Prospects for Utilization of Superconductors in the Power Industry

Session 8 — High Temperature Superconductors

PROSPECTS FOR UTILIZATION OF SUPERCONDUCTORS IN THE POWER INDUSTRY

N.A. CHERNOPLEKOV

Kurchatov Institute of Atomic Energy, Moscow ABSTRACT. Utilization of superconducting technology is greatly influenced by die discoveiy of the so-called high-temperature superconductors (HTS). The present report considers to what extent there is a need for HTS in up-to-date engineering and how much they are prepared for practical applications. The work on practical use of superconductors was started about 30 years ago. As a result, two fields of die high-current superconductivity have emerged. The first category is the field in which other alternatives were inconceivable from techno-econotnk points of view (magnets of large merino-nuclear installations, MHD generators, inductive energy storage systems, etc.). The second category involves areas where superconductors must demonstrate the ability to compete with existing technologies (electrical devices, magnetic separators, etc.). The present overview discusses development of various low temprature superconducting devices, estimates their potential and evaluates future applications of HTS based on the experience accumulated in the USSR and other countries.

INTRODUCTION MODERN COMMERCIAL Applied superconductivity and its most significant SUPERCONDUCTORS constituents, namely, the research and development Superconducting materials are necessary in electrical of superconducting electrical devices, have been devices to generate high magnetic fields in large studied for 30 years. This period follows the volumes while dissipating little power. theoretical prediction of Soviet physicists1 on the Technological feasibility of such materials should be possible existence of the so-called hard comparable to that of common conductors, e.g. superconductors. Such materials have sufficiently copper or aluminium. The stability of the high values of critical magnetic field and current superconducting state must also be ascertained for density to promise useful applications. This any particular environment within the prediction was followed by the discovery of such superconducting windings. substances by American physicists 2 (here we refer to How do modern industrial superconductors look the liquid helium temperature superconductors). like and what could be said about their industrial Almost immediately, these discoveries stimulated production (wires, cables, tapes, etc.)? In answering active research and development in many countries, these questions we consider only tow-temperature including the USSR. The subject of the superconductors, because the high-temperature investigations was the possible use of high current materials can hardly be anticipated on the market in low-temperature superconductors in magnets, the near future. electric power devices and physical research. Modem materials for superconducting windings The practical use of superconductors was are typically made of a quite complicated composite proclaimed a new technology which could radically structure; they comprise metals with different change electric power engineering and influence electrical, thermal and mechanical properties and are other branches of industry and science 3. Today it is usually covered with some insulation. The reasonable to evaluate the outcome of this effort. So cross-sectional area of a wire or a cable can vary the possible changes following the discovery of from a fraction of a few square milimeters to tens of "high-temperature superconductors" (or, more square centimeters. Rated currents can be, precisely, superconductors at liquid nitrogen accordingly, a few amperes or tens of kA. temperature level) and their possible use in Superconducting filament diameters in those wires or high-current technologies should be discussed. The cables vary from tens of micrometers to less than one present report deals with superconducting electrical micrometer, while the total number of filaments can engineering developments in the USSR and other vary from a few units to hundreds of thousands. parts of the world. Individual filaments could be separated by some

141 kma — USSR ENERCV CONFERENCE MAV 13-15.1991 intermediate material a few micrometers (or less) In spite of its technological complexity, the thick. These intermediate barriers ate made from commercial production of superconducting wires and high resistant alloys in order to minimize the cables is now performed in many developed transverse conductivity. The barriers can also serve countries; this production is at the level of several some technological purposes, for instance, to prevent hundreds of tons per year, with a steady growth mutual diffusion of different components of the trend. The cost of modern commercial conductor, etc. The wires are usually twisted around superconductors varies from a few to ten US dollars their axes so that each filament follows a spiral path. per one Idloamperemeter (kAm), depending on the In some high current conductors, individual strands particular design and the production scale. It is are transposed along the lengths so that every two pertinent to compare the price of one kAm of filaments could change their position. superconducting cable with that of common copper In general, commercial superconductors should wires, carrying a current with a standard density of meet specified requirements. The corresponding 107 A/m2. Whereas the current carrying capacity of technological principles of their design and a superconductor is about SO times mat of a copper production are now fairly well understood. They ate conductor, its effective price is smaller by a factor of based on the electric, thermal and mechanical two. processes which have been experienced in more than In the state of the art discussion on tens thousands of superconducting magnets4. superconducting materials and their prospects for Although many different superconductors are commercialization, two recent breakthroughs should known, only two of them are used in the high-current be mentioned. The first one, which took place in superconducting technologies. One is a group of 1983, is connected with the use of superconducting Nb-Ti alloys with slightly varying composition, and magnets to generate AC fields with frequencies of the other is the intermetaUic compound, Nb^Sn. 50-60 Hz. Until then, the superconductors were Their critical parameters are listed in Table 1. Nb-Ti extensively used only for steady field generation or superconductors became commercially available in for fields changing rather slowly (characteristic the late sixties and NbjSn conductors in the middle frequency of 10'2 Hz), e.g. for magnets of seventies. superconducting synchrotrons. Of these two superconductors, Nb-Ti alloys are The reasons for such limitations on AC more technologically feasible (for their ductility); applications of superconductors are the following. they are also cheaper and thus preferred for use in As the magnetic field partly penetrates the bulk of a superconducting magnets. They yield overall current superconductor, changes of the field lead to densities that are about ten times higher than those of dissipation. Additional losses could arise due to traditional conductors (without any power Table 1. Critical parameters of superconductors dissipation). Supenxnducton A. J*(10»A/m2) As can be seen from the •K (nm) (nm) (5T) (1CT) data in Table 1, the use of Nb3Sn conductors Lo» Temperature allows the building of Nb-Ti 95 12 250 6.2 15/4.5 — magnets with larger fields Nb-Zr 10.5 10 160 6.7 20 or higher operating 18 24 180 6.5 50/15 25 temperatures. These 23.2 35 133 3.6 180 94 magnets can be more 18.2 42 150 3.3 100 50 reliable, since their critical v Ga 15.5 23 168 4 90 45 parameters could be much * 14.2 59 244 2.8 100 63 higher than those of nMoe>* operating ones, at 4.2K. HkhTtmpmtKn However, due to the natural ~T brittleness of NbjSn. all 94 50 200 corresponding technologies mt4.2K are more intricate. The 85 100-200 300-1000 1.5-30 targe scale production of 110 2 10 Nl^Sn conductors was first realized in the USSR in the it 773 K Tokamak T-1S installation B—critical magnetic induction for fusion. J — critical current density

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turbulent currents in stabilizing metals with high point—77.3 K at 1 atm) for refrigeration. conductivity (Cu, Al). The removal of power Some authors place these discoveries at the same dissipated at liquid He temperatures require 500 to level as die development of semiconductor 1000 times more power than at room temperature. technology or even that of the nuclear fission Accordingly, the AC applications of superconductors reactions which form the base for atomic power should become economical if the losses in generation. They envisage radical changes in superconducting windings could be made less than electricity and power generation technologies due to 10"3 of corresponding losses in traditional practical application of high temperature conductors at room temperature. superconductors. To what extent ate these Investigations of different dissipative AC expectations justified? mechanisms in superconducting cables have First of all it should be stressed that currently even established the main relationship between losses and "high-temperature" superconductivity is still a the parameters of a cable. This allows the cryogenic superconductivity. Thus superconducting development of procedures serving to reduce the devices must be cooled, the cables or wires must be dissipation rate. Hysteresis losses in superconductors stabilized, magnets and their windings should be are proportional to the superconducting filament protected against possible damage from quenching to diameter. These losses can be significantly reduced normal (resistive) state, etc. Consequently, the main by using very thin filaments. Turbulent and coupling advantage of high-temperature superconductors, currents in normal components of the cable can be associated with the use of much less expensive liquid also suppressed by using alloys with high resistivity nitrogen instead of liquid helium, increases and by making the twist pitch sufficiently small. refrigeration efficiency, simplifies the construction of the cryostat, etc. The properties of these two Table 2. Parameters of superconducting wire for AC cryogenic liquids are illustrated in Table 3. application in high resistive matrix It is obvious that high temperature Diameter of wire (mm) 0.2 -0.5 superconductors will be used on high-current technology as soon as we would be able to produce Diameter of filaments (utn) 0.4-1 conductors with such properties. Particularly, their Number of filaments 75,000 current carrying capacity at working temperature (at Critical current density (A/m2) (1.6-2)»109 4.2 K) should not fall far below the characteristic (SC-alloy)at4.2KandST values of new commercially available Twist pitch (mm) 1.3 -3.3 low-temperature superconductors and their prices should also be comparable. The problem is mat we have to produce quite long and flexible conductors These measures led to dissipation rates of the order from substances which mechanically are very much of 104 for the corresponding values of traditional conductors at room temperature. T«Me3. Comparison cf coolants: liquid nitrogen vs liquid helium The characteristics of typical AC Liquid Nitrogen liquid Helium superconducting wire are presented in Table Physical properties 2. Development of such cables opens new • Latent heat at (MJ/nr) 161 2.58 possibilities for using high current • Heat transfer (kW/m2) 115-180 9-10 o superconductors in the power engineering. q,«Tlurf«e-Ti( K) 6-18 0.5-1 The production of such cables in small • Specific heat of copper experimental quantities began in France, at T. (I/kg-oR) 230 0.09 Japan, USA and USSR. • Electrical breakdown strength (kV/mm) 2 1.9 HIGH-TEMPERATURE Technical and economic SUPERCONDUCTORS — STATE OF aspects THE ART • Cost ($/litre) 0.11-0.3 5-12 The second and clearly the most significant • Coefficient of breakthrough in superconductivity was the power removal (Watts at 300°K/Watt at T.) 450-800 discovery of the so-called high-temperature 8-12 • Advantages easier thermal easier superconductors made by Bemorz and Mulle- insulation evacuation in 1986*. This revelation was followed by the of stored synthesis of superconducting oxide ceramics easier operation energy (with critical temperatures of < 125 K), Bom large allowing the use of liquid nitrogen (boiling magnets

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like porcelain. This simplified comparison illustrates conductors in the accelerator's bending and focusing the complexity of just one purely mechanical magnets (a huge machine) would be about twice that problem, to say nothing about others, also not very of superconducting magnets made of Nb-Ti alloys. trivial ones9. These estimates are based on experience gained with The efforts to produce current-carrying structures the 0.8 TeV proton synchrotron at Fermilab in the with high-temperature superconductors, which have USA. In a circular tunnel of this accelerator, which is been performed during the five years mat elapsed 6.3 km long, there are 774 superconducting dipoles, since their discovery, have greatly enhanced our each 6 meters long and weight of 3.8 tons. The understanding of the problem. Some promising maximum field of these magnets reaches 4.5 Teslas results were obtained with miniature samples of (T). Two hundred and fifty superconducting high-temperature superconductors. But, these quadrupole and sextupole lenses are also installed in samples could hardly serve as a model of conductors the accelerator. Superconducting magnets of this or cables; they may be considered only as a accelerators turned out to be more reliable than demonstration of the possibility of attaining high traditional electric devices. critical current density values. In most cases, Larger accelerators are now being developed however, the developed prototypes of long wires and (Table 4): tapes made of high temperature superconductors are • proton accelerator for the DESY complex in characterized by unacceptably low critical current Hamburg, Germany (energy — 0.82 TeV; 422 density and steep decreasing of that value, as the superconducting dipoles, each 8.9 m long with magnetic field becomes stronger 4. maximum field of 4.62 T; 224 quadrupoles); Nevertheless, optimistic estimates suggest that by • accelerating and storage ring complex in the end of this century high-temperature Protvino, USSR (3 TeV, 21 km long ring. 2000 superconductors will be sufficiently developed to dipoles with maximum field of 5 T). compete with standard conductors and • superconducting supercollider (SSC) in the USA low-temperature superconductors. (20 TeV, 86 km long ring, 10,000 dipoles with maximum field of 6.6 T); SUPERCONDUCTING TECHNOLOGIES AND • large hadron collider (LHC) in Cern, DEVICES Switzerland (20 TeV, 27 km long ring, 1,600 Superconducting electric devices are yet to become dipoles with a maximum field of 10 T, probably widely used in modem industry. A few pilot units of made from Nl^Sn). This accelerator is already such devices have been developed and tested and in operation. their economical and technological characteristics It should be stressed that, in case of huge have been investigated. Applied superconductivity traditional accelerators, the use of superconducting however, is used in so-called industrial physics, i.e. magnets is the only alternative for proton energy, building accelerators and thermonuclear surpassing 0.5 TeV. installations. It is also utilized in medicine for The use of high-temperature superconductors, if constructing nuclear magnetic resonance imaging they could be made at a price comparable to that of systems (NMR) with superconducting coils. In this low-temperature ones, would mean transition from

S Place State Length, Number of Maximum field took place — more than 1,500 (km) magnets In dipole (T) superconducting NMR imaging Tevatron completed 6 774 dipoles 4.5 systems have been built and are Fermilab.USA in 1983 216 quadrup. extensively used for medical research and diagnostics. New Hera,Desy, to be 6 422 dipoles 5 construction and further Germany completed 224 quadrup. improvements of such systems are in steady progress. Various UNK.USSR under 21 2160 dipoles 5 applications of superconductors Serpukhov construction 350 quadrup. will be discussed in detail below. LHC, Cent under 27 1,800 dipole* 10 1. Particle Accelerators Switzerland development 600 quadrup. Research in high-energy physics requires accelerators reaching SSC. USA under 86 8000 dipoles 6.6 particles energy of over 1 TeV development 1400 quadrup. (1012 eV). The cost of traditional

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liquid helium to liquid nitrogen. Since the cost of countries. Some results of this activity are presented superconducting magnets and the cryogenic system in Table 5. accounts for less than 30% of the total accelerator If we once again consider a possible transition cost, and the cost of the cryogenics is about 1/5 of from low-temperature to high-temperature that, about 3% capital saving of the total capital cost superconductors with comparable parameters, only could be envisaged by a change of the coolant The about 3% t.wings in capital and operational costs operational cost savings could also be of the same could be obtained. The cost of the superconducting order, even if possible helium losses ate taken into magnets and the cryogenic system is estimated at account. These relatively modest figures could be, about 30% of the total capital cost. But, as in the however, associated with very large absolute previous case, a relatively small savings could be 13 values . associated with rather large absolute figures. To conclude again, the use of superconducting magnets 2. Fusion Installations with Magnetic is die only alternative for thermonuclear reactors Confinement of the Plasma with magnetic confinement The problem of controlled thermonuclear fusion reaction to produce industrial power challenges the 3. Magnetohydrodynamic Generators minds of scientists for obvious reasons. 'Taming" of for Power Production this reaction could mean the development of a power Magnetohydrodynamic (MHD) generation of electric source with infinite resources of cheap fuel and very power promises to raise the efficiency of thermal mild environmental effects in comparison with energy conversion up to 45-50% (in power plants uranium fission reaction (due to absence of fission based on fossil fuels). This should be compared to fragments). 33-38% efficiency for the traditional steam turbine The toroidal reactor, or Tokamak, proposed in the cycle. The method involves hot, partly ionized gases USSR at the early fifties, is now the most developed (combustion products) streaming through a concept of fusion reactor. The magnets for such transverse magnetic field, thus generating electric installations, or more precisely, magnets for the main power. The value of the power that could be insulating toroidal field would require 400-800 MW produced per unit volume of the plasma duct is of electric power or even more. Estimates show that proportional to the square of the magnetic flux if traditional conductors were used, almost all the density. It turns out once again, that the use of MHD power produced by the plant would be consumed for method for energy conversion is justified industrially feeding these magnets. Accordingly, the only if a high magnetic field is maintained by the development of superconducting Tokamaks began in superconducting windings. Only then the power the early seventies and it is going on in many needed for generating the field will consume a small

Table 5. The main characteristics of SMS Tokamaks T-7 TRIAM-1M Tore Supra T-15 HER Plasma major radius (nt) 1.22 0.8 2.25 2.43 5.8 Plasma minor radius (m) 0.35 0.12x0.18 0.7 0.7 2.2 Plasma current Ip (mA) 0.4 0.5 1.7 1.4-23 22 Toroidal magnetic field at the plasma center BT (T) 3 8 4.5 3.5-5.0 53 Maximum field in the winding BmCT) 5 11 9 6.9-94 12 Average current density in the winding J (mA/m2) 47 57.4 40 33 -30 Cooling mode forced flow bath cooling superfluid He, forced flow forced flow 1.8 K Coil shape round D-shiped round round D-ihaped Number of coils 48 16 18 24 16 Superconductor Nb-Ti UbjSn Nb-Ta-Ti NbjSn NbjSn Stored energy (MJ) 20 76 600 380-790 40,000 Year of completion 1978 1986 1988 1989 projected, end of century

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Table 6. Parameters of super-conducting magnets for MHD generators Designer Julich, Hitachi, Argoime- CFFF, IVTAN, Industrial Germany Japan IVTAN USA USSR prototype Diameter of warm bore (m) 0.22x0.29 0.39 x 1.3 0.4 — 0.6 0.8 x 1.0 1.8 3—5 Length of magnet (m) 2 2.77 4 3 11.2 20 Conductor cross section (cm) 0.29x0.14 0.8x0.35 1.0x0.2 _ 1.5x3 1.5x3 Magnetic field in the winding (T) 6.5 6.75 6 6 5.7 6

Stored energy (MJ) 10 60 168 216 940 10,000 fraction of the whole power output. Therefore, as in Only superconductors can be used for inductive previous cases the use of superconductivity in MHD energy storage, since it is difficult to imagine such a power plants has no alternatives. device with resistive conductors. Some estimates1*, The development of superconducting magnets for however, show that inductive storage with low MHD generators started in the middle seventies in temperature superconductors could become the USSR, USA, Japan and Germany (Table 6). A promising at energies in the order of 1013J (5 GWh). significant step was the joint Soviet-USA experiment Essential experience has been accumulated in the on the by-pass duct of the U-2S installation at the USSR and other countries in construction and Institute for High Temperatures, USSR, which was development of superconducting storages with carried out in the late seventies16. Maximum steady moderate energies (10'-10°J). A storage coil was power reached was 575 kW. successfully used for transmission line. The stored The experience gained in the developing steady energy amounted up to 30 MJ and the maximum and pulsed MHD generators opens the way for power reached 10 MW20. construction of an experimental MHD plant Schemes of giant superconducting storages, for an However, due to the latest financial and technical energy of ten thousand times that of the maximum difficulties, the construction of this plant has been values reached in any superconducting magnet, are frozen. well known. Such projects could probably be further As can be seen from Table 6, very large magnets developed only in the next century. This must be built for industrial applications. development should be preceded, however, by Construction of such magnets is a rather hard, but studies of demonstration modules and their ability to still conceivable engineering problem. If the supply the desirable energy for an acceptable price. construction of an industrial MHD power plant In the USSR, the construction of an experimental without superconducting magnets would be multi-purpose installation for 100 MJ of stored impossible, the shift to high-temperature energy can be considered a step in this direction21. superconductors at the liquid nitrogen temperature level would lead to a capital savings of 3-5%; the 5. Superconducting Motors and Generators same value saving is estimated for operational The potential use of superconductors in a new costs". generation of electric motors, generators and other equipment have been well understood from the very 4. Superconducting Inductive Energy Storages beginning of the research in applied Inductive storage of energy in the magnetic field of superconductivity. Their use would allow a drastic superconducting magnets is one of the promising cut in the size and weight of the equipment at directions in applied superconductivity. The device increasing efficiency (by 0.5% to 1.5%) and lower could be used for energy storage, load control and energy costs. Some parameters of tested or projected stabilization of power networks. This method has cryogenic alternators are included in Table 7, which some evident advantages over other ways to store illustrates the developments in the field. Very useful energy (such as water dams, gas compressing experience has been gained in the Leningrad power stations, etc.) due to its high efficiency, iast reaction, network with a superconducting alternator of 20 better ability to network connection and little MVA, which started in 198223. Tests conducted environmental effect. within the framework of this project have confirmed

146 NA CHBMOPIBOV UTILIZATION OF SUPE JCTOfS

Table 7. Superconducting cryoturbogenerators with power above 1 MVA temperature superconductors with liquid Country Designer Power, MVA State nitrogen refrigeration will reduce capital and operational costs by 20%. It will also simplify the separator MTT 3 Tested in 1975 10 Tested in 1975 construction, making it more suitable USA Westinghouse 5 Tested in 1972 for the mining industry. Works in mis 300 Projected direction are actively continuing in the GE 20 Tested in 1982 USSR. 1200 Projected Toshiba 6 Tested in 1977 7. Power Transmission Lines Hitachi JAPAN 50 Tested in 1982 The feasibility of superconducting Mitsubishi 30 Tested power lines have been demonstrated Fuji 70—200 Projected even with helium temperature Electromasch 1.5 Tested in 1972 superconductors24. However, an 20 Tested in 1981 industrial application of USSR 1200 Projected superconducting power lines is not Electrosila 300 Under testing anticipated in the near future. Alternative methods of transporting the reliability of the corresponding equipment. electric power, namely high voltage lines and According to some estimates, alternators using insulated oil or gas underground cables, require helium level superconductors could become lower capital and maintenance costs. The appearance advantageous over traditional ones at power level of high-temperature superconductors with nitrogen exceeding approximately 500 MVA18- Transition to refrigeration will not change this picture17. high temperature superconductors with liquid Therefore, limited efforts are being applied towards nitrogen refrigeration could lower the corresponding the development of superconducting lines. marginal power level to 100-300 MVA. Superconducting motors have advantages over CONCLUSIONS standard electric motors, which are similar to those The aforementioned examples of superconductor's of superconducting alternators. Many different types use in electric power equipment do not cover all of motors have been built and tested; the power of possible applications. We did not mention here rather one model constructed in the USSR reached 11 successful efforts made in Israel in developing MVA. The estimates on commercial feasibility of superconducting magnets which are very effective in motors with low-temperature superconductors are scientific research and in medical diagnostic rather contradicting, so the marginal power value processes (namely, magnets for NMR imaging). varies in different sources from 1 to 100 MVA6. The main conclusions of this overview are: The use of high-temperature superconductors in • The technology of high-current low-temperature electric motors will shift the marginal power value, superconductivity has applications in scientific probably, to 1 MVA. This estimate justifies the research, "industrial" physics and medical development of a new system of naval propulsion diagnostics. It was used to build pilot electric that was started in some countries, including the power devices, demonstrating the possibility of USSR. drastic changes in power technology. • The scale up to industrial installations is now 6. Magnetic Separation mainly a matter of economics. Until This new branch of applied superconductivity has high-temperature industrial conductors with taken just a few steps towards commercialization. Its parameters rivaling those of low-temperature main advantage over traditional technologies using ones become available, tow-temperature resistive magnets lie in the significant power savings superconductors should be used for building of up to 90%. It also could be used to treat weak different prototypes and industrial installations. magnetic ores, separate organic fuels from sulfur and • The development of modern industrial other undesirable impurities, treat industrial wastes, superconducting products (in form of wires, etc. These possibilities explain the growing efforts in cables, tapes, etc.) and me discovery of superconducting magnetic separation (Table 8). The high-temperature superconductors will influence subject is of great significance for the USSR, some other possible applications. Further because its well-developed separation industry reduction in capital and maintenance costs consumes substantial amounts of electric power. would help the penetration of superconductor According to certain estimates, shifting to high technology into the power industry.

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Table 8. Superconducting separators Lengths Diameter B.T NOTES (cm) (cm) I. LABORATORY SEPARATORS Research separators (developed by developed by: USA, Great Britain, IMET, GIREDMENT Germany.Finland, CSFR, Japan, MEKHANOB., IAE China, USSR and others) H. LARGE SCALE SEPARATORS VGMS DFPE. (France) 80.0 12.0 5.0 separation of hematite e = 98%, b = 68* ERIES MAGN. (USA) 94.0 15.0 5.0 OGMS DIPOLE. Hels.Un. 30.0 28.0 75T2/m separation Tech. (Finland) HELMG.ATF. (Austria) 50.0 20.0 100T2An height select CRYOJIN. (USSR) 100.0 12 coils 70T2/m benefication m. INDUSTRIAL SEPARATORS VGMS CSFR 150.0 56.0 5.0 caolin rectification ERIES MAGN. (USA) 400.0 200.0 2.0 caolin 200 t/h rectification ERIES MAGN. (USA) 4000 300.0 2.0 current 500 t/h on/off

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