Enhanced shear damping and ion transit resonance damping of electron drift waves in toroidal plasmas

Abstract

Shear stabilization of low mode number, toroidally uncoupled electron drift waves is shown to be larger in axisymmetric tori than in cylinders, in sharp contrast to theoretical predictions obtained at higher mode numbers. When the wave frequency is larger than the thermal ion transit frequency, the enhancement factor, which is related to the toroidicity induced ∇B and curvature drifts of passing ions across the flux surfaces, is naturally reminiscent of the results of neoclassical diffusion theory in the Pfirsch–Schlüter regime. Even larger ion transit resonance damping arises in the opposite limit. The ratio of the sum of the revisited shear and Landau damping rates (resulting from interaction with passing ions and passing electrons, respectively) to the trapped electron excitation rate of these low frequency modes turns out to be approximately constant throughout the confinement zone of tokamaks with parabolic density profiles. An attempt is made to justify this remarkable result, as well as the transition from global gyro-Bohm to global Bohm scaling at very low collisionality.