Effect of Low-Z Impurity Injection Location and Recycling Coefficient on Tungsten Leakage from the DIII-D SAS-VW Divertor

Recent SOLPS-ITER and DIVIMP modeling work shows that the injection of low-Z impurities into the plasma boundary is a viable actuator to manipulate tungsten erosion and leakage from the new closed, V-shaped tungsten-coated divertor in DIII-D and ultimately the amount of tungsten ending up in the core. This optimization is driven in part by a balance between the friction force and the ion temperature gradient force acting on tungsten ions in the Scrape-off-Layer (SOL). Critically, the injection of low-Z impurities upstream in the SOL increases frictional forces which prevent tungsten from leaking out of the divertor. New results show that the SOL forces are sensitive to the poloidal location where low-Z impurities are injected, even when these impurities are assumed to be fully recycling. Furthermore, if the electron temperature at the target is initially high then the radiative cooling also results in a reduction of the tungsten sputtering source and in turn the total amount of tungsten reaching the core. The magnitude of the tungsten source is highly sensitive to the recycling properties of the injected low-Z impurities. With significant recycling at the divertor targets, low-Z impurities build up in the slot divertor and increase W sputtering, narrowing the window of operation for mitigation of tungsten leakage. An assessment of which low-Z impurity injection location and species (N vs. Ne) provides the greatest potential for the mitigation of tungsten leakage from the SAS-VW divertor will also be presented.