Page 206 - Fundamentals of Magnetic Thermonuclear Reactor Design

188 Fundamentals of Magnetic Thermonuclear Reactor Design

and τ = 2 s, then R = 0.88. This means that one only needs to pump
He
He
out 1 − R = 0.12 of He atoms incident on the wall to achieve a 10% He
He
relative concentration in the plasma. If there is a better He confinement, for
example, τ = 10 s, the proportion of pumped out He should increase to 62%.
He
At τ ≈ 16 s, all He atoms incident on the wall should be pumped out, and
He
where τ is greater, it is impossible to achieve a 10% He concentration with
He
any pumping equipment [6].
As the speed of protium formation is approximately 100× lower than that of
He formation, protium accumulation in the plasma only leads to a fusion power
3
decrease at cycles longer than 10 s. It is therefore necessary to equip long-
cycle reactors with pumping systems allowing protium to be evacuated at a rate
two orders of magnitude slower than He pumping rate.
Impurities getting into the plasma as part of a material’s thermal gas emis-
sion are minimised by wall conditioning. Hydrogen isotopes injected into the
chamber are subjected to a pre-treatment, after which the concentration of heavy
and light gases is within 0.001% and 0.1%, respectively [12].
Impurities coming from the shared vacuum pumping duct are brought down
to an acceptable level using oil-free pumps, differential pumping systems, as
well as ultra-high vacuum technology and processes [5].
Impurity gases coming through uncontrolled leaks in welded joints are
generally put at 10% of the desorption flow from the walls. It is important to
emphasise here that special attention should be given to an appropriate sealing
of the vacuum pumping duct at every stage of reactor design, manufacturing
and operation. The high priority of this task emanates from the tritium safety
considerations. The most vulnerable part with respect to safety issues are the
water cooling channels. Notably, a rather exotic laser scanning of water-coolant
seal faces allowing optical detection of leaking water vapours was considered as
a candidate method for detecting potential leaks under the ITER project. Leak
detection by this technique should allow urgent measures to be taken to prevent
a reactor emergency shutdown.

6.4.2 Impurity Control Methods: The Magnetic Divertor
Among the possible methods that can be used to control impurity genera-
tion in a fusion device, the magnetic divertor is believed to be the most
efficient. (Table 6.1). The pump limiter is also believed to be promising.
Although inferior to magnetic divertor in many respects, the pump limiter
offers the indisputable advantage of structural simplicity. Other methods
enable the control of impurity flows, but cannot help remove synthesised He
from plasma.
To understand the physics behind the divertor and pump limiter operation,
let us remember that impurity ions accumulate in the plasma boundary layer. A
divertor, producing a local deformation of magnetic lines covering the bound-
ary layer, allows the removal of in-layer ions, including impurity ions, from the