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Atomistic Mechanism from Vacancy Trapped H/He Atoms to Initiation of Bubble in W under Low Energy Ions Irradiation

With the first principles calculations of H and He induced energetics change we demonstrate that in W the accumulation of H (up to 9) and He (up to 4) in a single vacancy (V) surprisingly reduce the formation energy of first and second nearest vacancy (as low as 0 eV), which gives the direct evidence of V-H (He) complex mutation mechanism from V-Hn (Hen) to V2-Hn (Hen) and with the potential to lead to the growth of H (He)-vacancy complexes: an initial step to H and He bubble. This finding well explains the long-standing problem of why H and He bubbles being produced on W surface exposed to low-energy (far lower than displacement threshold energy) D or He ions irradiation. The further identified repulsive (attractive) interaction between V-H12 (V-He14) and additional H (He) illustrates the experimentally observed big difference of deposition depth of H (micron) and He (100 angstrom) bubbles in W even the migration rate of He is far larger than that of H.