Deciphering the Underlying Physical Processes Responsible for Synergistic Interactions Between Hydrogen and Noble Gases in Materials under Fusion Relevant Conditions

The objective of this research is to understand the underlying physical processes responsible for the synergistic interactions between hydrogen and noble gases in materials under fusion relevant conditions. The formation of He bubbles within plasma-facing components is a common phenomena that is expected to occur in the structural materials and first-wall, as well as the divertor, and is has been shown to strongly modify the retention properties for hydrogen of these materials. Molecular dynamics simulations were performed to assess the behavior of H around high pressure helium bubbles. The simulations revealed that hydrogen is strongly trapped to the periphery of helium bubbles. These results motivated more accurate assessments of the trapping energy and diffusion behavior of hydrogen around over-pressurized bubbles with density functional theory. These simulations modeled the edge of the bubbles with a slab geometry of various surface orientations of tungsten and iron. It was found that the de-trapping energy, the energy required of hydrogen to escape the interface of the tungsten - noble gas interface, is on the order of 2.0 eV, with a slight surface orientation but is largely indenpendent of noble gas density. As well, the migration barriers in the sub-surface region are found to be highly dependent on the pathway, increasing by as much as twice the bulk value and deceasing to nearly 0.0 eV. Understanding the trapping energetics of H to noble gas interfaces allows for better interpretation of experimental results. This study will quantify interactions of H with large He bubbles, which to-date has not been modeled and is not easily extractable through most simulation methods.