ELSEVIER Journal of Nuclear Materials 220-222 (t995) 410-414 Impurity transport modelling and simulation analysis of impurity behavior in JT-60U K. Shimizu, H. Kubo, T. Takizuka, M. Azumi, M. Shimada, S. Tsuji, N. Hosogane, T. Sugie, A. Sakasai, N. Asakura, S. Higashijima Japan Atomic Energy Research Institute, Naka Fusion Research Establishment, Naka-Machi, Naka-Gun, Ibaraki-Ken, 311-01, Japan Abstract A two-dimensional impurity code based on Monte Carlo techniques (IMPMC) has been developed, in order to study the impurity behavior in the divertor plasma. The model includes (1) impurity generation at the divertor plate, (2) ionization of sputtered neutrals, (3) parallel motion of impurity ions along field lines, (4) Coulomb scattering, (5) cross-field diffusion, and (6) atomic processes. This model is applied to the carbon impurity behavior in a JT-60U NB heated plasma. The impurity generation mechanism, the effect of the thermal force on impurity shielding, and the contribution of charge exchange recombination to C 3+ ion density near the plates are clarified. The measured spatial profile of the CIV line indicates that the diffusion coefficient, D ± is around 1 m2/s. The ion temperature measured by Doppler broadening of the CIV line is found to be shifted to lower temperature side due to the overspread distribution of C 3+ ions in the SOL by the thermal force. 1. Introduction Analysis of impurity transport in the divertor region involves a difficult requirement that the motion of impurity ions contains various characteristic times, i.e. diffusion time, parallel transit time along the magnetic field line, ionization time, recombination time and collision time between the impurity ions and the back- ground particles. Since such characteristic times strongly depend on the plasma parameters, the fluid description [1,2] cannot always be applied accurately to the impurity transport in the SOL. The Monte Carlo approach [3,4] is beneficial for incorporating the paral- lel motion and various collisional effects, and for mod- elling impurity generation and the interactions be- tween impurities and walls. A two-dimensional impurity code based on Monte Carlo techniques (IMPMC) has been developed. A feature of the IMPMC code is to simulate the diffusion process in velocity space for Coulomb scattering, in- stead of the classical parallel diffusion model which was used in the DIVIMP code [4]. (The parallel diffu- sion model is restricted to the high collisionality plasma [5].) And the impurity transport can be calculated 0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights SSDI 0022-3115(94)00499-4 under an MHD equilibrium configuration with real divertor plates and wall. This treatment facilitates a detailed comparison of simulation results with experi- mentally measured quantities. The model is described in the next section. The simulation results for the carbon impurity behavior in the NB heated divertor plasma of JT-60U are presented in Section 3. Conclu- sions are given in Section 4. 2. Impurity transport modelling We solve the kinetic equations with the Coulomb collision operator for the distribution functions fz(r, v, t) using the orbit following Monte Carlo tech- nique. (1) Impurity generation: impurity neutrals are origi- nally sputtered from a divertor plate by deuterium physical sputtering. The sputtered outflux is deter- mined by FD~C=nDdCsYD, where Yo ----JD'rOVD ..... 1 (E D) is the sputtering yield, fo is an enhancement factor of the normal incidence sputtering yield Y~, .... 1, in order to take account of angular dependence, E D is the incident energy, and C s the sound velocity of reserved