Dual-Time-Scale Ion Acceleration Dynamics in Hall Thrusters

We report the first direct observation using a retarding potential analyzer of time-resolved ion energy distribution functions (IDFs) in a Hall thruster (HT) on two distinct time scales corresponding to the breathing mode (∼kHz) and the ion transit time oscillation (ITTO, ∼100 kHz). We combined the dynamic time warping technique with a calculation of the cumulative quadratic distance applied to measure weakly nonperiodic oscillations to achieve a previously inaccessible fine resolution of ion-acceleration dynamics driven by ITTOs and revealing the footprint of multiple ion bunches on the IDF in accordance with theory. The analysis demonstrates the significant role of high-frequency instabilities in shaping the IDF and explains fundamental mechanisms of axial wave-particle interactions and ion transport in HTs.