Effect of boronization on plasma-facing graphite surfaces and its correlation with the plasma behavior in NSTX-U

Highlights: • Boronization on graphite in NSTX-U leads to surfaces with low oxygen concentration (5%) and B/C ratio near 0.5. • Exposure of boronized graphite to deuterium plasmas progressively increases the oxygen content of the surfaces. • The increments in oxygen concentration measured with XPS on the surface can be observed in the plasma edge using filterscopes. • Under the right conditions low oxygen concentrations at the plasma edge and on the first wall surfaces leads to better plasma performance. In the same way, high concentrations lead to poor performance. - Abstract: Boronization is a Plasma Facing Component (PFC) conditioning technique widely used in tokamak machines. The National Spherical Torus Experiment-Upgrade (NSTX-U) applied this conditioning, using a plasma glow with a deuterated Trimethyl-boron (d-TMB) and He mixture. The use of boronization during the campaign improved the plasma performance, allowing longer plasma discharges and H-mode access. The chemical state of an ATJ graphite sample, used as a proxy for the NSTX-U PFCs, was monitored in-situ using the Materials Analysis Particle Probe (MAPP) diagnostic and X-ray Photoelectron Spectroscopy (XPS). The XPS data showed a progressive rise (from + fluence increased. Filterscopes were used to measure the light emitted by oxygen impurities in the plasma near the surface of the PFC. An increase in the registered magnitude of the OII line, normalized to the D_γ intensity, was observed as the concentration of O on the ATJ surface increased. The plasma performance was found to be strongly correlated to oxygen impurity concentrations at the plasma edge and on the PFC surface, as measured by the discharge length and access to the H-mode regime. In this work, we present a quantitative analysis of the evolution of the chemistry of the ATJ surface, and the oxygen presence in the plasma-material interface, and report relevant plasma parameters observed during the same period of time.