Superconducting coil and superconducting magnet device

Abstract

This superconducting coil superconductively connects both ends of a superconducting coil winding (4) and both ends of a permanent current switch (5), wherein: a pair of power leads (6) for supplying current to the superconducting coil are included; the both ends are each configured such that a superconducting wire (main wire (7)) constituting the superconducting coil winding, a superconducting wire (PCS wire (8)) constituting the permanent current switch (5), and a superconducting connecting section (9) that superconductively connects the main wire and the PCS wire are disposed on the power lead (6); the superconducting wire constituting the superconducting coil winding (4) and the power lead (6) are electrically coupled; and a heater (emergency demagnetization heater (10)) for heating the superconducting connecting section (9) is provided; a diode (13) is disposed between the pair of power leads (6); and the pair of power leads are electrically coupled via the diode.

Description

Superconducting coil and superconducting magnet device

[0001] The present invention relates to a superconducting coil capable of being demagnetized in an emergency, and a superconducting magnet device using the same.

[0002] The basic circuit of a conventional superconducting electromagnet device consists of a superconducting coil, an excitation power supply that supplies current to the superconducting coil, a persistent current switch (PCS) that forms a closed circuit for persistent current operation, and a protective resistor and diode installed in parallel with the superconducting coil. The persistent current switch is heated to cause a normal-conducting transition, and current is supplied to the superconducting coil from the excitation power supply. The persistent current switch is then cooled to a superconducting state, and the current supply from the excitation power supply is then stopped, resulting in persistent current operation, in which current continues to flow in the superconducting closed circuit consisting of the superconducting coil and the persistent current switch. This allows the superconducting electromagnet to maintain a magnetic field for a long period of time.

[0003] This property has been utilized in devices such as MRI (Magnetic Resonance Imaging) and NMR (Nuclear Magnetic Resonance) devices. Furthermore, because the superconducting coil can be moved independently of the excitation power source, it is expected to be used in rotating machines such as generators and motors.

[0004] When it becomes necessary to quickly eliminate the magnetic field in a conventional superconducting magnet, the superconducting coil or persistent current switch is heated by a heater to cause a normal conducting transition. This diverts the current to the protective resistor and diode connected in parallel to the superconducting coil and persistent current switch, dissipating energy through Joule heating and attenuating the coil current, allowing the magnetic field to be quickly eliminated. This operation is called emergency demagnetization.

[0005] In the case of a device with multiple superconducting coils, a persistent current switch is placed in parallel with the multiple superconducting coils connected in series, and the connections between them are made using superconducting connections that have zero electrical resistance when cooled. This configuration makes it possible to achieve persistent current operation, which allows a magnetic field to be maintained for a long period of time without current decay, even in a device with multiple superconducting coils. However, because superconducting connections cannot be remade like ordinary solder connections, if one of the multiple superconducting coils or persistent current switches fails, it is impossible to repair it by simply replacing it.

[0006] Therefore, a modular superconducting coil has been proposed in which both ends of one superconducting coil and one persistent current switch are superconductively connected (for example, Non-Patent Document 1). Even when multiple modular superconducting coils are used, persistent current operation in which current continues to flow in the closed circuit within each modular superconducting coil is possible by operating them according to the above procedure. As described above, by using modular superconducting coils, it is possible to treat superconducting coils as replaceable general-purpose parts.

[0007] Weijun Yao et al.,“MgB2 Coils for MRI Applications”, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY,VOL.20,NO.3,JUNE 2010

[0008] In conventional superconducting magnet devices, in the event of an emergency demagnetization, a superconducting element such as a superconducting coil or persistent current switch is heated by a heater to cause a normal conducting transition, and the current is attenuated by a protective resistor and diode connected in parallel. However, when multiple superconducting coils are used, it is necessary to increase the size of the protective resistor or provide some kind of cooling means to prevent the protective resistor from being damaged by excessive Joule heat. Furthermore, when high-temperature superconductors are used as superconducting elements, it is difficult to heat the entire element evenly to cause a normal conducting transition due to their high critical temperature. If only a portion of the superconducting element were to undergo a normal conducting transition, localized Joule heat could occur, potentially damaging the element.

[0009] In view of the above, an object of the present invention is to provide a superconducting coil and a superconducting magnet device capable of persistent current operation, which makes it possible to omit protective resistors and their cooling devices that are installed outside the device, and which can prevent damage to the superconducting coil and persistent current switch in the event of emergency demagnetization.

[0010] In order to solve the above problems, the superconducting coil of the present invention has a pair of power leads for supplying current to the superconducting coil, in which both ends of a superconducting coil winding and a persistent current switch are superconductively connected, and each end has a superconducting wire constituting the superconducting coil winding, a superconducting wire constituting the persistent current switch, and a superconducting joint superconductively connecting them, which are arranged on the power leads, the superconducting wire constituting the superconducting coil winding and the power leads are electrically connected, a heater is provided for heating the superconducting joint, and a diode is arranged between the pair of power leads, and the pair of power leads are electrically connected via the diode. Other aspects of the present invention will be described in the embodiments described later.

[0011] According to the present invention, in a high-temperature superconducting magnet device capable of persistent current operation, it is possible to omit the protective resistor and its cooling device that are installed outside the device, and to prevent damage to the superconducting coil and persistent current switch in the event of emergency demagnetization.

[0012] FIG. 1 is a diagram schematically showing a circuit configuration of a superconducting magnet device according to a first embodiment. FIG. 2 is a diagram showing wiring on a power lead according to the first embodiment. FIG. 3 is a cross-sectional view of a superconducting connection according to the first embodiment. FIG. 4 is a diagram schematically showing the appearance of a modular superconducting coil according to the first embodiment. FIG. 5 is a diagram schematically showing a cross-section of a modular superconducting coil according to the first embodiment. FIG. 6 is a diagram schematically showing operation during first emergency demagnetization. FIG. 7 is a diagram showing wiring on a power lead according to a second embodiment. FIG. 8 is a cross-sectional view of wiring on a power lead according to the second embodiment. FIG. 9 is a diagram showing wiring on a power lead according to a third embodiment. FIG. 10 is a cross-sectional view of a superconducting connection according to the third embodiment. FIG. 11 is a diagram schematically showing a circuit configuration according to a fourth embodiment.

[0013] Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. (First Embodiment) A first embodiment to which the present invention is applied will be described below with reference to Figs. 1 to 6. Fig. 1 is a diagram showing a schematic diagram of the circuit configuration of a superconducting magnet device according to the first embodiment. The superconducting magnet device 1 to which the present invention is applied is composed of a modular superconducting coil 3 contained in a cryostat 2, an excitation power supply 11 provided outside the cryostat 2, a circuit breaker 12, and a diode 13 that electrically couples the power leads.

[0014] The modular superconducting coil 3 comprises a superconducting coil winding 4, a persistent current switch 5 arranged in parallel therewith, a PCS heater 14 in thermal contact with the persistent current switch, power leads 6 (power leads 6a, 6b) for supplying current to the modular superconducting coil, a wire for the superconducting coil (hereinafter referred to as a main wire 7), a wire for the persistent current switch (hereinafter referred to as a PCS wire 8), superconducting joints 9 (superconducting joints 9a, 9b) connecting these in a superconducting state, and an emergency demagnetization heater 10 in thermal contact with the superconducting joints. All superconducting elements contained in the cryostat 2 are kept at or below their critical temperature by cooling means (not shown) and in a superconducting state. Superconducting materials applicable to the above superconducting elements include bismuth-based wire, rare earth-based wire, and magnesium diboride.

[0015] During persistent current operation, first, the persistent current switch 5 is heated by a heater to cause a normal conducting transition, and then a current is supplied from the excitation power supply 11 to the modular superconducting coil 3. Then, the heater is stopped, and the persistent current switch 5 is returned to its superconducting state. Then, the current supply from the excitation power supply 11 is stopped, and the persistent current switch 5 is returned to its superconducting state. This results in persistent current operation, in which current continues to flow through the closed circuit consisting of the superconducting coil winding 4 and the persistent current switch 5 with almost no attenuation. This allows the superconducting magnet device 1 to maintain a magnetic field for a long period of time. To cancel the persistent current operation, current is supplied again from the excitation power supply 11 while the persistent current switch 5 is still in the superconducting state. Then, the persistent current switch is heated by a heater to cause a normal conducting transition, directly connecting the excitation power supply 11 and the superconducting coil winding 4, and the persistent current mode is canceled.

[0016] When emergency demagnetization is required to quickly eliminate the magnetic field of a superconducting magnet device due to some reason, conventional superconducting magnet devices use a heater to heat the superconducting coil or persistent current switch to normal conduction, and then the current is attenuated by dissipating energy through protective resistors and diodes connected in parallel to the superconducting coil. However, when multiple superconducting coils are used, it is necessary to either increase the size of the protective resistors themselves or provide some kind of cooling means to prevent the protective resistors from being damaged by excessive Joule heat. Furthermore, when high-temperature superconductors are used for the superconducting coils and persistent current switches, their high critical temperatures make it difficult to uniformly heat the entire element to achieve normal conduction. If only a portion of the superconducting element undergoes normal conduction, localized Joule heat may occur, potentially damaging the element.

[0017] Therefore, in this embodiment, as shown in FIG. 1, a main line 7, a PCS line 8, and a superconducting connection part 9 are arranged on a power lead 6 that supplies current to the modular superconducting coil 3.

[0018] Fig. 2 is a diagram showing wiring on a power lead 6 according to the first embodiment. Fig. 3 is a cross-sectional view (cross-section A-A') of a superconducting joint according to the first embodiment. Fig. 4 is a diagram schematically showing the appearance of a modular superconducting coil 3 according to the first embodiment.

[0019] 2, the main line 7 and the PCS line 8 are electrically connected to a power lead 6 made of a good conductor such as copper in a section just before they enter the superconducting joint 9. The superconducting joint 9 is provided with a heater for causing a normal conducting transition (hereinafter referred to as the superconducting joint heater (emergency demagnetization heater 10)).

[0020] As shown in the cross-sectional view of Fig. 3, inside the superconducting joint 9, the main wire 7 and the PCS wire 8 are connected via a superconducting bulk 70 inside a joint container (superconducting joint container 71) made of stainless steel or the like. To start emergency demagnetization during persistent current operation, an emergency demagnetization heater 10 is first used to quickly heat the superconducting joint, which has a relatively small heat capacity, so that it can undergo a normal conducting transition. As a result, the current flowing through current path 51 shown in Fig. 2 begins to flow through the power lead as shown in current path 52.

[0021] As shown in Figures 1 and 4, a pair of power leads 6 (power lead 6a and power lead 6b) are electrically coupled via a diode 13. The electrical coupling is achieved by using solder 21 for the main line 7 and PCS line 8, and by fastening a current cable made of a good conductor to the diode with bolts. Current is supplied from outside the device by fastening a current cable 26 made of a good conductor to the power leads with bolts, as shown in Figures 1 and 4.

[0022] 5 is a diagram showing a cross section (cross section B-B' and cross section C-C' in FIG. 4) of the modular superconducting coil 3 according to the first embodiment. As shown in FIG. 5, the main wires 7 and the PCS wires 8 other than those on the power leads 6 are fixed with epoxy resin or the like to a cooling copper plate 23 that cools the superconducting coil winding 4 in the coil bobbin 24, and are kept below the critical temperature, just like the superconducting coil winding 4.

[0023] The operation during emergency demagnetization in this embodiment will be described with reference to FIG. 6. FIG. 6 is a diagram schematically illustrating the operation during the first emergency demagnetization. To start emergency demagnetization during persistent current operation, the emergency demagnetization heater 10 is first used to quickly heat the superconducting joint 9, which has a relatively small heat capacity, to cause a normal conducting transition. As a result, the current flowing through the current path 51 shown in FIG. 2 begins to flow through the power lead 6 as shown in current path 52. Thereafter, the Joule heat of the current flowing through the power lead 6 warms the main wire 7 and the PCS wire 8, expanding the normal conducting region, and the voltage across the diode 13 increases.

[0024] Then, when the voltage across the diode 13 reaches the turn-on voltage, the current flowing through the persistent current switch 5 is diverted to the diode 13 side. If the PCS heater 14 is turned on in this state, Joule heat due to the normal conducting transition can be reduced, and it becomes possible to prevent damage to the persistent current switch 5. Thereafter, the persistent current switch 5 is heated to cause the normal conducting transition, and almost all of the current flowing through the persistent current switch 5 is diverted to the diode 13 side. This allows the current in each modular superconducting coil 3 to be rapidly attenuated, and emergency demagnetization becomes possible without providing a protective resistor outside the device.

[0025] If it is desired to accelerate the diode turn-on start time, the soldering area of ​​the PCS wire 8 can be reduced as shown in FIG. 2. That is, the area of ​​the solder 21a electrically connecting the superconducting wire (PCS wire 8) constituting the persistent current switch 5 to the power lead 6 is smaller than the area of ​​the solder 21b electrically connecting the superconducting wire (main wire 7) constituting the superconducting coil winding 4 to the power lead 6. This increases the electrical resistance of the current path 52, and the Joule heat generated can be increased, accelerating the expansion rate of the normal-conducting portion. Furthermore, the diode voltage shown in FIG. 6 decreases after turn-on. This is due to the temperature dependency of the turn-on voltage, and indicates that the temperature of the diode 13 is rising due to Joule heat.

[0026] Second Embodiment Fig. 7 is a diagram showing wiring on a power lead 6 according to a second embodiment. Fig. 8 is a cross-sectional view (cross-section taken along line DD') of the wiring on a power lead 6 according to the second embodiment. As shown in Figs. 7 and 8, the modular superconducting coil according to the second embodiment differs from the first embodiment in that the PCS wires 8 are electrically coupled to the power leads 6 via a metal material 54 having a high resistivity.

[0027] This not only achieves the same effects as in the first embodiment, but also makes it possible to increase the Joule heat generated in the power lead 6 after the emergency demagnetization heater 10 is turned on, thereby shortening the time required for the diode 13 to turn on.

[0028] (Third embodiment) Fig. 9 is a diagram showing wiring on a power lead 6 according to a third embodiment. Fig. 10 is a cross-sectional view (cross-section E-E' and cross-section F-F' in Fig. 9) of a superconducting joint 9 according to the third embodiment. As shown in Figs. 9 and 10, the modular superconducting coil 3 according to the third embodiment differs from the first and second embodiments in that the PCS wires 8 are not electrically coupled to the power leads 6 by soldering.

[0029] This not only provides the same effects as those of the first and second embodiments, but also reduces the time required for the diode 13 to turn on, whereas when the emergency demagnetization heater 10 is turned off, the main wires 7 and the PCS wires 8 are connected in a superconducting state via the superconducting bulk 70 provided inside the superconducting connection container 71, but by turning on the emergency demagnetization heater 10 and causing the superconducting bulk 70 to transition to normal conduction, the current path changes from 51 via the superconducting bulk 70 to current path 52 via the superconducting connection container 71. This makes it possible to increase Joule heat more than when it is via the power lead 6, and shorten the time required for the diode 13 to turn on.

[0030] 11, a modular superconducting coil 3 according to a fourth embodiment has a plurality of superconducting coils 3, each of which is connected to a current cable 26 made of good conductor, in comparison with the first embodiment. Another difference is that the plurality of emergency demagnetization heaters 10 are wired in series.

[0031] This allows for the simultaneous persistent current operation of multiple modular superconducting coils 3 in the same procedure as in the first to third embodiments. Emergency demagnetization is also possible in the same procedure as in the first to third embodiments, with the coil current being attenuated by the diodes 13 of each modular superconducting coil. In other words, the energy consumption during emergency demagnetization is shared among the modular superconducting coils 3, making it possible to avoid the need for larger protective resistors or additional cooling means as in conventional superconducting magnets. Furthermore, by wiring multiple emergency demagnetization heaters 10 in series, they can be energized simultaneously, enabling the simultaneous emergency demagnetization of multiple modular coils.

[0032] As described above, the modular superconducting coil 3 of this embodiment is a modular superconducting coil in which the superconducting coil winding 4 and the persistent current switch 5 are superconductively connected at both ends thereof, and includes a superconducting coil wire (hereinafter referred to as a main wire 7), a persistent current switch wire (hereinafter referred to as a PCS wire 8), and a superconducting connection that connects them in a superconducting state, arranged on a power lead 6 that supplies current to the modular superconducting coil. Either the main wire 7 and the PCS wire 8 or only the main wire 7 is electrically connected to the power lead in a section just before entering the superconducting connection 9. The pair of power leads 6 are electrically connected via a diode 13. A heating means (emergency demagnetization heater 10) is provided in the superconducting connection 9 to cause a normal conductive transition.

[0033] With the above structure, in the event of emergency demagnetization, the superconducting joint 9, which has a small heat capacity, can be quickly brought to a normal conducting transition, and part of the current flowing through the persistent current switch 5 can be diverted to the diode 13 provided between the power leads 6. The persistent current switch 5 is then heated to bring about a normal conducting transition, and almost all of the current flowing through the persistent current switch 5 is diverted to the diode 13 side. This allows the current in each modular superconducting coil 3 to be quickly attenuated, and emergency demagnetization becomes possible without providing a protective resistor outside the device. Furthermore, the persistent current switch 5 can be brought to a normal conducting transition while the current is sufficiently limited, making it possible to prevent damage due to excessive Joule heat.

[0034] REFERENCE SIGNS LIST 1 Superconducting magnet device 2 Cryostat 3 Modular superconducting coil (superconducting coil) 4 Superconducting coil winding 5 Persistent current switch 6, 6a, 6b Power lead 7 Main wire (wire for superconducting coil) 8 PCS wire (wire for persistent current switch) 9, 9a, 9b Superconducting joint 10 Emergency demagnetization heater (heater) 11 Excitation power supply 12 Circuit breaker 13 Diode 14 PCS heater 21, 21a, 21b Solder 23 Cooling copper plate 24 Coil bobbin 26 Current cable 51 Current path (current path during persistent current operation) 52 Current path (current path during emergency demagnetization) 54 Metal material with high resistivity 70 Superconducting bulk 71 Superconducting joint container

Claims

1. A superconducting coil in which both ends of a superconducting coil winding and a persistent current switch are superconductively connected, the superconducting coil having a pair of power leads for supplying current to the superconducting coil, each of the two ends having a superconducting wire constituting the superconducting coil winding, a superconducting wire constituting the persistent current switch, and a superconducting connection portion superconductively connecting them, the superconducting wire constituting the superconducting coil winding and the power leads are electrically connected, and a heater is provided for heating the superconducting connection portion, a diode is arranged between the pair of power leads, and the pair of power leads are electrically connected via the diode.

2. A superconducting coil as claimed in claim 1, characterized in that the area of ​​the solder electrically connecting the superconducting wire constituting the persistent current switch and the power lead is smaller than the area of ​​the solder electrically connecting the superconducting wire constituting the superconducting coil winding and the power lead.

3. A superconducting coil as claimed in claim 1, characterized in that the superconducting wire constituting the persistent current switch and the power lead are electrically connected via a metal having a higher resistivity than the metal constituting the power lead.

4. A superconducting coil as described in claim 1, characterized in that the superconducting connection portion is composed of a superconducting bulk material and a superconducting connection container containing the superconducting bulk material, and the superconducting wire constituting the superconducting coil winding arranged on the power lead and the superconducting wire constituting the persistent current switch are electrically connected at the superconducting connection portion via the superconducting bulk material and the superconducting connection container.

5. A superconducting magnet device, characterized in that a plurality of superconducting coils according to any one of claims 1 to 4 are arranged in a cryostat, and the plurality of superconducting coils are connected with each other by normal conducting current cables.

6. A superconducting magnet device according to claim 5, characterized in that the heaters for heating the multiple superconducting joints are connected in series.

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