Hall thrusters can support a wide range of instabilities, many of which remain poorly understood but are known to play a critical role in the fundamental operation of these devices. In this work, the dominant low-frequency oscillations known as the “breathing mode” is investigated to provide a more analytically rigorous yet intuitive description of the instability. The new understanding of Hall thruster oscillations yielded by this effort can improve the reliability of these devices.

Time-resolved laser-induced fluorescence paired with an ion kinetic analysis is used to characterize the near-field and internal thruster plasma during breathing oscillations. A frequency scaling study indicates that several existing theories for the breathing mode are consistent with observed oscillation trends. However, an examination of the dynamic properties of the discharge reveals that these same theories are fundamentally inconsistent with the experimental data.

A novel physical process for the breathing mode is proposed and found to agree with the experimental findings. A model corresponding to this process is developed and shown to predict positive linear growth and realistic real frequencies. A simpler model is derived and used to produce simple analytical descriptions of the real frequency and growth of the breathing mode.