110
6 Material Modification by High-Power Load …
alloys. Since the volume expansion coefficient of W is much less than these materials,
adhesion in the joining is not easy. Furthermore, cyclic heat load or heat shock could
destroy the joint. To make ITER divertor with W armors on a heat sink material is
one of the most difficult engineering tasks. Carbon materials, if they will be used
as PFM, have the similar problems in brazing to the heat sink material. In addition,
the carbon materials will swell or expand volumetrically due to neutron irradiation,
which also gives large stress on the joint.
Because of concerns on large T retention, C was excluded to use in ITER. Nevertheless, the use of carbon tiles as armor is beneficial to reduce T permeation in
structure materials comparing to W armor, because most of the injected hydrogen
will retain in near surface and not migrate deep [26].
6.4 Summary
The power load to PFS appears as plasma materials interactions (PMI), which are
similar to what the surface of a rocket running into the sun would be exposed to and,
at present, the physical and chemical phenomena expected in PMI in a fusion reactor
would be very difficult to study directly.
In this chapter, after introduction of estimated power load to PFM in a fusion
reactor,
material response to the power load which influences plasma is summarized.
Spontaneous response of PFM to the power load appears as emission of particles
and photons, including sputtered atoms and ions of PFM and surface impurities,
sublimated atoms, secondary electrons and photons, and radiation from PFM. Among
all, sputtered materials would give significant influence on plasma.
Owing to power load given by the energetic photons and particles, surface and
subsurface of PFM are modified or damaged, sometimes resulting in surface melting
or massive sublimation. Although these surface modifications are not spontaneous,
they gradually change materials properties which in turn changes response to the
power load of PFM. Since the power load is given by energetic photons (radiation) and
energetic fuel (H) particles, fuel recycling between plasma and PFM also significantly
influence plasma confinement. The material modification influences behavior of fuel
in PFM, resulting in changes in fuel recycling. Hence, material response to the
power load is synergistic of heating, modification or damaging of PFM, and the fuel
behavior, which makes understanding of PMI quite difficult.
Among presently operating plasma apparatus, only JET can realize similar or a
little lower levels of power load, therefore, understanding of PMI in a fusion reactor
must be extrapolated from observations on the currently operating apparatus and
laboratory experiments for basic understanding of individual processes of damaging
and hydrogen recycling.
6 Material Modification by High-Power Load …
alloys. Since the volume expansion coefficient of W is much less than these materials,
adhesion in the joining is not easy. Furthermore, cyclic heat load or heat shock could
destroy the joint. To make ITER divertor with W armors on a heat sink material is
one of the most difficult engineering tasks. Carbon materials, if they will be used
as PFM, have the similar problems in brazing to the heat sink material. In addition,
the carbon materials will swell or expand volumetrically due to neutron irradiation,
which also gives large stress on the joint.
Because of concerns on large T retention, C was excluded to use in ITER. Nevertheless, the use of carbon tiles as armor is beneficial to reduce T permeation in
structure materials comparing to W armor, because most of the injected hydrogen
will retain in near surface and not migrate deep [26].
6.4 Summary
The power load to PFS appears as plasma materials interactions (PMI), which are
similar to what the surface of a rocket running into the sun would be exposed to and,
at present, the physical and chemical phenomena expected in PMI in a fusion reactor
would be very difficult to study directly.
In this chapter, after introduction of estimated power load to PFM in a fusion
reactor,
material response to the power load which influences plasma is summarized.
Spontaneous response of PFM to the power load appears as emission of particles
and photons, including sputtered atoms and ions of PFM and surface impurities,
sublimated atoms, secondary electrons and photons, and radiation from PFM. Among
all, sputtered materials would give significant influence on plasma.
Owing to power load given by the energetic photons and particles, surface and
subsurface of PFM are modified or damaged, sometimes resulting in surface melting
or massive sublimation. Although these surface modifications are not spontaneous,
they gradually change materials properties which in turn changes response to the
power load of PFM. Since the power load is given by energetic photons (radiation) and
energetic fuel (H) particles, fuel recycling between plasma and PFM also significantly
influence plasma confinement. The material modification influences behavior of fuel
in PFM, resulting in changes in fuel recycling. Hence, material response to the
power load is synergistic of heating, modification or damaging of PFM, and the fuel
behavior, which makes understanding of PMI quite difficult.
Among presently operating plasma apparatus, only JET can realize similar or a
little lower levels of power load, therefore, understanding of PMI in a fusion reactor
must be extrapolated from observations on the currently operating apparatus and
laboratory experiments for basic understanding of individual processes of damaging
and hydrogen recycling.