Interpretation of the impurity distribution in the divertor during divertor plate biasing using the DIVIMP code

Simulations of carbon transport using the DIVIMP code [P.C. Stangeby, J.D. Elder, J. Nucl. Mater. 196–198 (1992) 258] are compared with 2D toroidal images of CII and CIII radiation near the external divertor plates in TdeV ohmic plasmas (Ip=170 kA, ne=3×1019 m−3, BT=1.4T). The main plasma parameters in the SOL and divertor are calculated by the onion skin model (OSM) [K. Shimizu et al., J. Nucl. Mater. 196–198 (1992) 476] included in DIVIMP, the neutrals being calculated by EIRENE [D. Reiter, Internal Report, KFA, Julich, 1947 (1984), 2599 (1992)] in an iterative loop. The results show that the carbon is mainly created by chemical sputtering, with a considerable fraction coming from the external oblique plate. By interpreting experimental CII and CIII distributions, it is found that carbon is affected by the biasing (−125 to +125 V) through a combination of at least three processes: the ion flux to the plates, the E×B drift velocity, and the cross field diffusion.