6.2 Material Response to Power Load …
99
finding motivated to use high Z materials after long period of exclusion of them as
PFM in tokamaks. Now, ITER has decided to use W as armor tiles of divertor.
In large tokamaks, the power load to PFS is so large that huge amount of PFM
could be sublimated. Accordingly, Carbon blooms or Beryllium blooms as appeared
in high-power discharges in JET and TFTR as discussed in Sect. 6.3.1.
It should be also noted that huge amount of vapor in front of the PFS could shield
succeeding plasma exposure to reduce the power load, which is referred to as vapor
shielding and is expected to reduce power load to the divertor. The vapor shielding
by sublimation of target materials is confirmed in linear plasma machines [19, 20],
while plasma detachment near divertor target is caused by high compression or gas
puffing in the divertor region. Impurity seeding like N and Ne to reduce the power
load to divertor is extensively studied to use in ITER divertor [21].
6.2.3 Hydrogen Recycling
As described in Chap. 7 in detail, behavior of hydrogen injected in PFM changes
with temperature. Most influential phenomena on plasma performance is reduction
of hydrogen retention with temperature rise caused by power load. Because of very
large incident hydrogen flux to PFM in a reactor, hydrogen concentration in nearsurface region of PFM would be easily saturated. Since the saturation concentration
decreases with temperature, the temperature rise enhances hydrogen release from
PFM. Different from current plasma devices which are mostly operated at RT or
ambient temperature, the temperature of PFM of a fusion reactor in operation should
be around 700-800 K, and hydrogen moves in PFM rapidly. Therefore, even small
PFM temperature rise during plasma discharge results in release of significant amount
of hydrogen, which makes density control quite difficult and sometimes could lead to
disruption. Another effect is wall saturation. In the current tokamak discharges, wall
pumping, i.e. small hydrogen recycling or injected hydrogen being hardly released,
is preferred to keep good plasma performance, or easy density control by fueling. In
contrast, under higher temperature and high incident flux, hydrogen concentration in
near-surface region of PFM easily comes up to saturation. This kind of wall saturation
or large hydrogen recycling was attained in JT-60U of which first wall was operated
around 520 K as seen in Fig. 6.3 [22, 23]. In a fusion reactor, long pulse or continuous
discharge should be performed under saturated wall. Hence, it is remained as a future
work to keep steady burning plasma with hydrogen saturated wall.
6.3 Damaging and Degradation of PFM
Material damages caused by very high-power loads appear in PFM as softening,
recrystallization, melting, and sublimation in heating phases, and hardening, embrittlement, cracking, and fracture in cooling phases. The material lifetime of plasmafacing components (PFC) is limited mainly by material loss by erosion and the
99
finding motivated to use high Z materials after long period of exclusion of them as
PFM in tokamaks. Now, ITER has decided to use W as armor tiles of divertor.
In large tokamaks, the power load to PFS is so large that huge amount of PFM
could be sublimated. Accordingly, Carbon blooms or Beryllium blooms as appeared
in high-power discharges in JET and TFTR as discussed in Sect. 6.3.1.
It should be also noted that huge amount of vapor in front of the PFS could shield
succeeding plasma exposure to reduce the power load, which is referred to as vapor
shielding and is expected to reduce power load to the divertor. The vapor shielding
by sublimation of target materials is confirmed in linear plasma machines [19, 20],
while plasma detachment near divertor target is caused by high compression or gas
puffing in the divertor region. Impurity seeding like N and Ne to reduce the power
load to divertor is extensively studied to use in ITER divertor [21].
6.2.3 Hydrogen Recycling
As described in Chap. 7 in detail, behavior of hydrogen injected in PFM changes
with temperature. Most influential phenomena on plasma performance is reduction
of hydrogen retention with temperature rise caused by power load. Because of very
large incident hydrogen flux to PFM in a reactor, hydrogen concentration in nearsurface region of PFM would be easily saturated. Since the saturation concentration
decreases with temperature, the temperature rise enhances hydrogen release from
PFM. Different from current plasma devices which are mostly operated at RT or
ambient temperature, the temperature of PFM of a fusion reactor in operation should
be around 700-800 K, and hydrogen moves in PFM rapidly. Therefore, even small
PFM temperature rise during plasma discharge results in release of significant amount
of hydrogen, which makes density control quite difficult and sometimes could lead to
disruption. Another effect is wall saturation. In the current tokamak discharges, wall
pumping, i.e. small hydrogen recycling or injected hydrogen being hardly released,
is preferred to keep good plasma performance, or easy density control by fueling. In
contrast, under higher temperature and high incident flux, hydrogen concentration in
near-surface region of PFM easily comes up to saturation. This kind of wall saturation
or large hydrogen recycling was attained in JT-60U of which first wall was operated
around 520 K as seen in Fig. 6.3 [22, 23]. In a fusion reactor, long pulse or continuous
discharge should be performed under saturated wall. Hence, it is remained as a future
work to keep steady burning plasma with hydrogen saturated wall.
6.3 Damaging and Degradation of PFM
Material damages caused by very high-power loads appear in PFM as softening,
recrystallization, melting, and sublimation in heating phases, and hardening, embrittlement, cracking, and fracture in cooling phases. The material lifetime of plasmafacing components (PFC) is limited mainly by material loss by erosion and the