  Molecular dynamics simulations were performed to study the dependence of
migration behaviours of single helium atoms near tungsten surfaces on the
surface orientation and temperature. For W{100} and W{110} surfaces, He atoms
can quickly escape out near the surface without accumulation even at a
temperature of 400 K. The behaviours of helium atoms can be well-described by
the theory of continuous diffusion of particles in a semi-infinite medium. For
a W{111} surface, the situation is complex. Different types of trap mutations
occur within the neighbouring region of the W{111} surface. The trap mutations
hinder the escape of He atoms, resulting in their accumulation. The probability
of a He atom escaping into vacuum from a trap mutation depends on the type of
the trap mutation, and the occurrence probabilities of the different types of
trap mutations are dependent on the temperature. This finding suggests that the
escape rate of He atoms on the W{111} surface does not show a monotonic
dependence on temperature. For instance, the escape rate at T=1500 K is lower
than the rate at T=1100 K. Our results are useful for understanding the
structural evolution and He release on tungsten surfaces and for designing
models in other simulation methods beyond molecular dynamics.
