Effect of Backpressure on Ion Current Density Measurements in Hall Thruster Plumes

The effects of facility backpressure and localized electric fields on the measured ion current densities of Hall thrusters are investigated. Langmuir probe measurements are taken in the near-field plasma surrounding a nude Faraday probe, which is located 1 m from the exit plane of the University of Michigan/U.S. Air Force Research Laboratory P5 Hall thruster. The thruster is operated at an anode flow rate of 5.30 mg/s, at backpressures of 1.5 × 10−3 Pa (1.1 × 10−5 torr) and 4.8 × 10−4 Pa (3.6 × 10−6 torr), corrected for xenon. The effect of the facility backpressure is clearly seen in the wings of the plume. A combination of charge-exchange collisions and vacuum chamber gas ingestion into the thruster is believed to be the cause of this phenomenon. The Langmuir probe results indicate that the electric fields near the nude Faraday probe are functions of facility backpressure and the angle from the thruster centerline. The plasma potential measured within 20 mm of the probe varied by no more than 3 V. Thus, the electric fields near the nude Faraday probe are not large enough to explain the increased collection of charge-exchange ions at elevated facility background pressures and large angles from the thruster centerline