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

•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.