EP2885792B1 - Superconducting coil device comprising a coil winding - Google Patents

Description

The present invention relates to a coil device having a coil winding of a superconducting band conductor.

In the field of superconducting machines and superconducting magnet coils, there are known coil devices in which superconducting wires or band conductors are wound in coil windings. For traditional low-temperature superconductors such as NbTi and Nb ₃ Sn, wire-type conductors are commonly used. The high-temperature superconductors or even high-T _c superconductors (HTS), however, superconducting materials with a transition temperature above 25 K and in some classes above 77 K. These HTS conductors are typically in the form of flat strip conductors, which is a band-shaped substrate strip and a superconducting layer disposed on the substrate tape. In addition, the band conductors often have further layers such as stabilization layers, buffer layers and in some cases also insulation layers.

The most important material class of the so-called second-generation HTS conductors (2G-HTS) are compounds of the type RE-Ba ₂ Cu ₃ O _x , where RE stands for a rare-earth element or a mixture of such elements. Many superconducting tape conductors with such ceramic superconducting layers are very sensitive to mechanical stresses and therefore must be protected from mechanical stresses such as tensile, compressive or shear stresses during manufacture as well as during operation of the superconducting coils.

When electrical coils are fabricated from superconducting tape conductors, successive windings of the tape conductors are typically either glued together by an impregnating resin during winding, or finally wound Coil is then potted with a casting agent. Typical casting agents here are epoxy resins, with which the coil can be cast, for example, with a Vakuumvergussverfahren. The gluing or casting of the coil turns causes the finished coil to be protected from mechanical stresses, for example due to Lorentz forces in strong magnetic fields and / or due to centrifugal forces during fast rotation.

A problem with the use of superconducting coils is the differential thermal contraction of the various materials in the coils during cooling to operating temperature. When cooling to an operating temperature of, for example, 30 K to 70 K, especially the polymeric constituents of the adhesive and / or the potting compound as well as any existing insulating materials of a greater thermal shrinkage than the metallic and ceramic components of the strip conductor. The differential thermal contraction leads to the formation of stresses during and after cooling, which can result in damage to the superconducting layer. Also, the use of a winding carrier with a greater thermal contraction than that of the strip conductor can cause the formation of radial tensile stresses perpendicular to the plane of the strip conductor and thus a compression of the superconducting layer. Above all, the radial tensile stresses lead much easier than any radial compressive stresses to damage the superconducting properties to a delamination of the superconducting layer of the substrate of the strip conductor. A radial pull causes inner layers of the coil winding to be pulled towards the coil interior, and thereby compressing the strip conductor in the longitudinal direction. The damage hereby can lead to a decrease of the maximum operating current of up to 60%, which makes the conventional winding methods for superconducting coils with the today's 2G-HTS materials unsuitable.

In the not previously published application with the official file number 102011077457.2 (published as DE 10 2011 077 457 A1 ), a coil winding is described in which a superconducting strip conductor is wound on a winding support such that there is a positive radial pressure between the layers of the coil winding both at room temperature and at an operating temperature of the coil. This can be achieved by a suitable choice of the winding support and the winding tension and by a weak connection of winding and winding support. However, even with appropriately manufactured coils, in which the winding support does not contribute to the formation of tensile stresses, only by the differences in the thermal contractions of the various materials in the winding unfavorable tensile stresses can arise. Especially with large windings with more than, for example, 100 windings, this effect can result in high tensile stresses which severely impair the superconducting properties of the coil.

DE 10 2011 118 465 A1 relates to a superconducting coil. Non-circular coil layer portions are adjacent to each other at boundary portions having an adhesive force smaller than that in other portions. JP 2010 267 835 A1 discloses a cylindrical superconducting coil of coaxial superconducting segments separated by regions of reduced adhesion.

The object of the present invention is to provide a superconducting coil device which avoids the disadvantages mentioned.

This object is achieved by the coil device described in claim 1. The coil device according to the invention comprises at least one superconducting band conductor which has a band-shaped substrate band and a superconducting layer arranged on the substrate band. The coil device is subdivided into a plurality of segments, wherein within each segment adjacent turns are cast or glued together, and wherein in the intermediate region between two adjacent segments at least in one subregion the adjacent ones Windings are at most weakly connected or glued together.

This causes the coil device according to the invention has a substantially reduced radial tensile stress of the strip conductor when cooled to its operating temperature. For suitable geometries and materials, the subdivision into segments causes the coil winding according to the invention to have at its operating temperature a substantially reduced tensile stress in the strip conductor, which is advantageously in the range of the tensile stress that the strip conductor of a coil with the winding number of a single segment would have. The invention is thus based on the finding that the voltage caused by thermal shrinkage increases with the number of turns, and that this increase can be reduced by a subdivision into weakly connected segments. The operating temperature of the superconductor is for example between 25 K and 77 K.

Advantageous embodiments and further developments of the coil device according to the invention will become apparent from the dependent claims. Accordingly, the coil device may additionally have the following features:
In the intermediate region between two adjacent segments, the adjacent windings may be connected at least in a partial area with such a weak adhesive that the compound is separated at a tension below 10 MPa. In this embodiment, the weak connection in the subregion is designed so that a radial tensile stress occurring on cooling of the superconductor to its operating temperature leads to a separation of the compound in this subregion, before the tensile stress can cause damage or even delamination of the superconducting layer. Advantageously, the separation of the compound even at 5 MPa, particularly advantageous at 3 MPa. Currently used 2G-HTS materials can withstand a tensile stress of several MPa.

In the intermediate region between two adjacent segments, at least a portion of the gap between adjacent turns may be free of adhesive bonding or potting compound. If the adjacent turns of the segments in the partial area are therefore not connected in this embodiment, then the segments in this partial area can deform independently of one another from the beginning. Even at low radial tensile stresses, the individual segments behave at least in the subregions as individual, independently thermally shrinking units.

In a further embodiment, the coil means may comprise a potting compound that envelops the adjacent turns within the segments. This potting compound may advantageously be an epoxide. The same potting compound may also be present between the segments in those sections that are outside the subregions with at most weakly connected adjacent turns.

In a further embodiment, the coil device may have a coating with a release agent or an inserted band of a release agent at least in a partial region of the intermediate region between two adjacent segments. The coating or the inserted band of a release agent then advantageously prevent the wetting with the potting compound or the adhesive in these areas, so that then the potting or bonding is either completely prevented or the bond in comparison to other areas of the winding only extremely weak is. The release agent may advantageously be PTFE.

In a further embodiment, in the intermediate region between two adjacent segments, the strip conductor may be provided, at least in a partial region, with an additional layer which is formed from a material having a coefficient of thermal expansion which is smaller than the coefficient of thermal expansion of the strip conductor is. It is advantageous if the thermal shrinkage of the additional layer by cooling to the operating temperature is below 0.3%, particularly advantageously below 0.1%. In this embodiment, there is no void between the adjacent segments in said subregion because the region between the unconnected or weakly connected band conductors is now filled by the less shrinkable interlayer. This intermediate layer behaves like an effectively expanding layer in comparison to the other materials and thus has an increased relative space requirement during and after cooling. This results in no cavities and thus causes greater mechanical stability of the coil winding after cooling. For example, the additional layer may be formed of graphite having a very low coefficient of thermal expansion. Particularly advantageously, the material for the additional layer has a negative coefficient of thermal expansion.

In a further embodiment, in the intermediate region between two adjacent segments, the band conductor can be provided, at least in one subregion, with an additional layer which is formed from a flexible material with a tensile strength of less than 10 MPa. In this embodiment, the stresses between the segments can be compensated by yielding the flexible material of the additional layer. If the adjacent strip conductors are still weakly connected in this area, then the weak connection can advantageously remain even after cooling. In this embodiment, the coil winding is mechanically more stable than in the complete absence of connection and in the formation of cavities.

The coil winding is formed in a first embodiment of the invention as a racetrack coil or as a rectangular coil.

If the coil winding is designed as a racetrack coil or as a rectangular coil, then there are several partial areas at most weak connection of the adjacent turns of adjacent segments within the curved portions of the racetrack or rectangular coil. In particular, the subregions with at most weak connection can advantageously lie in the four corners of the racetrack or rectangular coil. This embodiment has the advantage that on the straight portions of the coil, which form a large proportion of the entire length of the winding, all turns can be potted or glued together. This leads to a significantly improved mechanical stability of the coil winding. This embodiment is based on the finding that the tensile stresses arising due to thermal shrinkage arise primarily in the curved regions and therefore can best be reduced there as well by dividing them into segments. In the straight sections of a rectangular or racetrack coil, the winding can shrink relatively stress-relieved. This is comparable to the thermal shrinkage of a flat stack of tape conductors in which differences in the thermal expansion coefficients of the different materials can be compensated for by varying degrees of contraction in the tape conductor plane and perpendicular to the tape conductor plane.

In an alternative, non-inventive embodiment, the sub-regions with at most weak connection of the adjacent turns of adjacent segments may lie within the regions which comprise the curved regions of the coil winding and transition regions bordering on both sides. In this embodiment, even at the curved areas adjacent, straight transition areas are provided in which there is at most a weak connection between the segments. This has the advantage that where the strong connection of the segments merges into a weak connection of the segments, there are still no large radial tensile stresses due to the cooling. It is thus avoided kinking of the strip conductor in the area where the strong connection of the segments merges into a weak connection of the segments.

In a further embodiment of the invention, the coil winding is formed as an approximately cylindrical winding and the segments are formed as radial segments.

If the coil device is designed as a cylindrical winding with radial segments, then the sections with at most weak connection of the adjacent turns in a non-inventive embodiment can extend over at least one complete turn of 360 degrees. This embodiment has the advantage that a radial tensile stress which arises between the segments as a result of cooling is compensated as far as possible. The effective strain relief due to the weak connection between the segments is particularly effective wherever the coil winding is curved, ie in the case of a cylindrical coil over the entire circumference of the coil.

According to the invention, the approximately cylindrical coil is also formed from alternating straight areas and bent areas. Depending on the number of total existing areas or angle segments, the cylindrical shape is then given only more or less approximately. In this embodiment, the subregions with at most weak connection of the adjacent turns of adjacent radial segments are advantageously located in the region of the bent regions. However, it should not be ruled out that the subregions with at most weak connection extend into transition regions on both sides of the bent regions, so that a bending of the strip conductor is advantageously avoided.

The superconducting layer of the coil device may comprise a high-temperature superconductor of the second generation, in particular ReBa ₂ Cu ₃ O _x .

The coil means may comprise a cooling system, wherein the segments of the coil winding individually to the cooling system can be coupled. This embodiment is particularly advantageous if the segments are connected to one another either over the entire circumference of the coil or over relatively large portions at most weakly. Then it is particularly important to ensure that the individual segments are thermally well coupled to the cooling system for cooling to the operating temperature of the superconductor.

Fig. 1

einen schematischen Querschnitt eines supraleitenden Bandleiters zeigt,

Fig. 2

einen Ausschnitt einer Spulenwicklung gemäß einem ersten Ausführungsbeispiel zeigt und

Fig. 3

eine Spulenwicklung gemäß einem zweiten Ausführungsbeispiel in schematischer Draufsicht zeigt.

The invention will be described below with reference to two preferred embodiments with reference to the appended drawings, in which:

Fig. 1

shows a schematic cross section of a superconducting strip conductor,

Fig. 2

a section of a coil winding according to a first embodiment shows and

Fig. 3

shows a coil winding according to a second embodiment in a schematic plan view.

Fig. 1 shows a cross section of a superconducting strip conductor 1, in which the layer structure is shown schematically. The strip conductor in this example comprises a substrate strip 2, which here is a 100 μm thick substrate made of a nickel-tungsten alloy. Alternatively, steel bands or bands of an alloy such as Hastelloy can be used. Above the substrate strip 2, a 0.5 μm thick buffer layer 4 is arranged, which here contains the oxidic materials CeO ₂ and Y ₂ O ₃ . This is followed by the actual superconducting layer 6, here a 1 μm thick layer of YBa ₂ Cu ₃ O _x , which in turn is covered with a 50 μm thick cover layer 8 made of copper. As an alternative to the material YBa ₂ Cu ₃ O _x , it is also possible to use the corresponding compounds REBa ₂ Cu ₃ O _{x of} other rare earths RE. On the opposite side of the substrate strip 2, a further 50 .mu.m thick cover layer 8 made of copper is arranged here, followed by an insulator 10, which in this example is designed as a Kapton strip 25 .mu.m thick. The Insulator 10 may also be constructed of other insulating materials such as other plastics. In the example shown, the width of the insulator 10 is slightly larger than the width of the remaining layers of the strip conductor 1, so that windings W _i , W _{i + 1} coming over one another are reliably insulated from one another in a winding of the coil device. As an alternative to the example shown, the strip conductor 1 may also comprise insulator layers on both outer surfaces, or the lateral regions of the superconducting strip conductor 1 may additionally be protected by insulating layers. It is also possible to wrap an insulator tape only in the preparation of the coil winding as a separate band in the coil device. This is particularly advantageous when several strip conductors are wound in parallel, which need not be isolated from each other. Then, for example, a packet of 2 to 6 superimposed strip conductors without their own insulator layer can be wound together with an additionally inserted insulator strip in common turns.

Typically, the substrate tape 2, the buffer layer 4, the superconducting layer 6 and the cover layers 8 in their entirety undergo a thermal contraction of about 0.3% when cooling from about 300 K to about 30 K. For conventional materials of the insulator 10 and the epoxies used as potting compound or adhesive, however, the thermal contraction is much higher, at about 1.2%. For flat stacks of strip conductors and on the straight sections of a coil winding, these differences can be compensated for by varying in-plane shrinkage and perpendicular to the plane of the strip conductor. In the curved areas, however, they lead to the formation of radial tensile stresses. In the following two embodiments it is shown how the radial tensile stresses can be reduced by the division into segments. It is particularly advantageous if the layers with high thermal contraction are made as thin as possible, especially in the curved regions. The in Fig. 1 illustrated band should be for both subsequent embodiments as a winding material based. Here, the insulator 10 with 25 microns is advantageously designed relatively thin compared to the rest of the total thickness of the strip conductor 1.

Fig. 2 shows a section of a first coil winding 12 according to a first embodiment. In this example, the coil winding 12 is designed as a rectangular coil. The clipping in Fig. 2 shows an area around one of the four curved corners of the rectangular coil. It puts Fig. 2 only a part of the coil winding 12, namely a portion of the winding with six superimposed turns of strip conductors 1, each according to the example in Fig. 1 are constructed. In this case, three of the turns are part of an inner segment S _i , and three of the turns shown are part of an outer segment S _{i + 1} . As indicated, each segment includes even more than the three turns exemplified. For example, each segment may comprise between 10 and 200 turns, more preferably between 50 and 100 turns. The entire coil winding may for example comprise between 2 and 50 such segments, more advantageously between 5 and 10 segments. Within each segment S _i , S _{i + 1} , all windings W _{i are} potted with an encapsulation compound 14 of epoxy in this exemplary embodiment. The potting compound 14 has been introduced in this embodiment after the winding of the coil (so-called dry winding) by means of Vakuumverguss. Alternatively, an impregnating resin or an adhesive may also be introduced during the winding of the coil winding (so-called wet winding), wherein the band conductor is typically wetted on both sides with the impregnating resin or adhesive before winding. Also in the intermediate regions 20 between the segments S _i , S _{i + 1} , the adjacent windings W _i-1 , W _i are shed in several subsections in this exemplary embodiment. Of the four straight sections 28 of the rectangular coil are in Fig. 2 two shown schematically. Within these sections 28 all windings W _{i of} the entire coil with the potting compound 14th firmly connected to each other, even in the intermediate region 20 between two adjacent segments S _i , S _{i + 1} . In contrast, in the curved regions 24, of which the entire rectangular coil comprises four, the adjacent turns W _i-1 , W _{i of} different segments S _i , S _{i + 1 are} not connected to one another by potting compound 14. The same applies to the transition regions 26 adjoining each curved region 24 on both sides, in which no potting compound 14 is also arranged between the adjacent turns W _i-1 , W _{i of} different segments S _i , S _{i + 1} . Instead, in this entire portion 22 between the segments S _i , S _{i + 1,} a PTFE tape 16 is inserted, which prevents that in the casting of the wound coil of this portion 22 is filled with potting compound 14. In this example, the PTFE tape 16 has a similar layer thickness as the average thickness of the potting compound introduced during casting, here a thickness of 25 microns. The inserted PTFE tape 16 thus advantageously prevents the adhesive bonding of the strip conductors 1 of adjacent turns W _i-1 , W _i to the potting compound 14 in said partial region 22, since the interposed PTFE strip 16 is not wetted by the potting compound 14. This also prevents the formation of a strong connection of the adjacent strip conductor 1 in this sub-area 22. In this embodiment, no chemical adhesive bond is formed in this portion 22. As an alternative to this example, in the partial region 22, the band conductor may also be coated with a release agent such as, for example, PTFE. Depending on the properties of the coating, either no adhesive bond or only a weak adhesive bond between the adjacent strip conductors 1 can then be formed. As an alternative or in addition to the release agent 16 illustrated here, a further layer may also be incorporated in the intermediate region 20. The material of this further layer can either have a low or even negative thermal expansion coefficient, and / or the layer can have a flexible material with a tensile strength of less than 10 MPa. In both embodiments, the further layer contributes to reducing radial tensile stresses in the intermediate regions 20 and that the mechanical strength of the coil in the curved regions 24 and the adjacent transition regions 26 is increased.

All the variants described above have in common that the tensile stress on the turns W _{i of} the entire coil is reduced by the at most weak connection of the adjacent strip conductor 1 in the partial regions 22. Due to the maximum weak connection in these subregions 22, the maximum tensile stress on the strip conductor 1 by thermal contraction of the various materials behaves approximately as in a coil winding having only the number of turns of a single segment S _i . The rectangular coil of the exemplary embodiment shown has four relatively long straight regions 32 and four relatively short curved regions 24, each having transition regions 26 adjoining on both sides. For the reduction of the tensile stress on the strip conductor, especially a mechanical decoupling and strain relief of the segments in the curved regions 24 is effective. Therefore, the rectangular coil can be completely potted in the straight portions 32 as in conventional methods and thereby retains much of the mechanical stability achieved with these methods. Advantageously, the maximum weak connection of the adjacent strip conductor 1 between two adjacent segments S _i , S _{i + 1 in} addition to the curved portions 24 also in both adjacent transition regions 26, so that the transition of the straight portions 32 in the curved portions 24 and Transition of the strongly connected to the weakly connected intermediate areas do not form too high tensile, compressive or shear stresses.

Fig. 3 shows a second coil winding 30 according to a second embodiment in a schematic plan view. This second coil winding 30 is formed as an approximately cylindrical winding, in which example the cylindrical shape is composed only approximately of straight portions 32 and curved portions 24. In the example shown here, the coil winding each comprises eight straight areas 22 and eight curved areas 24, however, the number of individual areas may also be much larger. In the illustrated second embodiment, the coil winding comprises only two segments Si and Si + 1. However, the number of segments can also be significantly greater, for example, it can be between 2 and 50 and more preferably between 5 and 10. In the entire potted region 34 of the second embodiment shown, all adjacent turns are firmly connected to each other by potting compound, even across the boundary 36 of the two segments. Only in the eight sections 22 at the boundary 36 of the segments, the potting compound between the adjacent strip conductors 1 is interrupted. In this second exemplary embodiment, the strip conductors 1 adjoining the partial regions 22 are coated with the release agent PTFE, which acts as a wetting agent for the potting compound and thereby causes voids to be formed without potting compound in the partial regions 22. In the subregions 22, therefore, the adjacent semiconductors are not connected to one another in this example, and the formation of the cavities effectively brings about a strain relief of the radial tensile stresses which increase in the curved regions 24. By spreading or compression of the cavities with changes in temperature, both tensile and compressive stresses on the strip conductors 1 of the coil winding 30 can be reduced.

Claims (9)

1. Superconducting coil device comprising a coil winding (12, 30) consisting of a plurality of turns (W_i) comprising at least one superconducting tape conductor (1), which has a strip-shaped substrate tape (2) and a superconducting layer (6) arranged on the substrate tape (2), wherein the coil winding (12, 30) is subdivided into a plurality of segments (S_i), neighboring turns (W_i, W_i+1) within each segment (S_i) being encapsulated together or adhesively bonded to one another, and, in the intermediate region (20) between two neighboring segments (S_i, S_i+1), neighboring turns (W_i-1, W_i) being at most weakly connected or adhesively bonded to one another at least in a subregion (22), wherein the coil winding (12) is configured either as a racetrack coil or a rectangular coil, wherein a plurality of subregions (22) having at most a weak connection of the neighboring turns (W_i-1, W_i) of neighboring segments (S_i, S_i+1) lie within the curved regions (24) of the racetrack or rectangular coil, or as an approximately cylindrical winding in which the segments (S_i) are configured as radial segments (S_i) and which is formed from straight regions (32) and curved regions (24) alternating with one another, the subregions (22) having at most a weak connection of the neighboring turns (W_i-1, W_i) of neighboring radial segments (S_i) lying in the region of the curved regions (24).
2. Coil device according to Claim 1, wherein, in the intermediate region (20) between two neighboring segments (S_i, S_i+1), the neighboring turns (W_i-1, W_i) are at most connected by an adhesive so weak that the connection is broken at a stress below 10 MPa at least in a subregion (22).
3. Coil device according to Claim 1, wherein, in the intermediate region (20) between two neighboring segments (S_i, S_i+1), at least one subregion (22) in the intermediate space between neighboring turns is free of adhesive bonding or encapsulation compound.
4. Coil device according to one of the preceding claims, having an encapsulation compound (14) which encloses the neighboring turns (W_i, W_i+1) within the segment (S_i).
5. Coil device according to one of the preceding claims, which has a coating of a separating medium (16) or an inlaid tape of a separating medium (16) at least in a subregion (22) in the intermediate region (20) between two neighboring segments (S_i, S_i+1).
6. Coil device according to one of the preceding claims, characterized in that, in the intermediate region (20) between two neighboring segments (S_i, S_i+1), the tape conductor (1) is provided at least in a subregion (22) with an additional layer which is formed from a material having a thermal expansion coefficient lower than the effective thermal expansion coefficient of the tape conductor (1).
7. Coil device according to one of the preceding claims, characterized in that, in the intermediate region (20) between two neighboring segments (S_i, S_i+1), the tape conductor is provided at least in a subregion (22) with an additional layer which is formed from a flexible material having a tensile strength of less than 10 MPa.
8. Coil device according to one of the preceding claims, characterized in that the superconducting layer (6) comprises a second-generation high-temperature superconductor, in particular ReBa₂Cu₃O_x.
9. Coil device according to one of the preceding claims, comprising a cooling system, wherein the segments (S_i) of the coil winding (12, 30) are respectively coupled individually to the cooling system.

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