GB1576416A

GB1576416A - Stabilised super-conductor

Info

Publication number: GB1576416A
Application number: GB52000/76A
Authority: GB
Original assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Priority date: 1975-12-15
Filing date: 1976-12-13
Publication date: 1980-10-08
Also published as: DE7602004U1; DE2602728A1; CH601900A5

Description

(54) STABILISED SUPER-CONDUCTOR (71) We, BBC BROWN BOVERI & COMPANY LIMITED., a Swiss company of Baden Switzerland., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a stabilised superconductor suitable for the production of magnet coils and comprising a multiplicity of filaments of super conductive material which are embedded in a matrix consisting of material of normal electrical conductivity at the operating temperature of the superconductor.

For example, in order to generate the magnetic fields which are required in conjunction with atomic fusion, superconductors are required to produce high magnetic field strengths of up to 12 Tesla and must carry electric currents of up to 50,000 A and more.

However, superconductive materials suitable for such strong fields and heavy electric currents and consisting of an intermetallic A15 compound, such as Nb3 Sn, V3Ga or V3 Si, exhibit the great disadvantage that they are very brittle after heating to produce the compound has been carried out, and that they may be only slightly bent and stretched if their superconductive properties are not to be impaired.

The production of magnet coils consisting of superconductors involves the great disadvantage that any damage to the filaments of superconductive material before or during the winding of the magnet coil can be detected only when the magnet is completed and has been cooled to its operating temperature. In particular, the use of superconductive filaments consisting of one of the very brittle intermetallic A15 compounds involves extremely great danger of damage to said filaments when the superconductor is further pressed in order to make a magnet coil, that is to say, when the superconductor is bent.

According to the present invention there is provided a stabilised superconductor suitable for the production of magnet coils and comprising a multiplicity of filaments of superconductive material which are embedded in a matrix consisting of material of normal electrical conductivity at the operating temperature of the superconductor, the superconductor being so constructed that if it is excessively bent, to such an extent as to damage so many of the filaments of superconductive material that the superconducting properties of superconductor are significantly impaired, an irreversible electrical or visual change, ascertainable from outside at normal ambient temperature, is produced.

In order to be able to ascertain immediately from outside whether a bent superconductor has been bent to beyond the permissible extent and thus at least some of the filaments of superconductive material have been damaged, it is advantageous for the superconductor to be of such configuration that if it is excessively bent, thus damaging too many of the filaments of superconductive material, easily visible deformations in a transverse direction of the superconductor, such for example as inward and outward swellings and/or inward and outward kinks, appear on the outside of the superconductor. For this purpose, it may be advantageous for at least one U-section steel reinforcing member on the outside of the superconductor to be fastened to the superconductor by means of its base portion and one limb, and for the second limb of the reinforcing member to be of such dimensions as to kink outwards if the superconductor is excessively bent. However, it is also possible to provide at least one cooling pipe abutting the outside of the superconductor, and of such dimensions and configuration that it swells inwards or outwards if the superconductor is bent to an excessive extent.

In order if possible to be able to be able to avoid excessive bending of the superconductor altogether, it is advantageous for it to be reinforced against bending by means of reinforcing elements extending in its longitudinal direction, so that in the presence of the maximum bending moments normally to be expected in the expected direction of bending during production and further processing it can be bent only to such an extent that the filaments of superconductive material do not undergo any such damage as would lead to a reduction in their electrical superconductive properties with the conductor in operation.

In this connection, it is advantageous for the superconductor to be reinforced against bending in such a manner, and for the filaments of superconductive material to be so arranged, that bending during production and further processing of the superconductive is limited so that stretching of the outermost and most heavily stressed superconductive filaments does not exceed 2%. The critical amount of current which superconductive filaments can carry is known to fall noticeably in the presence of any stretch exceeding 0.2%, so that it is advantageous so to choose the reinforcing materials, and to give them such a cross-section, that the superconductive filaments are not stretched by more than 0.2% in their longitudinal direction in the presence of the stresses to be expected in the production, further processing and operation of the superconductor.

The fact that the outermost filaments may be allowed to be stretched by up to 2% when the conductor is bent arises from the fact that in this case the majority of the filaments disposed further inwards undergo less stretch, or are even subjected to a compressive force.

In many cases, it may be advantageous for the superconductor to be provided with a multiplicity of superconductive wires constituting a cable, in which wires a large number of filaments of superconductive material is embedded in a matrix consisting of material of normal electrical conductivity, In order to impart slight flexibility to the cable when the superconductor is bent, it is expedient for the wire constituting the cable to be plastically joined to one another and to the remaining adjoining parts of the superconductor, for example by soft-soldering.

It is furthermore advantageous for the filaments of superconductive material to be surrounded by a supporting carrier structure and for the joints between the various parts of the carrier structure which are relevant as regards strength to be made by brazing or welding.

In order to impart as little mechanical stress as possible to the filaments of superconductive material when the superconductor is bent, it is preferable for these filaments to be as near as possible to the surface which is neutral when the superconductor is bent.

In order to cool the superconductor to its operating temperature, it is advantageous for it to comprise at least one cooling pipe, preferably seamless, extending along its longitudinal axis, or at least one cooling duct.

For the purpose of electrically stabilising the superconductor, it is convenient for it to comprise at least one stabilising element extending in its longitudinal direction and consisting of material, such as copper or aluminium, having normal electrical conductivity at the operating temperature of the superconductor, in which case it is advantageous, in order to minimise the eddy-current losses which occur when the superconductor is being used in very quickly varying magnetic fields, for the stabilising material to be subdivided into a plurality of wires separated by a coating of high ohmic resistance consisting for example of a CuNi alloy. In tihis case, it may be expedient for the superconductive wires also to be surrounded by a coating of high ohmic resistance consisting for example of a CuNi alloy.

The invention will be explained by way of example hereinafter with reference to the drawing, in which: Figures 1 to 4 are sections through respect tive forms of superconductor according to the invention, given by way of examples.

In the case of the form of embodiment illustrated in Figure 1, the superconductor is provided with a multiplicity of superconductive wires 2 constituting a cable 1, in which a large number of filaments of a brittle intermetallic A15 compound, for example Nb3 Sn, V3Ga or V3 Si, are embedded in a matrix consisting of material of normal electrical conductivity at the operating temperature of the superconductor, such as Cu-Sn for Nb3Sn filaments or Cu-Ga for V3 Ga filaments etc. This matrix may also comprise material of high electrical conductivity, such for example as copper or aluminium, for the purpose of electrical stabilisation.

In addition to the superconductive wires 2, the cable 1 comprises wires 3 consisting of stabilising material and reinforcing wires 4 consisting of steel.

Whether or not stabilising or reinforcing wires are included in the matrix together with the superconducting wires, all the wires in the matrix may be cabled, roped or braided with one another, or transposed.

In order to stabilise the superconductor electrically, stabilising and supporting portions 5 and 6 of material of good electrical conductivity, such for example as copper or alu minium, are advantageously arranged to extend along the cable 1.

In order additionally to increase the mechanical strength, the unit made up to the cable 1 and the stabilising and supporting portions 5 and 6 is completely surrounded by a steel jacket 7. Two further stabilising and supporting portions 8 and 9 consisting of copper or aluminium are arranged on the outside of the steel jacket 7.

All the wires 2, 3 and 4 of the cable 1 are joined to one another by means of a Cu-Ag brazing composition, of which the melting point is lower than that of the matrix and which is at least substantially as strong as the matrix at the operating temperature of the superconductor, and constitute a compact component which is resistant to deformation.

In this case, the brazing time and the melting point of the brazing composition used, in the region of about 700"C, lie in such a range that the superconductive properties of the filaments of superconductive material are not impaired.

Furthermore, the cable 1, the stabilising and supporting portions 5, 6, 8 and 9 and the reinforcing sheath 7 are firmly joined to one another by way of brazing composition, and thus constitute a component of great mechanical strength which is suitable for producing the magnet coils required for carrying out atomic fusion. Reinforcing material may be embedded in each stabilising portion and metallurgically joined to it.

This superconductor is reinforced against bending by two C-shaped reinforcing elements 8, 9 extending in its longitudinal direction, in such a manner as to exhibit in the expected direction of bending a bending stiffness such that in the presence of the maximum bending moments to be expected during production and further processing, said moments being limited on the one hand by the machines used and on the other hand by persons in the vicinity of the superconductor, the latter can be bent only to such an extent that the filaments of superconductive material are not subjected to-any damage which would lead to a reduction in the electrical superconductive properties with the conductor in operation.

It has transpired in practice that it is sufficient for the superconductor to be reinforced in such a manner that when it is being produced and further processed bending is so limited that the stretch imparted to the superconductive filaments in the cable 1 disposed furthest from the neutral surface, and thus most heavily stressed, does not exceed 2%, since then practically no reduction in the electrical superconductive properties can be detected when the conductor is in operation.

In order to indicate visually from outside any excessive bending of the finished superconductor for unforeseen reasons, and leading to a reduction in the electrical superconductive properties at the corresponding point, the outside ot the superconductor is treated with a material, for example a lacquer, which changes in colour or reflectivity if the superconductor is excessively bent and immediately indicates the damage.

The outside of the superconductor may also be provided with a very thin ceramic coating capable of being deformed within certain limits, and becoming detached or tearing at the relevant point if the superconductor is excessively bent.

It is also possible to apply a so-called brittle lacquer to the outside of the superconductor, the density of the fine cracks which appear in the coating of laquer when the superconductor is bent being quantitatively measurable measure of the stress to which the conductor has been subjected.

Alternatively, at least those portions of the superconductor which are most exposed to damage when the coil is being wound may be provided with at least one extension-responsive member which'is stressed beyond its normal reversible working range if the superconductor is excessively bent. Thus, for example, it is possible to apply to the outside of the superconductor a coating consisting of electrically conductive or semiconductive material whereof the electrical conductivity is altered under tensile stress, for example as a result of cracks being formed, so that the degree of deformation may be continuously measured electrically by means of two contact rollers, preferably at least after the last bending operation carried out on the superconductor.

In order to be able to detect electrically and at normal room temperature whether the superconductor has been excessively deformed during winding or operation of the magnet coil, it is also possible, instead of the above-named coating, to fasten to the surface of the superconductor, for example three (for reasons of safety) electrically conductive wires 17, 18 and 19 embedded in a foil 16 of electrically nonconductive material. The wires 17, 18 and 19 consist of such a material, and are so tightly fastened to the surface of the superconductor that if the latter is excessively bent the wires 17, 18 and 19 break, which can be immediately detected electrically. In order to ensure that such a measuring wire shall not be broken by chance, there are preferably three such wires in the case of the form of embodiment illustrated in Figure 1.

In the case of the form of embodiment illustrated in Figure 2, a cable 1 consisting of a multiplicity of superconductive wires 2 is likewise used.

In order to stabilise the superconductor electrically, two stabilising and supporting portions 5 and 6 of symmetrical configuration and consisting of material of good electrical conductivity, such for example as copper or aluminium, are arranged to extend along the cable 1, and are so shaped that two cooling pipes extending symmetrically with respect to the cable 1 and intended to carry a coolant can be inserted into them.

Abutting the outer surfaces of the two stabilising and supporting portions 5 and 6, for the purpose of reinforcing the superconductor against forces acting on it, there are provided two U-shaped steel reinforcing members 12 and 13, mutually interfitting and extending in the longitudinal direction of the superconductor, reinforcing members 12, 13 are fastened to members 5, 6 by way of their base portions 12a and 13a respectively and by way of one limb 12b and 13b respectively, so that the second limb 1 2c and 1 3c respectively of each of the U-section reinforcing pieces 12 and 13 projects freely.

In the case of this embodiment of the superconductor, the superconductive wires 2 constituting the cable 1 are soft-soldered to the adjoining parts 5 and 6 of the carrier structure so that when the superconductor is bent the superconductor wires 2 can be slightly displaced with respect to one another and to the parts 5 and 6, so that the danger of damage to the brittle superconductor filaments is greatly reduced.

The stabilising and supporting portions 5 and 6 and the profiled reinforcing pieces 12 and 13 are brazed or welded to one another in order to attain sufficient bending stiffness.

This superconductor is of such dimensions and configuration, taken as a whole, as to exhibit, in the expected direction of bending, a bending stiffness such that, in the presence of the maximum bending moments normally to be expected during production and further processing, it can be bent only to such an extent that the filaments of superconductive material undergo no such damage as would lead to a reduction in the electrical superconductive properties of the conductor when in operation.

The exposed, free-standing limbs 12c and 1 3c of the profiled reinforcing pieces 12 and 13 are so arranged and of such dimensions that if the superconductor is nevertheless unexpectedly subjected for unforeseen reasons to so great a bending moment that excessive curvature is imparted to it, and as a result damage to the filament of brittle superconductive material leads to a reduction in the superconductive properties when the conductor is in operation, these exposed limbs 1 2a and 1 3c kink outwards and visually indicate from outside the damage to the superconductive filaments at this point.

In the case of the embodiment illustrated in Figure 3, the cable 1 consisting of mutually transposed superconductor wires 2 is arranged in the middle of the superconductor, and reinforced by means of a steel reinforcing wire 4 extending along the axis of the superconductor.

In order satisfactorily to stabilise the superconductor electrically, there are provided three stabilising wires 5 extending along the cable 1 consisting of material of normal electrical conductivity at the operating temperature of the superconductor, such as copper or aluminium, and in order to cool the superconductor three seamless-drawn steel cooling pipes 14 are provided adjoining the cable 1 and the outside of the superconductor.

The steel cooling pipes 14 serve to reinforce the superconductor, and the latter is also surrounded by a steel jacket 15 to provide reinforcement purposes.

The superconductive wires 2 of the cable 1 are plastically joined to one another and to the remaining adjoining parts 4, 5 and 14, for example by way of soft solder, and all the parts 5, 14 and 15 of the carrier structure are given corresponding dimensions and brazed or welded to one another in order to attain the desired bending stiffness.

The cooling pipes 14 adjoining the outside of the superconductor are of such dimensions and configuration that if the superconductor undergoes unforseen excessive bending, for example due to the winding machine being incorrectly adjusted, the cooling pipes 14 swell inwards or outwards at the excessively curved point, as a result visually indicate any damage to the filaments of superconductive material such as would lead to a reduction in the electrical superconductive properties of the conductor when in operation, so the coil to be produced is not wound any further.

The embodiment illustrated in Figure 4 differs from that illustrated in Figure 3 chiefly in a different geometrical arrangement of the various parts of the superconductor.

A cable 1 containing superconductive wires 2 and of rectangular cross-section is abutted on its longer sides be electrical stabilising and supporting members 5, 6, while cooling pipes 14, are applied to the ends of the assembly thus formed, which is surrounded by a sheath 15.

Here again, the cooling pipes 14 are of such dimensions and configuration that unforseen excessive bending of the superconductor, for example during winding of a magnet coil, would become visible from outside as a result of the cooling pipes 14 swelling inwards or outwards at the corresponding point.

In order to obtain minimum eddy-current losses when the superconductor is being used in very quickly varying magnetic fields, the supporting and stabilising members 5 and 6 consisting of stabilising material of normal electrical conductivity at the operating temperature of the superconductor, such for example as copper or aluminium, are subdivided into a plurality of wires 5a, 5b, 5e and 5d, and 6a, 6b, 6c and 6d, extending parallel to one another, and these wires 5a to 5d and 6a to 6d are individually separated by a coating of high ohmic resistance consisting for example of a CuNi alloy and brazed to one another.

In addition, the individual superconductive wires 2 of the cable 1 may also be surrounded by a coating of high ohmic resistance.

It will be understood that in all the desired embodiments the filaments of superconductive material are, at least substantially, concentrated around the surface which is neutral as the superconductor is intended to be bent. In alternative embodiments the filaments of superconductive material are so arranged that, in the finally bent superconductor they are disposed in that part of its cross-section that is compressively stressed when the superconductor is bent in its intended direction.

Reference is drawn to our applications 52001/76 and 52002/76 (Serial Nos. 1569983 and 1576417) which relate to superconductors the components of which are connected together by plastically deformable solder and by hand solder, respectively.

WHAT WE CLAIM IS: 1. A stabilised superconductor suitable for the production of magnet coils and comprising a multiplicity of filaments of superconductive material which are embedded in a matrix consisting of material of normal electrical conductivity at the operating temperature of the superconductor, the superconductor being so constructed that if its is excessively bent, to such an extent as to damage so many of the filaments of superconductive material that the superconducting properties of superconductor are significantly impaired, an irreversible electrical or visual change, ascertainable from the outside at normal ambient temperature, is produced.

2. A superconductor in accordance with claim 1, so constructed that said excessive results in the appearance of visible deformations in a transverse direction of the superconductor, such for example as inward and outward swellings and/or inward and outward kinks, on the outside of the superconductor.

3. A superconductor in accordance with claim 2, wherein at least one U-shaped steel reinforcing piece is fastened to the outside of the superconductor by way of its base portion and one limb and in that the second limb of the profiled reinforcing piece is of such dimensions as to kink outwards if the superconductor is excessively bent.

4. A superconductor in accordance with claim 1, wherein at least one cooling pipe adjoins the outside of the superconductor, and is of such dimensions and configuration as to swell inwards or outwards if the superconductor is excessively bent.

5. A superconductor in accordance with claim 1, having its outer surface treated with a material which, if the superconductor is excessively bent undergoes a change in colour, reflectivity or electrical conductivity, or becomes detached or tears, at the relevant point.

6. A superconductor in accordance with claim 1, wherein at least those portions which are most exposed to damage when the coil is being wound are provided with at least one extension-responsive member which is stressed beyond its normal reversible working range if the superconductor is excessively bent.

7. A superconductor in accordance with claim 1, wherein the superconductive filaments are supported by a carrier structure of such configuration that if the superconductor is excessively bent, the elastic limit of at least part of the carrier structure is exceeded, so that a permanent and externally visible kink is formed at the relevant point.

8. A superconductor in accordance with claim 1, which is reinforced against bending by reinforcing elements extending in its longitudinal direction, so as to exhibit in the expected direction of bending a bending stiffness such that, in the presence of the maximum bending moments normally to be expected in its production and further processing, it can only be bent to such an extent that the filaments of superconductive material are not subjected to any damage which would lead to a reduction in the electrical superconductive properties with the conductor in operation.

9. A superconductive in accordance with claim 8, which is reinforced against bending in such a manner, and the filaments of superconductive material are so arranged, that bending during production and further processing of the superconductor is so limited that stretch in the outermost and most heavily stressed superconductive filaments does not exceed 2%.

10. A superconductor in accordance with any one of claims 1, 8 or 9, and including a multiplicity of superconductive wires constituting a cable, in which wires a large number of filaments of superconductive material is embedded in a matrix consisting of material of normal electrical conductivity.

11. A superconductor in accordance with claim 10, wherein the wires constituting the cable are plastically joined to one another and to the remaining adjoining parts of the superconductor, for example by soft soldering.

12. A superconductor in accordance with claim 1 or any one of claims 8-11, wherein the filaments of superconductive material are surrounded by a supporting carrier, and the joints which are relevant as regards strength between the various parts of the carrier structure are made by brazing or welding.

13. A superconductor in accordance with claim 1 or any one of claims 8-12, wherein the filaments of superconductive material are at least substantially concentrated round a surface which is neutral when the superconductor is bent in a direction in which it has been designed to be bent.

14. A superconductor in accordance with claim 1 or any one of claims 8-13, and com

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. of a CuNi alloy and brazed to one another. In addition, the individual superconductive wires 2 of the cable 1 may also be surrounded by a coating of high ohmic resistance. It will be understood that in all the desired embodiments the filaments of superconductive material are, at least substantially, concentrated around the surface which is neutral as the superconductor is intended to be bent. In alternative embodiments the filaments of superconductive material are so arranged that, in the finally bent superconductor they are disposed in that part of its cross-section that is compressively stressed when the superconductor is bent in its intended direction. Reference is drawn to our applications 52001/76 and 52002/76 (Serial Nos. 1569983 and 1576417) which relate to superconductors the components of which are connected together by plastically deformable solder and by hand solder, respectively. WHAT WE CLAIM IS:

1. A stabilised superconductor suitable for the production of magnet coils and comprising a multiplicity of filaments of superconductive material which are embedded in a matrix consisting of material of normal electrical conductivity at the operating temperature of the superconductor, the superconductor being so constructed that if its is excessively bent, to such an extent as to damage so many of the filaments of superconductive material that the superconducting properties of superconductor are significantly impaired, an irreversible electrical or visual change, ascertainable from the outside at normal ambient temperature, is produced.

prising at least one preferably seamless cooling pipe or at least one cooling duct extending along its longitudinal axis.

15. A superconductor in accordance with claim 1, and comprising at least one stabilising portion extending along its longitudinal axis and consisting of a material of normal electrical conductivity at the operatng temperature of the superconductor, such as copper or alu maximum.

16. A superconductor in accordance with claim 15, wherein reinforcing material is embedded in the or each stabilising portion and is metallurgically joined to it.

17. A superconductor in accordance with claim 1 or any one of claims 8-14 and including at least one reinforcing portion extending parallel to its longitudinal axis, and of greater strength than the stabilising material.

18. A superconductor in accordance with claim 1 or any of claims 8-14, wherein the matrix of the superconductive wires consists of reinforcing and/or stabilising material.

19. A superconductor in accordance with claim 10, wherein the cable made up of superconductive wires comprises at least one reinforcing wire and/or at least one stabilising wire extending parallel to the longitudinal axis of the superconductor.

20. A superconductor in accordance with claim 10 or 19, wherein the wires constituting the cable are cabled, roped or braided with one another, or transposed.

21. A superconductor in accordance with claim 10, 19 or 20, wherein the individual wire of the cable are joined to one another by a heat-conductive jointing material, preferably a brazing composition, the melting point of which is lower than that of the matrix, and which is at least substantially as strong as the matrix at the operating temperature of the superconductor.

22. A superconductor in accordance with claim 21, wherein the cable is brazed to the adjoining parts of the superconductor.

23. A superconductor in accordance with claim 1 or claim 8, wherein the filaments of superconductive material are so arranged as to be disposed in that part of the cross-section of the finally bent superconductor that is compressively stressed when the superconductor is bent in its intended direction of bending.

24. A superconductor in accordance with claim 8 which is sheathed and reinforced by a welded-on steel jacket.

25. A superconductor in accordance with claim 15, wherein the stabilising material is subdivided into a plurality of wires which are separated by a coating of high ohmic resistance consisting for example of a CuNi alloy.

26. A superconductor in accordance with claim 10, wherein each of the superconductive wires is surrounded by a coating of high ohmic resistance consisting for example of a CuNi alloy.

27. A superconductor in accordance with claim 5, provided on its surface with at least one electrically conductive wire extending over its whole length, and embedded in an electrically non-conductive matrix, said wire being so arranged and of such configuration as to break if the superconductor is excessively bent.

28. A superconductor in accordance with claim 8, wherein the reinforcing materials are so chosen and of such cross-sectional dimensions that the filaments of superconductive material are not stretched by more than 0.2% in their longitudinal direction in the presence of the stresses to be expected in the production, further processing and operation of the superconductor.

29. A superconductor in accordance with.

any one of claims 1-28, wherein the superconductive material consists of an intermetallic A15 compound, for example of Nb3 Sn, V3Ga or V3Si.

30. A superconductor substantially as described with reference to Figure 1, 2, 3 or 4 of the drawings.