The divertor in fusion reactors will be subject to intense particle bombardment. Light gases such as hydrogen and helium implanted into solids can form gas bubbles in the target. Under appropriate conditions these bubbles may grow into larger bubbles, becoming visible on the target surface as small bumps or blisters. Under prolonged implantation these blisters may rupture, ejecting gas atoms and fragments of the solid. Blistering is harmful in fusion reactors, especially when
the plasma-facing materials contain heavy chemical elements such as tungsten, since energy loss in the plasma is proportional to the proton number of impurity nuclei.
However, hydrogen and helium are not on an equal footing concerning the blistering behaviour in tungsten. Experiments show that even at temperatures where the migration rates of these gas atoms are of similar order, He atoms will form bubbles right at the surface, at depths ~ 100 Å, while H atoms cluster at micrometer
depths. The reason for this huge difference has been unclear.
We have used density-functional theory calculations, molecular dynamics simulations, and kinetic Monte Carlo simulations, using parameters from implantation experiments, to show that the difference in bubble depths is explained by the different self-trapping behaviour of H and He atoms in W. Helium can trap with other He atoms to form bubble seeds [1], while the H-H interaction in the W lattice is repulsive at short H-H distances, preventing self-trapping [2]. This leads to fast bubble formation close to the surface in the case of helium, whereas hydrogen atoms migrate for a long time before becoming trapped by defects deeper down in the irradiation target.
[1] K. O. E. Henriksson, K. Nordlund, and J. Keinonen, Nucl. Instr. Meth. B 244, 377 (2005)
[2] K. O. E. Henriksson, K. Nordlund, A. Krasheninnikov, and J. Keinonen, Appl. Phys. Lett. 87, 163113 (2005)
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