Gradient-drift instability in partially ionized, partially magnetized plasmas*

Partially ionized, partially magnetized plasmas exhibit low-frequency, large-scale (1-10 kHz; cm order) self-organization patterns and high-frequency, small-scale (1-10 MHz; mm order) oscillations. In particular, low-frequency rotating spokes have been observed in various cross-field discharge plamsas, including Penning discharge, magnetron discharge, and Hall effect thrusters. One of the most accepted hypotheses is that the modified Simon-Hoh instability, which occurs when E·▽n>0 (where E is the electric field and n is the the plasma density), drives such low-frequency spoke propagation. However, various recent measurements demonstrate that the spokes can propagate in both +E×B and -E×B directions (where B is the magnetic field). In this talk, we will present a gradient-drift instability theory that accounts for the electron inertia, which is neglected in the Simon-Hoh instability theory. The updated theory shows that the gradient-drift instability occurs in the direction of the diamagnetic drift  and predicts an additional mode that occurs when the diamagnetic drift is large compared to the electron thermal speed.