Neoclassical polarization

Abstract

Sheared zonal flow is known to be the predominant saturation mechanism of plasma turbulence. Rosenbluth and Hinton(R-H) have shown that the zonal flow level is inversely proportional to the plasma radial polarizability due to magnetic drift departure from a flux surface. In another calculation, Hinton and Rosenbluth (H-R) considered the weakly collisional case in the banana regime and calculated the neoclassical polarization and associated zonal flow damping in the high frequency and low frequency limits. The work presented here extends R-H's calculation in several aspects. We calculate the neoclassical polarization for arbitrary radial wavelength zonal flows so that finite ion banana width and ion gyroradius are retained. We also add plasma shape effects into the R-H collisionless calculation and find the influence of elongation and triangularity on neoclassical polarization and zonal flow damping. In addition, we extend the H-R collisional calculation using an exact eigenfunction expansion of the collision operator to calculate neoclassical polarization for the entire range of frequencies. A semi-analytical fit of the exact results is obtained that gives the polarization to within 15% and allows the collisional zonal flow damping rate to be evaluated for arbitrary collisionality.