DE202012102334U1 - NbTi superconductors with circumferentially distributed Al blocks for weight reduction - Google Patents

Abstract

Superconductor wire (1; 31), containing NbTi superconductor material and Cu, with - a cladding tube (2), in particular a Cu cladding tube, - with at least three Al blocks (3a-3c) distributed in the circumferential direction in the cladding tube (2) are arranged, and with at least three NbTi-containing sections (4a-4c), which are also distributed in the circumferential direction in the cladding tube (2), and which separate the Al blocks (3a-3c) in the circumferential direction, the Al -Blocks (3a – 3c) each lie flat against their neighboring NbTi-containing sections (4a – 4c).

Description

The invention relates to a superconducting wire containing NbTi superconductor material and Cu.

Niobium titanium (NbTi) is an important superconducting material which, because of its good plastic deformability (both in the finished material and in respect of precursors), can be used in a variety of ways, for example in superconducting magnet coils or superconducting cables. The NbTi superconductor material is typically integrated into a superconducting wire, with a multiplicity of NbTi filaments generally running along the direction of extension of the superconducting wire.

In the context of wire production, it is often intended to make a plastic deformation into a hexagonal outer cross-section in order to simplify bundling processes in single-filament NbTi filaments (or other structures and intermediate structures), as part of a drawing process. The resulting hexagonal elements can then be arranged very compactly, for example in a copper cladding tube, cf. the aforementioned US 5,088,183 A ,

The superconducting properties of NbTi are achieved only at a particularly low temperature, below about 9 K, so that the superconductor must be cooled, such as with liquid helium. Likewise, the superconducting state is no longer maintained at too high magnetic field strength or too high current density.

NbTi superconducting wires are typically stabilized with copper (Cu), usually at high levels of purity. The copper runs parallel to superconducting filaments in the superconductor wire; For example, the copper may form a matrix for superconducting NbTi filaments. The copper is a good conductor of heat through which effective cooling of the superconducting NbTi filaments can take place. Furthermore, the copper has a high electrical conductivity. In the case of local loss of superconductivity in the superconducting wire, the copper provides a parallel current path which limits an increase in electrical resistance as well as associated heat buildup. Due to the effective cooling, the local normal conducting region in the superconductor can finally become superconducting again (stabilization function). Although recovery of superconductivity is not possible, the copper protects against damage ("burn-through") of the superconductor by limiting the increase in resistance and thereby limiting heat generation, and at the same time efficiently dissipating the resulting heat (quenching). The copper also improves the mechanical strength of the superconducting wire.

On the one hand, copper is an expensive material, and on the other hand, with a density of about 8.9 g / cm ^{3, it is} relatively heavy, which is undesirable in many applications. Therefore, alternative materials for the stabilization of NbTi superconductors are sought, which can at least partially replace the copper. Aluminum (Al) in pure form would be a suitable material in terms of heat conduction and electrical resistance, which is cheaper than copper and significantly lighter at a density of 2.7 g / cm ³ .

In the WO 2008/121764 A1 For example, an NbTi superconductor structure having a Cu ferrule, an Al core, and multifilament NbTi rods interposed therebetween has been proposed.

While Cu and NbTi are both relatively hard and behave similarly when plastically deformed, Al is much softer compared to NbTi and Cu. In wire drawing, when using Al structures, particularly an Al core, cracks were observed across the extension direction of the wire in an NbTi superconducting wire. A cracked wire is unusable for superconductor application.

From the US 5,189,386 For example, a superconductor structure has been known in which an Al core is divided into six sectors by layers of a Cu-Ni alloy. This structure is relatively complex.

In the US 4,652,697 Considered as the closest prior art, a superconductor structure is described in which three Al wires and four multifilament wires, for example with NbTi filaments and a copper matrix, have been bundled. One of the multifilament wires is arranged in the center of the superconductor structure. The structure is held together by a PbSn solder. A solder is difficult to process in the wire production and can further aggravate the cracking problem as another material.

The JP 09 282953 A describes an Nb ₃ Al superconducting wire in which Nb ₃ Al filaments are incorporated into a copper matrix. For reinforcement embedded additional structures may be provided such as Nb or Ta. In one embodiment, additional structures with an Al rod in a Cu tube are proposed.

Object of the invention

The invention has for its object to propose a stabilized NbTi superconducting wire, which has a low weight, the production of which is inexpensive and wherein the superconducting wire has a reduced tendency to cracking (especially when Drahtzeihen).

Brief description of the invention

• – einem Hüllrohr, insbesondere einem Cu-Hüllrohr,
• – mit mindestens drei Al-Blöcken, die in Umfangsrichtung verteilt im Hüllrohr angeordnet sind,
• – und mit mindestens drei NbTi-haltigen Sektionen, die ebenfalls in Umfangsrichtung verteilt im Hüllrohr angeordnet sind, und die die Al-Blöcke in Umfangsrichtung voneinander trennen,

wobei die Al-Blöcke jeweils flächig an ihren benachbarten NbTi-haltigen Sektionen anliegen.This object is achieved by a superconducting wire containing NbTi superconductor material and Cu, with

• A cladding tube, in particular a copper cladding tube,
• - With at least three Al blocks, which are arranged distributed in the circumferential direction in the cladding tube,
• - And with at least three NbTi-containing sections, which are also distributed in the circumferential direction in the cladding tube, and which separate the Al blocks in the circumferential direction from each other,

wherein the Al blocks each lie flat against their adjacent NbTi-containing sections.

In the invention, copper is partially replaced by aluminum in the superconducting wire for stabilization, at least three Al blocks being provided. The Al blocks are distributed in the cross-section of the wire (perpendicular to the longitudinal direction of the wire) in the circumferential direction and separated by NbTi-containing sections, d. H. Al blocks and NbTi-containing sections alternate in the circumferential direction. The Al blocks are significantly lighter and also less expensive than copper material of the same volume. In particular, the weight of the meter (that is to say the mass of the superconducting wire per length in the direction of extent) can be significantly reduced compared with the exclusive use of copper for stabilizing the superconducting material.

By a surface contact according to the invention between the NbTi-containing sections and the Al blocks a good, direct heat and power transition is ensured; Typically, the two-dimensional contact in cross section is over at least 20% of the wire diameter at approximately radially extending interfaces of NbTi-containing sections and Al blocks. This ensures on the one hand a very good stabilizing function, but on the other hand also keeps the overall structure of the superconducting wire simple.

The cladding tube can also improve the stabilization, especially if this is designed as a Cu cladding tube. The cladding tube can still hold together the Al blocks and the NbTi-containing sections well.

The superconducting wire according to the invention can readily be formed without soldering (in particular without a PbSn solder). In particular, no solder is needed to provide sufficient heat conduction or power line contact between the Al blocks and the NbTi superconductor material in the NbTi-containing sections; Likewise, no solder is needed to hold the overall structure of the superconducting wire together. Impurities of the superconducting wire with foreign particles can be kept very low in the invention.

The Al blocks are made of solid aluminum, typically with a purity of 99 atomic% or higher. The cross-sectional area of a superconducting wire according to the invention typically has a content of at least 10% aluminum, preferably at least 20% aluminum. The NbTi-containing sections are preferably composed only of Nb, Ti and Cu (apart from doping and impurities below 1 atom%). Each NbTi-containing section contains at least one, but typically a plurality of NbTi filaments extending in the longitudinal direction of the superconducting wire. Copper (Cu) structures in the superconducting wire are also typically of high purity (preferably> 99 atomic% Cu); but they can also be formed with a copper-containing alloy (preferably ≥ 80 atom% Cu, more preferably ≥ 90 atom% Cu in the Cu alloy); this applies in particular to a Cu cladding tube, a Cu profile tube, Cu filling plates and Cu blocks (see below).

The contact surfaces between the differently deformable materials aluminum on the one hand and copper and niobium titanium on the other hand according to the invention are divided into at least three azimuthally distributed (in the circumferential direction) Al blocks. As a result, a mechanical stress build-up in the deformation (for example during wire drawing) in the circumferential direction with respect to the entire superconducting wire is correspondingly often interrupted, which counteracts the formation of cracks. On the other hand reduces the formation of Al blocks of solid material, the contact surface with aluminum; Material inside the Al blocks hardly takes part in a tension build-up. The structure of the invention has been shown in practice to be highly resistant to cracking. In particular, with a small number of azimuthally distributed Al blocks (maximum six Al blocks, preferably three or four Al blocks), a maximum of one additional, central Al block (with a diameter less than or equal to 30% of the diameter of the superconductor wire, preferably less than or equal to 25% of the diameter of the superconducting wire) and typically no further Al structures in the superconducting wire are obtained with very good results.

The superconducting wire according to the invention as a whole typically has an approximately circular cross section, but may also have a different (outer) contour, for example a hexagonal contour.

Overall, with the invention, a weight-reduced NbTi-based superconducting wire can be provided, which at low production costs only limited interface between Al on the one hand and Cu and NbTi on the other condition, so that a favorable, low-cracking deformation behavior (especially in extrusion and Ziehumformung) is obtained. In the context of the manufacturing process, the risk of introducing foreign particles is also low, which, on the one hand, reduces the quench risk and, on the other hand, allows a high heat and current conductivity of the protective structures of Al and Cu.

Preferred embodiments of the invention

In a preferred embodiment of the superconducting wire according to the invention, the NbTi-containing sections are formed by one or more deep-hole drilled Cu blocks, wherein NbTi structures, each containing one or more NbTi filaments, are inserted in holes of the one or more Cu blocks , In this way, NbTi-containing sections can be formed very easily and inexpensively; the Cu block or blocks are easy to handle during the manufacturing process. The bores and the associated NbTi structures typically have a round cross-section. The one or more Cu blocks also participate in the stabilization of the NbTi filaments (cooling / replacement current paths).

Preferred is a development of this embodiment, in which the NbTi-containing sections are formed by only one deep-hole drilled Cu block, wherein the Cu block has at least three radially inwardly formed grooves, each containing an Al block. This development is particularly easy to handle in the manufacturing process; in the grooves of a Cu block, only the Al blocks need to be inserted or inserted. The Al blocks are then held securely by the Cu block and the cladding tube.

In an alternative development of the above embodiment, it is provided that the NbTi-containing sections are each formed by a deep-hole drilled Cu block, and that a central Cu profile tube is provided, to which the Al blocks and the Cu blocks are adjacent the profile tube contains an additional Al block. Through the profile tube and the additional Al block, the proportion of aluminum in the cross section can be further increased, and so the weight of the superconducting wire can be further reduced.

Also preferred is an embodiment in which the NbTi-containing sections are each formed as a cluster of contiguous NbTi hexagonal elements each containing one or more NbTi filaments. This can be used in a confined space very many NbTi filaments; the fabrication of the NbTi-containing sections can easily be adapted (via the number and arrangement of bundled NbTi hexagonal elements) to different wire geometries.

Advantageous is a development of this embodiment, in which a central Cu profile tube is provided, to which the Al blocks and the clusters adjoin, wherein the profile tube contains an additional Al block, and wherein the Cu profile tube an outer profiling suitable for Plant has the NbTi-hexagonal elements, in particular wherein the Cu-profile tube has an all-round outer profiling. Through the profile tube and the additional Al block, the proportion of aluminum in the cross section can be further increased, and so the weight of the superconducting wire can be further reduced. The profiles allow a dense (low-backlash or even backlash-free) packing of the structures of the superconducting wire, in particular in its central region.

Also preferred is a development in which the cladding tube contains a plurality of Cu filling plates, wherein the Cu filling plates have a profiling suitable for a system of NbTi hexagonal elements of the cluster. The filler plates in turn allow a dense (low-backlash or even backlash-free) packing of the structures of the superconducting wire, in particular in the region of the (typically round) inner contour of the cladding tube.

Also preferred is a development in which the Al blocks have a profiling suitable for the system of NbTi hexagonal elements of the cluster, in particular wherein the Al blocks have an all-round profiling. The profiling in turn allows a dense (low-backlash or even play-free) packing of the structures of the contact surfaces between the Al blocks and the clusters.

Particularly preferred is an embodiment of the superconducting wire according to the invention, which provides that exactly N Al blocks are provided, which are arranged distributed in the circumferential direction in the cladding tube, and that exactly N Nb Ti-containing sections are provided, which are also distributed in the circumferential direction arranged in the cladding tube in the circumferential direction, where N = 3, 4, 5 or 6. This number range has proven particularly useful in practice; the entire interface between Al on the one hand and Cu and NbTi on the other hand remains limited, since the Al is distributed over comparatively few Al blocks.

In a preferred embodiment, it is finally provided that the superconducting wire has a circular cross-section. This can be force peaks on the superconducting wire - be it in operation or in drawing processes - minimized. Note, however, that other wire cross sections, in particular a hexagonal Wire cross section, according to the invention are possible, optionally where initially a production of the superconducting wire with a round cross-section, and in a subsequent drawing process, the other wire cross-section is adjusted.

Further advantages of the invention will become apparent from the description and the drawings. Likewise, according to the invention, the above-mentioned features and those which are still further developed can each be used individually for themselves or for a plurality of combinations of any kind. The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention.

Detailed description of the invention and drawing

The invention is illustrated in the drawing and will be explained in more detail with reference to embodiments. Show it:

1 a schematic cross-sectional view of a first embodiment of a superconducting wire according to the invention, with three Al blocks and three NbTi-containing sections, which are formed by a deep-hole drilled Cu block;

2 a schematic cross-sectional view of a second embodiment of a superconducting wire according to the invention, with three Al blocks and three NbTi-containing sections, which are formed by three deep-hole drilled Cu blocks, and with an additional central Al block;

3 a schematic cross-sectional view of a third embodiment of a superconducting wire according to the invention, with three Al blocks and three NbTi-containing sections, each formed by clusters of NbTi-hexagonal elements, and with an additional central Al block.

The 1 illustrates a first embodiment of a superconducting wire according to the invention 1 in a cross section perpendicular to the longitudinal direction of the superconducting wire 1 , The superconducting wire 1 has a circular cross section here.

In a cladding tube 2 , here a Cu cladding tube (with at least 80 atomic% Cu content, preferably at least 99 atomic% Cu content), are several (here three) Al blocks 3a - 3c made of high-purity aluminum (with at least 99 atom% Al content) azimuthally distributed (in the circumferential direction) evenly, in which case the Al blocks 3a - 3c directly to the cladding tube 2 adjoin. The Al blocks 3a - 3c become azimuthal from (here also three) NbTi-containing sections 4a - 4c separated from each other, also in the cladding tube 2 are arranged here and also directly to the cladding tube 2 adjoin.

The NbTi-containing sections 4a - 4c border directly and area to the Al blocks 3a - 3c , see. the interfaces 5 , In particular, no solder mediates between the Al blocks 3a - 3c and the NbTi-containing sections 4a - 4c , The direct, surface contact between the Al blocks 3a - 3c and the NbTi-containing sections 4a-4c ensures a good heat transfer and a low electrical resistance, so that the Al blocks 3a - 3c very efficient for stabilizing the superconducting material in the NbTi-containing sections 4a - 4c can contribute.

The NbTi-containing sections 4a - 4c be here by a single Cu block 6 (with at least 80 atom% Cu content, preferably at least 99 atom% Cu content) formed in which a plurality of through holes 7 are formed (deep hole drilled Cu block). The holes 7 are each with a NbTi structure 8th filled. Every NbTi structure 8th contains one or more, in the longitudinal direction of the superconductor wire 1 (ie perpendicular to the plane of the 1 ) extending NbTi filaments (the latter are not shown). The holes 7 are in the NbTi-containing sections 4a - 4c evenly distributed azimuthally, here on three radii R1, R2, R3 (in each case with respect to the bore center points).

The approximately radially extending interfaces 5 between the Al blocks 3a - 3c and the NbTi-containing sections 4a - 4c extend over about 55% of the radius R _{tot of} the superconducting wire 1 , see. Share A _Al . About 15% of the radius R _tot accounts for the wall thickness of the cladding tube 2 , and about 30% of the radius R _tot account for a central portion of the Cu block 6 , In general, it is preferable if the interfaces 5 extending in the radial direction at least over 40% of the radius R _ges (note that the interface 5 does not need to run exactly parallel to a radius vector; if necessary, the interface can be projected onto a radius vector).

The center of the superconductor wire 1 is in the embodiment of 1 essentially of Cu material of the Cu block 6 taken; towards the center are the Al blocks 3a - 3c here in each case an approximately perpendicular to the direction of a radius vector extending, further interface 9 out.

The interfaces 5 and the other interfaces 9 each bound three radially inward into the Cu block 6 milled grooves 10 into which the Al blocks 3a - 3c are inserted.

The 2 illustrates a second embodiment of a superconducting wire according to the invention 1 in a cross section perpendicular to the longitudinal direction of the superconducting wire 1 , The following are the main differences from the embodiment of 1 presented.

In this embodiment, the NbTi-containing sections 4a . 4b . 4c each by its own, deep hole drilled Cu block 6a . 6b . 6c formed, ie there are three Cu blocks here 6a - 6c , according to the number of NbTi-containing sections 4a - 4c , In the holes 7 the Cu blocks 6a - 6c is each a NbTi-Sturktur 8th contain. The holes 7 here are arranged evenly distributed on two radii R2 and R3 azimuthally. The NbTi-containing sections 4a - 4c or Cu blocks 6a - 6c separate the Al blocks 3a - 3c in the azimuthal direction, with the NbTi-containing sections 4a - 4c and the Al blocks 3a - 3c again with interfaces 5 immediately adjacent to each other. This is a good stabilization contribution of the Al blocks 3a - 3c guaranteed.

In the center of the superconductor wire 1 is a Cu profile tube 11 arranged. In the Cu profile tube 11 is an additional, here round Al-block 12 inserted; the radius R _{Al of} the additional Al block 12 here is about 25% of the radius R _{ges of} the superconducting wire 1 , The Cu profile tube 11 lies outside on the Cu blocks 6a - 6c and the Al blocks 3a - 3c flat, compare interfaces 13 and 9 , The inner and outer contours of the Cu profile tube 11 is doing the Al blocks 3a - 3c , the Cu blocks 6a - 6c and the additional Al block 12 adapted so that no voids between said structures of the superconducting wire 1 stay. In the embodiment shown, the Cu profile tube 11 to a hexagonal outer contour on. The Al blocks 3a - 3c and the Cu blocks 6a - 6c are still with round outer contours close to the cladding tube 2 on, so that here too no cavities remain.

3 shows a third embodiment of a superconducting wire according to the invention 31 , again in cross-section perpendicular to its longitudinal extent.

The superconducting wire 31 points (similar to the embodiment of 2 ) in a cladding tube 2 namely a Cu cladding tube, three Al blocks 3a - 3c in the circumferential direction alternately with three NbTi-containing sections 4a - 4c are arranged. The NbTi-containing sections 4a - 4c each through a cluster 34a - 34c of (in cross section) adjacent NbTi hexagonal elements 35 educated. Each NbTi hexagonal element 35 has a hexagonal cross-section and contains one or more NbTi filaments (the latter not shown) extending in the longitudinal direction of the superconducting wire 31 (perpendicular to the plane) run; every cluster 34a - 34c contains thirty-six NbTi hexagonal elements here 35 ,

In the center of the superconductor wire 31 is a Cu profile tube 32 arranged outwardly profiled in a hexagonal scheme jagged, cf. the all-round external profiling 32a , Inside the profile tube 32 is an additional Al block 33 arranged.

On the inside of the cladding tube 2 are Cu filling plates 36 arranged outwardly profiled around the inside and jagged in hexagonal scheme. Furthermore, the Al blocks 3a - 3c (in cross section) profiled on all sides jagged in hexagonal scheme. The profilings of the Cu filling plates 36 , the central Cu profile tube 32 and the Al blocks 3a - 3c enable together with the clusters 34a - 34c an overall compact (in particular void-free) and mechanically stable overall structure of the superconducting wire 1 ,

The Al blocks 3a - 3c lie in the area of the jagged boundary surfaces 5 to the NbTi-containing sections 4a - 4c or clusters 34a - 34c flat on this, which in turn ensures a good heat and electricity transfer; This will be a good stabilization contribution of the Al blocks 3a - 3c ensured, with the Al blocks 3a - 3c less weight to the superconductor wire 1 contribute as comparable Cu structures of the same volume. The interfaces 5 extend here in the radius direction over about 40% of the radius R _{ges of} the superconducting wire 1 , see. Proportion A _Al (which is determined here in projection on a radius vector). The central, additional Al block 33 also contributes to weight reduction and stabilization function.

Note that in general, after assembling a superconducting wire structure according to the invention with Al blocks and NbTi-containing sections, preferably no heat treatment above 200 ° C occurs to avoid the formation of brittle intermetallic phases of Cu and Al. Typically, only cold deformations then take place (if desired). The NbTi superconductor material in the superconducting wire is preferably of the "artificial pinning" type.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 5,088,183 A [0003]
• WO 2008/121764 A1 [0007]
• US 5189386 [0009]
• US 4652697 [0010]
• JP 09282953 A [0011]

Claims (10)

Superconducting wire ( 1 ; 31 ) containing NbTi superconductor material and Cu, with - a cladding tube ( 2 ), in particular a Cu cladding tube, - with at least three Al blocks ( 3a - 3c ) distributed in the circumferential direction in the cladding tube ( 2 ), and - with at least three NbTi-containing sections ( 4a - 4c ), which is also distributed in the circumferential direction in the cladding tube ( 2 ) and that the Al blocks ( 3a - 3c ) in the circumferential direction, the Al blocks ( 3a - 3c ) in each case flat on their adjacent NbTi-containing sections ( 4a - 4c ) issue. Superconducting wire ( 1 ; 31 ) according to claim 1, characterized in that the NbTi-containing sections ( 4a - 4c ) through one or more deep-hole drilled Cu blocks ( 6 ; 6a - 6c ), wherein NbTi structures ( 8th ), each containing one or more NbTi filaments, in holes ( 7 ) of the one or more Cu blocks ( 6 ; 6a - 6c ) are inserted. Superconducting wire ( 1 ; 31 ) according to claim 2, characterized in that the NbTi-containing sections ( 4a - 4c ) by only one deep hole drilled Cu block ( 6 ), wherein the Cu block ( 6 ) at least three radially inwardly formed grooves ( 10 ), each having an Al block ( 3a - 3c ) contain. Superconducting wire ( 1 ; 31 ) according to claim 2, characterized in that the NbTi-containing sections ( 4a - 4c ) each through a deep hole drilled Cu block ( 6a - 6c ) are formed, and that a central copper profile tube ( 11 ) to which the Al blocks ( 3a - 3c ) and the Cu blocks ( 6a - 6c ), wherein the profile tube ( 11 ) an additional Al block ( 12 ) contains. Superconducting wire ( 1 ; 31 ) according to claim 1, characterized in that the NbTi-containing sections ( 4a - 4c ) each as a cluster ( 34a - 34c ) of contiguous NbTi hexagonal elements ( 35 ) each containing one or more NbTi filaments are formed. Superconducting wire ( 1 ; 31 ) according to claim 5, characterized in that a central Cu profile tube ( 32 ) to which the Al blocks ( 3a - 3c ) and the clusters ( 34a - 34c ), wherein the profile tube ( 32 ) an additional Al block ( 33 ) and wherein the Cu profile tube ( 32 ) an external profiling ( 32a ) suitable for the installation of NbTi hexagonal elements ( 35 ), in particular wherein the Cu profile tube ( 32 ) an all-round external profiling ( 32a ) having. Superconducting wire ( 1 ; 31 ) according to claim 5 or 6, characterized in that the cladding tube ( 2 ) several Cu filling plates ( 36 ), the Cu filling plates ( 36 ) a profiling suitable for a system of NbTi hexagonal elements ( 35 ) the cluster ( 34a - 34c ) exhibit. Superconducting wire ( 1 ; 31 ) according to one of claims 5 to 7, characterized in that the Al blocks ( 3a - 3c ) a profiling suitable for the installation of NbTi hexagonal elements ( 35 ) the cluster ( 34a - 34c ), in particular wherein the Al blocks ( 3a - 3c ) have an all-round profiling. Superconducting wire ( 1 ; 31 ) according to one of the preceding claims, characterized in that exactly N Al blocks ( 3a - 3c ) are provided, distributed in the circumferential direction in the cladding tube ( 2 ) are arranged, and that exactly N NbTi-containing sections ( 4a - 4c ) are provided, which are also distributed in the circumferential direction in the cladding tube ( 2 ) and that the Al blocks ( 3a - 3c ) in the circumferential direction, where N = 3, 4, 5 or 6. Superconducting wire ( 1 ; 31 ) according to one of the preceding claims, characterized in that the superconducting wire ( 1 ; 31 ) has a circular cross-section.

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