Experimental characterization of the near-wall plasma in a 6-kW hall thruster and comparison to simulation
In order to better understand interactions between the plasma and channel walls of a Hall thruster, the near-wall plasma was characterized within the H6 Hall thruster using five flush-mounted Langmuir probes. These probes were placed within the last 15% of the discharge channel and were used to measure plasma potential, electron temperature, and ion number density near the inner and outer channel walls. These data were then compared to prior internal measurements inside the channel using a High-speed Axial Reciprocating Probe stage. Comparison of these data has shown that, at the nominal operating condition of 300 V and 20 mg/s anode flow rate, the plasma near the wall begins to accelerate further upstream than plasma closer to centerline. This shift in acceleration zone creates large radial electric fields (~ 40-50 V/mm) that tend to defocus ions and drive them towards the walls. The shift is likely caused by large plasma density gradients between centerline and the channel walls, creating a significant deviation of equipotentials from magnetic field lines near the walls. Electron temperature axial profiles were found to be largely consistent across the channel, supporting the isothermal assumption along magnetic field lines. The experimental results were also compared to simulation results from the hybrid-PIC program HPHall-2. General agreement was found between simulation and experiment for axial profiles of plasma potential, electron temperature, and ion number density, with minor differences occurring in peak locations. Slight asymmetries in properties were found between the inner and outer channel walls despite the use of a symmetric magnetic field topology. This asymmetry was caused by a difference in the location of the maximum radial magnetic field, resulting in axial shifts of acceleration zone and peak electron temperature. This result is supported by asymmetric erosion profiles after 334 hours of operation, showing increased erosion along the outer wall where acceleration began further upstream. © 2011 by Rohit Shastry. Published by the American Institute of Aeronautics and Astronautics, Inc.
