Despite active study for decades, the stability of Hall thrusters is still not completely understood from first principles. Specifically, these devices exhibit several instabilities that are not completely characterized theoretically. These stability issues are problematic in two ways: the presence of instabilities may influence performance in-flight, and the lack of understanding of these phenomena makes it difficult to reproduce them with numerical simulation.

Hall thruster stability is studied in several ways. First, it can be examined experimentally in terms of empirical trends in oscillation characteristics with operating parameters. For example, the evolution of oscillation frequency with discharge voltage can be mapped experimentally to understand the stability of a thruster as a function of voltage. Second, it can be examined experimentally in terms of high-level performance metrics. For example, the evolution of power-normalized thrust with oscillation amplitude can be determined to illuminate the effect of instabilities on performance. Finally, it can be examined theoretically and numerically to understand the mechanism governing an instability. For example, a theoretical model can be developed and compared to low-level experimental data to assess the stability criteria for a certain oscillatory mode.

At PEPL, we have approached Hall thruster stability in all three ways. The work of McDonald et al. performed initial characterization of the Hall thruster breathing mode and spoke mode using high-speed imaging. Aside from identifying important trends in frequency and amplitude of these modes, McDonald found evidence that azimuthal spokes might play a role in electron transport in Hall thrusters. Sekerak et al. evaluated the changes in thruster performance with low-frequency oscillation strength and found that the breathing mode typically corresponds to low thrust-to-power. Finally, Dale et al. explored the breathing mode theoretically and numerically, finding that existing theories for it are insufficient. Dale performed additional experimental characterization of this mode and found evidence for the involvement of an upstream ionization process.