Analysis of the tokamak ripple-blocked ion distribution with abrupt changes of a radial electric field

Abstract

Convective transport of non-thermal, ripple-blocked ions in the presence of a rapidly varying radial electric field (Er) is investigated by numerically solving a relevant kinetic equation in tokamak geometry. Near the plasma periphery, at small poloidal angles, a strong suppression in the deeply ripple-blocked ion distribution is observed in the absence of a radial electric field for particle energies exceeding a collisional threshold. The deficit of the ripple-blocked ions is found to be rapidly filled by the onset of an inward radial electric field of a sufficient magnitude. The time scale for this filling can be explained by the different blocked-ion drift orbit topologies generated by the radial electric field, and it is determined by the convective drift time of ripple-blocked ions from the inner, well-filled ripple region to the depleted region along these orbits. Consequently, the time scale can be much faster than the collisional time scale, and the blocked ion distribution function should faithfully follow the changes in the radial electric field. Thus the present mechanism gives a physical basis for the earlier interpretation of experimental results [W. Herrmann and ASDEX Upgrade Team, Phys. Rev. Lett. 75, 4401 (1995)] that the measured changes in the neutral fluxes from charge exchange processes with ripple-blocked ions are caused by changes in the radial electric field. The dependence of the neutral flux changes on collisionality as well as on the width and magnitude of a radial electric field with a Gaussian potential profile are studied.