Characterizing Vacuum Facility Backpressure Effects on the Performance of a Hall Thruster

* Presented as Paper IEPC-01-045 at the 27 International Electric Propulsion Conference, Pasadena, CA, 15-19 October 2001. † Copyright © 2001 by the Electric Rocket Propulsion Society. All rights reserved. Vacuum facility backpressure is known to affect the performance of Hall thrusters through ingestion of background gases by the thruster. Ingested gases may be ionized and subsequently accelerated, artificially increasing the measured thrust. This study seeks to characterize how backpressure affects performance by changing the pumping speed of the facility, not by the commonly used technique of bleeding additional propellant in to the vacuum chamber. Performance is measured at xenon pumping speeds of 140,000 and 240,000 l/s, on a nominally 5 kW Hall thruster at voltages and flow rates of 300-600 V and 5-15 mg/s. At the highest pumping speed, performance data are collected while matching the discharge current observed at the lower pumping speed. Thrust, specific impulse, and efficiency are presented at both pumping rates. The anode mass flow at zero backpressure is calculated by extrapolation from the observed change in flow rate with pressure. On average, the correction is 4% of the anode flow rate at the maximum pumping speed, which is comparable to the uncertainty in the flow controllers.