Molecular dynamics simulations of cumulative helium bombardments on tungsten surfaces

Molecular dynamics (MD) simulations were performed to study the cumulative bombardments of low-energy (60-200 eV) helium (He) atoms on tungsten (W) surfaces. The behaviour of He and the response of the W surface were investigated. The He incident energy and tungsten temperature play important roles on the formation and growth of He clusters. The temperature can promote the coalescence of He clusters and increase the size of the He clusters. A stratification phenomenon of the He depth distribution has been observed. The rupture of the He clusters and, at the same time, the escape and re-deposition of the W atoms have been observed. During the formation of He clusters, the interstitial W atoms are produced and evolve into bundles of <111> crowdions, which would be constrained around the He clusters for a long time. However, they will finally move onto the top surface along the <111> direction, which results in stacking the W atoms on the surface. The complex combination effects of the He clusters and the interstitial atoms result in the growth of the surfaces.