Diverted tokamak carbon screening: Scaling with machine size and consequences for core contamination

Plasma impurity content depends on the impurity sources, fuelling efficiency, and confinement. In JET, carbon is the primary impurity, and its fuelling efficiency has been studied using methane gas injection and modelled with the scrape-off-layer (SOL) codes: DIVIMP and EDGE2D. In this paper, EDGE2D modelling of similar ASDEX Upgrade experiments and projections to ITER are described. The parameters that govern the size scaling of carbon screening have been identified. Size scaling is complex. For carbon injected from the main chamber, the important factors include: the SOL temperature, the magnitude of the thermal force at the divertor entrance, and the parallel distance to the divertor. For carbon injected at the strike points, the intersection of the carbon ionization region with the region of strong thermal force determines the carbon fuelling efficiency. ITER projects to have much better carbon screening than JET. The ITER SOL is hotter so that main chamber carbon is ionized further from the separatrix making the calculated carbon fuelling efficiency lower. Also, the carbon originating near the strike point has less chance of escaping the divertor since the ITER divertor is larger. The carbon sputtering is projected to be larger, making the ITER core contamination difficult to estimate. A general result is that the core contamination at fixed total sputtering rate and core impurity confinement increases when the fraction of carbon ionized in the main chamber SOL increases, and decreases for larger machine size and higher density operation.