Modeling of a Hall thruster plume: influence of plume boundary conditions and size, and cathode location

Abstract

The current-free plasma plume of a Hall effect thruster is investigated through a 2D axisymmetric hybrid (particle-in-cell/fluid) model and code. First, since the simulated plume is necessarily finite, attention is paid to the boundary conditions at the plume boundary P. A novel global plume condition (GPC) model, which directly imposes the global current-free condition at P and determines the potential far downstream, is implemented. This is compared with the widely used local plume condition (LPC) model, which over-forces the current-free condition by imposing zero current density locally at P, thus artificially distorting plasma plume characteristics. Second, the influence of the cathode location on plume characteristics, electron current paths, and the cathode-to-plume coupling voltage is studied in detail. Configurations with centrally and laterally-mounted cathodes are compared. Central cathodes have better discharge performance due to the improved cathode-beam coupling. Laterally-mounted cathodes behave very differently depending on whether they are located inside or outside a magnetic separatrix (MS) surface, generally present in the plume. MS-external cathodes present much worse cathode-plume coupling. These trends with the cathode location are in line with previous experimental and numerical studies. Finally, the influence of the plume size has been assessed within all the above studies: the GPC, with a much better physical basis than the LPC, makes the plume solution less dependent on plume size; and the MS-external lateral cathodes are more affected by plume sizes and thus require working with larger plume domains.