induced modification of the tungsten lattice in the vicinity of the dislocations,
formation of dynamic platelet-like hydrogen-reach structures with the hydrogen
trapping energy E tr ~ 1 eV. The amount of hydrogen trapped in such platelet
structures depends on the tungsten temperature and the hydrogen concentration. In
contrast to hydrogen trapping in a vacancy, at low temperatures, hydrogen trapping
in platelets related to a dislocation can greatly exceed 100 hydrogen atoms per a
dislocation segment. It is conceivable that the temporal evolution of such platelets
can account for the growth of the blisters observed experimentally under hydrogen
irradiation of tungsten and some other metals at relatively low temperatures (see
Fig. 3.11). Formation of similar platelet-like structures could also be triggered by the
presence in the sample of the non-hydrostatic tensile and shear stress components.
We notice that an impact of a strain applied to the tungsten lattice on the hydrogen
solution energy, found from the DFT simulations, was also reported in [54].
Neutron irradiation of tungsten, unavoidable in fusion reactors, results in tungsten
lattice damage and the formation of a large amount of additional hydrogen traps.
However, experiments with neutron-irradiated samples can only be done just in a
few laboratories (e.g. see [91, 92]). Therefore, some energetic ion (such as Si, He,
Cu, W, etc.) irradiation is often used as a proxy for the neutron damage [6, 52, 63,
92–95]. The available experimental data demonstrate that the lattice damage causes a
large increase of trapped hydrogen although the comparison of hydrogen retention in
the neutron- and ion-damaged tungsten samples shows a significant difference in the
corresponding TDS peaks [92]. This could suggest different structures of the lattice
damage imposed by neutrons and energetic ions. However, annealing of the damaged samples at high temperature before hydrogen irradiation strongly reduces the
impact of the lattice damage on hydrogen retention [94].
In fusion plasma, hydrogen is always accompanied by helium (the fusion ash).
Possible synergistic effects of tungsten irradiation by low energy hydrogen with
small admixture of helium ions on hydrogen retention were studied both experimentally and theoretically [63, 96–103]. All these studies show that an admixture of even
small (~5%) of helium results in a strong reduction of the amount of retained
hydrogen, which could be attributed to the binding of hydrogen to helium nanobubbles close to the surface, as seem to be indicated by some experimental data and
simulations. However, this effect could also be related to possible interconnections
of helium nano-bubbles resulting in a back leakage of hydrogen [104].
Fig. 3.11 Blisters produced
on monocrystalline tungsten
by 1.5 keV deuterium ion
irradiation at the sample
temperature of 400 K.
(Reproduced with
permission from [90],
© Elsevier 2011)
3.2 Basic Features of Hydrogen/Helium Transport in Plasma-Facing Materials
63
formation of dynamic platelet-like hydrogen-reach structures with the hydrogen
trapping energy E tr ~ 1 eV. The amount of hydrogen trapped in such platelet
structures depends on the tungsten temperature and the hydrogen concentration. In
contrast to hydrogen trapping in a vacancy, at low temperatures, hydrogen trapping
in platelets related to a dislocation can greatly exceed 100 hydrogen atoms per a
dislocation segment. It is conceivable that the temporal evolution of such platelets
can account for the growth of the blisters observed experimentally under hydrogen
irradiation of tungsten and some other metals at relatively low temperatures (see
Fig. 3.11). Formation of similar platelet-like structures could also be triggered by the
presence in the sample of the non-hydrostatic tensile and shear stress components.
We notice that an impact of a strain applied to the tungsten lattice on the hydrogen
solution energy, found from the DFT simulations, was also reported in [54].
Neutron irradiation of tungsten, unavoidable in fusion reactors, results in tungsten
lattice damage and the formation of a large amount of additional hydrogen traps.
However, experiments with neutron-irradiated samples can only be done just in a
few laboratories (e.g. see [91, 92]). Therefore, some energetic ion (such as Si, He,
Cu, W, etc.) irradiation is often used as a proxy for the neutron damage [6, 52, 63,
92–95]. The available experimental data demonstrate that the lattice damage causes a
large increase of trapped hydrogen although the comparison of hydrogen retention in
the neutron- and ion-damaged tungsten samples shows a significant difference in the
corresponding TDS peaks [92]. This could suggest different structures of the lattice
damage imposed by neutrons and energetic ions. However, annealing of the damaged samples at high temperature before hydrogen irradiation strongly reduces the
impact of the lattice damage on hydrogen retention [94].
In fusion plasma, hydrogen is always accompanied by helium (the fusion ash).
Possible synergistic effects of tungsten irradiation by low energy hydrogen with
small admixture of helium ions on hydrogen retention were studied both experimentally and theoretically [63, 96–103]. All these studies show that an admixture of even
small (~5%) of helium results in a strong reduction of the amount of retained
hydrogen, which could be attributed to the binding of hydrogen to helium nanobubbles close to the surface, as seem to be indicated by some experimental data and
simulations. However, this effect could also be related to possible interconnections
of helium nano-bubbles resulting in a back leakage of hydrogen [104].
Fig. 3.11 Blisters produced
on monocrystalline tungsten
by 1.5 keV deuterium ion
irradiation at the sample
temperature of 400 K.
(Reproduced with
permission from [90],
© Elsevier 2011)
3.2 Basic Features of Hydrogen/Helium Transport in Plasma-Facing Materials
63