Reduction in transport by the parallel velocity shear instability due to reversed magnetic shear

A nonlocal theory of the electrostatic parallel velocity shear instability in a three-dimensional slab with a uniformly sheared magnetic field has bee n developed. It is shown that in the limit of a weak parallel velocity grad ient, the linear growth rate can be increased depending upon the direction of the magnetic shear ((s) over cap) with respect to the radial curvature o f the parallel velocity profile (d(2)v(parallel to)/dx(2)). When these para meters have the same sign, the growth rate can actually be stronger than in the limit of no magnetic shear. In this limit of increased instability, th e eigenmode is broadened, thus producing enhanced transport. This effect sh ould be observable when the scale length of the curvature is of order simil ar to rootL(s)rho (s). For strong parallel velocity gradients that are more typical of flows in tokamaks, the effect of the varying Doppler shift beco mes more prominent on the stability of the mode, the net result being that the sensitivity of the growth rates on the sign of the magnetic shear becom es insignificant. This effect, however, is effectively offset when a finite density gradient is included. When the density scale length is of order th e scale length of v(parallel to), the growth rate is moderately reduced, bu t becomes dependent again upon the sign of the magnetic shear. (C) 2001 Ame rican Institute of Physics.