Internal propagation dynamics of breathing oscillations in Hall thrusters

The axial propagation characteristics of the breathing oscillation in Hall thrusters have recently become a focal point of research. Existing studies reveal that the wave propagates outside the discharge channel at the velocity of accelerated xenon ions. However, the spatial transit time corresponding to this high velocity is only on the order of a few microseconds, presenting a significant timescale discrepancy with the macroscopic breathing oscillation period of tens of microseconds. This discrepancy indicates that external propagation characteristics fail to capture the critical wave evolution process inside the discharge channel. Therefore, an experimental study was conducted to investigate the internal propagation dynamics of the breathing oscillation. By utilizing the discharge current phase as a first-order proxy for the local plasma fluctuation phase at the ionization core, the propagation time inside the channel was reconstructed, which accounts for the macroscopic oscillation period. The analysis demonstrates that inside the discharge channel, the wave actually propagates axially at the neutral gas transport velocity. Furthermore, the evolution of the internal propagation time under various operating conditions was investigated, revealing that this variation is jointly governed by the axial shift of the ionization zone and the change in neutral gas velocity. Finally, additional vacuum-condition tests suggested that the observed trends were not dominated by residual gases, thruster outgassing, or Xe accumulation within the investigated operating range.