Diverted Tokamak Carbon Screening: Scaling with Machine Size and Consequences to Core Contamination
Authors: J.D. Strachan, G. Corrigan, A. Kallenbach, G.F. Matthews, H. Meister, R. Neu, V. Rohde, and J. Spence
Date of PPPL Report: January 2004
Published in: Nuclear Fusion 44 (July 2004) 772-787
Plasma impurity content depends upon the impurity sources, fuelling efficiency, and confinement. In JET [Joint European Torus], carbon is the primary impurity, and its fuelling efficiency has been studied using methane gas injection and modeled with the SOL [scrape-off layer] codes: DIVIMP and EDGE2D. In this paper, EDGE2D modeling of similar AUG [ASDEX-Upgrade] experiments and projections to ITER are described. The parameters have been identified which govern the size scaling of carbon screening.
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 by factors of about 100. The carbon sputtering is projected to be larger by similar factors, making the projected ITER core contamination similar to JET. However, that result is based upon the assumption that the wall materials have similar composition and behavior as observed on JET. 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.