The Effects of Nude Faraday Probe Design and Vacuum Facility Backpressure on the Measured Ion Current Density Profile of Hall Thruster Plumes

The effects of dissimilar probe design and facility backpressure on the measured ion current densities of Hall thrusters are investigated. JPL and GRC designed nude Faraday probes are used to simultaneously measure the ion current density of a 5 kW Hall thruster in the Large Vacuum Test Facility (LVTF) at the University of Michigan. The probes are located one meter from the exit plane of the Hall thruster, which is operated over the range of 300-500 V and 5-10 mg/s. In addition, the effect of facility background pressure is evaluated by varying the nominal pumping speed from 70,000 l/s to 240,000 l/s on xenon, corresponding to backpressures of 4.3x10-6 Torr to 2.3x10-5 Torr, corrected for xenon. Detailed examination of the results has shown that the GRC probe measured a greater ion current density than the JPL probe over the range of angular positions investigated for each operating condition. Yet, both probes measure similar thruster plume profiles for all operating conditions. Because all other parameters are identical, the differences between ion current density profiles measured by the probes are contributed to material selection and probe design. Moreover, both probes measured the highest ion current density near thruster centerline at the lowest facility pumping speed. A combination of charge exchange collisions and vacuum chamber gas ingestion into the thruster is believed to be the cause of this phenomenon.