Plasma Simulations In 2-D (R-Z) Geometry For The Assessment Of Pole Erosion In A Magnetically Shielded Hall Thruster

Magnetic shielding of Hall thrusters has been shown to reduce erosion of the channel walls by at least a few orders of magnitude, thereby enabling the use of these devices in deep space missions. Wear tests of magnetically shielded thrusters, such as the H6 and HERMeS, have revealed that some sputtering occurs at the surface of the inner pole, a phenomenon not observed in unshielded thrusters. Even though the sputtering rates measured at the inner pole during ground testing are typically an order of magnitude lower than those found in the acceleration channel of unshielded thrusters, it is critical to understand how the source(s) of this erosion may change with operating conditions during flight. Hall2De is a 2-D axisymmetric code that makes use of a hydrodynamics formulation for both electrons and ions and assumes a quasi-neutral plasma. Since its computational domain is large enough to account for the discharge channel, poles, cathode, and plume regions, Hall2De can be used to investigate the physical mechanisms that produce the erosion of the poles. The simulation results are compared with experimental laser-induced fluorescence measurements of the ion velocity along the discharge channel of the H6 thruster. We find that the erosion of the poles in the magnetically shielded H6 is a result of the acceleration region moving outside of the discharge channel, which in turn is a consequence of a shift in the location of the maximum magnetic field along the channel centerline that occurs when magnetic shielding is implemented. When the acceleration region moves downstream, the plasma potential contours at the edges of the beam allow for high energy ions to be accelerated radially toward the pole surfaces. Published under license by AIP Publishing.