Simulating the role of intrinsic carbon impurities in the divertor detachment of ASDEX Upgrade

As part of a general effort in validating SOL fluid Monte Carlo codes, well diagnosed ohmic discharges have been done in ASDEX Upgrade reaching from the lowest accessible density up to densities at which both targets were at least partially detached. The numerical code B2.5-EIRENE has been used to simulate these experiments with a particular focus on detachment. Whilst it is possible to qualitatively reproduce the general experimental observations when transiting from attached to detached plasmas, the simulations do not reproduce the detachment threshold observed in experiment quantitatively. The strong asymmetry of the ion flux density between the inner and the outer target with increasing density and in particular the vanishing ion flux, defined as full detachment, at the inner target cannot be reproduced. The influence of various parameters such as chemical sputtering yields and the effect of the activation of drift terms on the results is analyzed. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.