Growth of the Modified Two-stream Instability in the Plume of a Magnetically Shielded Hall Thruster

The electrostatic dispersion relation of the Modified Two-Stream Instability has been solved numerically in the near-plume of a magnetically shielded Hall thruster. The plasma conditions have been obtained by 2-D (r-z) multifluid-PIC simulations. The maximum growth rate γ_max is found to be > 0 in most regions of the domain, with the fastest growth approaching the fluid-limit value of ω_LH/2, at angles ≳ π/6 relative to the z-axis and near the magnetic poles where ω_pe/ω_ce ~1. In this region the relative drift between beam ions perpendicular to B and magnetized electrons is u/u_Ti≳3, while that between counter-streaming beam and cathode ions is ≳4. The latter suggests that the ion-ion cross-field lower hybrid instability may also be active here. The presence of these instabilities can lead to anomalous heating of ions striking nearby surfaces.