Discharge oscillation mode transition of a Hall thruster

The discharge plasma of Hall thrusters exhibits either a stable or oscillatory mode depending on operation conditions such as mass flow rate, magnetic field, discharge voltage, and wall materials. A one-dimensional hybrid-direct kinetic solver is used to model the axial transport of the Hall thruster discharge plasma.[1] The predicted results including mean discharge current, discharge current oscillation, and breathing mode frequency show good agreement with experimental data.[2] As the magnetic field strength decreases, the azimuthal and axial electron drift velocities increase. The increase in axial electron drift results in larger Joule heating that triggers an ionization instability and causes the breathing mode oscillation. The electron thermal energy decreases due to the increase in electron kinetic energy, and thus the effect of plasma-wall interaction that stabilizes the ionization instability becomes smaller at low magnetic fields. It is suggested that the occurrence of a space charge limited sheath is not the direct mechanism of stable discharge mode but is the mechanism that generates the stable mode in a wide range of magnetic fields. The numerical results support the experimental observation that axial discharge oscillations are dominant over azimuthal rotating structure in the oscillatory breathing mode. The present investigation suggests that the electron current must be optimal to achieve a stable discharge mode.