Theory for the anomalous electron transport in Hall effect thrusters. II. Kinetic model

TLDR

Hall effect thrusters exhibit anomalously high cross‑field electron transport that is correlated with azimuthal electrostatic instabilities causing enhanced electron scattering. This study introduces a kinetic theory that predicts the enhanced scattering rate and yields an electron cross‑field mobility consistent with experimental observations. The theory attributes the increased mobility to a strong azimuthal electron drift that excites an instability which saturates through ion‑wave trapping and wave‑convection. The resulting mobility is many orders of magnitude above classical diffusion predictions and varies strongly with plasma density rather than following simple 1/B0 scaling laws.

Abstract

In Paper I [T. Lafleur et al., Phys. Plasmas 23, 053502 (2016)], we demonstrated (using particle-in-cell simulations) the definite correlation between an anomalously high cross-field electron transport in Hall effect thrusters (HETs), and the presence of azimuthal electrostatic instabilities leading to enhanced electron scattering. Here, we present a kinetic theory that predicts the enhanced scattering rate and provides an electron cross-field mobility that is in good agreement with experiment. The large azimuthal electron drift velocity in HETs drives a strong instability that quickly saturates due to a combination of ion-wave trapping and wave-convection, leading to an enhanced mobility many orders of magnitude larger than that expected from classical diffusion theory. In addition to the magnetic field strength, B0, this enhanced mobility is a strong function of the plasma properties (such as the plasma density) and therefore does not, in general, follow simple 1/B02 or 1/B0 scaling laws.