Page 224 - Fundamentals of Magnetic Thermonuclear Reactor Design

206 Fundamentals of Magnetic Thermonuclear Reactor Design

is the ability to ‘absorb’ incident molecules with a probability equal to the cap-
ture coefficient of the substituted sorbent, and that attributed with respect to the
sorbent itself is the ability to ‘absorb’ molecules hitting the sorbent from inside
with a unitary probability.
Equivalent surfaces are also attributed the ability to ‘emit’ molecular flows
that are divided, according to whether they are treated with respect to the cham-
ber or the sorbent, into a flow identical to one coming out of the substituted
sorbent, and a flow identical to that coming to the surface from the chamber.
Generally, the equivalent surface is a virtual surface at the interface of two
regions of a system under design, which is attributed different characteristic
functions depending on the region it is assigned to. This approach allows one
to deal with a system’s simplified simulation (containing actual functional ele-
ments and a number of equivalent surfaces) rather than an actual system at the
first stage of the design process. Then, the model’s molecular characteristics are
determined and compared with initial ones, taken to be used when establishing
equivalent surfaces. If there are significant differences between the two, the
flows in the substitutes are analysed prior to the second iteration step. Iteration
proceeds until the prescribed convergence between the calculated and experi-
mental molecular flow densities is reached. This method enables an optimisa-
tion design of a VS of a virtually infinite structural and geometrical complexity.
To estimate the gas-kinetic conductance of a vacuum line with a discretely
varying profile, we take structures containing ‘elemental’ channels connected in
series with tabulated conductance values, as well as intermediate chambers with
axial length decreasing towards zero as a limit state design.
Let us now discuss the principles and methods of a comprehensive energy
performance and structure/parameter optimisation of surface-action pumps,
which are the most important means of ensuring the required vacuum param-
eters and efficient operation of an MFR high-vacuum system. These principles
and methods are based on the concept of active centres.
The term ‘active centre’ refers to a micro-region on a sorbing surface that
is accessible to incident gas molecules. The interaction between incident mol-
ecules and active centres, which results in the ‘neutralisation’ of the latter, is
described by the statistically determinate parameters η (the probability of irre-
versible absorption by the active centre of a molecule that has entered its field
of force), and ϑ (the probability of any molecule to reach this field of force).
The concept of active centres as a generalised model of a molecule-sorb-
ing surface interaction has straightforward physical analogies (Table 6.7).
Cryogenic pumps’ active centres exist as long as the required temperature of
the absorbing surface is maintained by means of a cryogenerator. In adsorption,
chemisorption and implantation pumps’ active centres are formed naturally or
artificially before pumping begins and are ‘spent’ (neutralised) during pumping
at a rate proportional to the incident molecular flux.
In pumps using continuously regenerated getter films, the flow of active cen-
tres is reproduced by a vaporiser (atomiser) [16]. Thus, a pump or some other