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

The UEDGE modeling code has been used to study the effect of varying the carbon yield from the plasma facing surfaces on the core plasma carbon contamination 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 an approximate factor of two variation in the source. This weak dependence is in agreement with the analysis of spectroscopic data from DIII-D [1]. Examination of the carbon transport shows a general flow pattern of carbon as follows: (1) parallel flow from the divertors to the near scrape off layer (SOL) near the separatrix, (2) cross field diffusion from the near SOL to the far SOL (near the wall), and (3) parallel flow from the far SOL to the far region of the inner divertor. The carbon flux from the divertors to the near SOL drops as the sputtering rate is reduced. In the far SOL, background plasma parameters adjust in small ways to produce an increasing carbon density with decreasing sputtering yield. This increasing density of carbon in the far SOL is consistent with a reduction in the parallel velocity of carbon ions flowing from the far SOL back to the inner divertor. Since the carbon density near the separatrix is constant as the sputtering yield is reduced, the increasing density in the far SOL reduces the radial gradient and therefore the diffusive radial flow. A balance in the outward radial diffusive flow from the near SOL and the flow from the divertor into the near SOL maintains the carbon density in the near SOL nearly constant, even though the carbon throughput changes.