Page 136 - Fundamentals of Magnetic Thermonuclear Reactor Design

120 Fundamentals of Magnetic Thermonuclear Reactor Design

The BCS theory extend this behaviour model to a multi-electron system
with the so-called Cooper pairs, a pair of electrons bound together and interact-
ing in a space of length from ∼100 to several nanometres, are responsible for
superconductivity, as described in the BCS theory. Note, by the way, that the
space is filled with many similar pairs of electrons. Under certain conditions, a
Cooper pair of electrons has a lower energy than a pair of ‘unbound’ electrons.
Cooper pairs are attracted to each other through an indirect energy exchange
(via crystal lattice ions).
The electron–phonon mechanism ‘forces’ all conduction electrons to be-
have as a single entity, allowing them to get through the crystal lattice layers
without energy losses, that is, without resistance.
The BCS theory extend this behaviour model to a multi-electron system
with an assumption that the interaction between Cooper paired electrons is all
that matters for superconductivity. Other electrons only limit the number of
states in which Cooper pairs can scatter. The BCS theory derived several impor-
tant predictions with respect to SC properties.
An important parameter of an individual SC is an energy gap—a binding
energy per one electron. The binding energy has a maximum at absolute zero
and decreases monotonously with increasing temperature down to zero at point
T . The larger the gap, the higher the SC’s critical temperature.
C
The development of commercial low-temperature SCs paved the way for the
establishment of technical superconductivity as an applied science and an in-
dustry with a great potential for civilisation. The superconductivity applications
cover the production of superconducting wires (strands) and cables, supercon-
ducting magnetic systems and other precision equipment operated at cryogenic
temperatures.
By the last quarter of the 20th century, a number of countries had created a
research and manufacturing basis for the design and serial production of com-
mercial superconducting systems—for developing new superconducting com-
positions, strands and cables. Today, we know of more than 1000 superconduct-
ing compositions based on alloys, intermetallic materials and metal chemical
compounds. These include V Ga (Т = 4.8 K), Nb Sn (Т = 18.0 K), Nb Ge
С
С
3
3
3
(Т = 23.3 K), Nb Al (Т = 18.9 K) and the NbTi solid solution (Т = 10.0 K).
С
С
С
3
Since the 1960s, based on technical and economic indicators, NbTi and Nb Sn
3
have been the materials of choice for commercial SCs. Their total global produc-
tion exceeded 4000 t in 2013 and continues to grow. A powerful impetus for in-
novation in this area was provided by the world-famous ITER project (Table 5.1).
Russia is one of the leaders in superconductivity research and manufacture.
The Bochvar Research Institute of Inorganic Materials (Moscow, Russian acro-
nym: VNIINM) has been developing and manufacturing NbTi and Nb Sn multi-
3
filament superconducting strands for more than four decades.
These strands were used in the former USSR to manufacture world-class
winding SCs for the T-7 [1] and T-15 tokamaks, as well as the CS-250, the
world’s largest combined solenoid with a magnetic field of 25 T, and the UNK