Page 208 - Fundamentals of Magnetic Thermonuclear Reactor Design

190 Fundamentals of Magnetic Thermonuclear Reactor Design

TABLE 6.1 Methods for Controlling Plasma Impurities (cont.)
Key strengths and
Physical methods Design/Technical solutions weaknesses
Sputtering and first Division of the plasma sealing Structural complexity;
wall thermal loading and the plasma corpuscular and need for frequent
elimination by a radiant energy flow resistance screen replacement.
mechanical screening functions between the vacuum Operational and
envelope itself and one of the structural complexity
following screen types: Operational and
materials with low atomic structural complexity
number,
screen in the form of jet streams
of liquid metal or continuous
screen consisting of balls,
a ‘sweating’ porous metal wall,
through which a lithium-
containing liquid is pressed
through.
Reduction of the Stochastisation of edge Structurally more
energy of particles magnetic field using helical complex magnetic
bombarding the first coils. system.
wall by speeding
up the peripheral
plasma diffusion (the
‘turbulent plasma
blanket’)

plasma chamber to the divertor chamber (DC), as shown in Fig. 6.2. In the DC,
the ions get neutralised, with generated neutral gas partially evacuated. Another
part of the gas goes back to the plasma chamber via a divertor channel.
There are counter flows of plasma and atoms in the divertor channel; the
result being the neutrals’ charge exchange. For this reason, the divertor chan-
nel’s ‘traffic handling capacity’ in respect of the neutral gas leaving the DC and
travelling towards the plasma is different in cases where a plasma flow is there
and where it is absent, therefore
*
⋅
)
Gdc*=(1−R)⋅Gdc, G dc = (1 − R G ,
dc
*
Gdc* where G is the divertor channel conductivity with respect to the neutral gas,
dc
corrected for the latter’s ionisation by the plasma oncoming flow, G is the
dc
divertor channel’s gas kinetic conductivity, and R is the probability of the neu-
trals’ ionisation. At R = 1, the divertor channel is closed for the neutral particles,
*
Gdc/Gdc* so that none can re-enter the plasma (the ‘plasma plug’). Potential G dc G
dc
values are estimated to vary from several units to 100. Because the charge
exchange cross section for He is smaller than that for hydrogen, one can main-
tain high He concentration in the DC and provide a more intensive He pumping