Migration behaviors of helium atoms near tungsten surfaces: A molecular dynamics study

摘要

Under the fusion environment, the behavior of helium atoms near tungsten (W) surfaces plays a crucial role in surface morphological evolution and near-surface structural evolution of tungsten. In this paper, the migration behaviors of single helium atoms near W (2 1 1) and W (3 1 1) surfaces were investigated using molecular dynamics simulations. The results show that these single atoms can be well-described by the theory of continuous diffusion of particles in a semi-infinite medium, and different types of trap mutations occur for both surfaces. Although the temperature has an impotent impact on the occurrence probabilities of different types of trap mutations and the probabilities of helium atoms escaping from trap sites, there is no a strict relationship between the depth and temperature for trap mutations occurring. For the W (2 1 1) surface, an approximately one-dimensional diffusion may take place along the 1 1 1 directions when the helium atoms are trapped near 2 layers below the W surface. For the W (3 1 1) surface, the behaviors of trap mutations seem more significant than (2 1 1) and (1 1 1) surfaces at T = 1000 K. To investigate the diffusion and trap mutation processes, the nudged elastic band (NEB) method was also applied. These results can be helpful for understanding of helium release, helium retention, subsurface helium clustering, as well as surface morphological evolution.