Turbulence and structures related to lower-hybrid and ion-sound instabilities in Hall thrusters

Plasmas in Hall thrusters are typically in regimes where the electron motion across the magnetic is strongly constrained by the Lorentz force and the ion Larmor radius is large so the ions are only weakly affected by the magnetic field. Such plasmas support lower-hybrid and ion-sound modes. In the presence of the ExB electron drift, these modes are easily destabilized by plasma and magnetic field gradients, electron collisions, ion flows, resonance and finite length effects. Interaction of these various effects create a complex picture of turbulence in such plasmas. We analyze these destabilization mechanisms in application to typical conditions of Hall thrusters and discuss their roles in anomalous transport and structure formation. The fluid model of plasma dynamics in these conditions has been developed and numerically implemented in the BOUT++ framework. Wave-particle resonant interaction effects have also been included via the kinetic closures implemented via non-Fourier representation. Results of numerical simulations with this model will be presented in this talk. It is shown that the lower-hybrid instability growth rate is maximized at the wavelength on the order of the electron Larmor radius. In the large wavelength region, formation of coherent large scale structures is observed in the simulations. The relative role of the inherent large scale instability (Simon-Hoh) and the inverse energy cascade (from small to large scales) is being investigated.