Environmental Engineering Reference
In-Depth Information
This formula makes clear the danger posed by possible W contamination by
tungsten walls. Since W has
Z
74, with
Z
i
¼
5476), entering Equation 4.38 if W
ions should be eroded from the wall and end up in the plasma, these ions, even in
small amounts, would cool the plasma. This makes graphite an attractive wall
material, with
Z
only 6.
It is not clear what fraction of this radiationmight be re
ected back by the walls into
the plasma, because of its very short wavelength. This energy is probably more likely
absorbed by the walls, making it part of the useful power output, but its nature makes
it more likely to reduce the temperature of the plasma, than do the fusion events,
which actually heat the plasma by thermalization of the trapped energetic charged
fusion products.
The distribution of this radiation peaks near
hf
¼
¼
0.2
T
¼
2586 eV, corresponding to
gamma ray photons of wavelength 0.48 nm, where
T
is the particle energy in eV. The
frequency of this emitted acceleration radiation is far above the electron plasma
frequency
N
D
e
2
1
=
2
v
p
¼ð
=
m
e
e
0
Þ
;
ð
:
Þ
4
39
10
11
rad/s for this case, corresponding to 89.7 GHz. The
blanket in the Tokamak intercepts this radiation and turns it into heat, protecting the
superconducting coils from the damage that the gamma rays might in
ict.
For this DT plasma at 10
20
m
3
, the Debye screening length
l
D
, discussed in
conjunction with Equation 3.71a, is 84.5
which works out to 5.63
. The relation of the Tokamak plasma to the
sun and other cases was summarized in Figure 3.18.
It should be pointed out that the power density of the plasma is adjustable from the
D,Tdensity. Going from 10
20
to 2
m
10
20
will increase the power by a factor of 4. So,
the difference between the full torus volume and the hot plasma volume, perhaps a
factor of 2 in effective volume and power output, can easily be recouped by increasing
the deuteron density.
4.3.4
Summary, a Correction, and Further Comments
In our desire to make an opaque subject clearer to the nonexpert, we have focused on
the essential parts of the nanophysics of fusion. The cross section for electron
scattering from the ions, related to the resistivity of the plasma, it turns out, has been
treated too simply, and needs a correction factor, which increases the plasma
resistance. The correction factor known as ln(
) multiplies the strong scattering
cross section
p r
2
, whichwhenmultiplied by
T
Gamow
is the fusion cross section. In the
scattering of electrons from ions, the cumulative effect of many small-angle scat-
tering events make this substantial correction. This occurs because the Coulomb
force has a long range. The correction does not apply to fusion, but only to the plasma
resistance, because in that case the strong scattering is the only way to set up the
fusion event, an accumulation of weak scattering events is of no use. The importance
of the small scattering events is controlled by the Debye length,
L
