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

Abstract

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 Te, the electron density ne, as well as the plasma potential Vp during a time period that corresponds to one cycle of a breathing-type plasma oscillation with f≈15–30kHz. Although Te(t) stays constant in time, ne(t) and Vp(t) oscillate with the discharge current waveform frequency. The observed time delay between ne and anode discharge current (Ida) waveforms, which is of approximately 7μs, is linked to the ion transit time from the ionization layer to the probed near-field region. The same time gap is measured between Vp(t) and Ida(t), however Vp(t) and ne(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 ne is the highest.

Modulating action of low frequency oscillations on high frequency instabilities in Hall thrusters

“…The electric field in electron drift region is large when the low plasma density is low in the ionization region, and decreases when the plasma density increases. This is also seen in the experiments results of Albarède et al 26 and the numerical simulations of Boeuf and Garrigues. 8 Our numerical simulation results in Figure 4 also show that the plasma density in ionization region lead to the discharge current with a time of several microseconds, and the electric field in electron drift region is large when the low plasma density is low in the ionization region.…”

Dispersion relations of electron density fluctuations in a Hall thruster plasma, observed by collective light scattering

“…Most measurements of fluctuations in a Hall thruster plasma have been done using electrostatic probes and antennas, 7,8 or magnetic probes. 9 Since probes cannot withstand high-energy particles inside the plasma, these devices are placed around the channel external wall, outside the plasma; in addition, these probes cannot resolve small-scale spatial structures.…”

Low-frequency model of breathing oscillations in Hall discharges

“…The breathing mode is known to involve time scales that are much longer than the characteristic relaxation and flight times of charged species [14], but comparable to the flight time of neutrals. This observation readily leads to the conjecture that for low-frequency phenomena, charged species effectively behave as if they were at any moment in a quasisteady equilibrium with a slowly evolving background of neutrals.…”