Initial studies of plasma facing component surface conditioning in the national spherical tokamak experiment upgrade with the materials analysis particle probe. (August 2017)

• Record Type: Journal Article
• Authors: Bedoya, F.; Allain, J.P.; Kaita, R.; Skinner, C.H.; Koel, B.E.; Scotti, F.
• Is Part Of: Nuclear materials and energy. Volume 12(2017)
• Page Start: 1248
• Page End: 1252
• Publication Date: 2017-08
• Subjects: XPS -- Boronized graphite -- Conditioning -- Plasma facing components -- NSTX-U; Nuclear energy -- Periodicals; Nuclear fuels -- Periodicals; Nuclear reactors -- Materials -- Periodicals; Radioactive substances -- Periodicals; 621.4833
• DOI: 10.1016/j.nme.2017.03.035
• Languages: English
• ISSNs: 2352-1791

Abstract

Highlights: In vacuo analysis of plasma facing components in a tokamak is reported for the first time. Coatings deposited during boronization in NSTX-U were found to be B4 C. XPS data reveal oxidation and erosion of B4 C coatings exposed to D + plasmas. The MAPP facility was proven to be a powerful surface diagnostic for tokamak machines. Abstract: An innovative PFC diagnostic, the Materials Analysis Particle Probe (MAPP) was used to study the chemistry of Plasma Facing Components (PFCs) in the National Spherical Tokamak Experiment Upgrade (NSTX-U). NSTX-U used boronization as conditioning strategy during the 2015–2016 experimental campaign. Deposition with ∼9.1 g of deuterated tri-methyl boron (d-TMB) in a helium glow discharge resulted in coatings with an equivalent thickness of 7.0 nm at the lower outer divertor region. MAPP was used to capture for the first time the in-vacuo evolution of the chemical state of ATJ graphite PFCs after boronization and plasma exposure via X-ray Photoelectron Spectroscopy (XPS) on a day-to-day basis. The XPS data shows the formation of B4 C on the PFCs surface during boronization. We observed the gradual increase of the B2 O3 fraction in the coatings with plasma exposures. In contrast, we measured the formation of non-stoichiometric oxides when the samples were only exposed to residual gases and an argon vent. MAPP data revealed erosion and oxidation of deposited boron coatings over the course of tens of shots (a time resolution improved several orders-of-magnitude compared to post-campaign PFC characterization) that is correlated with the transitory nature of plasma performance enhancement with boron conditioning.