Structures and dynamics of helium in liquid lithium: A study by deep potential molecular dynamics

Current experimental techniques still face challenges in clarifying the structural and dynamic properties of helium (He) in liquid lithium (Li). A critical example of this technical hurdle is the formation of He bubbles, which significantly affects the transport of He within liquid Li — a vital aspect when considering liquid Li as a plasma-facing material in nuclear fusion reactors. We develop a machine-learning-based deep potential (DP) with ab initio accuracy for the Li–He system and perform molecular dynamics simulations at temperatures ranging from 470 K to 1270 K with a wide range of He concentrations. We observe that He atoms exhibit a tendency to aggregate and form clusters and bubbles in liquid Li. Notably, He clusters exhibit a significant increase in size at elevated temperatures and high concentrations of He, accompanied by the phase separation of Li and He atoms. We also observe an anomalous non-linear relationship between the diffusion coefficient of He and temperature, which is attributed to the larger cluster size at higher temperatures. Our study provides a deeper understanding of the behavior of He in liquid Li and further supports the potential application of liquid Li under extreme conditions.