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Progress report on superconductivity
Progress report on superconductivity I. M o d e n a ,
C. R i z z u t o a n d N . S a c c h e t t i
Applications de la supraconductivit6 Le transport b terre ~ grande vitesse, la propulsion des navires, la production d'#lectricit~, les mesures de grande sensibilitY, /a chirurgie, les r~acteurs ~ fusion, /'enrichissement des min~raux et la lutte contre la pollution sont quelques unes des applications oD /'utilisation de mat#riaux supraconducteurs commerciaux est en cours d'~tude. I/est c/air actue//ement que la r#a/isation ~conomique et technique de que/ques uns des objectifs ~num~r~s
High-speed ground transport, ship propulsion, electrical power production, high sensitivity measurements, surgery, fusion reactors, mineral enrichment and pollution control are some of the applications where the use of commercial superconducting materials is n o w under research. It is n o w clear that the economical and technical feasibility of some of the developments listed above will only be made possible by the use of superconducting materials which will help to overcome the intrinsic limitations of normally
Superconductivity which has been, for half a century, a laboratory curiosity and an unexplained phenomenon, has emerged, in the last decade and after the explanation of its physical origin, as an important and attractive technology which can be applied in very different fields ranging from heavy machinery for energy production or conversion (fusion reactors, MHD and ac generators) or passenger railtransport (magnetic levitation trains) up to extremely sensitive measurements of weak signals (eg the outside detection of the magnetic signal of a foetus heart-beat). In the following we will give an overview of the applications of superconductivity which are now either developed or proposed and we will try to suggest which are the areas of interest for the development of refrigeration techniques and materials.
ci-dessus ne sera possible qu'avec /'uti/isation des supraconducteurs qui aideront ~ surmonter /es //mites intrins~ques des techniques utilis#es normalement. Ce rapport donne des renseignements sur /es mat#riaux commerciaux disponib/es actuel/ement et sur ceux qui sont en cours de mise au point, ainsi que sur les principales applications ~tudi#es en ce moment. On insiste sur /es prob/#mes techno/ogiques Ii~s b I'utilisation f/able des temperatures voisines de OK. On donne aussi des renseignements sur /es #tudes grande ~chelle dans divers pays, avec indication des applications les plus avanc#es.
employed techniques. This paper gives data on commercial materials presently available and those which are at the development stage, as well as major applications currently under research. Emphasis is made on the technological problems related to the reliable use of near zero Kelvin temperatures. Some data are also given on large scale undertakings in various nations, with indications of the more mature applications.
and currents; and, superconductive devices - the detection and generation of very low level signals. The first branch is connected to the economic gain and the technical feasibility of generating intense magnetic fields (up to above 10 T) in large volumes (several m 3) by the use of Iossless superconducting windings. The most promising applications of this kind are listed in Table 1 with an indication of the year of industrial application as forecast now. We have also included superconductive power lines as a particular case.
IM is at the Istituto di F/s/ca, Universita di Roma, Italy, CR is at the Istituto di F/s/ca e di Elettrotecnica, Universita di Genova, Italy, Gruppo Nazionale di Struttura della Materia (GNSM) del CNR and NS is at the Associazione EURATOM-CNEN sulla Fusione, Centro di Frascati, CP 65-00044 Frascati, Italy.
We will look in some detail at the technical reasons which suggest the study of these applications and the problem areas to be explored before the technical and economic feasibilities are assessed. Table 2 shows a list of the second branch applications: it differs from Table 1 because all the applications listed (except the computer ones) are now well advanced in daily laboratory use and are very near to (or have already reached) the marketing stage. Only a few of the small scale applications (the last three listed in Table 2) make use of the zero-resistance property of superconductors: most are, instead, based on the large scale coherence of the electron states which is typical of the superconducting state.
Volume 2 Number 5 September 1979
0140-7007/79/050133-06 802.00 © 1979 IPC Business Press Ltd. and IIR.
The applications of superconductivity can be divided in two main branches: superconductive windings the generation and handling of intense magnetic fields
133
It is outside the scope of this review to give an introductory account and details on the physical origin of the superconductive state. We leave the reader to find more about this in the various excellent books now available 1-4. In some of these references an indication of the increasing financial commitment by governments and industries to applied superconductivity is also given.
,.~.,iO7Acm~
Jc Acm-2
/
,o.
"~'18K~ / z ~ ~
H kG
Generation of intense magnetic fields: materials and applications Materials and refrigeration At present commercial sc cables are available, which have the characteristic parameters listed in Table 3, together with some materials (Nb3 Ge and Nb3 AI) potentially very interesting, but whose development is still in progress and the last one (Pbl.lo Mo5.1 S6) which is only at the laboratory level. The first alloy (Nb-Zr) has now been superseded by Nb-Ti alloy, which is at present by far the most popular. Several windings, mainly those working in the most stringent conditions (high field magnets, MHD and fusion projects), have already been made by either Nb3 Sn or V 3 Ga; both show advanced properties which, however, are counterbalanced at present by a more costly winding due to the brittleness of the intermetallic compound. In Table 3 we may note the main point in using superconducting windings: they are able to carry, without any ohmic loss, dc currents with a density much higher than copper windings, up to magnetic
T,k
Fig. 1Jc' Hc' Tsurfacefor a typicalsuperconductingmaterial Fig. 1 Surface de Jc" He, T pour un supraconducteur typique
fields well in excess of those attainable with iron core magnets (approx. 2 T). This immediately explains why the use of superconductive windings is convenient every time the generation of fields in excess of about 2 T in large volumes is required: in these conditions and also for smaller magnetic fields the power saving outweighs the problems and the power requirements of the refrigerators as is evident by comparing the approximate power requirements of comparable 'normal' and 'superconducting' windings given in Table 4. We note that the working temperature of a superconducting cable is always lower than the
Table 1. Large scale applications and development stage Tableau 1 Applications ~ grande #chelle et ~tat de leur d~veloppement Type of application Fusion (Tokamak)
Present power/size built
Predicted final power/size
Year of predicted application
3.5 x 2.5 m, D shape torus. H = 8T
9 x 6 m D shape up to 12 T
MHD
2.5x0.7m, H=
20x3m, H = 6T
1990
ac generators
25 MVA
1500 MVA
1990
dc machines
3 MW
1-
1975
Transportation
20 tons at 314 kmh -1
Speed >400 kmh -1
1990
>100 m in diameter >101° J in energy
>2000
max
5T
Energy storage Magnetic separation and purification
Small demonstration plants
Medical application (nmr) 4.7 m diam. 3.5 T, 90 MJ
Energy transmission
10 kW - 100 kW 35 m line length
134
1985 (?) 0.3 T in 2 m length high uniformity
High energy physics
High field magnets (hybrid magnets)
10 MW
30 T
roughly 1990 demonstration ~>2000 commercial
1985 1975
2.500 - 5000 MW few km
>2000 1976
International Journal of Refrigeration
transition temperature T , if useful fields and currents are to be reached The charactenst~c diagram which relates the critical values of current density, field and temperature is shown in Fig. 1. It is quite evident that present materials are to be used below 10 K and close C
.
.
.
.
Table 2. Small scale applications
Tableau 2 Applications ~ petite #chel/e Type of device
Sensitivity and/or characteristics
1 Magnetometers
10- 16 T Hz- '/'
2 Gradiometers
10- 14 T cm- 1 Hz Biomagnetism, earth magnetic field gradients
3 Susceptometers
210- 11 emu
4 Voltmeters (dc)
10 -is V
5 Ammeters (dc - 10 kHz)
10- 16 A
6 Voltage standards
Stability better than 10- a
7 Computer logic and memory
Switching time < 10-1o s
8 Parametric amplifiers
20 K
9 Mixers and oscillators
Up to 10 TM Hz
10 Bolometers and detectors (ir and microwave)
Sensitivity 10 -13 W Hz -'~
11 Comparators and dc transformers
Linearity
12 High Q cavities
Q
Applications Biomagnetism, magnetotelluric
to 4.2 K (boiling point of liquid He). At present there is great effort being put into developing superconducting cables which could allow a safe working temperature close to 6 K: this would help ease the strong refrigeration problems encountered especially in rotating machinery, as will be discussed later. For the largest coil sizes and field intensities mechanical problems are important because the forces arising from the interaction between currents and fields become larger than the yield stress of the superconducting cables. The useful current density for a technically reliable operation is the 'stabilized' current density listed in Table 3 (last column) - this leads on to a few details about the techniques used in the construction of superconducting windings. The superconductors to be used are 'hard' alloys or compounds (ie full of defects, dislocations, chemical impurities, etc.) which have a rather high electrical resistivity in their normal state: this, together with the very low specific heat and the limited refrigeration power allowable (a few hundred W at most), would cause dangerous runaway conditions (instabilities) each time a small section of the cable becomes accidentally normal. To circumvent this problem the superconducting section of the cables is divided into many thin filaments embedded into a highly conductive matrix (Cu or AI) so that if one or several filaments are driven normal, the current is carried b y the matrix with no excessive heat generation, thus avoiding catastrophic thermal runaway. Sudden heat pulses could be easily released by the friction between adjacent turns due to relative motions originated by electro-dynamical forces: this can be avoided either by vacuum impregnation of the windings with epoxy resin or by adopting a construction technique and a cable structure such that these movements are impossible.
10- 8
10 l°
The use of superconductive windings is at present limited to maximum frequencies of about 1 Hz, and
Table 3. Values of critical t e m p e r a t u r e , critical field and critical current density for various materials
Tableau 3 Temp6rature critique, champ critique et densit~ critique de courant pour divers supraconducteurs
Material
Critical temperature, K
Critical current density at 5 T, Am -2
Critical field, T Bare
Stabilized
Nbe ~ Zre ~
10.6
8
0.5 x 10g
0 2 x 10°
Nbo., Tio ,j
9.4
14
1.5 x 10g
0.4 - 1 x 10g
Nb3 Sn
18.3
24.5
5 x 109
1 x 10s
V3 Ga
16.8
24
4 x 10'
Nb~ Ge
20 - 23
Nb 3 AI
16 - 18.7
Pbl ,e Mo. 1 Se
14.4
Volume 2 Number 5 September 1979
> 30 27 - 32
2.5 x 109
> 51
135
Table 4. A m o u n t of p o w e r needed to generate magnetic fields (order of magnitude)
Tab/eau 4 Puissance n~cessaire pour produire des champs magn#tiques (ordre de grandeur) Field and volume 3 T i n 1 m3
15Tin5cm 3
Normal Superconductingtechnique technique (includingrefrigeration) >1 MW
2-
3MW
410kW ~20kW
power transportation at line frequency is not economically viable because the superconducting materials, under an applied ac field, show a frequency dependent power dissipation which would require an extremely high refrigeration power. The refrigeration problems are not related only to the limitations due to the physical properties of materials and fluids at low temperatures (which limit heat exchange and specific heat) but also the low efficiency of the refrigerators and liquefiers available today (around 10% of the theoretical value). Also the long term reliability of the compressors is one of the problem areas to be covered before industrial applications are fully started. We have only briefly pointed out some of the problems related to superconducting materials and to refrigerators. However it should be realized that a whole area of development and characterization studies is still to be covered for many other materials involved in the superconducting electromechanical constructions like epoxy resin composites, stainless steels, normal conductors, insulating materials, etc., with selection to be made after testing their performance under extreme conditions of temperature cycling, mechanical stresses, magnetic fields, etc.
Fusion studies A great part of the researches to reach energy production by controlled nuclear fusion is based on magnetic containement of the hot plasma mainly in tokamaks even though other solutions (eg mirror machines) are possible. In both cases the size of the magnetic 'bottle', which is predicted by extrapolating various experiments now under way, calls for a field of order of 10 T over several m 3. This, if generated by conventional room temperature windings, would dissipate most, if not all, of the useful power. It is therefore necessary to use superconducting windings and the next generation of tokamaks to be built in USA, Europe, USSR and Japan will have these. A peculiar aspect of these applications of superconductors is that the materials will be exposed to neutron irradiation, a situation which has not so far been encountered in large scale applications. An experimental programme to test large superconducting coils arranged in a toroidal configuration is in progress in USA (Icp = large coils project) with international collaboration with Euratom, Switzerland and Japan. The Icp programme aims to get as much information as possible on the performance and reliability of a system of six D-shaped superconducting coils (external dimensions 3.7 x 4.7 m, max. field 8 T) and its associated cryogenic equipment. Superconducting coils are also
1316
envisaged for the so called NET (Next European Torus) which should be integrated into the lnternatior~ Torus INTOR, a joint project among USA, Euratom, USSR and Japan. To get an idea of the effort required the preliminary design parameter for NET are: D-shaped coils 6 x 9 m (external), maximum field on the superconductor 8 or 12 T. Apart from dimensions it will differ from Icp because it will be designed as a complete plasma machine.
Magnetohydrodynamic conversion (MHD) The energy of the hot exhaust gas of a thermoelectric power plant could be partly converted into electrical power by 'seeding' it with ionizable particles and collecting them after diversion by an intense magnetic field. This energy conversion scheme has proved feasible and is estimated to be able to increase the overall efficiency of a thermal power plant from the present approximate 40% up to a maximum of 60% with a net gain up to 50% in produced power. The transverse field necessary for a full scale MHD plant is of the order of 6 T over a cylindrical volume of about 2 m in diameter and several m in length. Here again the use of superconducting windings is compulsory. At present major efforts are under way in Japan and in USSR. The latter is in close cooperation with the USA on magnet technology. The main problem areas lie in developing materials to contain the hot exhaust gas and to build chemically inert electrodes.
ac generators The power generated by conventional turboalternators is roughly proportional to the excitation field and to the length and the square of the rotor's diameter, the speed being fixed by the mains frequency and by the number of poles (50 or 60 Hz for a two pole and 25 or 37.6 Hz for a four pole machine). At present the biggest machines, in the power range from 1300 to 1500 MVA, have nearly reached the limits imposed by the stiffness and the yield strength of the forged steel rotor-blocks, the excitation field being limited by the saturation of iron. If superconducting windings are used, instead of iron nucleus rotors, an estimated reduction of volume from three to five can be reached by raising the excitation field, allowing either higher unitary power or a greater transportability of the ironless machine. There are strong efforts now under way in all major industrial countries after the feasibility demonstration of few 1 MVE machines in USA, USSR and Japan. At this moment a 6.25 MVE machine is under test in Japan and the construction of machines in the 100 MVE range is being started in USA and in Europe. Although this is an application that will reach maturity within the next decade, there are still problem areas in the electromagnetic shielding of the superconducting windings against the ac reaction field from the stator, in the liquid He circulation in a rotating vessel and in the structural materials for the winding anchorage. dc machines (motors and generators) It is not possible to build a conventional dc machine having a power of more than approximately 10 MW (at about 1.7 Hz) or 1 MW (at nearly 17 Hz) and, close to these power
International Journal of Refrigeration
limits, the conventional machines are very heavy and costly. Several superconducting dc motors and generators have been built and tested satisfactorily, with designs of the homopolar type which allows for a steady magnetic excitation field and a relatively light weight low inertia rotor. At present these machines are being considered for ship-propulsion as an electric power transfer between the main power generator (a turbine) and the propellers, avoiding the mechanical rigidity and the weight of conventional gearboxes few motor/generator systems have been built, mainly in England and USA, and are at present under test. The possibility of reaching high powers at low voltage and high current with these generators is being considered for electrochemical plants. Peculiar problems of these machines are the high current collectors and their long term durability.
Ground transporation One of the most spectacular applications currently under test (mainly in Japan and in Germany) is high speed ground transport by means of magnetically levitated vehicles. The advantage of using superconducting windings is that a levitation gap of about 20 cm can be obtained by using the image field induced on a conductive rail by a moving field coil. The minimum speed to obtain such an effect is about 70 kmh TM and the advantages with respect to much smaller gaps obtainable with ferromagnetic cushions and with respect to the noise problem of air cushions are quite evident. The scheme that is now being studied provides for the motive power to be delivered directly by the rail through a magnetic 'pull' on the superconductive windings and therefore there is no prime motor on board. Problem areas as far as superconductors are concerned, are at present, with the reliability of longtime persistent current mode operation of the superconducting coils or with the possiblity of allowing the on board refrigerators.
Magnetic storage and magnetic separation Preliminary studies on the energy storage capability of very large superconducting coils have being made but no application is foreseen for the near future. Projects for magnetic separation are closer. In fact, by making use of the sizable forces which can be induced in paramagnetic materials by intense field gradients it is possible to separate ores or materials and to obtain attractive enrichment factors. This is particularly interesting because it may allow the exploitation of poor minerals.
Scanning nmr Nuclear magnetic resonance of the human body can be an useful diagnostic tool: it requires a very homogeneous magnetic field over the volume to be explored and presently superconducting windings are being considered to generate these fields.
High energy physics At present considerable use of solenoid, dipole and multipole coils made by superconducting materials is made in this field and it can be considered the testing ground for superconducting
Volume 2 Number 5 September 1979
and refrigeration techniques on a large scale. Several large coils weighing tens of tons and producing fields of a few tesla over many m 3 are in use in the main high energy physics laboratories throughout the world. We mention also the various superconducting particle accelerators designed or under construction (the 1000 GeV Energy doubler at Fermilabo, the Isabelle storage ring at Brookhaven, etc.).
Energy transmission The possibilities of superconducting cables seem to be at present limited to the handling of very high power densities where the land space is either very limited or very expensive as, eg, close to big cities. The limitation in the economics is mainly due to the decreased characteristics of superconducting cables with ac currents and the additional cost of ac - dc double conversion. Generation of very high magnetic field At present superconducting magnets above 15 T have been built essentially for laboratory use and are commercially available. Moreover fields up to 30.1 T have been reached by using hybrid magnets built at MIT National Magnet Lab. A similar project is now under development at Grenoble in a joint effort by Grenoble and Karlsruhe.
Superconducting devices The space we will dedicate to this part will be rather short in comparison to the number of different applications listed in Table 2: this is because the impact on refrigeration technology and on materials will probably be smaller and the apparatus which are being built will have a very specialized market. While it is easy to understand how a zero-resistance conductor can be profitably employed in coil winding, the principle of operation of most of the superconducting devices is due to a phenomenon (the Josephson effect) which is rather complicated to explain in classic electromagnetic wording and a detailed account of it should be sought in one of the introductory books 2,3,4 we have listed in the references. It may suffice here to say that, in any superconductor, the conduction electrons are in a macroscopic coherent quantum state which extends to the whole piece of material. This state can be roughly described by coupled pairs of electrons which rotate around each other at a distance of many interatomic lengths. By introducing a weak ('thin' from the superconducting point of view) link in a superconducting ring it is possible to insert or extract in a controlled way a number of magnetic flux 'quanta' connected to these elementary current vortices. The value of each flux quantum which can be so handled one by one in the SQUID (Superconducting quantum interference device) is h/2e = 2 x 10-15 Weber (h = Planck's constant, e = charge on the electron). To give an idea of the extremely small magnitude of this flux, we note that it is of the same order as the magnetic flux connected with the 'winding' equivalent to a single electron rotating around an hydrogen atom.
137
The possibility of measuring such low magnetic fluxes, connected to macroscopic circuits, allows the measurement of very weak magnetic fields and by suitable coil arrangements very weak field gradients, dc currents over very low impedances and hence very small dc voltages. The above principle is applied in most of the applications listed in Table 2 (1 - 5, 7, 8, 10). In some other devices, such as 6 and 9 in Table 2, the interaction between microwave fields and quantized flux transition is used: it is interesting to note that today the International Voltage Standard is defined by superconducting devices, which have the advantage of relating well defined voltage-step characteristics to definite values of frequency measurements, thus avoiding the long term drift problems typical of standard cells which limit the accuracy in maintaining a voltage standard. In some applications (10, 11 and 12) the zero resistivity property is used, or (in 10) the very steep transition that occurs at a well defined temperature from zero resistivity to a normal relatively high resistance. We will not go into further details about these applications; it should only be pointed out that the main develpoments required to extend the use of these devices are related to the availability of reliable low power and vibration free refrigerators, operating below 10 K and to the construction of long duration non-magnetic cryostats: this has already brought about the appearance on the market of plastic cryostats with advanced characteristics. Another area of development which is strongly connected with these devices is the possible use of solid state active components around liquid He temperatures (preamplifiers, oscillators, etc.). Conclusions From the present status and the outlook for the near future superconductivity seems to be ready to occupy a firm position as an applied technology of great interest in many different areas of electromechanical
138
engineering and in the manufacture of extremely sensitive devices. Its development is going to bring new impetus to the development of refrigeration and of new materials to be used at very low temperatures. References 1 Foner, S., Schwartz, B.B. (Eds) Superconducting machinesand devices - large system applications, Plenum (1974) 2 Foner, S., Schwartz, B.B. (Eds) Superconductor applications: SQUIDS and machines, Plenum (1977) 3 Burger, J.P. La supraconductivitedes metaux, des alliageset des films minces, Masson et cie, Paris (1974) 4 Parks, R.D. (Ed.) Superconductivity, Marcel Dekker Inc., New York, I, II (1969)
Professor Carlo Rizzuto was born in Genova, Italy, in 1937 and graduated in Physicsin 1961 at his home University. He was working at various Laboratories in Europeand America, and collaborates with McGill University, Montreal; Imperial College, London; and the Institute of Physics, Zagreb. He holds a chair in Solid State Physicsat the Faculty of Engineering, University of Genova.
Professor Rizzuto's field is low temperature physics and applied cryogenics with the main emphasison superconductivity and the magnetic, thermal and electronic properties of metals, alloys, intermetallic compounds, metal glasses and applied materials. He is also involved in the development of new techniques and materialsfor low temperature applications, including cryomedicine, applied superconductivity and research cryostats. Prof. Rizzuto is a member of the Low Temperature Physics Committee of the Condensed Matter Division (E.P.S.) of Commission At/2 (I.I.R.) and of the I.C.E.C. He was recently elected Presidentof G.N.S.M being the Italian Group on Structure of Matter, of most Italian researchersin the field of applied and basic research in condensed matter.
International Journal of Refrigeration
C. R i z z u t o a n d N . S a c c h e t t i
Applications de la supraconductivit6 Le transport b terre ~ grande vitesse, la propulsion des navires, la production d'#lectricit~, les mesures de grande sensibilitY, /a chirurgie, les r~acteurs ~ fusion, /'enrichissement des min~raux et la lutte contre la pollution sont quelques unes des applications oD /'utilisation de mat#riaux supraconducteurs commerciaux est en cours d'~tude. I/est c/air actue//ement que la r#a/isation ~conomique et technique de que/ques uns des objectifs ~num~r~s
High-speed ground transport, ship propulsion, electrical power production, high sensitivity measurements, surgery, fusion reactors, mineral enrichment and pollution control are some of the applications where the use of commercial superconducting materials is n o w under research. It is n o w clear that the economical and technical feasibility of some of the developments listed above will only be made possible by the use of superconducting materials which will help to overcome the intrinsic limitations of normally
Superconductivity which has been, for half a century, a laboratory curiosity and an unexplained phenomenon, has emerged, in the last decade and after the explanation of its physical origin, as an important and attractive technology which can be applied in very different fields ranging from heavy machinery for energy production or conversion (fusion reactors, MHD and ac generators) or passenger railtransport (magnetic levitation trains) up to extremely sensitive measurements of weak signals (eg the outside detection of the magnetic signal of a foetus heart-beat). In the following we will give an overview of the applications of superconductivity which are now either developed or proposed and we will try to suggest which are the areas of interest for the development of refrigeration techniques and materials.
ci-dessus ne sera possible qu'avec /'uti/isation des supraconducteurs qui aideront ~ surmonter /es //mites intrins~ques des techniques utilis#es normalement. Ce rapport donne des renseignements sur /es mat#riaux commerciaux disponib/es actuel/ement et sur ceux qui sont en cours de mise au point, ainsi que sur les principales applications ~tudi#es en ce moment. On insiste sur /es prob/#mes techno/ogiques Ii~s b I'utilisation f/able des temperatures voisines de OK. On donne aussi des renseignements sur /es #tudes grande ~chelle dans divers pays, avec indication des applications les plus avanc#es.
employed techniques. This paper gives data on commercial materials presently available and those which are at the development stage, as well as major applications currently under research. Emphasis is made on the technological problems related to the reliable use of near zero Kelvin temperatures. Some data are also given on large scale undertakings in various nations, with indications of the more mature applications.
and currents; and, superconductive devices - the detection and generation of very low level signals. The first branch is connected to the economic gain and the technical feasibility of generating intense magnetic fields (up to above 10 T) in large volumes (several m 3) by the use of Iossless superconducting windings. The most promising applications of this kind are listed in Table 1 with an indication of the year of industrial application as forecast now. We have also included superconductive power lines as a particular case.
IM is at the Istituto di F/s/ca, Universita di Roma, Italy, CR is at the Istituto di F/s/ca e di Elettrotecnica, Universita di Genova, Italy, Gruppo Nazionale di Struttura della Materia (GNSM) del CNR and NS is at the Associazione EURATOM-CNEN sulla Fusione, Centro di Frascati, CP 65-00044 Frascati, Italy.
We will look in some detail at the technical reasons which suggest the study of these applications and the problem areas to be explored before the technical and economic feasibilities are assessed. Table 2 shows a list of the second branch applications: it differs from Table 1 because all the applications listed (except the computer ones) are now well advanced in daily laboratory use and are very near to (or have already reached) the marketing stage. Only a few of the small scale applications (the last three listed in Table 2) make use of the zero-resistance property of superconductors: most are, instead, based on the large scale coherence of the electron states which is typical of the superconducting state.
Volume 2 Number 5 September 1979
0140-7007/79/050133-06 802.00 © 1979 IPC Business Press Ltd. and IIR.
The applications of superconductivity can be divided in two main branches: superconductive windings the generation and handling of intense magnetic fields
133
It is outside the scope of this review to give an introductory account and details on the physical origin of the superconductive state. We leave the reader to find more about this in the various excellent books now available 1-4. In some of these references an indication of the increasing financial commitment by governments and industries to applied superconductivity is also given.
,.~.,iO7Acm~
Jc Acm-2
/
,o.
"~'18K~ / z ~ ~
H kG
Generation of intense magnetic fields: materials and applications Materials and refrigeration At present commercial sc cables are available, which have the characteristic parameters listed in Table 3, together with some materials (Nb3 Ge and Nb3 AI) potentially very interesting, but whose development is still in progress and the last one (Pbl.lo Mo5.1 S6) which is only at the laboratory level. The first alloy (Nb-Zr) has now been superseded by Nb-Ti alloy, which is at present by far the most popular. Several windings, mainly those working in the most stringent conditions (high field magnets, MHD and fusion projects), have already been made by either Nb3 Sn or V 3 Ga; both show advanced properties which, however, are counterbalanced at present by a more costly winding due to the brittleness of the intermetallic compound. In Table 3 we may note the main point in using superconducting windings: they are able to carry, without any ohmic loss, dc currents with a density much higher than copper windings, up to magnetic
T,k
Fig. 1Jc' Hc' Tsurfacefor a typicalsuperconductingmaterial Fig. 1 Surface de Jc" He, T pour un supraconducteur typique
fields well in excess of those attainable with iron core magnets (approx. 2 T). This immediately explains why the use of superconductive windings is convenient every time the generation of fields in excess of about 2 T in large volumes is required: in these conditions and also for smaller magnetic fields the power saving outweighs the problems and the power requirements of the refrigerators as is evident by comparing the approximate power requirements of comparable 'normal' and 'superconducting' windings given in Table 4. We note that the working temperature of a superconducting cable is always lower than the
Table 1. Large scale applications and development stage Tableau 1 Applications ~ grande #chelle et ~tat de leur d~veloppement Type of application Fusion (Tokamak)
Present power/size built
Predicted final power/size
Year of predicted application
3.5 x 2.5 m, D shape torus. H = 8T
9 x 6 m D shape up to 12 T
MHD
2.5x0.7m, H=
20x3m, H = 6T
1990
ac generators
25 MVA
1500 MVA
1990
dc machines
3 MW
1-
1975
Transportation
20 tons at 314 kmh -1
Speed >400 kmh -1
1990
>100 m in diameter >101° J in energy
>2000
max
5T
Energy storage Magnetic separation and purification
Small demonstration plants
Medical application (nmr) 4.7 m diam. 3.5 T, 90 MJ
Energy transmission
10 kW - 100 kW 35 m line length
134
1985 (?) 0.3 T in 2 m length high uniformity
High energy physics
High field magnets (hybrid magnets)
10 MW
30 T
roughly 1990 demonstration ~>2000 commercial
1985 1975
2.500 - 5000 MW few km
>2000 1976
International Journal of Refrigeration
transition temperature T , if useful fields and currents are to be reached The charactenst~c diagram which relates the critical values of current density, field and temperature is shown in Fig. 1. It is quite evident that present materials are to be used below 10 K and close C
.
.
.
.
Table 2. Small scale applications
Tableau 2 Applications ~ petite #chel/e Type of device
Sensitivity and/or characteristics
1 Magnetometers
10- 16 T Hz- '/'
2 Gradiometers
10- 14 T cm- 1 Hz Biomagnetism, earth magnetic field gradients
3 Susceptometers
210- 11 emu
4 Voltmeters (dc)
10 -is V
5 Ammeters (dc - 10 kHz)
10- 16 A
6 Voltage standards
Stability better than 10- a
7 Computer logic and memory
Switching time < 10-1o s
8 Parametric amplifiers
20 K
9 Mixers and oscillators
Up to 10 TM Hz
10 Bolometers and detectors (ir and microwave)
Sensitivity 10 -13 W Hz -'~
11 Comparators and dc transformers
Linearity
12 High Q cavities
Q
Applications Biomagnetism, magnetotelluric
to 4.2 K (boiling point of liquid He). At present there is great effort being put into developing superconducting cables which could allow a safe working temperature close to 6 K: this would help ease the strong refrigeration problems encountered especially in rotating machinery, as will be discussed later. For the largest coil sizes and field intensities mechanical problems are important because the forces arising from the interaction between currents and fields become larger than the yield stress of the superconducting cables. The useful current density for a technically reliable operation is the 'stabilized' current density listed in Table 3 (last column) - this leads on to a few details about the techniques used in the construction of superconducting windings. The superconductors to be used are 'hard' alloys or compounds (ie full of defects, dislocations, chemical impurities, etc.) which have a rather high electrical resistivity in their normal state: this, together with the very low specific heat and the limited refrigeration power allowable (a few hundred W at most), would cause dangerous runaway conditions (instabilities) each time a small section of the cable becomes accidentally normal. To circumvent this problem the superconducting section of the cables is divided into many thin filaments embedded into a highly conductive matrix (Cu or AI) so that if one or several filaments are driven normal, the current is carried b y the matrix with no excessive heat generation, thus avoiding catastrophic thermal runaway. Sudden heat pulses could be easily released by the friction between adjacent turns due to relative motions originated by electro-dynamical forces: this can be avoided either by vacuum impregnation of the windings with epoxy resin or by adopting a construction technique and a cable structure such that these movements are impossible.
10- 8
10 l°
The use of superconductive windings is at present limited to maximum frequencies of about 1 Hz, and
Table 3. Values of critical t e m p e r a t u r e , critical field and critical current density for various materials
Tableau 3 Temp6rature critique, champ critique et densit~ critique de courant pour divers supraconducteurs
Material
Critical temperature, K
Critical current density at 5 T, Am -2
Critical field, T Bare
Stabilized
Nbe ~ Zre ~
10.6
8
0.5 x 10g
0 2 x 10°
Nbo., Tio ,j
9.4
14
1.5 x 10g
0.4 - 1 x 10g
Nb3 Sn
18.3
24.5
5 x 109
1 x 10s
V3 Ga
16.8
24
4 x 10'
Nb~ Ge
20 - 23
Nb 3 AI
16 - 18.7
Pbl ,e Mo. 1 Se
14.4
Volume 2 Number 5 September 1979
> 30 27 - 32
2.5 x 109
> 51
135
Table 4. A m o u n t of p o w e r needed to generate magnetic fields (order of magnitude)
Tab/eau 4 Puissance n~cessaire pour produire des champs magn#tiques (ordre de grandeur) Field and volume 3 T i n 1 m3
15Tin5cm 3
Normal Superconductingtechnique technique (includingrefrigeration) >1 MW
2-
3MW
410kW ~20kW
power transportation at line frequency is not economically viable because the superconducting materials, under an applied ac field, show a frequency dependent power dissipation which would require an extremely high refrigeration power. The refrigeration problems are not related only to the limitations due to the physical properties of materials and fluids at low temperatures (which limit heat exchange and specific heat) but also the low efficiency of the refrigerators and liquefiers available today (around 10% of the theoretical value). Also the long term reliability of the compressors is one of the problem areas to be covered before industrial applications are fully started. We have only briefly pointed out some of the problems related to superconducting materials and to refrigerators. However it should be realized that a whole area of development and characterization studies is still to be covered for many other materials involved in the superconducting electromechanical constructions like epoxy resin composites, stainless steels, normal conductors, insulating materials, etc., with selection to be made after testing their performance under extreme conditions of temperature cycling, mechanical stresses, magnetic fields, etc.
Fusion studies A great part of the researches to reach energy production by controlled nuclear fusion is based on magnetic containement of the hot plasma mainly in tokamaks even though other solutions (eg mirror machines) are possible. In both cases the size of the magnetic 'bottle', which is predicted by extrapolating various experiments now under way, calls for a field of order of 10 T over several m 3. This, if generated by conventional room temperature windings, would dissipate most, if not all, of the useful power. It is therefore necessary to use superconducting windings and the next generation of tokamaks to be built in USA, Europe, USSR and Japan will have these. A peculiar aspect of these applications of superconductors is that the materials will be exposed to neutron irradiation, a situation which has not so far been encountered in large scale applications. An experimental programme to test large superconducting coils arranged in a toroidal configuration is in progress in USA (Icp = large coils project) with international collaboration with Euratom, Switzerland and Japan. The Icp programme aims to get as much information as possible on the performance and reliability of a system of six D-shaped superconducting coils (external dimensions 3.7 x 4.7 m, max. field 8 T) and its associated cryogenic equipment. Superconducting coils are also
1316
envisaged for the so called NET (Next European Torus) which should be integrated into the lnternatior~ Torus INTOR, a joint project among USA, Euratom, USSR and Japan. To get an idea of the effort required the preliminary design parameter for NET are: D-shaped coils 6 x 9 m (external), maximum field on the superconductor 8 or 12 T. Apart from dimensions it will differ from Icp because it will be designed as a complete plasma machine.
Magnetohydrodynamic conversion (MHD) The energy of the hot exhaust gas of a thermoelectric power plant could be partly converted into electrical power by 'seeding' it with ionizable particles and collecting them after diversion by an intense magnetic field. This energy conversion scheme has proved feasible and is estimated to be able to increase the overall efficiency of a thermal power plant from the present approximate 40% up to a maximum of 60% with a net gain up to 50% in produced power. The transverse field necessary for a full scale MHD plant is of the order of 6 T over a cylindrical volume of about 2 m in diameter and several m in length. Here again the use of superconducting windings is compulsory. At present major efforts are under way in Japan and in USSR. The latter is in close cooperation with the USA on magnet technology. The main problem areas lie in developing materials to contain the hot exhaust gas and to build chemically inert electrodes.
ac generators The power generated by conventional turboalternators is roughly proportional to the excitation field and to the length and the square of the rotor's diameter, the speed being fixed by the mains frequency and by the number of poles (50 or 60 Hz for a two pole and 25 or 37.6 Hz for a four pole machine). At present the biggest machines, in the power range from 1300 to 1500 MVA, have nearly reached the limits imposed by the stiffness and the yield strength of the forged steel rotor-blocks, the excitation field being limited by the saturation of iron. If superconducting windings are used, instead of iron nucleus rotors, an estimated reduction of volume from three to five can be reached by raising the excitation field, allowing either higher unitary power or a greater transportability of the ironless machine. There are strong efforts now under way in all major industrial countries after the feasibility demonstration of few 1 MVE machines in USA, USSR and Japan. At this moment a 6.25 MVE machine is under test in Japan and the construction of machines in the 100 MVE range is being started in USA and in Europe. Although this is an application that will reach maturity within the next decade, there are still problem areas in the electromagnetic shielding of the superconducting windings against the ac reaction field from the stator, in the liquid He circulation in a rotating vessel and in the structural materials for the winding anchorage. dc machines (motors and generators) It is not possible to build a conventional dc machine having a power of more than approximately 10 MW (at about 1.7 Hz) or 1 MW (at nearly 17 Hz) and, close to these power
International Journal of Refrigeration
limits, the conventional machines are very heavy and costly. Several superconducting dc motors and generators have been built and tested satisfactorily, with designs of the homopolar type which allows for a steady magnetic excitation field and a relatively light weight low inertia rotor. At present these machines are being considered for ship-propulsion as an electric power transfer between the main power generator (a turbine) and the propellers, avoiding the mechanical rigidity and the weight of conventional gearboxes few motor/generator systems have been built, mainly in England and USA, and are at present under test. The possibility of reaching high powers at low voltage and high current with these generators is being considered for electrochemical plants. Peculiar problems of these machines are the high current collectors and their long term durability.
Ground transporation One of the most spectacular applications currently under test (mainly in Japan and in Germany) is high speed ground transport by means of magnetically levitated vehicles. The advantage of using superconducting windings is that a levitation gap of about 20 cm can be obtained by using the image field induced on a conductive rail by a moving field coil. The minimum speed to obtain such an effect is about 70 kmh TM and the advantages with respect to much smaller gaps obtainable with ferromagnetic cushions and with respect to the noise problem of air cushions are quite evident. The scheme that is now being studied provides for the motive power to be delivered directly by the rail through a magnetic 'pull' on the superconductive windings and therefore there is no prime motor on board. Problem areas as far as superconductors are concerned, are at present, with the reliability of longtime persistent current mode operation of the superconducting coils or with the possiblity of allowing the on board refrigerators.
Magnetic storage and magnetic separation Preliminary studies on the energy storage capability of very large superconducting coils have being made but no application is foreseen for the near future. Projects for magnetic separation are closer. In fact, by making use of the sizable forces which can be induced in paramagnetic materials by intense field gradients it is possible to separate ores or materials and to obtain attractive enrichment factors. This is particularly interesting because it may allow the exploitation of poor minerals.
Scanning nmr Nuclear magnetic resonance of the human body can be an useful diagnostic tool: it requires a very homogeneous magnetic field over the volume to be explored and presently superconducting windings are being considered to generate these fields.
High energy physics At present considerable use of solenoid, dipole and multipole coils made by superconducting materials is made in this field and it can be considered the testing ground for superconducting
Volume 2 Number 5 September 1979
and refrigeration techniques on a large scale. Several large coils weighing tens of tons and producing fields of a few tesla over many m 3 are in use in the main high energy physics laboratories throughout the world. We mention also the various superconducting particle accelerators designed or under construction (the 1000 GeV Energy doubler at Fermilabo, the Isabelle storage ring at Brookhaven, etc.).
Energy transmission The possibilities of superconducting cables seem to be at present limited to the handling of very high power densities where the land space is either very limited or very expensive as, eg, close to big cities. The limitation in the economics is mainly due to the decreased characteristics of superconducting cables with ac currents and the additional cost of ac - dc double conversion. Generation of very high magnetic field At present superconducting magnets above 15 T have been built essentially for laboratory use and are commercially available. Moreover fields up to 30.1 T have been reached by using hybrid magnets built at MIT National Magnet Lab. A similar project is now under development at Grenoble in a joint effort by Grenoble and Karlsruhe.
Superconducting devices The space we will dedicate to this part will be rather short in comparison to the number of different applications listed in Table 2: this is because the impact on refrigeration technology and on materials will probably be smaller and the apparatus which are being built will have a very specialized market. While it is easy to understand how a zero-resistance conductor can be profitably employed in coil winding, the principle of operation of most of the superconducting devices is due to a phenomenon (the Josephson effect) which is rather complicated to explain in classic electromagnetic wording and a detailed account of it should be sought in one of the introductory books 2,3,4 we have listed in the references. It may suffice here to say that, in any superconductor, the conduction electrons are in a macroscopic coherent quantum state which extends to the whole piece of material. This state can be roughly described by coupled pairs of electrons which rotate around each other at a distance of many interatomic lengths. By introducing a weak ('thin' from the superconducting point of view) link in a superconducting ring it is possible to insert or extract in a controlled way a number of magnetic flux 'quanta' connected to these elementary current vortices. The value of each flux quantum which can be so handled one by one in the SQUID (Superconducting quantum interference device) is h/2e = 2 x 10-15 Weber (h = Planck's constant, e = charge on the electron). To give an idea of the extremely small magnitude of this flux, we note that it is of the same order as the magnetic flux connected with the 'winding' equivalent to a single electron rotating around an hydrogen atom.
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The possibility of measuring such low magnetic fluxes, connected to macroscopic circuits, allows the measurement of very weak magnetic fields and by suitable coil arrangements very weak field gradients, dc currents over very low impedances and hence very small dc voltages. The above principle is applied in most of the applications listed in Table 2 (1 - 5, 7, 8, 10). In some other devices, such as 6 and 9 in Table 2, the interaction between microwave fields and quantized flux transition is used: it is interesting to note that today the International Voltage Standard is defined by superconducting devices, which have the advantage of relating well defined voltage-step characteristics to definite values of frequency measurements, thus avoiding the long term drift problems typical of standard cells which limit the accuracy in maintaining a voltage standard. In some applications (10, 11 and 12) the zero resistivity property is used, or (in 10) the very steep transition that occurs at a well defined temperature from zero resistivity to a normal relatively high resistance. We will not go into further details about these applications; it should only be pointed out that the main develpoments required to extend the use of these devices are related to the availability of reliable low power and vibration free refrigerators, operating below 10 K and to the construction of long duration non-magnetic cryostats: this has already brought about the appearance on the market of plastic cryostats with advanced characteristics. Another area of development which is strongly connected with these devices is the possible use of solid state active components around liquid He temperatures (preamplifiers, oscillators, etc.). Conclusions From the present status and the outlook for the near future superconductivity seems to be ready to occupy a firm position as an applied technology of great interest in many different areas of electromechanical
138
engineering and in the manufacture of extremely sensitive devices. Its development is going to bring new impetus to the development of refrigeration and of new materials to be used at very low temperatures. References 1 Foner, S., Schwartz, B.B. (Eds) Superconducting machinesand devices - large system applications, Plenum (1974) 2 Foner, S., Schwartz, B.B. (Eds) Superconductor applications: SQUIDS and machines, Plenum (1977) 3 Burger, J.P. La supraconductivitedes metaux, des alliageset des films minces, Masson et cie, Paris (1974) 4 Parks, R.D. (Ed.) Superconductivity, Marcel Dekker Inc., New York, I, II (1969)
Professor Carlo Rizzuto was born in Genova, Italy, in 1937 and graduated in Physicsin 1961 at his home University. He was working at various Laboratories in Europeand America, and collaborates with McGill University, Montreal; Imperial College, London; and the Institute of Physics, Zagreb. He holds a chair in Solid State Physicsat the Faculty of Engineering, University of Genova.
Professor Rizzuto's field is low temperature physics and applied cryogenics with the main emphasison superconductivity and the magnetic, thermal and electronic properties of metals, alloys, intermetallic compounds, metal glasses and applied materials. He is also involved in the development of new techniques and materialsfor low temperature applications, including cryomedicine, applied superconductivity and research cryostats. Prof. Rizzuto is a member of the Low Temperature Physics Committee of the Condensed Matter Division (E.P.S.) of Commission At/2 (I.I.R.) and of the I.C.E.C. He was recently elected Presidentof G.N.S.M being the Italian Group on Structure of Matter, of most Italian researchersin the field of applied and basic research in condensed matter.
International Journal of Refrigeration