Impurity screening behavior of the high-field side scrape-off layer in near-double-null configurations: prospect for mitigating plasma–material interactions on RF actuators and first-wall components

LaBombard, B.; Kuang, A. Q.; Brunner, D.; Faust, I.; Mumgaard, R.; Reinke, M. L.; Terry, J. L.; Howard, N.; Hughes, J. W.; Chilenski, M.; Lin, Y.; Marmar, E.; Rice, J. E.; Rodriguez-Fernandez, P.; Wallace, G.; Whyte, D. G.; Wolfe, S.; Wukitch, S.

doi:10.1088/1741-4326/aa6dd2

Title: Impurity screening behavior of the high-field side scrape-off layer in near-double-null configurations: prospect for mitigating plasma–material interactions on RF actuators and first-wall components

Journal Article · Wed May 24 00:00:00 EDT 2017 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/aa6dd2· OSTI ID:1894445

LaBombard, B. ^[1]; Kuang, A. Q. ^[1]; Brunner, D. ^[1]; Faust, I. ^[1]; Mumgaard, R. ^[1]; Reinke, M. L. ^[2]; Terry, J. L. ^[1]; Howard, N. ^[1]; Hughes, J. W. ^[1]; Chilenski, M. ^[1]; Lin, Y. ^[1]; Marmar, E. ^[1]; Rice, J. E. ^[1]; Rodriguez-Fernandez, P. ^[1]; Wallace, G. ^[1]; Whyte, D. G. ^[1]; Wolfe, S. ^[1]; Wukitch, S. ^[1]

Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center

The impurity screening response of the high-field side (HFS) scrape-off layer (SOL) to localized nitrogen injection is investigated on Alcator C-Mod for magnetic equilibria spanning lower-single-null, double-null and upper-single-null configurations under otherwise identical plasma conditions. L-mode, EDA H-mode and I-mode discharges are investigated. HFS impurity screening is found to depend on magnetic flux balance and the direction of B × $$\nabla$$B relative to the most active divertor. Impurity 'plume' emission patterns indicate that both parallel and perpendicular (E × B) flows in the SOL contribute to the 'flushing' of impurities towards the active divertor, thereby affecting the overall impurity screening behavior. Despite the fact that the HFS SOL is extremely narrow in near-double-null configurations, this SOL is able to screen locally injected nitrogen at least as effectively as the low-field side (LFS) SOL—up to a factor of 10 more effective, depending on specific plasma conditions and whether the magnetic geometry produces parallel flows that work with or against E × B flows. For situations in which the E × B drift of the impurity ions opposes parallel flow toward the primary divertor, HFS impurity screening is found to be least effective. When E × B drifts assist parallel flow toward the primary divertor, HFS impurity screening is found to be very effective. These data support the idea of placing RF actuators and close-fitting wall components on the HFS of the tokamak. Finally, with this configuration, near-double-null magnetic topologies may be used for active control of plasma parameters at the antenna/plasma interface for optimal RF coupling, mitigate the generation of local impurities from plasma–material interactions and, taking advantage of favorable plasma flows and good screening properties of the HFS SOL, further minimize the impact of wall-born impurity sources on the plasma core.

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Research Organization:: Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES)

Grant/Contract Number:: FC02-99ER54512

OSTI ID:: 1894445

Journal Information:: Nuclear Fusion, Vol. 57, Issue 7; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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An experimental assessment of methods used to compute secondary electron emission yield for tungsten and molybdenum electrodes based on exposure to Alcator C-Mod scrape-off layer plasmas McCarthy, W.; LaBombard, B.; Brunner, D. Plasma Physics and Controlled Fusion, Vol. 60, Issue 3 https://doi.org/10.1088/1361-6587/aaa51c	journal	February 2018
Filament identification in wide-angle high speed imaging of the mega amp spherical tokamak Farley, T.; Walkden, N. R.; Militello, F. Review of Scientific Instruments, Vol. 90, Issue 9 https://doi.org/10.1063/1.5109470	journal	September 2019
The dependence of divertor power sharing on magnetic flux balance in near double-null configurations on Alcator C-Mod Brunner, D.; Kuang, A. Q.; LaBombard, B. Nuclear Fusion, Vol. 58, Issue 7 https://doi.org/10.1088/1741-4326/aac006	journal	May 2018
Potential Impacts of Liquid-Metal Plasma-Facing Components on Heating and Current Drive Actuators for a Fusion Nuclear Science Facility Wallace, G. M.; Kessel, C. E.; Hosea, J. Fusion Science and Technology, Vol. 76, Issue 1 https://doi.org/10.1080/15361055.2019.1629253	journal	July 2019
High field side lower hybrid wave launch for steady state plasma sustainment Bonoli, P. T.; Wallace, G. M.; Shiraiwa, S. Nuclear Fusion, Vol. 58, Issue 12 https://doi.org/10.1088/1741-4326/aae7e9	journal	November 2018
Blob velocity scaling in diverted tokamaks: A comparison between theory and simulation Paruta, Paola; Beadle, C.; Ricci, P. Physics of Plasmas, Vol. 26, Issue 3 https://doi.org/10.1063/1.5080675	journal	March 2019

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