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This Week at Physics

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Events on Monday, April 22nd, 2019

Plasma Physics (Physics/ECE/NE 922) Seminar
Diagnosing impurity transport in the scrape-off layer of DIII-D using outer-midplane collector probes with isotopically enriched tungsten tracer particles
Time: 12:05 pm
Place: 2241 Chamberlin Hall
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.
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Physics Department Colloquium
Special Colloquium
Photoinduced reaction mechanisms in molecular systems probed with elemental specificity via ultrafast X-ray spectroscopy
Time: 3:30 pm
Place: 2103 Chamberlin Hall
Speaker: Nils Huse, Hamburg University
Abstract: Understanding the interplay of structural, electronic and spin degrees of freedom is paramount for mechanistic insights into how matter transforms upon external stimuli such as light. The elemental specificity of X-ray spectroscopy provides unique complimentary information to vibrational and electronic spectroscopy or non-resonant scattering techniques when studying reaction mechanisms in chemical and materials science. This can be especially useful for metal atoms and their nearest neighbors in transition-metal complexes [1,2] or so-called heteroatoms in functional groups of organic molecules [3,4] where specific atomic sites are of particular importance for the physical and chemical properties of molecular or solid state systems. I will introduce basic concepts of X-ray absorption spectroscopy and RIXS, the X-ray analogue to resonant Raman scattering, as methods to probe electronic structure on ultrafast time scales before presenting two examples of photo-induced reactions to show how X-ray spectroscopy of ligand- and heteroatoms can (i) unravel the role of ligand-field states in transition-metal complexes and (ii) identify transient excited states and new species in sulfur-containing molecules.

[1] B. Van Kuiken et al., J. Phys. Chem. Lett. 7, 465 (2016)
[2] A. A. Cordones et al., Nat. Comm. 9, 1989 (2018)
[3] M. Ochmann et al., J. Am. Chem. Soc. 139, 4789 (2017)
[4] M. Ochmann et al., J. Am. Chem. Soc. 140, 6554 (2018)
Host: Matt Herndon
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