Ph.D. Thesis Proposal
by
Jason D. Frieman
(Advisor: Professor Mitchell Walker)

CHARACTERIZATION OF BACKGROUND NEUTRAL FLOWS IN VACUUM TEST FACILITIES AND IMPACTS ON HALL EFFECT THRUSTER OPERATION

9:30 AM Thursday, October 6, 2016
Guggenheim Building
Room 246

ABSTRACT:
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 facility size and 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 controls HET neutral ingestion and the concomitant impacts on performance and plume characteristics. The first portion of this work seeks to determine if a bulk background flow exists inside ground test facilities, and, if it does, characterize 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 background neutral flows 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. An experiment is proposed in order to obtain equivalent empirical measurements of the background flow field inside a ground test facility in order to further validate the analytic results generated by the model.
The second portion of the proposed 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 of a HET are proposed in order to determine if description of the background flow field can explain the observed sensitivity of HET performance and plume properties to the facility pressure environment.