Considerations on the role of the Hall current in a laboratory-model thruster

Hall current magnitude and spatial distribution are presented for the plasma discharge in the University of Michigan/Air Force Research Laboratory P5 5 kW laboratory-model Hall thruster. The data are calculated from direct, probe-based measurements of the electric field, static magnetic field, and charged particle number density. Thruster discharge voltage was fixed at 300 V and two current levels investigated: 5.4 A (1.6 kW) and 10 A (3 kW). The results indicate that, for both cases, the bulk of the Hall current is confined to a region centered several millimeters upstream of the exit plane and is asymmetric about the centerline of the discharge channel. At 1.6 kW, the axial plasma potential drop occurs over a much shorter distance, resulting in a more sharply peaked Hall current zone, as compared to the 3-kW case. Comparison of the Hall current and ion number density distribution suggests that the azimuthal electron drift may contribute significantly to the ionization process in the discharge channel. Integration of the Hall current over the entire discharge volume yields total current values that are a factor of 3.5-4.6 times larger than the discharge current. Estimates of the self-magnetic field induced by the drifting electrons indicate no significant modification to the applied magnetic field during thruster operation, at the power levels considered. Using the Hall current density distribution derived from probe measurements, the electromagnetic body force on the ions was calculated and compared to measured engine thrust for both power levels.