Universal Instability for Wavelengths below the Ion Larmor Scale

Matt Landreman, Thomas M. Antonsen, William Dorland
2015 Physical Review Letters  
We demonstrate that the universal mode driven by the density gradient in a plasma slab can be absolutely unstable even in the presence of reasonable magnetic shear. Previous studies from the 1970s that reached the opposite conclusion used an eigenmode equation limited to L_x ≫ρ_i, where L_x is the scale length of the mode in the radial direction, and ρ_i is the ion Larmor radius. Here we instead use a gyrokinetic approach which does not have this same limitation. Instability is found for
more » ... icular wavenumbers k_y in the range 0.7 ≲ k_y ρ_i ≲ 100, and for sufficiently weak magnetic shear: L_s / L_n > 17, where L_s and L_n are the scale lengths of magnetic shear and density. Thus, the plasma drift wave in a sheared magnetic field may be unstable even with no temperature gradients, no trapped particles, and no magnetic curvature.
doi:10.1103/physrevlett.114.095003 pmid:25793821 fatcat:wzfvci4lybgcjnzthypzrkjxra