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Title	90: Carbon-13 in He Plasma erosion/deposition experiment to benchmark DIVIMP code being used by ITER
Name:	Christopher Chrobak cchrobak@cfs.energy	Affiliation:	Commonwealth Fusion Systems
Research Area:	Plasma-material Interface	Presentation time:	Not requested
Co-Author(s):	P. Stangeby, A. Leonard, G. Tynan	ITPA Joint Experiment :	No
Description:	Provide experimental data for low-Z (carbon) physical erosion and deposition under simple as possible helium plasmas in LSN configuration for benchmarking plasma material interaction codes DIVIMP and WBC-Redep.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	The ideal material to expose for the purposes of these experiments is Be, but due to its extreme safety hazards, a suitable substitute may need to be used. Al has been identified as a suitable proxy for Be, but after testing in 2012 was found unable to withstand high power loads before melting. Thus, carbon is left, requiring the use of He plasmas to remove the chemical erosion factor and a Carbon-13 enriched erosion sample is required to detect its presence over the carbon background. It is proposed here to expose a specially-designed sample with a carbon-13 enriched surface layer to dedicated, repeat, well-characterized plasma shots. Measurements of the sputtering yield and material influx plume would be done spectroscopically using the MDS spectrometer view chords and narrow band filtered visible light cameras, as well as post-mortem measurements of the exposed surface by ion beam nuclear reaction analysis.
Background:	Theory predicts that for erosion of high-Z materials, prompt-local deposition of eroded material is dominant, and for low-Z materials, long-range transport and deposition is dominant. However, initial measurements of Mo erosion in 2011 found that only 20% of the net eroded material was found immediately surrounding the sample [4,5]. By contrast, for the low-Z Be DiMES sample in the 1996 experiment [3], only about half the Be that was eroded from the Be sample was found on the graphite surface of the DiMES head. These discrepancies indicate gaps in the current theory, and further stress the need for accurate measurements of gross and net erosion from PFCs. From our 13C-methane injection experiments in DIII-D [6] we know that much of the low-Z launched from the main wall is transported long range, e.g. from the top of DIII-D to the bottom. About half the total 13C that was injected at the top of the LSN discharges was found in the bottom divertor. We have less of a handle on the other half of the injected 13C, but it appears to have been deposited short-range, on the main wall (short as distinguished from local) The idea that the wall is a source of sputtered impurity which all ends up in the divertor sink is too simplistic: parts of the wall are in a state of net erosion while other parts are in a state of net deposition from wall sources elsewhere. Additionally, within the region immediately surrounding the strike points there are net erosion and net deposition zones that change with varying strike point location and plasma confinement mode [7]. ITER urgently needs proper benchmarking of edge impurity transport codes DIVIMP for low-Z impurities to estimate the net erosion of the Be wall armor and the tritium retention by Be co-deposition [1]. Experiments prepared on EAST and JET to benchmark low-Z erosion/deposition (C in EAST, Be in JET) examine only limiter-type plasma contact. Although most of the area of the ITER Be wall will experience limiter-type contact, most of the actual erosion will occur at the upper, second divertor, and will therefore involve divertor-type plasma contact. Hence, it is essential to properly benchmark the DIVIMP code for low-Z materials in divertor-type plasma contact. DIII-D is in a unique position to provide these results due to the high quality divertor and edge diagnostic suite and DiMES material exposure system.
Resource Requirements:	13C-coated Sample Fabrication PIGE Nuclear Reaction Analysis with Ion Beam
Diagnostic Requirements:	DiMES TV camera view with spectral filter for C emission MDS spectrometer views 1) on DiMES and 2) just off DiMES Tangential TV camera views of He and C emission Langmuir probes Divertor-Thomson DiMES Thomson CER
Analysis Requirements:	TBD
Other Requirements: