Universal instability for wavelengths below the ion Larmor scale

Abstract

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 $L_x\gg {\rho }_i$, where $L_x$ is the scale length of the mode in the radial direction, and ${\rho }_i$ is the ion Larmor radius. Here we instead use a gyrokinetic approach which does not have this same limitation. Instability is found for perpendicular wavenumbers $k_y$ in the range $0.7\lesssim k_y{\rho }_i\lesssim 100$, and for sufficiently weak magnetic shear: $L_s/L_n\ge 17$, 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.

References in corpus (1)

• Linearized model Fokker-Planck collision operators for gyrokinetic simulations. I. Theory