Theory of drift waves in the presence of parallel and perpendicular flow curvature. I. Slab model

Abstract

It has recently [S. Sen and M. G. Rusbridge, Phys. Plasmas 2, 2705 (1995)] been shown that, contrary to the usual belief, parallel flow curvature (V∥″) can stabilize drift-like microinstabilities. Here the earlier work is extended to include the effect of the perpendicular flow curvature (V⊥″), which is known to have a stabilizing role on the drift-like microinstabilities. The full analytic stability analysis shows that the ratio of the stabilizing influences of the perpendicular to the parallel flow curvature scales as Ls/2Ln, where Ls and Ln are the magnetic shear and the density variation scale length, respectively. Thus, at the plasma edge (since Ls≫Ln) the perpendicular flow may play a crucial role in stabilizing microinstabilities and turbulence in the improved regimes of confinement [like the high (H) modes]. However, in the core confinement improvement [like the very high (VH) mode] both the parallel and the perpendicular flow curvatures are important, since Ls∼2Ln. Furthermore, as the confinement improvement in the core is usually related to the toroidal velocity and since V∥ coming from the toroidal flow is much more than V⊥ (V⊥∼εV∥, here ε is the inverse aspect ratio), this implies that it is the parallel component of the toroidal flow and not the perpendicular component, as is usually thought, which is responsible for the core confinement improvement.