The Influence of Divertor Plasma Parameter on Tungsten Screening in High Recycling Regime for CFETR

Since tungsten is considered as the optional divertor target material for the future fusion devices, e.g., China Fusion Engineering Test Reactor (CFETR), it is crucial to have a good screening of the tungsten impurity to keep a low core tungsten concentration. In this paper, based on the two-point model, possible divertor plasma parameters are predicted in high recycling regime for CFETR. Further analysis of tungsten ionization lengths shows that when target temperature is low enough (less than 15 eV), the increase of upstream density may cause an increase of ionization length, and thus a degradation of divertor tungsten screening effect may happen. The 2-D simulations of tungsten transport are then performed by using Osm-Eirene-Divimp edGE [onion skin model (OSM)-EIRENE-DIVvertor IMPurities (DIVIMP)] code package. The target plasma parameters, in the region of those predicted by the two-point model, are set as input in OSM-EIRNEN to generate the background plasma. DIVIMP code is used to further simulate the tungsten impurity transport process. The results reveal that although the decrease of target plasma temperature may reduce the tungsten impurity prompt redeposition rate, the benefit of a lower erosion rate can make compensation. Therefore, to control the core tungsten concentration, lower target plasma temperature is preferred in high recycling regime.