of a strong material mixture in both the tendrils and the bulk [75, 76] seem to support
this idea.
Let us now discuss hydrogen transport in tungsten. Even though interstitial
hydrogen does not exhibit self-trapping effects, the experimental data show its rather
strong accumulation in the tungsten lattice (e.g. see Fig. 3.9). Therefore, some other
hydrogen trapping mechanism(-s) should exist. One of the possible candidates is
hydrogen trapping in the vacancies. The DFT simulations (e.g. see [77–80] and the
references therein) show that a mono-vacancy at room temperature could trap ~6
hydrogen atoms with the binding energy ~1 eV. Moreover, a combination of the
DFT results and thermodynamic consideration suggests that a large amount of
hydrogen embedded into a metal lattice (including tungsten) strongly promotes the
vacancy formation and buildup of the so-called superabundant vacancies (SAV).
Experimental results, although involving the metals with the face-centered cubic
lattice such as palladium, nickel, and some others, seem to support the hydrogeninduced formation of SAV (see [81, 82]). The trapping energies ~1 eV, similar to
those found from the DFT calculations for hydrogen trapping in a mono-vacancy in
tungsten, are usually inferred from the TDS data (e.g. see [44, 83–85]), although in
some cases, other traps, in particular, with a higher, ~2 eV, trapping energy, are
needed to fit the entire TDS. Traps with E tr ~ 2 eV are usually attributed to hydrogen
trapping in voids.
Other possible trapping sites for hydrogen are related to dislocations. The DTF
simulations show that three hydrogen atoms can be bound to the jogs of a screw
dislocation (e.g. see [86]) in tungsten with the binding energy ~1.4 eV [87, 88]
although the further increase of the number of the hydrogen atoms decreases the
binding energy significantly. The TEM images demonstrate indeed the presence of
screw dislocations decorated by hydrogen clusters [87]. Moreover, they also suggest
that the formation of these dislocations, even in a monocrystalline sample, is
facilitated by hydrogen [64]. In [89], the MD modeling of hydrogen interactions
with both the edge and screw dislocations in tungsten demonstrated a hydrogenFig. 3.10 On the left: SEM micrograph of the fuzz grown on a single crystal after 1-hour irradiation
by 40 eV helium ions. (Reproduced with permission from [71], © Elsevier 2010) and on the right:
individual tree of nano-tendrils. (Reproduced with permission from [72], © Elsevier 2017) grown
on a tungsten surface exposed to 50 eV helium ion irradiation
62
3 Plasma-Material Interactions in Magnetic Fusion Devices
this idea.
Let us now discuss hydrogen transport in tungsten. Even though interstitial
hydrogen does not exhibit self-trapping effects, the experimental data show its rather
strong accumulation in the tungsten lattice (e.g. see Fig. 3.9). Therefore, some other
hydrogen trapping mechanism(-s) should exist. One of the possible candidates is
hydrogen trapping in the vacancies. The DFT simulations (e.g. see [77–80] and the
references therein) show that a mono-vacancy at room temperature could trap ~6
hydrogen atoms with the binding energy ~1 eV. Moreover, a combination of the
DFT results and thermodynamic consideration suggests that a large amount of
hydrogen embedded into a metal lattice (including tungsten) strongly promotes the
vacancy formation and buildup of the so-called superabundant vacancies (SAV).
Experimental results, although involving the metals with the face-centered cubic
lattice such as palladium, nickel, and some others, seem to support the hydrogeninduced formation of SAV (see [81, 82]). The trapping energies ~1 eV, similar to
those found from the DFT calculations for hydrogen trapping in a mono-vacancy in
tungsten, are usually inferred from the TDS data (e.g. see [44, 83–85]), although in
some cases, other traps, in particular, with a higher, ~2 eV, trapping energy, are
needed to fit the entire TDS. Traps with E tr ~ 2 eV are usually attributed to hydrogen
trapping in voids.
Other possible trapping sites for hydrogen are related to dislocations. The DTF
simulations show that three hydrogen atoms can be bound to the jogs of a screw
dislocation (e.g. see [86]) in tungsten with the binding energy ~1.4 eV [87, 88]
although the further increase of the number of the hydrogen atoms decreases the
binding energy significantly. The TEM images demonstrate indeed the presence of
screw dislocations decorated by hydrogen clusters [87]. Moreover, they also suggest
that the formation of these dislocations, even in a monocrystalline sample, is
facilitated by hydrogen [64]. In [89], the MD modeling of hydrogen interactions
with both the edge and screw dislocations in tungsten demonstrated a hydrogenFig. 3.10 On the left: SEM micrograph of the fuzz grown on a single crystal after 1-hour irradiation
by 40 eV helium ions. (Reproduced with permission from [71], © Elsevier 2010) and on the right:
individual tree of nano-tendrils. (Reproduced with permission from [72], © Elsevier 2017) grown
on a tungsten surface exposed to 50 eV helium ion irradiation
62
3 Plasma-Material Interactions in Magnetic Fusion Devices