Page 198 - Fundamentals of Magnetic Thermonuclear Reactor Design

180 Fundamentals of Magnetic Thermonuclear Reactor Design

The issues of the tritium circuit design and operation fall into the radio-
chemical domain and are beyond the framework of this book.
The vacuum equipment of the VTS is designed to execute two physical and
technological functions: (1) to ensure an ‘oil-less’ ultra-high vacuum (a gas-
eous environment free of carbon–hydrogen and other impurities) in the vacuum
chamber before the beginning of the operating cycle, and (2) to pump fusion
reaction products, part of the fuel mix (neutral hydrogen molecules) and a rela-
tively small flow of heavier gases generated under plasma interaction with the
plasma-facing components out of the chamber during the operating cycle.
The execution of these functions is absolutely necessary for a magnetic
fusion reactor (MFR) to operate effectively, and the Academician Kurchatov’s
comment that ‘no sound fusion reactor can be made without a revolution in
vacuum engineering’ was not in vain.
Many factors influence the design and technological solutions for MFR vac-
uum systems, as well as the latter’s parameters and performance characteristics.
The most important factors are as follows [2]:

l radioactive tritium as one of the main gas load components;
l the changing charge of hydrogen and impurity ions as a result of plasma
interaction with residual gas (recombination, charge exchange, additional
roughing, etc.);
l stimulated processes on chamber walls driven by heat flows, plasma’s elec-
tromagnetic and corpuscular radiation (physical and chemical sputtering,
re-deposition, surface morphological evolution, desorption, etc.);
l structural and functional materials’ activation by plasma neutron fluxes;
l non-uniform and non-stationary temperature fields; and
l localised and distributed gas sources.
These factors may qualitatively alter the MFR vacuum systems, imparting
them with properties unrivaled by other products of the vacuum engineering
industry. A number of plasma processes and the behaviour of the plasma col-
umn itself are critically dependent on seemingly narrow, vacuum-specific char-
acteristics. These include the physical and chemical surface condition of the
plasma-facing reactor elements (the first wall, FW), and the in-chamber ‘thin’
gas continuum, that is, the spatial distribution of the residual gas molecular
concentration. The importance of these factors was realised in the early days of
plasma experimenting.
This is valid not only for MFRs. Progress in elementary particle physics,
including the successful clashing beam experiments, would not have been pos-
sible without the introduction of ultra-high vacuum processes. Incidentally,
they are no less important for areas such as electronics, thin-film technologies,
surface physics, research machine engineering and many other sectors of sci-
ence and industry.
There is also an important economic point. Present-day electric and physical
engineering R&D centres need thousands of pieces of vacuum equipment, and