Theory of drift waves in the presence of parallel and perpendicular flow curvature. II. Toroidal model

The combined effect of the perpendicular and the parallel flow curvatures on the toroidal branch of the collisionless drift waves is investigated. The full analytical stability analysis shows that the ratio of the stabilising influences of the perpendicular to the parallel flow curvature scales as α2Lv2/Ln2, where α is a numerical factor whose value is usually around 3 and Lv, Ln are respectively the velocity shear and the density variation scale length. Thus at the plasma edge (since Lv∼Ln∼ρiθ, where ρiθ is the ion poloidal gyroradius) the perpendicular flow is expected to play the major role in stabilising microinstabilities and turbulence in the improved regimes of confinement (like the high confinement-modes). However, at the plasma core (since Ln∼a>Lv∼ρiθ, where a is the minor radius of the cross section) it is the parallel component of the flow which plays the dominant role. Furthermore, since the confinement improvement in the core is usually related to the toroidal velocity and Vparallel coming from the toroidal flow is much more that Vperpendicular coming from the toroidal flow, our result clearly indicates that it is the parallel component of the toroidal flow and not the perpendicular component, as is usually thought, that is responsible for the core confinement improvement. This conclusion is consistent with our earlier result calculated in a sheared slab configuration [S. Sen and A. Sen, Phys. Plasmas 3, 2224 (1996)]. copyright 1997 American Institute of Physics