7.3 Injection of Energetic Hydrogen
119
until they are thermalized to the wall temperature. Afterwards they diffuse into deep
or back to the injected surface according to the gradient of chemical potential or
concentration of H in the wall. During the diffusion, some of them are trapped.
Energy loss processes of high energetic hydrogen injected in a metal are twofold,
one is energy loss by excitation of electrons in the metal (electron energy loss, or
electron stopping), the other is nuclear collision (nuclear stopping). If the energy
of the hydrogen particles is high enough to displace a lattice atom, the lattice atom
is displaced remaining a Frenkel pair, i.e. an interstitial atom and a vacancy. If
the incident energy is not large, the nuclear collision occurs at surface resulting in
sputtering and surface recession. Most of the Frenkel pars are recombined. However,
some remain as defects and defect clusters, such as stacking fault, interstitial lops,
vacancy clusters, vacancy loops, and so on (see Sect. 4.2). Usually, these defects trap
hydrogen to be immobilized. In Chap. 8, hydrogen trapping in various defects of
plasma-facing candidate materials are described in detail.
Owing to hydrogen trapping in defects produced by injected hydrogen themselves,
implanted hydrogen diffusion or migration is strongly influenced by the newly introduced defects. Since the depth of the defects induced is a little shallower than the
implanted hydrogen depth, the implanted hydrogen move to the defects resulting
in the depth profile of the implanted hydrogen being shallower than the implanted
profile. Since the amount of trapped hydrogen is far larger than that of dissolved
hydrogen, hydrogen detrapping dominates in hydrogen recycling at lower temperature and wall pumping could continue long. However, trapping decreased with
increasing temperature which makes low recycling difficult for high-temperature
wall.
In a reactor vessel, residual hydrogen gases escaping from plasma are always
existing with pressure P (around 1–100 Pa), and dissolve into PFM following the
Sievert’s law with concentration c given by
c = S · P
1 / 2 ,
(7.1)
where S is the Sievert’s constant with temperature dependence of
S = S 0 exp(−Es,/RT),
(7.2)
where S 0 , Es, R, and T are a constant, heat of solution, gas constant, and
temperature (K).
Although S increases with temperature in most of the metals except hydride
forming ones, H uptake in metals from the residual fuel gas is much less than those
given by energetic H injection except at elevated temperatures. For carbon materials,
owing to their porous nature, molecular hydrogen can penetrate deep and tap on the
surface of crystallites given homogenous depth profile as shown in Sect 9.4.1 and
Fig. 9.2 in Chap. 9.
119
until they are thermalized to the wall temperature. Afterwards they diffuse into deep
or back to the injected surface according to the gradient of chemical potential or
concentration of H in the wall. During the diffusion, some of them are trapped.
Energy loss processes of high energetic hydrogen injected in a metal are twofold,
one is energy loss by excitation of electrons in the metal (electron energy loss, or
electron stopping), the other is nuclear collision (nuclear stopping). If the energy
of the hydrogen particles is high enough to displace a lattice atom, the lattice atom
is displaced remaining a Frenkel pair, i.e. an interstitial atom and a vacancy. If
the incident energy is not large, the nuclear collision occurs at surface resulting in
sputtering and surface recession. Most of the Frenkel pars are recombined. However,
some remain as defects and defect clusters, such as stacking fault, interstitial lops,
vacancy clusters, vacancy loops, and so on (see Sect. 4.2). Usually, these defects trap
hydrogen to be immobilized. In Chap. 8, hydrogen trapping in various defects of
plasma-facing candidate materials are described in detail.
Owing to hydrogen trapping in defects produced by injected hydrogen themselves,
implanted hydrogen diffusion or migration is strongly influenced by the newly introduced defects. Since the depth of the defects induced is a little shallower than the
implanted hydrogen depth, the implanted hydrogen move to the defects resulting
in the depth profile of the implanted hydrogen being shallower than the implanted
profile. Since the amount of trapped hydrogen is far larger than that of dissolved
hydrogen, hydrogen detrapping dominates in hydrogen recycling at lower temperature and wall pumping could continue long. However, trapping decreased with
increasing temperature which makes low recycling difficult for high-temperature
wall.
In a reactor vessel, residual hydrogen gases escaping from plasma are always
existing with pressure P (around 1–100 Pa), and dissolve into PFM following the
Sievert’s law with concentration c given by
c = S · P
1 / 2 ,
(7.1)
where S is the Sievert’s constant with temperature dependence of
S = S 0 exp(−Es,/RT),
(7.2)
where S 0 , Es, R, and T are a constant, heat of solution, gas constant, and
temperature (K).
Although S increases with temperature in most of the metals except hydride
forming ones, H uptake in metals from the residual fuel gas is much less than those
given by energetic H injection except at elevated temperatures. For carbon materials,
owing to their porous nature, molecular hydrogen can penetrate deep and tap on the
surface of crystallites given homogenous depth profile as shown in Sect 9.4.1 and
Fig. 9.2 in Chap. 9.