Low-frequency electron dynamics in the near field of a Hall effect thruster

Time-resolved electrostatic probe measurements were performed in the near field of a SPT100-ML Hall effect thruster in order to investigate electron properties changes on a microsecond time scale. Such measurements allow one to monitor the electron temperature T-e, the electron density n(e), as well as the plasma potential V-p during a time period that corresponds to one cycle of a breathing-type plasma oscillation with f approximate to 15-30 kHz. Although T-e(t) stays constant in time, n(e)(t) and V-p(t) oscillate with the discharge current waveform frequency. The observed time delay between n(e) and anode discharge current (Id(a)) waveforms, which is of approximately 7 mu s, is linked to the ion transit time from the ionization layer to the probed near-field region. The same time gap is measured between V-p(t) and Id(a)(t), however V-p(t) and n(e)(t) are in phase opposition. The electron density reaches its highest value at the very moment ions are ejected out of the thruster discharge chamber, which also corresponds to the instant the cathode potential is the most negative. Such a behavior images the need for ion beam neutralization. Further, it is shown that there is a strong correlation between the electron dynamics and the presence of high frequency (HF) plasma oscillations in the megahertz range: HF fluctuations are the strongest when n(e) is the highest.