Extended Investigations of the Lower Hybrid Drift Instability in the Near-pole Plume of a Magnetically Shielded Hall Thruster

Previous analytical and numerical investigations conducted at JPL have predicted that the Modified Two-Stream (MTSI) and Lower Hybrid Drift (LHDI) instabilities can become active in the plume region downstream of the front magnet pole pieces of a magnetically shielded Hall thruster. The findings were based on the solution of the electrostatic dispersion equation for waves in a long-wavelength (k_y r_e≲1), low-frequency (ω_ci<ω<ω_ce) branch, and used the plasma conditions from 2-D axisymmetric multi-fluid/PIC simulations with the Hall2De code. In that previous work the solution for the LHDI was obtained in only a limited region of the plume. In this article we expanded that solution domain. The region of positive growth is now found to extend at least a pole-cover distance downstream of the front pole, and increases away from the magnet pole pieces towards the edges of the main ion beam. Here the damping effect of classical collisions is found to be relatively small (<15%). As expected the wavenumbers that maximize LHDI growth yield k_y r_e~1 with the (thermal) electron Larmor radius ranging 0.1 ≲r_e≲0.5 mm in the regions studied. Finally, the effects of the magnetic field and electron temperature gradients have been compared to those associated with the density gradient. The former can become important as the ion beam is approached but are not as significant near the cathode plume