High temperature superconductor magnet coils

Abstract

A method of winding a magnetic field coil. An HTS tape having a surface coated with a bonding material is provided. The bonding material is indium or an alloy chiefly composed of indium. One or more tapes, including the coated HTS tape, are wound into a coil. Pressure is applied to the coil during winding 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. Also provided are a magnetic field coil, a method of manufacturing a cable, a cable, and tapes for winding into a field coil.

Description

[0001] High Temperature Superconductor Magnet Coils

[0002] Field of the Invention

[0003] The present invention relates to high temperature superconductor, HTS, coils, methods of winding such coils, and tapes for winding into such coils.

[0004] 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).

[0005] 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.

[0006] 38259192-2 “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.

[0007] An HTS cable comprises one or more HTS tapes. 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.

[0008] 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:

[0009] • Insulated, having electrically insulating material between the turns (so that current can only flow in the “spiral path” through the HTS cables).

[0010] • Non-insulated, where the turns are electrically connected radially, as well as along the cables

[0011] • 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 turns.

[0012] Non-insulated coils could also be considered as the low-resistance case of partially insulated coils.

[0013] 38259192-2 HTS coils can be manufactured as shown in Figure 2, by providing a spool 201 of HTS cable 210, with a magnetic brake 202 or other means 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 (e.g. 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.

[0014] 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 excessive 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.

[0015] For large coils, it may be 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.

[0016] 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.

[0017] 38259192-2 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.

[0018] 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.

[0019] 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 impregnating the coil to improve mechanical strength and provide desired thermal and electrical properties. Impregnation (also known as “potting”) is the process of filling the coil with a liquid medium (“impregnation medium”) which penetrates between turns of the coil and then cures or cools to form a solid. Common impregnation mediums include solder or epoxy resin. This consolidates the loose turns of the coil into a solid, durable structure.

[0020] However, there are a number of challenges associated with solder impregnation. Where a coil has varying radius of curvature around the coil, the smaller radius parts of the coil are subject to higher radial forces from the winding tension, which leads to tighter compaction of the windings at these regions. 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 impregnation, 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.

[0021] 38259192-2 Thus, using current techniques, achieving a desired structure when winding with tension alone is a significant technical challenge, and the heat of subsequent solder impregnation can warp that structure.

[0022] Given these challenges, it is desirable to improve the manufacture of consolidated coils..

[0023] Summary

[0024] The invention is set out in the accompanying claims.

[0025] Brief Description of the Drawings

[0026] Figure 1 is a schematic illustration of a typical HTS tape.

[0027] Figure 2 is a schematic illustration of a system for winding a magnetic field coil.

[0028] Figure 3 is a schematic illustration of an alternative system for winding a magnetic field coil.

[0029] Figure 4 is a schematic diagram illustrating joints between HTS tapes.

[0030] Figure 5 is a schematic diagram illustrating a system for winding a magnetic field coil with coated tape.

[0031] Figure 6 is a schematic diagram of a pressure roller.

[0032] Figure 7 is a schematic diagram of a multiple spool winding apparatus.

[0033] Figures 8A, 8B, 80, and 9 each illustrate a respective insulating tape adapted for bonding to adjacent tapes;

[0034] Figure 10 illustrates a partially insulating tape adapted for bonding to adjacent tapes.

[0035] Figure 11 illustrates a process for forming an insulating or partially insulating tape.

[0036] 38259192-2 Figure 12 illustrates a cross section of an HTS coil.

[0037] Detailed Description

[0038] 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 impregnation process.

[0039] Some of these challenges can be addressed by including indium or an indium alloy between the HTS tapes and / or between other tapes of the coil such as insulating or partially insulating tapes.

[0040] 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 a similar manner. 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.

[0041] Indium is commonly used to make joints and electrical connections for cryogenic applications due to its malleability and ductile behavior even at temperatures below 120 K where many lead-based solders cease to be ductile. 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.

[0042] 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 without the need for further impregnation with solder. Cold welding is a solid-state welding process that requires little or no heat to join two clean metal surfaces together. Instead, energy used for creating a weld is provided in the form of pressure.

[0043] Coils comprising HTS tapes may also comprise other tapes, for example:

[0044] 38259192-2 conductive tapes, formed from a highly electrically conductive material such as copper, which may be used as spacers and to provide additional electrical stabilisation within the coil; insulating tapes, comprising an insulating material which prevents current flow within part of the coil, such as radial current flow between turns of the coil; partially insulating tapes, having a resistance across the tape intermediate between the conducting tapes and the insulating tapes (e.g. being formed from steel or another metal with an electrical conductivity much lower than copper, a semiconductor, or comprising an insulating material with conductive paths through it), used to control the turn-turn resistance of the coil.

[0045] While the description primarily focuses on the case of joining HTS tapes to HTS tapes, these other tapes may also be joined using cold welding techniques as discussed below, either to HTS tapes or to other non-HTS tapes.

[0046] In practice, to put this into effect, the basic steps are as follows:

[0047] • Coat HTS tape with indium (or obtain pre-coated HTS tape);

[0048] • Use this coated HTS tape to wind an HTS coil.

[0049] • Apply pressure during winding to bring together two surfaces, at least one of which is indium, and cause them to bond by cold welding.

[0050] These steps are now discussed in more detail, interspersed with other steps which may assist in optimising the process.

[0051] Indium and copper react with one another over time in a continuous process, resulting in formation of indium-copper (lnCu2) interfacial intermetallic compound followed by Cu11 ln9 for prolonged storage. At room temperature it is estimated that it takes a few months for an indium coating on copper of about 5pm thickness to be fully converted to InCu intermetallics.

[0052] Unlike indium, InCu intermetallics will not cold-weld to itself or other metals under reasonable pressures. This means there is a finite time within which HTS tape (or non- 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 of indium or storing the tape at low temperatures.

[0053] 38259192-2 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 m to about 10pm thick. This ensures that HTS tape coated with indium has a much longer shelf life before it must be wound into a coil.

[0054] 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.

[0055] 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 5-10pm. If this coating is applied on both sides of the tape, this provides a spacing between tapes of the order of about 10-20pm. The plating may be carried out by applying an indium film to the HTS tape directly or by using an electroplating or other suitable process.

[0056] The texture and surface roughness of the indium surface may be readily controlled up to about 5pm, which may enhance subsequent bonding. Such texture optimisations are similar to those usually employed when bonding other electrical elements using indium. Where the surface roughness of the indium can be improved, such as by using more involved plating methods than is typical, and the formation of InCu intermetallics can be limited or prevented, then the thickness of the initial indium coating may be reduced below 5pm. An indium thickness as low as 2-3pm can provide adequate bonding with practicable process controls in place, and in principle could be even lower. Alternatively, a thicker coating could be applied if there is expected to be a lengthy storage period before using the HTS tape, with no mitigation against the formation of InCu intermetallics. Provided there is a thickness of at least 2pm-5pm of unconverted indium, depending on the texture optimisations employed, at the time of using the HTS tape, then adequate bonding can still be expected.

[0057] An example of an electroplating approach is to use an indium sulphamate electroplating bath at current densities of 5-20mA / cm2. The liquid in the bath may be a solution of

[0058] 38259192-2 indium in sulfamic acid (as the electrolyte) with optional additives such as inhibitors and brighteners for optimized performance.

[0059] The indium may be provided in a strip only along a central portion of each face of the HTS tape, and not around the edges. This may be achieved by, for example, selective deposition of the indium or by applying a mask to the edges of the tape prior to coating the tape with indium. Limiting the indium coating to only a central strip reduces the chance of the indium bridging turns of the coil during subsequent winding.

[0060] Following the step of plating the HTS tape 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.

[0061] 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 surface 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 (e.g. transferring the tape through deionised water as the final cleaning step). An oxide layer should prevent self-welding in most conditions.

[0062] 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.

[0063] 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.

[0064] 38259192-2 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.

[0065] 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.

[0066] 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.

[0067] As a further alternative, the indium-coated HTS tape may be wound onto the coil with the oxide layer still present, and then the oxide layer may be removed following winding, e.g. by immersing the coil in flux, prior to applying pressure to the coil to cold-weld the indium.

[0068] 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 surfaces cold-weld 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 10-50 MPa as long as the indium has been cleaned by the flux bath, or around 100 MPa for uncleaned indium. Higher pressures may be applied if required, although once the pressure reaches about 300-400 MPa there is a likelihood of damaging HTS tape.

[0069] 38259192-2 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, or other components of the coil (e.g. copper and insulation tapes) may be deformed.

[0070] 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 radial component of the winding tension in these straight sections so some additional force or pressure is required.

[0071] 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.

[0072] 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 clean warm 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 over-pressure.

[0073] It will be noted that this cleaning process is much simpler than that required for solder impregnated coils, subjected to high temperature impregnation, 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 impregnated coil is ready for use only after a chemical de-bridging and final clean.

[0074] 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.

[0075] 38259192-2 The wheel 601 may have (or be coated with) a surface to which indium does not adhere, such as PTFE, a polyimide film such as Kapton®, or anodized aluminium.

[0076] 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.

[0077] 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, or providing conductive spacer tapes. Examples of such tapes may include steel tape, partial insulation tape (as described for example in W02019150123), or an electrical insulator. These are described in more detail below in connection with Figures 8-10.

[0078] 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.

[0079] As previously discussed, winding a coil with HTS tape with tension alone is a significant technical challenge, and the heat of subsequent solder impregnation warps that structure. An advantage of plating HTS tapes with indium before winding is that, during

[0080] 38259192-2 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 impregnation.

[0081] 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.

[0082] 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.

[0083] As a further example, and as noted above, indium may be plated onto other tapes of the HTS coil, such as conductive spacer tapes, partially insulating tapes, or insulating tapes, and they may be wound as described for the HTS tapes above.

[0084] 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 tape in an electroplating bath, or using some deposition process on only one side of the tape).

[0085] 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

[0086] 38259192-2 thickness per turn spacing (typically about 10-20pm) 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.

[0087] Indium also cold-welds to other metals and may also cold-weld to some 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] As described further in the “Cables” section below, the technique described above could also be used to form an HTS cable rather than a coil. A further advantage of the use of indium compared to other joining methods (e.g. non-indium solder) is that it can be easily sheared at room temperature, while retaining the connection between tapes. During bending of the HTS cable, this allows the HTS tapes of the cable to slide past each other, allowing a greater degree of damage-free bending of the cable after the HTS tapes are joined.

[0092] 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

[0093] 38259192-2 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).

[0094] A further option may include the application of ultrasound during winding. This may significantly 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. This may be achieved, for example, by the use of an ultrasound bath, or by the compression wheel vibrating at ultrasonic frequencies. However, this may increase the complexity of the winding step itself.

[0095] 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.

[0096] 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 make it possible to achieve cold welding under conditions that do not damage the tape.

[0097] 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 be used in certain circumstances in place of indium in the process described above.

[0098] 38259192-2 Insulating Tape

[0099] An insulating tape will now be described, which may be used with the indium winding method as disclosed above or for other known winding methods, as well as partially insulating tapes which may be made as modifications to the insulating tape. An insulating tape is a tape which does not allow substantial current to pass between its faces (i.e. between its two largest faces, as the side surfaces are referred to as “edges” in this disclosure), i.e. it acts as an insulator at least in that direction, as there is no current path through conductive material of the insulating tape between the faces. A partially insulating tape is a tape having a resistance between its faces intermediate between a the resistance of an equivalently sized tape formed of an electrical conductor and the resistance of an equivalently sized insulating tape.

[0100] Typically, in an insulated HTS coil, the turns are separated by layers of insulating material, for example an organic insulator such as Kapton™ . However, there are several problems with electrically insulating materials in an HTS coil:

[0101] • Solder does not generally bond well to insulating materials, leading to poor results when solder impregnating the coil (i.e. consolidating the coil with solder), resulting in structural weaknesses and / or voids in the solder.

[0102] • Electrical insulators are usually poor thermal conductors, which may result in poor thermal performance of the magnet. For example, when a “hotspot” forms in a magnet due to HTS material becoming non-superconducting (e.g. due to damage or a thermal event), that hot-spot will not spread as quickly through the magnet due to the poorer thermal conductivity. This results in a greater likelihood of a quench, and more damage if the HTS magnet quenches, due to the magnet’s energy being dumped into a smaller non-superconducting region than would be present with more even heating.

[0103] • Common organic insulating materials, which are cheap, effective insulators, and readily available, have low thermal conductivity and low stiffness. The low stiffness reduces the radial stiffness of the coil pack, which in turn means the coil is less able to share the electromagnetic forces radially throughout the pack, and hence is less suitable for high stress magnets. This can also result in failure of the insulator, leading to undesirable short circuits.

[0104] 38259192-2 Figure 8A illustrates, in cross section, an insulating tape 800 for use in winding an HTS magnet coil. The insulating tape comprises an insulator 801 in the form of an elongate tape, having a length (in a z direction as indicated on the figure) much greater than its width w in a y direction, and a width w much greater than its thickness t in a x direction. For example, the length may be on the order of several meters up to several kilometres, the width may be on the order of a few millimetres up to a few centimetres, and the thickness may be on the order of a micrometre up to a millimetre. It will be appreciated that the thickness is exaggerated in Figure 8A for clarity of illustration. The insulating tape has first and second faces, each generally facing in the x direction, which oppose each other, and first and second edges (i.e. faces generally facing in the y direction, referred to as edges as they are very thin compared to the first and second faces) which oppose each other and which separate the first and second faces.

[0105] On each of the first and second faces there is provided a bonding strip 802, arranged such that the bonding strips are not in contact with each other. No bonding strip is present on either of the first and second edges.

[0106] In this regard, the insulating tape retains its insulating properties, as the bonding strips 802 are separated by the insulator 801 , and the insulator 801 mechanically supports the bonding strips 802.

[0107] The bonding strips 802 are used to facilitate or improve the connection to adjacent tapes in the coil (e.g. adjacent HTS tapes) to increase the durability and radial thermal conductivity of the coil. For example, the bonding strips 802 may provide a solderable surface to ensure good bonding to insulating tapes when solder impregnating. Alternatively, the bonding strips 802 may provide a surface suitable for cold-welding to adjacent tapes to ensure good bonding when using the cold-weld consolidation methods disclosed above in coils that include insulating tapes.

[0108] Note that while figure 8A shows an insulator having a rectangular cross section, the insulator may have a different cross section, e.g. an elliptical cross section as shown in Figure 8B, or having rounded rather than flat edges. Figure 8B illustrates an insulating tape 810 having an elliptical insulator 811 and bonding strips 812. In these cases, the “first and second faces” are those areas covered by the first and second bonding strips,

[0109] 38259192-2 and the “first and second edges” are the areas between the first and second faces, which are not covered by the first and second bonding strips.

[0110] A material may be considered an insulator if it has a conductivity less than 1000 S / m.

[0111] Important properties for the insulator include:

[0112] • breakdown voltage (as required by the magnet design, some magnets need up to a few hundred volts per turn; other magnets as low as 10V per turn);

[0113] • thermal conductivity;

[0114] • thermal contraction coefficients, e.g. the proportional shrinkage of the material from room temperature to operating temperature of the magnet, e.g. 300K to 4K; this is ideally close to the thermal contraction coefficient of the HTS tape (e.g. of the tape substrate) and the material of the bonding strips

[0115] • ability to be bonded to the bonding strips by a suitable method o Preferably, the bonding between the insulator and the bonding strips does not require an adhesive, as the adhesive may deform under the forces involved in winding the coil or the forces experienced during operation. o Example methods for bonding the insulator and the bonding strips are discussed in more detail below

[0116] Ceramic, inorganic or crystalline insulators are generally preferred over plastic, organic, or polymer insulators (such as Kapton™ or mylar), as they typically have higher stiffness, thermal conductivity and breakdown voltage than plastic, organic, or polymer insulators. Ceramic, inorganic, or crystalline insulators are typically more brittle, and may be prone to cracking when under tensile strain or stress - but in the context of an HTS coil this is unlikely to be a significant issue, as the HTS coil must already be designed to avoid putting excessive strain on the HTS material of the HTS tapes. The strain limits of ceramic insulators are well known in the art, and would not present any additional design challenge.

[0117] Suitable insulators include alumina, titanium oxides, tantalum oxides, diamond (or diamond-like carbon), quartz, glass (e.g. glass fibre or a solid glass strip), or mica.

[0118] Important properties for the bonding strip include:

[0119] • Providing either a solderable surface, or a surface suitable for cold welding

[0120] 38259192-2 • Ability to be bonded to the insulator by a suitable method.

[0121] The bonding strips may be formed from a metal, such as copper, silver, nickel, titanium, or indium.

[0122] Figure 8C shows a further insulating tape 820, comprising an insulator 821 as described with reference to figure 8A, and bonding strips 822A. The insulating tape further comprises a connection layer 822B, between the bonding strips and the insulator. Such a construction may allow for use of a metal with preferred properties for bonding to other tapes (e.g. improved solderability, or improved properties for cold welding) for the bonding strips, and a different metal with preferred properties for bonding to the insulator for the connection layer - i.e. the metal of the bonding strips need not be selected to bond to the insulator, and the metal of the connection layer need not be selected to bond to other tapes. The metal for the bonding strips may be indium, providing a surface for cold welding as discussed previously.

[0123] The connection layer is applied to the insulating tape in the same manner as disclosed for the bonding strips of Figure 8A and 8B - e.g. the connection layer is provided on first and second surfaces, and does not connect those surfaces. The bonding strips are then applied to the connection layer, e.g. by electroplating.

[0124] The use of a metal suitable for cold-welding on the outer surface of the insulating tape (i.e. as the metal of the bonding strip or the first metal of a bonding strip formed from multiple metal layers) makes the insulating tape disclosed in this section particularly suited for the cold-welding techniques disclosed elsewhere in the specification.

[0125] Figure 9 illustrates, in cross section, a further insulating tape 900. The insulating tape 900 comprises a core 903 in the form of an elongate tape, having a length (in a z direction as indicated on the figure) much greater than its width w in a y direction, and a width w much greater than its thickness t in a x direction. The core 903 has first and second sides, and first and second edges. An insulator 901 is provided, covering at least the first and second sides and first and second edges. The insulator 901 may also cover the ends of the core (i.e. the faces of the core that face in the z direction), but this need not be the case.

[0126] 38259192-2 Also on the first and second sides are provided first and second bonding strips, such that the first and second bonding strips are outside of the insulator (i.e. the portion of the insulator on each of the first and second sides is sandwiched between the core and the respective first or second bonding strip).

[0127] As with the insulating tape 800, the insulating tape 900 provides electrical insulation in the x direction, while the bonding strips enable it to be easily connected to other tapes such as HTS tapes having metal surfaces, via solder or the like. The core provides additional structural support to the insulator, and results in thinner layers of insulator for the same overall level of insulation, which makes the insulator less likely to break under strain, improves thermal properties of the insulating tape, and retains some insulating properties in the event of a break in the insulator on only one side of the core.

[0128] The important properties for the core include:

[0129] • Stiffness (Young’s modulus preferably around 200 GPa, e.g. at least 100 GPa or at least 50 GPa)

[0130] • Yield strength

[0131] • Thermal conductivity (e.g. greater than that of the insulator)

[0132] • Thermal contraction coefficient, e.g. the proportional shrinkage of the material from room temperature to operating temperature of the magnet, e.g. 300K to 4K; this is ideally close to the thermal contraction coefficient of the HTS tape (e.g. of the tape substrate) and the material of the bonding strip. o Where a core is present, the overall thermal contraction of the insulating tape will be primarily determined by the thermal contraction coefficient of the core.

[0133] • Ability to be readily provided with a coating of the insulator, and substantially non- reactive with that coating (i.e. contact between the insulator and the core does not cause degradation of the insulator over time)

[0134] The core 903 may be formed of a metal, e.g. Hastelloy™, steel, stainless steel, titanium, tantalum, aluminium or copper. The core may be formed from multiple metals, e.g. as aluminium-plated copper, with the aluminium plating being in contact with the insulator.

[0135] 38259192-2 The insulator may be an oxide or other ceramic compound of the metal forming the core (or the outer layer of the core, in the case that the core comprises multiple metals). For example, the core may be formed of aluminium, and the insulator may be alumina.

[0136] The core 903 may be electrically conducting, in which case the core 903 may be configured to provide an unbroken electrical connection along the length of the insulating tape. This electrical connection may be used, for example, to measure the non-inductive start-to-end voltage of the magnet, by measuring the total start-to-end voltage of the HTS of the magnet coil and subtracting the start-to-end voltage of the core 903. As the core 903 is co-wound with the HTS, it will experience the same inductive voltage (assuming a single continuous insulating tape, or multiple tapes connected end-to-end, which has the same number of turns as the HTS), and therefore the difference between the voltages will be the non-inductive start-to-end voltage of the coil. Alternatively or additionally, the insulating tape 900 may be used as a quench heater. Current may be provided to the core 903 to heat the magnet, providing even heating throughout the magnet e.g. to cause all of the HTS in the magnet to quickly become normally conducting, quenching the whole magnet, in the event of a localised quench, mitigating against “hot-spots” forming during the localised quench.

[0137] The core may have a thickness of, for example, 20 micrometres to 100 micrometres. The insulator coated onto the core may have a thickness of, for example, 1 micrometre to 20 micrometres on the first and second faces of the core and the first and second edges of the core. The width of the core may be substantially the width of the insulating tape, e.g. may be the width of the insulating tape minus the 1-20 micrometre thickness of the insulator applied to each edge.

[0138] Figure 10 illustrates a partially insulating tape 1000. The partially insulating tape 1000 is similar to the insulating tape 900 of Figure 9, except that the insulator has a plurality of windows 1010 allowing contact between the bonding strips and the core on each of the first and second faces. In detail, the partially insulating tape 1000 comprises a core 1003 similar to the core 903 of Figure 9, an insulator 1001 covering the first and second edges of the core, and partially covering the first and second faces of the core as described in more detail below. Bonding strips 1002 are provided on the first and second faces, with the insulator 1001 being between the bonding strips and the core on each face, except as noted below.

[0139] 38259192-2 On each face, the insulator has a plurality of “windows” through it, i.e. gaps through which the bonding strips and the core come into contact. This contact may involve extension of either the bonding strips or the core into the windows, or an intermediate material such as a solder that bridges the gap. The windows may be the only electrical connection through the insulator. The windows may have any shape and may extend to the edges of the bonding strips. The location of the windows on either side of the core may be staggered either in the width direction of the partially insulating tape 1000 (as illustrated in Figure 10) or in the length direction, or both, such that current must take an indirect path between the windows to travel through the partially insulating tape, increasing the resistance of the partially insulating tape. The resistance of the partially insulating tape will depend on the size, shape, and spacing of the windows in a similar manner to the windows provided within the partially insulating tape disclosed in WO / 2019 / 150123, as discussed in that document. It will be noted that the structure of the insulating layers in that document differs from the structure of the insulating coating over the core in the present disclosure, but equivalent patterns of windows may be used, treating the portion of the insulating coating between the core and the bonding strips as analogous to the insulating layers in WO / 2019 / 150123.

[0140] It will be appreciated that many of the teachings and features of the insulating tapes described above in connection with Figure 8 and 9 can be implemented in the partially insulating tape illustrated in Figure 10.

[0141] Manufacture of Insulating Tapes

[0142] For an insulating tape 800, 810, 820 as described above with reference to Figures 8A- C, the insulator may be formed as an elongate tape by any suitable method as appropriate to the chosen insulator, e.g. extrusion, deposition onto a removable substrate, rolling, or other methods as known in the art.

[0143] For an insulating tape 900 or a partially insulating tape 1000 having a core 903, 1003 as described above with reference to Figure 9 or 10, the core may be formed as an elongate tape by any suitable method as appropriate to the chosen core material(s), e.g. extrusion, deposition onto a removable substrate, rolling, or other methods as known in the art. Where multiple core materials are used, an elongate tape may be formed of a first core

[0144] 38259192-2 material and the second core material subsequently plated onto it, or a block may be formed having a relatively thick rod of the first core material encapsulated in the second core material, and this block may be shaped (e.g. rolled) to shape it into an elongate tape as required.

[0145] The insulator 901 , 1001 may be coated onto the core by any suitable method as known for applying thin (e.g. 1-20 micrometre) coatings of the insulator material onto the (outer) core material. For example, where the core is Hastelloy or a similar metal, and the insulator is alumina, the alumina may be coated onto the Hastelloy by processes well known in the art of ReBCO HTS tape manufacture (as alumina is used as an HTS tape buffer layer, so is coated onto the HTS tape substrate which is usually Hastelloy during HTS tape manufacture). Such processes include sputtering, I BAD (ion beam assisted deposition), etc.

[0146] Alternatively, where the insulator is an oxide or other compound of a metal constituent of the (outer) core material, the insulator may be formed directly onto the core. For example, where at least the outer layers of the core comprise or are primarily composed of aluminium, the aluminium may be anodised or otherwise oxidised to create an insulating layer of alumina around the core.

[0147] As a further alternative, where the insulator is an oxide or other compound of a metal constituent of the (outer) core material, the insulator may be formed directly onto a sheet of the core material, which is then cut into tapes having the insulator on their first and second faces. The edges of those tapes will have exposed core material, which can then be insulated by the application of a suitable insulator (which may be the same as the insulator formed on the faces, or different to that insulator).

[0148] Where windows 1010 through the insulator are required such as in the partially insulating tape 1000 discussed with reference to Figure 10, these windows may be created, for example, by etching through the insulator after it is applied to the core, or by masking off portions of the core (by methods as appropriate to the application method for the insulator) prior to application of the insulator, such that the insulator is not applied to the areas under the windows. To ensure electrical connection through the windows, they may be pre-filled with solder or other conductive material prior to application of the bonding strips to the insulator.

[0149] 38259192-2 For application of the bonding strips 802, 812, 822A, 822B, 902, 1002 to the insulator to form any of the above described insulating or partially insulating tapes, this may be by any suitable method to apply the metal of the bonding strip (or, in the case of a bonding strip comprising multiple metals, the second metal 822B as described above) to the material of the insulator.

[0150] The bonding strips may be applied only to the first and second faces, e.g. by the use of targeted deposition methods or by masking the first and second edges and optionally the ends of the tapes. Alternatively, the insulator may be encapsulated in the (second) metal of the bonding strips, and that metal may be chemically etched or otherwise removed from the first and second edges, and optionally the ends of the tapes.

[0151] Figure 11 illustrates an example of such etching, where the tape is provided as an insulator in the form of an elongate tape (having a core in this example, though it will be appreciated that the method does not require a core) encapsulated in a layer of metal (e.g. copper and / or silver). One edge of the insulator is dipped into an etching solution, i.e. a solution which chemically etches the metal encapsulating the insulator (e.g. hydrogen peroxide or ammonium peroxide, where the metal is copper and / or silver), to remove the copper from that edge, and the tape is then flipped and the other edge dipped into the etching solution.

[0152] Suitable deposition methods for the bonding strips will depend on the insulator and the metal used. Where the insulator is alumina or another ceramic, the methods used for encapsulating HTS tapes in silver and copper as part of their usual manufacture will also work in this case, or the metal may be applied by, for example, electroless plating / deposition or sputter / thin film deposition.

[0153] Where the bonding strip comprises multiple metals as described above, the second metal 822B may be applied to the insulator, and then the first metal 822A applied to the second metal (or further metals applied to the second metal or to previous further metals, and then the first metal applied to the outer of the further metals). This is of particular use, for example, where the deposition methods available to provide the first metal onto the insulator are not suitable for large scale production, but there are more suitable methods available to apply the second metal to the insulator and to apply the first metal

[0154] 38259192-2 to the second metal (or to an intermediate further metal which can be applied to the second metal by a suitable method). In particular, it may be possible to electroplate the first metal 822A onto the second metal 822B. Where the first metal 822A is indium or an indium alloy for use in cold-weld winding, electroplating the indium onto a base layer of a second metal 822B may enable better control of desired surface characteristics to improve subsequent bonding.

[0155] It will be appreciated that, while several options have been provided for the provision of an insulator or core in the form of an elongate tape, the application of an insulator to a core, the creation of windows into the insulator, and the application of bonding strips to the insulator, these may be combined in various ways to produce examples of insulating tapes as described above with reference to figures 8 through 10.

[0156] In particular, at each stage, which methods are available depend primarily on the material which is being applied, and the material which it is being directly applied to - e.g. when applying the bonding strips to the insulator, it will not generally matter what method was chosen to apply the insulator to the core, or what material the core is formed from. The exception to this is where a bonding method requires an elevated temperature, and this elevated temperature would be above the melting point of a material already applied (e.g. where application of the bonding strips would require a temperature above a melting point of a core material), where additional cooling may be required to avoid such melting.

[0157] It is advantageous that the methods used to bond the materials of the insulating tape do not result in adhesives present between the materials after bonding, as such adhesives generally have significantly less stiffness (e.g. under 10 GPa) than the other materials of the insulating tape and may result in unwanted compression of the tape during winding and / or operation of the HTS coil. Where adhesives are present after bonding, the layers of adhesive should be as thin as possible, so as to have minimal impact on the structural behaviour of the insulating tape.

[0158] Figure 12 shows a cross section of a coil constructed using the techniques described above. The coil comprises a plurality of HTS tapes 1201 , a plurality of insulating tapes 1202, an inner bobbin 1203, and an outer bobbin 1204. Only one of each type of tape is labelled, for clarity, and the dotted line 1210 in the figure indicates an arbitrary number of further turns of the coil. The HTS tapes are arranged with three HTS tapes per turn,

[0159] 38259192-2 i.e. with each set of three HTS tapes separated by an insulating tape, though in practice this may be any number of HTS tapes. The HTS tapes are bonded to adjacent HTS tapes (and, where appropriate, to the insulating tapes or the bobbins) by layers of indium or solder 1211 depending on the consolidation technique used.

[0160] The cold-weld winding 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 a high radial component of winding tension at the apexes makes solder-impregnation challenging. The wind itself is easier to control, achieving more uniform turn thickness and tension in the tapes, which are 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, improving current sharing between turns. There is no need to heat coils to consolidate them, which avoids problems with coils warping due to differential thermal expansion, removes significant amount of thermal stress, and saves on cost and additional complex equipment processes such as hot plates, control units, fume cupboards. Fewer ‘high risk’ processes are required. De-bridging processes after solder impregnation to remove excess solder are no longer needed. The lead usually present in solder is not required. Indium is not brittle at cryogenic temperatures.

[0161] Cables

[0162] The techniques described for manufacturing an HTS coil by cold-welding HTS tapes together can also be used to form HTS cables. As with HTS coils, the cables may comprise other tapes, such as conductive tapes, insulating tapes, and partially insulating tapes. In addition, the cables may comprise substrates as described in more detail below, which act to support the cable and to ensure proper positioning of the tapes.

[0163] The basic steps of forming a cable are similar to those for an HTS coil:

[0164] • Coat HTS tape with indium (or obtain pre-coated HTS tape)

[0165] • Arrange the coated HTS tape into the cable

[0166] • Apply pressure to bring together two surfaces, at least one of which is indium, and cause them to bond by cold welding.

[0167] 38259192-2 The details of manufacturing, storing, and handling the coated HTS tape are the same whether that tape is used for an HTS coil or an HTS cable, as such the previous disclosure still applies, except for the actual winding steps (with references to “winding the coil” replaced with “assembling the cable”, e.g. when a step is described as being “to prepare for winding”).

[0168] Figure 13 shows a mechanism for manufacturing an HTS cable. In the example shown, this is a simple stacked tape cable (i.e. where the HTS tapes are laid parallel on top of each other to form the cable), but similar principles apply for other kinds of cable. In this example it is assumed that indium coated HTS tape 1300 has already been wound onto a plurality of spools 1301. The tape 1300 is unwound from each spool 1301 and passes through respective tension rollers or pulleys 1302 and a respective flux bath 1303 which provides a flux coating to prevent oxidation as described for the winding of HTS coils. The clean tapes are then brought together to form the cable 1305, and a pressure roller 1306 applies pressure to the tapes. Pressure roller 1306 may be a pressure roller such as that depicted in Figure 6, with a former, a passive roller or a further pressure roller 1304 provided on the opposite side of the cable to the pressure roller. The indium surfaces cold-weld instantly by the aid of the external pressure, which expels the liquid flux coating from between the tapes. Suitable pressures are the same as for the winding of HTS coils. The cable may be shaped during assembly, e.g. to provide a curved cable. This may be achieved by providing a former opposed to the pressure roller, configured to cause the cable to conform to a desired shape as it passes over the former. The cable may be bent to some extent after pressure is applied, as the indium can shear at room temperature while retaining the connection between tapes, but excessive bending from the shape set during cold welding may cause buckling and / or excessive strain. This allows some bending of the cable in use, or for the cable to be wound onto a reel for transportation, provided the radius of curvature is sufficiently large to prevent buckling or excessive strain.

[0169] Figure 14 shows a further mechanism for manufacturing an HTS cable. As with figure 13, a plurality of spools 1401 of HTS tape 1400 are provided. Also provided is a substrate 1411 , as will be described in more detail below. The tape 1400 is unwound from each spool 1401 and passes through respective tension rollers or pulleys 1402 and a respective flux bath 1403. The clean tapes are then arranged into or onto the substrate 1410 to form the cable 1405, and a pressure roller 1406 applies pressure to the tapes.

[0170] 38259192-2 The substrate 1410 and the tension rollers or pulleys 1402 move relative to each other (i.e. either the tension rollers or the substrate may move) so that the tape is progressively laid into or onto the substrate. This may be achieved by providing the tension rollers or pulleys 1402, the spools 1401 , and the flux bath 1403 on a movable cart which moves along the substrate, or by providing further tension rollers 1412 which move the substrate.

[0171] The substrate may be provided in the required shape of the cable, e.g. straight or curved along its length, or it may be shaped prior to the cables being laid into or onto the substrate, e.g. by shaping rollers 1413.

[0172] The substrate acts to support the HTS tapes in the cable. The substrate may be formed from a conductive material, e.g. copper or steel, in which case it may also act to provide current-sharing paths between HTS tapes in the cable.

[0173] Example transverse cross sections of suitable substrates are shown in Figure 15A-C, with the locations where the HTS tapes are to be laid shown by the dotted profiles 1500.

[0174] As shown in Figure 15A, the substrate 1510 may be a simple bar of material, onto which the tapes are laid.

[0175] As shown in Figure 15B, the substrate 1520 may be a “u-shaped” channel, having a rectangular trench 1521 surrounded on three sides by the material of the substrate, with an opening into which the HTS cables are laid on the other side.

[0176] As shown in Figure 15C, the substrate 1530 may be a core having a substantially circular cross section, with a plurality of trenches 1531 evenly spaced at the circumference of the circular cross section and either extending straight along the length of the substrate or helically winding around it.

[0177] The substrate may be coated with indium on the face(s) which come into contact with the tapes of the HTS cable.

[0178] As an alternative shown in Figure 16, the cable 1600 may be assembled without the application of pressure (e.g. using a mechanism such as that shown in Figure 13 or 14,

[0179] 38259192-2 but with the pressure roller 1306 I 1406 replaced by a passive roller which guides the tapes into position but does not apply sufficient pressure to cold-weld the tapes), and then pressure 1601 may be applied to the cable following formation of the cable. This pressure may be applied evenly over the length of the cable, e.g. by a suitably shaped press, or may be applied progressively along the cable, e.g. by passing the cable through a pressure roller. The HTS tapes forming the cable may be coated in flux prior to assembling the cable, or flux may be impregnated into the cable between assembly of the cable and the application of pressure to the cable. An advantage of the latter approach, where flux is applied after assembling the cable, is that it avoids unintentional cold-welding and allows the tapes to move easily relative to each other.

[0180] Clauses

[0181] Further examples of the present disclosure are presented in the following numbered clauses:

[0182] Clause 1. An insulating tape for winding into a superconducting magnetic field coil, the tape comprising: an insulator in the form of an elongate tape having first and second faces and first and second edges, wherein the first face is opposite the second face, and each edge separates the first and second faces, the insulator comprising an electrically insulating material on at least the first and second faces and the first and second edges; a bonding strip provided on each of the faces of the insulator without extending around the edges of the insulator, the bonding strip comprising a first metal.

[0183] Clause 2. A tape according to clause 1 , wherein the electrically insulating material is a ceramic.

[0184] Clause 3. A tape according to clause 2, wherein the ceramic is one of: alumina; titanium oxide; tantalum oxide.

[0185] Clause 4. A tape according to clause 1 , wherein the electrically insulating material is an inorganic crystalline material.

[0186] 38259192-2 Clause 5. A tape according to clause 4, wherein the electrically insulating material is one of: diamond; diamond-like carbon; quartz; mica; glass.

[0187] Clause 6. A tape according to any of clauses 1 to 5, wherein the insulator comprises a core, the core comprising a core material, the core material being a material other than the electrically insulating material, and wherein the electrically insulating material surrounds the core on each of the first and second face and the first and second edge.

[0188] Clause 7. A tape according to clause 6, wherein the core material is a material having one of: a greater thermal conductivity than a thermal conductivity of the electrically insulating material; a greater Youngs modulus than a Youngs modulus of the electrically insulating material; a greater yield strength than a yield strength of the electrically insulating material.

[0189] Clause 8. A tape according to clause 6 or 7, wherein the core material is an electrically conductive material.

[0190] Clause 9. A tape according to clause 8, wherein the core material is a metal, more preferably one of: aluminium;

[0191] Hastelloy™; steel; stainless steel; copper; titanium; tantalum.

[0192] 38259192-2 Clause 10. A tape according to any one of clauses 6 to 9, wherein the electrically insulating material is an oxide of a constituent metal of the core material.

[0193] Clause 11. A tape according to any one of clauses 6 to 11 , wherein the core comprises a plurality of different core materials.

[0194] Clause 12. A tape according to any one of clauses 8 to 10 wherein the core material is aluminium and the electrically insulating material is alumina.

[0195] Clause 13. A tape according to any preceding clause, wherein each bonding strip further comprises a second metal, the second metal being bonded to the insulator and the first metal having an exposed face parallel to the respective face of the insulator.

[0196] Clause 14. A tape according to any preceding clause, wherein the first metal is a metal capable of cold-welding.

[0197] Clause 15. A tape according to any preceding clause, wherein the first metal is indium.

[0198] Clause 16. A tape according to any preceding clause, wherein the tape does not comprise an adhesive.

[0199] Clause 17. A method of manufacturing a tape for winding into a superconducting magnetic field coil, the method comprising: providing an insulator in the form of an elongate tape having first and second faces and first and second edges, wherein the first face is opposite the second face, and each edge separates the first and second faces, the insulator comprising an electrically insulating material on at least the first and second faces and the first and second edges; applying a bonding strip to each of the first and second faces of the insulator, each bonding strip comprising a metal.

[0200] Clause 18. A method according to clause 17, wherein providing the insulator comprises: providing a core in the form of an elongate tape;

[0201] 38259192-2 coating the core in the electrically insulating material on at least the first and second face and the first and second edges.

[0202] Clause 19. A method according to clause 18, wherein coating the core in the electrically insulating material comprises: depositing the electrically insulating material onto the core.

[0203] Clause 20. A method according to clause 18 wherein coating the core in the electrically insulating material comprises: oxidising the core to form the electrically insulating material.

[0204] Clause 21 . A method according to clause 18 wherein the core comprises a metal and coating the core in the electrically insulating material comprises: anodising or oxidising the core to form an electrically insulating metal oxide layer on the surface of the core.

[0205] Clause 22. A method according to clause 17, wherein providing the insulator comprises: providing a sheet of a core material, the core material comprising a metal; oxidising first and second faces of the sheet to form the electrically insulating material; cutting the sheet of core material to form a plurality of insulators in the form of elongate tapes; and coating the first and second edges of each insulator in further insulating material.

[0206] Clause 23. A method according to any one of clauses 18 to 22, and comprising forming a plurality of windows in the insulating material on each of the first and second face, by one of: masking areas of the core prior to coating the core in the electrically conductive material; or removing the insulating material from areas of the core following coating the core in the electrically conductive material.

[0207] Clause 24. A method according to any one of clauses 17 to 23, wherein applying the bonding strips to the insulator comprises one of:

[0208] 38259192-2 masking the first and second edges prior to application of the bonding strips; encapsulating the insulator in the metal of the bonding strips and subsequently removing the metal of the bonding strips from the first and second edges.

[0209] Clause 25. A method according to any one of clauses 17 to 24, wherein applying the bonding strips to the insulator comprises applying a first metal to the insulator, and applying a second metal to the first metal, the first and second metals together forming the bonding strip.

[0210] Clause 26. A method according to clause 25 wherein applying the second metal to the first metal comprises electroplating the second metal onto the first metal.

[0211] Clause 27. A tape for winding into a superconducting magnetic field coil, the tape comprising: a metal core in the form of an elongate tape having first and second faces and first and second edges, wherein the first face is opposite the second face, and each edge separates the first and second faces; an electrically insulating material surrounding the core on each of the first and second face and the first and second edge, the electrically insulating material comprising a ceramic; wherein the electrically insulating material on each of the first and second faces has a plurality of windows through the electrically insulating material for making electrical contact to the core.

[0212] Clause 28. A tape according to clause 27 further comprising: a metal strip provided on each of the exterior faces of the tape without extending around the edges of the tape and in electrical contact with the core through the windows.

[0213] Clause 29. A tape according to clause 27 or 28 wherein the core comprises aluminium and the insulating material comprises aluminium oxide.

[0214] Clause 30. A superconducting magnetic field coil comprising a plurality of turns of High Temperature Superconductor (HTS) separated by an insulating tape according to any one of clauses 1 to 16.

[0215] 38259192-2 M&C PX221 184W0

[0216] 34

[0217] Clause 31. A method of heating a superconducting magnetic field coil according to clause 30 wherein the insulating tape comprises an electrically conductive core, the method comprising driving an electrical current through the electrically conductive core of the insulating tape to heat the superconducting magnetic field coil.

[0218] Clause 32. A superconducting magnetic field coil comprising a tape according to any one of clauses 27 to 29.

[0219] 38259192-2

Claims

1. 35CLAIMS:1 . A method of winding a magnetic field coil, comprising: providing an HTS tape having a surface coated with a bonding material; winding one or more tapes, including the coated HTS tape, into a coil; and applying pressure to the coil during winding 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; wherein the bonding material is indium or an alloy chiefly composed of indium.

2. The method of claim 1 , further comprising, before winding the HTS tape into the coil, cleaning the bonding material to remove an oxidation layer.

3. The method of claim 2, wherein cleaning the bonding material comprises passing the HTS tape through a bath or waterfall comprising an acid-based flux.

4. The method of claim 2 or 3, wherein cleaning the bonding material comprises creating a thin protective film on the surface of the bonding material.

5. The method of any preceding claim, wherein providing the HTS tape having the surface coated with the bonding material comprises coating the surface of the HTS tape with the bonding material.

6. The method of claim 5, wherein coating the surface of the HTS tape with the bonding material comprises electroplating the surface with the bonding material.

7. The method of claim 5 or 6, further comprising, before coating the surface of the HTS tape with the bonding material, applying a diffusion barrier layer to the surface of the HTS tape.

8. The method of any preceding claim, wherein the bonding material coated on the surface of the HTS tape has a thickness in the region of about 2- 10pm.

9. The method of any preceding claim, wherein applying pressure to the coil comprises applying pressure to the HTS tape as it is wound into the coil.38259192-23610. The method of claim 9, wherein applying pressure to the HTS tape as it is wound into 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.

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

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

13. The method of claim 12, 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.

14. The method of any of claims 1 to 11, wherein only one surface of the HTS tape is coated with the bonding material.

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

16. A method of winding a magnetic field coil, comprising providing a High Temperature Superconductor, HTS, tape having a surface coated with a bonding material which is cold-weldable 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.

17. The method of claim 16, wherein the bonding material is chiefly composed of indium.

18. The method of claim 16 or 17, wherein the bonding material is indium.

19. A magnetic field coil wound using the method of any preceding claim.38259192-220. A magnetic field coil comprising at least one HTS tape having a surface coated with a bonding material, wherein the bonding material is indium or an alloy chiefly composed of indium, and the surface coated with the bonding material is cold-welded to a surface of an adjacent tape.21 . A magnetic field coil comprising a plurality of HTS tapes wound around a central axis, wherein adjacent tapes are separated by a layer of indium or an allyo chiefly composed of indium having a thickness between about 4pm and about 20pm.

22. 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; and during winding, causing the surfaces of adjacent tapes to cold-weld to each other.

23. A method of forming an HTS cable, comprising: providing a High Temperature Superconductor, HTS, tape having a surface coated with a bonding material, wherein the bonding material is indium or an alloy chiefly composed of indium; arranging one or more tapes, including the HTS tape, into a cable; applying pressure to the cable 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 coldweld to each other.

24. A method according to claim 23 wherein the cable further comprises a substrate into which the one or more tapes are arranged.

25. A method according to claim 24 wherein the substrate comprises a groove within which the one or more tapes are arranged.

26. A method according to any of claims 23 to 25 wherein applying pressure to the cable comprises applying pressure to each tape as it is arranged into a cable.

27. A high temperature superconducting, HTS, cable comprising at least one HTS tape having a surface coated with a bonding material, wherein the bonding material is38259192-2indium or an alloy chiefly composed of indium, and the surface coated with the bonding material is cold-welded to a surface of an adjacent tape.

28. An insulating tape for winding into a superconducting magnetic field coil, the tape comprising: an insulator in the form of an elongate tape having first and second faces and first and second edges, wherein the first face is opposite the second face, and each edge separates the first and second faces, the insulator comprising an electrically insulating material on at least the first and second faces and the first and second edges; a bonding strip provided on each of the faces of the insulator without extending around the edges of the insulator, the bonding strip comprising indium or an alloy chiefly composed of indium.

29. A tape according to claim 28, wherein the electrically insulating material is a ceramic or an inorganic crystalline material.

30. A tape according to claim 28 or 29, wherein the insulator comprises a core, the core comprising a core material, the core material being a material other than the electrically insulating material, and wherein the electrically insulating material surrounds the core on each of the first and second face and the first and second edge.

31. A tape according to claim 30, wherein the core material is a material having one of: a greater thermal conductivity than a thermal conductivity of the electrically insulating material; a greater Youngs modulus than a Youngs modulus of the electrically insulating material; a greater yield strength than a yield strength of the electrically insulating material.

32. A tape according to claim 30 or 31 , wherein the core material is an electrically conductive material.

33. A tape according to any one of claims 30 to 32, wherein the electrically insulating material is an oxide of a constituent metal of the core material.38259192-23934. A tape according to any one of claims 28 to 33, wherein the tape does not comprise an adhesive.

35. A tape according to any one of claims 28 to 34, further comprising a connection layer provided between the insulator and the bonding strip on each of the faces of the insulator without extending around the edges of the insulator, the connection layer being formed from a metal other than indium.

36. A method of manufacturing a tape for winding into a superconducting magnetic field coil, the method comprising: providing an insulator in the form of an elongate tape having first and second faces and first and second edges, wherein the first face is opposite the second face, and each edge separates the first and second faces, the insulator comprising an electrically insulating material on at least the first and second faces and the first and second edges; applying a bonding strip to each of the first and second faces of the insulator, each bonding strip comprising indium or an alloy chiefly composed of indium.

37. A method according to claim 36, wherein providing the insulator comprises: providing a core in the form of an elongate tape; coating the core in the electrically insulating material on at least the first and second face and the first and second edges.

38. A method according to claim 37, wherein coating the core in the electrically insulating material comprises: depositing the electrically insulating material onto the core; or oxidising or anodising the core to form the electrically insulating material on the surface of the core.

39. A method according to any one of claims 36 to 38, wherein applying the bonding strips to the insulator comprises one of: masking the first and second edges prior to application of the bonding strips; encapsulating the insulator in the metal of the bonding strips and subsequently removing the metal of the bonding strips from the first and second edges.38259192-2M&C PX221 184W04040. A method according to any one of claims 36 to 39, and comprising, prior to applying the bonding strip, applying a connection layer to each of the first and second faces of the insulator, the connection layer comprising a metal other than indium, wherein each bonding strip is applied to the respective connection layer.41 . A method according to any one of claims 36 to 40 wherein applying the bonding strip to the connection layer comprises electroplating indium or the alloy chiefly composed of indium onto the connecting layer.38259192-2

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