High and low efficiency subregimes of breathing mode oscillations in Hall thrusters

Breathing mode oscillations are one of the most prominent plasma oscillations observed in Hall thrusters and are known to appear at a variety of operational conditions. In this study, we employ a 1D fluid numerical model of a Hall discharge to perform extensive parametric investigations with regards to discharge voltage and magnetic field strength. The parametric investigations reveal two distinct regions of breathing mode occurrence in the parametric space: an efficient submode present in the region of moderate discharge voltage and relatively high magnetic field strength and a second, highly inefficient submode present in the region of high discharge voltage and low magnetic field. Besides efficiency, the submodes differ in terms of prevailing mechanisms driving the instability. The efficient submode is found to be driven largely by electron temperature effects, causing the ionization rate to significantly fluctuate during the breathing mode cycle. The inefficient submode is associated with a significant overlap of the ionization region and the region of negative ion velocity, causing the bulk part of the instability to happen in the ion backflow region. As a result, the inefficient mode's instability mechanism consists of cyclic ionization and anode plasma recombination events. It is also found that the inefficient submode is related to operation in a space charge-saturated wall sheath regime. Further analysis reveals that the efficient submode is associated with high current and propellant utilization and moderate voltage utilization. On the other hand, the low-efficiency submode is associated with very low current utilization (due to a large ion backflow current), which outweighs the high propellant and voltage utilization efficiencies by a large margin.