US 3639672 A
A cryogenic electric conductor formed from a plurality of superconducting wires arranged in a twisted configuration. The conductor is enclosed in a vacuumtight sheath and the gaps between conductor and the sheath and between the individual wires are filled with a low-temperature cooling medium, preferably helium. Each of the wires has one or more superconducting cores surrounded by a metal e.g. copper, in efficient heat-conducting contact therewith. The sheaths may be formed of a low-conducting metal or from a plastic.
Claims available in
Description (OCR text may contain errors)
United States Patent Kafka 541 ELECTRICAL CONDUCTOR  Inventor: Wilhelm Kafka, Tennenlohe, Germany  Assignee: lnstitut fur Plasmaphysik G.m.b.H.,
Garching, Germany  Filed: Feb. 20, 1970 21 Appl. No.: 13,123
 Foreign Application Priority Data Feb. 21, 1969 Germany ..P 19 08 885.8
 US. Cl. ..l74/15 C, 174/27, 174/126 R, 174/126 CP, 174/128, 174/129 R, 174/DIG. 6,
 Int. Cl ..H0lb 7/34, H01b 5/00  Field ofSearch ..l74/15, 15 C,DIG.7, 126, 174/26, 27, 34, 102-107, 113, 115, 99, 126,129,
 References Cited UNITED STATES PATENTS 2,344,501 3/1944 ..174/106 3,218,693 11/1965 .174/D1G. 7 3,291,898 12/1966 ..174/15 X 3,343,035 9/1967 ....174/15 X 3,366,728 l/1968 174/1 13 3,431,347 3/1969 Kafka et a1 ..174/15 IIIII' 1 Feb. 1, I972 Primary Examiner-Thornas J. Kozma Assistant ExaminerA. T. Grimley Attorney-Spencer & Kaye [5 7] ABSTRACT A cryogenic electric conductor fonned from a plurality of superconducting wires arranged in a twisted configuration. The conductor is enclosed in a vacuumtight sheath and the gaps between conductor and the sheath and between the individual wires are filled with a low-temperature cooling medium, preferably helium. Each of the wires has one or more supcrconducting cores surrounded by a metal e.g. copper, in efficient heat-conducting contact therewith. The sheaths may be formed of a low-conducting metal or from a plastic.
24 Claims, 10 Drawing Figures PATENIEUFEB 1m 3.639.672
SHEET E OF 2 l 67a "'67b 67c T 68 L 66 T 68 70a 70b 70c CRYOGENIC COOLING AND C/RCULATING SYSTEM FIG. 70
INVHN'I ()R: Wilhelm Kafka Attorneys.
ELECTRICAL CONDUCTOR BACKGROUND OF THE INVENTION The present invention relates to cryogenic electric conductors for superconducting windings or switching paths (cryotrons). More particularly this invention relates to cryogenic electric conductors having a plurality of superconductor wires which are arranged alongside one another in a twisted configuration.
It is known to operate superconducting windings in a bath of liquid helium in order to maintain the required low working temperature of, for example, 4.? K., and to carry off both the heat flowing in from the surrounding outer environment and that generated in the winding. It is also known to construct the winding from tubular conductors which internally carry the cooling medium, for example liquid or gaseous helium. In this manner the winding requires only heat insulation outside the conductor, but does not require a helium vessel. Such directly cooled hollow conductors consist of a tube of normally conducting metal, such as copper or aluminum, in which the superconductor wires are embedded as separate wires, or which carries the superconductor as a surface film either on its inside or outside. A tube of this kind naturally has a diameter of several millimeters, since otherwise the cooling medium required for cooling purposes can not flow through the center thereof. In conductors for current intensities of more than 100 a. and in windings for magnetic field intensities of more than 1,000 Oe, such dimensions result in appreciable losses upon changes in current andfield, that is, for example, in windings for alternating current. Such is also the case however, with windings for direct current which are energized and deenergized within a limited time. The resulting eddy-current and hysteresis losses increase with the square of the diameter and the intensity of the magnetic field. Accordingly with a high field intensity, these losses may play a decisive part in the designing of the cooling system for the windings.
It is also known that in order to fully utilize the superconducting material, the superconductor should be fully stabilized. This is effected by combining the superconductor with highly pure copper or aluminum having a cross section which is several times larger than that of the superconductor. The effect of this combination is that in the event of surges of flux in the superconductor, the local losses remain small because the current can pass from the superconductor into a sufficiently large cross section of the stabilizing metal, the losses of which can be carried off continuously by the cooling medium. This method, however, fails to produce the desired results in directcurrent windings which are exposed to pronounced variations of magnetic field, for example energizing windings of large machines, in alternating-current windings and in cryotrons, because in all of these applications the greater the cross-sectional area of the stabilizing metal the greater the eddy-current losses. l
SUMMARY OF THE INVENTION It is accordingly an object of this invention to provide a superconducting cryogenic electrical conductor which overcomes the above cited disadvantages.
Accordingly, in order to avoid the difficulty in designing the cooling system for windings subjected to a high field intensity while at the same time also achieving the advantage of the above-described tubular type of conductor, according to the invention, a cryogenic electric conductor is provided having a plurality of superconductor wires which are arranged alongside one another in a twisted configuration and enclosed in a vacuum-tight sheath, and the gaps between the individual wires and between the bundle of wires and the sheath is filled with a low-temperature cooling medium, which may be, for example, liquid or above-critical helium. The construction of the conductor in the form of a large number of individual wires which are separated from one another and twisted avoids appreciable eddy-current and hysteresis losses and permits large-area contact with the cooling medium. Furthermore, a readily flexible conductor which can also be used for windings of small bending radius is obtained.
In order to overcome the problem of increased eddy-current losses with increased cross section for stabilized superconductor electric conductors, according to an embodiment of the invention the cross section of the wires is preferably so reduced that the eddy-current losses are negligible. Hysteresis losses are avoided for all practical purposes by utilizing superconductor cores for the wires which are very thin, insulated from one another and twisted. If a wire contains a plurality of superconducting cores, these cores are also twisted, but are only separated from one another by the normally conducting stabilizing metal. In order to render harmless any residual flux surges nevertheless still occurring when there are variations in flux, which surges are attended by localized heating, the wires are surrounded by a heat-storing agent which can absorb these localized amounts of heat without any increase in temperature worth mentioning. Liquid or above-critical helium is particularly suitable as such a heat-storing agent since in the temperature range of 4-5 K. its heat-storing capacity, referred to the unit of volume, is more than 1,000 times greater than that of the metals copper and aluminum heretofore generally employed. These metals are not able to absorb any amount of heat worth mentioning and accordingly must transfer this heat immediately to the cooling medium in order that their temperature not rise to a value which is detrimental to the superconductivity. For this reason the construction which has heretofore been customary, in which large amounts of stabilizing metal are used, is reliable in operation only when no gas bubbles of below-critical pressure accumulate at the surface of the stabilizing metal. This condition is also decisive for the reliability of operation of the type of conductor according to the invention. In the case of cooling with above-critical helium, this condition is satisfied from the start. In the case of cooling with liquid helium, that is with below-critical pressure of the cooling medium, this condition can be observed by design precautions, for example a powerful flow of the liquid cooling medium or perpendicular arrangement of the conductor.
In a preferred embodiment of the invention, superconductor wires of small diameter are accordingly brought into efficiently heat-conducting contact with a certain amount of metal, so that the area of the wires is increased relative to the area of the superconducting cores of the wires. For efficient cooling the major portion of the wire surface is in direct contact with the cooling medium, and accordingly the insulation for the wires does not therefore form a continuous covering. Separation or insulation is effected most simply by covering the wires quite loosely with insulating threads. For example, glass fiber thread of at least 50 pm. in diameter may be wound helically around the wire, with the pitch of the helix being several times as large as the thickness of the thread. The wires are then wound into a conductor, the successive or adjacent layers of which are twisted in opposite directions. The layers may be wound on an insulating helium-permeable core, for instance a coil spring made from a plastic filament. For good utilization of space, it is advantageous to enclose a round or oval conductor in a rectangular sheath in such manner that free flow cross sections are created at the four corners. The conductor can nevertheless be supported in all four directions. The result of the design of the conductor according to the invention is that the internal space of the sheath is filled by the cooling medium to the extent of at least one quarter, but preferably to the extent of more than one half of its cross-sectional area. With this construction favorable effective current densities are nevertheless obtained. This is true even in the case of direct-current windings, wherein the effective current densities are more favorable than in fully stabilized windings of conventional type.
The vacuumtight sheaths are advantageously extruded over the finished conductor. With fairly short lengths of conductor, the wires may alternatively be drawn into the sheath. The
.the;low operating temperature. Suitable as extrudablemetals' are, for example, aluminum of low purity, aluminum-zinc alf loys or nonsuperconducting lead alloys. Extruded plastic sheaths can be rendered vacuumtight by a metal coating. For example, a vacuumtight sheathmay be formed by extruding a thin supplementary coverin'g consisting of a plastic over the conductor coating the plastic covering with a firmly adhering nickel layer by means of currentless reduction, and thereafter applying another pore-free coating of metal, for example nickel or chromium afew urn. thick, thereto by electrodeposi- ,tion..
1 If the bundle of wires or conductor is enclosed by a metal sheath, it isexpedient to cover the conductor loosely with insulating threads, so that'the conductor is insulated from the sheath, whileatthe same time allowing the helium or cooling medium to pass out of the free flow cross sections and penetrate between the wires. This has the advantage that the sheaths need not be externally insulated even for turns which are disposed side by side and contacting one another, sincethe natural oxide films between the individual turns of a complete winding. are sufficient to prevent large eddy-current losses in the sheaths. The entire winding thus forms an efficiently heatcondu'cting body which, in the case of directcurrent windings,
need only be cooled at its surface and brought to'the operating temperature. In such an arrangement the cooling medium inside the sheath does-not have to be circulated and serves only to-absorb, by reason of itshigh specific heat storing capacity,
any amounts of heat generated when'flux'surges and variations such an arrangement wherein the winding is cooled externally, it is sufficient to connect the conductor 'sheaths to a helium supply bottle at one or both of the ends of the' winding. On
cooling of the initially gaseous charge of the sheath, more hell: um can then flow in and the same pressure can always be maintained If the lead-ins to the winding are at room temperature, heli um gas bottles at high pressure are advantageously employed and this pressure is then lowered by means of reducing valves to the operating pressure inside the conductor sheath. If the operating pressure is below-critical, the helium inside the sheath becomes liquid on cooling to 42 I(., if it is above criti- CaLthat is, for example, 3 ata, the helium remains gaseous at high density and no gas bubbles can form.
If the conductor according to the invention is employed for alternating current, for direct current whose magnitude varies frequently or as a magnetically controlled switching path (cryotron), i.e., of appreciable losses are to be expected during operation, it is not sufficient merely to prevent the heat flowing in from the outside from reaching the surface of the entire winding by means of a cooled shield. In such case, every conductor inside the sheath must be cooled continuously, and accordingly the cooling medium inside the sheath is continuously replaced by freshly cooled cooling medium. The fresh cooling medium is supplied to the conductor sheath at suitable intervals along the length thereof and then flows through the conductor sheath and out again after covering a certain desired distance. If the conductor sheaths are at ground potential, the supply and removal of cooling medium may be effected by means of metal tubes inside the surrounding heat in- I sulation. If metallic conductor sheaths are in electrical contact with the bundle of wires of the conductor, the repeated supply and removal of cooling medium is carried out by. way of insu- Iating tubes or metal tubes with an insulating intermediate piece. The conductor sheath of eachturn'must then also be electrically insulated from the other turns.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a cryogenic electric conductor according to the invention for direct current of great intensity;
FIG. 2 is a cross-sectional view of a wire of the conductor according to FIG. 1;
FIG. 3 is a cross-sectional view of a direct current winding composed of conductors according to FIGJI;
FIG. 4- is a diagrammatic representation of a winding 'according to FIG. 3, including the cooling system;
FIG. 5 is a cross-sectional view of a conductor according to the invention for an alternating-current winding;
FIG. 6 is a cross-sectional view of a wire for a conductor according to FIGS; I I
FIG. 7 is a cross-sectional view of a cryotronconductor according to the invention;
FIG. 8 is a sectional view illustrating a connecting piece for two conductors according to FIGS. 1 and 7;
FIG. 9 is a cross-sectional view of a conductor according to the invention having a plurality of wire bundles;
FIG. 10 is a sectional view similar to FIG. 8 illustrating a conductor according to the invention and a schematic view of an associate cryogenic cooling system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS applied to the finished conductorby extrusion. Due to the relative shapes of conductor and sheath which is of a polygonal shape, free flow cross sectionsv 4 for the cooling medium are left at the four corners of the sheath. Additionally due to the forming of the support 1 the cooling medium can also flow through the passages therein. The cooling medium can also penetrate between the individual wires through the loose covering 3 which may for example be a loosely wound helical thread. The cooling area for the wires is therefore large and preferably comprises at least one-fourth'of the cross-sectional area of the sheath.
FIG. 2 Ba cross-sectional view of a wire of the conductor according to FIG. 1, wherein 6 represents the superconducting core, The core 6 consists, for example, of the hard superconductor niobium-titanium. The core 6 is surrounded by a shell or jacket 7 of highly pure copper which may be given a sufficiently large cross section that complete stabilization occurs. In order to achieve high effective current densities, however, the cross section may be substantially reduced without suffering a decrease in the reliability of operation. Of course, if quenching of the superconductivity occurs in a winding dimensioned in this manner, the winding current must be reduced rapidly in order to prevent all the magnetic energy of the winding from being converted into heat within the conductor. To insulate the individual wires from one another, they are provided with a loose covering consisting of a helically wound insulating thread 8, whereby the cooling medium can contact the major portion of the surface of the wire.
FIG. 3 is a cross section of a winding which is built up from conductors according to FIG. 1 and serves, for example, to energize a turbogenerator. The reference 10 designates a turn formed by a conductor according to FIG. 1. Due to the presence of the insulating layer 3 within the sheaths, the turns are wound side by side and one over the other and mutually support each other. The turns of the winding are enclosed by a copper shield 12 which, however, does not have to be vacuumtight and advantageously has longitudinal and transverse slots formed therein to reduce eddy-current losses. One such longitudinal slot is designated by the reference numeral 16. The shield 12 is in heat-conducting contact with a flat cooling tube 13 carrying liquid or above-critical helium inside it. The shield 12 and the cooling tube 13 are at ground potential, and are held in position within a vacuum vessel 14 by means of lateral supports 15.
The vacuum vessel 14 is formed of metal which is a poor conductor, for example stainless steel, and is also maintained at ground potential. The lateral supports 15 may be so constructed from pressure-resistant ceramic discs that there are always only a few points of contact between the individual parts thereof so that the transfer of heat remains slight.
FIG. 4 illustrates diagrammatically a winding such as shown in FIG. 3 including the cooling system therefor. A winding 21 composed of conductors according to FIG. 1, is enclosed within a shield 22, which in turn is enclosed within the outer vacuum vessel 28 of the heat insulation. The lateral support pieces between the vacuum vessel 28 and the shield 22 are not shownQThe gap between the shield and the vacuum vessel is advantageously filled with superinsulation, that is reflecting foils, so that heat absorption is reduced. The cooling medium is introduced into the structure by means of inlets 23 extending from a reflux condenser 24. Connected to the direct current terminals 27 are the current leads 25 of the winding. Pressure bottles 26, preferably for gaseous helium are connected to the current leads 25 in order to maintain the cooling medium within the sheath of the conductor.
FIG. 5 is a cross section through a conductor which is also suitable for alternating current. The conductor consists of a plurality of wires 31 which are placed around a central wire 33 in a plurality of layers, adjacent ones of which are twisted in opposite directions. The conductor which is. circular, is placed in an extruded plastic sheath 32 of rectangular cross section, thus again providing free flow cross sections at the corners of the sheath. The sheath 32 may be made vacuumtight by coating the outer surface thereof with a metal coating 34 in a manner known in the art. Due to the fact that the illustrated sheath 32 is made of a plastic material no insulation around the entire conductor is required.
The construction of the wires 31 is illustrated in FIG. 6 wherein it is seen that each wire 31 contains a large number of superconducting cores 41 which are embedded in a metal 42, e.g., pure copper. Each wire 31 is twisted about its own axis, so that the individual cores 41 extend helically around the axis of the wire. Insulation between the adjacent wires of the cable is provided by means of an insulating thread 43 wound helically round the wire with the pitch of the helix being several times as large as the'thickness of the thread, whereby the cooling medium can easily contact the surface of the wires.
- FIG. 7 is a cross-sectional view through a conductor which can be used for a cryotron in machines or switches. The con ductor consists in this case of two layers of wires 51 which are twisted in opposite directions, and wound on a coil spring 52 made from a plastic filament. A loose or open covering 53 is disposed around the conductor as insulation and the entire arrangement is surrounded by a sheath 54. If a metal is used for this sheath, it should have a relatively poor conductivity.
The wires of the cable of FIG. 7 are constructed similarly to the wire shown in FIG. 6, but the embedding metal 42 must not have a cross section substantially larger than the sum of the cross section of the superconducting cores 41. The cores 41 consist of a soft superconductor, forexample niobium, or a hard superconductor, such as lead-bismuth eutectic with a critical field strength of about 13.8 K gauss that is below the saturation of iron. The embedding metal should have as high a specific resistance as possible. Alloys of nickel with copper, iron or chromium, for example, are suitable therefore. The
wires 51 are produced by providing a block of the embedding metal with holes into which the superconductor rods are insorted and, if necessary, soldered. The complete assembly is then worked down by hammering, rolling or drawing to a cross section so small that the individual cores are only a few microns ([1.) thick. Where soft superconductors are employed, it is particularly important to achieve as small a thickness as possible for the cores, because in this way the critical current density j and, as a result of the path-length effect, the specific resistance p are increased. To reduce the blocking losses of a cryotron, it is important to make the product j -p as large as possible. Because the critical field intensity increases with decreasing thickness of the superconducting core, there is moreover a lower limit for the thickness. In this embodiment, the twisting of the wire about its longitudinal axis is also advantageous.
FIG. 8 is a longitudinal section through a connecting piece between a conductor according to FIG. 1 and a conductor according to FIG. 7. The reference 61 designates the conductor according to FIG. 1 and the reference 62 designates the conductor according to FIG. 7. In order to establish a connection the sheath is removed from both conductors over a distance of several centimeters, and likewise the insulation between the wires. The wires 63 of the conductor 61 and the wires 64 of the conductor 62 are then inserted into each other, enclosed by a copper bushing 65, squeezed together and soldered. At this point, therefore, the wires are not insulated from one another and increased eddy-current losses are to be feared. For this reason, such connections or joints within a winding are located in a zone of small magnetic field strength. The sheaths of the two conductors 61 and 62 are interconnected in vacuumtight fashion by a sheath 66. FIG. 8 also shows the supply" of the cooling medium, which is advantageously effected at such connecting pieces so as to remove the heat produced here in a reliable manner. A pipe coupling 67 is soldered in vacuumtight fashion to a resilient intermediate metal piece 68 and thus secured to a ceramic tube 69. The tube 69 is also secured in the same manner to a feed pipe 70 for the cooling medium. In this manner, an insulation is interposed between the grounded feed pipe 70 and the metal sheath 66, which is important in cases where there is contact between the conductor and the sheath of the conductor. 7
FIG. 9 shows a conductor with a plurality of wire bundles which is suitable for alternating current or a cryotron. The individual wires are constructed, for example, as in FIG. 6. They form six single-layer bundle of wires 72 which are wound on coil springs 73 of plastic material. The bundles of wires rest in common on a coil spring 74 of plastic material. The plurality of wire bundles are enclosed in vacuumtight fashion by a sheath 75. In this case, the major portion of the internal space of the sheath is available for the flow of cooling medium.
The features of the above-described drawings may also be combined with one another in other ways. In machines with hard superconductors and cryotrons, these two conductors may also be accommodated in a common sheath or the hard superconductor may also be accommodated in the internal hollow space of a cryotron in accordance with FIG. 7 or FIG.
Reference is now made to FIG. 10 which illustrates in section a length of a conductor according to the invention. Conductor 80 may be similar as shown in FIG. 9 and has a sheath 82 of an electrically conducting material and an inner braided multiple conductor 84 which is shown only schematically and may comprise elements 71 to 74 shown in FIG. 9. No insulation is provided between sheath 82 and conductor 84. Sheath 82 is provided with pipe couplings 67a, 67b and 67c at spaced locations along the conductor 80. Each of pipe couplings 67a to 670 is connected to an individual feed pipe 70a to 700, respectively, by connecting systems as described with reference to FIG. 8 and each comprising a ceramic tube 69 electrically insulating the pipe couplings 67 from the corresponding feed pipe 70. Feed pipes 70a and 700 are connected to outlet means 85 of a cryogenic cooling and circulat- 7 ing system 86 which may comprise a source and supply of liquid helium, and a circulation pump, as known in the cryogenic art. Feed pipe 70b is connected to an inlet connection 88 of system 86 which in operation provides circulation of ,a:cryogenic cooling medium, e.g., liquid helium through perconductive windings and magnetically controlled switching paths comprising a bundle of wires formed from a plurality of 8 11. A cryogenic electric conductor as defined in claim 1, wherein said sheath is formed of an extruded plastic material having a vacuumtight metal coating on the surface thereof.
12. A cryogenic electric conductoras defined in claim 1, wherein'above-critical helium is maintained within said sheath of said conductor in that the ends of the conductor are connected to pressure bottles filled with helium gas.
13. A cryogenic electric conductor as defined in claim 1, wherein said sheath has inlet and outlet arrangements for the coolingmedium at a plurality of points along the length of the conductor, and means connected to said inlet and outlet arelectrically separated superconductor wires arranged alongside one another in a twisted configuration, a vacuumtight sheath enclosing and directly supporting said bundle of wires, and a low-temperature cooling medium filling the gaps between the individual wires and between the bundle of wires and-the sheath.
2. A cryogenic electric conductor as defined in claim 1, wherein each wire includes at least one core formed of superconducting material and a metal surrounding said core in effiv ciently heat-conducting contact therewith.
3. A cryogenic electric conductor as defined inclaim 2 wherein each of said wires includes a plurality of cores of superconducting material.
4. A cryogenic electric conductor as defined in claim 3 wherein each of said wires is twisted about its own axis so that said plurality of cores extend helically around saidaxis. I
5. A cryogenic conductor as defined in claim 2, wherein each of said wires is covered with insulating threads which are sufficiently loosely wound thereabout so that said. cooling medium comes into direct contact with the ma or portion of the surface of the said metal.
6; A cryogenic electric conductor as defined in claim 1, wherein said cooling medium occupies at least one quarter of the internal cross-sectional area of said sheath. 7
7. A cryogenic electric conductor as defined in claim 1, wherein said bundle of wires has a circular or oval cross section and is enclosed in a sheath of polygonal cross section, so that free flow cross sections are provided at the comers between the bundle of wires and the sheath.
8. A plurality of cryogenic electric conductors as defined in claim 7, wherein the conductors are arranged alongside one another between the sheaths thereof.
9. A plurality of cryogenic electric conductors as defined in claim 7, wherein the conductors are located within a metal shield of sufficiently high heat conductivity to be capable of being kept at the low operating temperature by means of tubes through which liquid or above-critical helium flows during operation.
10. A cryogenic electric conductor as defined in claim 7, wherein the sheath has a rectangular cross-sectional configuration.
rangements for maintaining the flow of cooling medium within said sheath.
14. A cryogenic electric conductor as defined inclaim 1, wherein the sheath is formed of a metal of low thermal con ductivity from the group consisting of impure aluminum alloly.
15. A cryogenic electric conductor asdefined in claim wherein said bundle of wires is loosely covered with insulating threads, so that the bundle of wires is insulated from said sheath, but, said cooling medium can pass out of the free flow cross section and penetrate easily between said wires.
16. A cryogenic electric conductor as defined in claim 5 wherein the end of said conductor is connected to the end of another similar conductor, the ends of each conductor having their-sheaths and insulations removed for a distance of several centimeters, and the uninsulated portions of the wires of the two conductors being axially intermeshed with one another, squeezed together and soldered to form said connection.
17. A cryogenic electric conductor as defined in claim 5 wherein said bundle of wires comprises a plurality of layers of wires, each layer of wires being twisted in the opposite direction from any adjacent layer.
18. A cryogenic electric conductor as defined in claim 17,
wherein said wires are wound about a nonconductive support, said support being provided with openings so that said cooling medium may freely flow within the center of said bundle of.
19. A cryogenic electric conductor as defined in claim 18, wherein said sheath is formed from a metal of low conductivity, and wherein said bundle of wires is loosely covered with insulating threads.
20. A cryogenic electric conductor as defined in claim 19 wherein each of said wires has only a single core formed of superconducting material.
2l.'A cryogenic electric conductor as defined in claim 18, wherein each of said wires includes a plurality of cores of superconducting material. 1
22. A cryogenic electrical conductor as defined in claim 21 wherein said sheath is formed from a metal of low conductivity, and wherein said bundle of wires is loosely covered with insulating threads.
23. A cryogenic electric conductor as defined in claim 17,
wherein said layers of wires are wound about a further wire extending along-the axis of the bundle of wires.
24. A cryogenic electric conductor as defined in claim 23, wherein said sheath is formed of an extruded plastic material.