Superconductors, an exotic thing yesterday, have become commonplace today, same as semiconductors used to be. For the most part, big hopes are set upon them when electrotechnical equipment is being developed. Superconducting technologies of a new generation have been discussed in a recent publication by N. A. Chemoplekov, Corresponding Member of the Russian Academy of Sciences, Director of the Institute of Superconductivity and Solid-State Physics of the "Kurchatov Institute" Research Center.
The phenomenon of superconductivity was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes. However, it was only 40 years later that low-temperature superconducting technologies were developed on the basis of superconducting materials and cryogenic facilities. These technologies, in their turn, enabled the development of some unique research equipment such as super high-energy accelerators, detectors for nuclear and elementary- particle physics, laboratory magnetic systems, supersensitive and high-precision instruments, the equipment which has become the object of immediate commercial interest, something that deserves special mention.
At this point we get either products of an entirely new quality like, for example, a magnetic resonance imaging scanner (a NMR tomograph) and a high-gradient magnetic separator or entirely new equipment, such as superconducting energy inductive storing devices with an unlimited period of storage.
Dr. Chemoplekov also dwelled on the superconductors' physical properties of which there are four. First, under certain conditions (when the strength of the magnetic field is lower than the so-called Meissner* value) superconductors possess ideal diamagnetism, i.e. the magnetic field does not penetrate into the conductor. Secondly, the magnetic momentum of geometric bodies like a superconducting ring or a hollow cylinder, can change by the value of a magnetic flux quantum equal to 2x 10 -7 G x cm 2 . The third very important property is the following: superconductors' surface resistance at under-critical frequencies is 10-100 times lower than that of the well-conducting metals (copper, aluminum) at the same temperatures. Finally, due to the Josephson effect**, the current can flow in them without voltage drop through a tunnel contact formed by two superconductors separated by a thin dielectric layer.
The first two properties provide the basis for high-current superconducting technologies meant for manufacturing high power and accumulated energy devices (condensers). Such "monsters" are used in power industry, and slightly modified ones-in other branches of industry and transport.
The other properties have become applicable to the low-current superconducting technologies used in the manufacture of telecommunication facilities, scientific and medical equipment and in the precision tool making.
The high-current trend in the superconducting technologies began to grow rapidly in the mid-1970s, with the USSR and the USA being in the lead. Our country launched the industrial production of the appropriate materials and cryogenic equipment operating at liquid helium temperature (below -270 0 C). Academic and specialized institutes worked hard on the devices themselves - from laboratory magnets for scientific research in physics, chemistry and biology to large industrial-grade magnetic containment equipment for hot thermonuclear plasma or high power pulsed power sources on the basis of inductive storage.
The basis of the superconducting materials are two substances. First, a deformable niobium-titanium alloy with the following parameters: 9.6 К critical temperature at zero magnetic field and current; 12 Tl critical magnetic field at 4.2 К (this is the boiling point of liquid helium at the normal pressure), zero current and critical current density equal to 3x10 9 A x m -2 at 4.2 К and 5 Tl magnetic field.
*The Meissner Effect-displacement of magnetic field from material upon its transition into superconducting state.- Ed.
**The Josephson Effect - the superconducting current Hows through a thin (about 10 A) dielectric layer between two superconductors. - Ed.
The second superconductor developed by the industry later became the intermetallic compound of niobium and zinc. It broadened slightly the range of operating temperatures and magnetic fields of superconductor devices. The material based on the above compound had 18.3 К critical temperature at zero magnetic field and current, and critical magnetic field was about 22 Tl at 4.2 К and zero current.
The superconductors in their essence are composite structures made of heterogeneous materials with superthin (up to micron fractions) filaments, the production of which was launched by the USSR, the USA, Japan, Germany and other developed countries. Their characteristics covered all the expected requirements for electric and power generating equipment.
As far as applied superconductivity is concerned, research in this field began in the mid-1960s. And innovations poured down as from a horn of plenty They were superconducting modifications of nearly all the main facilities, which generate, transmit, convert and consume electric energy on an industrial scale. A case in point was the Russian programe on strong-current superconducting technology of liquid helium temperature range for electric engineering and electric power production.
Since then research scientists have managed to develop and test models and production prototypes of superconducting turbine generators with capacities of 1 to 20 MW or even 300 MW. Also placed into operation have been brush and homopolar* (otherwise they are called unipolar) generators with the capacity of up to 10 Mw, transformers, current limiters, flexible and rigid power transmission lines, inductive energy storing devices and so forth.
The development of non-traditional kinds of electric equipment involved a broad range of associated research and the development of fundamentally new methods of electromagnetic, mechanic, thermophysical and hydrodynamic analysis accompanied by their verification on models. As a result, three types of superconducting devices with liquid helium range temperatures got their "lease of life". They were: magnetic resonance (NMR) tomographs with superconducting magnets, superconducting separators and low-inductive energy storing devices.
The production of NMR tomographs started in the 1980s. Thanks to their high-quality diagnostics due to high induction of the magnetic field, time stability and space homogeneity of the so- called trapped magnetic field of the superconducting solenoid, they gained much ground by the mid-1990s from the cheaper tomographs with resistive or permanent magnets. Nowadays the annual output of superconducting magnetic resonance devices amounts to about 1,000 pieces, and the profit from their sales has already exceeded 2 bin dollars a year.
As regards the other two devices, they are only making first steps on their way of development. Say, in Russia a spatial gradient magnetic separator for dressing of low-grade ferruginous quartzite has been developed.
On the whole the forty-year development work on the superconducting technology in the liquid helium temperature range has resulted in some unique electric physical units. However, all of these things have been unable to change essentially the face of the industrial power production.
The situation changed radically in 1986 when high-temperature superconductors (HTSC) were discovered, their critical temperatures exceeded considerably the nitrogen boiling point (77.3 К) at normal pressure, which seemingly made it possible to use the latter as a cooling agent instead of liquid helium which is non-renewable and expensive. By the mid-1990s HTSC first- generation conductors were developed and their pilot-scale production was launched in the USA, Japan and number of European countries, Russia including. Such conductors are mainly produced by the so-called "powder-in-tube" (PIT)
* Homopolar engine (generator) - a brushless direct-current electric machine, the operation of which is based on the phenomenon of unipolar induction, i.e. the appearance of emf (electromotive force) of induction in the magnetized body which moves at a certain angle with respect to the axis of magnetism.- Ed.
method. During the thermal, mechanic and chemical processing of a silver tube, filled with the powder-like HTSC, a tape with 4х0.3 mm 2 in section and up to 1,000 m long is being formed. It is based on the compound of five components (bismuth, strontium, calcium, copper and oxygen), packed up, as was said above, into the silver casing. Notwithstanding the comparably low critical temperature of this conductor (about 90 К), its technological properties allow to attain high density of the critical current in strong magnetic fields at 20-30 К temperatures, thus exceeding the potentialities inherent in the materials developed hitherto at 4.2 К temperature.
At the same time cryogenic technology was being developed. There appeared compact microcoolers which possess a large resource of non-stop operation. Their reliability approaches that of the domestic refrigerator. Technological achievements in this field have made possible broad applications of NMR tomographs of liquid helium temperature range in clinics, promoted the development of the first industrial-grade superconducting separators and small inductive energy storing devices for systems ensuring the continuity of energy supply to the main consumers.
Today strong-current superconducting technologies have reached the level which opens up the way to the development of electric power equipment of a new generation. This equipment will excel the traditional one by its higher efficiency, twice or three times lesser mass and overall dimensions and, consequently, the specific consumption of materials, greater reliability and service life, ecological compatibility etc. True, the process of assimilation of the new equipment is unlikely to be explosive. More probably is an evolutionary way, though with a certain degree of acceleration. The wide use of superconducting electric equipment for generating, transmission and consuming electric energy will bring a 5-7 percent rise in the efficiency of its use and, consequently, will lead to a reduction of the primary power consumption to the same extent. However, the importance of the new technology significance is not confined to that. The transformations will cover machine-building, metallurgy, mining and processing industries as well as different types of transport, etc.
Russia keeps on collaborating in such prestigious international projects as the experimental thermonuclear reactor (ITER) and the superconducting accelerator - "The Large Hadron Collider" (LHC). Moreover, Russian research scientists and engineers working on superconducting materials, magnetic systems for various applications and on non-traditional equipment are ready to join the industry in efforts for significant improvement of electric and power equipment on the basis of modern superconducting technologies and cryogenic technology competitive on the international market.
"Superconducting Technologies: Modem State and Prospects of Practical Use", Vestnik RAN, Vol. 71, No. 4, 2001