Impurity leakage and radiative cooling in the first nitrogen and neon seeding study in the slot divertor at DIII-D

Impurity seeding experiments were performed in the SAS divertor at DIII-D using N and Ne as radiative species showing a strong dependence of leakage behavior on impurity species [1]. The experimental results indicate that Ne radiates more upstream than N reducing compression in the divertor while increasing the pedestal pressure profile. The different dissipative behavior between the two radiative species is confirmed by SOLPS-ITER modelling which for the first time at DIII-D includes multiple impurity species and a treatment of full drifts, SOL currents and n-n collisions. The impurity transport in the SOL is studied in terms of the parallel momentum balance showing that N is mostly retained in the divertor whereas Ne leaks out of the divertor consistent with its higher ionization potential and longer mean free path producing the lower divertor enrichment found for Ne. The strong ionization source and particle drifts characterizing the SAS contribute significantly in the shift of the stagnation point, thus affecting impurity leakage. The results demonstrate the impact of flow reversal and drifts in closed divertor structure with important consequences on SOL impurity transport and pedestal performance. [1] L. Casali et al PoP 2020.