Impact of axial instabilities on the time evolution of the ion energy distribution function in Hall thrusters*

Hall thrusters (HT) are low-pressure cross-field devices in which numerous instabilities can develop. These instabilities can take the form of regular or transient oscillations. It has been experimentally shown that instabilities can propagate in both axial and azimuthal directions and cover a large range of frequencies (from a few Hz up to several GHz). The results presented here focus mainly on axial instabilities known as breathing mode instabilities (BM) and ion transit time oscillations (ITTO) and their effect on the time evolution of the ion energy distribution function (IEDF). Experiments are carried out in a tunable magnetic field Hall thruster (PPS Flex) and time-resolved IEDFs are obtained from RPA measurements. As BM instabilities and ITTO are not regular enough in amplitude and frequency to obtain sufficient time resolution after averaging, a code based on quadratic distance has been developed to detect the most similar patterns on the trigger discharge current (DC). The time-resolved IEDF are inferred from the corresponding RPA ion current. We highlight a time evolution of the IEDF on two distinct time scales: firstly a modulation of the ion energy with the BM oscillation (reduced ion acceleration during the growth phase of the DC) and then an oscillation of ion energy associated with the ITTO showing an acceleration of ions on the ITTO time scale. These temporal evolutions observed on the IEDF suggest that the ionization and ion acceleration zones evolve spatially in an uncoordinated manner.