Experimental characterization of Hall thruster breathing mode dynamics

Abstract

The dynamic properties of the Hall thruster breathing mode are investigated experimentally. Non-invasive time-resolved laser diagnostics are combined with a quasi-one-dimensional non-Maxwellian modeling technique to yield the high-speed evolution of a variety of plasma properties over the course of a breathing oscillation. The fluctuations of both ion and neutral densities are quantified in terms of amplitude and dispersion and are interpreted in the context of a simplified dispersion relation. It is found that the oscillations associated with the breathing mode are likely ion and neutral waves propagating with phase velocities commensurate with the local species drift speed. Further, the amplitudes of the waves are shown to decay with distance from the anode. This effect can be explained largely by the influence of the expansion of the background plasma and neutral gas. This monotonic decay combined with the downstream propagating nature of the waves suggests that these oscillations are influenced by conditions upstream of the acceleration and ionization regions. By comparing the presence of these waves to inferred electron temperature fluctuations, the hypothesis that the breathing mode is governed by a process in which the modulation of the neutral density near the anode sheath or the anode itself is coupled to a downstream ionization instability is qualitatively explored. The possibility of these waves relating to cyclical recombination of ions at the anode is also discussed.