3.2 Estimation of Power Load and Its Distribution in a Fusion Reactor
41
is much shorter than that given on the spaceplane. Accordingly, the transient power
load in a fusion reactor becomes much higher and easily causes ablation or material
damages.
Materials’ tolerance to such high-power load depends on their thermo-physical
properties, such as thermal conductivity, melting point, heat capacity, thermal shock
resistance, and so on as described in Chap. 4. In addition, these material parameters
are degraded by power loads especially neutron load.
Candidate PFM having the highest melting point can tolerate the power load of
about 20 MW m
−2 under intensive cooling but not more. Therefore, two critical
issues are remaining, one is the reduction of the power load to PFS to be less than
their tolerant level by cooling boundary plasma, and the other is the development or
selection of PFM. At present, even for the highest melting temperature materials, W
and C being used as armor tiles, the power load to the armor tiles in a fusion reactor
seems high enough to damage them. Plasma detachment with high divertor pressure
and impurity seeding [4] to enhance radiation cooling is encouraged. Still, they are
not enough to ensure the robustness of the divertor.
Power load by particle fluxes are another concern. Suppose 1/3 of the power flux
of 1 MW/m
2 was carried by fuel particles with an energy of around 100 eV, fuel
particle fluxes would be 6 × 10
22 m
2 s
−1 . Based on the power load given in Fig. 3.1,
the fuel particle flux to the first wall and divertor plate would be 10
23 and 10
24 m
2
s
−1 , respectively. Compared to the surface atomic density of a solid material (around
10
19 m
−2 ), at least every 10
−4 s to 10
−5 s, one plasma fuel particle passes nearby
or collides with a surface atom. If the surface atom gets energy from the plasma
particles by collision, it takes a millisecond or more to be relaxed. This means that
all surface atoms are not in thermal equilibrium with bulk temperature but at excited
or higher temperature states. Furthermore, such excited atoms or higher temperature
atoms could emit photons (radiation) to be relaxed. Therefore, the mitigation of the
power load to the divertor plates by particles is also mandatory. Power exhaust from
burning plasma and mitigation of power load to PFS are the most important targets
in recent PMI researches.
3.3 Steady-State Power Load
At steady state or normal operation, various particles carry power to PFS: charged
particles including fuel ions, He and other impurity ions, energetic neutrals produced
by charge exchange, electrons, neutrons produced by D-T reactions, and radiation
(photons) from plasma. In addition to the steady power load, periodic power load
caused by saw tooth, blobs, and edge localized mode (ELM) activities are superposed.
These power loads are often very much localized to a specified area, and sometimes
resulting in extremely high-power load in the narrow area. In particular, very large
ELM (often referred to as giant ELM) is concerned [5]. The giant ELM is discussed
as a transient power load in the next section.
41
is much shorter than that given on the spaceplane. Accordingly, the transient power
load in a fusion reactor becomes much higher and easily causes ablation or material
damages.
Materials’ tolerance to such high-power load depends on their thermo-physical
properties, such as thermal conductivity, melting point, heat capacity, thermal shock
resistance, and so on as described in Chap. 4. In addition, these material parameters
are degraded by power loads especially neutron load.
Candidate PFM having the highest melting point can tolerate the power load of
about 20 MW m
−2 under intensive cooling but not more. Therefore, two critical
issues are remaining, one is the reduction of the power load to PFS to be less than
their tolerant level by cooling boundary plasma, and the other is the development or
selection of PFM. At present, even for the highest melting temperature materials, W
and C being used as armor tiles, the power load to the armor tiles in a fusion reactor
seems high enough to damage them. Plasma detachment with high divertor pressure
and impurity seeding [4] to enhance radiation cooling is encouraged. Still, they are
not enough to ensure the robustness of the divertor.
Power load by particle fluxes are another concern. Suppose 1/3 of the power flux
of 1 MW/m
2 was carried by fuel particles with an energy of around 100 eV, fuel
particle fluxes would be 6 × 10
22 m
2 s
−1 . Based on the power load given in Fig. 3.1,
the fuel particle flux to the first wall and divertor plate would be 10
23 and 10
24 m
2
s
−1 , respectively. Compared to the surface atomic density of a solid material (around
10
19 m
−2 ), at least every 10
−4 s to 10
−5 s, one plasma fuel particle passes nearby
or collides with a surface atom. If the surface atom gets energy from the plasma
particles by collision, it takes a millisecond or more to be relaxed. This means that
all surface atoms are not in thermal equilibrium with bulk temperature but at excited
or higher temperature states. Furthermore, such excited atoms or higher temperature
atoms could emit photons (radiation) to be relaxed. Therefore, the mitigation of the
power load to the divertor plates by particles is also mandatory. Power exhaust from
burning plasma and mitigation of power load to PFS are the most important targets
in recent PMI researches.
3.3 Steady-State Power Load
At steady state or normal operation, various particles carry power to PFS: charged
particles including fuel ions, He and other impurity ions, energetic neutrals produced
by charge exchange, electrons, neutrons produced by D-T reactions, and radiation
(photons) from plasma. In addition to the steady power load, periodic power load
caused by saw tooth, blobs, and edge localized mode (ELM) activities are superposed.
These power loads are often very much localized to a specified area, and sometimes
resulting in extremely high-power load in the narrow area. In particular, very large
ELM (often referred to as giant ELM) is concerned [5]. The giant ELM is discussed
as a transient power load in the next section.