Patent Description:
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 critical temperature (the temperature above which the material cannot be superconducting even in zero magnetic field) below about <NUM>. The behaviour of HTS material is not described by BCS theory, and such materials may have critical temperatures above about <NUM> (though it should be noted that it is the physical differences in composition and superconducting operation, rather than the critical temperature, which define HTS and LTS material). 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 diboride (MgB<NUM>).

ReBCO is typically manufactured as tapes, with a structure as shown in <FIG>. Such tape <NUM> is generally approximately <NUM> microns thick, and includes a substrate <NUM> (typically electropolished nickel-molybdenum alloy such as Hastelloy™, approximately <NUM> microns thick), on which is deposited by Ion Beam Assisted Deposition (IBAD), magnetron sputtering, or another suitable technique a series of buffer layers known as the buffer stack <NUM>, of approximate thickness <NUM> microns. An epitaxial ReBCO-HTS layer <NUM> (deposited by Metal Oxide Chemical Vapour Deposition (MOCVD) or another suitable technique) overlays the buffer stack, and is typically <NUM> micrometer thick. A <NUM>-<NUM> micrometer silver layer <NUM> is deposited on the HTS layer by sputtering or another suitable technique, and a copper stabilizer layer <NUM> is deposited on the tape by electroplating or another suitable technique. The silver layer <NUM> and copper stabilizer layer <NUM> are deposited on the sides of the tape <NUM> and the substrate <NUM> too, so that these layers extend continuously around the perimeter of the tape <NUM>, thereby allowing an electrical connection to be made to the ReBCO-HTS layer <NUM> from either face of the tape <NUM>. These layers <NUM>, <NUM> may therefore also be referred to as "cladding". Typically, the silver cladding has a uniform thickness on both the sides and edges of the tape of around <NUM>-<NUM> microns. Providing a silver layer <NUM> between the HTS layer <NUM> and the copper layer <NUM> prevents the HTS material contacting the copper, which might lead to the HTS material becoming poisoned by the copper. The parts of the silver layer <NUM> and copper stabilizer layer <NUM> on the sides of the tape <NUM> are not shown in <FIG> for clarity, but are shown in the cross section view provided in <FIG>. <FIG> also does not show the silver layer <NUM> extending beneath the substrate <NUM>, as is normally the case (see <FIG>, for example). The silver layer <NUM> makes a low resistivity electrical interface to, and an hermetic protective seal around, the ReBCO layer <NUM>, whilst the copper layer <NUM> enables external connections to be made to the tape (e.g. permits soldering) and provides a parallel conductive path for electrical stabilisation.

The substrate <NUM> provides a mechanical backbone that can be fed through the manufacturing line and permit growth of subsequent layers. The buffer stack <NUM> provides a biaxially textured crystalline template upon which to grow the HTS layer, and prevents chemical diffusion of elements from the substrate to the HTS which damage its superconducting properties. The silver layer <NUM> provides a low resistance interface from the ReBCO-HTS layer <NUM> to the stabiliser layer <NUM>, and the stabiliser layer <NUM> provides an alternative current path in the event that any part of the ReBCO ceases superconducting (enters the "normal" state).

In addition, "exfoliated" HTS tape can be manufactured, which lacks a substrate (e.g. Hastelloy™ substrate) and buffer stack, but typically has a "surround coating" of silver, i.e. layers on both sides and the edges of the HTS layer. Tape which has a substrate will be referred to as "substrated" HTS tape.

HTS tapes may be arranged into HTS cables. 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-<NUM> (with the HTS layers facing each other), type-<NUM> (with the HTS layer of one tape facing the substrate of the other), or type-<NUM> (with the substrates facing each other). Cables comprising more than two 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-<NUM> pairs or a stack of type-<NUM> pairs and (or, equivalently, type-<NUM> pairs). HTS cables may comprise a mix of substrated and exfoliated tape.

When describing coils in this document, the following terms will be used:.

Broadly speaking, there are two types of construction for HTS field coils - by winding, or by assembling several sections. Wound coils, as shown in <FIG>, are manufactured by wrapping an HTS cable <NUM> (shown as a solid line) around a former <NUM> in a continuous spiral. The former is shaped to provide the required inner perimeter of the coil, and may be a structural part of the final wound coil, or may be removed after winding. Sectional coils, as shown schematically in <FIG>, are composed of several sections <NUM>, each of which may contain several cables or preformed busbars <NUM> (shown as solid lines) and will form an arc of the overall coil. The sections are connected by joints <NUM> to form the complete coil. While the turns of the coils in <FIG> and <FIG> are shown spaced apart for clarity, there will generally be material connecting the turns of the coil - e.g. they may be consolidated by potting with epoxy.

The coils may be "insulated" - having electrical insulating material between the turns of the coil (e.g. so that there is a turn-to-turn resistance of around <NUM> Ohm or greater), or "non-insulated", where the turns of the coil are electrically connected radially (i.e. connected by an electrical conductor material), as well as along the cables (e.g. by connecting the copper stabiliser layers of the cables by soldering or by direct contact). Insulated HTS coils can be used in applications in which large, rapid changes in magnetic field are required, such as plasma control in fusion magnets. By contrast, non-insulated coils are generally not suitable for large field coils, due to extremely high ramp-up times. A middle ground option is a "partially insulated" coil, where the material between turns has resistance intermediate between that of a traditional electrical conductor, e.g. a metal, and a that of a traditional electrical insulator, such as ceramic or organic insulator, e.g. having a resistivity between <NUM> and <NUM><NUM> times that of copper or between <NUM>-<NUM> and <NUM><NUM> Ohm metre. Partially insulated HTS magnets, i.e. HTS magnets with a partially conductive turn-to-turn connection (i.e. a turn-to-turn connection with material of an "intermediate" resistivity between <NUM>-<NUM> and <NUM><NUM> Ohm-metre), have a reduced field sweep rate but offer increased thermal and electrical stability under operational conditions, e.g. because the coils allow heat and/or electrical current to be conducted both around and between the windings of the coil. Partial insulation may be achieved by the selection of materials with an appropriate resistivity, or by providing partially insulating structures, which provide the required resistance. Such structures are described in detail in <CIT>.

Insulated and partially insulated magnets require components to be introduced into the coil to either insulate the turns (windings) from one another or provide a turn-to-turn connection. Insulated magnets are often made by co-winding HTS tapes with insulating materials such as polyimide. Partially insulated magnets are made in a variety of ways such as co-winding with metallic tapes (e.g. stainless steel tape), applying edge coatings, or co-winding with a specially fabricated flexible printed circuit board (PCB) containing conductive tracks.

Each of these methods of manufacture suffers from one or more drawbacks. For example, co-winding HTS tapes with other types of tape (or PCB) reduces the current density in the magnet windings. Introduction of organic insulators (such as polyimide) may also reduce the mechanical performance of the coil, e.g. by reducing its Young's modulus. Poor mechanical performance may also occur in some cases because the windings of the magnet are not bonded together (the windings are "non-consolidated"), such that contact pressure between the turns is relied on to provide turn-to-turn contacts. For example, dry wound non-insulated or metal insulated coils have been shown to suffer conal deformation (i.e. deformation causing the coil to adopt a cone-like geometry) because of induced screening currents that occur when the coils are operated. Some organic insulating materials may also be poor choices for use in fusion applications because of their poor neutron tolerance.

Another problem with existing methods of making insulated or partially insulated coils is that the turn-to-turn resistivity value may be too low to be implemented on large (high inductance) coils. As the electromagnetic time constant associated with a coil is determined by the ratio of the coil's inductance to its resistance (L/R), high inductance coils require correspondingly "high" turn-to-turn resistances so that the coil can respond to changing electrical current/magnetic field on a suitable timescale. A further issue is that, in some cases, the geometry of the turn-to-turn connection, or its limited thermal connection to the magnet, can result in excessive Ohmic-heating during discharge, leading to electrical burnout of the connection. This may occur, for example, when a partially insulated magnet is formed by introducing a PCB between the turns: the thin metal tracks of the PCB are generally thermally insulated from the rest of the magnet by the polyimide insulation.

<FIG> shows a radial cross section of a specific type of wound coil known as a "pancake coil", in which HTS cables (tapes) <NUM> are wrapped to form a flat coil, in a similar manner to a spool of ribbon. Pancake coils may be made with an inner perimeter which is any two dimensional shape. Often, pancake coils are provided as a "double pancake coil", as shown in the radial cross section of <FIG>, which comprises two pancake coils <NUM>, <NUM> wound in an opposite sense to one another, with insulation <NUM> between the pancake coils, and with the inner terminals connected together <NUM>. This configuration means that voltage only needs to be supplied to the outer terminals <NUM>, <NUM> (which are generally more accessible) to drive current through the turns of the coil and thereby generate a magnetic field.

One application of HTS field coils is in tokamak plasma chambers ("tokamaks"). A tokamak features a combination of strong toroidal magnetic field, high plasma current and, usually, a large plasma volume and significant auxiliary heating, to provide hot, stable plasma in which fusion can occur. The auxiliary heating (for example via tens of megawatts of neutral beam injection of high energy hydrogen, deuterium or tritium) is necessary to increase the temperature to the sufficiently high values required for nuclear fusion to occur and/or to maintain the plasma current.

An issue with using HTS field coils in tokamak plasma chambers is that, because of the large size, large magnetic fields, and high plasma currents that are generally required, build and running costs are high and the engineering has to be robust to cope with the large stored energies present, both in the magnet systems and in the plasma, which has a risk of 'disrupting' - mega-ampere currents reducing to zero in a few thousandths of a second in a violent instability.

The situation can be improved by contracting the donut-shaped torus of a conventional tokamak to its limit, having the appearance of a cored apple - the 'spherical' tokamak (ST). The first realisation of this concept in the START tokamak at Culham demonstrated a huge increase in efficiency - the magnetic field required to contain a hot plasma can be reduced by a factor of <NUM>. In addition, plasma stability is improved, and build costs may be reduced.

To obtain the fusion reactions required for economic power generation (i.e. much more power out than power in), a conventional tokamak would have to be huge so that the energy confinement time (which is roughly proportional to plasma volume) can be large enough so that the plasma can be hot enough for thermal fusion to occur.

<CIT> describes an alternative approach, involving the use of a compact spherical tokamak for use as a neutron source or energy source. The low aspect ratio plasma shape in a spherical tokamak improves the particle confinement time and allows net power generation in a much smaller machine. However, a small diameter central column is required, which presents challenges for design of the plasma confinement vessel and associated magnets.

The magnet coils in a tokamak can be divided into two groups. The poloidal field coils are horizontal circular coils wound with their centre located on the central column of the tokamak, and produce a poloidal field (i.e. a magnetic field which is substantially parallel to the central column). The toroidal field coils are wound vertically through the central column, and around the outside of the plasma chamber (the "return limbs") to produce a toroidal field (i.e. a magnetic field which is circular around the central column). The combination of the poloidal and toroidal fields produces a helical magnetic field within the plasma chamber that keeps the plasma confined.

The electric currents required to generate the toroidal field are very large. Designs for tokamaks therefore increasingly involve the use of superconducting materials in the field coils. For a compact spherical tokamak, the diameter of the central column should be as small as possible. This presents conflicting requirements, as the current density which can be achieved, even with superconducting materials, is limited.

<FIG> shows a vertical cross section through of a spherical tokamak <NUM> comprising a toroidal field magnet (TF) <NUM> formed from a plurality of D-shaped TF coils 603A,B (only two of which are shown in <FIG>) arranged around a central column <NUM> orientated along an axis A-A', and a plurality of poloidal field (PF) magnets 605A-F, each encircling the central column <NUM>. Electrical current applied to the TF and PF magnets 603A,B, 605A-F generates a closed magnetic field that, when the tokamak is in use, confines, shapes and controls a hot plasma <NUM> inside a toroidal vacuum vessel <NUM>.

<FIG> shows a cross section of the central column <NUM>, which comprises a plurality of current carrying assemblies <NUM>, through which the central column sections of the TF coils pass. Space in the central column <NUM> is taken up by both the current carrying assemblies and non-current carrying components, including, for example:.

<FIG> shows one of the sectors <NUM> of the central column <NUM>. The sector <NUM> comprising three current carrying assemblies 801A-C which make up the TF coil <NUM>. Each assembly comprises one or more HTS field coils <NUM>, e.g. a pancake coil or double pancake coil. It is apparent from <FIG> that the rectangular cross sections of the HTS field coils do not fit well into the arc-shaped cross section of the sector <NUM> and a large amount of space is wasted that more than could be usefully used for other components, such as additional cooling channels or sensors. The arrangement of the current carrying assemblies 801A-C within the sector <NUM> also results in high stresses at the corners of the cross section of the HTS coils (as discussed below in relation to <FIG>).

Another issue with the current carrying assemblies 801A-C in sector <NUM> is that the current carrying assemblies 801A, 801C closest to the tapered sides of the sector <NUM> have fewer turns than the centrally located current carrying assembly 801B. These differences in the number of turns of HTS tape results in a high level of "ripple" in the magnetic field (as discussed below in relation to <FIG>), which causes the magnetic field within and close to the central column to deviate from the desired circular symmetry. There is therefore a need for an alternative construction for the TF coils in the central column which avoids these effects.

<CIT> describes joining first and second HTS coated elements via their cap layers. <CIT> describes superconducting articles including stacks of superconductor segments. <CIT> describes laminated superconductor wire.

<NPL>, describes a study into the effect of substrate thickness on the interfacial adhesive strength of <NUM> HTS tape using peel test modelling.

<CIT> describes HTS coils having a partially insulating layer that comprises an electrically conducting layer, and first and second insulating layers. The electrically conducting layer is coated on one side with the first insulating layer and on the other side with the second insulating layer. Each insulating layer has one or more windows through which electrical contact can be made between the turns and the electrically conducting layer. The windows in the first insulating layer are offset in the plane of the electrically conducting strip from the windows in the second insulating layer.

<CIT> and <CIT> describe constructing coils by wrapping HTS cable around a former.

It is an object of the present invention to provide a method of fabricating an HTS field coil that addresses, or at least alleviates, the problems described above. This object is addressed by the appended claims.

The present invention is directed to a method of manufacturing a High Temperature Superconductor, HTS, field coil from one or more HTS tapes according to the independent method claim <NUM> and to a resulting HTS field coil according to claim <NUM>.

Removing material from the axial edge of the one or more HTS tapes may comprise reducing an extent of the HTS material layer along the axis of the field coil.

In embodiments in which the HTS tape has a layered structure (such as the HTS tape <NUM> shown in <FIG>), the HTS tape is typically wound such that the layers are parallel to the axis of the field coil, i.e. the tape is wound such that the layers are arranged concentrically about the axis of the field coil. In such an arrangement, removing material from the axial edge of the HTS tape (e.g. by mechanical means, such as machining) may comprise removing material from each of the layers simultaneously. For HTS tapes with a rectangular cross section (such as the HTS tape <NUM>, shown in <FIG>), the axial edge of the HTS tape typically corresponds to the smaller side of the rectangular cross section, i.e. to the edge of the HTS tape corresponding to the "thickness" of the HTS tape.

The material may be removed such that the extent of the HTS layer along the axis of the coil varies radially across the field coil and/or circumferentially around one or more of the windings. Removing the material may comprise removing material across a whole of a face of the field coil. The material may be removed so as to provide the field coil with at least one axial surface that is frustoconical. Removing the material comprises removing electrically conductive cladding in electrical contact with the HTS material and extending across at least the axial edge of the HTS tape. The cladding may be removed from the axial edge of the one or more HTS tapes across the whole of a face of the field coil to expose the HTS material layer at the axial edge of the one or more of the HTS tapes.

The or each HTS tape may comprise a flexible substrate (e.g. a substrate comprising Hastelloy™ or another metal or alloy) and an electrical insulator layer (e.g. a buffer stack <NUM>) provided on a face of the flexible substrate, with the HTS material layer being provided on the electrical insulator layer. Alternatively, exfoliated tape may be used, in which case there is no electrical insulator layer and the HTS material layer is provided on the flexible substrate (which may be made of silver, for example).

The intermediate layer may be or may comprise an electrical insulator layer (e.g. buffer stack <NUM>) or a semiconductor layer, e.g. a layer of silicon and/or gallium arsenide, the semiconductor layer optionally being incorporated as a layer with a buffer stack <NUM>. The HTS field coil may be radially "insulated" or "partially insulated" (as described above) depending on the resistivity of the intermediate layer, as described above. For example, in some embodiments, the resistivity of the intermediate layer may be between <NUM>-<NUM> and <NUM><NUM> Ohm-metre to provide a partially insulated coil.

Before the cladding is partially or wholly removed from the edge of the one or more HTS tapes, the cladding may extend continuously around the entire perimeter of the HTS tape.

The method may comprise bonding an electrical conductor element to the cladding on the axial edge of the one or more HTS tapes before removing the metal cladding. The method may comprise partially removing the electrical conductor element to leave an electrical contact through which to supply electric current to at least a portion of at least one of the windings through the axial edge of the one or more HTS tapes.

In embodiments in which the intermediate layer is an electrical insulator layer, the electrical insulator layer may, for example, have a thickness of less than <NUM> micrometers, or less than <NUM> micrometer, and preferably less than <NUM> micrometers. The electrical insulator layer may comprise one or more layers of ceramic material. The substrate may have a thickness of less than <NUM> micrometers, or less than <NUM> micrometers, and preferably less than <NUM> micrometers. Generally speaking, the HTS material layer may comprise ReBCO material, wherein Re is a rare earth element such as Y or Gd. The HTS material layer may have a thickness of less than <NUM> micrometers, or less than <NUM> micrometer. The electrically conductive cladding may comprise a metal, such as copper and/or silver. The electrically conductive cladding of the or each HTS tape may extend over each face of the HTS tape.

The windings of the HTS field coil may be arranged in two or more layers stacked along the axis of the field coil and the material is removed from an axial edge of the one or more HTS tapes in one or more of the layers. The HTS field coil may, for example, be a double pancake coil.

The material may be removed around a whole of one or more of the windings.

The HTS field coil may comprise two HTS tapes arranged as a type-<NUM> pair in which the HTS layers of the two HTS tapes face each other and the substrates of the HTS tapes are separated by the HTS layers.

The method may further comprise sealing the edge of the one or more of the HTS tapes with an insulator or conductor material.

Removing the material from the axial edge of the one or more of the HTS tapes may comprise mechanically removing the material, preferably by machining the axial edge of the one or more HTS tapes. Mechanically removing the material may comprise one or more of: cutting; drilling, laser cutting, plasma cutting, water jet cutting, grinding, sanding, wire erosion, turning, laser ablation, ion milling, sputtering, and electrical discharge machining.

Alternatively or additionally, the material may be partially or wholly removed from the axial edge of the one or more of the HTS tapes chemically. For example, if the cladding comprises copper, the cladding may be chemically removing by dissolving the copper using ferric chloride solution. If the cladding comprises silver, the cladding may (preferably) be chemically removed by dissolving the silver in a solution comprising an oxidising agent such as hydrogen peroxide.

The method may comprise cooling the HTS field coil during the step of removing the material from the axial edge of the one or more HTS tapes. This cooling may, for example, prevent heat damage or degradation of the HTS material layer.

The method may further comprise polishing the axial edge of the one or more HTS tapes after removing the material.

The method may further comprise removing material from another axial edge of the one or more HTS tapes, the other axial edge being provided on a face of the field coil opposite a face of the field coil on which the axial edge is provided.

Removing the material may comprise cutting through the HTS field coil to divide the HTS field coil into two or more HTS field coils. For example, the HTS field coil may be divided by a cutting surface (e.g. a plane) passing through each of the one or more HTS tapes. For example, the HTS field coil may be may divided by a cutting plane that is substantially perpendicular to the axis of the coil, the plane preferably bisecting the coil. The method may further comprise forming one or more electrical connections between the two or more HTS field coils to form a solenoid.

The method may further comprise, before removing material from the axial edge of the one or more of the HTS tapes, potting the field coil.

Winding the one or more HTS tapes about an axis to form a field coil may comprise winding a cable having two outer HTS tapes of the one or more HTS tapes and one or more internal HTS tapes. The internal HTS tape(s) is or are arranged between the outer HTS tapes and have metal cladding that provides an electrically conductive pathway between the HTS layers of the two outer HTS tapes. Before and/or after removal of the material, the internal HTS tape(s) may be narrower than the external HTS tapes along a direction parallel to the axis of the coil. Before and/or after removal of the material, respective axial edges of the internal HTS tape(s) and the outer HTS tapes may be aligned with one another along an edge of the cable.

A solution to some of the above issues is proposed here, in which an HTS field coil is modified after it has been wound using one or more HTS tapes, by removing material from an axial edge of the one or more HTS tapes. For example, in one embodiment, HTS material is removed from the HTS tapes to vary the superconductor properties of.

the tape in different regions of the coil. In another embodiment, material is removed so that the buffer stack of the HTS tape provides turn-to-turn electrical insulation in an insulated or partially insulated field coil. Such a field coil can be produced, by fully or partially removing the metal cladding from the edges of HTS tape, e.g. after the HTS tape has been wound into a coil. The resulting coil may be referred to as a "Buffer Layer Insulator" (BLI) coil.

<FIG> shows a radial cross section through a ReBCO tape <NUM> that is similar to the ReBCO tape <NUM> of <FIG>. Elements which are the same as in the tape of <FIG> are given the same reference numerals. However, in this example, the copper stabilizer layer <NUM> and the silver layer <NUM> can be seen to extend around the other layers of the tape <NUM> to enclose them completely, i.e. the copper stabilizer layer <NUM> and silver layer <NUM> act as cladding for the layered structure of the tape <NUM>.

As mentioned above, the substrate <NUM> is typically Hastelloy™ and has a thickness of around <NUM>-<NUM>. The buffer stack <NUM> is a series of four ceramic layers of materials such as MgO, LaMaO3, YSZ, etc. which typically have a combined thickness of around several hundred nanometres to a few microns. Details of the composition and thickness of the layers of the buffer stack <NUM> generally vary according to supplier. The copper cladding <NUM> is typically around <NUM> to <NUM> thick (including at the edges). Buffer stacks with more than (or fewer than) four ceramic layers could also be used.

<FIG> shows a radial cross section view of a pancake coil <NUM> formed by winding two ReBCO tapes 900A,B about an axis Z (the axis of the coil). For clarity, only two windings <NUM>, <NUM> of the tapes 900A, B are shown in the figure, but any number of windings can be used (see <FIG>, for example). The windings <NUM>, <NUM> of the coil <NUM> are nested within one another to form a generally planar coil <NUM>. In this particular example, the tapes 900A,B are arranged in a type-<NUM> configuration, such that their ReBCO layers face one another, which allows some electrical current to be shared between the two tapes 900A,B when the coil <NUM> is operated. Other configurations are of course possible, as discussed above. The coil <NUM> is generally solder potted with e.g. PbSn solder, to form a non-insulated coil.

<FIG> shows a radial cross section view of a coil <NUM> made by removing the copper and silver cladding <NUM>, <NUM> from both faces of coil <NUM> to expose the layered structure of the ReBCO tape <NUM>.

The cladding can be removed mechanically by several methods, including turning or wire erosion. In one preferred embodiment, a copper plate is soldered across a face of the coil <NUM> and the plate subsequently machined away by milling, grinding, turning (e.g. on a lathe), or other means of mechanical metal removal, to leave one or more metal joint rings for injecting current into the coil <NUM> through the edges of the HTS tapes 900A,B. In this case, the cutting tool penetrates the metal plate and continues into the HTS tapes 900A,B to remove around <NUM> of the edges of the HTS tapes 900A,B. This process can of course be carried out on one or both of the faces of the coil <NUM>, depending on the desired turn-to-turn resistance for the coil <NUM> (which is maximised by removing the cladding entirely from both faces). The turn-to-turn resistance can also be varied by changing the thickness of one or more of the layers <NUM> of the buffer stack of the HTS tapes 900A,B (e.g. a thinner buffer stack can be used to make a coil that is only partially insulating). Other types of metal plate (other than copper) can also be used. Current can also be injected into the coil by means other than an "edge connected" plate. For example, metallic components (contacts) at the inner diameter and/or the outer diameter of the coil onto which the cable is terminated e.g. by soldering.

Alternatively or additionally, chemical processes can be used to dissolve the copper and silver cladding. For example, the copper cladding <NUM> can be dissolved using ferric chloride (FeCl<NUM>) solution. To dissolve the silver cladding <NUM>, a solution of <NUM> part ammonium hydroxide and <NUM> part hydrogen peroxide, optionally diluted with methanol to reduce reaction rate, can then be used. Other reagents can also be used to dissolve the silver cladding, including nitric acid and/or hydrochloric acid, or hydrogen peroxide solution in combination with an acid or base. The surface of the ReBCO layer <NUM> is generally unaffected by removing the copper and silver cladding <NUM>, <NUM> using a solution of hydrogen peroxide and ammonium hydroxide. In some cases, the use of other reagents to remove the silver cladding <NUM> may dissolve or react with the ReBCO layer <NUM> at its exposed edges. However, a small amount of degradation of the edge of the ReBCO layer <NUM> is generally acceptable for many applications. Optionally the edges can then be sealed using an insulating material, such as an epoxy resin, to prevent ingress of contaminants to the exposed tape edges and permit thermal contact to be made. Portions of the coil at the internal diameter and outer diameter can be omitted from the edging process in order to leave the metallic layers in place for the purposes of current injection (not shown).

Other methods for removing the copper and silver cladding <NUM>, <NUM> include sputtering (e.g. using ion milling) and/or laser ablation.

The structure of the BLI coil <NUM> has a number of advantages. As the BLI coil <NUM> is wound only from ReBCO (HTS) tape the current density of the magnet is maximised (in contrast to other types of insulated or partially insulated magnet made by co-winding the HTS tape with another tape or PCB). In particular, the buffer layers <NUM> of the tapes 900A,B are very effective turn-to-turn resistors because they are very thin, have maximal cross sectional area, and are in intimate thermal contact with the ReBCO (HTS) layers <NUM>. The Young's modulus of the BLI coil <NUM> and the structural integrity of the coil also remain high because the coil <NUM> contains (essentially) only HTS tape.

The buffer stack layers <NUM> present an insulating barrier between the turns of the coil <NUM> without the need for additional layers of insulating material to be introduced between the windings, e.g. with without the use of low modulus organic insulation. This allows the insulated coil to have both a high Young's modulus and a high density of turns, i.e. a density of turns that is determined substantially by only the thickness of the HTS tape. A high (e.g. maximal) density of turns may allow the coil <NUM> to provide higher current densities compared to other coils that incorporate additional material between the windings of the HTS tape. The cross sectional area of the turn-to-turn connection is also maximal, spanning the entire width of the HTS tapes, which maximises the opportunity for heat to be transferred between the windings, e.g. in the event of a quench. The rigidity of the coil <NUM> and the consolidation of the windings may also reduce or minimise the strain (or variation in strain) within the HTS tape, which may, for example, prevent degradation of the HTS material when the coil is used and/or make the coil <NUM> easier to maintain. A lack of additional material within the coil (such as organic insulation) also avoids the possibility that the additional material degrades, which may be of particular concern if the coil is exposed to high neutron fluxes, such as those produced in fusion reactors.

<FIG> shows a schematic radial cross section through a coil <NUM>, which is identical to coil <NUM>, except that an electrical contact <NUM> has been made to a central electrical conductor region <NUM> located in between the centres (in the radial direction) of the respective windings <NUM>, <NUM>. The central region <NUM> comprises the metal cladding (i.e. copper cladding <NUM> and silver cladding <NUM>) and the substrates <NUM> of each tape 900A,B (i.e. tape 900B of winding <NUM> and tape 900A of winding <NUM>). The metal cladding <NUM>, <NUM> and the substrate <NUM> of each tape 900A,B in the conductor region <NUM> are electrically connected to one another, but electrically isolated from the HTS layers <NUM> of each tape 900A,B immediately adjacent the conductor region <NUM> by the buffer stacks <NUM>. In other words, electrical contact <NUM> is made to a central region <NUM> of the windings <NUM>, <NUM> that is electrically insulated (along the radial direction) from the HTS layers <NUM> of the tapes. The electrical contact <NUM> can be made by inserting a metal (e.g. copper) contact between the windings <NUM>, <NUM> when the coil <NUM> is being wound. Alternatively, or additionally, the electrical contact <NUM> may be made by soldering a metal contact to the axially facing edge of the electrical conductor region <NUM>.

When electrical current is supplied to HTS layers <NUM> of the windings <NUM>, <NUM>, an inductive voltage is generated within the coil <NUM>. This voltage is "picked up" by the electrical conductor region <NUM> and can be measured using a potentiometer connected between the electrical contact <NUM> and another electrical contact made to another part of the electrical conductor region <NUM>, e.g. a contact to the radially innermost turn or radially outermost turn of the coil <NUM>. The measured inductive voltage between the two contacts can then be subtracted from a voltage measured across the coil <NUM> as a whole, i.e. between the innermost turn of the coil <NUM> and the outermost turn of the coil <NUM> and comprising the HTS layers <NUM>. The remaining voltage therefore gives the resistive (rather than inductive) contribution to the voltage drop across the coil <NUM>, which is a good indicator of whether the HTS layers <NUM> are superconducting. In other words, the difference between the voltage across the coil <NUM> and the inductive voltage measured for the electrical conductor region <NUM> is indicative of a resistive voltage building in the coil <NUM>, which is itself indicative of a non-superconducting region developing or having been developed in the HTS material, which may lead to rapid heat generation within the coil (i.e. an incoming quench).

<FIG> shows a radial cross section view of a coil <NUM> comprising a plurality of windings of HTS tape <NUM> arranged to form a predominantly planar pancake coil and two ring conductors 1303A, B covering the faces of the coil across the edges of the HTS tape <NUM> on either side of the coil <NUM>. Each ring conductor 1303A,B comprises an annulus or ring made from a conducting material, preferably a metal such as copper, which extends radially across the coil to form an electrical connection between the windings of the coil <NUM>. The ring conductors 1303A,B are soldered to faces of the coil <NUM> to provide good electrical contact with the edges of the HTS tape <NUM>.

<FIG> shows a radial cross section view of a coil <NUM> obtained by removing the ring conductors 1303A,B and the cladding <NUM> from a portion of the coil <NUM> shown in <FIG>. For example, material from the ring conductor and the cladding can be machined away to leave portions of the ring conductors 1403A,B bonded to the cladding <NUM>, whilst exposing the HTS layer <NUM> at the edge of the HTS tape <NUM>. Electric current can be supplied to the coil <NUM> through the radially innermost end of the HTS tape <NUM> using the top ring conductor 1403A. The current flows around successive windings of the coil <NUM> before being received by the bottom ring conductor 1403B at the radially outermost end of the HTS tape <NUM>, i.e. the ring conductors 1403A,B function as electrical contacts (or "current connection points") through which to transfer electrical current to or from the coil <NUM>. It is believed that during the initial stages of current injection the current injected into the coil <NUM> penetrates the turns covered by the ring conductor 1403A (which act as a small "non-insulated" coil) to produce a current distribution that minimises impedance across the turns. Over time, the current distribution within the coil <NUM> then adjusts from a distribution that (i) initially reduces the inductive voltage (such that the current flows from the radially outermost edge of the top ring conductor 1403A through the turns of the coil <NUM> in between the two ring conductors 1403A,B and out through the radially innermost edge of the bottom ring conductor 1403B) to (ii) a current distribution that minimises Ohmic voltage (such that current will penetrate all the turns of the ring conductors 1403A,B evenly).

The coil <NUM> may, in some examples, have a ring conductor 1403A,B on only one face. Although the ring conductors 1403A,B are shown as being on opposite faces of the coil <NUM>, they may be provided on the same face, in which case electrical current can be injected or removed from only one side of the coil <NUM>, which can be advantageous in space-constrained environments, for example.

Although it is preferred to remove the metal cladding after the HTS tape 900A,B, <NUM>, <NUM> has been wound into a coil, a BLI coil may also be formed by removing the metal cladding from the sides of the HTS tape before winding the coil.

The coils <NUM>, <NUM>, <NUM>, <NUM>, <NUM> described above are wound from a cable formed from two HTS tapes arranged in a type-<NUM> configuration (with the HTS layers facing each other), which allows electrical current to be shared between the two HTS tapes 900A,B. Coils may also be formed from cable comprising more than two HTS tapes 900A,B. However, for these types of coil, removing the copper <NUM> and silver <NUM> cladding from the axial edges of the tapes 900A,B may prevent electrical current being shared between all the tapes 900A,B in the cable. This is because removal of the cladding means that current is only shared between tapes for which there is a type-<NUM> configuration, i.e. for which the HTS layers <NUM> face one another. A solution to this problem may be achieved by winding the coil from cable comprising HTS tapes of different widths, as described below.

<FIG> shows a radial cross section of a coil <NUM> that comprises windings <NUM>, <NUM> of a cable comprising four HTS tapes 1501A-D. The central axis Z of the coil (not shown) is to the right of part of the coil illustrated in <FIG>. Two of the tapes 1501A, D have a width that is greater than the width of the other two tapes 1501B,C. In <FIG>, the "widths" of the tapes 1501A-D (i.e. the second shortest dimension of the tapes) extend vertically as <FIG> show radial cross sections through the coils. Each of the narrower tapes 1501B,C is provided in the cable between the wider tapes 1501A,D and arranged in a type-<NUM> configuration with a respective one of the wider tapes 1501A,D, i.e. the wider tapes 1501A,D are oriented such that their HTS layers <NUM> are closer to the centre of the cable than their buffer stacks <NUM>, while the narrower tapes 901B,C are orientated such that their HTS layers <NUM> are further from the centre of the cable than their buffer stacks <NUM>. The windings of the coil <NUM> (as exemplified by the windings <NUM>, <NUM>) are aligned on one side 1506A of the coil <NUM> (in the direction transverse to the radial and circumferential directions) to give the coil a flat face on this (bottom) side 1506A of the coil <NUM> and a radially "crenelated" face on the other (top) side 1506B of the coil <NUM> as a result of the wider tapes 1501A,D protruding from the coil <NUM> relative to the narrower tapes 1501B,C. The silver cladding <NUM> and the copper cladding <NUM> in the coil <NUM> is intact.

<FIG> shows a radial cross section of a coil <NUM> obtained by removing the silver cladding <NUM> and copper cladding <NUM> from the flat face of the coil <NUM> on the bottom side 1006B of the coil <NUM> and from the wider tapes 1501A,D on the top side 1506B of the coil ("narrower" and "wider" referring here to the extent of the tapes along the axis of the coil <NUM>). The cladding <NUM>, <NUM> of the narrow tapes 1501B,C is not removed on the top side 1506B. Consequently, there is a conductive pathway provided by the cladding <NUM>, <NUM> between the narrow tapes 1501B,C that allows electrical current to be shared between the narrow tapes 1501B,C, and therefore between each of the tapes 1501A,D making up the cable. There is, however, no electrically conductive pathway between adjacent windings <NUM>, <NUM>, so the coil <NUM> is a BLI coil.

Alternatively, the coil <NUM> may not have the edges of the HTS tapes aligned on one side 1506A, i.e. both sides of the coil <NUM> may be stepped or "crenelated", in which case, is may not be necessary to remove any of the cladding <NUM>, <NUM> from the narrower HTS tapes 1501B,C.

This approach (i.e. using HTS tapes of different widths) allows insulated or partially insulated coils to be constructed using cables comprising more than two HTS tapes, thereby expanding the scalability of this fabrication method to larger coils.

In some cases, the coil <NUM> may be "potted" with solder to fill in the gaps between the wider tapes 1501A,D as this facilitates removal of the cladding <NUM>, <NUM> from the wider tapes 1501A, D on the top side 1506B of the coil <NUM>. The solder covering the narrower tapes 1501BC provides mechanical stability for the wider tapes 1501A,D along the radial direction, which prevents them being damaged when the top side 1506B of the coil <NUM> is machined to remove the cladding <NUM>, <NUM>.

In one specific example, the wider tapes 1501A,D have a width of <NUM> whereas the narrower tapes 1501B,C have a width of <NUM> or <NUM>. Preferably, the HTS layer <NUM> and buffer stack <NUM> of the wider tapes 1501A,D should extend above the narrower tapes 1501B,C so that the cladding <NUM>, <NUM> can be removed from the wider tapes 1501A,D without damaging the cladding <NUM>, <NUM> of the narrower tape 1001B,C. In some cases, the cladding <NUM>, <NUM> of the tapes 1500A-D may extend beyond the HTS layer <NUM> and the buffer stack <NUM> by around <NUM> micrometers, such that around <NUM> micrometers of material (i.e. cladding) would need to be removed from either face of the coil <NUM> to produce an insulated coil <NUM>. However, in practice, variation in the width of the HTS tape width and the alignment of the windings with respect to one another (i.e. the flatness of the coil) means that it may be preferable to remove <NUM> microns or more of material (i.e. the cladding and some of the HTS layer <NUM> and buffer stack <NUM>) from each face of the coil to ensure that the coil is fully insulated.

Cables comprising more than four HTS tapes can also be used to produce insulated coils, provided that the HTS tapes 1501A,D providing the exterior faces of the cable are wider than the HTS tapes 1501B,C closer to the centre of the cable.

<FIG> is a flowchart of a method of manufacturing an HTS coil from one or more HTS tapes. Each HTS tape comprises a flexible substrate, an electrical insulator layer (e.g. a buffer stack <NUM>) provided on a face of the flexible substrate, an HTS material layer provided on the electrical insulator layer, and electrically conductive cladding in electrical contact with the HTS material and extending across at least an edge of the HTS tape. The electrical insulator layer may be only "partially insulating", in which case the electrical insulator may, for example, have a resistivity between <NUM> and <NUM><NUM> times that of copper or between <NUM>-<NUM> and <NUM><NUM> Ohm metre, and so forth.

While the above discussion has focussed on removing the cladding from one or more axial edges of an HTS field coil <NUM>, <NUM>, <NUM> it is also possible to remove further material from the axial edge(s) in order to reduce the width (i.e. extent along the axis of the coil) of the field coil. Surprisingly, it has been found that HTS field coils can be very resilient to physical damage, such that even large changes to the shape of the coil can be made after it has been wound. For example, it has been found that HTS field coils can function effectively even after several holes have been drilled through them or after the coil has been shaped via standard machining techniques, such as sawing, grinding and sanding). In particular, the electrical current may continue to be able to circulate around the HTS material of such coils even when there are interruptions to or damage within some of the coil windings. This may occur as a result of current sharing between neighbouring windings, which allows the current to "bypass" the affected parts of the windings. Current may continue to circulate around the windings of the coil so long as the total current does not exceed the minimum critical current of the HTS material in any part of the coil, or potentially even beyond the minimum critical current if the turn to turn resistance is sufficiently low and cooling is provided to cool the resistive heat load.

These findings present significant opportunities for situations in which the ease of assembly of a double pancake coil is desired, but the rectangular cross section of such a coil presents disadvantages, such as in the central column of a toroidal field coil (as described above). As discussed below in connection with <FIG>, the cross section of the coils can be modified after winding (and preferably after potting the coil in solder or epoxy, for example) so that the cross section fits better in the required space. "Potting" a coil refers to filling (or encasing) the coil with a material, e.g. solder or epoxy, following formation of the coil by winding one or more lengths of HTS tape (cable). The potting material fills gaps in the coil structure, and improves the structural integrity of the coil, e.g. to reduce the likelihood of the windings towards the inner radius or the outer radius of the coil from coming loose or being damaged during machining of the coil. Potting with a conductive material (e.g. solder) also improves the electrical connection between the turns of the coil. Potting the coil with a structurally effective material such as resin or solder provides effective transmission of radial stress, which may enable higher currents to be supplied to the coil to generate larger magnetic fields without exceeding the strain limit of the coil. In addition, the rigidity of the potted coil may reduce the amount of support that the coil requires. For example, the potted coil may be supported by a rigid supporting structure at its outer radius (or conversely, on its inner radius) only.

By varying the width (i.e. the extent along the axis of the field coil) of the HTS material layer <NUM>, the superconducting properties of the HTS field coil <NUM>, <NUM>, <NUM> such as the critical current can be varied. The further material that is removed generally comprises material from the axial edges of each of the layers in the HTS tape <NUM>, i.e. the substrate <NUM>, buffer stack <NUM>, HTS material layer <NUM>, and cladding <NUM>, <NUM>. The further material may be removed by (for example) machining one or both faces of the HTS field coil <NUM>, <NUM> after the cladding <NUM>, <NUM> has been removed from the axial edge(s) of the field coil <NUM>, <NUM>, <NUM>. Alternatively (or additionally) reduced width HTS field coils may be made by cutting through the HTS field coil, e.g. by cutting across the HTS field coil <NUM>, <NUM>, <NUM> in a plane that is perpendicular to the coil axis Z, thereby dividing the coil into two HTS field coils of reduced width. The plane may bisect the field coil <NUM>, <NUM>, <NUM> so that each of the narrower field coils has a width that is approximately half that of the original field coil <NUM>, <NUM>, <NUM>. For example, one or more <NUM> wide HTS tapes may be wound into a pancake coil, which is then divided into two pancake coils each having a width of approximately <NUM>. Cutting through the field coil <NUM>, <NUM>, <NUM> may be accomplished by electrical discharge machining (also known as spark machining or wire erosion), for example. The axial edge(s) of the HTS field coils may be polished after machining to ensure that there are no "shorts" (i.e. no radial electrical connections between the windings) and/or to gently remove any damaged edges that might be caused by the cutting process.

The axial edges (or potentially all the outer surfaces) of the HTS field coils may preferably be hermetically sealed to prevent ingress of contaminants into the exposed laminar structure. This can be done by applying an insulator coating such as a ceramic or resin (e.g. epoxy resin). An insulating coating may be preferred to avoid forming unwanted electrical connections between turns. However, in some cases, such as where the HTS field have a low turn-to-turn resistivity (e.g. "partially insulated" coils), one or more metal layers may be applied instead (or applied beneath the insulator coating) to protect the layers of the HTS tape. For example a layer of nickel may be applied to the HTS coils, e.g. by deposition onto the surfaces of the HTS field coils via electroplating or other chemical or physical deposition methods.

This method of manufacturing HTS field coils of reduced width may be contrasted with the conventional method in which HTS field coils are made by selecting or preparing HTS tape <NUM> that has a width that corresponds to the desired width of the field coil. In particular, cutting the field coil <NUM>, <NUM>, <NUM> to the desired width after the coil has been wound avoids the width of the coil being limited by the widths of HTS tape <NUM> that are available from manufacturers. It may also avoid the need to produce narrower HTS tape <NUM> by cutting commercially available HTS tape <NUM> down to size, which is generally time consuming, difficult to do accurately, and may induce edge damage (cracks) that may lead to long term conductor degradation, particularly in high field coils. Cutting through the HTS tape <NUM> after it has been wound into a field coil <NUM>, <NUM>, <NUM> also allows the profile of the HTS field coil to be shaped into geometries that are more complex than that of a planar pancake coil. For example, one or both of the axial surfaces of the field coil may be convex or concave when viewed along the axis of the coil. In some examples, one or both of the axial surfaces of the field coil may be frustoconical i.e. shaped such that vectors normal to the exterior axial surface of the field coil either diverge from the axis or converge towards it. In some examples, only one of the axial surfaces is non-planar. Varying the width of the HTS field coil in this way allows the superconducting properties of the field coil to be varied (or "graded") for the different windings of the coil. The magnetic field that is produced by the coil when it is in use can also be shaped to some extent by varying the width of the field coil, which may be useful in applications in which fine control of magnetic fields are needed, such as in magnetic resonance imaging (MRI).

The reduced width HTS field coils made by the above method may, for example, be electrically connected to one another, e.g. to form a double pancake coil. One or more reduced width HTS field coils may be incorporated into an HTS solenoid formed from a stack of pancake coils. Preferably, the widths of the coils (i.e. their extent along the axis of the solenoid) may be adapted according to their position within the stack. For example, coils located in or close to the mid-plane of the HTS solenoid may have a reduced width compared to those located further away from the mid-plane. In general, the width of the coils may increase with increasing distance from the mid-plane of the solenoid. It is preferable to have thinner coils nearer to the mid-plane because the magnetic field in this part of the solenoid is parallel to HTS layer <NUM> and so the critical current density tends to be high, unlike at the extremities of the solenoid where field angles diverge away from parallel and critical current densities are commonly lower. It is therefore desirable for inner pancake coils (i.e. pancake coils near the mid-plane of the solenoid) to be thin, and the outer pancake coils to be wider, to equilibrate critical current across all the pancake coils and to make optimal use of conductor throughout the solenoid.

The above discussion of reduced width HTS field coils has referred to manufacturing the coils using "conventional" HTS tape comprising cladding <NUM>, <NUM>. However, in some cases, reduced width HTS field coils be manufactured from HTS tape <NUM> that does not comprise cladding <NUM>, <NUM>. Alternatively, in some other examples, the HTS tapes may comprise cladding <NUM>, <NUM> that does not extend over the axial edges of the HTS tape.

<FIG> shows a sector <NUM> of an exemplary central column. As with the sector <NUM> shown in <FIG>, the sector <NUM> is a cross section through a D-shaped HTS coil comprising six pancake coils, arranged as three double pancake coils. The HTS coil has been cut to provide a cross section in the central column which is a sector of an annulus. In this example, this is achieved by leaving the four pancake coils <NUM> towards the centre of the sector unmodified, and cutting inclines <NUM> and <NUM> into the two pancake coils <NUM> at the left and right sides of the sector <NUM>, such that the cross section of the pancake coils <NUM> tapers towards and away from the central axis of the central column. The inclines <NUM> and <NUM> are formed such that the pancake coils <NUM> (and the other half of the corresponding double pancake coils) can be made larger, that is the extent of the coil in a direction towards the axis of the central column can be increased (i.e. more windings can be accommodated), compared to those of <FIG>, with the incline <NUM> adapted (i.e. fitting close) to the boundaries of the sector <NUM>. The inclines <NUM>, <NUM> may be cut exclusively in the straight parts of the coils <NUM> which pass through the central column (i.e. the "uprights" of each D-shaped coil), with the "return limbs" (i.e. the parts outside of the central column) being left unmodified.

Multiple such coils may be arranged to form a TF magnet, with the annular sectors <NUM> of the central column sections of each coil <NUM>, <NUM> brought together to form the central column of the TF magnet.

Comparing Figure <NUM> to Figure <NUM>, the outer double pancake coils <NUM> have approximately <NUM>% more area in their cross section, even accounting for the loss of area due to the cutting of the inclines, and the total cross sectional area of the pancake coils <NUM>, <NUM> in the current carrying element is increased by about <NUM>%. This is a significant increase to the current density of the current carrying element <NUM> compared to the current carrying elements <NUM> of <FIG>. Further shaping of the coils, and/or the addition of more shaped coils (e.g. into one or more gaps <NUM>) would potentially allow for even further improvements.

<FIG> shows results of a stress simulation of a cross section <NUM> of a TF magnet, showing the windings of HTS tape <NUM> of a single pancake coil having a rectangular (i.e. unmodified) cross section. The stress at the radially inner part of the HTS stack <NUM> is unacceptably high (over <NUM> MPa in places, compared to a maximum acceptable compressive stress for commonly available HTS tape of <NUM> MPa), and would lead to degradation and likely permanent damage to the HTS tape.

<FIG> shows results of a stress simulation of a cross section <NUM> of a TF magnet, showing the windings of HTS tape <NUM> of a single pancake coil which has had an incline <NUM> cut into it. Comparing this to <FIG>, it can be seen that the stress is considerably reduced - being less than <NUM> MPa throughout the structure.

The coils <NUM>, <NUM> (and the HTS field coils <NUM>, <NUM>, <NUM> described above) can be cut and shaped by essentially any suitable method for cutting metal or metal composites, e.g. electrical discharge machining (EDM), grinding, sanding, etching, laser cutting, plasma cutting, water jet cutting, etc. Shaping methods which generate significant heat may cause local degradation of the HTS material close to the cut and HTS material will permanently degrade if it is brought above a degradation temperature, which depends on the exact HTS material and manufacturer, but is generally on the order of <NUM> to a few hundred degrees centigrade. This degradation can be mitigated or avoided by providing suitable cooling (e.g. water cooling) as the coils are cut/shaped. In addition, the high thermal conductivity of a partially insulated coil (particularly if solder potted) means that it acts as an effective heat sink during the cutting process. EDM may be preferred in many case as it can minimise damage imparted to the tapes, and is readily adaptable to produce curved geometries by appropriate shaping of the electrodes used to create the discharge. Combinations of these methods can also be employed as required. For example, the bulk of the material may be removed by one method (which may be referred to as a "roughing" process) that may damage the axial edge of the HTS tapes, and another method (which may be termed a "finishing" method) used to remove the damaged material remaining after the roughing process.

In general, an HTS coil will be particularly suited for cutting and shaping as described above if the coil is:.

The coil can then be cut as described above to form any desired shape, provided that, once cut, there is at least one current path around the windings of the coil, i.e. from a radially outermost end of the HTS tape (cable) to a radially innermost end of the HTS tape.

While the above examples have been directed towards TF coils in tokamaks, it will be appreciated that the technique of cutting and shaping HTS coils may be applied to any application of HTS coils, and is particularly useful in applications where the space available for the coils is limited and/or irregularly shaped, e.g. in aircraft and/or spacecraft. Also, while the above description has focussed on pancake coils, the method applies equally to other partially- or non-insulated coil constructions, e.g. jointed coils.

A further advantage to shaping HTS field coils as described above is that greater control over the magnetic fields produced by the coils is possible. <FIG> shows a contour plot of the magnitude of a magnetic field produced close to the central column <NUM> of a TF magnet comprising <NUM> TF coils <NUM> of rectangular cross section. Whilst the magnetic field away from the central column <NUM> is essentially circularly symmetric, the magnetic field close to and within the central column <NUM> has a high level of "ripple" (i.e. lack of symmetry). Shaping the coils <NUM> to be "wedge shaped" allows them to be packed together more closely, which significantly reduces the ripple. Use of shaped coils <NUM> as described herein is not limited to the central column of a tokamak, but may also include other types of plasma chambers (e.g. stellarators) or apparatus for generating a precisely controlled magnetic field, such as MRI machines, NMR spectrometers and charged particle accelerators.

<FIG> is a flowchart of a method of manufacturing an HTS coil. In step <NUM>, a partially insulated coil is formed, e.g. by winding an HTS cable and a partially insulating layer around a former. In step <NUM>, the resulting coil is then potted, e.g. in solder or epoxy. In step <NUM>, the coil is machined into the desired shape by the removal of material, including HTS material, such that after shaping there is a current path around the coil.

<FIG> is a flowchart of a method of manufacturing a TF magnet comprising a plurality of HTS field coils for use in a tokamak plasma chamber. The method comprises:.

For example, although the coils above have been described as having HTS tapes <NUM> arranged in a "type-<NUM>" configuration, other configurations can also be used, e.g. "type-<NUM>" and "type-<NUM>" (as described, for example, in <CIT>).

Similarly, while the disclosure is exemplified with reference to "pancake" coils, i.e. largely planar coils formed of nested concentric windings, it will be understood from the discussion above that the disclosure is not limited to such coils. Similarly, although the above description refers to ReBCO tapes <NUM>, other types of HTS tape that are constructed with one or more insulating (or partially insulating) buffer layers can be used instead of, or in addition to, ReBCO tapes <NUM>. Properties of the buffer layer stack, such as composition of the layers and/or the thickness of the layers may also be varied in order to make the coils insulating or partially insulating. For example, the buffer layer may consist of or comprise a semiconductor material, such as silicon and/or gallium arsenide. In some cases, the buffer layer stack may consist of or comprise one or more metal-insulator transition (MIT) materials, such as vanadium oxide (e.g. VO, V<NUM>O<NUM>, V<NUM>O<NUM>, V<NUM>O<NUM>, V<NUM>O<NUM>, etc.), to provide a turn-to-turn resistance that can be varied by based on a metal to insulator phase transition within the material, e.g. as a result of changing the temperature of the material.

The HTS field coils described above (or, equivalently, HTS field coils manufactured according to the above methods), are particularly advantageous for use in aerospace applications. For example, the HTS field coils may be included in aircraft, unmanned aerial vehicles, satellites, spacecraft, rocket-propelled vehicles, and autonomous exploration vehicles. In such (and other) applications, removing material from an axial edge of the one or more of the HTS tapes allows the shape of the coil to be adapted so that it takes up less volume and is lighter. This is particularly beneficial for technologies in which there are space and weight constraints, such as in satellites. Buffer layer insulated HTS field coils are also advantageous in this regard because they are able to provide partially insulated (PI) or fully insulated coils without requiring additional layers of insulating material to be introduced between the windings, therefore further reducing the volume/weight of the coils. In addition, because the HTS tapes are generally unmodified when the HTS field coils are wound (with the material then being removed after winding), it is possible to form tightly wound, consolidated coils, which are able to withstand large forces, such as those generated during take-off of an aircraft or launch of a satellite and so forth. By contrast, producing HTS field coils with a high degree of mechanical stability is generally more difficult using other manufacturing methods in which material is removed from the HTS tapes before the coil is wound, or which require additional layers to be introduced between the windings.

HTS field coils may, in general, be provided to generate a magnetic field that has a particular strength and spatial distribution. However, in some cases, it is possible to remove material from the axial edge of the one or more of the HTS tapes to reduce the size and mass of the coil, whilst largely preserving the strength and/or spatial distribution generated by the coil. The amount and/or location of the material to be removed may be determined by trial and error, or preferably from computer simulations (e.g. finite element models) of the coils and their associated magnetic field. In the latter case, evolutionary or genetic algorithms may be used to optimise the removal of the material. These algorithms may, for example, be subject to one or more constraints (e.g. acceptable tolerances) relating to the strength and spatial distribution of the magnetic field and/or the operating parameters of the coil, such as current and temperature.

The shape of the HTS field coils may also (or alternatively) be varied by removing material from the HTS tapes to obtain a particular current distribution within the tapes when the coil is in use. For example, the material may be removed from the HTS tapes (e.g. by machining the coil), so that the ratio of the current to the critical current (I/ IC) is approximately constant for a particular region of the coil or throughout the coil. This kind of optimisation may reduce the "excess" current (i.e. the current exceeding the critical current) and allow the coil to be operated in a "saturated" mode in which the current is substantially equal to the critical current to minimise resistive heating within the coil. An example of this type of optimisation is an HTS pancake coil that has been divided into two or more HTS field coils of reduced width, i.e. reduced extent along the axis of the coil (see the discussion above of reduced width HTS field coils), e.g. by "wire slicing" a pancake coil into two smaller (reduced width) pancake coils. A further benefit associated with coils manufactured this way is that the electrical current required from a power supply is reduced, thereby allowing a smaller and/or lighter power supply and non-superconducting current carrying components to be used. The reduced power consumption of the HTS field coils may also extend the lifetime of the power supply, which may be an electrochemical cell (battery), for example.

Claim 1:
A method of manufacturing a High Temperature Superconductor, HTS, field coil (<NUM>, <NUM>) from one or more HTS tapes (<NUM>, <NUM>, 900A,B, <NUM>, 1501A-D), each HTS tape comprising a flexible substrate, an intermediate layer (<NUM>) provided on a face of the flexible substrate, an HTS material layer (<NUM>) provided on the intermediate layer, and electrically conductive cladding (<NUM>, <NUM>) in electrical contact with the HTS material and extending across at least the edges of the HTS tape, the method comprising:
winding (<NUM>) the one or more HTS tapes about an axis (Z) to form a field coil (<NUM>, <NUM>) comprising windings (<NUM>, <NUM>) of HTS tape; and
characterised by:
after winding, removing (<NUM>) material from an axial edge of the one or more of the HTS tapes around at least a part of one or more of the windings to reduce the extent of the one or more HTS tapes along the axis of the field coil, wherein removing material from an axial edge of the one or more of the HTS tapes comprises partially or wholly removing the cladding from an edge of the one or more of the HTS tapes around at least a part of one or more of the windings to increase the electrical resistance between the HTS material layer in the winding and the HTS material layer in an adjacent winding.