Kinetics of impurity charge-state distributions in tokamak plasmas

Abstract

An analysis of impurity behavior in tokamak plasmas with the use of the observation results on impurity emission shows that it is necessary to distinguish between the ion dynamics (for example, ion transport) and ion kinetics, i.e., the processes related to the motion of ions on the charge states and/or excited states due to atomic processes in plasma. This paper presents a systematic analysis of the kinetics of impurity chargestate distributions and the related effects, as well as their typical scales and conditions for their observation. The quantitative analysis is performed in terms of the lowest moments of charge-state distributions such as the average charge m and dispersion D. Analytic approaches to solving charge-state kinetic equations are considered. An approach based on the symmetry properties of the kinetic matrix is proposed for the first time. The simplest types of impurity charge-state kinetics and the most important limiting cases are considered. A detailed analysis of the nonstationary behavior of the function of the moments D(m) of the charge-state distribution is presented. A quantitative analysis of the available experimental and model charge-state distributions of C, O, Ne, and Ar impurities in the JET, DIII-D, TORE SUPRA, ALCATOR-C, TEXTOR, PLT, TFR, and DAMAVAND tokamaks is performed in terms of the moments D(m). It is shown that the moments D(m)of the model charge-state distributions of the above impurities in the plasma core are essentially insensitive to the empirical diffusion coefficient. The equivalent curves D(m) obtained for the plasma periphery can be attributed to the convective fluxes of ionizing and/or recombining impurity ions.