D retention in C- and O-contaminated tungsten during D+ irradiation

Deuterium retention during 10 eV and 500 eV/D+ irradiations at 300 and 500 K was compared for 99.96% pure polycrystalline tungsten (PCW) foils and similar PCW foils contaminated with carbon and oxygen impurities. The contaminated foils showed reduced deuterium retention at 500 K compared with pure foil specimens while there was no difference for 300 K irradiations. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) of the unirradiated specimens showed regions of highly localised, non-homogeneous C and O contamination as well as a broader region of low level contamination of about 1–10% areal coverage. It is hypothesised that the C/O contamination creates a diffusion barrier in the PCW foil preventing deuterium from diffusing from the near surface into the bulk, thus reducing overall D retention at 500 K. This finding has implications for tritium retention in ITER; if carbon contamination even at low levels can impede T diffusion into the bulk it would mean reduced T loading in W plasma-facing components.