Post-mortem surface chemistry analysis of boronized graphite PFC samples and their links to PMI behavior in NSTX-U *

In the 2015-2016 experimental campaign the National Spherical Torus Experiment Upgrade (NSTX-U) used ATJ -graphite as a first wall and divertor material. Wall conditioning was completed with deuterated trimethylboron (d-TMB). Post-mortem samples taken from graphite tiles extracted from various locations in the NSTX-U divertor region after the campaign were analyzed at the IGNIS (Ion-Gas-Neutral Interactions with Surfaces) facility at UIUC using X-ray Photoelectron Spectroscopy (XPS). According to the XPS analysis, samples contain on average 15-30% B. At Penn State, additional post-mortem samples are analyzed using both XPS and Scanning Auger Electron Spectroscopy (AES) to establish an elemental mapping and chemical characterization of the extracted tiles. In addition, samples of varying B concentration will then be exposed to high flux D+ ions, mimicking plasma material interactions in a tokamak, and later analyzed with XPS and Thermal Desorption Spectroscopy (TDS) to decipher PMI behavior linked to NSTX-U plasma operation. XPS and TDS will also be used to investigate the effects of the D+ irradiation on the surface chemistry and retention of the samples. The acquisition of data through XPS, AES and TDS will further investigate the chemical composition of the tiles after deuterium exposure.