Recent experiments and modeling on the influence of helium on deuterium transport and retention in tungsten

Recent laboratory experiments revealed that He admixture to deuterium (D) plasmas reduces D retention at elevated temperature for tungsten [1]. This change in retention is accompanied by reduced blistering and the growth of He nano bubbles below the surface. While there are several speculative attempts to explain these observations, the actual cause for the reduced retention remains unclear. One possibility is that He might act as diffusion barrier for D. Likewise nano-sized bubbles might open up additional pathways for D to reach the surface thereby decreasing its transport into the bulk. Contrary to these experimental findings density functional theory (DFT) calculations show strong attraction between He and hydrogen [2], indicating increased trapping and hence retention of D around He clusters should be expected. In order to unravel this mystery we took a novel experimental approach. We separated the surface effect from the He effect by moving the He interaction zone into depth. Moreover, He implantation and D transport was decoupled. With this we show for the first time unambiguously that the presence of He does locally increase D trapping. He does not act as diffusion barrier. Rate equation modelling is presented which can explain the observed effects without the need for free fitting parameters. ([1] M. J. Baldwin et al. Nucl. Fusion 51 (2011) 103021 [2] H-B. Zhou, Nucl. Fusion 50 (2010) 115010)