Superconducting magnet and alternating magnetic field generator

Abstract

Provided is a superconducting magnet that can be operated with AC in a state where AC loss is reduced. This superconducting magnet includes: a superconducting coil 102 that is housed in a cryostat 101; and a superconducting current transformer 103 that is housed in the cryostat 101 and includes a primary winding 104 connected to an AC power supply 109 and a secondary winding 105 connected to the superconducting coil 102. The superconducting coil 102 and the secondary winding 105 are configured by winding a superconducting cable 20 in which superconducting wires 10, 90 are spirally wound along the axis of a core material 9.

Description

Superconducting magnets and AC magnetic field generators

[0001] The present invention relates to a superconducting magnet and an AC magnetic field generating device equipped therewith.

[0002] High-temperature superconductivity is attracting attention as a technology for efficiently generating, transmitting, converting, using, and storing electrical energy. For example, by using wires made from high-temperature superconducting materials (hereinafter referred to as high-temperature superconducting wires) or superconducting cables made by assembling them as coils (windings) of superconducting magnets, it becomes possible to pass large currents through the coils. This makes it possible to generate high-intensity magnetic fields that cannot be obtained with normal conducting magnets.

[0003] On the other hand, when superconducting wires are used with alternating current, AC losses occur due to the alternating magnetic field. Generally, when an electric current flows through a superconductor or a magnetic field is applied, magnetic flux penetrates the superconductor in the form of magnetic flux quanta. Under operating conditions where a direct current or direct current magnetic field is applied, the magnetic flux quanta remain stationary, but under operating conditions where an alternating current or alternating magnetic field is applied, the distribution of magnetic flux changes at the position of the superconductor, forcing the magnetic flux quanta to move. When the magnetic flux quanta move, something like friction occurs, and the equivalent of this frictional heat is the AC loss.

[0004] Due to the AC losses generated in superconductors by alternating magnetic fields, the high-intensity magnetic fields obtained by superconducting magnets have so far been limited to DC magnetic fields (static magnetic fields). The range of applications for superconducting magnets has also been limited to measurements by nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI). For example, Patent Document 1 discloses a magnetic field generator that generates a DC magnetic field by using a superconducting coil connected to a DC power supply.

[0005] Japanese Patent Publication No. 2018-164028

[0006] If a high-intensity alternating magnetic field can be generated by driving a superconducting magnet with alternating current while reducing AC losses, it is expected that this will lead to applications in electromagnetic metallurgy technologies such as electromagnetic induction heating, and the miniaturization of electrical equipment such as electric motors, thereby expanding the range of applications for superconducting magnets. For this reason, it is necessary to operate superconducting coils with alternating current while reducing AC losses.

[0007] The present invention aims to provide a superconducting magnet that can operate on AC power while reducing AC losses.

[0008] The inventors, in diligent research to solve the above problems, have discovered that in a superconducting coil and superconducting current transformer housed in a cryostat, the AC loss generated in the superconducting coil operating on AC can be reduced by winding the superconducting coil and the secondary winding of the superconducting current transformer connected to the superconducting coil with a superconducting cable in which a superconducting wire is wound spirally along the axis of a core material. This makes it possible to convert the entire magnetic field generating device, including the power supply, to AC, and to provide an AC magnetic field generating device that generates a high-intensity AC magnetic field that changes rapidly over time. Such a high-intensity AC magnetic field generating device in which the entire device is converted to AC has not been realized to date.

[0009] In other words, the present invention for solving the above problems includes, for example, the following embodiments: (1) A superconducting magnet comprising: a superconducting coil housed in a cryostat; and a superconducting current transformer housed in the cryostat, the primary winding of which is connected to an AC power source and the secondary winding of which is connected to the superconducting coil, wherein the superconducting coil and the secondary winding are constructed by winding a superconducting cable in which a superconducting wire is wound spirally along the axis of a core material. (2) The superconducting magnet according to claim 1, wherein the superconducting cable is made of multiple superconducting wires wound spirally along the axis of a core material and then stacked in multiple layers. (3) The superconducting magnet according to claim 1 or 2, wherein the AC power source is located outside the cryostat, and the primary winding is connected to the AC power source via a current introduction terminal provided at the boundary between the inside and outside of the cryostat. (Item 4) The superconducting magnet according to any one of Items 1 to 3, wherein the superconducting cable is configured by connecting a plurality of the superconducting cables in series via a connecting member having electrical resistance, the electrical resistance value of the connecting member is an electrical resistance value corresponding to the AC frequency of the AC power supply, and the electrical resistance value is such that the superconducting coil can generate an AC magnetic field in a superconducting state. (Item 5) The superconducting magnet according to any one of Items 1 to 4, wherein the cryostat maintains the superconducting coil and the superconducting winding of the superconducting current transformer in a superconducting state. (Item 6) The superconducting wire comprises: a plurality of superconducting layers extending in the longitudinal direction of a substrate and arranged in parallel in the short direction of the substrate; and an insulating portion extending in the longitudinal direction of the substrate and arranged between the plurality of superconducting layers to electrically insulate the plurality of superconducting layers, and when wound in a spiral shape, the following conditions A superconducting magnet according to any one of claims 1 to 5, wherein D is the diameter of the helix, P is the length of the helical pitch along the winding axis, and L is the length of the insulating portion along the longitudinal direction. (Clause 7) An AC magnetic field generating device comprising: a cryostat; an AC power source disposed outside the cryostat; and a superconducting magnet according to any one of claims 1 to 6 housed inside the cryostat. (Clause 8) A superconducting magnet according to claim 6, wherein the superconducting wire is disposed in the insulating portion along the longitudinal direction of the substrate and further comprises a plurality of connection portions that superconductively electrically connect the plurality of adjacent superconducting layers. (Clause 9) A superconducting magnet according to claim 6, wherein the superconducting wire satisfies the following conditions. Here, g is the length of the connection portion along the longitudinal direction, and n is a natural number of 1 or more. (Item 10) The superconducting magnet according to any one of items 6, 8, and 9, wherein the superconducting wire satisfies the following conditions. Here, g is the length of the connecting portion along the longitudinal direction, and w ｆ is the length of the superconducting layer along the shorter direction. (Item 11) The superconducting magnet according to item 10, wherein the superconducting wire satisfies the following conditions. Here, lol ｔ is the length of the superconducting wire along the shorter direction. (Clause 12) The superconducting magnet according to any one of Clauses 6, 8, and 9, wherein the superconducting wire satisfies the following conditions: Here, g is the length of the connecting portion along the longitudinal direction, and w ｆ(13) The superconducting magnet according to any one of claims 6 and 8 to 12, wherein the superconducting wire comprises a plurality of insulating portions, each insulating portion being positioned between the plurality of superconducting layers arranged in parallel. (14) The superconducting magnet according to claim 13, wherein the plurality of connection portions are positioned away from the line along the short direction across the plurality of insulating portions arranged in parallel. (15) The superconducting magnet according to any one of claims 6 and 8 to 14, wherein the insulating portion is a groove that exposes the substrate. (16) The superconducting magnet according to any one of claims 6 and 8 to 15, wherein the superconducting wire further comprises a conductive layer covering the superconducting layer. (17) The superconducting magnet according to claim 16, wherein the conductive layer further covers the insulating portion and the connection portion.

[0010] According to the present invention, it is possible to provide a superconducting magnet that can operate on AC power while reducing AC losses.

[0011] This is a diagram for schematically illustrating the circuit and device configuration of an AC magnetic field generator 100 according to one embodiment of the present invention. This is a diagram schematically showing the configuration of a superconducting wire 10A according to the first embodiment. This is a diagram schematically showing the configuration of a superconducting cable 20A constructed using the superconducting wire 10A according to the first embodiment. This is a plan view of the superconducting wire 10A according to the first embodiment, cut out in a plane including the superconducting layer 2 and the connection part 4, showing an example in which the superconducting wire 10A does not satisfy the conditions for achieving both a reduction in AC loss and an improvement in robustness. This is a plan view of the superconducting wire 10A according to the first embodiment, cut out in a plane including the superconducting layer 2 and the connection part 4, showing an example in which the superconducting wire 10A satisfies the conditions for achieving both a reduction in AC loss and an improvement in robustness. This is a schematic side view of the superconducting cable 20A according to the first embodiment, showing a state in which the superconducting wire 10A achieves the conditions necessary to simultaneously reduce AC loss and improve robustness by spirally winding the superconducting wire 10A along the axis of the core material 9. This is a schematic diagram showing the configuration of the superconducting wire 10B according to the second embodiment. This is a plan view of the superconducting wire 10B according to the second embodiment, cut out in a plane including the superconducting layer 2 and the connecting portion 4, showing an example in which the superconducting wire 10B satisfies the conditions necessary to simultaneously reduce AC loss and improve robustness. This is a schematic diagram showing the configuration of the superconducting wire 10C according to the third embodiment. This is a schematic diagram showing the configuration of the superconducting wire 10D according to a modification of the first embodiment. This is a schematic diagram of the superconducting wire 80 for explaining the meaning of terms used in this specification. This is a schematic diagram for explaining the superconducting wire 90 without the connecting portion 4. This is a schematic diagram for explaining the superconducting wire 90 without the connecting portion 4.

[0012] Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the following description and drawings, the same reference numerals indicate the same or similar components, and therefore, redundant explanations of the same or similar components will be omitted.

[0013] In this specification, alternating current (AC) refers to a physical quantity or value that changes over time. The waveform of AC includes both periodically repeating waveforms and single, non-repeating waveforms. The waveform of AC can be any waveform that changes arbitrarily over time, regardless of whether it is periodic or not. [AC Magnetic Field Generator]

[0014] Figure 1 is a diagram illustrating the circuit and device configuration of an AC magnetic field generator 100 according to one embodiment of the present invention.

[0015] An AC magnetic field generator 100 (hereinafter also simply referred to as "device 100") according to one embodiment comprises a cryostat 101, a superconducting coil 102, a superconducting current transformer 103, and an AC power supply 109. The superconducting coil 102 and the superconducting current transformer 103 are housed inside the cryostat 101. The AC power supply 109 is located outside the cryostat 101.

[0016] In the following, among the components 101, 102, 103, and 109 that constitute the device 100, the superconducting coil 102 and the superconducting current transformer 103 housed within the cryostat 101 will be referred to as the superconducting magnet.

[0017] The cryostat 101 maintains the superconducting coil 102 and superconducting current transformer 103 at a predetermined temperature in which the superconductors constituting the superconducting coil 102 and superconducting current transformer 103 exhibit a superconducting state. In this embodiment, the cryostat 101 maintains the superconducting coil 102 and superconducting current transformer 103 in a superconducting state. The cryostat 101 maintains the superconducting coil 102 and superconducting current transformer 103 at a temperature of about 70 Kelvin or less. The cryostat 101 can more preferably maintain the superconducting coil 102 and superconducting current transformer 103 at a temperature of about 40 Kelvin or less, and even more preferably at a temperature of about 20 Kelvin or less.

[0018] In this embodiment, the AC power supply 109 is a power source that supplies alternating current, and may be a commercial AC power supply provided by a power company or the like, or a device that receives power from it and supplies alternating current. The AC power supply 109 applies a three-phase or single-phase alternating current to the primary winding 104 of the superconducting current transformer 103. For example, the value of the alternating current applied by the AC power supply 109 to the primary winding 104 is, for example, about 100 A, and the frequency is, for example, about 50 Hz. [Superconducting Magnet]

[0019] Refer to Figure 1 again. One embodiment of the superconducting magnet is a superconducting magnet comprising a superconducting coil 102 and a superconducting current transformer 103 housed in a cryostat 101, wherein both the superconducting coil 102 and the secondary winding 105 of the superconducting current transformer 103 are constructed by winding a superconducting cable 20 in which a superconducting wire 10 is wound spirally along the axis of a core material. Details of the superconducting cable 20 and the superconducting wire 10 will be described later.

[0020] The superconducting coil 102 is constructed by winding a superconducting cable 20, which will be described later. The superconducting coil 102 is excited while maintaining a superconducting state. For example, the current flowing through the superconducting coil 102 and the secondary winding 105 is approximately 10 kA.

[0021] The superconducting current transformer 103 comprises a primary winding 104 connected to an AC power supply 109 and a secondary winding 105 connected to a superconducting coil 102. The superconducting current transformer 103 converts the magnitude of the current flowing through the primary winding 104 and sends it to the secondary winding 105. The superconducting current transformer 103 amplifies the current input to the primary side and outputs it to the secondary side. By amplifying the current input to the primary side and outputting it to the secondary side, the superconducting current transformer 103 reduces the value of the AC current penetrating the inside and outside of the cryostat 101 (room temperature and low temperature sections), making it possible to use thinner copper wires such as the current introduction terminal 106, which is the current path. As a result, the intrusion of heat into the cryostat 101 is reduced. For example, the magnitude of the current flowing through the primary winding 104 is approximately 100A, and the magnitude of the current flowing through the secondary winding 105 is approximately 10kA. In this embodiment, the primary winding 104 is constructed by winding a single superconducting wire 10 (described later) around it, and the secondary winding 105 is constructed by winding a superconducting cable 20 (described later) around it.

[0022] Both ends of the secondary winding 105 are connected to a superconducting coil 102 located inside the cryostat 101. Current input terminals 106 are provided at the boundary (penetration) between the inside and outside of the cryostat 101, and both ends of the primary winding 104 are electrically connected to an AC power supply 109 located outside the cryostat 101 via the current input terminals 106. The current input terminals 106 are conductors for supplying alternating current from the AC power supply 109 located outside the cryostat 101 to the superconducting coil 102 located inside the cryostat 101. The current input terminals 106 are constructed using a normal conductor (for example, copper). If the copper wire of the current input terminals 106 is thick, the intrusion of heat into the cryostat 101 will increase. In this embodiment, a superconducting current lead 107 is located in the low-temperature section inside the cryostat 101 between the current input terminals 106 and the primary winding 104. The superconducting current lead 107 is constructed using a superconductor. The superconducting current lead 107 has the property of conducting electric current but not much heat.

[0023] The AC power supply 109 and the current input terminal 106 are electrically connected by wiring made of a normal conductor (e.g., copper). The current input terminal 106 and the superconducting current lead 107 are electrically connected by wiring made of a normal conductor (e.g., copper). The superconducting current lead 107 and the primary winding 104 are electrically connected using conductive wiring 108. In this embodiment, the conductive wiring 108 is made of a normal conductor, but it may also be made of a superconductor. The AC current supplied from the AC power supply 109 flows through the current input terminal 106, the superconducting current lead 107, and the wiring 108 and is applied to the primary winding 104. <Connection of the superconducting cable>

[0024] When operating the superconducting coil 102 and secondary winding 105 in a superconducting state, the superconducting coil 102 and secondary winding 105 can be constructed by connecting multiple superconducting cables 20 in series via a connecting member 29 having electrical resistance. In this case, if the electrical resistance value of the connecting member 29 is of an appropriate magnitude according to the AC frequency of the AC power supply 109, the superconducting coil 102 can continuously generate an AC magnetic field in a superconducting state. Various connecting members can be used for the connecting member 29, such as copper terminals or solder (an alloy mainly composed of tin (Sn)).

[0025] The relationship between the electrical resistance of the connecting member 29 and the AC frequency of the AC power supply 109, which enables the superconducting coil 102 to continuously generate an AC magnetic field in a superconducting state, will be explained.

[0026] Now, if we let the resistance of the connecting member 29 be R and the inductance of the superconducting coil 102 be L, then if the characteristic time τ of the current change is sufficiently smaller than the value of equation (1) [unit: sec], then even if the connecting member 29 has a finite resistance, the current decay can be ignored and the AC magnetic field generator 100 can generate an AC magnetic field.

[0027] For example, the characteristic time τ of a current change that is changing at a frequency of 50 Hz ５０This is 0.01 s, which is half a period of the current change. Also, for example, the characteristic time τ of a current change that is changing at a frequency of 5 Hz. ５ This is 0.1 s, which is half a period of current change. On the other hand, when the inventor manufactured a connecting member 29 for connecting the superconducting cable 20 and measured the resistance R of the connecting member 29 at a temperature of 77 K, for example, the resistance R of the connecting member 29 was, for example, about 1 μΩ. At this time, assuming that the inductance L of the superconducting coil 102 of the AC magnetic field generator 100 is 10 μH, the value of equation (1) becomes 10 s. Characteristic time τ of current change in the cases of AC frequencies of 50 Hz and 5 Hz ５０， τ ５ These values ​​are 0.01s and 0.1s, respectively, both of which are sufficiently smaller than the value of 10s in equation (1). Therefore, for example, an AC magnetic field generator 100 configured using a superconducting coil 102 with an inductance of 10 μH or more can generate an AC magnetic field that changes at a frequency of 50 Hz or 5 Hz. [Superconducting Cables and Superconducting Wires]

[0028] The following describes an example of a superconducting cable 20 used to reduce AC losses in the superconducting coil 102 in a superconducting magnet according to one embodiment, with reference to Figures 2 to 13. The superconducting cable used in the configuration between the superconducting coil 102 and the secondary winding 105 of the superconducting current transformer 103 is not limited to a superconducting cable 20 constructed using the superconducting wire 10 as illustrated in Figures 2 to 10. The superconducting cable 20 can be constructed by spirally winding superconducting wires 10 (10A, 10B, 10C, 10D) equipped with connecting parts 4, as illustrated in Figures 2 to 10, along the axis of a core material 9, or by spirally winding superconducting wires 90 (90A, 90B, 90C) without connecting parts 4, as illustrated in Figures 12 to 13, along the axis of a core material 9. <Meaning of Superconducting Wire Terms>

[0029] This section explains the meanings of terms used in the following descriptions of superconducting wires and superconducting cables. Some terms have already been explained in their initial appearances, but are explained again for clarity.

[0030] The electromotive force is a "force" that acts in a loop to cause an eddy current 82 to flow in the superconducting wire 80 by electromagnetic induction when the magnetic field H ｅ (strictly speaking, among the applied magnetic fields, the component H ｅ perpendicular to the superconducting layer 81) varies with time. The length L of the longest part along the longitudinal direction of the superconducting wire 80 of the loop of the electromotive force 83 ｉ (hereinafter, for simplicity, referred to as "the length L of the loop of the electromotive force" ｉ ) is equal to the length of the longitudinal part of the superconducting wire 80 in which the magnetic field H ｅ is in the same direction, or more precisely, the time derivative dH ｅ / dt of the magnetic field H ｅ is in the same direction. However, even if an electromotive force is generated, an eddy current cannot flow without a conductor or a superconductor. Unless otherwise specified, in this specification, the electromotive force is illustrated using a dashed line.

[0031] The eddy current 82 means a current induced in a loop (vortex shape) in a conductor or a superconductor by the electromotive force 83 due to electromagnetic induction. The eddy current 82 is a concept that includes both the persistent eddy current and the coupled current described later. Unless otherwise specified, in this specification, the eddy current is illustrated using a solid line.

[0032] The length of the eddy current is, as shown in FIG. 11, the length of the longest part along the longitudinal direction of the superconducting wire 80 of the eddy current 82 shown by the solid line distributed in the superconducting wire 80, and is represented by the symbol L ｅ . The eddy current 82 can only flow within the length L ｉ of the loop of the electromotive force. That is, the length L ｅ of the eddy current does not exceed the length L ｉ of the loop of the electromotive force.

[0033] The width of the eddy current is, as shown in FIG. 11, the length of the longest part along the short-side direction of the superconducting wire 80 of the eddy current 82 shown by the solid line distributed in the superconducting wire 80, and is represented by the symbol w ｅThis is represented by the following. Referring to Figures 12(A) and (B), the eddy current 82 flows in a reciprocating direction as indicated by the symbols 99a and 99b, and the typical distance of the movement of the magnetic flux quantum 93 is the width of the eddy current w. ｅ It is equal to half of and therefore the width of the eddy current w ｅ Narrowing the gap can reduce AC losses.

[0034] Sustained eddy currents refer to eddy currents that flow only within a superconductor and can be considered to be non-changing (not decaying) over practical time scales (e.g., several hours to several years). Unless otherwise specified, sustained eddy currents are illustrated using solid lines in this specification.

[0035] A coupled current is a type of eddy current. For example, referring to Figure 13(B), the path of the coupled current 98 flowing through the superconducting wire 90C equipped with a copper shunt layer 97 is as follows: The coupled current 98 first flows longitudinally from the front to the back of the superconducting layer 92a located on the left side of the figure, and then flows to the right in the short direction of the copper shunt layer 97. Subsequently, the coupled current 98 flows longitudinally from the back to the front of the superconducting layer 92a located on the right side of the figure, then flows to the left in the short direction of the copper shunt layer 97, and returns to the superconducting layer 92a located on the left side of the figure. Note that the coupled current 98 decays over time because its path includes the copper shunt layer 97, which is a conductor. Alternatively, if the period of the magnetic field fluctuation is sufficiently long compared to the coupling time constant, which is the decay time constant of the coupled current, then the coupled current can be considered to flow almost nonexistent. Unless otherwise specified, the coupled current is shown using a dashed line in this specification. <Superconducting Wire - First Embodiment>

[0036] Figure 2 is a schematic diagram showing the configuration of the superconducting wire 10A according to the first embodiment. (A) is a perspective view of the superconducting wire, (B) is a plan view of the superconducting wire, and (C) is a cross-sectional view of the superconducting wire along the 1B-1B line shown in (B). In the illustrated embodiment, the direction of the Y axis is the longitudinal direction of the superconducting wire 10A, the direction of the X axis is the short direction of the superconducting wire 10A, and the direction of the Z axis is the thickness direction of the superconducting wire 10A.

[0037] The superconducting wire 10A (10) according to the first embodiment comprises a substrate 1, a plurality of superconducting layers 2, an insulating portion 3, and a plurality of connecting portions 4, and when wound in a spiral shape, satisfies the conditions for achieving both a reduction in AC loss and an improvement in robustness, as will be detailed below.

[0038] The substrate 1 is formed in a tape shape using, for example, a nickel-based alloy or stainless steel. Exemplary, Hastelloy® can be used as the material for the substrate 1. The substrate 1 is flexible, and the superconducting wire 10A is used wound in a spiral shape.

[0039] An intermediate layer (not shown) is formed on the surface of the substrate 1 as needed, which serves as the base for the superconducting layer 2. As the material for the intermediate layer, a material can be used whose physical properties, such as thermal expansion coefficient and lattice constant, are intermediate between those of the substrate 1 and the superconductor constituting the superconducting layer 2. For example, the intermediate layer material may be LaMnO ３ This can be used. In this embodiment, an intermediate layer is formed on the surface of the substrate 1, and in this description, the substrate 1 with the intermediate layer formed on its surface is collectively referred to as the substrate 1.

[0040] The superconducting layer 2 conducts current superconductively in the superconducting wire 10A. To reduce AC losses, the superconducting layer 2 is formed as a multifilament on the surface of the substrate 1. The superconducting layer 2 is stretched in the longitudinal direction of the substrate 1, and multiple superconducting layers 2 are arranged in parallel in the short direction of the substrate 1. Exemplarily, the superconducting layer 2 is formed using REBCO high-temperature superconductor, which is a ceramic. REBCO has the chemical formula REBa ２ Cu ３ O ７－δ This is a copper oxide superconductor having a compositional formula represented by (RE being a rare earth element such as Y, Gd, Eu, or Sm). In the following description, the multifilamented superconducting layer 2 will be referred to as superconducting filament 2, or simply as filament 2 or simply as superconducting layer 2.

[0041] The insulating portion 3 extends in the longitudinal direction of the substrate 1 and is arranged between a plurality of superconducting layers 2, 2, electrically insulating the plurality of superconducting layers 2, 2. Exemplarily, in this embodiment, the insulating portion 3 is formed as a groove that exposes the surface of the substrate 1 by, for example, three-dimensional patterning of the superconducting layer 2 using a known photolithography process. In this embodiment, the superconducting wire 10A has a plurality of insulating portions 3, and each insulating portion 3 is arranged between a plurality of superconducting layers 2, 2 arranged in parallel.

[0042] The connecting portion 4 is arranged in the insulating portion 3 along the longitudinal direction of the substrate 1 and electrically connects multiple adjacent superconducting layers 2, 2 in a superconducting manner. The superconducting wire 10A is provided with multiple connecting portions 4 in the insulating portion 3 along the longitudinal direction of the substrate 1. In this embodiment, the connecting portion 4 is formed integrally with the superconducting layer 2 using the same superconducting material as the superconducting layer 2.

[0043] By superconductively connecting multiple adjacent superconducting layers 2, 2 with the connecting portion 4, the superconductive current distribution of the current flowing through the superconducting layers 2 is improved, and the robustness of the superconducting wire 10A is enhanced. In other words, even if a local transition to a normal conducting state occurs in a certain superconducting layer 2 for some reason, the connecting portion 4 superconductively bridges the multiple adjacent superconducting layers 2, 2, and by distributing the current from the superconducting layer 2 that has transitioned to a normal conducting state to the adjacent superconducting layer 2, a quench of the entire superconducting wire 10A is prevented.

[0044] Exemplary, the length (width) of the superconducting wire 10A along its short side is about 2 mm to about 4 mm, preferably about 1 mm to about 4 mm. The length (width) of a single multifilamented superconducting layer 2 along its short side is preferably about 0.4 mm to about 1 mm, more preferably about 0.1 mm to about 1 mm. Exemplary, the overall thickness of the superconducting wire 10A including the substrate 1 and the superconducting layer 2 is in the range of about 150 μm to about 50 μm, preferably in the range of about 50 μm to about 30 μm. Since the superconducting wire 10A is used wound in a spiral shape, more preferably the overall thickness of the superconducting wire 10A including the substrate 1 and the superconducting layer 2 is less than about 30 μm.

[0045] In order to achieve both a reduction in AC loss and an improvement in robustness, when the superconducting wire 10A (10) is wound in a spiral shape, the following conditions are satisfied in order to reduce AC loss.

[0046] D is the diameter of the helix, P is the length of the helical pitch along the winding axis, and L is the length of the insulating portion 3 along the longitudinal direction. The diameter D of the helix and the length of the helical pitch P are illustrated in Figures 3 and 6, which will be described later. For example, the diameter D of the helix is ​​approximately 3 mm, the length of the helical pitch P is approximately 7.5 mm, and the length L of the insulating portion 3 along the longitudinal direction is approximately 10 mm.

[0047] Preferably, the superconducting wire 10A (10) satisfies the following conditions in order to reduce AC loss. g is the length of the connection portion 4 along the longitudinal direction, and n is a natural number of 1 or more. Exemplarily, the length g of the connection portion 4 along the longitudinal direction is approximately 2 mm.

[0048] Preferably, the superconducting wire 10A (10) satisfies the following conditions in order to reduce AC loss. ｆ This is the length of the superconducting layer 2 along the shorter direction.

[0049] Preferably, the superconducting wire 10A (10) satisfies the following conditions in order to improve robustness.

[0050] Preferably, the superconducting wire 10A (10) satisfies the following conditions in order to improve robustness. ｔ This is the length of the superconducting wire 10A (10) along the shorter direction.

[0051] The principles for achieving both reduced AC loss and improved robustness will be explained later with reference to Figures 4 to 6.

[0052] Figure 3 is a schematic diagram showing the configuration of a superconducting cable 20A constructed using the superconducting wire 10A according to the first embodiment.

[0053] The superconducting cable 20A (20) according to the first embodiment comprises a core material 9 and a superconducting wire 10A that is wound spirally along the axis of the core material 9.

[0054] In this embodiment, the core material 9 is a solid cylindrical member. For example, stainless steel or copper can be used for the core material 9.

[0055] The superconducting wire 10A (10) is wound in a spiral shape, and the length L of the loop of the electromotive force generated in the superconducting wire 10A (10) ｉ The length can be shortened. In this embodiment, the superconducting wire 10A is wound around the outer wall of the core material 9 in the direction in which the left-hand thread progresses, and the winding axis direction is the direction in which this left-hand thread progresses. In the illustrated embodiment, one layer of superconducting wire 10A (10) is wound spirally along the axis of the core material 9. In the illustrated embodiment, three superconducting wires 10A (10A １ , 10A ２ , 10A ３ Three superconducting wires 10A are wound in a trifiler pattern in a spiral around the outer wall of the core material 9, in parallel. １ , 10A ２ , 10A ３ Of these, superconducting wire 10A １ The detailed structure is shown in the diagram, but the superconducting wire 10A ２ , 10A ３ Detailed structural illustrations have been omitted. <Principles of Superconducting Wires>

[0056] Figures 4 and 5 are plan views of the superconducting wire 10A according to the first embodiment, cut out in a plane including the superconducting layer 2 and the connecting portion 4. Figure 4 shows an example in which the superconducting wire 10A does not satisfy the conditions for achieving both reduced AC loss and improved robustness. Figure 4(A) shows that the length L of the insulating portion 3 is short, that is, the spacing of the connecting portion 4 is short, and although robustness can be improved, the width w of the superconducting wire 10A is short. t This is an example where a wide, persistent eddy current 11 flows across the width w of each superconducting filament 2, and AC losses cannot be reduced. Figure 4(B) shows that the persistent eddy current 11 flows across the width w of each superconducting filament 2. ｆIn this example, the narrow, enclosed structure reduces AC losses, but the length L of the insulating section 3 is long, meaning the spacing between the connection sections 4 is long, and robustness cannot be improved. Figure 5 shows an example where the superconducting wire 10A satisfies the conditions for achieving both reduced AC losses and improved robustness.

[0057] Figure 6 is a schematic side view of the superconducting cable 20A according to the first embodiment, showing a state in which the superconducting wire 10A is wound spirally along the axis of the core material 9, thereby achieving conditions for both reduced AC loss and improved robustness. In Figure 6, only one superconducting wire 10A is shown for the sake of simplicity in illustration and explanation. In Figure 6, reference numeral 11a indicates a persistent eddy current 11a occurring in the region of the superconducting wire 10A located on the front side of the core material 9 in a side view of the superconducting cable 20A, and reference numeral 11b indicates a persistent eddy current 11b occurring in the region of the superconducting wire 10A located on the back side of the core material 9. • Reduction of AC loss

[0058] The principle for reducing AC losses while improving robustness will be explained below with reference to Figures 4 to 6.

[0059] In order to achieve a reduction in AC loss by multifilamentizing the superconducting layer 2, a wide (w) current flowing across multiple superconducting filaments 2, 2 is required. ｅ The large sustained eddy current 11 is interrupted in the insulating part 3, and the width w of one superconducting filament 2 ｆ A narrow (w) trapped inside ｅ The sustained eddy current 11 must be small. On the other hand, as will be described later, in order to improve robustness, it is desirable that the distance between the connection parts 4, i.e., the length L of the insulating part 3 be short.

[0060] A magnetic field H is applied perpendicularly to the superconducting layer 2 of the superconducting wire 10A. ｅ (More precisely, the component H perpendicular to the superconducting layer 81 of the applied magnetic field) ｅ When the voltage fluctuates over time, a loop-shaped electromotive force 19 is induced by electromagnetic induction, which attempts to create eddy currents within the superconducting wire 10A. The length of the electromotive force loop L ｉThis is a magnetic field H applied perpendicular to the superconducting layer 2, along the longitudinal direction of the superconducting wire 10A, regardless of the length L of the insulating portion 3. ｅ This is determined by the distribution. Here, in order to improve robustness, the connection parts 4 are arranged at short intervals, that is, the length L of the insulating part 3 is short, and the length L of the insulating part 3 is the length L of the electromotive force loop. ｉ Let's consider the case where the length is shorter. In this case, as shown by the solid line in Figure 4(A), a wide, sustained eddy current 11 continues to flow through the connection 4 across the multiple superconducting filaments 2, 2, and the effect of reducing AC losses due to the multifilamentization of the superconducting layer 2 is not realized.

[0061] In contrast, as shown in Figure 4(B), the length L of the insulating portion 3 is equal to the length L of the electromotive force loop. ｉ If the length is greater, the insulating portion 3 will block the flow of a sustained eddy current that is wide in the vertical direction in the figure across the multiple filamented superconducting layers 2, 2, and the sustained eddy current 11 will be limited by the width w of each superconducting filament 2. ｆ A narrow (w) trapped inside ｅ This results in smaller AC losses.

[0062] Length L of the electromotive force loop ｉ This is determined by the shape of the coil formed using superconducting wires or superconducting cables, but it often extends to the total length of the windings used to form the coil (e.g., tens to hundreds of meters) or even several meters. Therefore, in order to block wide, persistent eddy currents, such a large length L is required. ｉ If the length L of the insulating portion 3 is increased, the spacing between the connection portions 4 becomes too long, and no improvement in robustness can be expected.

[0063] In order to improve robustness by arranging the connection parts 4 at short intervals, and to reduce AC losses by blocking wide, sustained eddy currents with the insulating part 3, the length L of the electromotive force loop is determined by some means as shown in Figure 5. ｉ It is necessary to make it even shorter than the length L of the insulating section 3, which was shortened to meet the requirement for improved robustness.

[0064] In this invention, as illustrated in Figure 6, the superconducting wire 10 is wound spirally along the axis of the core material 9, thereby, as illustrated in Figure 5, the length L of the electromotive force loop is ｉ The length of the insulating part 3 is made shorter than the length L of the insulating part 3.

[0065] When the superconducting wire 10A (10) is wound spirally along the axis of the core material 9, the length P of the superconducting wire 10A corresponds to the length P of the spiral pitch. ｔ This can be expressed as shown in equation (2) below, using the diameter D of the spiral.

[0066] By winding the superconducting wire 10A (10) spirally along the axis of the core material 9, a magnetic field H is applied perpendicularly to the superconducting layer 2 of the superconducting wire 10A (10). ｅ (More precisely, the component H perpendicular to the superconducting layer 2 of the applied magnetic field) ｅ The direction of the spiral is given by equation (2), which is the length of the spiral pitch P of the superconducting wire 10A. ｔ It reverses every half of the time. As a result, the loop length L of the electromotive force, which is determined when a magnetic field is applied, is as shown in equation (3). ｉ Also, the length of the spiral pitch P of the superconducting wire 10A. ｔ It becomes half the length, making it significantly shorter.

[0067] In this way, by winding the superconducting wire 10A in a spiral shape, the length L of the electromotive force loop is ｉ This can be made shorter than the length L of the insulating part 3 which has been shortened to improve robustness, and as a result, the width (w) illustrated in Figure 5 can be made narrower. ｅ This makes it possible to realize a state of small sustained eddy currents 11 (11a, 11b). That is, the sustained eddy currents 11 flowing through the superconducting wire 10A are equal to the width w of each superconducting filament 2. ｆ The width w of the sustained eddy current 11 (11a, 11b) is confined within. ｅ The width becomes narrower. At this time, the width of each superconducting filament 2 is w ｆ The state of the narrow, persistent eddy currents 11 (11a, 11b) confined within is also illustrated in Figure 6. This improves robustness while reducing AC losses.

[0068] In other words, if the length P of the helical pitch and the diameter D of the helix satisfy equation (4) with respect to the length L of the insulating part 3, the AC loss can be reduced.

[0069] The condition in equation (4) can also be called the condition under which no persistent eddy currents 11 are induced within the helical 1 / 2 pitch. Equation (4) indicates the lower limit of the length L of the insulating portion 3. From the viewpoint of reducing AC loss, exemplary, the upper limit of the length L of the insulating portion 3 is preferably the width w of the superconducting wire 10A. ｔ It can be 10 times, more preferably 20 times, more preferably 50 times, and even more preferably 100 times.

[0070] Furthermore, from the viewpoint of improving robustness, it is preferable that the length L of the insulating portion 3 along the longitudinal direction be as short as possible, in addition to satisfying the conditions of equation (4). This is because the number of connection portions 4 that function as bypass circuits for the current increases.

[0071] Alternatively, the following formula (5) can be defined as another exemplary upper limit for the length L of the insulating portion 3.

[0072] When a superconducting cable 20A, formed by spirally winding a superconducting wire 10A along the axis of a core material 9, is further wound onto a winding frame such as a bobbin to form a coil, the superconducting wire 10A comes into contact with the winding frame at intervals of this upper limit, worsening the cooling state of the superconducting wire 10A. The parts of the superconducting wire 10A whose cooling state has deteriorated become potential candidates for a normal conducting transition, becoming a weak point from the standpoint of robustness. When the upper limit of the length L of the insulating portion 3 is defined by equation (5), connection portions 4 are provided at intervals of this upper limit, making it possible to maintain the superconducting current separation of the current flowing through the superconducting layer 2 via the connection portions 4 at intervals of this upper limit.

[0073] Furthermore, in relation to the conditions of equation (4), under the condition that the average lateral magnetic field with respect to the helix is ​​constant, it is possible to define conditions under which an electromotive force 19 that would cause a broad, persistent eddy current 11 to flow across multiple superconducting filaments 2, 2 via the connection portion 4 within one pitch of the helix is ​​not induced in the first place. Equation (6) is a more preferable condition for reducing AC losses. In equation (6), g is the length of the connection portion 4 along the longitudinal direction, and n is a natural number of 1 or more. Equation (6) assumes that the loop of the electromotive force 19 passes through the longitudinal center of the connection part 4. The connection part 4 has a length g in the longitudinal direction, and if the length of the helical pitch P and the diameter of the helix D satisfy the following equation (7) for a certain length L of the insulating part 3 along the longitudinal direction, then the condition is met that an electromotive force 19 that would attempt to generate a wide, persistent eddy current 11 flowing across the multiple superconducting filaments 2, 2 through the connection part 4 is not induced in the first place. When the left and middle sides of equation (7) are connected by an equals sign, the electromotive force 19 becomes zero for the loop passing through the inner edge of the connection part 4, and when the middle and right sides of equation (7) are connected by an equals sign, the electromotive force 19 becomes zero for the loop passing through the outer edge of the connection part 4. Thus, considering the longitudinal length g of the connection part 4, the condition for reducing the AC loss shown in equation (6) can be defined as the condition shown in equation (7).

[0074] By shortening the length g of the connection portion 4, the current flowing through the connection portion 4 superconductively can be limited, thereby limiting wide, persistent eddy currents. The length g of the connection portion 4 along the longitudinal direction is equal to the length (width) w of the superconducting layer 2 along the short direction. ｆ The following is preferable:

[0075] The condition in equation (8) can also be described as the condition that the persistent eddy current 11 just saturates or does not saturate the outermost superconducting layer 2. When this condition is met, the superconducting wire 10A has at least a width w ｔ No sustained eddy currents can flow through the connection 4 throughout the entire structure. Equation (8) shows that, from the perspective of reducing AC losses, the width w of a single superconducting filament 2 is...ｆ This indicates the upper limit of the length g of the connection part 4, which is determined by its relationship with [the other element]. Note that, if we were to tentatively show a lower limit for length g in equation (8), then 0 ≤ g, and the lower limit of length g is zero. • Principle of error magnetic field reduction

[0076] The principle of reducing the error magnetic field will be explained with reference to Figures 12 and 5.

[0077] As illustrated in Figure 12(A), in a superconducting wire 90A in which the superconducting layer 92 is not multifilamentized, a magnetic field H is applied perpendicular to the superconducting layer 92. ｅ When the magnetic field H fluctuates over time, according to Lenz's law, the magnetic field H extends across the entire width of the superconducting layer 92. ｅ Eddy currents 99 flow in a direction that opposes the fluctuations. Eddy currents 99 flow distributed within the superconducting layer 92. That is, on the left side of the figure, the bundle of eddy currents 99a flows in the longitudinal direction of the superconducting layer 92 from the front to the back with a width w ｅ The current flows in one direction at a rate of / 2, and on the right side of the diagram, the bundle of eddy currents 99b flows from the back to the front in the longitudinal direction of the superconducting layer 92 with a width w ｅ It flows in one direction at a rate of / 2.

[0078] These eddy currents 99 (99a, 99b) flow within the superconducting layer 92 as persistent eddy currents. On the other hand, since persistent eddy currents are unexpected currents that are not taken into consideration when designing electromagnets, they are a source of error in the magnetic field in electromagnets that require the generation of high-precision magnetic fields, such as electromagnets for nuclear magnetic resonance (NMR) devices, electromagnets for nuclear magnetic resonance imaging (MRI), and electromagnets for particle beam accelerators.

[0079] In contrast, with a superconducting wire 10 in which the superconducting layer 2 is made of multiple filaments and the connecting portion 4 superconductively bridges multiple adjacent superconducting filaments 2, 2, robustness is improved while the error magnetic field is reduced, as will be explained below.

[0080] As illustrated in Figure 5, even in the superconducting wire 10 in which the superconducting layer 2 is multifilamentized, the magnetic field H applied perpendicular to the superconducting filament 2 is the same as in the superconducting wire 90A in which the superconducting layer 92 is not multifilamentized. ｅWhen the temperature fluctuates over time, persistent eddy currents 11 flow according to Lenz's law. These persistent eddy currents flow confined within each superconducting filament 2 (2a, 2b, 2c, 2d). When considering the direction of the persistent eddy currents 11 between multiple adjacent superconducting filaments 2, 2, the directions of the persistent eddy currents 11 located close together on the adjacent side are opposite, and the magnetic fields they create cancel each other out.

[0081] For example, consider the direction of the persistent eddy currents 11a in close proximity between adjacent filaments 2a and 2b. In filament 2a, the direction of the persistent eddy current 11a adjacent to filament 2b is to the left in the figure, and in filament 2b, the direction of the persistent eddy current 11a adjacent to filament 2a is to the right in the figure. Thus, between multiple adjacent filaments 2a and 2b, the directions of the persistent eddy currents 11a located close to each other are opposite.

[0082] Regarding the direction of the adjacent persistent eddy currents 11a between filaments 2b and 2c, similar to the case of filaments 2a and 2b described above, the directions of the adjacent persistent eddy currents 11a located close together are opposite. The same applies to filaments 2c and 2d. The same applies to persistent eddy currents 11b as to persistent eddy currents 11a described above. As a result, in the example in Figure 5, the magnetic field created by the upper component of the loop of persistent eddy current 11 in filament 2a and the magnetic field created by the lower component of the loop of persistent eddy current 11 in filament 2d remain.

[0083] Thus, in the superconducting wire 10, the directions of the persistent eddy currents 11 located close together on adjacent superconducting filaments 2, 2 are opposite, and the magnetic fields they create cancel each other out. As a result, only the magnetic field created by the outer component of the loop of persistent eddy currents 11 flowing through the superconducting filaments 2a, 2d located on the outermost sides (uppermost and lowermost in the figure) of the superconducting wire 10 remains. In this way, in the superconducting wire 10 in which the superconducting layer 2 is multifilamentized, the error magnetic field generated by persistent eddy currents, which are currents not anticipated in the design, is reduced. ・Improved robustness

[0084] To improve robustness, it is necessary to facilitate the diversion of current from one superconducting filament 2 to an adjacent superconducting filament 2. For this purpose, it is desirable that the length L of the insulating portion 3 is short, that is, that the distance between the connecting portions 4 is short, and that the length g of the connecting portions 4 is long.

[0085] When localized normal conduction occurs in a certain superconducting layer 2, the maximum value of the current that bypasses the normal conducting portion is half of the current flowing through the entire superconducting wire 10A (10). Therefore, as defined in the following equation (9), the length g of the connection portion 4 for diverting the current is equal to the width w of the superconducting wire 10A (10). ｔ If at least half of it is present, it is possible to bypass the maximum possible current at one connection point 4. Equation (9) shows that, from the viewpoint of improving robustness, the width w of the superconducting wire 10A (10) ｔ The lower limit of the length g of the connection part 4, which is determined by its relationship with the other factors, is shown as an example.

[0086] Furthermore, if the length g of the connection portion 4 along the longitudinal direction is too short, this length g becomes a bottleneck, preventing the current flowing in from the adjacent superconducting layer 2 by bypassing the normal conducting portion. The conditions for current to bypass defects and normal conducting portions within a single superconducting filament 2 through a single connection portion 4 can be defined as shown in the following equation (10). Equation (10) is defined in terms of the width w of a single superconducting filament 2 from the viewpoint of improving robustness. ｆ The lower limit of the length g of the connection part 4, which is determined by its relationship with the other factors, is shown as an example.

[0087] Needless to say, a longer length g of the connection part 4 is desirable in terms of current distribution. However, in the connection part 4, the space between the superconducting layers 2, 2 is not divided by the insulating part 3, and as a result, the width w of the persistent eddy current 11 ｅ As the area widens, localized AC losses increase. If we were to indicate an upper limit for the length g of the connection part 4 from the perspective of improving robustness, one example that can be considered is the condition in the following equation (11), which is the limit at which the effect of reducing AC losses due to division almost disappears.

[0088] In the superconducting wire 10, a connection portion 4 exists between the superconducting layers 2, 2, and the adjacent, divided superconducting layers 2, 2 are electrically connected superconductingly. Therefore, even if, for example, a defect exists in a certain superconducting layer 2, or a localized normal conducting area occurs in a certain superconducting layer 2, and the superconducting layer 2 cannot conduct current superconductingly, the current can be rerouted superconductingly through the connection portion 4. For this reason, unlike the superconducting wire 90B, which does not have a connection portion 4 and has the superconducting layers 2 completely electrically separated, and the superconducting wire 90C, which has the superconducting layers 2, 2 electrically connected normally by a copper current divider layer 97, the superconducting wire 10, which has a connection portion 4, does not have its robustness compromised by the division of the superconducting layers 2 due to multifilamentization. <Effects of the superconducting wire>

[0089] As described above, the superconducting wire 10A and superconducting cable 20A according to the first embodiment can achieve both a reduction in AC loss and error magnetic field, and an improvement in robustness. The superconducting wire 10A according to the first embodiment has a plurality of connection parts 4 that superconductively electrically connect a plurality of adjacent superconducting layers 2, 2, and when wound in a spiral shape, it satisfies the conditions for achieving both a reduction in AC loss and an improvement in robustness, as explained with reference to Figures 4 to 6.

[0090] In the superconducting wire 10A according to the first embodiment, by shortening the length L of the insulating portion 3, the connecting portion 4 superconductively connects multiple adjacent superconducting layers 2, 2 at a sufficiently short interval. This improves the superconductive current distribution of the current flowing through the superconducting layer 2, and improves the robustness of the superconducting wire 10A. That is, even if a local transition to a normal conducting state occurs in a certain superconducting layer 2 for some reason, the connecting portion 4 superconductively bridges multiple adjacent superconducting layers 2, 2, and divides the current from the superconducting layer 2 that has undergone a normal conducting state to the adjacent superconducting layer 2, thereby preventing a quench of the entire superconducting wire 10A.

[0091] Furthermore, in the first embodiment, the superconducting wire 10A is wound spirally along the axis of the core material 9, thereby increasing the length L of the electromotive force loop. ｉ The length P of the superconducting wire 10A corresponds to the length P of the helical pitch. ｔ The length is shortened by half, and made shorter than the length L of the insulating section 3. This reduces AC loss and also reduces the error magnetic field. <Superconducting wire - second embodiment>

[0092] The superconducting wire 10B (10) according to the second embodiment differs from the superconducting wire 10A according to the first embodiment in that it further comprises a conductive layer 5a (5) covering the superconducting layer 2, the insulating portion 3, and the connecting portion 4. The configuration of the superconducting wire 10B according to the second embodiment, as described below, is the same as that of the superconducting wire 10A according to the first embodiment unless otherwise specified, so redundant explanations will be omitted.

[0093] Figure 7 is a schematic diagram showing the configuration of the superconducting wire 10B according to the second embodiment. (A) is a perspective view of the superconducting wire, (B) is a plan view of the superconducting wire, and (C) is a cross-sectional view of the superconducting wire along the 6B-6B line shown in (B).

[0094] In the second embodiment, the superconducting wire 10B (10) further comprises a conductive layer 5a (5) that covers the superconducting layer 2, the insulating portion 3, and the connecting portion 4. In the illustrated embodiment, the conductive layer 5a is formed to cover not only the superconducting layer 2, but also the superconducting layer 2, the insulating portion 3, and the connecting portion 4. The conductive layer 5a functions as a current diversion layer that bypasses the current flowing through the superconducting layer 2 when an abnormality occurs in the superconducting layer 2. Exemplarily, the conductive layer 5a is formed of copper. In the illustrated embodiment, the insulating portion 3 is formed as a groove whose bottom surface reaches the surface of the substrate 1, and the groove is filled with copper that functions as the conductive layer 5a.

[0095] Figure 8 is a plan view of the superconducting wire 10B according to the second embodiment, cut out in a plane including the superconducting layer 2 and the connecting portion 4. Figure 8 shows an example in which the superconducting wire 10B satisfies the conditions for achieving both a reduction in AC loss and an improvement in robustness.

[0096] The superconducting wire 10B according to the second embodiment has improved robustness compared to the superconducting wire 10A according to the first embodiment, due to the inclusion of a conductive layer 5a.

[0097] According to the superconducting wire 10B of the second embodiment, the same AC loss reduction effect as the superconducting wire 10A of the first embodiment can be obtained in a limited, but practically sufficient, operating frequency range.

[0098] Unlike the superconducting wire 10A according to the first embodiment, in the superconducting wire 10B according to the second embodiment, a wide coupling current 12 flows across multiple superconducting layers 2 (2a, 2b, 2c, 2d) in the vertical direction in the figure, as shown by the dashed line in Figure 8, via the conductive layer 5a provided on the upper part of the superconducting layer 2, insulating portion 3, and connecting portion 4. Unless this coupling current 12 is attenuated, the effect of multifilamentizing the superconducting layer 2 is not realized, and the AC loss remains large.

[0099] Referring to FIG. 8, the state of the coupling current generated in the superconducting wire 10B and the state of the persistent eddy current after the decay of the coupling current will be described in detail. In the superconducting wire 10B in the state shown in FIG. 8, the coupling current 12 flows across the conductive layer 5b provided on the upper part of the insulating portion 3, and the width of the eddy current with respect to the coupling current 12 is wide in the vertical direction in the figure across the plurality of superconducting layers 2 (2a, 2b, 2c, 2d), and the AC loss is large when the coupling current 12 has not decayed. If the period of the applied magnetic field fluctuation is sufficiently longer than the coupling time constant τ ｃ which is the decay time constant of the coupling current, the coupling current 12 decays, and the persistent eddy current 11 confined in each superconducting filament 2 starts to flow. The width w ｅ of the eddy current with respect to the persistent eddy current 11 confined in this superconducting filament 2 is narrow and the AC loss is small.

[0100] The coupling current decays at the coupling time constant τ ｃｃ which is the ratio of the self-inductance L ｃｃ determined by its path and the resistance R ｃ . The self-inductance L ｃｃ is proportional to the length L ｅ of the eddy current (coupling current), and the resistance R ｃｃ is inversely proportional to the length L ｅ of the eddy current (coupling current). Therefore, the coupling time constant τ ｃ is proportional to the square of the length L ｅ of the eddy current (coupling current). Thus, when the length L ｅ of the eddy current (coupling current) is long, the conductive layer 5a impairs the effect of reducing the AC loss due to the multi-filamentation of the superconducting layer 2.

[0101] In the superconducting wire 10B according to the second embodiment, when the superconducting wire 10B is wound spirally like the superconducting cable 20A according to the first embodiment, the length L ｅ of the eddy current with respect to the coupling current 12 can be shortened. Thereby, the coupling time constant τ ｃ can be shortened.

[0102] The period of the magnetic field variation is the reciprocal of the operating frequency. In the second embodiment, when the operating frequency is sufficiently lower than the characteristic frequency f ｃ = 1 / (2πτ ｃ ), the AC loss is reduced and the stray magnetic field becomes small.

[0103] Regarding the generation of persistent eddy currents 11a in the region of the superconducting wire 10B located on the front side of the core material 9 and persistent eddy currents 11b in the region of the superconducting wire 10B located on the back side of the core material 9 in a side view of the superconducting cable in the second embodiment, it is the same as in the first embodiment described with reference to FIGS. 5 and 6.

[0104] Similar to the first embodiment, the superconducting wire 10B according to the second embodiment can form a superconducting cable by being wound spirally along the axis of the core material 9. - Consideration of the operating frequency at which the effect of reducing AC loss is exhibited

[0105] When a superconducting wire 90C without the connection part 4 shown in FIG. 13 is wound spirally, the decay time constant (coupling time constant) τ of the coupling current 98, which is an eddy current flowing between the plurality of superconducting layers 92a, 92a through the copper shunt layer 97, ｃ１ is experimentally confirmed to be given by the following formula (12) (in the case of a temperature of 77 K).

[0106] Here, t Ｃｕ is the thickness of the copper shunt layer 97 by copper plating that gives conductivity between the multi-filamentary superconducting layers 92a, 92a.

[0107] When the superconducting wire 10B according to the second embodiment is wound spirally, considering that one side of the loop of the coupling current 12 passes through the connection part 4, the resistance of the loop of the coupling current 12 is halved, and as shown in the following formula (13), the coupling time constant τ ｃ２ becomes twice that of formula (12).

[0108] When the coupling time constant is τ ｃ and the characteristic frequency f ｃ = 1 / (2πτ ｃ ), with the operating frequency being the characteristic frequency f ｃIf it is about one-tenth of that, the coupling current will be attenuated, reducing AC losses and error magnetic fields. In other words, in the superconducting wire 10B according to the second embodiment in which the connection part 4 is provided, if the operating frequency is less than or equal to the value of the following equation (14), the effect of reducing AC losses and the effect of reducing error magnetic fields due to sustained eddy currents will be exhibited.

[0109] The frequency calculated by equation (14) is, for example, 500 Hz or higher, which is higher than the operating frequency of many electrical devices that operate on AC. In other words, many electrical devices have an operating frequency that is less than or equal to the value of equation (14), satisfying the operating frequency conditions for the AC loss reduction effect and error magnetic field reduction effect of the superconducting wire 10 to be exhibited. Therefore, the superconducting wire 10 and the superconducting cable 20 can be applied to electrical devices that operate on AC with improved robustness, and with the AC loss reduction effect and error magnetic field reduction effect to be exhibited. <Superconducting Wire - Third Embodiment>

[0110] The superconducting wire 10C (10) according to the third embodiment differs from the superconducting wire 10A according to the first embodiment in that it further comprises a conductive layer 5b (5) covering the superconducting layer 2. The configuration of the superconducting wire 10C according to the third embodiment, as described below, is the same as that of the superconducting wire 10A according to the first embodiment unless otherwise specified, so redundant explanations will be omitted.

[0111] Figure 9 is a schematic diagram showing the configuration of the superconducting wire 10C according to the third embodiment. (A) is a perspective view of the superconducting wire, (B) is a plan view of the superconducting wire, and (C) is a cross-sectional view of the superconducting wire along the 8B-8B line shown in (B).

[0112] In the third embodiment, the superconducting wire 10C (10) further comprises a conductive layer 5b (5) covering the superconducting layer 2. In the illustrated embodiment, the conductive layer 5b is formed to cover only the superconducting layer 2, without covering the insulating portion 3 and the connecting portion 4. The conductive layer 5b functions as a current diversion layer that bypasses the current flowing through the superconducting layer 2 when an abnormality occurs in the superconducting layer 2. Exemplarily, the conductive layer 5b is formed of copper. In the illustrated embodiment, the insulating portion 3 is formed as a groove that exposes the surface of the substrate 1, but the conductive layer 5b is formed to cover only the superconducting layer 2, and the groove is not filled with copper that functions as the conductive layer 5b.

[0113] In the third embodiment, as in the first embodiment described with reference to Figures 5 and 6, a sustained eddy current 11a is generated in the region of the superconducting wire 10C located on the front side of the core material 9, and a sustained eddy current 11b is generated in the region of the superconducting wire 10B located on the back side of the core material 9, in a side view of the superconducting cable. The superconducting wire 10C according to the third embodiment can obtain the same AC loss reduction effect as the superconducting wire 10A according to the first embodiment.

[0114] The superconducting wire 10C according to the third embodiment has improved robustness compared to the superconducting wire 10A according to the first embodiment, due to the inclusion of a conductive layer 5b. More specifically, the rise in hot spot temperature can be suppressed by diverting the current from the superconducting layer 2 to the conductive layer 5b provided on top of the superconducting layer 2.

[0115] Similar to the first embodiment, the superconducting wire 10C according to the third embodiment can be wound spirally along the axis of the core material 9 to form a superconducting cable. [Effects]

[0116] As described above, the superconducting magnet according to one embodiment of the present invention allows the superconducting coil to be operated with alternating current while reducing AC losses. This makes it possible to generate a high-intensity alternating magnetic field.

[0117] For example, in tokamak-type fusion reactors currently under development, it is required to operate superconducting coils with alternating current while reducing AC losses. The superconducting magnet according to one embodiment of the present invention makes it possible to provide superconducting coils that can be used in such tokamak-type fusion reactors.

[0118] Furthermore, according to an AC magnetic field generator equipped with a superconducting magnet according to one embodiment of the present invention, it becomes possible to make the entire magnetic field generator, including the power supply, AC, and it becomes possible to provide an AC magnetic field generator that generates a high-intensity AC magnetic field that changes rapidly over time. [Other embodiments]

[0119] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the embodiments described above.

[0120] In the above-described embodiment, the primary winding 104 of the superconducting current transformer 103 is constructed by winding a single superconducting wire 10, but the material of the primary winding 104 is not limited to this. In other embodiments, the primary winding 104 may also be constructed by winding a superconducting cable 20, similar to the secondary winding 105. In yet another embodiment, the primary winding 104 can be constructed using a normal conductor (for example, copper). The conductive wiring 108 may also be constructed by winding a single superconducting wire 10, similar to the primary winding 104.

[0121] Since both the primary winding 104 and the wiring 108 are connected to the AC power supply 109, there is no need to construct these primary windings 104 and wiring 108 using superconductors. The materials for the primary windings 104 and wiring 108 can be any electrically conductive material, regardless of whether they are normal conductors or superconductors.

[0122] In the embodiment described above, the alternating current supplied from the AC power supply 109 is applied to the primary winding 104 of the superconducting current transformer 103 via the current input terminal 106, but the connection configuration between the AC power supply 109 and the primary winding 104 is not limited thereto. In other embodiments, a transformer (voltage transformer or current transformer) not shown may be further provided between the AC power supply 109 and the primary winding 104.

[0123] In the above-described embodiment, the superconducting wire 10 comprises four multifilamented superconducting layers 2, but the number of superconducting layers 2 is not limited. The superconducting wire 10 only needs to have at least one insulating portion 3 between the multiple superconducting layers 2, 2, and the superconducting wire 10 only needs to have at least two superconducting layers 2. Similarly, in the above-described embodiment shown in the figure, the superconducting wire 10 comprises three insulating portions 3, but the number of insulating portions 3 is not limited, and the superconducting wire 10 only needs to have at least one insulating portion 3.

[0124] In the above-described embodiment, the superconducting layer 2 is formed using REBCO high-temperature superconductor, but the material of the superconducting layer 2 is not limited. For example, the high-temperature superconductor used for the superconducting layer 2 is chemical formula YBa ２ Cu ３ O ７－ｙ Yttrium-based high-temperature superconductors (where y is the unstoichiometric amount of oxygen) and bismuth-based high-temperature superconductors can be used in the superconducting layer 2. Furthermore, the superconductor used in the superconducting layer 2 is not limited to high-temperature superconductors with a transition temperature above the liquid nitrogen temperature (77 K); superconductors with a transition temperature below the liquid nitrogen temperature can be used, and in the future, superconductors with a transition temperature closer to room temperature (approximately 300 K) can also be used. In other words, the superconducting layer 2 can use superconductors that exhibit superconductivity.

[0125] In the embodiment described above, the insulating portion 3 is exemplary formed as a groove that exposes the surface of the substrate 1, but the insulating portion 3 is not limited to a groove. The insulating portion 3 can be configured as a physical member made of, for example, various insulating materials. The insulating portion 3 only needs to be positioned between a plurality of superconducting layers 2, 2 to electrically insulate these superconducting layers 2, 2, that is, to divide their conductivity. The insulating portion 3 can also be described as a conductivity division portion that divides the superconducting conductivity of the plurality of superconducting layers 2, 2. Furthermore, the insulating portion 3 formed as a groove only needs to be able to electrically insulate the plurality of superconducting layers 2, 2, and if an intermediate layer (not shown) is formed on the surface of the substrate 1, the insulating portion 3 only needs to be a groove that exposes the intermediate layer.

[0126] In the above-described embodiment, the connecting portion 4 is formed integrally with the superconducting layer 2, but the connecting portion 4 may be formed as a separate component from the superconducting layer 2. Also, in the above-described embodiment, the connecting portion 4 is formed using the same superconductor as the superconducting layer 2, but the superconductor used for the connecting portion 4 and the superconductor used for the superconducting layer 2 may be different. In other words, the connecting portion 4 only needs to be able to superconductively electrically connect a plurality of adjacent superconducting layers 2, 2. Also, in the above-described embodiment, the connecting portion 4 is arranged in alignment in the short direction across a plurality of parallel insulating portions 3, but as illustrated in Figure 10, the connecting portion 4 may also be arranged at a position away from the line along the short direction across a plurality of parallel insulating portions 3.

[0127] In the embodiment described above, the conductive layer 5 (5a, 5b) is made of copper, but the material of the conductive layer 5 is not limited to copper. In addition to the example copper, the conductive layer 5 can be made of a material with high electrical conductivity, such as silver or gold, which is used for electrical wiring.

[0128] In the embodiment described above, the core material 9 is a solid member, but the core material 9 can also be a hollow member. For example, if the core material 9 is a solid member, the superconducting wire 10 can be wound spirally around the outer wall of the core material 9. For example, if the core material 9 is a hollow member, the superconducting wire 10 can be wound spirally around the outer wall of the core material 9, or it can be wound spirally along the inner wall of the core material 9. In other words, the core material 9 only needs to be able to spirally wind the superconducting wire 10 along the axis of the core material 9. The core material 9 can also be made of a stranded wire made by twisting together multiple strands. The strands may or may not be insulated from each other. For example, metal wires such as copper or stainless steel can be used for the strands. Exemplarily, the cross-sectional shape of the core material 9 and the strands can be a honeycomb shape in which copper is separated by a high-resistance material such as a copper-nickel alloy.

[0129] In the embodiment described above, the core material 9 is a cylindrical member, but the cross-sectional shape of the member used for the core material 9 is not limited to a circle. The cross-sectional shape of the member used for the core material 9 may be, for example, an ellipse, a regular polygon such as a regular hexagon or an equilateral triangle, a rectangle, a regular polygon with rounded corners, or a rectangle with rounded corners. Furthermore, in the case of a regular polygon, for example, by increasing the number of corners of the regular polygon representing the cross-sectional shape of the member, the cross-sectional shape of the member can be approximated to a circle more closely.

[0130] In the embodiment described above, the superconducting wire 10 is wound along the axis of the core material 9 in the direction in which a left-hand thread progresses, but the superconducting wire 10 may also be wound in the direction in which a right-hand thread progresses.

[0131] In the embodiment described above, a single layer of superconducting wire 10 is wound spirally along the axis of the core material 9, but the number of layers of superconducting wire 10 wound spirally along the axis of the core material 9 is not limited. By increasing the number of layers of superconducting wire 10 and thereby increasing the number of superconducting wires 10 used to manufacture the cable, a superconducting cable with increased current capacity can be manufactured. For example, by arranging n superconducting wires 10 in parallel on the same layer (for example, if n=3, it is a trifilar winding), winding them spirally along the axis of the core material 9, and stacking multiple such layers, a superconducting cable with increased current capacity can be manufactured. Alternatively, for example, by stacking multiple superconducting wires 10 in multiple layers and then winding them spirally along the axis of the core material 9, a superconducting cable with increased current capacity can be manufactured. Thus, when winding multiple superconducting wires 10 spirally along the axis of the core material 9, it is possible to combine the method of stacking multiple layers in which multiple superconducting wires 10 are wound in parallel on the same single layer, and the method of stacking and winding multiple superconducting wires 10 in multiple layers.

[0132] 100 AC magnetic field generator 101 Cryostat 102 Superconducting coil 103 Superconducting current transformer 104 Primary winding (of superconducting current transformer) 105 Secondary winding (of superconducting current transformer) 106 Current input terminal 107 Superconducting current lead 108 Conductive wiring 109 AC power supply 1 Substrate 2 (2a, 2b, 2c, 2d) Superconducting layer 3 Insulation 4 Connection 5 (5a, 5b) Conductive layer 9 Core material 10 (10A, 10B, 10C, 10D) Superconducting wire 11 (11a, 11b) Sustained eddy current 12 Coupling current 19 Electromotive force 20 (20A) Superconducting cable 29 Connecting component 80 Superconducting wire 81 Superconducting layer 82 Eddy current 83 Electromotive force 90 (90A, 90B, 90C) Superconducting wire without connection 91 Substrate 92 Superconducting layer 92a Multifilamented superconducting layer 93 Magnetic flux quantum 95 Normal conducting state 96 Current 97 Copper shunt layer 98 Coupling current 99 (99a, 99b) Eddy current

Claims

1. A superconducting magnet comprising: a superconducting coil housed in a cryostat; and a superconducting current transformer housed in the cryostat, the primary winding of which is connected to an AC power source and the secondary winding of which is connected to the superconducting coil, wherein the superconducting coil and the secondary winding are constructed by winding a superconducting cable in which a superconducting wire is wound spirally along the axis of a core material.

2. The superconducting magnet according to claim 1, wherein the superconducting cable is made up of multiple superconducting wires that are spirally wound along the axis of a core material and then stacked in multiple layers.

3. The superconducting magnet according to claim 1, wherein the AC power supply is located outside the cryostat, and the primary winding is connected to the AC power supply via a current introduction terminal provided at the boundary between the inside and outside of the cryostat.

4. The superconducting magnet according to claim 1, wherein the superconducting cable is configured by connecting a plurality of the superconducting cables in series via a connecting member having electrical resistance, the electrical resistance value of the connecting member is an electrical resistance value corresponding to the AC frequency of the AC power supply, and the electrical resistance value is such that the superconducting coil can generate an AC magnetic field in a superconducting state.

5. The superconducting magnet according to claim 1, wherein the cryostat maintains the superconducting coil and the superconducting winding of the superconducting current transformer in a superconducting state.

6. The superconducting wire comprises: a plurality of superconducting layers stretched in the longitudinal direction of the substrate and arranged in parallel in the short direction of the substrate; and an insulating portion stretched in the longitudinal direction of the substrate and arranged between the plurality of superconducting layers to electrically insulate the plurality of superconducting layers, and when wound in a spiral shape, the following conditions The superconducting magnet according to claim 1, which satisfies the following, where D is the diameter of the helix, P is the length of the helical pitch along the winding axis direction, and L is the length of the insulating portion along the longitudinal direction.

7. An AC magnetic field generating device comprising: a cryostat; an AC power supply located outside the cryostat; and a superconducting magnet according to any one of claims 1 to 6 housed inside the cryostat.

Images