Speaker: Professor David C. Donovan, University of Tennessee-Knoxville (UTK)
Abstract: The impurity transport chain begins at the plasma facing component (PFC) target, connects through the edge (Scrape-Off Layer (SOL) and Divertor) plasma, and finally contaminates the core plasma. Understanding the nature of impurity transport in the SOL is therefore of crucial importance to control core contamination; however, the SOL link in the transport chain is the most weakly understood due to diagnostic limitations. CPs provide the unique capability to sample the impurity content of the SOL and their use is greatly enabled by high sensitivity material characterization techniques and interpretive modeling tools. Two isotopically distinct sources of W were installed on the lower divertor for the DIII-D MRC, which allowed the W impurities collected by the mid-plane CPs to be traced back to the poloidal location on the divertor from which they came. The CPs were combined with W source spectroscopy to determine a leakage proxy for impurities escaping the divertor and reaching the SOL. Results will be presented demonstrating that impurity leakage from the outer strike point increases nearly linearly with power across the separatrix. Experiments varying the ELM frequency identified that leakage from the far-target region is reduced significantly as ELM size decreases. The DIVIMP-OEDGE-WALLDYN code suite was utilized as an interpretive modelling tool for the edge plasma region. Experimental results have been obtained that are consistent with computational models hypothesizing an accumulation of impurities in the crown region of the SOL created by ion temperature gradients. This unique combination of impurity exposure capabilities, SOL diagnostic coverage, ex-situ material characterization, and interpretive modeling tools have provided a potent framework with which to better elucidate the SOL link in the impurity transport chain.
*Work supported by US DOE under DE-SC0016318, DE-SC0019256, DE-AC05-00OR22725, DE-FG02-07ER54917, DE-FC02-04ER54698, DE-AC04-94AL85000.