Impurity transport and divertor retention in Ar and N seeded SOLPS 5.0 simulations for ASDEX Upgrade

Impurity seeding will be an important tool to reduce the peak power loads and temperatures at the divertor targets in future tokamak devices. To improve the physics understanding and provide predictive capabilities for the impact of impurities on the plasma, different impurity species have to be investigated and compared to each other. For this purpose SOLPS 5.0 simulations of argon (Ar) and nitrogen (N) seeded H-mode plasmas for ASDEX Upgrade have been performed. The (purely numerical) investigations extend previous studies dedicated to impurity transport and to the divertor impurity retention. An analysis of mixed Ar and N impurity seeding reveals that a trade-off between pedestal top temperature drop and fuel dilution can be achieved by an adjustment of the impurity mixture. Due to the impact of the impurities on the temperatures, the impurity seeding reduces the main ion ionization rates in the divertor regions, and therefore, the ion particle sources. Accordingly, this modification of the particle sources results in a main ion background flow inversion at higher seeding levels, which also strongly affects the impurity flow patterns. This mechanism explains a modification of the impurity density distribution at higher seeding levels, where Ar impurities are observed to be redistributed from the outer to the inner divertor. A less pronounced effect is observed for N, which can be explained by the radiation efficiency. The divertor impurity retention is determined by the relative positions of the ionization front of the neutral impurities and the impurity stagnation point. The impact of impurity seeding on the stagnation point position is studied in detail for the first time. Under the investigated conditions, decreasing the target temperature (i.e. increasing impurity seeding) always results in a reduction of the divertor impurity retention. This is a critical result making power exhaust even more challenging.