Friday, March 24, 2017
Abstract: First-of-its-kind experiments using isotopically-enriched, W-coated divertor tiles coupled with midplane collector probes (CPs) have been performed on DIII-D to understand divertor impurity production and transport. Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) results are presented characterizing the isotopic ratios of deposited W on the mid-plane CPs and give quantitative information on the transport of W from specific poloidal locations within the lower outer divertor region. The setup includes two toroidal tile arrays (5 cm wide) of W-coated, TZM inserts in the lower outer divertor with the remaining plasma facing materials (PFMs) being carbon tiles. The inner ring was coated in natural-W (with 26.5% W-182) and the outer ring was coated with 93% isotopically enriched W-182. The unique "isotopic fingerprints" for the W impurities released from each coating in a dominant C PFM environment enables their use as tracer particles to be collected and distinguished at other locations.
Rutherford Backscatter (RBS) analysis of these CPs has provided areal densities of elemental W content along the length of each CP face, which were compared with DIVIMP modelling of the far-SOL to better understand impurity transport in the edge plasma. ICP-MS analysis of the CPs has successfully identified the presence of the enriched W isotopes and yielded isotopic ratios of the deposited W. By using a two-source Stable Isotope Mixing Model (SIMM), the amount of W from each of the divertor rings that contributed to the total W deposition on the CP has been determined and shown to vary with the given plasma conditions, particularly ELM amplitude as examined through divertor spectroscopy and CP deposition profiles. An analysis of deposited W profiles with strike point positioning, H-mode/L-mode, ELM frequency, and forward/reverse Bt is reviewed.
Bio: David Donovan is an assistant professor in the Nuclear Engineering Department at the University of Tennessee-Knoxville. He received his PhD in Nuclear Engineering from the University of Wisconsin-Madison in 2011 and his BS in Nuclear Engineering at the University of Illinois at Urbana-Champaign. His PhD work was in the area of Inertial Electrostatic Confinement (IEC) Fusion for the purpose of creating and utilizing small-scale neutron generating devices to detect explosives and other illicit materials. He was a post-doctoral research associate at Sandia National Laboratories-California in the Hydrogen and Metallurgy Sciences Department. His work at Sandia was in the area of plasma-surface interactions in magnetically confined fusion devices. He collaborated extensively with the DIII-D tokamak operated by General Atomics in San Diego, CA as well as with the Tritium Plasma Experiment located at Idaho National Laboratory. He developed expertise in grazing incidence x-ray diffraction and atomic force microscopy, and use of laboratory scale RF and microwave plasmas. His work since joining UTK includes plasma and heat flux diagnostic development and analysis with the Proto-MPEX experiment at ORNL, development of a low-flux He implantation stage for testing damage to fusion materials, and continued collaborations with DIII-D on diagnostic development and ex-situ material characterization studies. Dr. Donovan is also introducing a new series of undergraduate and graduate courses at UTK in the area of Nuclear Fusion Technology.