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It was shown experimentally for tungsten, that He retained close to the surface influences transport and retention of hydrogen isotopes (HI). Namely, experiments using He seeded D plasmas, showed that the addition of He leads to reduced blistering accompanied by reduced D retention [1].
Recently, we have performed a systematic series of D exposures in tungsten where He was pre-implanted near the surface [2]. 20 MeV W irradiation was performed before or after He implantation to create defects within the first 3 µm. The defects created by W ions were used to trap D that was penetrating through the He surface layer. Using 3He nuclear reaction analysis (NRA) made it hence possible to quantify D transport beyond the He layer. D and He depth profiling showed increased D retention at the depth where He was implanted, but D retention in the bulk was reduced five times as compared to a He-free reference sample. Transmission electron microscopy showed nanometer-sized bubbles where He was implanted. From this we concluded that the observed D retention in He irradiated samples is due to D trapping at He bubbles and that the He containing microstructure enhances D re-emission. The same methodology was applied to EUROFER97 to clarify the effect of surface-near helium on deuterium transport into and retention in the bulk. To quantify the influence on D uptake at the surface, He was implanted into EUROFER97 samples close to the surface with 1 keV ions with different fluences and at different temperatures. Samples were then exposed to a low flux, low energy (100 eV/D) D ion beam at 370 K. One He-free W-irradiated EUROFER97 reference sample was also exposed only to low energy D ions for comparison. The defects created by W ions trap penetrating D and make it hence possible to quantify D transport below the He layer using NRA. Measured D depth profiles show reduced D uptake in the He-irradiated samples with major D retention near the surface where He is implanted. The D uptake is reduced by a factor of two compared to He-free sample. Surface analysis of the sample with the largest He fluence of 5.5×10^{21} He/m2 showed He bubbles near the surface. Results will be discussed and compared to tungsten.
[1] M. Baldwin et al. Nucl. Fusion 51 (2011) 103021; Nucl. Fusion 57 (2017) 076031
[2] Markelj et al. Nucl. Mater. Energy 45 (2025) 101981