Influence of an ergodic magnetic limiter on the impurity content in a tokamak

Abstract

This work deals with the properties of an ergodic magnetic limiter and presents calculations concerning the reduction of the impurity rate in a tokamak by a boundary sheath with decreased confinement time. The layer is produced by resonant helical windings superposed on an equilibrium magnetic field with closed magnetic surfaces [1]. The transport coefficients in the boundary layer, which yield the temperature and density distribution, are obtained from the movement of particles along a stochastic magnetic field [2,3]. The resulting line density can be made a factor of ten higher than is expected for a poloidal divertor experiment. From this we conclude that all impurities coming from the wall will be ionized in the boundary layer. The concentration of the impurities in the plasma center is calculated according to a model which uses an anomalous diffusion coefficient [4] being consistent with the ergodization in the boundary layer. The resulting concentration can be reduced proportional to the factor ( n e D e ) −1 where n e and D e are electron density and diffusion coefficient in the boundary layer.