6
1 Introduction
plasma confined to satisfy the Lawson condition. The initial input power would be
1/3 to 1/4 of the fusion output power. Since a fusion reactor will be designed to
produce the power of a few GW, each reactor may require a power station with the
power of a few hundred MW to start up. In a fission reactor, no such high power is
required, and only removing control rods from the reactor core starts fission reactions
and continue steady-state burning.
Energy conversion systems for fission and fusion are also completely different.
In a fission reactor, the energy produced by the fission reaction of an uranium atom
(U) and a neutron is carried mainly by fission products (FPs) and transformed to
thermal heat of coolant for electric power generation, while, in a fusion reactor,
the energy carried by 14 MeV neutrons must be converted to the thermal heat of
the coolant in the blanket. At the same time, the 14 MeV neutron is used to breed
T to sustain fuel self-sufficiency as described below. Both fission and fusion leave
nuclear wastes. Compared to long-life nuclear wastes in fission including FPs and
trans-uranium elements like U, Np, and Pu, which are serious concerns for radiation
safety, activated structure materials in a fusion reactor by neutron irradiation are less
hazardous and even they could be re-used.
Any power sources require power removal through cooling system to convert their
generated energy to heat or electricity. In a fusion reactor, nearly 1/4 of generated
fusion power is used to sustain burning plasma (either initial heating power from
outside and heating power given by He) and must be removed or recovered. Except
energy carried by the 14 MeV neutrons, all power used for plasma heating is loaded to
plasma-facing materials (PFM). Divertor is introduced to remove such high-power
load and He ash. Still the tolerance of PFM to the power load is concerned. PFS
of the main chamber is also exposed to the significant power load. Depending on
the location of the plasma-facing components, power loads significantly differ. Midplane of the central pole (inner first wall) would be the highest except divertor target
area.
Power load is given to PFM by radiation or energetic photons, and energetic
particles including tritons, deuterons, neutrons, helium ions, and electrons escaping
from boundary plasma in addition to neutrals produced by charge exchange. The
radiation consists of the Bremsstrahlung emission from burning plasma, and radiation
from impurities in plasma and seeded gas required for cooling the boundary plasma.
The power load to PFM is so high that no simple material can tolerate without active
cooling. Still there is a limit in the removal of the loaded power. Accordingly, the
maximum of the power load to materials having a high melting point is limited to
10–20 MW m
−2 under efficient cooling.
The main subjects of this book entitled “Plasma wall interactions in a fusion
reactor” are to introduce/discuss responses of PFS to the power load and modification
of PFM by the power load, both of which significantly influence the performance of
burning plasma.
In the early days of PMI studies, when the first proceedings of the International
Symposium on Plasma Wall Interaction were published in 1977 [1], the main interest
was focused on material response to the injection of high-energy ions appearing as
sputtering, and little works had done on high-power load, because confined energy
1 Introduction
plasma confined to satisfy the Lawson condition. The initial input power would be
1/3 to 1/4 of the fusion output power. Since a fusion reactor will be designed to
produce the power of a few GW, each reactor may require a power station with the
power of a few hundred MW to start up. In a fission reactor, no such high power is
required, and only removing control rods from the reactor core starts fission reactions
and continue steady-state burning.
Energy conversion systems for fission and fusion are also completely different.
In a fission reactor, the energy produced by the fission reaction of an uranium atom
(U) and a neutron is carried mainly by fission products (FPs) and transformed to
thermal heat of coolant for electric power generation, while, in a fusion reactor,
the energy carried by 14 MeV neutrons must be converted to the thermal heat of
the coolant in the blanket. At the same time, the 14 MeV neutron is used to breed
T to sustain fuel self-sufficiency as described below. Both fission and fusion leave
nuclear wastes. Compared to long-life nuclear wastes in fission including FPs and
trans-uranium elements like U, Np, and Pu, which are serious concerns for radiation
safety, activated structure materials in a fusion reactor by neutron irradiation are less
hazardous and even they could be re-used.
Any power sources require power removal through cooling system to convert their
generated energy to heat or electricity. In a fusion reactor, nearly 1/4 of generated
fusion power is used to sustain burning plasma (either initial heating power from
outside and heating power given by He) and must be removed or recovered. Except
energy carried by the 14 MeV neutrons, all power used for plasma heating is loaded to
plasma-facing materials (PFM). Divertor is introduced to remove such high-power
load and He ash. Still the tolerance of PFM to the power load is concerned. PFS
of the main chamber is also exposed to the significant power load. Depending on
the location of the plasma-facing components, power loads significantly differ. Midplane of the central pole (inner first wall) would be the highest except divertor target
area.
Power load is given to PFM by radiation or energetic photons, and energetic
particles including tritons, deuterons, neutrons, helium ions, and electrons escaping
from boundary plasma in addition to neutrals produced by charge exchange. The
radiation consists of the Bremsstrahlung emission from burning plasma, and radiation
from impurities in plasma and seeded gas required for cooling the boundary plasma.
The power load to PFM is so high that no simple material can tolerate without active
cooling. Still there is a limit in the removal of the loaded power. Accordingly, the
maximum of the power load to materials having a high melting point is limited to
10–20 MW m
−2 under efficient cooling.
The main subjects of this book entitled “Plasma wall interactions in a fusion
reactor” are to introduce/discuss responses of PFS to the power load and modification
of PFM by the power load, both of which significantly influence the performance of
burning plasma.
In the early days of PMI studies, when the first proceedings of the International
Symposium on Plasma Wall Interaction were published in 1977 [1], the main interest
was focused on material response to the injection of high-energy ions appearing as
sputtering, and little works had done on high-power load, because confined energy