Investigation of Surface Conditioning on Plasma-facing Components in the National Spherical Torus Experiment Upgrade (NSTX-U)

Heather Sandefur, University of Illinois at Urbana-Champaign

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The National Spherical Torus Experiment Upgrade (NSTX-U) has been used to investigate the effect of wall tile surface conditioning on plasma performance during operation. Previous campaigns have demonstrated the enhanced suppression of edge-localized modes (ELMs) and reduced divertor recycling when reactor walls were conditioned with lithium. Performance was also improved when wall tile conditioning via boronization was performed and high-confinement (H-mode) operating conditions were routinely achieved during operation after conditioning. In order to better understand the impacts of surface conditioning and subsequent plasma exposure on the wall materials, cored samples of the exposed NSTX-U wall tiles were obtained and their surface chemistry was analyzed. The Ion-Gas-Neutral Interactions with Surfaces (IGNIS) In-situ Surface Science Facility was used to analyze boronized NSTX-U samples using X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LEISS). Boronized samples were also analyzed in the presence of lithium and changes in surface composition and morphology were observed.

Abstract Author(s): H. Sandefur, H. Schamis, J.P. Allain