Page 211 - Fundamentals of Magnetic Thermonuclear Reactor Design

Vacuum and Tritium System Chapter | 6 193

FIGURE 6.4 Tokamak vacuum chamber pumping speed necessary to achieve the ‘back-
ground’ level. (I) experimental facilities, (II) ‘large-scale’ tokamaks (III) fusion reactors.

After integration, it takes the following form:
>
cn 2 < συ τ He   1 − R He  
−
n He () =t  1exp  − t  
41 )   τ He   nHet=cn <σ>τHe41−RHe1−ex
2
( − R He
For the values of с, n, συ< > and τ , specified earlier, a 10% He con- <σ> p−1−RHeτHet.
He
centration in the plasma is achieved in around 80 s. External pieces of high-
vacuum pumping equipment may be placed not closer than 10 m from the
chamber. Evacuation ports should be around 2 m across. Gas kinetic conduc-
tivity, P, of any such port, in terms of molecular processes, should be ≤0.2.
Therefore, to achieve the earlier specified He plasma return coefficient, the
evacuation ports should occupy around 60% of the wall area. A reactor with
such a sizeable wall perforation would have unacceptably weak economic and
performance parameters. This highlights once again the need for a method
of He removal that would be more effective than the external high-vacuum
pumping. One possible solution is to use an integrated vacuum pumping duct,
with high-vacuum pump built-in directly in the evacuation ports close to the
divertor chamber [6].
Now, let us make similar estimates for the divertor option. Assuming that
the divertor layer ions are fully captured, we obtain the following approximate
relationships between the plasma characteristics and the vacuum pumping duct
th
parameters for the n concentration of plasma’s j component:
i
nV = R nV + Sn ,
j
j
τ j d τ j ddj njVτj=RdnjVτj+Sdndj,
*
( −1 µ)G dk
R d =
S d +G * dk Rd=1−µGdk*Sd+Gdk*,
where R is the coefficient of plasma as neutral gas returns from the DC back
d
to the plasma, S is the DC pump-down speed, µ is the coefficient of ion
d