Particle transport and rotation damping due to stochastic magnetic field lines

Abstract

It is suggested that sound wave propagation along stochastic magnetic field lines contributes to particle transport, described by a quasilinear diffusion coefficient D=Dmcs, where Dm is the magnetic field line diffusion coefficient [Nucl. Fusion 6, 297 (1966)]. In this calculation, the perturbed magnetic field δB is taken to be specified, either due to global modes or due to external perturbations, such as toroidal ripple, divertor coils, or an ergodic magnetic limiter. The transport, which proceeds by the propagation of electrostatic sound waves along the stochastic field lines, is not intrinsically ambipolar. The importance of this process is that it represents the shorting out of a radial electric field in the presence of δBr, and therefore provides a mechanism for damping of plasma rotation, and may therefore be an important factor for L–H transition in tokamaks. This part of particle transport and momentum transport are therefore related and are of the same order of magnitude. However, experiments (without an ergodic magnetic limiter) appear to indicate that another source of particle transport, perhaps by E×B advection due to electrostatic modes, can dominate the particle transport due to sound wave propagation along stochastic field lines.