Environmental Engineering Reference
In-Depth Information
Figure 4.10 Poloidal magnetic field [52] due to the combined toroidal plasma current and vertical
field.
6
Li
4
He
þ
n
!
þ
T
ð
4
:
37
Þ
7
Li
4
He
þ
!
þ
þ
n
T
n
In practice, the blanket can be maintained as a liquid, for example, as a Pb
þ
Li
mixture, or as a solid ceramic, such as Li
4
SiO
4
,Li
2
ZrO
3
,orLi
2
TiO
3
.
Figures 4.9 and 4.10 indicate that there is a temperature distribution in the plasma
and it is necessary that the
first walls be maintained safely far below their melting
temperatures. For reference, graphite and tungsten are viable up to the vicinity of
3000 K, but the plasma as we have seen is 150 million K. The heat radial transfer by
particle diffusion from the plasma toward the walls is inhibited by the magnetic
con
nement discussed above. The plasma has no neutral particles, and all charged
particles are magnetically con
ned to move principally in the toroidal direction,
around the circle, and this greatly diminishes the ordinary heat
flow by diffusion, in
the radial direction.
Radiation transfer from the plasma to the walls is important, even though we
ignored it in our simple estimate of the necessary heating power, Equation 4.26. Since
the DTplasma is simple, containing only electrons and singly positive deuteron and
triton positive ions, it does not emit black body radiation as we might suppose. The
radiation fromthe plasma is known as
bremsstrahlung
(acceleration radiation) and can
be estimated as follows.
The radiated power density
P
Brem
per cubic meter from
bremsstrahlung
is sum-
marized [51] for a plasma of equal numbers
N
D
of electrons and ions of charge
Z
i
.At
thermal energy
T
(in eV), it is
2
W
Z
i
N
i
N
e
T
1
=
2
10
18
m
3
P
Brem
¼
=ð
7
:
69
Þ
=
ð
4
:
38
Þ
for
N
i
ions of charge
Z
i
and
N
e
electrons. If we evaluate this for the Tokamak plasma,
where
Z
i
¼
1, with equal numbers of deuterons and electrons, we get
10
20
2
1
=
2
10
18
2
W
m
3
m
3
P
Brem
¼ð
Þ
ð
12930
Þ
=ð
7
:
69
Þ
=
¼
19
:
23 kW
=
:
ð
4
:
38a
Þ
This radiation density
P
Brem
is fortunately quite small compared to the fusion
power density for the case we are considering (for the DT case) that is 634MW/
500m
3
1268 kW/m
3
.
¼
