Interpretation of Wall-mounted Probe Characteristics in Hall Thrusters

Abstract

Wall-mounted Langmuir probes are a promising internal diagnostic for Hall thrusters that can help understand critical plasma-wall interactions. However, the non-ideal environment within the discharge channel can complicate the implementation and interpretation of resulting probe characteristics. The effects of magnetic field and flowing plasma are experimentally investigated on a boron nitride coupon instrumented with a wall-mounted probe and placed in the far-field plume of a Hall thruster. Magnetic field settings were varied from 0 – 200 G, and orientations of 0 – 90 degrees were tested. Typical plasma number densities measured were around 3×10 14 m -3 , while electron temperatures were around 1.5 eV. The ratio between cyclotron radius and the Debye length ranged from 0.3 to 11, allowing comparisons to be made between this experiment and typical Hall thruster conditions where this ratio ranges from 10-15. It was found that magnetic field and ion flow angle effects were negligible for all orientations except 90° (parallel to the wall). Based on the results, it is concluded that magnetic fields likely have a negligible effect on measured properties, but the effects of ion flow and magnetic field oriented parallel to the wall requires further investigation.