In divertor plasmas, characterizing of the impurity flows is important for understanding the energy balance. In LHD plasma, a significant reduction of metallic impurity radiation was observed when the stainless steel divertor plate was replaced carbon. This suggests that the backstreaming into the plasma core affects the metallic impurity concentration in the LHD divertor configuration. The question is whether impurity backstreaming actually occurs in the divertor plasma. In this study we examine the backstreaming process of impurity ions.
Simulation experiments for impurity transport are conducted in TPD-II (Test Plasma by Direct current discharge) device at NIFS. TPD-II is a steady-state high-density plasma (n_e≈10¹⁹m^-3 and T_e≈10eV) source. The plasma is produced with discharge current of more than 100A, and blows out from the anode hole with flow velocity of about 10³~10⁴m/s. The plasma is sustained with magnetic field of 0.2T and terminates on a carbon target.
In TPD-II plasma, the carbon impurity distributions are steady state. The observed spatial distributions of carbon emissions are attributed not only to the successive ionization of carbon flow from the carbon target source but also to recombination processes of carbon ions which flow into the target. In front of the carbon target, a strong spectral line of CI (247.9nm) and CII (283.7nm) were observed, and these spectral intensity decreases immediately toward the upstream side within the scope of about 0.8m and 1.2m from target. The spectral line of CIII (229.7nm) was observed within the scope of about 2m taking with the peak at about 1m from target.
We are analyzing the observed spatial distributions with a 1-dimensional transport model. In the model, the parallel impurity fluxes consist of parallel diffusion and convection. Also this model includes ionization and recombination process of carbon impurities. The collisions between impurity neutrals and plasma ions are considered to have hard sphere cross section, the collisions between impurity ions and plasma ions have coulomb scattering cross section. The radial diffusion is considered as the classical diffusion. We tried to fit the calculated results with the experimental ones. We assumed that plasma ion, electron temperature and density are constant in space and that T_i=T_z=2eV, T_e=8eV and n_e=n_i=8×10¹⁸m^-3. In this case, the parallel diffusion coefficients of CI, CII and CIII are 147m²/s, 24m²/s and 6m²/s. If the convective velocity is small value, the carbon impurities are clustered at the front of the target. When the convective velocity is the same order of the sound speed of 10⁴m/s, the calculated results coincide with the experimental results. The origin of the convective velocity is not clarified for the present.