Winding coils with HTS tapes

Abstract

A method of winding a magnetic field coil comprises winding one or more tapes, including at least one High Temperature Superconductor (HTS) tape, into a coil and causing surfaces of adjacent tapes to

Description

Field of the Invention The present invention relates to high temperature superconducting, HTS, tapes, methods of winding coils from such tapes, and coils formed from such tapes. Background Superconducting materials are typically divided into “high temperature superconductors” (HTS) and “low temperature superconductors” (LTS). LTS materials, such as Nb and NbTi, are metals or metal alloys whose superconductivity can be described by BCS theory. All low temperature superconductors have a self-field critical temperature (the temperature above which the material cannot be superconducting even in zero external magnetic field) below about 30K. The behaviour of HTS material is not described by BCS theory, and such materials may have self-field critical temperatures above about 30K. The most commonly used HTS are “cuprate superconductors” - ceramics based on cuprates (compounds containing a copper oxide group), such as BSCCO, or ReBCO (where Re is a rare earth element, commonly Y or Gd). Other HTS materials include iron pnictides (e.g. FeAs and FeSe) and magnesium diborate (MgB2). ReBCO superconductors are typically manufactured as tapes approximately 100 micrometres thick and with a width of between about 2mm and 12mm. The structure of a typical tape 100 is illustrated in Figure 1 and includes a substrate 101 (typically an electropolished nickel-molybdenum alloy, e.g Hastelloy™ approximately 50 micrometres thick), on which is deposited a series of buffer layers known as the buffer stack 102, of approximate thickness 0.2 micrometres. An epitaxial ReBCO-HTS layer 103 overlays the buffer stack, and is typically 1 micrometre thick. A 1-2 micrometre silver layer 104 and a copper stabilizer layer 105 are deposited on and often completely encapsulate the tape. The silver layer 104 and copper stabilizer layer 105 extend continuously around the perimeter of the tape 100 (not illustrated in Figure 1 for clarity) and may therefore also be referred to as “cladding”. The silver layer 104 makes a low resistivity electrical interface to, and an hermetic protective seal around, the ReBCO layer 103, whilst the copper layer 105 enables external connections to be made to the tape (e.g. by soldering) from either face and provides a parallel conductive path for electrical stabilisation. “Exfoliated” HTS tape can be manufactured, which lacks a substrate and buffer stack, but typically has a “surrounding coating” of silver on both faces of the ReBCO layer, and optionally a copper cladding around the silver layer. Tape which has a substrate can be referred to as “substrated” HTS tape. An HTS cable comprises one or more HTS tapes, which are connected along their length via conductive material (normally copper). The HTS tapes may be stacked (i.e. arranged such that the HTS layers are parallel), or they may have some other arrangement of tapes, which may vary along the length of the cable. Notable special cases of HTS cables are single HTS tapes, and HTS pairs. HTS pairs comprise a pair of HTS tapes, arranged such that the HTS layers are parallel. Where substrated tape is used, HTS pairs may be type-0 (with the HTS layers facing each other), type-1 (with the HTS layer of one tape facing the substrate of the other), or type-2 (with the substrates facing each other). Cables comprising more than 2 tapes may arrange some or all of the tapes in HTS pairs. Stacked HTS tapes may comprise various arrangements of HTS pairs, most commonly either a stack of type-1 pairs or a stack of type-0 pairs and (or, equivalently, type-2 pairs). HTS cables may comprise a mix of substrated and exfoliated tape. A superconducting magnet is formed by arranging HTS cables (or individual HTS tapes, which for the purpose of this description can be treated as a single-tape cable) into coils, either by winding the HTS cables or by providing sections of the coil made from HTS cables and joining them together. HTS coils come in three broad classes: • Insulated, having electrically insulating material between the turns (so that current can only flow in the “spiral path” through the HTS cables). • Non-insulated, where the turns are electrically connected radially, as well as along the cables • Partially insulated, where the turns are connected radially with a controlled resistance, either by the use of materials with a high resistance (e.g. compared to copper), or by providing intermittent insulation between the coils. Non-insulated coils could also be considered as the low-resistance case of partially insulated coils. HTS coils are typically manufactured as shown in Figure 2, by providing a spool 201 of HTS cable 210, with a magnetic brake 202 to apply tension. Then, by moving the spool around the coil (starting with a former or support structure 203 which defines the shape of the coil) or rotating the coil around its axis while keeping the spool stationary, the cable is wound onto the coil turn-by-turn. Additional layers such as insulators, partially insulating layers (i.e. insulators having current paths within them, or materials with a resistance intermediate between a typical insulator and a conductor), quench detection components, or similar, may be wound along with the HTS cable. This is not suitable for all coil shapes and cable constructions. In particular, stacked tape cables (comprising several parallel HTS tapes which run tangential to the coil at all points) cannot be wound this way on coils with sharp turns, as this will result in heavy strain on tapes at the outside of the turns. For such coils, an alternative winding method may be used as shown in Figure 3, where the stacked tape cable is built up in-situ by providing a plurality of spools 301 a-d of HTS tape, so the HTS tape is wound simultaneously from several spools onto the coil 302. Similarly to the previous case, additional layers may be wound between the layers of HTS tape which form each cable (e.g., between each turn) and may be dispensed by one or some of the spools 301a-d. It is generally difficult to obtain HTS tape of sufficient length that each of the spools of HTS tape in Figure 3 can hold enough tape for an entire coil. However, HTS tapes may be replaced as each runs out or with a predetermined pattern. The result is a pattern of tape-end to tape-end “butt” joints as shown schematically in Figure 4 (with the coil “straightened out” and lengths significantly shortened), where each layer of HTS tapes includes a butt joint 401 where the HTS tape stops, and HTS tapes 402 of other layers overlap this butt joint, resulting in an overall pattern similar to typical bricklaying. As noted, the lengths in Figure 4 are significantly shortened - normally each HTS tape would have a length on the order of meters to hundreds of meters, and a thickness on the order of hundredths to tenths of millimetres. An alternative winding method is disclosed in WO 2023 / 83956 A1, in which a plurality of HTS tapes are laid on the coil in an offset pattern, such that for each tape other than the radially innermost HTS tape, each end of the HTS tape is offset in the first direction from the corresponding end of an adjacent HTS tape which is radially inward of the said HTS tape. Winding a coil from HTS tapes presents challenges in terms of achieving a consistent turn density (or thickness) across the whole coil. Consistent winding relies on tension in the HTS tapes. Although this may be adequate for small, circular coils, as coil size increases, tension alone may be insufficient: turns can slip azimuthally, and variations in turn density build up. In coils with several hundred turns, variations on the scale of micrometres can result in 5% to 10% variation in coil size or pack density. For non-circular coils, the difficulties increase because the winding tension varies around the perimeter of the coil. For straight sections, there may be no winding tension at all. One approach to solving this problem is to wind the coil as well as possible and then compact and tighten the turns afterwards to try to achieve a desired turn density. This is time-consuming and imprecise. Once an HTS coil has been wound by any of the above methods, it is important to ensure durability of the coil. In operation at the high magnetic fields achievable with HTS, loosely wound turns can deform and the individual tapes delaminate. One approach involves “potting” the coil to improve mechanical strength and provide desired thermal and electrical properties. “Potting” (also known as “consolidating”) is the process of filling the coil with a liquid medium (“potting medium”) which penetrates between turns of the coil and then cures or cools to form a solid, such as solder or electrically conductive epoxy resin. This consolidates the coil into a solid, durable structure. However, there are a number of challenges associated with solder potting in some coils. Apexes or tight turns often have high winding tension, making solder penetration difficult. The coil must be heated, which can damage or warp it, and damage the individual HTS tapes. There are a number of potential safety hazards involved in the process such as heat, lead-based solders and flux fumes. Excess solder must be removed after potting, which is a time consuming process. Tin-lead and many other suitable solders become brittle at cryogenic temperatures, which may compromise the durability of the coil in operation. Thus, using current techniques, achieving a desired structure when winding with tension alone is a significant technical challenge, and the heat of subsequent solder potting can warp that structure. It is desirable to wind coils that address some of the challenges involved in the coil winding and coil consolidation processes described above. Summary The invention is set out in the accompanying claims. Brief Description of the Drawings Figure 1 is a schematic illustration of a typical HTS tape. Figure 2 is a schematic illustration of a system for winding a magnetic field coil. Figure 3 is a schematic illustration of an alternative system for winding a magnetic field coil. Figure 4 is a schematic diagram illustrating joints between HTS tapes. Figure 5 is a schematic diagram illustrating a system for winding a magnetic field coil with coated tape. Figure 6 is a schematic diagram of a pressure roller. Figure 7 is a schematic diagram of a multiple spool winding apparatus. Figure 8 illustrates an insulating or partially insulating tape adapted for bonding to adjacent tapes. Detailed Description As noted in the background, there are challenges involved in winding and consolidating HTS magnetic field coils. These challenges include obtaining a consistent turn density, especially for non-circular coils, and in the solder potting process. Some of these challenges can be addressed by including indium or an indium alloy between the HTS tapes. The following discussion uses the term “indium” as the material used. However, in practice various alloys of indium combined with other materials may have properties sufficiently similar to indium to behave in the same way. Indium may be combined with small quantities of silver, for example (a typical arrangement is 3%wt Ag). For the purposes of the discussion, the term “indium” is intended to include alloys that are malleable at room temperature, whose chief component is elemental indium, and which will support cold welding. Indium is commonly used to make electrical joints and connections due to its ductility and malleability. Two surfaces to be connected can be plated with indium, or an indium foil can be placed between them, and pressed together to make an excellent electrical connection. In the case of winding HTS tapes, the tapes can be plated with indium before winding. Pressure may then be applied during the winding process so as to cause the the tapes to cold weld and bond to one another to achieve a consolidated coil. Cold welding is a solid-state welding process that requires little or no heat of fusion to join two clean metal surfaces together. Instead, energy used for creating a weld is provided in the form of pressure. In practice, to put this into effect, the basic steps are as follows: • Coat HTS tape with indium; • Apply a protective surface film to remove and prevent formation of an oxidation layer on the indium. • Use this treated HTS tape to wind an HTS coil. • Apply pressure during winding to bring together two clean surfaces, at least one of which is indium, and cause them to bond by cold welding. These steps are now discussed in more detail, interspersed with other steps which may assist in optimising the process. Before HTS tape is coated with indium it may be beneficial to apply a barrier layer to the HTS tape. This is because indium and copper react with one another over time in a continuous but slow process, resulting in indium-copper (InCu) compounds and / or intermetallics. At room temperature it is estimated that it may take about 2 months for an indium coating on copper of about 5um thickness to be completely converted to an InCu intermetallic. Unlike indium, InCu will not cold-weld to itself or other metals under reasonable pressures. This means there is a finite time within which HTS tape coated with indium must be used if it is to be used for cold-weld winding. This finite time can be increased by applying a thicker coating or storing the tape at low temperatures. A permanent solution is to apply a diffusion barrier to the HTS tape before coating with indium. Suitable barrier materials include nickel or chromium in a layer about 0.1 urn to about 10um thick. This enables that HTS tape coated with indium has a much longer shelf life before it must be wound into a coil. Once a coil is wound (if a diffusion barrier is not present), the In / Cu reaction will continue. This in itself is not expected to degrade or affect the performance of the coil, but a diffusion barrier may nevertheless provide an additional benefit of ensuring consistent operation over the lifetime of the coil. In order to prepare for winding (with or without a diffusion barrier), HTS tapes are plated with indium to achieve an indium coating, which may have a thickness of the order of about 5-1 Oum. If this coating is applied on both sides of the tape, this provides a spacing between tapes of the order of about 10-20um. The plating may be carried out by applying an indium film to the HTS tape or by using an electroplating or other suitable process. The texture and surface roughness of the indium surface may be controlled up to about 5um, which may enhance subsequent bonding. Such texture optimisations are similar to those usually employed when bonding other electrical elements using indium. An example of an electroplating approach is to use an indium sulphamate electroplating bath at current densities of 10-20mA / cm2. The liquid in the bath may be a solution of indium in sulfamic acid (as the electrolyte) with optional additives such as inhibitors and brighteners for optimized performance. Following the step of plating the HTS with indium, the plated tape may be wound onto a spool for storage. This step is not always necessary, but provides the option of storing the plated tape, and removes the need to run a full end-to-end process from Inelectroplating to coil winding. It will be noted that there may be a risk of unintentional self-welding, or bonding to other surfaces or to tooling or tools, when winding newly In-plated HTS onto a spool (and during subsequent coil winding). This risk may be reduced by removing any residual plating solution, and by ensuring that the indium is well oxidized. This can be achieved for example by exposure to air for a few minutes, or by exposure to a high oxygen environment. An oxide layer should prevent self-welding in most conditions. For long storage or for transport under uncontrolled conditions (for example high temperatures such as may be caused by long exposure to hot sun, or extreme vibration), a removable protective film may be applied to the plated tape. Materials that do not adhere to indium include Kapton® and PTFE. When ready to wind a coil, the HTS tape may be unspooled from the storage spool(s) and transferred to winding spool(s). Winding may be done directly from storage spools, but transferring to winding spools enables final quality control checks, preparing desired lengths of tape on the winding spool, and orienting the tape as required (e.g. whether the HTS layer is on the inside or outside of the wind relative to the substrate). Any protective film can be removed while unspooling and the Indium oxide surface will prevent self-welding. It is desirable to remove any oxide immediately before winding, and prevent the formation of an oxide layer as the winding takes place. This can be achieved using a mildly acidic liquid (such as, for example, acid-based fluxes as a surface activator) which creates a thin protective film on the HTS tape surface preventing further oxidation. This can be applied by passing the tape through a flux bath or waterfall immediately prior to winding. There is a time limit from applying the flux to use of the tape, though this may be as much as a few hours. A similar effect can be achieved if the winding takes place in an acidic gas environment. As a further alternative, if an oxide layer has previously been removed and a removable protective film applied, the film may be removed and winding performed in a low-pressure or low-oxygen environment. The approaches described above are intended to achieve a clean indium surface when two indium surfaces, or an indium surface and another metal surface, are brought together. This reduces the pressure required to cause cold welding to take place. The winding proceeds by bringing indium coated tapes into contact, preferably under pressure, while winding an HTS coil. A mechanism to achieve this is shown in Figure 5. In this example it is assumed that indium coated HTS tape 500 has already been wound onto a spool 501. The tape 500 is unwound from the spool 501 and passes through tension rollers or pulleys 502 and a flux bath 503 which provides a flux coating to prevent oxidation as described above. The clean tape is then wound onto a mandrel, bobbin or former 504 to form a coil 505. A pressure roller 506 applies pressure to the tape as soon as possible after the tape contacts the mandrel 504 or coil 505. The clean indium coldwelds instantly by the aid of the external pressure, which expels the liquid flux coating from between the tapes. A suitable pressure is of the order of 10-30 MPa as long as the indium has been cleaned by the flux bath. Higher pressures may be applied if required, although once the pressure reaches about 300-400 MPa there is a likelihood of damaging the tape. However, at higher pressures (even if below 300 MPa), there is still a risk that indium may be squeezed out, making it difficult to control the wind and turn spacing. In certain circumstances (for example when winding circular coils) it may be possible that winding tension alone can expel the flux and enable a cold weld, removing the need for the pressure rollers 506. But for coils with non-regular geometry the winding tension varies around the coil. For coils with straight sections (e.g., D-coils or racetrack coils) there is little or no winding tension in these straight sections so some additional force or pressure is required. The pressure (and, if necessary, temperature) is controlled during winding to ensure a good bond without providing pressure high enough to squeeze indium out from between the turns. This could cause unwanted electrically conductive bridges across multiple turns which would need to be removed. Once the HTS coil has been wound, external surfaces of the coil may be cleaned to prepare the coil for use. Flux may simply be rinsed off with water since the coil has not been heated. It may also be necessary to remove excess indium by manual and / or chemical cleaning if some has been squeezed out from between the turns due to overpressure. It will be noted that this cleaning process is much simpler than that required for solder potted coils, subjected to high temperature potting, in which burnt on flux residue is removed by placing the coil in an ultrasonic bath with a solvent or cleaning solution. In such coils solder bridges are common. Large bridges must be removed in a manual, hot de-bridging process, which is followed by further cleaning. Even then, a solder potted coil is ready for use only after a chemical de-bridging and final clean. Figure 6 is a schematic drawing of an exemplary pressure roller 606 which may be used to apply pressure to HTS tape as it is wound onto a mandrel, and may be used as the pressure roller 506 shown in Figure 5. The roller 606 comprises a wheel 601 which rotates around an axle 602. The axle is connected to a rod 603 and piston 604 which can be used to provide a controllable pressure to the axle 602 and thus to the wheel 601. The wheel 601 may have (or be coated with) a surface to which indium does not adhere, such as PTFE or a polyimide film such as Kapton®. As previously discussed, in some circumstances it is desirable to include multiple tapes in each turn of the coil. If the tapes are coated and pressed together in such a way that they cold weld, they cannot then move relative to each other, which can lead to damage caused by buckling and / or stretching. This means that it is not practical to make a cable of multiple tapes first and then wind that cable. Instead, if multiple tapes per turn are required, multiple spools and rollers may be spaced around the coil. Figure 7 is a schematic illustration of a multiple spool winding apparatus, in which three spools 701a, 701b, 701c are spaced around a single mandrel 704 in order to wind a coil 705. Each spool 701 a-c is associated with tension rollers 702a-c, a flux bath 703a-c and a pressure roller 706a-c which immediately bonds individual tapes 700a-c to the surface of the coil and prevents buckling. This approach also facilitates delivering tapes of material other than HTS: for example, two spools may provide HTS tape in different orientations, for example to create a pair of tapes with the HTS face-to-face (known as type-0 pairs) with the third spool delivering non-HTS tape (optionally also plated with indium) for insulating or partially insulating the turns. Examples of such tapes may include steel tape, partial insulation tape (as described for example in WO2019150123), or an electrical insulator. These are described in more detail below in connection with Figure 8. It will be appreciated that the three spools shown in Figure 7 is simply an example, and the number of spools and associated rollers depends on the desired number of HTS tapes per turn. The number of spools is likely to be limited by size and shape restrictions. For example, it may not be possible to fit more than four individual rollers around a compact circular coil (less than 30cm diameter, for example). This restriction reduces with size and shape of coil, and larger coils and non-circular coils (e.g. racetrack coils, D-coils) may support 10 or more spools and rollers and therefore 10 or more tapes per turn. As previously discussed, winding a coil with HTS tape with tension alone is a significant technical challenge, and the heat of subsequent solder potting warps that structure. An advantage of plating HTS tapes with indium before winding is that, during winding, each tape can be placed in the optimal position and bonded in place as winding takes place. This process is more controllable and predictable and simpler overall than tension winding and solder potting. The benefits of this approach are particularly significant for for larger, unusually shaped coils. These may include racetrack coils, D-coils, coils with straight sections, and flimsy coils (e.g. coils having an inner diameter that is large compared to the outer diameter) having a low inherent structural strength. In larger and non-circular coils, achieving consistent turn density across the entire coil is nearly impossible using traditional techniques reliant on winding tension and subsequent pack compression. With coldwelding, individual tapes can be locked in place as they are wound, with precisely controlled tension and pressure to position them as desired. It will be appreciated that the process described above is provided by way of example and there are modifications which can be made which remain within the scope of the present disclosure. Indium will adhere to many different surfaces, especially if clean: cold welding is not restricted to indium-indium contact. In particular, indium readily cold-welds to copper. This means that, when winding HTS tapes where only one surface of the two surfaces in contact with one another is plated with indium (which may be the case where only every other tape in a multi-tape turn is plated), fluxing / surface preparation of both tapes and the application of low pressure may form a bond between the indium plating on one tape and the copper surface, or a metal diffusion barrier surface, of the adjacent tape. The possibility of In-Cu cold welding also means that (if all of the tapes are plated), only one surface of the tapes needs to be plated with indium. This may require masking one surface of the HTS tape in an electroplating bath, or using some deposition process on only one side of the HTS). It will be appreciated that these approaches would not necessarily reduce the amount of indium used, since the single plated layer needs to be thicker to achieve the same total thickness per turn spacing (typically about 10-20um) of coating both sides of all of the tapes. However, this approach could reduce the total amount of tape requiring indium plating, and increase shelf-life before the thicker indium layer converts to InCu compounds. Indium also cold-welds to other metals and may also cold-weld to some ceramic and / or electrically insulating materials that could be used as insulation, or partial insulation, layers between turns. This may enable a bond to non-conductive layers that cannot be electroplated with indium. In an embodiment, a camber may be added to the indium, layer so that it is thicker in the middle of tape, so as to force flux out from between the tapes as they are pressed together. A similar result may be achieved by including a camber on the roller or by increasing the pressure in the middle of the tape. In an embodiment, winding may take place in a cold environment, for example at 77K (liquid nitrogen). Pressure for cold-welding may be provided by thermal expansion afterwards as coil rises to room temperature. In an embodiment, winding may take place without additional pressure. The whole coil may then be compressed once winding is complete, for example by placing a hot overbind around the coil that shrinks as it cools to apply pressure. This approach may be suitable for small, compact, circular coils. The technique described above could also be used to form an HTS cable rather than a coil, although the solid cable could not be bent to any significant degree afterwards without straining the tapes on the outside of the bend. In addition, as discussed above, pressure is applied to indium-coated and pre-cleaned surfaces. The minimum pressure required will be highly dependent on the level of cleanliness of the surfaces, and in some circumstances indium surfaces may self-weld to each other even if they touch at very low pressures. So there are a wide range of options to make surfaces oxide-free or nearly-free depending on pressure restrictions. There are various approaches to ensure cleanliness and reduce the pressure required in addition to or instead of passing the tape through a flux bath as described above. These approaches may include processing in a vacuum, processing in a reducing atmosphere, for example using acidic gases, processing in an inert atmosphere, and processing in ambient atmosphere but under a blanket of protective liquids (like acidic flux). A further option may include the application of ultrasound during winding. This may significantly the reduce pressure needed and may eliminate the need for reducing agents, since the ultrasound may constantly break the oxide films allowing "oxygen-free" surfaces to interact. However, this may increase the complexity of the winding step itself if the coil is required to sit in an ultrasound bath. The principles described above may in certain circumstances also be used to achieve consolidated coils without the use of indium. As mentioned above, indium may coldweld to copper, and indeed copper may cold-weld to copper under the right conditions, and other materials may also cold-weld. For such materials, it is likely that higher pressure will be required than in the case for indium-indium or indium-copper cold welding, and this higher pressure on its own may need to be so high that it would damage the tape. However, ultrasonic welding may also be employed to cause HTS tapes and other insulating or partially insulating tapes to bond to each other even in the absence of indium. Surface preparation and the introduction of powdered metal, (for example powdered copper) between the turns may increase the probability of achieving cold welding. Furthermore, other materials which have similar properties to indium {e.g., a melting point near room temperature and / or malleability and ductility at room temperature which enable cold-welding), although indium is the only material which is believed to be ductile at temperatures close to absolute zero. For example, some gallium alloys have a melting point close to room temperature. Such materials may in certain circumstances in place of indium in the process described above. Figure 8 illustrates, in cross-section, an insulating or partially insulating tape 801 adapted for cold-weld winding processes as described above. The tape 801 is provided with a layer or coating 802 of indium, or other metal or suitable bonding material, on at least one of its flat faces. The coating 802 does not extend over the edges of the tape 801 to avoid creating an unwanted electrical bridge around the tape 801. This may be achieved by masking the edges of the tape 801 such that the coating 802 is applied only to the flat faces, or by applying the coating 802 to the whole tape 801 and subsequently removing it from the edges, either mechanically or by chemical etching, for example. The tape 801 may be a partially insulating tape such as steel or a partially insulating layer such as the one described in WO2019150123, or may be a fully insulating tape comprising an electrically insulating material. Suitable electrical insulating materials may include organic insulators such as Kapton ® or ceramic insulators such as aluminium oxide (alumina). In the example of Figure 8, an insulating tape 801 comprises a metallic core 803 provided with an electrically insulating oxide layer 804. An aluminium tape 803, for example, may be anodised or otherwise oxidised to form an electrically insulating alumina layer 804 on its outer surfaces to form the insulating tape 801. The method of applying the indium coating 802 depends upon the material of the insulating or partially insulating tape 801. For a partially insulating tape or layer having an outer metallic surface, or at least metallic flat faces, the indium coating 802 may be electroplated onto the tape 801. For organic insulators that do not bond directly to indium, an adhesive may be used. For ceramic insulators, it is possible to apply and bond indium directly to a range of different material surfaces, including many metallic oxides. Where the faces of the tape 801 are non-metallic, so would not readily cold-weld to indium during winding, both faces may be provided with an indium coating 802 to ensure the tape 801 cold welds to tapes, such as HTS tapes, either side of it in the wound coil. 5 The techniques described in the present disclosure have a number of advantages over the current approach to winding field coils. Consolidation of a coil can be achieved even in coils with tight-turns where high winding tension at the apexes makes solder-potting challenging. The wind itself is easier to control, achieving more uniform turn thickness 10 and tension in the tapes, which bonded in place as they are wound. The coil manufacture process including complex shapes is simplified and sped up. The resistance between tapes may be reduced, improved current sharing between turns. There is no need to heat coils to consolidate them, which avoids problems with coils warping due to temperature cycling, removes significant amount of thermal stress, and saves on cost 15 and additional complex equipment processes such ashot plates, control units, fume cupboards. Fewer ‘high risk’ processes are required. De-bridging processes after solder potting to remove excess solder are no longer needed. The lead usually present in potting solder is not required. Indium is not brittle at cryogenic temperatures.

Claims

1. A method of winding a magnetic field coil, comprisingcoating a surface of a High Temperature Superconductor, HTS, tape with a bonding material which is ductile at or near room temperature;winding one or more tapes, including the HTS tape, into a coil;during winding, applying pressure to the coil so as to cause surfaces of adjacent tapes, at least one of which is the surface of the HTS tape coated with the bonding material, to cold-weld to each other.

2. The method of claim 1, wherein the bonding material is chiefly composed of indium.

3. The method of claim 1 or 2, wherein the bonding material is indium.

4. The method of any preceding claim, further comprising, before winding the HTStape into the coil, cleaning the bonding material to remove an oxidation layer.

5. The method of claim 4, wherein cleaning the bonding material comprises passing the HTS tape through a bath or waterfall comprising an acid-based flux.

6. The method of claim 4 or 5, wherein cleaning the bonding material comprises creating a thin protective film on the surface of the bonding material.

7. The method of any preceding claim, wherein coating the surface of the HTS tape with the bonding material comprises electroplating the surface with the bonding material.

8. The method of any preceding claim, wherein the thickness of the bonding material coated on the surface of the HTS tape has a thickness in the region of about 5-10um.

9. The method of any preceding claim, wherein applying pressure to the coil comprises applying a pressurised roller to the HTS tape at or close to a location at which the HTS tape contacts the coil.

10. The method of any preceding claim, wherein the pressure applied to the coil is in the region of about 10 to about 30 MPa.

11. The method of any preceding claim, wherein both surfaces of the HTS tape are coated with the bonding material.

12. The method of claim 11, wherein the one or more tapes include more than one HTS tapes in a turn of the coil, and wherein alternating HTS tapes are coated with the bonding material.

13. The method of any of claims 1 to 10, wherein only one surface of the HTS tape is coated with the bonding material.

14. The method of any preceding claim, further comprising, before winding the HTS tape into the coil, winding the HTS tape onto a storage spool.

15. The method of any preceding claim, further comprising, before coating the surface of the HTS tape with the bonding material, applying a diffusion barrier later to the surface of the HTS tape.

16. A method of winding a magnetic field coil, comprising:coating a surface of an HTS tape with indium;applying a protective surface film to remove and prevent formation of an oxidation layer on the indium;winding one or more tapes, including the coated HTS tape, into a coil; andapplying pressure to the coil during winding to so as to cause surfaces of adjacent tapes, at least one of which is the surface of the HTS tape coated with indium, to coldweld to each other.

17. A magnetic field coil wound using the method of any preceding claim.

18. A magnetic field coil comprising at least one HTS tape having a surface coated with indium, wherein the indium-coated surface is cold-welded to a surface of an adjacent tape.

19. A magnetic field coil comprising a plurality of HTS tapes wound around a central axis, wherein adjacent tapes are separated by an indium layer having a thickness between about 10um and about 20um.

20. A method of winding a magnetic field coil comprising:winding one or more tapes into a coil, the tapes including at least one HTS tape; andduring winding, causing the surfaces of adjacent tapes to cold-weld to each other.

21. A tape for winding into a superconducting magnetic field coil, comprising: an electrically insulating tape comprising an electrically insulating material on at least an outer surface of the electrically insulating tape;a metal layer provided on each of the faces of the electrically insulating tape without extending around the edges of the tape.

22. The tape according to claim 21 wherein the electrically insulating material is a ceramic.

23. The tape according to claim 22 wherein the ceramic is alumina.

24. The tape according to any of claims 21 to 23 wherein the metal layer chiefly comprises indium.

25. The tape according to any of claims 21 to 24 wherein the metal layer is directly bonded to the faces of the electrically insulating tape.

26. The tape according to any of claims 21 to 25 wherein the electrically insulating tape comprises a metal core.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:-CLAIMS:

1. A method of winding a magnetic field coil, comprising coating a surface of a High Temperature Superconductor, HTS, tape with a 5 bonding material which is ductile at or near room temperature;winding one or more tapes, including the HTS tape, into a coil;during winding, applying pressure to the coil so as to cause surfaces of adjacent tapes, at least one of which is the surface of the HTS tape coated with the bonding material, to cold-weld to each other.

102. The method of claim 1, wherein the bonding material is chiefly composed of indium.

3. The method of claim 1 or 2, wherein the bonding material is indium.

154. The method of any preceding claim, further comprising, before winding the HTS tape into the coil, cleaning the bonding material to remove an oxidation layer.

5. The method of claim 4, wherein cleaning the bonding material comprises passing 20 the HTS tape through a bath or waterfall comprising an acid-based flux.

6. The method of claim 4 or 5, wherein cleaning the bonding material comprises creating a thin protective film on the surface of the bonding material.25 7. The method of any preceding claim, wherein coating the surface of the HTS tapewith the bonding material comprises electroplating the surface with the bonding material.

8. The method of any preceding claim, wherein the thickness of the bonding material coated on the surface of the HTS tape has a thickness in the region of about 5-30 10um.

9. The method of any preceding claim, wherein applying pressure to the coilcomprises applying a pressurised roller to the HTS tape at or close to a location at which the HTS tape contacts the coil.

10. The method of any preceding claim, wherein the pressure applied to the coil is in the region of about 10 to about 30 MPa.

11. The method of any preceding claim, wherein both surfaces of the HTS tape are 5 coated with the bonding material.

12. The method of claim 11, wherein the one or more tapes include more than one HTS tapes in a turn of the coil, and wherein alternating HTS tapes are coated with the bonding material.1013. The method of any of claims 1 to 10, wherein only one surface of the HTS tape is coated with the bonding material.152514. The method of any preceding claim, further comprising, before winding the HTS tape into the coil, winding the HTS tape onto a storage spool.

15. The method of any preceding claim, further comprising, before coating the surface of the HTS tape with the bonding material, applying a diffusion barrier later to the surface of the HTS tape.

16. A method of winding a magnetic field coil, comprising:coating a surface of an HTS tape with indium;applying a protective surface film to remove and prevent formation of an oxidation layer on the indium;winding one or more tapes, including the coated HTS tape, into a coil; andapplying pressure to the coil during winding to so as to cause surfaces of adjacent tapes, at least one of which is the surface of the HTS tape coated with indium, to coldweld to each other.

17. A method of winding a magnetic field coil comprising:winding one or more tapes into a coil, the tapes including at least one HTS tape; andduring winding, causing the surfaces of adjacent tapes to cold-weld to each other.

18. A tape for winding into a superconducting magnetic field coil, comprising:an electrically insulating tape comprising an electrically insulating material on at least an outer surface of the electrically insulating tape;a metal layer provided on each of the faces of the electrically insulating tape5 without extending around the edges of the tape;wherein the metal layer chiefly comprises indium.

19. The tape according to claim 18 wherein the electrically insulating material is a ceramic.1020. The tape according to claim 19 wherein the ceramic is alumina.

21. The tape according to any of claims 18 to 20 wherein the metal layer is directlybonded to the faces of the electrically insulating tape.1522. The tape according to any of claims 18 to 21 wherein the electrically insulating tape comprises a metal core.

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