The tungsten are deemed to be the most promising candidates as plasma facing material due to its high melting temperature, good thermal properties, low sputtering yields[1]. In the near surface of plasma facing materials high densities of interstitials and vacancies are produced in addition to high concentrations of hydrogen and helium (He). He easily are trapped by vacancies, dislocations, grain boundaries to form He bubble nucleation. When no traps are available, He spontaneously form clusters, which result in strong lattice strain. It can be relieved by ejecting one or more matrix atoms to form one or more Frenkel Pairs, i:e:vacancies and self-interstitial atoms. He cluster will be trapped by the vacancy it created, this is a self-trapping event[2].
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