Theory for the anomalous electron transport in Hall effect thrusters. I. Insights from particle-in-cell simulations

Using a 1D particle-in-cell simulation with perpendicular electric, E₀, and magnetic, B₀, fields, and modelling the azimuthal direction (i.e., the E₀ × B₀ direction), we study the cross-field electron transport in Hall effect thrusters (HETs). For low plasma densities, the electron transport is found to be well described by classical electron-neutral collision theory, but at sufficiently high densities (representative of typical HETs), a strong instability is observed to significantly enhance the electron mobility, even in the absence of electron-neutral collisions. This instability is associated with correlated high-frequency (of the order of MHz) and short-wavelength (of the order of mm) fluctuations in both the electric field and the plasma density, which are shown to be the cause of the anomalous transport. Saturation of the instability is observed to occur due to a combination of ion-wave trapping in the E₀ × B₀ direction, and convection in the E₀ direction.