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Surface chemistry analysis of lithium conditioned NSTX graphite tiles correlated to plasma performance
Authors:CN Taylor  KE Luitjohan  B Heim  L Kollar  JP Allain  CH Skinner  HW Kugel  R Kaita  AL Roquemore  R Maingi
Affiliation:1. Purdue University, School of Nuclear Engineering, West Lafayette, IN 47906, USA;2. Birck Nanotechnology Center, Discovery Park, West Lafayette, IN 47907, USA;3. Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USA;4. Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Abstract:Lithium wall conditioning in NSTX has resulted in reduced divertor recycling, improved energy confinement, and reduced frequency of edge-localized modes (ELMs), up to the point of complete ELM suppression. NSTX tiles were removed from the vessel following the 2008 campaign and subsequently analyzed using X-ray photoelectron spectroscopy as well as nuclear reaction ion beam analysis. In this paper we relate surface chemistry to deuterium retention/recycling, develop methods for cleaning of passivated NSTX tiles, and explore a method to effectively extract bound deuterium from lithiated graphite. Li–O–D and Li–C–D complexes characteristic of deuterium retention that form during NSTX operations are revealed by sputter cleaning and heating. Heating to ~850 °C desorbed all deuterium complexes observed in the O 1s and C 1s photoelectron energy ranges. Tile locations within approximately ±2.5 cm of the lower vertical/horizontal divertor corner appear to have unused Lisingle bondO bonds that are not saturated with deuterium, whereas locations immediately outboard of this region indicate high deuterium recycling. X-ray photo electron spectra of a specific NSTX tile with wide ranging lithium coverage indicate that a minimum lithium dose, 100–500 nm equivalent thickness, is required for effective deuterium retention. This threshold is suspected to be highly sensitive to surface morphology. The present analysis may explain why plasma discharges in NSTX continue to benefit from lithium coating thickness beyond the divertor deuterium ion implantation depth, which is nominally <10 nm.
Keywords:Lithium  Deuterium  Retention  Carbon-facing components  Divertor  X-ray photoelectron spectroscopy  Plasma–surface interactions
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