Anomalous impurity transport: charge-state diffusion due to atomic processes in tokamak plasmas

Abstract

A new interpretation of anomalous impurity transport in tokamak plasmas is presented. An alternative (compared with conventional) approach reveals the relative importance of the transport of impurity particles and the diffusion of impurity charge states due to ionization and recombination processes. The set of coupled transport equations is reduced to a novel kinetic equation which is analysed in terms of charge-state transport including both the transport of particles and charge-state diffusion due to atomic processes. From the analysis of this equation and from available spectroscopic data the relationship between diffusion coefficients of these types of transport is found. It is concluded that charge-state diffusion exceeds the radial diffusion of particles significantly. Hence, the conventional empirical coefficients of anomalous diffusion DA and convective velocity VA mainly model charge-state diffusion rather than radial transport of impurity particles. A simple model is developed to describe the diffusion of impurity charge states with ionization rates enhanced by non-thermal effects of electron energy distribution and with recombination rates dominated by charge exchange of impurities on hydrogen atoms. The model is tested by comparing the simulated and measured radial profiles of line emission from oxygen, scandium and iron impurities taken from the TFR and PLT tokamaks. The radial profiles of neutrals and of superthermal electrons are obtained by modelling of spectroscopic data instead of the conventional empirical coefficients DA and VA.