Modeling of carbon transport in the divertor and SOL of DIII-D during highperformance plasma operation

The UEDGE modeling code has been used to study the effect of varying the ca rbon yield from the plasma facing surfaces on the core plasma carbon contam ination in DIII-D. The model of the lower single-null, ELMing H-mode plasma shows a remarkably weak dependence of the core carbon concentration over a n approximate factor of two variation in the source. This weak dependence i s in agreement with the analysis of spectroscopic data from DIII-D [1]. Exa mination of the carbon transport shows a general flow pattern of carbon as follows: (1) parallel flow from the diverters to the near scrape off layer (SOL) near the separatrix, (2) cross field diffusion from the near SOL to t he far SOL (near the wall), and (3) parallel flow from the far SOL to the f ar region of the inner divertor. The carbon flux from the diverters to the near SOL drops as the sputtering rate is reduced. In the far SOL, backgroun d plasma parameters adjust in small ways to produce an increasing carbon de nsity with decreasing sputtering yield. This increasing density of carbon i n the far SOL is consistent with a reduction in the parallel velocity of ca rbon ions flowing from the far SOL back to the inner divertor. Since the ca rbon density near the separatrix is constant as the sputtering yield is red uced, the increasing density in the far SOL reduces the radial gradient and therefore the diffusive radial flow. A balance in the outward radial diffu sive flow from the near SOL and the how from the divertor into the near SOL maintains the carbon density in the near SOL nearly constant, even though the carbon throughput changes. (C) 2001 Published by Elsevier Science B.V.