EA040624B1 - MAGNET WITH HIGH-TEMPERATURE SUPERCONDUCTOR - Google Patents

Description

Technical field

The present invention relates to high temperature superconductor (HTSC) magnets,

HTSC magnets. In particular, it relates to the supply of electric current to HTS magnets.

Background of the invention

Superconducting materials are typically divided into high temperature superconductors (HTSC) and low temperature superconductors (LTSC). LTSC materials such as Nb and NbTi are metals or metal alloys whose superconductivity can be described by the BCS (Bardin-Cooper-Schrieffer) theory. All low-temperature superconductors have a critical temperature (the temperature above which a material cannot be superconductive even in zero magnetic field) below about 30 K. The behavior of an HTS material is not described by BCS theory, and such materials may have critical temperatures above about 30 K (although it should be It should be noted that it is the physical differences in the work and composition of superconductors, and not the critical temperature, that determine the HTSC material). The most commonly used HTSCs 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, usually Y or Gd). Other HTS materials include iron pnictides (eg FeAs and FeSe) and magnesium diborate (MgB ₂ ).

ReBCOs are typically produced as tapes with the structure shown in FIG. 1. Such tape 100 is typically about 100 µm thick and includes a substrate 101 (typically an electropolished Hastelloy about 50 µm thick) on which is deposited by ion beam deposition (IBAD), magnetron sputtering, or other suitable technique, a series of buffer layers known as buffer package 102, approximately 0.2 µm thick. An HTS-ReBCO epitaxial layer 103 (deposited by metal organic chemical vapor deposition (MOCVD) or other suitable method) covers this buffer package 102 and is typically 1 µm thick. The HTS layer is coated with a 1-2 micron silver layer 104 by magnetron sputtering or other suitable method, and a stabilizing copper layer 105 (or sheath) is applied to the tape by electroplating or other suitable method, which often completely encapsulates the tape. Electrical current is typically applied to tape 100 through the sheath.

The substrate 101 provides a mechanical support that can be fed down the production line and allows subsequent layers to be grown. The buffer package 102 is required to provide a biaxially textured crystal template on which the HTS layer will grow, and to prevent chemical diffusion of elements from the substrate into the HTS, which destroys its superconducting properties. The silver layer 104 is required to provide a low resistance transition from the ReBCO to the stabilization layer, and the stabilization layer 105 provides an alternate current path in the event that any portion of the ReBCO ceases to be superconductive (goes to the normal state).

HTS magnets can be formed by winding an HTS tape, such as the ReBCO tape 100 described above, onto a spool. Common points of failure in such HTS magnets are where individual tapes or cables break away from the winding package at the junction (ie, electrical connection).

Fig. 2 schematically shows a conventional electrical interface with a coil 201 containing an HTS tape 100. The outer winding of the coil 201 has been partially pulled away from the coil stack to create a free end 202. coil 201.

In free-end joints, such as those shown in FIG. 2, HTS tapes are damaged due to cycling under electromagnetic (EM) forces and thermal contraction causing degradation during normal operation. Often these exposed sections of the HTS tape are also at additional risk because there are no adjacent HTS turns (windings) to share current with in the event of a critical current failure, meaning that such sections do not enjoy proximity to the main winding stack for dissipation of heat and/or current.

These riser regions are also prone to damage during magnet winding and assembly processes, as the individual ribbons are brittle and bend easily if mishandled. In addition, costly, precision-machined parts must be made in a free-end circuit to guide and support the free ends as they exit the winding package into the splice attachment.

Another problem that can occur in superconducting magnets is the disruption of superconductivity. Superconductivity breakdown occurs when part of a superconducting wire or coil goes into a resistive state.

This may be due to fluctuations in temperature or magnetic field, or physical damage or defects in the superconductor (for example, due to neutron exposure if the magnet is used in a fusion reactor). Due to the high currents present in the magnet, when even a small part of the superconductor becomes resistive, it quickly heats up. As mentioned above, the superconducting wires are provided with some kind of copper stabilizer to protect them from breaking the superconductivity. Copper provides an alternative current path if the superconductor becomes normal. The more copper is present, the slower the temperature rises in the hot spot that forms around the broken superconductivity region of the conductor.

Therefore, there is a need for an HTS magnet that eliminates or alleviates some or all of these disadvantages.

The essence of the invention

It is an object of the present invention to provide an HTS magnet that solves or at least alleviates the problems described above.

According to a first aspect of the invention, an HTS magnet is provided. The HTS magnet comprises a coil formed by nested concentric windings, each winding containing an HTS material; and a conductive element comprising an electrical contact surface through which an electrical current is supplied to a portion of at least one of the windings. This surface provides electrical contact between the conductive element and the axial edge of the coil practically along the path of said at least one of the windings.

Each winding may comprise an HTS tape and a sheath electrically connected to the HTS tape, electrical contact being made with the sheath.

The electrical contact surface may provide electrical contact with the axial edge of the coil over more than 20%, more than 50%, or more than 80% of the path of said at least one of the windings. The electrical contact surface may be annular.

The HTS magnet may include a plate extending along one or more of the other windings, the conductive element integral with or provided on the plate. The conductive element may protrude from the face of the plate, and the plate further comprises a dielectric or electro-resistive layer for electrically isolating that face of the plate from a portion of one or more of the other windings.

As used herein, the term electrical resistive layer means a layer that has an electrical resistance greater than the electrical resistance between the conductive element and the coil and the electrical resistance between the turns of the coil (ie, the radial electrical resistance of the coil). However, the electrical resistive layer may be thermally conductive, thereby allowing more efficient transfer of heat from (or to) the coil. The electrical resistive layer may or may not be a dielectric layer. A non-dielectric but electrically resistive layer may be preferred in cases where the dielectric would be susceptible to radiation damage, such as when the coils are part of a tokamak fusion reactor.

The HTS magnet may include an intermediate conductive layer passing through one or more other windings to transfer heat and/or electric current from the edge of said or each winding. The intermediate conductive layer may comprise brass and/or stainless steel. Other solderable metals can also be used, i.e. metals that solder can adhere to in order to make electrical contact. The intermediate conductive layer can be patterned (structured) by changing its thickness, for example, to obtain a web-like pattern.

The coil may contain electrical insulation between the windings.

The HTS magnet may include one or more sensors and/or one or more heaters located between the plate and the coil.

The electrical contact surface may provide electrical contact with either the innermost or outermost winding of the coil. The electrical contact surface may provide electrical contact through a discontinuity (discontinuity) in the windings. For example, if the coil is formed from two lengths of HTS tape, the electrical contact surface can act as an electrical interface to connect the tapes in series.

The HTS magnet may further comprise another conductive element comprising an electrical contact surface for receiving electrical current from a portion of the other at least one of the windings. This surface provides electrical contact with said or another axial edge of the coil practically along the path of the other at least one of the windings. The electrical contact surfaces may provide electrical contact with opposite sides (faces) of the coil.

The HTS magnet may further comprise one or more additional coils, said or each additional coil having conductive elements for providing electrical contact with opposite sides (faces) of this coil, the coils being axially stacked and electrically connected to each other through their respective conductive elements. Adjacent axially stacked coils may be wound in opposite directions.

The HTS magnet may comprise two or more concentrically nested coils each having respective conductive elements, each coil being electrically connected to an adjacent coil by an electrical connection between the respective conductive elements.

- 2 040624 coils. This electrical connection may be flexible to accommodate the movement of the coils relative to each other. The HTS magnet may include one or more intermediate supports positioned between adjacent coils to resist radial forces.

The respective HTS tapes of adjacent coils may differ in one or more of the thicknesses; composition; width and number.

According to a second aspect of the present invention, there is provided an HTS magnet comprising first and second coils, each coil being formed by nested concentric windings, and each winding containing HTS material; and first and second conductive elements, each conductive element providing an electrical connection between the coils. Each conductive element comprises a first electrical contact surface through which electrical current is transmitted to and from a portion of at least one of the windings of the first coil; and a second electrical contact surface through which electrical current is transmitted to or from a portion of at least one of the windings of the second coil. Each surface provides electrical contact between the respective conductive element and the axial edge of the respective coil substantially along the path of said at least one of the windings.

The electrical resistance of the electrical connection provided by the first conductive element divided by the electrical resistance of the electrical connection provided by the second conductive element may be greater than 1.5, greater than 3, or greater than 10. The electrical contact surface areas of the second conductive element may be greater than the electrical contact surface areas of the first conductive element.

The first conductive element may be positioned radially outward from the second conductive element. This may allow the first conductive element to be placed in a region of lower magnetic field.

The first or second conductive element may comprise a variable resistor or switch. The variable resistor or switch may comprise HTS material.

According to a third aspect of the present invention, there is provided a tokamak comprising the HTS magnet described above. The HTS magnet is configured to provide a toroidal magnetic field or a poloidal magnetic field.

According to a fourth aspect of the present invention, there is provided a method for generating a semi-continuous current in the HTS magnet described above. The method includes preparing each of the coils in a superconducting state; connecting the power supply in parallel with the coils and disconnecting the power supply.

The second conductive element may comprise an HTS material, and the method may include, after connecting a power source in parallel with the coils, converting the HTS material from a normal state to a superconducting state.

According to a fifth aspect of the present invention, there is provided a method for making an electrical connection to a HTS magnet comprising a coil formed by nested concentric windings, each winding containing an HTS material. The method includes applying a dielectric or electro-resistive layer to partially cover the face of the coil; applying the conductive plate to the dielectric or electro-resistive layer and forming an electrical contact between the conductive plate and the axial edge of the coil substantially along the path of at least one of the windings.

The method may further include depositing an intermediate conductive layer between the dielectric or electrical resistive layer and the coil, the intermediate conductive layer extending along one or more of the other windings to transfer heat or electric current from an edge of said or each winding.

According to a sixth aspect of the present invention, there is provided a conductive plate for supplying current to an axial edge of a coil formed by nested concentric windings. The conductive plate contains an annular conductive element integral with the plate or provided on it. The conductive element includes an electrical contact surface to provide electrical contact between the conductive element and the coil. The conductive element further comprises a dielectric or electro-resistive layer on the conductive plate to provide an electrically insulating barrier adjacent to the electrical contact surface.

The conductive plate may further comprise an intermediate conductive layer partially or completely extending over the dielectric or electro-resistive layer. The intermediate conductive layer is configured to transfer heat or electric current from the edge of said or each winding.

According to a seventh aspect of the present invention, there is provided a method for manufacturing a conductive plate for supplying current to an axial edge of a coil formed by nested concentric windings. The method includes providing an annular conductive element integral with or provided on the plate, the conductive element comprising an electrical contact surface for providing electrical contact between the conductive element and the coil; And

- 3 040624 curing the fiber-resin composite on the conductive plate to form a dielectric or electro-resistive layer on the conductive plate to provide an electrically insulating barrier adjacent to the electrical contact surface.

Curing may include heating the composite to a target temperature, maintaining the composite at the target temperature for a period of time, and cooling the composite.

The heating rate may be less than 1°C per minute, preferably less than 0.3°C per minute. The cooling rate may be less than 1°C per minute, preferably less than 0.4°C per minute. The target temperature may be greater than or equal to 180°C. The period may be greater than 1 hour and preferably greater than 2 hours.

Also described herein is a method for forming an electrical and/or thermal (thermal) connection to a copper surface, including providing a silver layer on the copper surface and providing an indium layer on the silver surface, whereby an electrical and/or thermal connection can be formed to the indium layer. Also described here is an electrical and/or thermal connection containing a copper surface, a silver layer and an indium layer, with the silver layer located directly between the copper surface and the indium layer.

Brief description of the drawings

Fig. 1 is a schematic perspective view of a prior art HTS tape;

fig. 2 is a schematic plan view of a joint with a free end of the prior art;

fig. 3A and 3B are schematic plan views of HTS magnets;

fig. 4 is a schematic sectional view of an HTS magnet;

fig. 5 is a schematic sectional view of an HTS magnet;

fig. 6 is a schematic view in radial section of a HTS magnet with a radial joint;

fig. 7 is a schematic radial sectional view of an HTS magnet with multiple stacked coils;

fig. 8 is a schematic sectional view of another HTS magnet;

fig. 9 is a schematic sectional view of the HTS magnet of FIG. 8 showing the electric current flowing through the magnet when the power supply is connected in parallel with the coils; and fig. 10 is a schematic sectional view of the HTS magnet of FIG. 8 and 9 showing the electric current flowing through the magnet after the power supply has been disconnected.

Detailed description of the invention

Here, a solution to the above problems is proposed in which the electrical connection to the coil of the HTS magnet is made through the axial edge of the coil so that electric current can be supplied or received through the face of the coil. This form of connection allows the stack of HTS tape windings to be kept tight, so that none of the HTS tapes must be pulled away from the coil. For example, the electrical connection may be provided by a ring-shaped conductor located over the face of the coil, with the conductor in contact with the upward facing edge of the winding around the circumference of the coil. This arrangement or ring connection (ring joint) can be used to minimize the risk of point failures in the magnet, both during assembly and during operation. It also allows the HTS coils to be energized or current to be drawn from the HTS coils without the need for risers, eliminating the need for many ancillary parts, reducing cost and complexity, and simplifying the manufacture of HTS magnets. Such connections or junctions may also improve the performance of HTS magnets, as described below.

Although this document makes reference to certain directions (eg, up, down) or uses relative terms (eg, above, above, below, etc.), it should be understood that such terms are used simply for the purpose of providing examples of the concepts described here. . Likewise, although the disclosure is illustrated by way of example with reference to disk spools (also sometimes referred to as plate spools), i.e. generally flat coils formed by nested concentric windings, it will be understood from the discussion below that the disclosure is not limited to such coils.

Incorporating circumferential joints into larger structures (described below as Electro-Thermal Interface, ETI, plates, or ETI-plates) also allows thermal connections, electrical insulation, and sensors, which are often traditionally installed, to be provided in a single unit. magnet separately. This simplifies the assembly process and allows these components to be manufactured independently of the HTS coil.

Fig. 3A and 3B show schematic plan views of two possible embodiments of an annular joint 300A, 300B for disc coil 301.

Coil 301 contains nested concentric windings of HTS tape 100 in a predominantly flat arrangement. The HTS tape 100 is wound face to face so that opposite edges of the tape 100 are projected along the reel axis 303 . Each complete winding corresponds to one complete revolution (coil) of the HTS tape 100 around the coil axis 303. The start and end points of the outermost winding are indicated in FIG. 3A at 301A and 301B.

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Circumferential joints 300A, 300B are formed by respective ring conductors 304A, 304B. For the sake of clarity, ring conductors 304A, 340B are shown behind coil 301 to show coil windings. Each ring conductor 304A, 304B comprises a ring-shaped structure or ring made of a conductive material, preferably a metal such as copper. Ring conductors 200A, 200B contact the top or bottom edge of the windings in order to make electrical contact with coil 201. Ring conductor 300A is located on the outer radius of coil 201, while ring conductor 200B is located on the inner radius of coil 201. Each ring conductor 200A, 200B covers only a portion of the windings so that electric current can be supplied to one end of the coil 201 and thereby circulate through the windings.

Because each of the ring conductors 300A, 300B makes electrical contact with different ends of the HTS tape 100, they can be used in pair to drive an electric current radially from the inside out of the coil 301 (or vice versa). For example, coil 201 may be provided (placed) between a pair of ring conductors 304A, 304B so that current can be supplied to one face (face) of coil 301 (for example, the top) by one ring conductor 304A, pass through the windings of coil 301 in order to to create a magnetic field, and then be received from the other face (face) of the coil by another ring conductor 304B.

The radial widths of the ring conductors 304A, 304B are chosen to compromise the joint resistance with the number of turns between joints.

The resistance of the joint can be reduced by making the ring conductor 304A, 304B wider to cover more turns of the coil 301. However, the result is that the magnetic field produced by the magnet per unit of current is reduced as the number of turns carrying the total current of the magnet is reduced. If the radial width decreases, then the opposite is true.

Since the annular joints can be opposed by distances on the order of the circumference of the coil, low resistance joints can typically be made with radially narrow ring conductors 304A, 304B that do not significantly reduce the field produced by the magnet. Although ring conductors 304A, 304B in FIG. 3A and 3B are shown to extend slightly beyond the outer/inner edges of coil 301, the shape of the ring conductors may alternatively be more closely aligned with the radial profile of coil 301 to minimize the radial indentation of coil 301 and ring joint 300A, 300B.

Although in FIG. 3A and 3B use round disc (biscuit) coils to illustrate the features of the ring splices 300A, 300B, it will be readily understood that these types of splices can be applied to other coil shapes such as D-shaped toroidal field coils such as those used in the tokamak. . In such cases, circumferential splices 300A, 300B need not be circular, but may be shaped to follow the path of the coil windings. Likewise, it is not necessary that the ring conductors 304A, 304B follow the entire path of the coil windings, and instead they may only partially follow the path (path) of the coil windings. For example, for magnets with large radii and/or those containing thick HTS tape, it may be possible to form a low resistance joint using a ring conductor that only travels 20%, 50%, or 80% of the way along the path of the windings, i.e. so that the ring conductor passes through an angle of less than 360°. Introducing such a discontinuity into the ring conductor (because it does not travel completely along the path of the windings) can preferably eliminate the formation of stray current loops in the ring conductor, which can be useful in applications such as nuclear magnetic resonance (NMR) or magnetic resonance imaging ( MRI). In other applications such as the tokamak (see below), for example, geometric constraints and/or the presence of other components may necessitate such a break.

The girth joints may be provided as part of a larger structure, which may be referred to as a conductive plate or an electrothermal interface (ETI) plate. Such ETI plates are generally composite metal/insulator/sensor plates that, as discussed below, can be attached to the edge faces of HTS coils to perform a number of functions:

means for creating electrical interfaces with and/or between the HTS coils;

means for introducing a controlled resistor in parallel with the HTS coil to achieve partial isolation.

The geometry of the resistor can be chosen to control the dynamic electrothermal behavior of the coil;

means for forming a thermal interface with the HTS coil for cooling;

means for introducing thin electrical insulation between the HTS coils and mechanically protecting such insulation from damage;

means for incorporating auxiliary elements (such as sensors or heaters) into the HTS magnet without disturbing the HTS windings.

Fig. 4 shows a cross-section of an exemplary magnet 400 comprising two ETI plates 400A, 400B applied to a single disc coil 401. In this example, coil 401 includes two lengths of HTS tape 100 wound together around magnet axis 303. 100 tapes are sheathed in 101 copper

- 5 040624 as a type 0 pair (as described, for example, in WO 2018/078326), so that each winding includes two tapes. An insulator 402 is provided between the windings of the HTS tapes 100 to prevent electrical current from flowing between the faces of the HTS tape, i. so that an electrical current injected at one end is forced to flow through the windings of coil 401. Each of the ETI plates 400A, 400B includes a respective ring conductor 404A, 404B forming an annular junction with either end of the HTS tapes 100. In this In this example, electrical current is applied to the coil 401 through the radially innermost end of the HTS tapes 100 using the lower ETI plate 400V. The current flows through the series windings of the coil 301 before being received by the top ETI plate 400A through the ring conductor 404A at the radially outermost end of the HTS tapes 100.

Although ETI plates 400A, 400B are in electrical contact with coil 401 only through ring conductors 404A, 404B, the plates themselves extend radially through the coil to form a base conductor layer 405A, 405B through which current is supplied to (or received from) the ring joints. and provides a heat conduction path away from the coil 301. In this example, the main conductors 405A, 405B are copper, although other conductive materials (eg, metals) may also be used. Ring conductors 404A, 404B may be integral with or attached to main conductors 405A, 405B, such as by soldering.

By making ring conductors 404A, 404B on (or integral with) ETI plates 400A, 400B, they can be made very narrow in their radial size (submillimeter if necessary) yet still easy to handle in a manner that it would be difficult to achieve if the ring conductor were an independent element. The large surface area provided by the main conductors 405A, 405B allows efficient heat dissipation from the coil 401 and provides more flexibility in where the electrical connections to the magnet can be placed.

The main conductor 405A, 405B in the ETI plates 400A, 400B can be made thin to minimize temperature rise across it and ensure that the coils are not spaced too far apart in the axial direction to avoid weakening the magnetic field. Alternatively, heat can also be extracted radially into the cooling bar at the outer or inner diameter of the coil 401, in which case the main conductor 405A, 405B must be made thick enough to meet the temperature requirements. ETI plates 400A, 400B may also be designed to provide more efficient cooling of one or more of the coil faces 401 (ie, without relying on heat conduction to the coil's radial edges). For example, ETI plates 400A, 400B may include channels or tubes through which gaseous or liquid refrigerant must flow in order to carry heat away from coil 401. Preferably, channels or tubes may be provided on or in one or more main conductors 405A. , 405V.

The use of ETI 400A, 400B cooling plates provides an alternative approach to methods such as thermal pastes, which have poorer thermal conductivity than solder and can be difficult to apply in thin layers properly and complicate the manufacturing process.

The ETI wafer 400A, 400B also includes a dielectric layer 406A, 406B to electrically isolate the edges of the HTS tape 100 from the base conductor portions 405A, 405B of the wafers. The dielectric layers 406A, 406B are made of a dielectric material, such as a glass fiber/resin composite such as prepreg.

ETI plates 400A, 400B further include an intermediate conductive layer 407A, 407B that can be soldered to coil 401 to provide excellent thermal and electrical contact. This layer acts as a radial resistor to control the thermal and electrical behavior of coil 401. This partial isolation, i. the introduction of a controlled turn-to-turn resistor provides the desired balance between thermal stability and coil rise time in HTS coils. The intermediate conductive layer is made of a conductive material, preferably brass or stainless steel, since this material can be soldered and has a much higher electrical resistivity than copper, allowing it to be thicker and thus more easily adjustable in thickness. Such partial isolation (from the English. Partial Insulation, PI), i.e. the introduction of a controlled turn-to-turn resistor provides the desired balance between thermal stability and coil rise time in HTS coils. In particular, the use of a layer passing through the windings of the coil can eliminate the need for other types of partial insulation, such as embedded layers of metals such as stainless steel.

In some cases, the intermediate conductive layer 407A, 407B may be bonded to the dielectric layer 406A, 406B with an adhesive. However, the adhesive must be able to withstand soldering temperatures without becoming structurally weakened, otherwise there is a tendency for these layers to peel off during soldering. One way to solve this problem is to use a fiberglass/resin composite such as prepreg as both the dielectric and the bonding medium. For example, a composite such as Prepreg MTC400 can be used,

- 6 040624 manufactured by SHD Composites. By performing a relatively long curing process, it is possible to raise the glass transition temperature (Tg) of the composite above the typical coil brazing temperature. For example, the composite can be post-cured (i.e., post-cured) by heating at a rate of about 0.3°C/min to 180°C, holding for 2 hours, and then cooling at a rate of about 0.3°C/min. This procedure can, for example, reach a Tg of 200° C., which allows the composite to withstand most brazing processes that occur at lower temperatures. However, it may not be possible to perform such a curing treatment with an in-situ ETI plate on a spool, because elevated temperatures and times would damage the spool (due to the continuous degradation of HTS tapes that occurs with temperature and time) and possible damage or deterioration of any solder joints that have already been made.

Although bare coils can also be used, the introduction of an insulator 402 increases the effectiveness of the turn-to-turn resistance by blocking the alternative low resistance path between the windings through the copper jacket of the HTS tapes 100.

The thickness of ETI plates 400A, 400B (i.e. total thickness including ring conductor) is typically in the range of 0.25-1.00 mm, while the thickness of the dielectric layer (if present) is typically in the range of 10-100 µm , and the thickness of the intermediate conductor (if any) is typically in the range of 10-100 µm.

The solder used to bond the interlayer 407A, 407B to the coil windings 401 is typically tin-lead (PbSn). However, this material is so conductive that even a thin coating of PbSn on the interlayer 407A, 407B will provide a very low resistance current path, forcing the current to bypass the interlayer 407A, 407B. To eliminate this problem, a high resistivity material is selected as the solder material, preferably one that has a higher resistivity than PbSn, such as solder whose resistivity is more than 10 times that of PbSn when the coil is used in magnet, for example, at a lower temperature than the critical temperature of the ReBCO tape. For example, the solder may be made from PbBi because this material has a resistivity that is about 50 times that of PbSn. Likewise, PbBiSn may also be used. The higher resistance of the lead-bismuth (PbBi) or tin-lead-bismuth (PbBiSn) solder coating (compared to the PbSn solder coating) means that more current flows in the intermediate layer 407A, 407B.

Partially insulating ETI plates offer the advantage of high flexibility. The turn-to-turn resistance can be adjusted by changing the thickness and composition of the intermediate conductor. The geometry of the intermediate conductor layer can be modified using lithography, such as etching fragmented cobweb patterns to cut off long range radial currents or impedance-providing inductive coils, both of which can allow for a balance between charge times and thermoelectric stability.

Thin electrical insulation between the disk coils is desirable to provide the necessary dielectric properties without introducing too much temperature rise caused by heat flowing through it. However, since many conventional dielectric materials (such as polyimide sheets) are soft, they are vulnerable to puncture from electromagnetic stresses encountered during magnet operation and assembly. By embedding the insulation inside the ETI plate, with metal sheets protecting the insulation on both sides, the risk of puncture is minimal.

Because the ETI plate is a separate entity from the coil, it can be changed to change the behavior of the coil. For example, you can initially attach ETI plates with a thick intermediate conductor to ensure safe operation of the coil and determine its critical current. Once the maximum operating current is determined, the ETI plate can be replaced with a plate that allows the magnet to be changed more rapidly within a known range of magnet characteristics.

Fig. 5 shows a cross section of a magnet 500 comprising two axially stacked magnets 400 each containing a disk coil 401A, 401B. Thermal and electrical connections between coils 401A, 401B can be made by bonding the base copper layers of adjacent ETI plates 402A, 402B together after stacking. This can be done either by axial pressing (i.e., along the axis 503 of the magnet) and the optional addition of a compliant conductive layer 504, such as an indium layer, between the magnets 400, or by soldering (although this requires heating the entire magnet). The NanoBond® method may also be used, in which a laminated foil is inserted between the ETI plates 402A, 402B and a heat generating chemical reaction is initiated in the foil to seal the foil to each of the plates.

In order for thermal and/or electrical interfaces between adjacent ETI plates 402A,

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402B are effective, it is preferable that the surface condition of the plates be good, for example clean of oxides. One way this can be achieved is by providing the base copper layer(s) of the ETI plates 402A, 402B with a layer (eg, coating) of a noble metal such as silver. The silver layer is preferred because silver has a low affinity for oxygen and is also chemically compatible with indium, allowing high quality pressed joints to be created using the compliant indium layer 504.

Additional plates 505A, 505B, such as copper plates, may be provided axially on either end of magnet 500 to provide additional cooling or electrical connections to magnet 500.

A power source (not shown) is connected to the plates 505A, 505B to supply electrical current to the magnet 500. In this example, the circumferential joints on each of the faces (faces) of the magnet are located on the radially outer edges of the coils, while the circumferential joints connecting the two coils , located on the radially inner edges of the coils. Therefore, the electric current flows radially inward through the windings of one of the coils, then axially between the coils, and then radially outward through the windings of the other coil. Because current flows in opposite directions through each coil, coils 401A, 401B are wound in opposite directions (i.e., clockwise/counterclockwise) so that the magnetic field generated by each of the magnets 400 has the same polarity, thereby allowing generate very large magnetic fields. For example, coils 401A and 401B may be prepared identically (i.e., wound in the same direction) and then one of the coils is reversed relative to the other before being stacked to form magnet 500. It will be appreciated that others may be stacked in the same manner. disc reels with additional ETI plates between them.

As discussed above, ETI plates may include channels or tubes through which a gas or liquid will flow in order to transfer heat from the coil(s). This embodiment is particularly effective for cooling axially stacked magnets 400, such as those shown in FIG. 5. In particular, the presence of cooling channels or tubes in the ETI plates 402A, 402B located between the coils allows more efficient heat transfer from the body of the coils 401A, 401B.

Ring conductors in ETI plates can also be used to create electrical interfaces between nested/concentric disc coils, i.e. joints that carry current in the radial direction rather than in the axial direction as described above. For example, a disk coil can be enlarged by wrapping a second piece of HTS tape around it, and in such a case, an electrical connection can be made between the ends of two HTS tapes using a ring conductor, preferably in an ETI wafer, i. the ring conductor is used to provide a joint through a discontinuity or break in the windings. An example of where this can be useful is to reduce stress in deep coils (those where the outer radius divided by the inner radius is large, eg greater than ~3). In such a situation, it is useful to subdivide the coil into multiple nested coils and cut off the forces generated by each of them, preventing the accumulation of mechanical stress in the windings. In such a case, a radial joint can be made between the nested coils using a suitable ring conductor in the ETI plate.

Fig. 6 shows a radial section of an HTS magnet 600 with two ring conductors 604A, 604B providing radial junctions between the inner coil 601 and the outer coil 602. A mechanical support 607, such as a cylinder, is provided between the inner and outer coils 601, 602 to stop (intercept) radial forces between them.

Fig. 7 shows a radial section of an HTS magnet 600 which is capable of generating very strong magnetic fields in a compact, reliable and simple manner.

HTS magnet 700 comprises radially nested stacks 701, 702, 703 of HTS magnets 400 described above. For example, the radially outermost stack contains axially arranged HTS magnets 400A-F, with adjacent magnets electrically connected via their respective ETI plates. The annular junctions formed by the ETI plates are positioned so that current flow alternates between axial flow (between adjacent coils) and radial flow (along the windings of each coil) as indicated by the arrows superimposed on the HTS magnets 400A-F in FIG. 6. With regard to the HTS magnet 400 described with reference to FIG. 4, coils of adjacent magnets are wound in opposite directions so that the magnetic field along magnet axis 704 is maximized. The other two nested stacks 702, 703 of magnets are similarly configured to enhance the magnetic field generated by the outer stack 701. Radial joints/connections 705, 706 are formed between the end plates of ETI stacks 705, 706 to allow electrical current to flow from one stack, flowing in to the next one. In the example shown in FIG. 6, current is applied to magnet 700 through the top ETI plate of topmost HTS magnet 400A of outer stack 701. After flowing through stack 701 as described above, current then flows radially through joint 705 into outer nested stack 702. Similarly, after flowing through this stack 702, the current then flows radially into the inner nested stack 703 through the joint 706. Finally, after flowing through the inner nested stack 703, the current exits the magnet 700 through the lower ETI plate of the lowermost HTS magnet of the inner nested stack 703.

Although the exemplary magnet 700 shown in FIG. 7, there are three stacks 701, 702, 703 each with 6 HTS coils, of course, any number of stacks and/or coils can be used. The stacks may also have varying numbers of coils to provide greater flexibility in magnet design.

Composing the HTS magnet 700 from many smaller HTS magnets (coil subdivision) provides a number of advantages. In particular, as described below, coil subdivision overcomes stress limitations of HTS tapes and provides a more optimal magnet design by grading the subdivided coils according to their location in magnet 700.

Given the mechanical stress limitations of HTS tapes, the maximum allowable transverse tensile strength of HTS tapes 100 is typically around 10-50 MPa, which provides a practical limit on the radial depth of windings that can be used. However, this problem can be avoided by subdividing the coil by dividing the coil into several radially nested coils, as shown in FIG. 7 and inserting mechanical supports 707, 708 between the coils. Likewise, axial forces can be suppressed by subdividing along the magnet axis and inserting axial support structures (not shown).

Given the gradation of subdivided coils in high field magnets, the magnetic field vector can vary significantly depending on the radial and/or axial location within the magnet. For HTS magnets, this means that the parameters that characterize the operation of a magnet as a superconductor, such as critical current, also depend on location. Therefore, a more optimal design of the magnet 700 can be obtained by grading the HTS tape 100 according to where it is in the magnet. For example, to achieve the highest possible magnetic field and to control the breakdown of superconductivity, it is desirable to maximize the ratio (gamma) of current to critical current (I/I _c ) throughout the magnet. Grading can be done by varying the number of HTS tapes per turn/winding, the width or thickness of the tape, or the type of tape (ie manufacturer or HTS materials used).

It is often the case that sensors (such as temperature or strain sensors) must be embedded in HTS magnets for monitoring purposes. Other elements, such as heaters, may also be required to protect against superconductivity breakdown. It is desirable that the HTS coil and accessories be separate entities so that they can be manufactured separately and replaced independently in the event of failure. A suitably adapted ETI wafer can be configured to accommodate many sensors or other elements that do not need to be built directly into the HTS coil windings.

Fig. 8 shows a section through an HTS magnet 800, which is similar to the HTS magnet 500 shown in FIG. 5, except that each of the adjacent ETI plates 801A, 801B includes an inner ring conductor 804A, 804B and an outer ring conductor 805A, 805B.

As shown in FIG. 9, the inclusion of two ring conductors on each plate provides two alternative paths 908, 909 for current to flow through magnet 800 when a power supply 907 is connected to the coil pair. One path 908 is the same as the path described with reference to FIG. 4, i.e. current flows through the windings of each coil 400 in turn, through the inner ring conductors 805A, 805B. Another path 909 shorts or bypasses path 908, providing an electrical connection between the outer windings of the coils 400. In this case, current flows into one of the coils 400 through its outer winding(s) and out of the coil through the outer ring conductor 804B instead of passing through the others. coil windings. Similarly, current flows into the outer windings of the other coil through the outer ring conductor 804A, between their axial edges, and out of the coil, without passing through the other windings of the other coil.

The proportion of current flowing in each path 908, 909 is determined by the relative electrical resistances of the paths, which can be controlled by varying the electrical resistances of the outer and inner ring conductors 804A, 804B, 805A, 805B and/or the electrical resistances associated with the electrical contacts of the ring conductors with the windings coils 400. The longer, bypass path 908 can be made preferable to the shorter, straighter path 909 so that more of the current flows radially in and out of the coils 400, making the electrical resistance of the straight path 909 much greater than that of the bypass path 908. This is achieved , for example, by making the areas of the outer ring conductors 804A, 804B smaller than the areas of the inner ring conductors 805A, 805B. Although a small amount of leakage current travels along path 909 and does not contribute to the magnetic field created by the current flowing through the windings of coils 400, coils 400 can be charged to full critical current on the internal windings and additional leakage current can be carried by the external windings because they have a higher critical current in this region of lower magnetic field. Once the magnet 800 has been charged, the power supply 907 may

- 9 040624 be disabled so that current flows through the coils 400 in a closed loop.

Fig. 10 shows the path 1010 of the trapped current flowing within the magnet 800 after the power supply 907 is turned off. In this case, the current flows in a closed loop through the coil windings 400 and through each of the ring conductors 805B, 805A, 804A, 804B in turn. Because coils 400 are superconductive, current is able to flow along path 1010 for extended periods before decaying, i. the circulating current and the magnetic field it creates are semi-undamped.

The time constant associated with the damping of the circulating current is determined by the ratio (L/R) of the magnetic self-induction of the coil (L) to its electrical resistance (R). Consider, for example, a magnet that has coils with an inner diameter of 50 mm and an outer diameter of 98 mm, a self-induction of ~2 mH, and electrical junctions of typical resistance of ~1-5 nΩ in liquid nitrogen (i.e., the calculated total resistance of internal and external electrical junctions ~10 nΩ). The time constant L/R of this magnet will be approximately equal to 200000 s, i.e. 2.3 days.

A large time constant is also associated with a large charging time, i.e. the time until a stable state of current sharing between paths 908, 909 is established when the power supply is connected. To minimize charging time, it is beneficial to increase the resistance of path 908 during charging (ie, in the configuration shown in FIG. 9). This can be achieved with a variable resistor or switch built into the external electrical connection provided by the outer ring conductors 804A, 804B. For example, between the outer ring conductors 804A, 804B, an HTS switch containing HTS material may be provided. During charging, this switch is off (in a non-superconducting state), providing high resistance and hence fast charging. This is achieved, for example, by heating the HTS material above its critical temperature. The switch is then turned on (eg no longer heating or rather cooling) to close the current path 1010 and the power supply is disconnected.

One important application of HTS magnets, such as those described above, is in a type of fusion reactor known as a tokamak. The tokamak combines a strong toroidal magnetic field, high plasma current and usually large plasma volume, and significant auxiliary heating to produce a hot stable plasma. This allows tokamaks to create conditions under which nuclear fusion can occur. Auxiliary heating (for example, by injecting a beam of tens of megawatts of neutral particles of high energy hydrogen, deuterium or tritium) is necessary to increase the temperature to high enough values required for nuclear fusion to occur and/or to maintain plasma current.

The magnetic coils on a tokamak can be divided into two groups. Poloidal field coils are horizontal circular coils wound with their center lying on the central column of the tokamak and produce a poloidal field (i.e., a field that is substantially parallel to the central column). The coils of the toroidal field are wound vertically through the central column and around the outer part of the plasma chamber (so-called bypasses or reverse branches) to create a toroidal field (ie, a field that is circular around the central column). The combination of poloidal and toroidal fields creates a spiral field inside the plasma chamber, which keeps the plasma in a limited space.

The currents required to create a toroidal field are very high. Therefore, the designs of tokamaks increasingly imply the use of superconducting materials in field excitation coils. For a compact spherical tokamak, the diameter of the central column should be as small as possible. This presents conflicting requirements because the current density that can be achieved even with superconducting materials is limited.

The HTS magnets described herein are particularly suitable for use in tokamaks, in particular spherical tokamaks, for example, to create one of the poloidal or toroidal fields (or both).

Although various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example and not limitation. It will be apparent to those skilled in the art(s) that various changes in form and detail may be made without departing from the spirit and scope of the invention. For example, although the coils above have been described as having HTS tapes 100 arranged in a type 0 configuration, other configurations such as type 1 and type 2 can also be used (as described, for example, in WO 2018/078326). Similarly, although in the examples described above, the coils are usually connected to the power supply in series, it is also possible to connect the coils to the power supply in parallel. Thus, the present invention should not be limited by any of the above-described exemplary embodiments, but should only be determined in accordance with the following claims and their equivalents.

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Claims (21)

CLAIM 1. A high temperature superconductor (HTSC) magnet comprising a coil formed by nested concentric turns wound around a coil axis, each turn containing an HTS material; and first and second conductive elements, each of which contains respective electrical contact surfaces through which electrical current is supplied to or electrical current is received from a portion of the corresponding at least one of the turns, each electrical contact surface providing electrical contact between the conductive element and the axial edge coil, at least partially, along corresponding at least one of the turns, while the electrical contact surface of the first conductive element is located closer to the axis than the electrical contact surface of the second conductive element, due to which the electric current supplied to the coil through the first conductive element , flows through successive turns of the coil before being received by the second conductive element. 2. The HTS magnet according to claim 1, wherein each turn comprises an HTS tape and a sheath electrically connected to the HTS tape, and electrical contacts are made to the sheath. 3. HTS magnet according to claim 1 or 2, wherein the electrical contact surfaces provide electrical contact with one or more of the axial edges of the coil along more than 50% or more than 80% of the path of the corresponding at least one of the turns. 4. An HTS magnet according to any one of the preceding claims, wherein one or more of the electrical contact surfaces are annular. 5. An HTS magnet according to any one of the preceding claims, comprising at least one plate extending along one or more of the other turns, the first or second conductive element being integral with or provided on the plate. 6. The HTS magnet of claim 5, wherein the first or second conductive element protrudes from the face of the wafer, and the wafer further comprises a dielectric or electroresistive layer for electrically isolating the face of the wafer from a portion of one or more of the other turns. 7. The HTS magnet of claim 6, comprising an intermediate conductive layer extending along one or more other turns to transfer heat and/or electric current away from the edge of the turns. 8. The HTS magnet according to claim 7, wherein the intermediate conductive layer comprises a solderable metal such as brass and/or stainless steel. 9. HTS magnet according to claim 7 or 8, wherein the intermediate conductive layer is patterned by changing its thickness. 10. An HTS magnet according to any one of claims 7 to 9, wherein the coil comprises electrical insulation between the turns. 11. HTS magnet according to any one of claims 5 to 10, comprising one or more sensors and/or one or more heaters located between the plate and the coil. 12. An HTS magnet according to any one of the preceding claims, wherein the electrical contact surface of the first or second conductive element makes electrical contact with either the innermost or outermost turn of the coil. 13. An HTS magnet according to any one of the preceding claims, wherein the electrical contact surface of the first or second conductive element provides electrical contact through a discontinuity in the coils. 14. An HTS magnet according to any one of the preceding claims, wherein the electrical contact surfaces provide electrical contact with opposite faces of the coil. 15. An HTS magnet according to any one of the preceding claims, further comprising one or more additional coils, said or each additional coil having conductive elements for providing electrical contact with opposite faces of this coil, the coils being axially stacked and electrically connected to each other. with a friend through their respective conductive elements. 16. The HTS magnet of claim 15, wherein adjacent axially stacked coils are wound in opposite directions. 17. An HTS magnet according to any one of the preceding claims, comprising two or more concentrically nested coils, each having respective conductive elements, each coil being electrically connected to an adjacent coil by an electrical connection between the respective conductive elements of the coils. 18. The HTS magnet according to claim 17, wherein the electrical connection is flexible. 19. HTS magnet according to claim 17 or 18, containing one or more intermediate supports located between adjacent coils, to suppress radial forces. 20. HTS magnet according to any one of claims 17 to 19, wherein the respective HTS tapes of adjacent coils differ in one or more of thicknesses; composition; width and number. 21. A tokamak containing an HTS magnet according to any one of the preceding claims, wherein the HTS magnet -