Impact of divertor material on neutral recycling and discharge fueling in DIII-D

Journal Article · Fri Mar 13 00:00:00 EDT 2020 · Physica Scripta (Online)

DOI:https://doi.org/10.1088/1402-4896/ab5a2e· OSTI ID:1860689

Bykov, I. ^[1]; Rudakov, D. L. ^[1]; Pigarov, A. Yu ^[1]; Hollmann, E. M. ^[1]; Guterl, J. ^[2]; Boedo, J. A. ^[1]; Chrobak, C. P. ^[3]; Abrams, T. ^[3]; Guo, H. Y. ^[3]; Lasnier, C. J. ^[4]; McLean, A. G. ^[4]; Wang, H. Q. ^[3]; Watkins, J. G. ^[5]; Thomas, D. M. ^[3]

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1. Univ. of California San Diego, La Jolla, CA (United States)
2. Oak Ridge Associated Univ., Oak Ridge, TN (United States)
3. General Atomics, San Diego, CA (United States)
4. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
5. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)

We report that experiments with the lower divertor of DIII-D during the Metal Rings Campaign (MRC) show that the fraction F of atomic D in the total recycling flux is material-dependent and varies through the ELM cycle, which may affect divertor fueling. Between ELMs, F_C ~ 10% and FW ~ 40%, consistent with expectations if all atomic recycling is due to reflections. During ELMs, F_C increases to 50% and F_W to 60%. In contrast, the total D recycling coefficient including atoms and molecules R stays close to unity near the strike point where the surface is saturated with D. During ELMs, R can deviate from unity, increasing during high energy ELM-ion deposition (net D release) and decreasing at the end of the ELM which leads to ability of the target to trap the ELM-deposited D. The increase of R > 1 in response to an increase in ion impact energy Ei has been studied with small divertor target samples using Divertor Materials Evaluation System (DiMES). An electrostatic bias was applied to DiMES to change E_i by 90 eV. On all studied materials including C, Mo, uncoated and W-coated TZM (>99% Mo, Ti, and Zr alloy), W, and W fuzz, an increase of E_i transiently increased the D yield (and R) by ~10%. On C there was also an increase in the molecular D₂ yield, probably due to ion-induced D₂ desorption. Despite the measured increase in F on W compared to C, attached H-mode shots with OSP on W during MRC did not demonstrate a higher pedestal density. About 8% increase in the edge density could be seen only in attached L-mode scenarios. The difference can be explained by higher D trapping in the divertor and lower divertor fueling efficiency in H- versus L-mode.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Sandia National Lab. (SNL-NM), Albuquerque, NM (United States); Univ. of California, San Diego, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: AC52-07NA27344; FG02-07ER54917; AC05-06OR23100; FC02-04ER54698; NA0003525
• OSTI ID: 1860689
• Report Number(s): LLNL-JRNL-830755; 1034355; TRN: US2305414
• Journal Information: Physica Scripta (Online), Vol. 2020, Issue T171; ISSN 1402-4896
• Publisher: IOP Publishing
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
DIII-D
tokamak
tungsten
recycling
fueling