Non-classical electron transport in the cathode plume of a Hall effect thruster

An experimental investigation is presented into the wave-driven electron transport in the near-field plume of a hollow cathode operating in a 300V, 4.5kW magnetically shielded Hall thruster. Correlational analysis of probe measurements in the cathode plume shows two types of electrostatic waves: ion acoustic turbulence propagating along the applied longitudinal magnetic field at frequencies from 500 to 1250kHz and coherent, azimuthal anti-drift waves with a fundamental frequency of 95kHz and mode numbers from m = 1 - 4. A quasilinear analysis is applied to quantify the impact of each wave on the electron transport in the near-field plume. It is found that the ion acoustic modes give rise to an enhanced effective collision frequency in the direction parallel to the applied magnetic field that exceeds the classical collision frequency by two orders of magnitude. The anti-drift waves promote an anisotropic collision frequency that depends on the direction of the electron drift. While the enhanced collision frequency from these waves is comparable to the classical frequency for motion along the applied magnetic field, the effective collision frequency in the azimuthal direction exceeds the classical by three orders of magnitude. These results are discussed in the context of their impact on the steady-state plasma gradients in the near-field cathode plume. Closure models for incorporating the effective collision frequencies from both types of waves into fluid-based codes are derived and shown to agree with the measured wave-driven collision frequencies.