Improved Hall Thruster Plume Simulation by Including Magnetic Field Effects

Abstract

Hall-effect thrusters (HETs) are affordable and efficient electric propulsion devices for space exploration, with higher specific impulse than conventional chemical propulsion and higher thrust at a given power compared to ion thrusters. A detailed understanding and an accurate characterization of the physical processes occurring in HET plume are critical from both the thruster performance and spacecraft integration perspectives. Therefore, a new electron model that includes full 2-D axisymmetric magnetic field effects is developed and incorporated within the framework of a 2-D axisymmetric hybrid particle-fluid code. The governing equation of this new electron model consists of an electron mobility coefficient tensor. The new electron model can simulate any shape magnetic fields.

The accuracy of the model is first assessed using the method of manufactured solutions and a Hall thruster test case to confirm 2nd order accuracy. Then, the simulation results of a 6-kW laboratory Hall thruster are directly compared with experimental measurements to validate the model. By including the magnetic field, modeling of the anomalous electron mobility is required. Since the anomalous electron mobility is still not yet well-understood, it is modeled using the Bohm coefficient. A parametric study of the Bohm coefficient is performed to examine its effect on plasma properties. Due to the concave shape of magnetic field lines, the plasma potential in the plume does not show a linear trend with the anomalous collision frequency. Comparisons with experimental data show that the new model with the magnetic field captures the detailed physics than without the magnetic field. In particular, the plasma potential profile agrees well with data by accurately capturing the strong negative gradient near the discharge channel exit of the thruster.

In order to extend the capability of the plume simulation, a sputter model is also implemented. The sputter model is applied to simulate the sputtering process of xenon propellants bombarding the surface of the "keeper" for the cathode, which can be an important failure mechanism in Hall thrusters. The steady-state mean erosion rate suggests that keeper erosion is as low as the erosion rate of the discharge channel walls in magnetically-shielded Hall thrusters.