Characterization of Fluctuations and Electron Transport in Two-Dimensional Simulations of Hall Thrusters using an Axial-Azimuthal Hybrid Model

Abstract

A two-dimensional model of the Hall thruster with kinetic, non-magnetized ions, and fluid electrons is presented. The model dynamically evolves azimuthal flows and fluctuations, differing from standard hybrid models that assume axisymmetry and resolve quantities in the radial and axial coordinates only. Unlike those descriptions, which typically use adhoc cross-field electron transport parameters in order to sustain the discharge, the present model relies on classical transport and fluctuations generated within the plasma. A number of low-frequency wave modes are captured in the simulation, from the lowest (few kHz) “breathing mode”, to waves on the order of a few hundred kHz. At issue is the characterization of the various modes with regards to their instability mechanisms, their spectral signatures, their dependence on plasma inhomogeneity along the channel, and their role in cross-field electron transport. Simulations show that gradient-driven drift instabilities emerge downstream of the peak of the magnetic field, as predicted by linear stability analysis, while strong, ionization driven fluctuations take place upstream. While fluctuations result in fluctuation-driven transport, they do not drive sufficient current to match experimental measurements. Electron transport is reduced in the strong magnetic field region to near classical levels, in qualitative agreement with experiments.