The interaction between ion transit-time and electron drift instabilities and their effect on anomalous electron transport in Hall thrusters

Abstract

Recent simulations and experiments have observed a transition from short to long-wavelength azimuthal instabilities that leads to enhanced electron transport in Hall thrusters. Here we make the hypothesis that this phenomenon stems directly from the interaction between the axial ion transit-time instabilities (ITTI), and the azimuthal electron drift instability (EDI). This interaction is studied using 2D axial-azimuthal self-consistent particle-in-cell simulations which include a 1D neutral dynamics solver. It is found that a short to long-wavelength transition only occurs if the breathing-mode and ITTI are captured in the simulation, and two distinct instability regions can be distinguished depending on the local ion Mach number. Upstream of the ion sonic point the EDI exhibits an ion-acoustic behaviour, and the associated instability-enhanced electron transport is well described by a previously developed model based on kinetic theory. Downstream of the ion sonic point however, the ITTI significantly changes the local plasma parameters, and this modifies the EDI while increasing the electron transport.