Superconducting magnet device

Abstract

A superconducting magnet device (10) comprises: a primary circuit comprising a superconducting coil (12) and a primary-side resistor connected to the superconducting coil (12); and a secondary circuit comprising a secondary coil (32) electromagnetically coupled to the superconducting coil (12), and a secondary-side resistor connected to the secondary coil (32). The time constant of the secondary circuit is greater than or equal to the time constant of the primary circuit. The coupling coefficient between the superconducting coil (12) and the secondary coil (32) may be 0.3 or more.

Description

Superconducting magnet device

[0001] The present invention relates to a superconducting magnet device.

[0002] Thermal runaway (quench) of superconducting coils is an undesirable phenomenon that can occur during the operation of superconducting magnet devices. When a quench occurs, the superconducting coil transitions from superconductivity to normal conduction, generating resistance within the coil. The large current flowing through the coil during its previous superconducting state can generate a large amount of Joule heat. A voltage increase within the coil and resulting discharge may also occur. Additionally, the transient current imbalance during a quench can exert a large electromagnetic force on the superconducting coil. Eddy currents can also be generated in conductors located near the coil, generating electromagnetic forces. The resulting heat, discharge, and electromagnetic force can damage the superconducting coil and its surrounding structures and equipment. Therefore, installing an induction coil near the superconducting coil has been proposed. During a quench, energy can be recovered from the superconducting coil to the induction coil through electromagnetic induction, allowing the energy stored in the superconducting coil to be released.

[0003] JP 2015-142044 A

[0004] When a superconducting coil experiences a quench, its temperature rises due to Joule heat, and it must be re-cooled to recover. The more energy that is recovered from the superconducting coil when a quench occurs, the more the temperature rise in the superconducting coil can be suppressed, and the shorter the time it takes to re-cool the superconducting coil, i.e., to recover.

[0005] One exemplary objective of certain aspects of the present invention is to efficiently recover energy from superconducting coils.

[0006] According to one aspect of the present invention, a superconducting magnet apparatus includes a primary circuit including a superconducting coil and a primary resistor connected to the superconducting coil, and a secondary circuit including a secondary coil electromagnetically coupled to the superconducting coil and a secondary resistor connected to the secondary coil, wherein the time constant of the secondary circuit is equal to or greater than the time constant of the primary circuit.

[0007] According to the present invention, energy can be efficiently recovered from a superconducting coil.

[0008] Fig. 5(a) and Fig. 5(b) are schematic diagrams showing a superconducting magnet device according to an embodiment; Fig. 5(a) is a graph showing the relationship between the time constant ratio τ2 / τ1 and the energy extraction efficiency η according to an analysis by the inventor of the present invention according to an embodiment; Fig. 5(b) is a graph showing the relationship between the coupling coefficient κ and the energy extraction efficiency η according to an analysis by the inventor of the present invention according to an embodiment; Fig. 5(a) is a diagram showing a superconducting magnet device according to another embodiment; Fig. 5(b ... schematic diagram of another example of a superconducting magnet device according to an embodiment;

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description and drawings, identical or equivalent components, parts, and processes are designated by the same reference numerals, and redundant explanations will be omitted as appropriate. The scale and shape of each part shown in the drawings are set for convenience to facilitate explanation, and should not be interpreted as limiting unless otherwise specified. The embodiments are merely examples and do not limit the scope of the present invention in any way. All features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0010] 1 is a schematic diagram showing a superconducting magnet device 10 according to an embodiment. The superconducting magnet device 10 is installed in high-magnetic field equipment as a magnetic field source for, for example, a single crystal pulling device, an NMR (Nuclear Magnetic Resonance) system, an MRI (Magnetic Resonance Imaging) system, an accelerator such as a cyclotron, a high-energy physics system such as a nuclear fusion system, or other high-magnetic field equipment (not shown), and can generate the high magnetic field required for the equipment.

[0011] The superconducting magnet device 10 comprises a superconducting coil 12, a vacuum vessel 14, a cryogenic refrigerator 16, a heat shield 18, a current introduction line 20, a first Joule heat generating element 22, and an energy extraction mechanism 30 having a secondary coil 32 and a second Joule heat generating element 34.

[0012] The superconducting coil 12 is disposed within a vacuum vessel 14 and is configured to generate a strong magnetic field when current is applied while the coil is cooled to an extremely low temperature below the superconducting transition temperature. The superconducting coil 12 may be, for example, a so-called low-temperature superconducting coil, or any other known superconducting coil. The superconducting coil 12 is connected to an external power supply 24 disposed outside the vacuum vessel 14 via a current introduction line 20. An excitation current is supplied from the external power supply 24 to the superconducting coil 12 via the current introduction line 20. This enables the superconducting magnet device 10 to generate a strong magnetic field.

[0013] The vacuum vessel 14 is an insulated vacuum vessel, also called a cryostat, that provides a cryogenic vacuum environment suitable for bringing the superconducting coil 12 into a superconducting state. Typically, the vacuum vessel 14 has a cylindrical shape or a hollow cylindrical shape with a central cavity. Therefore, if the vacuum vessel 14 has a cylindrical shape, it has a generally flat, circular top plate 14a and a bottom plate 14b, and a cylindrical outer wall connecting them. If the vacuum vessel 14 has a cylindrical shape, it has a generally flat, annular top plate 14a and a bottom plate 14b, and coaxially arranged cylindrical outer and inner walls connecting them. As an example, a cryogenic refrigerator 16 may be installed on the top plate 14a of the vacuum vessel 14. The vacuum vessel 14 is formed of a metallic material, such as stainless steel, or other suitable high-strength material to withstand ambient pressure. The ambient pressure may be atmospheric pressure, for example.

[0014] The cryogenic refrigerator 16 is configured to cool the heat shield 18 and the first Joule heat generating element 22 to a first cooling temperature and to cool the superconducting coil 12 to a second cooling temperature lower than the first cooling temperature. In this embodiment, the cryogenic refrigerator 16 is a two-stage Gifford-McMahon (GM) refrigerator and includes a first cooling stage 16a and a second cooling stage 16b. The first cooling stage 16a and the second cooling stage 16b are provided to surround the first expansion space and the second expansion space, respectively, within the cryogenic refrigerator 16 and are formed of a metal material such as copper or other material with high thermal conductivity. The first cooling temperature may be in the temperature range of about 20 K to about 100 K, for example, in the temperature range of about 30 K to about 50 K, and the second cooling temperature may be in the temperature range of about 3 K to about 20 K, for example, about 4 K.

[0015] The heat shield 18 is disposed within the vacuum vessel 14 to surround the superconducting coil 12. The heat shield 18 is formed of a metallic material, such as copper, or another material with high thermal conductivity. The heat shield 18 is directly attached to the first cooling stage 16a of the cryocooler 16 and thermally coupled thereto. Alternatively, the heat shield 18 may be attached to the first cooling stage 16a via a flexible or rigid heat transfer member. During operation of the superconducting magnet device 10, the heat shield 18 is cooled to a first cooling temperature by the first cooling stage 16a. The heat shield 18 can thermally protect low-temperature parts, such as the second cooling stage 16b of the cryocooler 16 and the superconducting coil 12, which are disposed inside the heat shield 18 and cooled to a lower temperature than the heat shield 18, from radiant heat from the vacuum vessel 14.

[0016] The superconducting coil 12 is thermally coupled to the second cooling stage 16b via a heat transfer member 26. The heat transfer member 26 is formed of a metal material, such as copper, or another material with high thermal conductivity, and connects the superconducting coil 12 to the second cooling stage 16b. The heat transfer member 26 may be a rigid member that rigidly connects the superconducting coil 12 to the second cooling stage 16b, or it may be flexible and connect the superconducting coil 12 to the second cooling stage 16b to allow relative displacement between them. Alternatively, the superconducting coil 12 may be directly attached to the second cooling stage 16b and thermally coupled to the second cooling stage 16b. During operation of the superconducting magnet apparatus 10, the superconducting coil 12 is cooled to the second cooling temperature by the second cooling stage 16b.

[0017] The current introduction line 20 includes an external wiring 20a, a feed-through portion 20b, an outer current lead portion 20c, and an inner current lead portion 20d, and forms a current path from an external power supply 24 to the superconducting coil 12. Typically, one current introduction line 20 on the positive electrode side and one current introduction line 20 on the negative electrode side are provided.

[0018] An external wiring 20a arranged outside the vacuum vessel 14 connects an external power source 24 to a feed-through portion 20b provided in the wall of the vacuum vessel 14. The external wiring 20a may be an appropriate power supply cable. The feed-through portion 20b is an airtight terminal for introducing current into the vacuum vessel 14, and connects the external wiring 20a to internal wiring within the vacuum vessel 14, i.e., an outer current lead portion 20c and an inner current lead portion 20d. The feed-through portion 20b allows the current introduction line 20 to penetrate the wall of the vacuum vessel 14 while maintaining the airtightness of the vacuum vessel 14.

[0019] The outer current lead 20c is disposed outside the heat shield 18 within the vacuum vessel 14, and connects the feedthrough 20b to the inner current lead 20d. The outer current lead 20c is formed of a metal material with excellent conductivity, such as pure copper, e.g., oxygen-free copper. The end of the outer current lead 20c connected to the inner current lead 20d is thermally coupled to the heat shield 18 and cooled to a first cooling temperature by the first cooling stage 16a.

[0020] The inner current lead 20d is disposed inside the thermal shield 18 and connects the outer current lead 20c to the superconducting coil 12. A first end of the inner current lead 20d connected to the outer current lead 20c is thermally coupled to the thermal shield 18 and is cooled to a first cooling temperature similar to the thermal shield 18. A second end of the inner current lead 20d connected to the superconducting coil 12 is cooled to a second cooling temperature similar to the superconducting coil 12.

[0021] The inner current lead portion 20d may include a high-temperature superconducting current lead connecting the first end and the second end. The high-temperature superconducting current lead may be formed, for example, of a copper oxide superconductor or other high-temperature superconducting material. Such a high-temperature superconducting current lead material has thermal insulating properties. Therefore, compared to when the inner current lead portion 20d is made of metal, the heat that can be transferred from the first Joule heat generating element 22 to the superconducting coil 12 through the inner current lead portion 20d as a heat transfer path can be reduced. This can reduce the thermal load on the second cooling stage 16b of the cryocooler 16 and help to effectively cool the superconducting coil 12.

[0022] The first Joule heat generating element 22 is capable of generating heat when energized and may be a general linear resistance element (i.e., obeying Ohm's law) or a non-linear resistance. In this embodiment, the first Joule heat generating element 22 is, for example, a first diode 22a. The first Joule heat generating element 22 is connected in parallel to the superconducting coil 12. One end of the first Joule heat generating element 22 is connected to the positive current introducing line 20, and the other end is connected to the negative current introducing line 20, thereby connecting the first Joule heat generating element 22 in parallel to the superconducting coil 12.

[0023] In this embodiment, the first Joule heat generating element 22 is cooled to a higher cooling temperature than the superconducting coil 12 during operation of the superconducting coil 12. The first Joule heat generating element 22 may be thermally coupled to the first cooling stage 16a of the cryogenic refrigerator 16 and cooled to the first cooling temperature. As shown in the example, the first Joule heat generating element 22 may be connected between the outer current lead portions 20c and installed in a portion cooled to the first cooling temperature, such as the heat shield 18.

[0024] When a quench occurs during operation of the superconducting coil 12, the voltage generated in the superconducting coil 12 is also applied to the first Joule heat generating element 22. At this time, a current can be passed from the superconducting coil 12 to the first Joule heat generating element 22, and at least a portion of the electromagnetic energy stored in the superconducting coil 12 can be converted into heat and consumed by the first Joule heat generating element 22. In this way, by extracting energy from the superconducting coil 12 using the first Joule heat generating element 22, the superconducting coil 12 can be protected in the event of a quench. By reducing the energy in the superconducting coil 12, it is possible to prevent or reduce damage to the superconducting coil 12 and its surroundings that may result from this.

[0025] If the temperature rise of the superconducting coil 12 due to a quench is large, the time required for recooling to recover will be extended, which may lead to an increase in downtime of the superconducting magnet device 10. In this embodiment, the first Joule heat generating element 22 is cooled to a higher cooling temperature than the superconducting coil 12. Therefore, recooling can be performed in a shorter time than in a typical existing design in which such a quench protection circuit is cooled to the same temperature as the superconducting coil (i.e., the second cooling temperature). This reduces the time required to recover the superconducting magnet device 10 from a quench.

[0026] Since the first Joule heat generating element 22 is cooled by the first cooling stage 16a of the cryocooler 16, the heat generated by the first Joule heat generating element 22 can be efficiently removed. This is because the cooling capacity of the first stage of the cryocooler 16 is usually larger (for example, several tens of times) than that of the second stage, and therefore has a relatively large margin. This is also advantageous in shortening the time required to restore the superconducting magnet device 10 from a quench.

[0027] Alternatively, the first Joule heat generating element 22 may be cooled to the same temperature as the superconducting coil 12. That is, the first Joule heat generating element 22 may be thermally coupled to the second cooling stage 16b of the cryogenic refrigerator 16 and cooled to the second cooling temperature. In this case, the first Joule heat generating element 22 may be connected between the inner current lead portions 20d and installed in a portion that is cooled to the second cooling temperature, such as the superconducting coil 12 or the heat transfer member 26. As a further alternative, the first Joule heat generating element 22 may be disposed in the ambient environment outside the vacuum vessel 14. In this case, the first Joule heat generating element 22 may be connected between the external wiring 20a.

[0028] As described above, the energy extraction mechanism 30 includes the secondary coil 32 and the second Joule heat generating element 34. The secondary coil 32 is disposed adjacent to or in close proximity to the superconducting coil 12 and is electromagnetically coupled to the superconducting coil 12. The secondary coil 32 may be a normal conducting coil made of a conductor such as copper. The secondary coil 32 may be a coil formed by winding a conducting wire, or may be a C-shaped ring formed of a conducting plate. The C-shaped ring can function as a coil with one turn, so to speak.

[0029] Similar to the first Joule heat generating element 22, the secondary coil 32 is cooled to a higher cooling temperature than the superconducting coil 12 during operation of the superconducting coil 12. The secondary coil 32 may be thermally coupled to the first cooling stage 16a of the cryogenic refrigerator 16 and cooled to the first cooling temperature. As shown in the illustrated example, the secondary coil 32 may be installed in a portion cooled to the first cooling temperature, such as the heat shield 18.

[0030] The secondary coil 32 is disposed facing the end face of the superconducting coil 12 with a gap therebetween. In the illustrated example, the secondary coil 32 is disposed facing the lower end face of the superconducting coil 12, but other arrangements are also possible, and the secondary coil 32 may be disposed facing the upper end face, outer peripheral face, or inner peripheral face of the superconducting coil 12. The gap between the secondary coil 32 and the superconducting coil 12 may be determined so as to avoid thermal contact between them.

[0031] The second Joule heat generating element 34 is connected across the secondary coil 32. The second Joule heat generating element 34 is capable of generating heat when current is applied, and may be a typical linear resistance element (i.e., obeying Ohm's law) or a non-linear resistance. In this embodiment, the second Joule heat generating element 34 includes, for example, a second diode 34a.

[0032] Like the first Joule heat generating element 22, the second Joule heat generating element 34 may be cooled to a higher cooling temperature than the superconducting coil 12 during operation of the superconducting coil 12. The second Joule heat generating element 34 may be thermally coupled to the first cooling stage 16a of the cryogenic refrigerator 16 and cooled to the first cooling temperature. The second Joule heat generating element 34 may be installed in a portion that is cooled to the first cooling temperature, such as the heat shield 18.

[0033] Alternatively, the second Joule heat generating element 34 may be cooled to the same temperature as the superconducting coil 12. That is, the second Joule heat generating element 34 may be thermally coupled to the second cooling stage 16b of the cryogenic refrigerator 16 and cooled to the second cooling temperature. As a further alternative, the second Joule heat generating element 34 may be disposed in the ambient environment outside the vacuum vessel 14.

[0034] The nonlinear resistance element used as the first Joule heat generating element 22 or the second Joule heat generating element 34 may have nonlinear characteristics such that the resistance value is high when the voltage across the nonlinear resistance is small and the resistance value is low when the voltage across the nonlinear resistance is large. That is, the nonlinear resistance may have a first resistance value when the voltage across the nonlinear resistance is a first value, and a second resistance value smaller than the first resistance value when the voltage across the nonlinear resistance is a second value greater than the first value.

[0035] The nonlinear resistor may be, for example, a rectifying element such as a diode or a thyristor. Alternatively, the nonlinear resistor may be a varistor. At least one of the first Joule heat generating element 22 and the second Joule heat generating element 34 may include both a linear resistor and a nonlinear resistor, and these may be connected in series, for example.

[0036] The secondary coil 32 and the second Joule heat generating element 34 are connected by electrical wiring 36. The electrical wiring 36 may be formed of a metallic material, such as copper, or other conductive material, or may comprise a high temperature superconducting current lead.

[0037] Generally, when a quench or a symptom thereof is detected in the superconducting coil 12, operation of the superconducting magnet device 10 is stopped and the superconducting coil 12 is demagnetized. The magnetic field generated by the superconducting coil 12 is rapidly reduced from a desired high magnetic field to zero. At this time, a current is induced in the secondary coil 32 by electromagnetic induction. This current flows from the secondary coil 32 through the electrical wiring 36 to the second Joule heat generating element 34, causing the second Joule heat generating element 34 to generate heat. In this way, the electromagnetic energy stored in the superconducting coil 12 is electromagnetically extracted by the secondary coil 32 and converted into heat by the second Joule heat generating element 34. The heat is removed by cooling using the cryogenic refrigerator 16.

[0038] In this way, at least a part of the energy of the superconducting coil 12 can be consumed by the energy extraction mechanism 30. By reducing the energy of the superconducting coil 12, it is possible to prevent or reduce damage to the superconducting coil and its surroundings that may occur as a result of this.

[0039] Furthermore, if the temperature rise of the superconducting coil 12 due to a quench is large, the time required for recooling to recover will be extended, which may lead to an increase in downtime of the superconducting magnet device 10. In this embodiment, the energy extraction mechanism 30 can extract energy from the superconducting coil 12, which is advantageous in that it is possible to suppress the temperature rise of the superconducting coil 12 and shorten the time required for recovery from a quench.

[0040] Since the energy take-off mechanism 30 is cooled by the first cooling stage 16a of the cryogenic refrigerator 16, the heat generated by the energy take-off mechanism 30 can be efficiently removed. As mentioned above, the first stage of the cryogenic refrigerator 16 usually has a larger cooling capacity (for example, several tens of times larger) than the second stage, so there is a relatively large margin. This is also advantageous in shortening the time required to recover the superconducting magnet device 10 from a quench.

[0041] The inventors have discovered that a large time constant τ2 of the secondary circuit, i.e., the energy extraction mechanism 30, in the above-described superconducting magnet device 10 is advantageous for extracting energy from the superconducting coil 12. The time constant τ2 of the secondary circuit is defined by the ratio L2 / R2 of the inductance L2 of the secondary coil 32 to the resistance R2 of the secondary-side resistor connected thereto. The larger the inductance L2, the larger the induced electromotive force in the secondary coil 32, and the smaller the resistance R2, the larger the current value in the secondary coil 32. In other words, a large time constant τ2 of the secondary circuit means that a large current can be passed through the secondary coil 32. This enables the energy extraction mechanism 30 to extract energy efficiently from the superconducting coil 12.

[0042] When determining the time constant τ2 of the secondary circuit, the resistance value R2 of the secondary-side resistor is the resistance value of the second Joule heat generating element 34. If the second Joule heat generating element 34 is a linear resistor, its constant resistance value is used. If the second Joule heat generating element 34 is a nonlinear resistor, such as the second diode 34a, the differential resistance value (dV2 / dI2) can be used. V2 and I2 represent the voltage and current acting on the second Joule heat generating element 34, respectively. If the resistance value R2 of the secondary-side resistor varies in this way, the minimum value may be used as a representative value to determine the time constant τ2 of the secondary circuit. If the second Joule heat generating element 34 includes multiple resistor elements, for example, both linear and nonlinear resistors, the secondary-side resistor is considered to be the combined resistance of these multiple resistor elements.

[0043] The time constant τ1 of the primary circuit in the superconducting magnet device 10 can be used as a criterion for indicating that the time constant τ2 of the secondary circuit is large. Therefore, as will be described later, the time constant τ2 of the secondary circuit may be equal to or greater than the time constant τ1 of the primary circuit. The primary circuit includes a superconducting coil 12 and a primary-side resistor connected thereto. The time constant τ1 of the primary circuit is defined by the ratio L1 / R1 of the inductance L1 of the superconducting coil 12 to the resistance value R1 of the primary-side resistor.

[0044] When determining the time constant τ1 of the primary circuit, the resistance value R1 of the primary-side resistor may be the resistance value of a resistive element connected to the primary circuit, for example, the first Joule heat generating element 22. If the first Joule heat generating element 22 is a linear resistor, its constant resistance value may be used. If the first Joule heat generating element 22 is a nonlinear resistor, such as the first diode 22a, the differential resistance value (dV1 / dI1) may be used. V1 and I1 represent the voltage and current acting on the first Joule heat generating element 22, respectively. If the resistance value R1 of the primary-side resistor varies in this manner, the maximum value may be used as a representative value to determine the time constant τ1 of the primary circuit. If the first Joule heat generating element 22 includes multiple resistive elements, for example, both linear and nonlinear resistors, the primary-side resistance may be considered to be the combined resistance of these multiple resistive elements. Where possible, other resistances included in the primary circuit, such as the resistance of the superconducting coil 12 during quenching and connection resistance, may also be taken into account in addition to the resistive elements connected to the primary circuit. That is, the combined resistance of these resistors may be used as the resistance value R1 of the primary side resistor.

[0045] Fig. 2 is a graph showing the relationship between the time constant ratio τ2 / τ1 and the energy extraction efficiency η, as analyzed by the inventors of the present invention. In Fig. 2, the horizontal axis represents the ratio τ2 / τ1 of the time constant τ2 of the secondary circuit to the time constant τ1 of the primary circuit, and the vertical axis represents the energy extraction efficiency η. The relationship between the time constant ratio τ2 / τ1 and the energy extraction efficiency η was calculated for a number of different values ​​of the coupling coefficient κ between the superconducting coil 12 and the secondary coil 32, and is shown in Fig. 2. Specifically, the relationship is shown for three coupling coefficient κ values ​​of 0.5, 0.7, and 0.9.

[0046] Here, the energy extraction efficiency η is defined as the ratio E2 / E1 of the electromagnetic energy E2 extracted by the secondary coil 32 to the electromagnetic energy E1 released from the superconducting coil 12 due to a quench. Therefore, a large value of the energy extraction efficiency η indicates that energy is efficiently extracted from the superconducting coil 12 by the energy extraction mechanism 30.

[0047] As can be seen from Fig. 2, the energy extraction efficiency η increases monotonically as the time constant ratio τ2 / τ1 increases. More specifically, the energy extraction efficiency η increases at a significant rate when the time constant ratio τ2 / τ1 is smaller than a time constant ratio threshold value, e.g., 1, and the rate of increase decreases and saturates when the time constant ratio τ2 / τ1 exceeds this threshold value. This change in the energy extraction efficiency η in response to an increase in the time constant ratio τ2 / τ1 is common regardless of the magnitude of the coupling coefficient κ. However, the larger the coupling coefficient κ, the larger the value of the energy extraction efficiency η.

[0048] In practice, it is desirable that the energy extraction efficiency η of the energy extraction mechanism 30 be at least 0.1. To achieve this, the time constant ratio τ2 / τ1 is preferably 1 or greater. In other words, it is preferable that the time constant τ2 of the secondary circuit be greater than or equal to the time constant τ1 of the primary circuit. To achieve a higher energy extraction efficiency η, the time constant τ2 of the secondary circuit may be 2 times or greater, 5 times or greater, or 10 times or greater than the time constant τ1 of the primary circuit. Furthermore, since the energy extraction efficiency η saturates as the time constant ratio τ2 / τ1 increases, excessively increasing the time constant ratio τ2 / τ1 does not increase the energy extraction efficiency η. Therefore, the time constant τ2 of the secondary circuit may be 20 times or less, 15 times or less, or 10 times or less than the time constant τ1 of the primary circuit.

[0049] In this embodiment, as described above, the secondary coil 32 is cooled to the first cooling temperature by the cryogenic refrigerator 16. By cooling the secondary coil 32 to a cryogenic temperature in this manner, it is possible to reduce the electrical resistivity of the secondary coil 32. Because the time constant τ2 of the secondary circuit is inversely proportional to the electrical resistivity of the secondary coil 32, it is possible to increase the time constant τ2 of the secondary circuit.

[0050] In this embodiment, the secondary coil 32 is described as a normal conducting coil, but the secondary coil 32 may also include a superconducting coil. When the secondary coil 32 is cooled to the first cooling temperature, the secondary coil 32 may also include a high-temperature superconducting coil. A superconducting coil is capable of passing a large current, which allows the secondary coil 32 to efficiently extract energy from the superconducting coil 12. When the secondary coil 32 is cooled to the second cooling temperature, the secondary coil 32 may also include a low-temperature superconducting coil. Furthermore, the secondary coil 32 may also include both a normal conducting coil and a superconducting coil.

[0051] Fig. 3 is a graph showing the relationship between the coupling coefficient κ and the energy extraction efficiency η according to an analysis by the inventors of the present invention. In Fig. 3, the horizontal axis represents the coupling coefficient κ between the superconducting coil 12 and the secondary coil 32, and the vertical axis represents the energy extraction efficiency η. The relationship between the coupling coefficient κ and the energy extraction efficiency η was calculated for a number of different values ​​of the time constant ratio τ2 / τ1, and is shown in Fig. 3. Specifically, the relationship is shown for three time constant ratios τ2 / τ1 of 2, 5, and 10.

[0052] As can be seen from FIG. 3 , the energy extraction efficiency η increases monotonically as the coupling coefficient κ increases. As described above, in practice, it is desirable that the energy extraction efficiency η of the energy extraction mechanism 30 be at least 0.1. To achieve this, the coupling coefficient κ is preferably 0.3 or more. To achieve a higher energy extraction efficiency η, the coupling coefficient κ may be 0.5 or more, or 0.7 or more. Furthermore, taking into account dimensional constraints in arranging the superconducting coil 12 and the secondary coil 32 closely, the coupling coefficient κ may be, for example, 0.95 or less, or 0.9 or less.

[0053] Fig. 4 is a schematic diagram showing a superconducting magnet device 10 according to another embodiment. As shown in the figure, the superconducting magnet device 10 includes a superconducting coil 12, a vacuum vessel 14, a thermal shield 18, and an energy extraction mechanism 30. The energy extraction mechanism 30 includes a secondary coil 32. Although not shown, the superconducting magnet device 10 shown in Fig. 4 may also include a cryogenic refrigerator 16, a current introduction line 20, a first Joule heat generating element 22, an external power supply 24, and a second Joule heat generating element 34, similar to the superconducting magnet device 10 described with reference to Fig. 1.

[0054] The secondary coil 32 is disposed outside the vacuum vessel 14. In this example, the vacuum vessel 14 has a cylindrical shape with a hollow portion 15 in the center. The vacuum vessel 14 has a generally flat, annular top plate 14a and bottom plate 14b, and cylindrical outer and inner peripheral walls 14c and 14d that are coaxially disposed to connect the top and bottom plates 14a and 14b. As an example, the secondary coil 32 may be disposed outside the vacuum vessel 14 so as to surround the outer peripheral wall 14c of the vacuum vessel 14, as shown in the figure.

[0055] When the secondary coil 32 is disposed inside the vacuum vessel 14 as in the above-described embodiment, it is often difficult to employ a large secondary coil 32 due to dimensional constraints imposed by the volume of the vacuum vessel 14 and the positional relationship with various other elements within the vacuum vessel 14. In contrast, when the secondary coil 32 is disposed outside the vacuum vessel 14 as shown in FIG. 4, it is easier to increase the size of the secondary coil 32 than when not disposed outside the vacuum vessel 14. Since the time constant τ2 of the secondary circuit is proportional to the cross-sectional area of ​​the secondary coil 32, the time constant τ2 of the secondary circuit can be increased by increasing the size of the secondary coil 32, such as by increasing the coil height or coil diameter. This enables efficient energy extraction from the superconducting coil 12 by the energy extraction mechanism 30.

[0056] The secondary coil 32 may be disposed outside the vacuum vessel 14 so as to be surrounded by the inner peripheral wall 14d of the vacuum vessel 14. The secondary coil 32 may be disposed outside the vacuum vessel 14 adjacent to the top plate 14a or the bottom plate 14b of the vacuum vessel 14. Alternatively, a combination of these is also possible. That is, the secondary coil 32 may be provided in multiple locations among the top plate 14a, the bottom plate 14b, the outer peripheral wall 14c, and the inner peripheral wall 14d of the vacuum vessel 14.

[0057] In addition, the energy extraction mechanism 30 may be provided with multiple secondary coils 32, with at least one secondary coil 32 being located inside the vacuum vessel 14 and at least one other secondary coil 32 being located outside the vacuum vessel 14.

[0058] 5(a) and 5(b) are diagrams schematically showing a superconducting magnet apparatus 10 according to another embodiment. Similar to that shown in FIG. 4, the superconducting magnet apparatus 10 includes a superconducting coil 12, a vacuum vessel 14, a heat shield 18, and an energy extraction mechanism 30. The energy extraction mechanism 30 includes a secondary coil 32. In this example, the secondary coil 32 is disposed outside the vacuum vessel 14, adjacent to each of the top plate 14a and bottom plate 14b of the vacuum vessel 14.

[0059] In the superconducting magnet device 10 shown in FIG. 5( a), the secondary coil 32 is provided with an iron core 38a. The iron core 38a is disposed in the hollow portion of the vacuum vessel 14. In the superconducting magnet device 10 shown in FIG. 5( b), the secondary coil 32 is provided with a yoke 38b. The yoke 38b is disposed adjacent to both the upper and lower sides of the secondary coil 32 and around the superconducting magnet device 10 so as to surround the outer wall 14c of the vacuum vessel 14. The iron core 38a or yoke 38b has the effect of increasing the magnetic permeability. Since the time constant τ2 of the secondary circuit is proportional to the magnetic permeability, the time constant τ2 of the secondary circuit can be increased. This enables the energy extraction mechanism 30 to extract energy efficiently from the superconducting coil 12.

[0060] When the superconducting magnet device 10 is started, an excitation current is supplied to the superconducting coil 12 from the external power supply 24 through the current introduction line 20, thereby exciting the superconducting coil 12. During excitation of the superconducting coil 12, the external power supply 24 may increase the excitation current from zero to the rated current value of the superconducting coil 12 at a predetermined current increase rate (for example, a constant current increase rate). After excitation of the superconducting coil 12 is completed, the external power supply 24 may maintain the excitation current at the rated current value. In this way, the superconducting coil 12 can generate a magnetic field in response to the excitation current.

[0061] During excitation of the superconducting coil 12, as the magnetic field generated by the superconducting coil 12 increases from zero to a desired high magnetic field, a current is induced in the secondary coil 32 by electromagnetic induction. This current flows from the secondary coil 32 through the electrical wiring 36 to the second Joule heat generating element 34, causing the second Joule heat generating element 34 to heat up. In this way, part of the energy input for excitation of the superconducting coil 12 is consumed in the secondary circuit. This undesirably increases the power consumption during excitation of the superconducting coil 12.

[0062] When a current flows through the secondary coil 32, the secondary coil 32 also generates a transient magnetic field. Such an irregular magnetic field due to the secondary coil 32 may have an undesirable effect on the magnetic field of the superconducting coil 12. When the secondary coil 32 includes a superconducting coil, there is a concern that it may take time for the current to decay and, in turn, for the irregular magnetic field to subside because of the low resistance due to superconductivity.

[0063] If the second Joule heat generating element 34 includes a rectifying element such as a second diode 34a, the rectifying element may be connected across the secondary coil 32 so as to block a current induced in a first direction in the secondary coil 32 when the magnetic field of the superconducting coil 12 increases and allow a current induced in a second direction opposite to the first direction in the secondary coil 32 when the magnetic field of the superconducting coil 12 decreases. Connecting the second Joule heat generating element 34 to the secondary coil 32 in this orientation blocks induced current that may flow in the energy extraction mechanism 30 while the superconducting coil 12 is excited. This avoids the adverse effects of increased power consumption and an irregular magnetic field, as described above. On the other hand, when a quench occurs and the superconducting coil 12 is demagnetized, the second Joule heat generating element 34 allows induced current to flow in the secondary coil 32. This allows the energy extraction mechanism 30 to extract energy while the superconducting coil 12 is demagnetized, thereby addressing the above-described problems caused by a quench.

[0064] However, from the perspective of increasing the time constant τ2 of the secondary circuit for efficient energy extraction by the energy extraction mechanism 30, employing a rectifying element for the second Joule heat generating element 34 may not necessarily be optimal. This is because the resistance of the rectifying element itself can be relatively large. As described above, the time constant τ2 is defined by the ratio L2 / R2 of the inductance L2 of the secondary coil 32 to the resistance value R2 of the secondary-side resistor connected thereto. Therefore, if the resistance of the rectifying element is large, the time constant τ2 can be small. If it is desired to avoid this, the second Joule heat generating element 34 may be equipped with a general linear resistive element (i.e., one that obeys Ohm's law) instead of a rectifying element such as a diode.

[0065] 6 is a diagram schematically illustrating another example of a superconducting magnet device 10 according to an embodiment. Similar to the above-described embodiment, the superconducting magnet device 10 includes a primary circuit including a superconducting coil 12 and a secondary circuit constituting an energy extraction mechanism 30. The energy extraction mechanism 30 includes a secondary coil 32 electromagnetically coupled to the superconducting coil 12 and a secondary resistor, such as a second Joule heat generating element 34, connected to the secondary coil 32. The secondary coil 32 and the second Joule heat generating element 34 are connected by electrical wiring 36. The energy extraction mechanism 30 also includes a switch 40 that blocks or allows current to flow through the secondary coil 32, as will be described in detail below.

[0066] The energy extraction mechanism 30 shown in Fig. 6 may be applied to the superconducting magnet device 10 described with reference to Fig. 1. Thus, the superconducting magnet device 10 shown in Fig. 6 may also include, although not shown, a vacuum vessel 14, a cryogenic refrigerator 16, a heat shield 18, a current introduction line 20, a first Joule heat generating element 22, and an external power supply 24, similar to the superconducting magnet device 10 described with reference to Fig. 1.

[0067] The switch 40 is connected in series to the secondary coil 32 and is configured to switch between an OFF state that cuts off current flowing through the secondary coil 32 and an ON state that allows current to flow through the secondary coil 32. As shown in the figure, the switch 40 may be provided on the electrical wiring 36 connecting the secondary coil 32 and the second Joule heat generating element 34 so as to electrically connect or disconnect the secondary coil 32 to the second Joule heat generating element 34. Note that Fig. 6 shows the switch 40 in the OFF state.

[0068] In order to allow a larger current to flow through the secondary coil 32 in order to extract energy more efficiently from the superconducting coil 12, it is desirable that the resistance of the switch 40 itself in the on state be as small as possible, ideally zero. From this perspective, the switch 40 may be a persistent current switch.

[0069] A known persistent current switch can be used as the switch 40. For example, the switch 40 may be a mechanical persistent current switch. Like a typical mechanical switch, a mechanical persistent current switch is turned on by mechanically bringing one contact into contact with the other contact, and turned off by mechanically separating the contacts. The switch 40 may also be a thermal persistent current switch. A typical thermal persistent current switch includes a superconducting wire and a heater for heating the wire, and is turned on when the superconducting wire assumes a superconducting state, and turned off when the superconducting wire is heated by the heater and loses its superconductivity.

[0070] Alternatively, the switch 40 may be a general-purpose switch such as a mechanical switch, a semiconductor switch, or any other type of switch capable of turning on and off the current flowing through the secondary coil 32 .

[0071] The switch 40 may be configured to be in an OFF state during excitation of the superconducting coil 12. The switch 40 may be configured to switch from an OFF state to an ON state when excitation of the superconducting coil 12 is completed or at a predetermined timing after completion of excitation. The switch 40 may be configured to be in an ON state after excitation of the superconducting coil 12 is completed.

[0072] With this configuration, when the switch 40 is in the OFF state while the superconducting coil 12 is excited, the induced current that may occur in the secondary coil 32 is cut off. Therefore, energy consumption by the energy take-out mechanism 30 while the superconducting coil 12 is excited can be suppressed, and the generation of an irregular magnetic field by the secondary coil 32 can be prevented. On the other hand, when a quench occurs and the superconducting coil 12 is demagnetized, the switch 40 is in the ON state, allowing current to flow through the secondary coil 32. The energy take-out mechanism 30 can take out energy while the superconducting coil 12 is demagnetized, and can address the above-mentioned problems caused by the occurrence of a quench.

[0073] The energy extraction mechanism 30 shown in FIG. 6 may be applied to the superconducting magnet device 10 described with reference to FIG. 4, FIG. 5(a), or FIG. 5(b).

[0074] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, and that various design changes and modifications are possible, and that such modifications are also within the scope of the present invention. Various features described in relation to one embodiment can also be applied to other embodiments. A new embodiment created by combining embodiments will have the combined effects of the respective combined embodiments.

[0075] In the above embodiment, the cryocooler 16 is described as a GM refrigerator, but the present invention is not limited thereto. In some embodiments, the cryocooler 16 may be another type of two-stage cryocooler having a first cooling stage 16 a and a second cooling stage 16 b, such as a Solvay refrigerator, a Stirling refrigerator, or a pulse tube refrigerator.

[0076] Although FIG. 1 shows one cryogenic refrigerator 16 as an example, if necessary, for example, when the superconducting coil 12 is large, the superconducting magnet device 10 may be provided with multiple cryogenic refrigerators 16 that cool one and the same superconducting coil 12.

[0077] In the above-described embodiment, the superconducting magnet device 10 is configured as a so-called conduction-cooled type in which the superconducting coil 12 is directly cooled by the cryogenic refrigerator 16, rather than as an immersion-cooled type in which the superconducting coil 12 is immersed in a cryogenic liquid refrigerant such as liquid helium. However, the superconducting magnet device 10 may also be an immersion-cooled type. In this case, the superconducting coil 12 may be immersed and cooled in a cryogenic liquid such as liquid helium, and at least one of the first Joule heat generating element 22 and the energy extraction mechanism 30 may be cooled using a refrigerant with a higher boiling point (e.g., liquid nitrogen). In this way, the first Joule heat generating element 22 may be cooled to a higher cooling temperature than the superconducting coil 12 during operation of the superconducting coil 12. Alternatively, at least one of the first Joule heat generating element 22 and the energy extraction mechanism 30 may be cooled with liquid helium, similar to the superconducting coil 12.

[0078] The superconducting magnet device 10 may have a plurality of superconducting coils 12, in which case a first Joule heat generating element 22 may be provided for each superconducting coil 12, connected in parallel to the superconducting coil 12. The superconducting magnet device 10 may also have a plurality of energy extraction mechanisms 30, in which case a corresponding energy extraction mechanism 30 may be provided for each superconducting coil 12.

[0079] The present invention has been described using specific terms based on the embodiments, but the embodiments merely illustrate one aspect of the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the concept of the present invention as defined in the claims.

[0080] The present invention can be used in the field of superconducting magnet devices.

[0081] 10 Superconducting magnet device, 12 Superconducting coil, 14 Vacuum vessel, 16 Cryogenic refrigerator, 16a First cooling stage, 16b Second cooling stage, 22 First Joule heat generating element, 32 Secondary coil, 34 Second Joule heat generating element, 40 Switch.

Claims

1. A superconducting magnet device comprising: a primary circuit having a superconducting coil and a primary resistor connected to the superconducting coil; and a secondary circuit having a secondary coil electromagnetically coupled to the superconducting coil and a secondary resistor connected to the secondary coil, wherein the time constant of the secondary circuit is greater than or equal to the time constant of the primary circuit.

2. The superconducting magnet apparatus of claim 1, further comprising a cryogenic refrigerator configured to cool the secondary coil to a first cooling temperature and to cool the superconducting coil to a second cooling temperature lower than the first cooling temperature.

3. The superconducting magnet apparatus of claim 2, wherein said secondary coil comprises a high temperature superconducting coil.

4. The superconducting magnet apparatus according to claim 1, further comprising a vacuum vessel housing said superconducting coil, said secondary coil being disposed outside said vacuum vessel.

5. A superconducting magnet apparatus according to claim 1, characterized in that the secondary coil is provided with an iron core or a yoke.

6. A superconducting magnet apparatus according to claim 1, characterized in that the coupling coefficient between said superconducting coil and said secondary coil is 0.3 or more.

7. A superconducting magnet apparatus according to any one of claims 1 to 6, characterized in that the secondary circuit is provided with a switch for cutting off or allowing the current flowing through the secondary coil.

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