Comparison of two-dimensional five-moment fluid and kinetic simulations for the discharge plasma in a Hall effect thruster

A two-dimensional, axial-radial, five-moment (5M) fluid model is developed to simulate the cross-field plasma discharge of a Hall effect thruster and is compared with a particle-in-cell Monte Carlo collision (PIC-MCC) simulation. In the 5M model, we account for a non-neutral plasma and inertial effects in the conservation of momentum, which are usually neglected in quasineutral drift-diffusion treatments for electrons. In addition, we propose a boundary condition that accounts for the virtual cathode formation due to secondary electron emission, which plays a critical role in obtaining a steady-state solution. Ions are treated with a multi-fluid approach, accounting for separate high- and low-energy subpopulations. Qualitative agreement is observed in the spatial profiles of many plasma properties. The steady-state discharge current in the PIC-MCC model is found to be higher than that of the 5M model, which is hypothesized to be due to the anisotropic pressure contributions absent in the 5M model. A sensitivity study is performed wherein the field-aligned thermal conductivity in the 5M model is reduced, leading to non-isothermal electron temperature profiles along the magnetic field lines, consistent with observations from the PIC-MCC results. The advantages and limitations of the 5M model compared to existing fluid approaches are discussed.