Comparing N versus Ne as divertor radiators in ASDEX-upgrade and ITER

• First attempt to analyze influence of machine scale size on the behavior of plasma with extrinsic seeded impurity (Ne and N) in the scrape-off layer (SOL) and divertor in SOLPS-ITER modeling: large (ITER) and small (ASDEX upgrade geometry) scale modeling cases are compared. • The distribution of main plasma, radiating impurity, and heat load between the inner and outer divertors is found to be much more symmetric on the larger scale machine at higher toroidal field due to the smaller influence of drifts and improved screening in the divertor. • The fraction of Ne radiation inside the separatrix is considerably higher for ASDEX upgrade than for ITER, making this impurity less attractive as a radiator for smaller size machines. • On the basis of this study, both N and Ne appear to be acceptable divertor radiators for ITER. The study aims to estimate the influence of machine scale size on the behavior of plasma with extrinsic seeded impurities in the scrape-off layer (SOL) and divertor. This is performed through the comparison of plasma boundary simulations using the SOLPS-ITER code including drifts and currents of nitrogen (N) and neon (Ne) injection in ITER and ASDEX Upgrade (AUG) geometries. Trends are examined between the two seeding species in each individual device and by a comparison of the differences between the two machines. In the modeling results, the radiated power peak is located near the X-point in the inner divertor for the AUG cases and in the vicinity of the strike points in both divertors in ITER. The simulations also find less Ne impurity ions in the divertor volume than N and more significant Ne radiation inside the separatrix for AUG, consistent with published experimental findings. In ITER, both species radiate mostly from the divertor, in agreement with the existing SOLPS-4.3 simulation database obtained without drifts and with a less sophisticated treatment of parallel impurity transport. Drifts are important players in determining the plasma background for AUG and are comparatively less important in ITER. In both devices, the spatial distribution of the impurity ion density is complex, with their parallel flow patterns correlating with the thermal and friction force balance. Within this isolated modeling study, the principal reasons for different behavior between N and Ne on AUG and ITER appear to be the combination of a stronger drift effect and reduced screening of recycled fuel and impurity from divertor to private flux region on AUG leading to a more extended, colder plasma than in ITER. The increased temperature in the confined region just inside the separatrix on ITER also means that impurity ions reaching this zone are fully ionized and do not contribute significantly to the radiation loss there. On the basis of this study, both N and Ne are found to be acceptable low Z radiators on ITER.