Non-dilute helium-related defect interactions in the near-surface region of plasma-exposed tungsten

A Weerasinghe and L Hu and KD Hammond and BD Wirth and D Maroudas, JOURNAL OF APPLIED PHYSICS, 128, 165109 (2020).

DOI: 10.1063/5.0023356

We report a systematic energetic analysis of helium-related defect interactions that mediate helium (He) segregation on surfaces of plasma- exposed tungsten at different levels of He ion implantation. We focus on high He fluences that increase the He content in the plasma-exposed material well beyond the dilute limit of He concentration and employ atomic configurations generated by large-scale molecular dynamics simulations of He-implanted tungsten. We perform systematic molecular statics computations of cluster-defect interaction energetics in the highly defect-rich near-surface region of plasma-exposed tungsten for small mobile helium clusters as a function of the clusters' distances from the surface. In this region, mobile clusters are also subjected to the stress fields generated by defects such as helium bubbles and other clusters, which govern cluster-defect interactions in addition to the cluster-surface interaction. Based on systematic investigation of individual cluster-defect interactions, we develop a mathematical framework to describe the interaction energy landscapes consisting of elastic interaction potential perturbations to finite-width square-well potentials, where the potential well accounts for cluster trapping by the defect at close range and subsequent coalescence and the perturbation potential is parameterized according to elastic inclusion theory. Superposition of all the relevant interaction potentials provides a comprehensive description of the interaction energy landscape that would be experienced by a small mobile cluster along its migration path toward the plasma-exposed surface at high He fluences. Such descriptions are particularly important for developing atomistically informed, hierarchical multi-scale models of helium cluster dynamics in plasma-facing materials.

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