2-D Fluid-PIC Simulations of Hall Thrusters with Self-consistent Resolution of the Space-charge Regions

Many hybrid simulations of Hall thrusters, where electrons and ions are solved using hydrodynamics and particle-in-cell methods, respectively, assume that the ionized gas is quasi-neutral everywhere in the computational domain and apply so-called thin-sheath approximations to account for space-charge effects near solid boundaries. These approximations do not hold along boundaries near the exit of the thruster or in the near plume regions, where the plasma conditions can lead to Debye lengths in the order of or higher than the local grid resolution. We present a numerical scheme that fully resolves the conditions of the ionized gas in space-charge regions of any thickness and that is coupled consistently to a global hybrid simulation of Hall thrusters. We verify the numerical results with the closed-form solution for a Langmuir sheath in a simplified one-dimensional example and then, again, in simulations where the model is integrated in a 2-D multifluid/PIC axial-radial code called Hall2De. The new capability to resolve numerically large sheaths around solid boundaries in Hall thrusters allows for significantly more accurate assessments of ion sputtering thus improving thruster lifetime predictions.