Surface Characterization of TFTR First Wall Graphite Tiles Used During DT Operations

Surface characterization studies were performed on graphite tiles used as first wall materials during DT operation of the Tokamak Fusion Test Reactor (TFTR) at the Princeton Plasma Physics Laboratory. These ex situ analysis studies revealed a number of interesting and unexpected features. In this work we examined the spatial and (where possible) the depth distribution of impurity species deposited onto the plasma facing surfaces using X-ray Photo-electron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS). This work determined that beyond the predominant species of carbon and oxygen, common impurities included silicon, boron, lithium and sulfur. Oxygen content in the plasma facing tile surfaces ranged from 20 to 50 atomic percent [excluding H-isotopes], clearly indicating an extensive zone of oxidized carbon. By contrast, carbon tile surfaces not exposed to the plasma have surface oxygen contents ranging from 2 to 6 atomic percent. Analytical measurements of the secondary impurities (B, Li, Si, S) levels were on the order of 1–4 atomic percent, (boron and lithium were injected for wall conditioning in TFTR.) The core level binding energies of these impurity species were consistent with the presence of common oxides or hydroxides (e.g., B_xO_y, Li₂O, LiOH, Silicates). XPS measurements performed in concert with depth profiling indicated that the tile oxidized zone was significantly deeper than 1 micrometer into the (averaged) surface. Surface analytical results clearly indicate that plasma operations clearly redeposit injected impurities (Li, B) and the depth profiles and distributions of the hydrogen isotopes may be impactedand/or influenced by this deposition process.An attempt at determining hydrogen isotope concentration distributions was made using positive ion SIMS. Specific regions of some surfaces clearly indicated the presence of m/z=3 (HD, T) and m/z=15 (CH₃, CHD, CT). Preliminary data examination using positive ion SIMS examination indicates that these mass markers are substantially higher in the near surface region when compared with spectra recorded deeper in the surface region. The deuterium and tritium concentrations were; however, sufficiently low or compromised bycommon isobaric interferencesthat accurate isotopic distributions using SIMS were not possible. These findings are in agreement with results reported by others. [Morimoto et al, Sun et al, reference 3 Haasz et al]