  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.
