High-temperature superconductor field coil, method for manufacturing the same, system including the same coil, and satellite, aircraft, or unmanned aerial vehicle

Abstract

A method for fabricating a field coil from high temperature superconductor (HTS) material, including one or more HTS tapes, each HTS tape including a layer of HTS material. The method includes winding the one or more HTS tapes around an axis to form a field coil including windings of the HTS tape; and removing material from one or more axial ends of the one or more HTS tapes around at least a portion of the one or more windings to reduce the extent of the one or more HTS tapes along the axis of the field coil.

Description

[Technical Field]

[0001] The present invention relates to high temperature superconductor (HTS) field coils, and more particularly to HTS field coils that include HTS tapes. [Background technology]

[0002] 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 explained by BCS theory. All low-temperature superconductors have a critical temperature below approximately 30 K (the temperature above which the material cannot become superconducting, even in zero magnetic field). The behavior of HTS materials is not explained by BCS theory, and such materials may have critical temperatures higher than approximately 30 K. (Note, however, that physical differences in composition and superconducting behavior, not critical temperature, define HTS and LTS materials.) The most commonly used HTS materials are "cuprate superconductors," ceramics based on cuprates (compounds containing a copper oxide group), such as BSCCO and ReBCO (Re is a rare-earth element, usually Y or Gd). Other HTS materials include iron pnictides (e.g., FeAs and FeSe) and magnesium diboron (MgB2).

[0003] ReBCO is typically fabricated as a tape having a structure as shown in FIG. 1. Such a tape 100 is generally about 100 microns thick and includes a substrate 101 (typically an electropolished nickel-molybdenum alloy such as Hastelloy®, about 50 microns thick) upon which a series of buffer layers, known as a buffer stack 102, about 0.2 microns thick, are deposited by ion-beam-assisted deposition (IBAD), magnetron sputtering, or another suitable technique. An epitaxial ReBCO-HTS layer 103 (deposited by metal-oxide chemical vapor deposition (MOCVD) or another suitable technique) covers the buffer stack and is typically 1 micron thick. A 1-2 micron silver layer 104 is deposited on the HTS layer by sputtering or another suitable technique, and a copper stabilizer layer 105 is deposited on the tape by electroplating or another suitable technique. The silver layer 104 and copper stabilization layer 105 are also deposited on the sides of the tape 100 and the substrate 101, extending continuously around the periphery of the tape 100 and allowing electrical connection to the ReBCO-HTS layer 103 from either side of the tape 100. These layers 104, 105 are therefore sometimes referred to as "cladding." Typically, the silver cladding has a uniform thickness of approximately 1 to 2 microns on both sides and edges of the tape. The silver layer 104 is provided between the HTS layer 103 and the copper layer 105 to prevent the HTS material from contacting the copper, which could be harmful to the HTS material. Portions of the silver layer 104 and copper stabilization layer 105 on the sides of the tape 100 are not shown in FIG. 1 for clarity but are shown in the cross-sectional view provided in FIG. 9. FIG. 1 also does not show the silver layer 104 extending under the substrate 101, as is typical (see, for example, FIG. 9). The silver layer 104 forms a low resistivity electrical interface and a surrounding hermetic protective seal to the ReBCO layer 103, while the copper layer 105 allows for external connection to the tape (e.g., to allow soldering) and provides a parallel conductive path for electrical stability.

[0004] The substrate 101 provides a mechanical backbone that can be fed through a manufacturing line and enables subsequent layer growth. The buffer stack 102 provides a biaxially oriented crystalline template for growing the HTS layers, preventing chemical diffusion of elements from the substrate into the HTS, which would impair superconducting properties. The silver layer 104 provides a low-resistance interface from the ReBCO-HTS layer 103 to the stabilization layer 105, which provides an alternative current path if any portion of the ReBCO stops superconducting (enters the "normal" state).

[0005] Additionally, "peeled" HTS tapes can be produced, which lack the substrate (e.g., Hastelloy® substrate) and buffer stack, but typically have a silver "surround coating," i.e., a layer on each side and edge of the HTS layer. Tapes with a substrate are referred to as "substrate" HTS tapes.

[0006] HTS tapes can be arranged in HTS cables. HTS cables consist of one or more HTS tapes, connected longitudinally via a conductive material (usually copper). HTS tapes may be stacked (i.e., arranged so that the HTS layers are parallel) or have some other arrangement of tapes that can vary along the length of the cable. A notable special case of HTS cables is a single HTS tape and an HTS pair. An HTS pair contains a pair of HTS tapes arranged so that the HTS layers are parallel. Where substrate tapes are used, the HTS pair can be either Type 0 (HTS layers facing each other), Type 1 (the HTS layer of one tape faces the substrate of the other tape), or Type 2 (the substrates face each other). Cables containing two or more tapes can have some or all of the tapes arranged in HTS pairs. Stacked HTS tapes can include 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 have a mixture of substrate tapes and release tapes.

[0007] The following terms are used to describe coils herein: - "HTS Cable": A cable made up of one or more HTS tapes. For purposes of this definition, a single HTS tape is an HTS cable. - "Turn": The part of the HTS cable that surrounds the inside of the coil (i.e., the part that can be modeled as a complete loop). - "Arc": A continuous length of coil that is shorter than the entire field coil. - "Inner / Outer Radius": The distance from the center of the coil to the inner / outer side of the HTS cable. - "Inner / Outer Circumference" The distance measured around the inner / outer circumference of the coil. - "Thickness": The radial depth of all turns of the coil, i.e. the difference between the inner and outer radius. - "Critical Current (Ic)": the current at which the HTS becomes cold at a given temperature and external magnetic field (the HTS is considered to have "come cold" at the characteristic point of the superconducting transition where the tape produces E0 volts per meter. The choice of E0 is arbitrary, but is usually taken to be 10 or 100 microvolts per meter). - Critical temperature: The temperature at which the HTS becomes normally conductive under a given magnetic field and current. - Peak critical temperature: The temperature at which the HTS becomes normally conducting in the absence of an external magnetic field and with negligible current. -Electrical insulator material: approx. 10 6 Materials with electrical resistivities above ohm-meters. For example, insulators such as MgO and diamond have resistivities of about 10 ohm-meters each (at room temperature). 12 It has a resistivity of ohm-meter or greater. -Conductive material: Electrical resistance is approximately 10 -6 Materials below ohm-meter.

[0008] Generally, HTS field coils are broadly divided into two types: "wound" and "assembled." As shown in Figure 2, wound coils are manufactured by continuously helically winding an HTS cable 201 (shown in solid lines) around a former 202. The former is shaped to provide the required inner circumference of the coil and may be a structural part of the final wound coil or may be removed after winding. As shown schematically in Figure 3, a sectioned coil is composed of several sections 301, each of which may contain several cables or preformed busbars 311 (shown in solid lines), forming the arc of the entire coil. The sections are connected by joints 302 to form the complete coil. The coil turns in Figures 2 and 3 are shown spaced apart for clarity, but typically there will be a material connecting the coil turns. For example, they may be joined by potting with epoxy.

[0009] The coils may be either "insulated," which has electrically insulating material between the coil turns (e.g., so that the turn-to-turn resistance is about 1 ohm or greater), or "non-insulated," which means that the coil turns are electrically connected (i.e., connected by an electrically conductive material) radially along the cable (e.g., by soldering or connecting the copper stabilizer layers of the cable with direct contact). Insulated HTS coils are used in applications requiring large, rapid magnetic field changes, such as plasma control in nuclear fusion magnets. In contrast, non-insulated coils are generally not suitable for large field coils due to their extremely long rise times.

[0010] An intermediate option is to use a material between the turns with a resistance intermediate between that of a conventional electrical conductor, such as a metal, and a conventional electrical insulator, such as a ceramic or organic insulator (e.g., 100 times that of copper to 10 times that of copper). 16 double, 10 -6 From 10 8 There are "partially insulated" coils with partially conductive turn-to-turn connections (i.e. 10 ohm meters). -6 From 10 8HTS magnets with turn-to-turn connections made of "intermediate" resistivity materials (between ohms and meters) have reduced field sweep rates but increased thermal and electrical stability under operating conditions, for example, because the coils can conduct heat and / or current both around and between the coil windings. Partial insulation can be achieved by selecting materials with appropriate resistivity or by providing a partially insulating structure that provides the required resistance. Such structures are described in detail in WO 2019 / 150123 A1, which is incorporated herein by reference.

[0011] Insulated and partially insulated magnets require the introduction of components into the coil to insulate or connect the turns. Insulated magnets are often made by co-wrapping insulating materials such as HTS tape and polyimide. Partially insulated magnets are manufactured in a variety of ways, including co-wrapping with metal tape (such as stainless steel tape), edge coatings, and specially processed flexible printed circuit boards (PCBs) that include conductive tracks.

[0012] Each of these manufacturing methods suffers from one or more drawbacks. For example, winding HTS tape with other types of tape (or PCBs) reduces the current density through the magnet windings. The introduction of organic insulators (e.g., polyimides) can reduce the coil's mechanical performance by, for example, lowering its Young's modulus. Furthermore, because the magnet windings are not bonded (the windings are "single-piece"), the contact pressure between turns depends on the contact between the turns, which can reduce mechanical performance. For example, dry-wound, uninsulated, or metal-insulated coils have been known to undergo conic deformation (deformation of the coil into a cone-like shape) due to induced shielding currents generated during coil operation. Furthermore, some organic insulating materials are unsuitable for fusion applications due to their poor resistance to neutrons.

[0013] Another problem with existing methods for creating insulated or partially insulated coils is that the inter-turn resistance is sometimes too low to implement in large (high-inductance) coils. Because 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 a correspondingly "high" inter-turn resistance so that the coil can respond to changing currents and magnetic fields on an appropriate time scale. Furthermore, in some cases, the geometry of the inter-turn connections and / or the limited thermal connection to the magnet can cause excessive ohmic heating during discharge, leading to electrical burnout of the connections. This can occur, for example, when a partially insulated magnet is created by introducing a PCB between the turns, where the thin metal tracks of the PCB are thermally insulated from the rest of the magnet by polyimide insulation.

[0014] FIG. 4 shows a radial cross section of a particular type of wound coil known as a "pancake coil," in which an HTS cable (tape) 401 is wound to form a flat coil in a manner similar to a spool of ribbon. Pancake coils can be made with an inner circumference that is any two-dimensional shape. Pancake coils are often provided as "double pancake coils," as shown in the radial cross section of FIG. 5, with two pancake coils 501 and 502 wound in opposite directions, with insulation 503 between the pancake coils and inner terminals connected by 504. This configuration allows current to flow through the coil turns, thereby generating a magnetic field, simply by applying a voltage to the (typically more accessible) outer terminals 521 and 522.

[0015] One application of HTS field coils is in tokamak plasma chambers (hereafter referred to as tokamaks). Tokamaks are characterized by a strong toroidal magnetic field, high plasma current, and typically a combination of large plasma volume and large auxiliary heating to provide the high-temperature, stable plasma required for fusion. Auxiliary heating (e.g., tens of megawatts of heating from high-energy hydrogen, deuterium, or tritium neutral particle beam injection) is required to raise the temperature high enough for fusion and to maintain the plasma current.

[0016] The challenges of using HTS field coils in tokamak plasma chambers are the large size, high magnetic fields, and high plasma currents typically required, which make them expensive to build and operate; the engineering must be robust to handle the large stored energies present in both the magnet system and the plasma; and the risk of "disruption" due to violent instabilities that can cause megaampere currents to go to zero in a few thousandths of a second.

[0017] To improve this situation, the "Spherical Tokamak (ST)" was developed by shrinking the doughnut-shaped torus of a conventional tokamak to its limit, creating a shape similar to an apple with a core. This concept was first realized in the START tokamak at Culham, and it has been demonstrated to significantly improve efficiency. It can reduce the magnetic field required to confine the high-temperature plasma by a factor of 10. Furthermore, it may improve plasma stability and reduce construction costs.

[0018] To obtain the fusion reaction required for economical power generation (i.e., power output much greater than power input), conventional tokamaks must be large enough to increase the energy confinement time (which is roughly proportional to the plasma volume) and heat the plasma high enough for thermonuclear fusion to occur.

[0019] WO 2013 / 030554 describes an alternative approach, involving the use of a compact spherical tokamak for use as a neutron or energy source. The low aspect ratio plasma shape of a spherical tokamak improves particle confinement time and allows for net power generation with a much smaller machine. However, it requires a small diameter central column, presenting challenges to the design of the plasma confinement vessel and associated magnets.

[0020] Tokamak magnet coils can be divided into two groups. Poloidal field coils are horizontal circular coils wound with their center positioned at the tokamak's central column, generating a poloidal magnetic field (a magnetic field substantially parallel to the central column). Toroidal field coils pass vertically through the central column and are wound on the outside (return side) of the plasma chamber, generating a toroidal magnetic field (a circular magnetic field centered on the central column). The combination of the poloidal and toroidal fields generates a spiral magnetic field within the plasma chamber, confining the plasma.

[0021] The current required to generate a toroidal magnetic field is very large. Therefore, tokamak designs increasingly use superconducting materials for the field coils. To achieve a compact spherical tokamak, it is desirable to minimize the diameter of the central column. However, even with superconducting materials, there is a limit to the current density that can be achieved, which creates conflicting requirements.

[0022] Figure 6 is a diagram of a longitudinal section through a spherical tokamak 600 comprising a toroidal field magnet (TF) 602 formed from a number of D-shaped TF coils 603A, B (only two of which are shown in Figure 6) arranged around a central column 604 oriented along an axis A-A', and a number of poloidal field (PF) magnets 605A-F, each surrounding the central column 604. Electric currents applied to the TF and PF magnets 603A, B, 605A-F generate a closure magnetic field that confines, shapes, and controls a high-temperature plasma 607 within a toroidal vacuum vessel 608 when the tokamak is in use.

[0023] 7 shows a cross section of the central column 604 containing multiple current-carrying assemblies 701 through which the central column section of the TF coil passes. The space within the central column 604 is occupied by both current-carrying assemblies and non-current-carrying components, such as: Neutron shielding 702: prevents heating of the central column and degradation of the critical current of the superconductor within the central column. Electrical isolation 703: electrically insulating current carrying assemblies from each other. Coolant channels 704: Carry heat away from the central column (for example using cryogens). Cooling ribs 705: carry heat from the individual current carrying assemblies to the coolant channels 704.

[0024] FIG. 8 shows one of the sectors 701 of the central column 604. The sector 701 includes three current-carrying assemblies 801A-C that make up the TF coil 602. Each assembly includes one or more HTS field coils 801, such as a pancake coil or a double-pancake coil. As is evident from FIG. 8, the rectangular cross-section of the HTS field coils does not fit neatly into the arc-shaped cross-section of the sector 701, wasting a large amount of space that could be usefully used for other components, such as additional cooling channels or sensors. Additionally, the placement of the current-carrying assemblies 801A-C within the sector 701 results in high stresses at the corners of the HTS coil cross-section (discussed below in connection with FIG. 19).

[0025] Another problem with the current-carrying assemblies 801A-C within sector 701 is that the current-carrying assemblies 801A, 801C closest to the tapered sides of sector 701 have fewer turns than the centrally located current-carrying assembly 801B. These differences in the number of turns in the HTS tape result in a high level of "ripple" in the magnetic field (discussed below in connection with FIG. 21), which causes the magnetic field in and near the central column to deviate from the desired circular symmetry.

[0026] Therefore, an alternative structure for the TF coils below the central column that avoids these effects is needed. Summary of the Invention

[0027] It is an object of the present invention to provide an HTS field coil that addresses or at least mitigates the above-mentioned problems.

[0028] According to a first aspect of the present invention, there is provided a method for fabricating a high temperature superconductor (HTS) field coil from one or more HTS tapes, each HTS tape comprising a layer of HTS material. The method includes winding one or more HTS tapes about an axis to form a field coil including windings of the HTS tape, and removing material from axial ends around at least a portion of the windings of the one or more HTS tapes to reduce the extent of the one or more HTS tapes along the axis of the field coil.

[0029] Removing material from the axial ends of the one or more HTS tapes may include reducing the extent of the HTS material layer along the axis of the field coil.

[0030] In embodiments in which the HTS tape has a layered structure (such as HTS tape 100 shown in FIG. 1 ), the HTS tape is typically wound with the layers parallel to the axis of the field coil, i.e., the tape is wound with the layers arranged concentrically relative to the axis of the field coil. In such an arrangement, removing material from the axial end of the HTS tape (e.g., by mechanical means such as machining) can consist of removing material from each layer simultaneously. For HTS tapes with a rectangular cross-section (such as HTS tape 100 shown in FIG. 1 ), the axial end of the HTS tape typically corresponds to the smaller side of the rectangular cross-section, i.e., the end corresponding to the "thickness" of the HTS tape.

[0031] Material may be removed such that the extent of the HTS layer along the coil axis varies radially and / or around one or more of the windings of the field coil. Removing material may include removing material across the entire face of the field coil. Material may be removed to provide the field coil with at least one axial surface that is conical. Removing material may include removing conductive cladding that is in electrical contact with the HTS material and extends across at least the axial ends of the HTS tapes. The cladding may be removed from the axial ends of one or more HTS tapes across the entire face of the field coil, exposing the HTS material layer at the axial ends of one or more HTS tapes.

[0032] The or each HTS tape may include a flexible substrate (e.g., a substrate comprising Hastelloy® or other metal or alloy) and an electrical insulator layer (e.g., buffer stack 102) disposed on a surface of the flexible substrate, with the HTS material layer disposed on the electrical insulator layer. Alternatively, a release tape may be used, in which case there is no electrical insulator layer and the HTS material layer is disposed on the flexible substrate (which may be made of silver, for example).

[0033] In certain embodiments, or each HTS tape, includes a flexible substrate, an intermediate layer disposed on a surface of the flexible substrate, an HTS material layer disposed on the intermediate layer, and a conductive cladding in electrical contact with the HTS material and extending across at least an axial end of the HTS tape. Removing material from one or more axial ends of the HTS tape may include removing some or all of the cladding from the axial end of the HTS tape around at least a portion of one or more windings to increase electrical resistance between the HTS material layer in the winding and the HTS material layer in an adjacent winding. Removing cladding from one or more ends of the HTS tape may include removing cladding to expose the HTS material layer at the axial end of the HTS tape around at least a portion of one or more windings.

[0034] The intermediate layer may be an electrical insulator layer (e.g., buffer stack 102) or a semiconductor layer, e.g., a layer of silicon and / or gallium arsenide, which may optionally be incorporated as a layer with buffer stack 102. The HTS field coil may be radially "insulated" or "partially insulated" depending on the resistivity of the intermediate layer (as described above). For example, in some embodiments, the resistivity of the intermediate layer may be less than 100 .mu.m to provide a partially insulated coil. -6 From 10 8 It may be between ohms and meters.

[0035] The cladding may extend continuously around the entire periphery of the HTS tape before the cladding is partially or completely removed from the ends of one or more of the HTS tapes.

[0036] The method may include bonding an electrical conductor element to the cladding on the axial end of the one or more HTS tapes before removing the metal cladding. The method may also include partially removing the electrical conductor element to leave an electrical contact for supplying current to at least one of the windings through the axial end of the one or more HTS tapes.

[0037] In embodiments in which the intermediate layer is an electrical insulator layer, the electrical insulator layer may have a thickness of, for example, less than 3 microns, or less than 1 micron, preferably less than 0.3 microns. The electrical insulator layer may be composed of one or more layers of ceramic material. The substrate may have a thickness of less than 100 microns, or less than 75 microns, preferably less than 50 microns.

[0038] Typically, the HTS material layer may be composed of a ReBCO material, where Re is a rare earth element such as Y or Gd. The HTS material layer may have a thickness of less than 10 microns, or less than 1 micron. The conductive cladding may be composed of a metal such as copper and / or silver, or the conductive cladding of each HTS tape may extend on each side of the HTS tape.

[0039] The windings of the HTS field coil may be arranged in two or more layers stacked along the axis of the field coil, with material removed from the axial ends of one or more layers of HTS tape. The HTS field coil may be, for example, a double pancake coil.

[0040] Material may be removed all around one or more windings.

[0041] The HTS field coil may include two HTS tapes arranged as a Type 0 pair with the HTS layers of the two HTS tapes facing each other and the substrates of the HTS tapes separated by the HTS layer.

[0042] The method may further include sealing the ends of one or more HTS tapes with an insulating or conductive material.

[0043] Removing material from one or more axial ends of the HTS tape may include mechanically removing material, preferably by machining one or more axial ends of the HTS tape. Mechanically removing material may include one or more of cutting, drilling, laser cutting, plasma cutting, water jet cutting, grinding, sanding, wire electrical discharge, turning, laser ablation, ion milling, sputtering, and electrical discharge machining.

[0044] Alternatively or additionally, material may be partially or entirely chemically removed from the axial ends of one or more HTS tapes. For example, if the cladding comprises copper, the cladding may be chemically removed by dissolving the copper with a ferric chloride solution. If the cladding comprises silver, the cladding may be chemically removed by dissolving the silver with a solution containing an oxidizing agent such as (preferably) hydrogen peroxide.

[0045] The method may include cooling the HTS field coil during the step of removing material from the axial ends of the one or more HTS tapes, which cooling may, for example, prevent thermal damage or degradation of the HTS material layer.

[0046] The method may further include polishing the axial ends of the one or more HTS tapes after removing the material.

[0047] The method may further include removing material from another axial end of the one or more HTS tapes, the other axial end being on a face of the field coil opposite the face of the field coil on which the axial end is provided.

[0048] Removing material may include cutting 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 plane (e.g., a plane) that passes through each of the one or more HTS tapes. For example, the HTS field coil may be divided by a cutting plane that is substantially perpendicular to the axis of the coil, preferably dividing the coil into two equal halves. The method may further include forming one or more electrical connections between the two or more HTS field coils to form a solenoid.

[0049] Winding one or more HTS tapes about an axis to form a field coil may include winding a cable having two outer HTS tapes and one or more inner HTS tapes of the one or more HTS tapes. The inner HTS tape is disposed between the outer HTS tapes or has a metal cladding disposed therebetween that provides a conductive path between the HTS layers of the two outer HTS tapes. Before and / or after material removal, the inner HTS tape may be narrower than the outer HTS tape along a direction parallel to the axis of the coil. Before and / or after material removal, axial ends of the inner and outer HTS tapes may be aligned with each other along an end of the cable.

[0050] According to a second aspect of the present invention, there is provided a method of manufacturing an electromagnet. The electromagnet includes a high temperature superconductor (HTS) field coil disposed within a recess or enclosed space. The method includes manufacturing an HTS field coil according to the method of the first aspect of the present invention, wherein the step of removing material from the axial ends of one or more HTS tapes of the HTS field coil includes removing material to fit the HTS field coil into the recess or enclosed space. The method further includes disposing the HTS field coil within the recess or enclosed space.

[0051] The fit of the HTS field coil within the recess or enclosed space may be such that the HTS field coil contacts each surface defining the recess or enclosed space (i.e., a tight fit). Alternatively, the relative size of the HTS field coil compared to the recess or enclosed space may be such that a small amount of movement of the HTS field coil within the recess or enclosed space (e.g., less than 1 mm) is permitted. The recess or enclosed space may be provided with a support structure, such as a rigid housing or casing, that provides mechanical support for some or all of the coil's windings. The support structure may prevent or limit deformation of the field coil when the field coil is operated.

[0052] According to a third aspect of the present invention, there is provided a method for fabricating a toroidal field (TF) magnet including a plurality of high temperature superconductor (HTS) field coils for use in a tokamak plasma chamber. The method includes fabricating a plurality of HTS field coils according to the method of the first aspect of the present invention. Each of the HTS field coils includes a section for installation in a respective sector of the central column of the TF magnet. The step of removing material from the axial ends of one or more HTS tapes of each HTS field coil includes removing material to match the cross section of the HTS field coil section to the cross section of the sector. The method further includes: Installing the coil sections for each HTS field in their respective sectors; and The sectors are arranged around a central axis to form a central column of TF magnets, and the windings in the sections of the HTS field coil installed in the sectors are arranged parallel to the central axis.

[0053] Removing material to match the cross-section of the section of the HTS field coil to the cross-section of the sector may include removing material such that the cross-section of the section of the HTS field coil tapers toward and / or away from the central axis.

[0054] Each sector may be comprised of multiple HTS field coils, with the axes of the HTS field coils arranged parallel to one another and perpendicular to the central axis of the central column.

[0055] According to a fourth aspect of the present invention, there is provided a high temperature superconductor (HTS) field coil including one or more windings of HTS tape about the axis of the coil, wherein the or each HTS tape comprises a flexible substrate, an intermediate layer disposed on a surface of the flexible substrate, and an HTS material layer disposed on the intermediate layer. At least one of the one or more HTS tapes (and optionally all of the HTS tapes) is configured such that there are no conductive paths extending radially across the intermediate layer for one or more windings, whereby the HTS material layer of at least one HTS tape of one or more windings is at least partially electrically insulated from the HTS layer of an adjacent winding by the intermediate layer.

[0056] The intermediate layer may be or consist of an electrical insulator or semiconductor layer (e.g., Si and / or GaAs). As described above for the first aspect of the invention, the HTS field coil may therefore be radially "insulated" or "partially insulated" depending on the resistivity of the intermediate layer. In other words, the HTS material layer in at least one HTS tape of one or more windings may be electrically insulated or partially electrically insulated from the HTS layer in an adjacent winding by an intermediate layer.

[0057] The HTS field coil may be formed according to the method described above for the first aspect.

[0058] The term "radial" means extending perpendicular to the axis of the coil (i.e., the axis encompassed by the windings of the coil) and in a direction toward or away from the axis. The absence of a radially extending conductive path across an intermediate layer (e.g., an electrical insulator layer) means that there is no conductive material (or materials) that extends continuously (i.e., spans) radially across the intermediate layer. Thus, the intermediate layer (e.g., an electrical insulator layer) radially electrically insulates (i.e., electrically insulates along the radial direction) or at least partially radially electrically insulates the HTS material layer of at least one HTS tape of one or more windings from the HTS layer of an adjacent winding.

[0059] The or each HTS tape may include a conductive cladding electrically connected to the HTS material layer, and in some examples, the cladding does not extend radially beyond the intermediate layer at least in a radial portion radially outward from the first current connection point and radially inward from the second current connection point.

[0060] The HTS field coil may be constructed from an electrically insulating material applied to one or more axial ends of one or more HTS tapes. Alternatively or additionally, the HTS field coil may be constructed from a conductive material applied to one or more axial ends of one or more HTS tapes.

[0061] The winding may include a winding of a cable including two outer HTS tapes and one or more inner HTS tapes of one or more HTS tapes, the inner HTS tape(s) having a metal cladding disposed between the outer HTS tapes to provide a conductive path between the HTS layers of the two outer HTS tapes. The conductive path between the HTS layers of the two outer HTS tapes provided by the metal cladding may extend radially across the middle layer(s) of the inner HTS tape on only one side (in some embodiments).

[0062] The HTS field coil may be constructed from a conductor element including an electrical contact surface for supplying current to at least one portion of the winding, the surface providing electrical contact between the conductor element and an axial end of the field coil.

[0063] The HTS field coil may be comprised of an electrical conductor disposed between intermediate layers of HTS tape of adjacent windings, the HTS material layer being radially separated from the electrical conductor by the intermediate layer. The HTS field coil may include an electrical conductor disposed between intermediate layers of HTS tape of adjacent windings, the HTS layer being radially separated from the electrical conductor by the intermediate layer. The electrical conductor may include two or more electrical contacts that allow electrical connection for measuring voltage across the radially separated portions of the electrical conductor.

[0064] According to a fifth aspect of the present invention, there is provided an electromagnet comprising one or more HTS field coils according to the fourth aspect of the present invention or one or more HTS field coils manufactured according to a method according to the first aspect of the present invention.

[0065] According to a sixth aspect of the present invention, there is provided a system comprising a plasma vessel and a set of field coils for generating a magnetic field within the plasma vessel, each field coil being an HTS field coil according to the fourth aspect of the present invention or an HTS field coil manufactured according to the method according to the first aspect of the present invention.

[0066] According to a seventh aspect of the present invention, there is provided a satellite, aircraft or unmanned aerial vehicle comprising one or more HTS field coils according to the third aspect of the present invention or one or more HTS field coils manufactured according to a method according to the first aspect of the present invention. [Brief explanation of the drawings]

[0067] [Figure 1] FIG. 1 is a schematic perspective view showing the internal structure of a ReBCO tape. [Figure 2] FIG. 2 is a schematic top view of a winding coil. [Figure 3] FIG. 3 is a schematic top view of a cross-sectional coil. [Figure 4] FIG. 4 is a schematic radial cross-sectional view of a pancake coil. [Figure 5] FIG. 5 is a schematic radial cross-sectional view of a double pancake coil. [Figure 6] FIG. 6 is a schematic vertical cross section of a tokamak. [Figure 7] FIG. 7 is a cross-sectional view of the central support. [Figure 8] FIG. 8 is a cross-sectional view of a sector of the central column of FIG. [Figure 9] FIG. 9 is a schematic cross-sectional view of a ReBCO tape. [Figure 10] FIG. 10 is a schematic radial cross-section of a coil containing a cable formed from two ReBCO tapes. [Figure 11] FIG. 11 is a schematic radial cross-sectional view of a coil according to an embodiment of the invention formed by removing the metal cladding from the axial ends of the coil of FIG. [Figure 12] FIG. 12 is a schematic radial cross-sectional view of the coil of FIG. 11 modified to include electrical contacts inserted between the windings of the coil. [Figure 13] FIG. 13 is a schematic radial cross-section of a coil including a pair of conductor plates extending to the axial ends of a coil including a cable formed from two ReBCO tapes. [Figure 14] 14 is a schematic radial cross-sectional view of a coil according to an embodiment of the present invention, in which a portion of the metal clad and conductive plate has been removed from the coil of FIG. 13; FIG. 14 is a schematic radial cross-sectional view of a coil according to an embodiment of the present invention, in which a portion of the metal clad and conductive plate has been removed from the coil of FIG. [Figure 15] FIG. 15 is a schematic radial cross-section of a coil containing a cable formed from four ReBCO tapes. [Figure 16] FIG. 16 is a schematic radial cross-sectional view of a coil according to an embodiment of the invention formed by removing metal cladding from the axial ends of the coil. [Figure 17] FIG. 17 is a flow chart of a method for manufacturing an HTS coil. [Figure 18] FIG. 18 is a cross-sectional view of a sector of an exemplary central column. [Figure 19] Figure 19 shows the results of a stress simulation of the HTS stack in the TF magnet. [Figure 20] FIG. 20 shows the results of a stress simulation of the HTS stack in an exemplary TF magnet. [Figure 21] FIG. 21 is a cross-sectional view of the central column showing the magnetic field. [Figure 22] FIG. 22 is a flowchart showing a method for manufacturing an HTS coil. [Figure 23] FIG. 23 is a flow chart of a method for manufacturing a TF magnet including multiple HTS field coils for use in a tokamak plasma chamber. DETAILED DESCRIPTION OF THE INVENTION

[0068] Several solutions to the above problems are proposed herein, in which HTS field coils are modified after they have been wound using one or more HTS tapes by removing material from the axial ends of one or more HTS tapes. For example, in one embodiment, HTS material is removed from the HTS tapes to change the superconducting properties of the tapes in different regions of the coil. In another embodiment, material is removed so that a buffer stack of HTS tapes provides electrical insulation between turns in an insulated or partially insulated field coil. Such field coils can be fabricated, for example, by completely or partially removing the metal cladding from the ends of the HTS tapes after they have been wound into the coil. Such coils are sometimes referred to as "buffer layer insulator" (BLI) coils.

[0069] Figure 9 shows a radial cross section through a ReBCO tape 900 similar to the ReBCO tape 100 of Figure 1. Elements that are the same as those in the tape of Figure 1 are given the same reference numerals. However, in this example, the copper stabilizer layer 105 and the silver layer 104 can be seen to extend around and completely surround the other layers of the tape 900, i.e., the copper stabilizer layer 105 and the silver layer 104 function as cladding for the layer structure of the tape 900.

[0070] As mentioned above, the substrate 101 is typically Hastelloy® and has a thickness on the order of 50 to 75 μm. The buffer stack 102 is a series of four ceramic layers of materials such as MgO, LaMaO3, or YSZ, typically having a combined thickness on the order of a few hundred nanometers to a few microns. The specific composition and thickness of the layers in the buffer stack 102 generally vary by supplier. The copper cladding 105 is typically about 10 to 20 μm thick (including the edges). Buffer stacks with more or less than four ceramic layers may also be used.

[0071] FIG. 10 is a radial cross-sectional view of a pancake coil 1000 formed by winding two ReBCO tapes 900A, B about axis Z (the axis of the coil). For clarity, only two windings 1002, 1004 of the tapes 900A, B are shown, but any number of windings can be used (see, e.g., FIG. 13). The windings 1002, 1004 of the coil 1000 are nested within one another to form a generally planar coil 1000. In this particular example, the tapes 900A, B are arranged in a type-0 configuration with their ReBCO layers facing one another, which allows for some current sharing between the two tapes 900A, B when the coil 1000 is operated. Of course, as mentioned above, other configurations are possible. The coil 1000 is typically solder potted, for example with PbSn solder, to form an uninsulated coil.

[0072] FIG. 11 is a radial cross-section of a coil 1100 made by removing the copper and silver cladding 104, 105 on both sides of the coil 1000 to expose the layered structure of the ReBCO tape 900.

[0073] The cladding can be mechanically removed by several methods, including lathing or wire EDM. In one preferred embodiment, a copper plate is soldered across the faces of the coil 1000, and then the plate is mechanically removed by milling, grinding, turning (e.g., lathe), or other means of mechanical metal removal, leaving one or more metal bond rings for injecting current into the coil 1000 through the ends of the HTS tapes 900A, B. In this case, a cutting tool penetrates the metal plate and removes approximately 0.5 mm of the HTS tapes 900A, B, followed by the ends of the HTS tapes 900A, B. This process can, of course, be performed on one or both faces of the coil 1000, depending on the desired winding resistance of the coil 1000 (which is maximized by completely removing the cladding from both faces). The winding resistance can also be varied by varying the thickness of one or more of the layers 104 of the buffer stack of the HTS tapes 900A, B (e.g., a thinner buffer stack can be used to create a coil that is only partially insulated). Other types of metal plates (other than copper) can also be used. Current can also be injected into the coil by means other than "end-connected" plates. For example, metal parts (contacts) can be provided on the inner and / or outer diameter of the coil, onto which cables can be terminated, e.g., by soldering.

[0074] Alternatively or additionally, chemical processes can be used to dissolve the copper and silver cladding. For example, the copper cladding 105 can be dissolved using a ferric chloride (FeCl3) solution. To dissolve the silver cladding 104, a solution of one part ammonium hydroxide and one part hydrogen peroxide, optionally diluted with methanol to slow the 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 a hydrogen peroxide solution combined with an acid or base. The surface of the ReBCO layer 103 is generally unaffected by removing the copper and silver cladding 104, 105 using a hydrogen peroxide and ammonium hydroxide solution. In some cases, using other reagents to remove the silver cladding 104 may dissolve or react the ReBCO layer 103 at its exposed edges. However, a small amount of degradation of the edges of the ReBCO layer 103 is generally acceptable for many applications. Optionally, the ends can then be sealed with an insulating material such as epoxy to prevent contaminants from entering the exposed tape ends and to allow thermal contact. Portions of the coil at the inner and outer diameters can be omitted from the end treatment (not shown) to leave a metal layer for current injection purposes.

[0075] Other methods for removing the copper and silver cladding 104, 105 include sputtering (eg, using ion milling) and / or laser ablation.

[0076] The construction of the BLI coil 1100 offers many advantages. Because the BLI coil 1100 is wound exclusively from ReBCO (HTS) tape, the current density of the magnet is maximized (as opposed to other types of insulated or partially insulated magnets made by co-winding HTS tape with other tapes or PCBs). In particular, the buffer layer 102 of tapes 900A, B is very thin, has the largest cross-sectional area, and is in intimate thermal contact with the ReBCO (HTS) layer 103, making it a very effective turn-to-turn resistor. The Young's modulus of the BLI coil 1100 and the structural integrity of the coil also remain high because the coil 1100 comprises (essentially) only HTS tape.

[0077] The buffer stack layer 102 presents an insulating barrier between the windings of the coil 600 without the need to introduce additional layers of insulating material between the windings, e.g., without the use of low modulus organic insulators. This allows the insulated coil to have both a high Young's modulus and a high winding density, i.e., a winding density determined substantially only by the thickness of the HTS tape. The high (e.g., maximum) winding density may enable the coil 600 to provide a higher current density compared to other coils that incorporate additional material between the windings of the HTS tape. The cross-sectional area of ​​the inter-turn connections is also maximized, spanning the entire width of the HTS tape, maximizing the opportunity for heat transfer between the windings, e.g., during a quench. The stiffness of the coil 600 and the winding connections may also reduce or minimize strain (or strain variations) within the HTS tape, which may, for example, prevent degradation of the HTS material as the coil is in use and / or make the coil 600 easier to maintain. The absence of additional materials (such as organic insulation) within the coil also avoids the potential for degradation of the additional materials, which may be of particular concern if the coil is exposed to high neutron fluxes such as those produced in a fusion reactor.

[0078] 12 is a schematic radial cross-section through coil 1200, which is identical to coil 1100, except that electrical contact 1201 is made to a central conductive region 1202 located radially between the centers of each of windings 1002, 1004. Central region 1202 is comprised of the metal cladding (i.e., copper cladding 105 and silver cladding 104) and substrate 101 of each tape 900A, B (i.e., tape 900B of winding 1004 and tape 900A of winding 1002). Metal claddings 104, 105 of conductive region 1202 and substrate 101 of each tape 900A, B are electrically connected to each other but are electrically isolated from the HTS layer 103 of each tape 900A, B immediately adjacent to conductive region 1202 by buffer stack 102. In other words, electrical contact 1201 is made to the central region 102 of the windings 1002, 1004, which are electrically isolated (along the radial direction) from the HTS layer 102 of the tape. The electrical contact 1201 can be made by inserting a metal (e.g., copper) contact between the windings 1002, 1004 as the coil 1200 is being wound. Alternatively, or additionally, the electrical contact 1201 can be made by soldering a metal contact to the axially facing edge of the electrically conductive region 1202.

[0079] When current is applied to the HTS layer 103 of the windings 1002, 1004, an induced voltage is generated in the coil 1200. This voltage is "picked up" by the electrically conductive region 1202 and can be measured using a potentiometer connected between the electrical contact 1201 and another electrical contact made to another portion of the electrically conductive region 1202, such as the radially innermost turn of the coil 1200 or the radially outermost turn of the coil 1200. The measured induced voltage between the two contacts can then be subtracted from the voltage measured across the coil 1200 as a whole, i.e., the voltage comprising the HTS layer 103 between the innermost turn of the coil 1200 and the outermost turn of the coil 1200. The remaining voltage therefore provides a resistive (as opposed to inductive) contribution to the voltage drop across the coil 1200, which is a good indicator of whether the HTS layer 103 is superconducting. In other words, the difference between the voltage across the coil 1200 and the induced voltage measured about the electrically conductive region 1202 indicates that a resistive voltage is building up in the coil 1200, which in itself indicates that a non-superconducting region has occurred or is occurring in the HTS material, which may lead to rapid heating in the coil (i.e., the occurrence of a quench).

[0080] 13 shows a radial cross-section of a coil 1300 including multiple windings of HTS tape 1301 arranged to form a primarily planar pancake coil, and two ring conductors 1303A, B that run across the edges of the HTS tape 1301 on either side of the coil 1300 and cover the face of the coil. Each ring conductor 1303A, B consists of an annulus or ring comprising a conductive material, preferably a metal such as copper, and extends radially across the coil to form an electrical connection between the windings of the coil 1300. The ring conductors 1303A, B are soldered to the face of the coil 1300 to provide good electrical contact with the ends of the HTS tape 1301.

[0081] FIG. 14 is a radial cross-sectional view of a coil 1400 obtained by removing the ring conductors 1303A, B and the cladding 105 from a portion of the coil 1300 shown in FIG. 13. For example, material from the ring conductor and cladding can be machined away, leaving portions of the ring conductors 1403A, B bonded to the cladding 105 while exposing the HTS layer 103 at the end of the HTS tape 1401. Current can be supplied to the coil 1400 through the radially innermost end of the HTS tape 1401 using the top ring conductor 1403A. The current flows around successive turns of the coil 1400 before being received by the bottom ring conductor 1403B at the radially outermost end of the HTS tape 1401; i.e., the ring conductors 1403A, B function as electrical contacts (or "current connections") for transferring current to and from the coil 1400. During the initial stages of current injection, the current injected into coil 1400 is believed to penetrate the turns covered by ring conductor 1403A (which acts as a small, "uninsulated" coil), creating a current distribution that minimizes the impedance between the turns. Then, over time, the current distribution in coil 1400 adjusts from (i) initially a distribution that reduces induced voltage (so that current flows from the radially outermost edge of top ring conductor 1403A through the turns of coil 900 between the two ring conductors 1403A,B and exits through the radially innermost edge of bottom ring conductor 1403B) to (ii) a current distribution that minimizes ohmic voltage (so that current penetrates uniformly through all turns of ring conductors 1403A,B).

[0082] The coil 1400 may, in some instances, have ring conductors 1403A, B on only one side. Although the ring conductors 1403A, B are shown on opposite sides of the coil 1400, they may be on the same side, in which case current may be injected or removed from only one side of the coil 1400, which may be advantageous in space-constrained environments, for example.

[0083] Although it is preferable to remove the metal cladding after winding the HTS tapes 900A, B, 1301, 1401 into a coil, a BLI coil can also be formed by removing the metal cladding from the sides of the HTS tapes before winding the coil.

[0084] The coils 1000, 1100, 1200, 1300, and 1400 described above are wound from cables formed from two HTS tapes in a Type 0 configuration (HTS layers facing each other), allowing current sharing between the two HTS tapes 900A, B. Coils may also be formed from cables containing more than two HTS tapes 900A, B. However, in these types of coils, removing the copper 105 and silver 104 cladding from the axial ends of the tapes 900A, B can prevent current sharing between all tapes 900A, B in the cable, because removing the cladding would mean current sharing would only occur between tapes with a Type 0 configuration, i.e., tapes with HTS layers 103 facing each other. A solution to this problem can be achieved by winding coils from cables containing HTS tapes of different widths, as described below.

[0085] Figure 15 is a radial cross-section of a coil 1500 comprising windings 1502, 1504 of a cable that includes four HTS tapes 1501A-D. The central axis Z (not shown) of the coil is to the right of the portion of the coil shown in Figure 15. Two of the tapes 1501A, D have a width that is greater than the width of the other two tapes 1501B, C. In Figures 15 and 16, the "width" of tapes 1501A-D (i.e., the second shortest dimension of the tape) extends vertically so that Figures 15 and 16 show radial cross-sections through the coil. Each of the narrow tapes 1501B, C is provided in the cable between the wide tapes 1501A, D and is arranged in a Type 0 configuration with a respective one of the wide tapes 1501A, D, i.e., the wide tapes 1501A, D are oriented so that their HTS layers 103 are closer to the center of the cable than their buffer stacks 102, and the narrow tapes 901B, C are oriented so that their HTS layers 103 are farther from the center of the cable than their buffer stacks 102. The windings of coil 1500 (exemplified by windings 1502, 1504) are aligned on one side 1506A of coil 1500 (orthogonal to the radial and circumferential directions) to give the coil a flat surface on this (bottom) side 1506A of coil 1500 and a radially "crenelled" surface on the other (top) side 1506B of coil 1500 as a result of the wider tapes 1501A, D protruding from coil 1500 relative to the narrower tapes 1501B, C. Note that the silver cladding 104 and copper cladding 105 of coil 1500 are intact.

[0086] FIG. 16 shows a radial cross-section of coil 1600 obtained by removing silver cladding 104 and copper cladding 105 from the flat surface of coil 1000 on bottom 1006B of coil 1500 and removing wide tapes 1501A and 1501D on top 1506B of coil 1600 ("wide" and "narrow" refer to the extent of the tapes along the axis of coil 1600). The cladding 104 and 105 of narrow tapes 1501B and 1501C are not removed on top 1506B. As a result, a conductive path exists between narrow tapes 1501B and 1501C, provided by cladding 104 and 105, allowing current sharing between narrow tapes 1501B and 1501C and, ultimately, between tapes 1501A and 1501D that make up the cable. However, because no conductive path exists between adjacent windings 1502 and 1504, coil 1600 is a BLI coil.

[0087] Alternatively, the coil 1500 may have HTS tape ends that are not flush with one side 1506A, i.e., the coil 1500 may be stepped or "crenelled" on both sides, in which case it may not be necessary to remove the cladding 104, 105 from the narrower HTS tapes 1501B, C.

[0088] This approach (i.e., using HTS tapes of different widths) allows cables containing two or more HTS tapes to be used to construct insulated or partially insulated coils, thereby extending the scalability of this manufacturing method to larger coils.

[0089] In some cases, to facilitate removal of cladding 104, 105 from wide tapes 1501A, D on top surface 1506B of coil 1500, coil 1500 may be "potted" with solder to fill gaps between wide tapes 1501A, D. Additionally, the solder covering narrow tapes 1501BC provides radial mechanical stability to wide tapes 1501A, D, preventing damage when top surface 1506B of coil 1500 is processed to remove cladding 104, 105.

[0090] In one embodiment, the wide tapes 1501A, D have a width of 12 mm, while the narrow tapes 1501B, C have a width of 11 mm or 10 mm. Preferably, the HTS layer 103 and buffer stack 102 of the wide tapes 1501A, D extend over the narrow tapes 1501B, C so that the cladding 104, 105 can be removed from the wide tapes 1501A, D without damaging the cladding 104, 105 of the narrow tapes 1501B, C. In some cases, the cladding 104, 105 of tapes 1500A-D may extend beyond the HTS layer 103 and buffer stack 102 by about 10 microns, such that about 10 microns of material (i.e., cladding) must be removed from either side of the coil 1500 to produce the insulated coil 1600. However, in practice, variations in the width of the HTS tape and the alignment of the windings relative to each other (i.e., the flatness of the coil) mean that it may be preferable to remove 100 microns or more of material from each side of the coil (i.e., the cladding and part of the HTS layer 103 and buffer stack 102) to ensure that the coil is fully insulated.

[0091] Cables containing four or more HTS tapes can also be used to manufacture the insulating coil, provided that the HTS tapes 1501A, D that provide the outer surface of the cable are wider than the HTS tapes 1501B, C that are closer to the center of the cable.

[0092] 17 is a flow chart illustrating a method for fabricating an HTS coil from one or more HTS tapes. Each HTS tape comprises a flexible substrate, an electrical insulator layer (e.g., buffer stack 102) disposed on a surface of the flexible substrate, an HTS material layer disposed on the electrical insulator layer, and a conductive cladding in electrical contact with the HTS material and extending across at least the edge of the HTS tape. The electrical insulator layer may be only "partially insulating," in which case the electrical insulator has a resistivity between 100 and 10 times that of copper, for example, or 10 -6 From 10 8It may have a resistivity between ohm-meter and ohm-meter. Step 1701 of the method includes winding one or more HTS tapes about an axis to form a field coil including windings of the HTS tape. Step 1702 of the method includes partially or fully removing cladding from axial ends of one or more HTS tapes around at least a portion of one or more of the windings to increase electrical resistance between the HTS material layer in the winding and the HTS material layer in an adjacent winding.

[0093] While the above discussion focused on removing cladding from one or more axial ends of the HTS field coils 1000, 1100, and 1200, it is also possible to remove more material from the axial ends to reduce the field coil's width (i.e., its extent along the coil's axis). Surprisingly, HTS field coils have proven highly resistant to physical damage, even allowing significant changes to the coil's shape after winding. For example, HTS field coils have been found to function effectively even after several holes have been drilled or the coil has been shaped by standard machining techniques such as sawing, grinding, and polishing. In particular, even if there is an interruption or damage to a portion of the coil winding, current may be able to continue circulating around the HTS material of such a coil. This can occur as a result of current sharing between adjacent windings, allowing current to "bypass" the affected portion of the winding. It is also possible that the minimum critical current may be exceeded if the resistance between the windings is sufficiently low and cooling is provided to cool the resistive heat load.

[0094] These findings offer significant opportunities for situations where the ease of assembly of double pancake coils is desirable but the rectangular cross-section of such coils presents disadvantages, such as the central column of a toroidal field coil (discussed above). As discussed below in connection with Figure 18, the coil cross-section can be modified after winding (preferably after potting the coil with solder or epoxy, for example) to better fit the required space. "Potting" a coil refers to filling (or encasing) the coil with a material such as solder or epoxy after winding one or more HTS tapes (cables) to form the coil. The potting material fills gaps in the coil structure and improves the structural integrity of the coil. This is done to reduce the likelihood of loosening or damaging the radially inner or outer windings of the coil during, for example, coil machining. Potting with a conductive material (e.g., solder) can also improve the electrical connection between the coil's windings. Potting with a structurally effective material such as resin or solder can effectively transfer radial stresses, potentially allowing the coil to be supplied with a higher current to generate a larger magnetic field without exceeding the coil's strain limit. The stiffness of a potted coil may also reduce the amount of support the coil requires. For example, a potted coil can be supported only at its outer radius (or conversely, its inner radius) by a stiff support structure.

[0095] Varying the width (i.e., extent along the axis of the field coil) of HTS material layer 103 can change the superconducting properties of HTS field coils 1000, 1100, 1200, such as the critical current. The additional material removed generally consists of material from the axial ends of each layer in HTS tape 100: substrate 101, buffer stack 102, HTS material layer 103, and cladding 104, 105. The additional material can be removed by (for example) machining one or both faces of HTS field coil 200, 500 after cladding 104, 105 has been removed from the axial ends of field coil 1000, 1100, 1200. Alternatively (or additionally), reduced-width HTS field coils can be produced by cutting the HTS field coil, for example, by cutting across HTS field coil 1000, 1100, 1200 in a plane perpendicular to coil axis Z, thereby dividing the coil into two reduced-width HTS field coils. The plane may bisect field coil 1000, 1100, 1200, so that each of the narrower field coils has approximately half the width of original field coil 1000, 1100, 1200. For example, one or more 12 mm wide HTS tapes may be wound into a pancake coil, and the pancake coil may be divided into two pancake coils each having a width of approximately 6 mm. The cut through field coil 1000, 1100, 1200 may be achieved, for example, by electrical discharge machining (also known as spark machining or wire electrical discharge machining). The axial end(s) of the HTS field coil may be polished after machining to ensure there are no "shorts" (i.e., radial electrical connections between windings) and / or to gently remove any damaged ends that may be caused by the cutting process.

[0096] The axial ends (or potentially all outer surfaces) of the HTS field coil may preferably be hermetically sealed to prevent the ingress of contaminants into the exposed layered structure. This can be done by applying an insulator coating, such as a ceramic or resin (e.g., epoxy resin). Insulator coatings may be preferred to avoid unwanted electrical connections between turns. However, if the HTS field coil has low inter-turn resistivity (e.g., a "partially insulated" coil), one or more metal layers may instead be applied (or applied underneath the insulator coating) to protect the layers of HTS tape. For example, a layer of nickel may be applied to the HTS field coil by depositing it on the surface via electroplating or other chemical or physical vapor deposition methods.

[0097] This method of manufacturing a reduced-width HTS field coil can be contrasted with conventional methods of manufacturing HTS field coils by selecting or preparing an HTS tape 100 having a width corresponding to the desired width of the field coil. Notably, cutting the field coil 1000, 1100, or 1200 to the desired width after winding the coil avoids being limited in width by the width of the HTS tape 100 available from the manufacturer. This also avoids the need to cut commercially available HTS tape 100 to produce narrower widths, which is generally time-consuming, difficult to perform accurately, and can potentially lead to edge damage (cracks) that can lead to long-term conductor degradation, especially in high-field coils. Cutting the HTS tape 100 after it has been wound into the field coil 1000, 1100, or 1200 also allows the profile of the HTS field coil to be shaped into more complex shapes than a flat pancake coil. For example, one or both of the axial faces 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 conical, i.e., shaped so that a vector normal to the outer axial surface of the field coil diverges from or converges toward the axis. In some examples, only one of the axial surfaces is non-planar. By varying the width of the HTS field coil in this manner, the superconducting properties of the field coil can be varied (or "graded") by winding the coil differently. Varying the field coil width also allows for some shaping of the magnetic field generated by the coil during use, which may be useful in applications requiring precise control of the magnetic field, such as magnetic resonance imaging (MRI).

[0098] The reduced-width HTS field coils fabricated by the above methods may be electrically connected to each other, for example, to form a double pancake coil. One or more reduced-width HTS field coils may also 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) can be adapted depending on their position within the stack. For example, coils located at or near the mid-plane of the HTS solenoid may have reduced widths compared to coils located further away from the mid-plane. Generally, the width of the coils may increase as the distance from the mid-plane of the solenoid increases. Because the magnetic field in this portion of the solenoid is parallel to the HTS layer 103, the critical current density tends to be higher, and it is preferable to have thinner coils near the mid-plane, unlike the ends of the solenoid, where the magnetic field angle diverges away from parallelism and the critical current density is generally lower. Therefore, to balance the critical current across all pancake coils and to make optimal use of the conductors throughout the solenoid, it is desirable to have thin inner pancake coils (i.e., pancake coils near the mid-plane of the solenoid) and wide outer pancake coils.

[0099] In the above discussion of narrow width HTS field coils, reference was made to fabricating the coils using "traditional" HTS tape that includes cladding 104, 105. However, in some embodiments, the reduced width HTS field coils are fabricated from HTS tape 100 that does not include cladding 104, 105. Alternatively, in some other examples, the HTS tape may be constructed with cladding 104, 105 that does not extend to the axial ends of the HTS tape.

[0100] FIG. 18 shows an exemplary central column sector 1800. Similar to sector 701 shown in FIG. 8, sector 1800 is a cross section through a D-shaped HTS coil containing six pancake coils arranged as three double pancake coils. The HTS coils have been cut to provide a cross section at the central column that is a sector of the toroid. In this example, this is achieved by leaving four pancake coils 1801 toward the center of the sector unchanged and cutting slopes 1803 and 1804 into two pancake coils 1802 on the left and right sides of sector 1800 so that the cross section of the pancake coils 1802 tapers toward and away from the central axis of the central column. The ramps 1803, 1804 are formed so that they conform to (i.e., fit closely to) the boundaries of the sector 1800, allowing the pancake coil 1802 (and the corresponding other half of the double pancake coil) to be larger, i.e., to increase the extent of the coil in the direction toward the axis of the central column (i.e., to accommodate more windings), compared to that of Figure 8. The ramps 1803, 1804 may be cut only in the straight portion of the coil 1802 that passes through the central column (i.e., the "uprights" of each D-shaped coil), leaving the "return limbs" (i.e., the portions outside the central column) unchanged.

[0101] A number of such coils may be arranged to form a TF magnet, with the annular sectors 1800 of the central column sections of each coil 1802, 1801 together forming the central column of the TF magnet.

[0102] Comparing Figure 18 with Figure 8, the outer double pancake coil 1802 has increased its cross-sectional area by approximately 50%, even accounting for the area reduction due to the tapered cuts, resulting in an approximately 25% increase in the combined cross-sectional area of ​​the energization element's pancake coils 1801, 1802. This significantly increases the current density of energization element 1800 compared to energization element 701 of Figure 8. Further shaping of the coils and / or the addition of more shaped coils (e.g., into one or more gaps 1805) could potentially allow for further improvements.

[0103] 19 shows the results of a stress simulation of a cross section 1901 of a TF magnet, showing a single pancake coil of HTS tape 1902 wound with a rectangular (i.e., unmodified) cross section. The stresses in the radially inner portion of the HTS stack 1902 are unacceptably high (over 500 MPa in some places, compared to the maximum allowable compressive stress of 450 MPa for commonly available HTS tapes), which would lead to degradation and possibly permanent damage of the HTS tape.

[0104] Figure 20 shows the results of a stress simulation of a cross section 2001 of a TF magnet, showing a single pancake coil of HTS tape 2002 wound with a slope 2003. Compared to Figure 19, it can be seen that the stress across the entire structure is significantly reduced to less than 330 MPa.

[0105] Coils 1802, 2002 (and the HTS field coils 1000, 1100, and 1200 described above) can be cut and shaped by essentially any suitable method for cutting metals or metal composites, such as electrical discharge machining (EDM), grinding, sanding, etching, laser cutting, plasma cutting, or waterjet cutting. Furthermore, HTS materials permanently degrade above their degradation temperature. This temperature depends on the exact HTS material and manufacturer, but is typically on the order of 150 to several hundred degrees Celsius. This degradation can be mitigated or avoided by providing appropriate cooling (e.g., water cooling) during cutting and shaping of the coil. Furthermore, partially insulated coils have high thermal conductivity (especially when soldered), making them effective heat sinks during cutting. Electrical discharge machining is often preferred because it minimizes damage to the tape and facilitates the creation of curved shapes by properly shaping the electrodes used for the discharge. These methods can also be used in combination. For example, the majority of the material may be removed by one method (sometimes referred to as a "roughing" process) that may damage the axial ends of the HTS tape, and then by another method (sometimes referred to as a "finishing" method) that is used to remove the damaged material remaining after the roughing process.

[0106] In general, an HTS coil will be particularly suitable for cutting and shaping as described above if the coil is: - Non-insulated or partially insulated, i.e., constructed so that current can be shared radially between the turns of the coil over substantially all of its length (e.g., including coils with regularly spaced radial current paths). -Immersed in a material such as solder or epoxy that hardens and provides additional structure to the coil (or additional current paths if conductive).

[0107] The coil can be cut to form the desired shape as described above, provided that once cut, there is at least one current path around the windings of the coil, i.e., from the radially outermost edge of the HTS tape (cable) to the radially innermost edge of the HTS tape.

[0108] While the above examples are directed to tokamak TF coils, it will be appreciated that the techniques for cutting and shaping HTS coils are applicable to any application of HTS coils, and are particularly useful in applications where the space available for the coils is limited or the coils have irregular shapes, such as aircraft and spacecraft. Also, while the above description focuses on pancake coils, the methods apply equally to other partial or non-insulated coil structures, such as joint coils.

[0109] An additional advantage of shaping HTS field coils as described above is that the magnetic field generated by the coils can be more controlled. FIG. 21 shows a contour plot of the magnitude of the magnetic field generated near the central column 2100 of a TF magnet containing 12 TF coils 2101 with rectangular cross-sections. While the magnetic field away from the central column 2100 is essentially circularly symmetric, the magnetic field near and within the central column 2100 has a high level of “ripple” (i.e., lack of symmetry). By shaping the coils 2101 into a “wedge” shape, the coils 2101 can be packed more closely together, significantly reducing the ripple. The use of shaped coils 2101 as described herein is not limited to the central column of a tokamak but can also include other types of plasma chambers (e.g., stellarators) or devices that generate precisely controlled magnetic fields, such as MRI machines, NMR spectrometers, and charged particle accelerators.

[0110] 22 is a flowchart of a method for fabricating an HTS coil. In step 2201, a partially insulated coil is formed, for example, by winding an HTS cable and a layer of partial insulation onto a former. In step 2202, the resulting coil is potted, for example, with solder or epoxy. In step 2203, the coil is machined to the desired shape by removing material, including the HTS material, so that a current path exists around the coil after forming.

[0111] 23 is a flow chart of a method for fabricating a TF magnet including multiple HTS field coils for use in a tokamak plasma chamber. The method comprises the following steps: Step 2301: Produce a plurality of HTS field coils according to the method described above by removing material from the axial end (or ends) of the coils. Each of the HTS field coils (which may be, for example, D-shaped coils as shown in FIG. 3) constitutes a section (such as the "busbar" 311 shown in FIG. 3) for installation in a respective sector of the central column of a TF magnet (such as current-carrying assembly 701 as shown in FIGS. 7 and 8). Material is removed from the axial end of each HTS tape to match the cross section of the HTS field coil section to the cross section of the sector. Step 2302: Installing each HTS field coil section in its respective sector. Step 2303: Arranging sectors around the central axis to form a central column of TF magnets, with the windings in the sections of the HTS field coil installed in the sectors being arranged parallel to the central axis.

[0112] While various embodiments of the present invention have been described above, it should be understood that they are presented by way of example and not limitation. It will be apparent to those skilled in the relevant art(s) that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, while the coils described above have been described as having HTS tape 100 arranged in a “Type 0” configuration, other configurations, such as “Type 1” and “Type 2” (e.g., as described in WO 2018 / 078326), may also be used. Similarly, while the present disclosure has been illustrated with reference to “pancake” coils, i.e., generally planar coils formed from nested concentric windings, it will be understood from the above discussion that the present disclosure is not limited to such coils. Similarly, while the above description refers to ReBCO tape 100, other types of HTS tapes comprised of one or more insulating (or partially insulating) buffer layers may be used in place of or in addition to ReBCO tape 100. Properties of the buffer layer stack, such as layer composition and / or layer thickness, may also be varied to render the coil insulating or partially insulating. For example, the buffer layer may consist of or include semiconductor materials such as silicon and / or gallium arsenide. In some cases, the buffer layer stack may consist of or include one or more metal-insulator transition (MIT) materials, such as vanadium oxides (e.g., VO, VO, VO, VO, VO, VO, VO, etc.), to provide a turn-to-turn resistance that can be varied based on a metal-to-insulator phase transition within the material, for example, as a result of varying the temperature of the material.

[0113] The HTS field coil described above (or equivalently, an HTS field coil manufactured according to the method described above) is particularly advantageous for use in aerospace applications. For example, HTS field coils can be included in aircraft, unmanned aerial vehicles, satellites, spacecraft, rocket-powered vehicles, and autonomous exploration vehicles. In these (and other) applications, removing material from the axial ends of one or more HTS tapes allows the coil shape to be tailored to occupy less volume and be lighter. This is particularly beneficial for space- and weight-constrained technologies such as satellites. Buffer-layer insulated HTS field coils are also advantageous because they can provide partially insulated (PI) or fully insulated coils without the need to introduce additional layers of insulating material between windings, thus further reducing the coil volume / weight. Furthermore, because the HTS tape is typically left unmodified during winding of the HTS field coil (i.e., material is removed after winding), it is possible to form tightly wound, linked coils that can withstand the large forces encountered during aircraft takeoff and satellite launch. In contrast, other manufacturing methods, such as removing material from the HTS tape before winding or adding layers between windings, generally make it difficult to produce HTS field coils with high mechanical stability.

[0114] HTS field coils are generally provided to generate a magnetic field with a specific strength and spatial distribution. However, in some cases, it is possible to remove material from the axial ends of one or more HTS tapes to reduce the size and mass of the coil while substantially maintaining the strength and / or spatial distribution generated by the coil. The amount and / or location of material to be removed can be determined by trial and error or, preferably, from a computer simulation (e.g., a finite element model) of the coil and its associated magnetic field. In the latter case, evolutionary or genetic algorithms may be used to optimize material removal. These algorithms can be subject to one or more constraints (e.g., tolerances) related to, for example, the strength and spatial distribution of the magnetic field and / or the operating parameters of the coil, such as current and temperature.

[0115] The geometry of an HTS field coil can also (or alternatively) be altered by removing material from the HTS tape to achieve a specific current distribution within the tape when the coil is in use. For example, material can be removed from the HTS tape (e.g., by machining the coil) so that the ratio of current to critical current (I / I) is approximately constant for specific regions of the coil or throughout the coil. Such optimization can reduce "excess" current (i.e., current above the critical current) and allow the coil to operate in a "saturated" mode where the current is substantially equal to the critical current, minimizing resistive heating within the coil. An example of this type of optimization is an HTS pancake coil that is split into two or more HTS field coils of reduced width, i.e., reduced extent along the axis of the coil, by, for example, "wire slicing" the pancake coil into two smaller (reduced-width) pancake coils (see the above discussion regarding reduced-width HTS field coils). An additional advantage associated with coils fabricated in this manner is the reduced current required from the power source, thereby enabling the use of smaller and / or lighter power sources and non-superconducting current-carrying components. Reducing the power consumption of HTS field coils can also extend the life of the power source, for example an electrochemical cell (battery).

[0116] As will be appreciated by those skilled in the art, the technique used to remove material from HTS coils may be selected depending on the application the HTS coil will be used in. However, in general, any of the techniques discussed above, such as EDM, grinding, sanding, etching, laser cutting, plasma cutting, water jet cutting, chemically removing material, etc., may be used.

Claims

1. 1. A method of fabricating a high temperature superconductor (HTS) field coil from one or more HTS tapes, each HTS tape including a flexible substrate, an intermediate layer disposed on a surface of the flexible substrate, an HTS material layer disposed on a surface of the intermediate layer, and a conductive cladding in electrical contact with the HTS material layer and extending across at least opposite ends of the HTS tape, the method comprising: winding the one or more HTS tapes around a shaft to form an HTS field coil including windings of the HTS tape; removing material from both axial faces of the HTS field coil after winding the HTS tape to remove the conductive cladding extending around both ends of the one or more HTS tapes around at least a portion of one or more of the windings; A method comprising:

2. The method of claim 1, wherein the step of removing the material increases the electrical resistance between the HTS material layer in a winding and the HTS material layer in an adjacent winding.

3. The method of claim 1 , wherein the intermediate layer is or comprises an electrical insulator layer.

4. A method described in any one of claims 1 to 3, wherein the step of removing material includes removing the conductive cladding extending from the entire uniaxial surface of the HTS field coil to both ends of the one or more HTS tapes.

5. A method described in any one of claims 1 to 3, wherein the step of removing material includes removing the conductive cladding extending from the entire axial surface of the HTS field coil to both ends of the one or more HTS tapes.

6. A method as described in any one of claims 1 to 3, further comprising the step of connecting the HTS field coil to join the windings of the HTS field coil together after winding the HTS tape and before removing the material from both axial faces of the HTS field coil.

7. 7. The method of claim 1, further comprising sealing both ends of the one or more HTS tapes with an electrical insulator material.

8. A method described in any one of claims 1 to 6, further comprising a step of sealing both ends of the one or more HTS tapes with a conductive material.

9. 9. The method of claim 1, wherein the removing material comprises cutting the HTS field coil to divide the HTS field coil into two or more HTS field coils.

10. A high temperature superconductor (HTS) field coil manufactured by the method of any one of claims 1 to 9.

11. 1. A high temperature superconductor (HTS) field coil comprising a winding of one or more HTS tapes about an axis of the HTS field coil, the or each HTS tape comprising a flexible substrate, an intermediate layer disposed on a surface of the flexible substrate, an HTS material layer disposed on the intermediate layer, and a conductive cladding in electrical contact with the HTS material layer and extending across at least opposite ends of the HTS tape, wherein material is removed from both axial faces of the HTS field coil after winding the HTS field coil to remove the conductive cladding extending across opposite ends of the one or more HTS tapes.

12. The HTS field coil of claim 11, wherein the HTS field coil is coupled and the windings of the HTS field coil are joined together prior to removing the conductive cladding.

13. 13. The HTS field coil of claim 11 or 12, wherein the intermediate layer is or includes an electrical insulator layer.

14. 14. The HTS field coil of claim 11, wherein the winding comprises a winding of a cable including two outer HTS tapes and one or more inner HTS tapes of the one or more HTS tapes, the inner HTS tape having a conductive cladding disposed between the outer HTS tapes and providing a conductive path between HTS layers of the two outer HTS tapes.

15. a conductor element including an electrical contact surface for supplying current to at least a portion of the winding; 15. The HTS field coil of claim 11, wherein the electrical contact surface provides electrical contact between the conductor element and one axial end of the HTS field coil.

16. 16. An electromagnet comprising one or more HTS field coils according to any one of claims 11 to 15.

17. 16. A system comprising a plasma vessel and a set of field coils for generating a magnetic field within the plasma vessel, each field coil being an HTS field coil according to any one of claims 11 to 15.

18. 16. A satellite, aircraft or unmanned aerial vehicle comprising one or more HTS field coils according to any one of claims 11 to 15.

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