Oak Ridge National Laboratory
Friday, February 7, 2020
Experiments carried out on DIII-D using isotopic tungsten (W) sources in the outer divertor have characterized how the W leakage from this region depends on both the exact source location and edge-localized mode (ELM) behavior. These studies help to elucidate the physics driving divertor impurity source rates and leakage, with and without ELMs, and provide better insight on the link in the chain connecting wall impurity sources to core impurity levels in magnetic fusion devices.
The DIII-D divertor sources are toroidally-symmetric and poloidally- localized to two regions: (1) the outer strike point (OSP) with natural abundance of W isotopes; and (2) the far-target with highly-enriched 182W isotopes. With the use of a dual-faced collector probe (CP) in the main scrape-off layer (SOL) near the outside midplane and source-rate spectroscopy, a proxy for divertor impurity leakage is developed. Using this proxy, global discharge parameter trends are found in power across the separatrix (PSEP), pedestal density and SOL collisionality. These trends are consistent with the effect of an increased upstream ion temperature parallel gradient force in the near-SOL as well as suggest that ELM size plays a key role in the leakage from the far-target location. Furthermore, indications for main SOL W contamination is evidenced by the measurement of large deposition asymmetries on the two opposite CP faces.