plume properties to changes in facility pressure than can pressure magnitude. Changes in the background flow field are shown to be unable to fully describe changes in discharge current oscillation characteristics, however, changes in facility pressure are shown to directly cause mode transitions, thus suggesting that optimal magnetic field settings may change between test facilities.
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