(TH/P2-24) Impurity Transport in ITER-like Plasmas

T.M. Fülöp1), J. Weiland
1) Radio and Space Science, Chalmers University of Technology, Göteborg, Sweden

Abstract.  The presence of impurities in the edge region of tokamaks can be beneficial because a strongly radiating boundary distributes plasma power loss. But many tokamak discharges suffer from unwanted impurity accumulation in the plasma core. Impurity transport is critical for ITER and therefore it should be given careful consideration. In this work we compute the impurity transport from neoclassical theory and compare it with transport calculated from the reactive drift wave model of turbulent transport for a specific ITER-like scenario. Neoclassical transport is driven by parallel friction dynamics, and is not affected significantly by the fact that the ion cross-field transport is dominated by fluctuations. Thus it is likely that both neoclassical and anomalous transport co-exist. The reactive model used here (usually called Weiland model) has been used extensively in describing the present database and also for making ITER predictions It uses an ``advanced" reactive fluid model where ``advanced" here refers to the rule for closure which allows us to use the model close to the fluid resonance in the collisionless case. In the version used here it has two independent ion species with the same physics included for both. The particle transport for the main species was tested successfully on JET discharges. The particle pinch depends particularly strongly on the magnetic drift frequency. Because of this, species with larger Z have a weaker particle pinch thus giving a favourable net effect on the effective Z. The particle pinch is particularly relevant for ITER since central fuelling will not be possible there. The particle pinch has been found to improve ITER performance significantly. We consider a specific high-Q ITER-like scenario and keep Zeff constant. Neoclassical and turbulent impurity transport have opposite signs. The turbulent transport leads to an inward pinch of the impurities, but the main ion inward pinch is much stronger. The total impurity transport, determined by a balance between turbulent and neoclassical transport, depends sensitively on the charge number of the impurity and on the ratio of the ion density and temperature scale lengths ηi.

Full paper available (PDF)