Collisionless drift waves in the H mode edge

Existing theories predict that in ohmic and/or L mode discharges the drift wave is stable in a sheared slab geometry. The representative experimental values for the plasma parameters show, however, that because of the very steep density gradient the shear stabilization criteria for collisionless drift waves are not likely to be satisfied in the H mode edge plasma. Magnetic shear is, therefore, not a candidate for stabilizing the slab branch of drift waves in the H mode. However, a radially sheared transverse (to the equilibrium magnetic field) velocity field is found to play a key role in stability. Velocity profiles corresponding to those observed in a region of a few centimeters inside the separatrix of an H mode plasma stabilize drift waves. On the other hand, velocity profiles corresponding to the L mode render drift waves unstable in the experimental range of parameters, even though the magnetic shear continues to play its stabilizing role. (author). 26 refs