JP2022041709A - Superconducting wire for permanent current switch and manufacturing method, and superconducting magnet device - Google Patents

Abstract

To provide a superconducting wire for a permanent current switch having a small heat capacity and good connectivity of a superconducting connection, a manufacturing method thereof, and a superconducting magnet device using the same.SOLUTION: A superconducting wire 100 for a permanent current switch includes a small diameter portion 120 between end portions 110, in which the cross-sectional area of a superconducting filament 10 is smaller than the cross-sectional area at the end portion 110. A manufacturing method of a superconducting wire for a permanent current switch includes the steps of: filling a metal tube with magnesium and boron powder; wire-drawing the metal tube; forming a small diameter portion 120 by performing aging processing between the end portions of the metal tube; and heat-treating the powder filled in the metal tube to produce magnesium diboride. A superconducting magnet device includes a superconducting coil, a permanent current switch composed of the superconducting wire for the permanent current switch, a cryostat, and a demagnetization mechanism that attenuates the energy stored when the superconducting coil is demagnetized.SELECTED DRAWING: Figure 1

Description

The present invention relates to a superconducting wire for a permanent current switch constituting a permanent current switch of a superconducting magnet, a method for manufacturing the superconducting wire, and a superconducting magnet device using the same.

Magnetic resonance imaging (MRI) equipment is used for diagnostic imaging in the medical field, and has become an indispensable tool for diagnosing pathological conditions. The MRI device is a device that utilizes a strong magnetic field created by a superconducting magnet, and is operated in a permanent current mode in order to suppress a time change of the magnetic field. The superconducting magnet includes a superconducting coil formed of a superconducting wire in the cryostat.

In a superconducting magnet, the positive electrode and the negative electrode of the superconducting coil are short-circuited by a permanent current switch (PCS). The superconducting wire has a core wire of a superconducting filament that becomes a superconductor at a critical temperature or lower, and the superconducting wire forming the superconducting coil and the superconducting wire constituting the permanent current switch form a closed circuit. ing.

The permanent current switch is an element that can switch between a high resistance state (off state) and a zero resistance state (on state), and a thermal type is generally used. The thermal type permanent current switch includes a superconducting wire wound around a bobbin or the like and a heater that heats the superconducting wire at the time of switching. The permanent current switch is turned on when the superconducting wire is cooled below the critical temperature, and turned off when the superconducting wire is heated.

When the positive and negative electrodes of the superconducting coil are connected to the power supply and the permanent current switch is turned off to increase the current to the superconducting coil, the superconducting coil is excited. After the superconducting coil is excited, when the permanent current switch is turned on to reduce the current to the superconducting coil, the permanent current flows through the closed circuit short-circuited by the permanent current switch with substantially zero resistance.

The superconducting coil and the permanent current switch are superconductingly connected via a superconductor so as to be connected to each other with zero resistance. Since the superconducting coil and the permanent current switch are cooled inside the cryostat, the resistance of the closed circuit becomes virtually zero, and the operation in the permanent current mode becomes possible. According to the permanent current mode, it is possible to generate a magnetic field in which the current is hardly attenuated and the time change is extremely small.

Conventionally, in commercially available superconducting magnet devices, niobium-titanium (NbTi) is used as the superconducting wire for forming the superconducting coil. Generally, liquid helium is used for cooling niobium-titanium, and 1500 to 2000 L of liquid helium is filled inside the cryostat. However, in recent years, it has become difficult to obtain helium and the price is soaring, so the development of alternative technologies is underway.

As a technology for reducing the amount of helium used, a superconducting magnet device equipped with a thermosiphon type cooling mechanism has been developed, and some of them have been commercialized. According to the thermosiphon type, since heat convection due to gravity is used, high cooling capacity can be obtained even with a small amount of liquid helium. In addition, magnesium diboride (MgB ₂ ) and high-temperature superconductors are being put into practical use.

Since MgB ₂ and high-temperature superconductors have a high critical temperature of several tens of K or more, it is expected that cooling with liquid helium will not be necessary. The superconducting wire using MgB ₂ or a high-temperature superconductor is characterized by having a large quenching margin as compared with the case of using conventional niobium-titanium. For the quench margin _Δe , the operating temperature of the device is Top, the composite heat capacity synthesized for the superconducting wire that is the composite material of the core and sheath is _Cp , and the rated current is the _upper limit temperature that can be energized with zero resistance as T1. Then, it is expressed by the following formula (1).

In Non-Patent Document 1, the quenching margin of superconducting wire is about 1 kJm ^-3 in the case of niobium-titanium, whereas it is 10 to 10,000 times that in the case of MgB2 or high _- temperature superconductor. It is stated that there is. In the case of niobium-titanium, quenching due to mechanical factors such as sliding becomes a problem, whereas in the case of MgB2 and high _- temperature superconductors, it means that such quenching does not occur substantially.

When MgB ₂ or a high-temperature superconductor having a high quenching margin is used, the risk of quenching is low, so that the need for immersion cooling with liquid helium is reduced. However, there is concern that new problems will arise if liquid helium is not used or if the amount of liquid helium used is reduced. Immersion cooling with liquid helium has a protective effect in abnormal situations such as loss of cooling and emergency demagnetization, but such an effect cannot be obtained.

Liquid helium keeps the cryostat at a very low temperature for several hours in the event of a loss of cooling. Such an action provides sufficient grace for restoration of the cooling function and safe demagnetization. In addition, liquid helium absorbs the heat inside the cryostat as latent heat of vaporization during emergency demagnetization. Due to such an action, a rapid temperature rise of the superconducting coil and the permanent current switch during emergency demagnetization is suppressed. Small temperature rises allow for rapid recovery by refilling liquid helium.

When liquid helium is not used or when the amount of liquid helium used is reduced, such a protective effect cannot be obtained, so that the grace period in the event of loss of cooling is shortened. In addition, the temperature rise during emergency demagnetization becomes large, and it takes time to recover. Therefore, when MgB ₂ or a high-temperature superconductor is used, it is necessary to take new measures instead of immersion cooling with liquid helium.

In particular, when MgB ₂ or a high-temperature superconductor having a large quenching margin is used, there is a problem that the normal conduction region is difficult to expand spontaneously as compared with the case of niobium-titanium. Since MgB ₂ and high-temperature superconductors do not easily expand the normal conduction region when the temperature rises, the stored energy is consumed in the narrow region unless a wide range is actively heated. In the event of an abnormal situation such as loss of cooling or emergency demagnetization, if energy is consumed in a narrow area, there is a problem that the superconducting wire is burnt out.

As a measure to safely demagnetize the superconducting coil in the event of an abnormal situation such as loss of cooling or emergency demagnetization, the use of an emergency demagnetization mechanism such as a protective resistor or an external power supply can be considered. If the permanent current switch is turned off to consume the energy stored in the superconducting coil with a protective resistor, or if a negative voltage is applied from an external power supply to the closed circuit of the superconducting coil, the superconducting wire will burn out. The risk can be reduced.

Since the emergency demagnetization mechanism such as the protection resistor and the external power supply is provided outside the cryostat, the temperature rise inside the cryostat is suppressed during the emergency demagnetization. In the case of the conventionally used niobium-titanium, quenching is likely to occur due to AC loss, so such an emergency demagnetization mechanism cannot be used. However, if MgB ₂ or a high-temperature superconductor having a high quenching margin is used, it can be safely demagnetized by a protective resistance, an external power source, or the like.

When using an emergency demagnetization mechanism such as a protective resistor or an external power supply, the remaining major problem is how to avoid burning of the permanent current switch. If the permanent current switch is switched to a high resistance state (off state) during emergency demagnetization, some current can be diverted to the permanent current switch shorting the closed circuit. When a large current flows through the permanent current switch in the off state, the superconducting wire constituting the permanent current switch may be burnt out due to Joule heat, and the disconnection of the superconducting wire hinders recovery after emergency demagnetization.

For example, in the most popular MRI apparatus having a magnetic field strength of 1.5 T, the inductance of the superconducting coil is about 10 to 80 H. In normal emergency demagnetization, it is said that the central magnetic field needs to be 0.02T or less within 60 seconds. Assuming that the time required to switch the permanent current switch is 10 seconds when the inductance L of the superconducting coil is 80H and the operating current I _op is 300A, the current is 6A · s ^-1 within the remaining 50 seconds. Must be reduced.

Under such conditions, the voltage V _p between the terminals of the permanent current switch becomes V _p = L · dI _op / dt = 480 V. The current I _p flowing through the permanent current switch is represented by I _p = V _p · R _p ^-1 when the resistance of the permanent current switch in the off state is R _p . The resistance R _p of the permanent current switch in the off state is D _p for the wire diameter of the superconducting wire constituting the permanent current switch, L _p for the length of the superconducting wire, and the resistivity of the superconducting wire which is a composite material. When ρ _p , R _p = 4ρ _p · L _p / π · D _p ² .

The possibility of burning of the superconducting wire constituting the permanent current switch can be evaluated based on the time change of the temperature. Assuming that the time change dT _p / dt of the temperature T _p of the superconducting wire constituting the permanent current switch is an adiabatic change, when the composite heat capacity per unit volume of the superconducting wire which is a composite material is _cp . , Expressed by the following formula (2).

Therefore, in order to prevent the permanent current switch from burning in the event of an abnormal situation such as loss of cooling or emergency demagnetization, the composite resistivity ρ _p of the superconducting wire constituting the permanent current switch should be increased, or the superconducting wire should be increased. It can be said that it is effective to lengthen the line length _Lp of. On the other hand, the wire diameter of the superconducting wire is irrelevant to the likelihood of burning under the assumption that it is an adiabatic change.

Japanese Patent No. 6047341 U.S. Patent Application Publication No. 2015/0018220

Supercond. Sci. Technol. 30 (2017) 053001 Supercond. Sci. Technol. 30 (2017) 014007

From the viewpoint of preventing burnout during emergency demagnetization, the superconducting wire for the permanent current switch is desired to have a high composite resistivity synthesized from the core superconducting filament and the sheath covering the core. Moreover, it is desired that the line length is long. However, if the wire length of the superconducting wire is long, the amount of heat input from the heater must be increased when the permanent current switch is switched off. If the amount of heat input is large, the temperature inside the cryostat will rise, which raises the risk of quenching and takes time to recover after emergency demagnetization.

From the viewpoint of reducing the amount of heat input from the heater, it is desirable that the superconducting wire constituting the permanent current switch has a small volume and a small heat capacity. The wire diameter of the superconducting wire is irrelevant to the likelihood of burning under the assumption that it is an adiabatic change. It can be said that the temperature rise of the cryostat can be suppressed while preventing it.

On the other hand, the superconducting wire for the permanent current switch needs to be superconductingly connected to the superconducting coil so that it can be energized with zero resistance in the on state. For superconducting connection, the end of the superconducting wire constituting the permanent current switch must be integrated with the superconducting wire forming the superconducting coil via the superconductor. However, if the wire diameter at the end of the superconducting wire is too small, another problem arises that breakage is likely to occur during the connection work.

As described above, regarding the superconducting wire for the permanent current switch, the volume and heat capacity of the wire and the mechanical strength of the end of the wire and the connectivity of the superconducting connection are in a trade-off relationship. Even in a superconducting wire having a high composite resistivity and a long wire length that can avoid burning during emergency demagnetization, there is a demand for a superconducting wire that has both a small heat capacity and good connectivity.

Therefore, the present invention provides a superconducting wire for a permanent current switch having a small heat capacity and good connectivity of a superconducting connection, a manufacturing method thereof, and a superconducting magnet device using the same. With the goal.

In order to solve the above problems, the superconducting wire for a permanent current switch according to the present invention is a superconducting wire for a permanent current switch constituting the permanent current switch of a superconducting magnet, and the ends of the superconducting wires are connected to each other. Has a small diameter portion between which the cross-sectional area of the superconducting filament is smaller than the cross-sectional area at the end.

Further, the method for manufacturing a superconducting wire for a permanent current switch according to the present invention includes a step of filling a metal tube with magnesium and boron powder, and a step of wire drawing the metal tube filled with the powder. A step of forming a small diameter portion in which the cross-sectional area of the superconducting filament is smaller than the cross-sectional area at the end portion by performing a swaying process between the end portions of the wire-drawn metal pipe and the metal pipe. Includes a step of heat-treating the powder filled in to produce magnesium diboride.

Further, the superconducting magnet device according to the present invention includes a superconducting coil, a permanent current switch composed of a superconducting wire for a permanent current switch, a cryostat accommodating the superconducting coil and the permanent current switch, and the above. A demagnetization mechanism that attenuates the energy stored in the superconducting coil when the superconducting coil is demagnetized is provided, and the superconducting wire for the permanent current switch is provided between the ends of the superconducting wire. It is a superconducting wire having a small diameter portion whose cross-sectional area of the superconducting filament is smaller than the cross-sectional area at the end, and the demagnetization mechanism attenuates the energy stored in the superconducting coil outside the cryostat.

According to the present invention, it is possible to provide a superconducting wire for a permanent current switch having a small heat capacity and good connectivity of a superconducting connection, a manufacturing method thereof, and a superconducting magnet device using the same. can.

It is a figure which shows typically the superconducting wire for the permanent current switch which concerns on embodiment of this invention. It is a figure which shows the equivalent circuit of the superconducting magnet apparatus provided with a permanent current switch. It is an image which shows the cross section of the precursor of a superconducting wire after wire drawing. It is an image which shows the cross section of a superconducting wire after heat treatment. It is a figure which shows the relationship between the critical current density of the superconducting wire material which concerns on an Example, and an external magnetic field. It is a figure which shows the relationship between the composite resistivity and the temperature of the superconducting wire material which concerns on an Example. It is a figure which shows the relationship between the composite heat capacity per unit volume, and the temperature of the superconducting wire material which concerns on an Example. It is a figure which shows the relationship between the ultimate temperature of a permanent current switch, and the length of a superconducting wire. It is a figure which shows an example of a permanent current switch schematically. It is sectional drawing which shows the main part of a permanent current switch in an enlarged manner. It is a figure which shows typically the superconducting wire for the permanent current switch which concerns on the modification of this invention.

Hereinafter, a superconducting wire for a permanent current switch according to an embodiment of the present invention, a method for manufacturing the superconducting wire, and a superconducting magnet device using the same will be described with reference to the drawings. In each of the following figures, common configurations are designated by the same reference numerals and duplicated description will be omitted.

FIG. 1 is a diagram schematically showing a superconducting wire for a permanent current switch according to an embodiment of the present invention.
As shown in FIG. 1, the superconducting wire 100 for a permanent current switch according to the present embodiment includes a superconducting filament 10, has a relatively large wire diameter, and has a relative cross-sectional area of the superconducting filament 10. It has a large end portion 110 and a small diameter portion 120 having a relatively small wire diameter and a relatively small cross-sectional area of the superconducting filament 10.

The superconducting wire 100 for a permanent current switch according to the present embodiment is particularly used as a superconducting wire constituting a thermal permanent current switch. The superconducting wire 100 includes a superconducting filament 10 that becomes a superconductor at a critical temperature or lower, and a sheath 11 that covers the periphery of the superconducting filament 10.

Since the superconducting wire 100 for a permanent current switch has a structure having an end portion 110 having a relatively large cross-sectional area of the superconducting filament 10 and a small diameter portion 120 having a relatively small cross-sectional area of the superconducting filament 10. The volume and heat capacity of the wire itself are reduced, and the mechanical strength at the end and the connectivity of the superconducting connection are improved.

The ends 110 having a relatively large cross-sectional area of the superconducting filament 10 are provided at both ends of the superconducting wire 100. The end portion 110 serves as a connection portion for superconductingly connecting the permanent current switch to a superconducting coil or the like. The length of the end portion 110 is not particularly limited, but it is preferable to secure the length required for the lead wire drawn from the bobbin constituting the permanent current switch.

The small diameter portion 120 having a relatively small cross-sectional area of the superconducting filament 10 is provided between the end portions 110 of the superconducting wire rod 100. In FIG. 1, the small diameter portion 120 is continuously reduced in diameter from the end portion 110 via the tapered portion. Such a small diameter portion 120 can be formed by aging processing as described later.

Further, in FIG. 1, the small diameter portion 120 is continuously provided between the end portions 110 along the longitudinal direction of the superconducting wire 100, and the cross-sectional area of the central portion in the longitudinal direction is the end portion. It is always smaller than the cross-sectional area. With such a small diameter portion 120, the superconducting filament 10 is unlikely to be distorted during aging processing, and unlike the case where the small diameter portion 120 is provided intermittently, the volume and heat capacity of the wire itself are uniform in the longitudinal direction. It will be reduced.

FIG. 2 is a diagram showing an equivalent circuit of a superconducting magnet device including a permanent current switch.
As shown in FIG. 2, the superconducting magnet device 200 includes a superconducting coil 1, a superconductingly connected connection portion 2, a permanent current switch 3, a cryostat 4, a current lead 5, and an exciting power supply 6. It is equipped with a protection resistor 7. The superconducting wire 100 for the permanent current switch shown in FIG. 1 can be used as a wire constituting the permanent current switch 3 of the superconducting magnet device 200 as shown in FIG.

The superconducting magnet device 200 includes a plurality of superconducting coils 1. The plurality of superconducting coils 1 are superconductingly connected in series with each other. The number of superconducting coils 1 is not particularly limited, but may be, for example, eight. The superconducting wire forming the superconducting coil 1 and the superconducting wire forming the permanent current switch 3 are superconductingly connected to each other, forming a closed circuit that can be energized with zero resistance.

The superconducting coil 1 is formed of a superconducting wire using MgB ₂ or a high-temperature superconductor. Since MgB ₂ and high-temperature superconductors have a large quenching margin, it is possible to use an emergency demagnetization mechanism such as a protective resistance 7 in the event of an abnormal situation such as loss of cooling or emergency demagnetization. Therefore, it is suitable for a superconducting magnet device 200 to which an emergency demagnetization mechanism such as a protective boathouse 7 is attached. Examples of the high-temperature superconductor include yttrium-based copper oxides such as YBCO and bismuth-based copper oxides such as BSCCO.

When the superconducting magnet device 200 is used in an MRI device or the like, the combined inductance of the superconducting coil 1 is about 10 to 80H. The total energy stored in the superconducting coil 1 is about 2 to 4 MJ. The operating current passed through the superconducting coil 1 is about 300 to 500 A. The superconducting wire 100 constituting the permanent current switch 3 is required to have a critical current value corresponding to a required operating current and an off resistance for appropriately performing excitation and demagnetization.

The permanent current switch 3 is a thermal type and includes a superconducting wire 100 for the permanent current switch and a heater for heating the superconducting wire 100. When the thermal type permanent current switch 3 is heated by a heater, it switches from an on state having zero resistance to an off state showing resistance. When the permanent current switch 3 is in the off state, the superconducting coil 1 is connected so as to form a closed circuit with respect to the exciting power supply 6 and the emergency demagnetization mechanism such as the protection resistor 7.

The cryostat 4 is provided as a heat insulating container, and includes a vacuum container, a heat radiation shield, and the like. The superconducting coil 1 and the permanent current switch 3 are housed inside the cryostat 4. The inside of the cryostat 4 is cooled to an extremely low temperature below the critical temperature so that an operating current can flow through the superconducting coil 1 with substantially zero resistance. Cooling of the cryostat 4 can be performed by, for example, conduction cooling by a Gifford McMahon (GM) type refrigerator.

The current lead 5 adiabatically superconducts the closed circuit of the superconducting coil 1 short-circuited by the permanent current switch 3 inside the cryostat 4, the exciting power supply 6 outside the cryostat 4, the protection resistance 7, and the like. You are connected. The current lead 5 is formed of a high-temperature superconductor, and is connected between the superconducting wires by soldering with superconducting solder. The current lead 5 can carry a large current with substantially zero resistance, but since it has a low thermal conductivity, it is possible to suppress heat intrusion into the cryostat 4.

The excitation power supply 6 is a power supply that supplies a current for exciting the coil to the superconducting coil 1. The excitation power supply 6 is installed outside the cryostat 4. Both terminals of the exciting power supply 6 are superconductingly connected to the superconducting coil 1 via the current lead 5. When the permanent current switch 3 is in the off state, the superconducting coil 1 is supplied with a current from the exciting power supply 6 to generate a predetermined magnetic field.

The protection resistance 7 constitutes an emergency demagnetization mechanism that attenuates the energy stored in the superconducting coil 1 when the superconducting coil 1 is demagnetized in an emergency. The protection resistance 7 is installed outside the cryostat 4. Both ends of the protection resistor 7 are superconductingly connected to the superconducting coil 1 via the current lead 5. When the permanent current switch 3 is switched to the off state at the time of emergency demagnetization, a current flows through the protection resistor 7 from the superconducting coil 1. The protection resistance 7 consumes the energy stored in the superconducting coil 1 as Joule heat or the like.

In FIG. 2, as the emergency demagnetization mechanism, only the protective resistance 7 for attenuating the current flowing through the superconducting coil 1 is provided. As the protection resistance 7, a solid resistor using a metal or the like, a liquid resistor using a conductive liquid, or the like can be used. However, as an emergency demagnetization mechanism, a power source that applies a negative voltage to the superconducting coil 1 can also be provided.

Here, the configuration and operation of the superconducting wire 100 for the permanent current switch will be described more specifically.

At the time of emergency demagnetization of the superconducting magnet device 200 as shown in FIG. 2, the permanent current switch 3 is switched to the off state, but the current from the superconducting coil 1 to the emergency demagnetization mechanism such as the protection resistor 7 flows. , Can be split into the permanent current switch 3. When a large current is diverted to the permanent current switch 3, the superconducting wire constituting the permanent current switch 3 may be burnt out due to Joule heat. If the superconducting wire breaks, recovery after emergency demagnetization becomes impossible. Therefore, it is preferable that the superconducting wire constituting the permanent current switch has a high resistivity ρ _p and a long wire length L _p , as expressed by the mathematical formula (2).

However, the material of the superconducting wire must be selected under various restrictions depending on the type of superconductor and the manufacturing method of the superconductor. Even if the wire length _Lp of the superconducting wire constituting the permanent current switch is lengthened, there are limits in manufacturing and practical use. Further, the longer the wire length _Lp , the larger the heat capacity. Therefore, when the permanent current switch is switched to the off state, the amount of heat input from the heater must be increased. If the amount of heat input is large, the temperature inside the cryostat rises, which causes problems such as an increased risk of quenching and a long recovery time after emergency demagnetization.

As expressed by the formula (2), the wire diameter of the superconducting wire is irrelevant to the likelihood of burning under the assumption that it is an adiabatic change. Therefore, it can be said that the superconducting wire constituting the permanent current switch is preferably provided with a small wire diameter from the viewpoint of reducing the heat capacity. However, the wire diameter of the superconducting wire that constitutes the permanent current switch must be thinned to the extent that the required operating current can flow in the ON state.

In the MRI apparatus generally distributed, the current density of the superconducting coil is about 100 to 150 A · mm ^-2 (see Non-Patent Document 2). The superconducting wire forming the superconducting coil of the MRI apparatus is insulated with an insulating coating made of formal or glass braid. Further, the gap between the superconducting wires in the superconducting coil is impregnated with an insulating resin in order to prevent mechanical disturbance.

Assuming that the volume ratio of the insulating coating and the impregnated resin is 15% in the cross section of the superconducting wire forming the superconducting coil, the current density of the superconducting wire forming the superconducting coil is 118 ^to 176 A · mm-. It is calculated as about ² . Since the operating current flowing through the superconducting coil is about 300 to 500 A, it can be said that the wire diameter of the superconducting wire for the superconducting coil is generally about 1.5 to 2.5 mm.

Therefore, the wire diameter of the superconducting wire 100 for the permanent current switch is preferably equal to or less than 2.5 mm, more preferably 1.5 mm or less, from the viewpoint of thinning the wire within the range in which the required operating current can flow. Equivalent or less. When a superconductor or a manufacturing method similar to that of a superconducting coil is used, at least such a wire diameter does not easily bend, so that the possibility of breakage is low. The diameter of the small diameter portion 120 needs to be smaller than the wire diameter of the superconducting wire 100 for the permanent current switch, preferably 2.5 mm or less, and more preferably 1.5 mm or less.

The superconducting wire 100 for the permanent current switch can be formed by using MgB ₂ or a high-temperature superconductor, similarly to the superconducting wire forming the superconducting coil 1. Since the superconducting wire using MgB ₂ or a high-temperature superconductor has a large quenching margin, it is possible to use an emergency demagnetization mechanism such as a protective resistance 7 in the event of an abnormal situation such as cooling loss or emergency demagnetization. Therefore, it is suitable for a superconducting magnet device 200 to which an emergency demagnetization mechanism such as a protective boathouse 7 is attached.

The superconducting wire 100 for the permanent current switch can be manufactured by the powder in-tube (PIT) method when MgB ₂ is used. In the PIT method, a metal tube is filled with the raw material powder of a superconductor, the metal tube is wire-drawn, and then heat-treated for reaction sintering or self-sintering to obtain a core superconductor. Form a superconducting filament.

The method for producing a superconducting wire using MgB ₂ is roughly classified into an in-situ method and an ex-situ method. The in-situ method is a method in which a mixed powder of magnesium and boron is filled in a metal tube and reaction sintering by heat treatment is performed to form a superconducting filament of MgB ₂ . The ex-situ method is a method in which a powder of MgB ₂ is filled in a metal tube and a superconducting filament of MgB ₂ is formed by self-sintering by heat treatment.

The superconducting wire 100 for the permanent current switch may be produced by either the in-situ method or the ex-situ method, but it is preferably produced by the in-situ method. The ex-situ method is said to facilitate the production of uniform wire rods. The in-situ method is effective in obtaining a high critical current density.

In the PIT method, a tube formed of a barrier material is used as the metal tube for filling the raw material powder. The barrier material is a material that does not easily react with magnesium and boron, and prevents the raw material powder from causing an unintended reaction during the heat treatment. When magnesium or boron reacts with other substances during the heat treatment, the formation of superconducting filaments is hindered and the critical current density of the superconducting wire becomes low. However, when a metal tube made of a barrier material is used, magnesium and boron react appropriately, so that a high critical current density can be obtained.

The metal tube filled with the raw material powder serves as a base material or a sheath for covering the superconducting filament in the superconducting wire obtained after the heat treatment. From the viewpoint of preventing the permanent current switch from burning, it is preferable that the composite resistivity of the superconducting wire 100 for the permanent current switch is high, so that the resistivity of the barrier material constituting the metal tube is also preferable.

As the barrier material, niobium (Nb), iron (Fe), tantalum (Ta), titanium (Ti), alloys thereof and the like can be used. As the barrier material, an alloy is preferable, and niobium-titanium is particularly preferable, from the viewpoint of increasing the composite resistivity of the superconducting wire 100 for a permanent current switch. Impurity scattering is unlikely to occur in pure metals. In addition, phonon scattering is unlikely to occur at extremely low temperatures below the critical temperature. On the other hand, when an alloy is used, the off resistance of the permanent current switch can be effectively increased by scattering impurities.

Here, an example in which the superconducting wire 100 for a permanent current switch is manufactured by the in-situ method is shown. As the material of the superconducting filament, MgB ₂ to which C was added was used. Further, niobium-titanium was used as the material of the sheath covering the superconducting filament, that is, the barrier material of the metal tube filled with the raw material powder.

First, magnesium powder (particle size <45 μm, purity 99.8%), boron powder (particle size <350 nm, purity 98.5%), and coronen powder as a carbon source (C ₂₄ H ₁₂ ) are mixed in elemental ratio. Was weighed so that Mg: B: C = 1: 1.94: 0.06, and the mixture was sufficiently mixed using a pot mill device.

Then, the obtained mixed powder was filled in a double tube having a diameter of 20 mm. In the double tube, the inner layer is niobium-titanium and the outer layer is copper, and a through hole is formed in the center of a bar made of niobium-titanium coated with copper. A bar made of niobium-titanium coated with copper corresponds to an intermediate material of a conventional superconducting wire using niobium-titanium.

After filling the double tube with the mixed powder, the double tube was wire drawn. As a result of wire drawing, the wire diameter of the double pipe could be reduced to 0.79 mm. After the wire drawing process, the double tube was immersed in 30% nitric acid to remove the copper in the outer layer. Then, it was heat-treated at 700 ° C. for 3 hours under an inert atmosphere or a vacuum atmosphere with argon gas to generate MgB ₂ . The obtained superconducting wire had a wire diameter of 0.64 mm and a volume fraction of MgB ₂ of 64%.

FIG. 3 is an image showing a cross section of a precursor of a superconducting wire after wire drawing.
FIG. 3 shows the results of photographing a backscattered electron image with a scanning electron microscope after filling a double tube having a copper outer layer and a niobium-titanium inner layer with a mixed powder of magnesium and boron and performing wire drawing. show. As shown in FIG. 3, the precursor of the superconducting wire after wire drawing is in a state where a mixed powder 210 of magnesium and boron is filled in a double tube having an inner layer 211 and an outer layer 212.

The outer layer 212 made of copper is mainly provided to prevent seizure during wire drawing. Copper reacts with magnesium and the like to prevent the formation of MgB ₂ , but since the mixed powder 210 of magnesium and boron is covered with the inner layer 211 formed of niobium-titanium, which is a barrier material, an unintended reaction during heat treatment is performed. Is prevented. The outer layer 212 made of copper can function as a stabilizing material after the production of the superconducting wire, but if it is unnecessary, it can be removed before the heat treatment or the like.

FIG. 4 is an image showing a cross section of the superconducting wire after heat treatment.
FIG. 4 shows the results of taking a reflected electron image with a scanning electron microscope after removing only the outer layer from the precursor of the superconducting wire after wire drawing and performing heat treatment. As shown in FIG. 4, the superconducting wire after heat treatment includes a superconducting filament 10 formed of MgB ₂ and a sheath 11 which is a barrier material made of niobium titanium for covering the superconducting filament 10.

The superconducting filament 10 formed of MgB ₂ forms a minute void inside the sheath 11 which is a barrier material. The reaction between magnesium and boron proceeds by the diffusion of molten and volatilized magnesium into the boron particles during the heat treatment. When MgB ₂ is produced by the reaction of magnesium and boron, the volume as a substance becomes small and voids are formed. However, mechanically milled mixed powders can be used to reduce the final porosity.

Next, the results of measuring the critical current density of the superconducting wire rod according to the produced example are shown. The critical current density was measured by the magnetization method using a Magnetic Property Measurement System (MPMS) (manufactured by Quantum Design). A superconducting wire having a wire length of about 4 mm was cut out, the magnetization curve was measured under a vertical magnetic field, and the critical current density _Jc was obtained based on the extended Bean model represented by the following mathematical formula (3).

In formula (3), d represents the diameter of the superconducting filament, and ΔM represents the width of the magnetic hysteresis loop (MH curve).

FIG. 5 is a diagram showing the relationship between the critical current density of the superconducting wire according to the embodiment and the external magnetic field.
As shown in _FIG . 5, the critical current density Jc of the superconducting wire rod according to the produced example showed a decreasing function with respect to the external magnetic field B. The critical current density _Jc of the superconducting wire according to the example showed magnetic field dependence, and the lower the temperature, the smaller the decrease with respect to the increase of the external magnetic field B.

A superconducting coil of a MRI apparatus having a magnetic field strength of 1.5 T, which is widely used, has a maximum empirical magnetic field of about 3 to 5 T. On the other hand, the permanent current switch is usually installed in a place where the external magnetic field is low, and the empirical magnetic field is about 1T or less. The superconducting wire according to the produced example had a magnetic field strength of 1 T and a critical current Ic of 864 A at 15 K. The superconducting wire according to the embodiment has a sufficient margin as a superconducting wire for a permanent current switch with respect to a general operating current of 300 to 500 A.

Therefore, it can be said that the superconducting wire according to the embodiment can be further thinned within the range in which the required operating current can flow. For example, when the wire diameter is reduced to 0.46 mm, the critical current _Ic is 432 A, so that if the operating current is about 300 A, there is a sufficient margin. The wire diameter of about 0.46 mm is only a fraction of the superconducting wire for superconducting coils, and the cross-sectional area is an order of magnitude smaller. However, since a certain amount of operating current can be passed, the wire diameter is preferable in that the heat capacity can be reduced.

From the viewpoint of reducing the heat capacity, the superconducting wire 100 for the permanent current switch is preferably as small as the wire diameter, and the diameter of the superconducting filament 10 is preferably 1.0 mm or less, more preferably 0.75 mm or less, and further. It is preferably 0.50 mm or less, and it can be said that it is particularly preferable that the wire is thinned to 0.46 mm or less. Since the superconducting wire 100 for the permanent current switch is installed in a lower magnetic field than the superconducting coil, it can be made thinner than the superconducting wire for the superconducting coil.

FIG. 6 is a diagram showing the relationship between the composite resistivity and the temperature of the superconducting wire according to the embodiment.
As shown in FIG. 6, the composite resistivity of the superconducting wire rod according to the produced example decreased with a decrease in temperature, and showed a superconducting transition at 37.7 K. The composite resistivity of the superconducting wire according to the example was 38.7 μΩcm at 40 K. The electrical resistance was measured using a Physical Property Measurement System (PPMS) (manufactured by Quantum Design).

The composite resistivity of the superconducting wire according to the embodiment can be increased by adding an impurity to the superconducting filament. For example, when a carbon source is added to the powder that is the raw material of MgB ₂ , a part of the honeycomb lattice of boron in the crystals of MgB ₂ can be replaced with carbon during the heat treatment. When substitution with a different element is performed, the periodicity of the atomic arrangement is disturbed, so that the residual resistance of the superconducting filament itself can be increased and burnout during emergency demagnetization can be prevented.

As the carbon source, hydrocarbons such as coronene, benzene, naphthalene and anthracene, organic acids such as stearic acid, magnesium salts of organic acids and the like, and inorganic carbon compounds such as _B4C and SiC can be used. .. The amount of the carbon source added is preferably 2 to 3% by mass per raw material of the superconducting filament. With such an addition amount, the effect of increasing the residual resistance can be sufficiently obtained while ensuring a high critical current.

FIG. 7 is a diagram showing the relationship between the combined heat capacity and the temperature per unit volume of the superconducting wire according to the embodiment.
In FIG. 7, the specific heat of MgB ₂ is based on the literature value (Phys. Rev. B 64 (2001) 172515). The specific heat of NbTi is based on the literature value (CERN Accelerator School-2013 lecture material <https://indico.cern.ch/event/194284/contributions/1472798/attachments/281498/393574/CAS_propmat_2013.pdf>). The specific heat of the MgB ₂ filament is a value calculated as 60% of the specific heat peculiar to the substance, considering that the superconducting filament usually produces a void of about 40%.

As shown in FIG. 7, the composite heat capacity per unit volume of the superconducting wire according to the embodiment is larger than that of the conventional MgB ₂ filament by using niobium-titanium having a large specific heat, and is 1.0 × 10 at 40K. It was ⁵ J ・ m ^-3・ K ^-1 or more and 1.0 × 10 ⁶ J ・ m ^-3・ K ^-1 or less. Therefore, the superconducting wire according to the embodiment has a larger quenching margin than the conventional MgB ₂ filament when compared at the same volume, and is reached by the time change or integration of the temperature represented by the mathematical formula (2). The temperature decreases.

That is, since the superconducting wire according to the embodiment uses niobium-titanium as the barrier material, it can be said that it is less likely to burn out than the conventional MgB ₂ filament when compared with the same wire length. When niobium-titanium is used as the barrier material, the wire length is more realistic than that of the conventional MgB ₂ filament, which means that it is possible to prevent burning during emergency demagnetization.

FIG. 8 is a diagram showing the relationship between the ultimate temperature of the permanent current switch and the length of the superconducting wire.
As shown in FIG. 8, the ultimate temperature Tf of the permanent current switch becomes lower as the wire length _Lp of the superconducting wire becomes longer. The ultimate temperature Tf of the permanent current switch is determined by the composite resistivity of the superconducting wire (see FIG. 6) and the combined heat capacity of the superconducting wire (see FIG. 7) based on the mathematical formula (2). It can be obtained as a function of the line length L _p .

When the wire length L _p of the superconducting wire constituting the permanent current switch is 250 m or more, the ultimate temperature Tf of the permanent current switch is 300 K or less, so that burning during emergency demagnetization does not occur substantially. Therefore, it can be said that the superconducting wire 100 for the permanent current switch preferably has a wire length of 250 m or more, although it depends on the composite resistivity.

On the other hand, in order to lower the ultimate temperature Tf of the permanent current switch for a realistic line length _Lp of about several hundred meters, a high resistivity is required. In order to increase the composite resistivity, it is important to add impurities to the superconducting filament and select the base material / sheath material. However, in an actual permanent current switch, an impregnated resin or bobbin exists around the superconducting wire, and heat conduction to the impregnated resin or bobbin occurs, so that the ultimate temperature Tf becomes low and the required wire length _Lp is also short. Become.

As shown in FIG. 8, from the viewpoint of preventing the superconducting wire constituting the permanent current switch from burning, it is desirable to provide the superconducting wire with a wire length of several hundred meters, but the wire length L of the superconducting wire is L. _{If p} is long, the volume of the superconducting wire becomes large and the heat capacity as a whole becomes large. Therefore, when the permanent current switch is switched, the amount of heat input from the heater must be increased.

Since the wire diameter of the superconducting wire is irrelevant to the likelihood of burning under the assumption that it is an adiabatic change, there is a degree of freedom in the wire diameter as long as a certain wire length is secured. Therefore, in the superconducting magnet device 200 that utilizes the emergency demagnetization mechanism, it is effective to make the superconducting wire 100 for the permanent current switch thinner in order to suppress the amount of heat input while preventing burning, and it is effective in manufacturing and practical use. It can be said that the effect of providing the small diameter portion 120 with respect to the range of the upper line length is large.

Conventionally, superconducting magnet devices using niobium-titanium for the superconducting coil have become widespread, but in the case of niobium-titanium, it is difficult to demagnetize the superconducting coil using the emergency demagnetization mechanism because the quenching margin is small. be. A superconducting coil made of niobium-titanium causes a quench due to AC loss when there is a sudden decrease in current, and consumes energy inside the cryostat.

Therefore, conventional superconducting magnet devices using niobium-titanium are designed on the premise of causing quenching, and are designed to consume energy in a wide area of the superconducting coil in order to prevent burning. ing. The superconducting wire that constitutes the permanent current switch of the conventional superconducting magnet device is different from the superconducting wire 100 for the permanent current switch in that a high composite resistivity is not required and that the small diameter portion 120 is not provided. ing.

The end of the superconducting wire constituting the permanent current switch needs to be superconductingly connected to the superconducting wire forming the superconducting coil. The superconducting filament of the superconducting wire constituting the permanent current switch and the superconducting filament of the superconducting wire forming the superconducting coil must be a combination of superconducting transition at the operating temperature. For example, as a method of superconductingly connecting superconducting wires using MgB ₂ , a method of forming a bulk of MgB ₂ between the wires (see Patent Document 2 and the like) can be used.

First, the ends of each superconducting wire to be superconductingly connected to each other are subjected to cutting, melting by etching, etc. to expose the superconducting filament. Then, each end side of the superconducting wire is inserted into the connecting container, and the exposed superconducting filaments are brought close to each other. After that, the powder as a raw material for MgB ₂ is filled around the end of the superconducting wire in the connection container, the powder is pressurized and compressed, and then the metal container is sealed and heat-treated. When the heat treatment is performed, a bulk of MgB ₂ is formed between the ends, so that the superconducting filaments are integrated and superconductingly connected to each other.

When superconducting connections of superconducting wires that make up a permanent current switch, if the wire diameter at the end of the superconducting wires is too small, there is a problem that breakage is likely to occur due to handling, compression of precursors, etc. .. Since superconducting filaments are brittle, excessive strain can cause damage that prevents them from passing a permanent current. Further, if the wire diameter at the end of the superconducting wire is too small, it becomes difficult to secure the contact area between the superconductors, so that a high critical current cannot be obtained.

On the other hand, since the superconducting wire 100 for the permanent current switch has an end portion 110 having a relatively large wire diameter and a relatively large cross-sectional area of the superconducting filament 10, it is assumed that the small diameter portion 120 is thinned. However, it is possible to secure the mechanical strength at the end, the workability of the superconducting connection, and the contact area between the superconductors. By changing the wire diameter in the longitudinal direction so that the end side of the wire has a large diameter and the center side of the wire has a small diameter, it is possible to deal with the trade-off between heat capacity and connectivity.

The cross-sectional area of the superconducting filament 10 in the small diameter portion 120 is preferably ½ or less of the cross-sectional area of the superconducting filament 10 in the end portion 110. When it is 1/2 or less, a large difference appears in the mechanical strength of the end portion 110 with respect to the small diameter portion 120, and the end portion 110 is less likely to be broken or damaged during handling, compression of the precursor, or the like.

Next, a method for manufacturing the superconducting wire 100 for a permanent current switch will be described more specifically by taking the case of using MgB ₂ as an example.

The method for manufacturing a superconducting wire for a permanent current switch according to the present embodiment includes a step of filling a metal tube with magnesium and boron powder, a step of drawing a metal tube filled with the powder, and a wire drawing process. A step of forming a small diameter portion in which the cross-sectional area of the superconducting filament is smaller than the cross-sectional area at the end portion by performing a swaying process between the ends of the metal pipes, and the powder filled in the metal pipe. It includes a step of heat-treating to produce magnesium diboride.

As the raw material powder, a mixed powder in which magnesium powder, boron powder, and a carbon source to be added as needed are weighed and uniformly crushed and mixed is used. The mixed powder is filled in a metal tube formed of a barrier material. The powder can be crushed and mixed with a ball mill device, a planetary mill device, a V-type mixer, a mortar and the like. The powder is preferably handled in an inert gas atmosphere such as argon gas or nitrogen gas or in a vacuum atmosphere.

The powder is preferably pulverized and mixed by a mechanical milling method. In the mechanical milling method, powder is violently collided with a medium such as a ball or the inner wall of a pot, collision energy is applied to such an extent that MgB ₂ is not clearly generated, and crushing / mixing is performed while performing strong processing. The fact that MgB ₂ is not clearly generated means that the peak of MgB ₂ is not substantially confirmed in the powder X-ray diffraction.

When the mechanical milling method is used, the B particles penetrate into the gaps between the Mg particles, and a powder structure in which B is finely dispersed in the Mg matrix is obtained, so that a superconducting filament having a high filling rate can be formed. Mechanical milling can be performed by a planetary mill device or the like using, for example, a ball medium made of zirconia or the like, or a pot made of zirconia or the like.

As with the superconducting wire that forms a superconducting coil, wire drawing of a metal tube filled with powder is performed within a range in which the required operating current can flow, for example, about 1.5 to 2.5 mm or so. Repeat until the wire diameter is as follows. The wire drawing process can be performed by drawing process, extrusion process, swage process, cassette roll process, groove roll process, or the like. For wire drawing, a draw bench, a hydrostatic pressure extruder, a wire drawing machine, a swager, a cassette roller die, a groove roll, or the like can be used.

The swaging process is a forging process in which the divided dies rotate and hit a bar to reduce the diameter, and is also called rotary forging. In the aging process, the central side in the longitudinal direction of the wire rod is formed so that the small diameter portion 120 having a relatively small wire diameter and a relatively small cross-sectional area of the superconducting filament 10 is formed between the end portions 110. It can be applied only to the end side and not applied to the end side, or it can be applied by changing the drawing force or the compressive force along the longitudinal direction of the wire.

For example, pass one end of the wire through the die of the processing device, and when it reaches the part to be thinned, change the drawing to form a small diameter part, and when it reaches the other end, return the drawing again and let it pass through the die. And, the small diameter portion 120 can be formed on the central side in the longitudinal direction of the wire rod. According to the aging process, only a part of the wire rod in the longitudinal direction can be selectively reduced in diameter, so that the small diameter portion 120 having an arbitrary length can be formed. Further, since the raw material powder can be sufficiently compressed, a superconducting filament having a high filling rate can be obtained.

For example, a metal tube filled with powder is thinned by drawing to a wire diameter of about 1.5 to 2.0 mm, and then aged to about 0.46 mm or less between the ends of the metal tube. The diameter can be reduced. When a double pipe is used, these wire diameter values mean the diameter excluding the outer layer of the double pipe. When a double tube is used, the outer layer of the metal tube is removed after aging. The copper in the outer layer can be removed by dissolving it with nitric acid.

The swaged metal tube is heat-treated to generate MgB ₂ in an atmosphere of an inert gas such as argon gas or nitrogen gas. The heat treatment temperature can be, for example, 550 to 800 ° C, preferably 580 to 700 ° C. At such a temperature, while the diffusion of magnesium progresses, the grain growth of the produced MgB ₂ is suppressed, so that the grain boundary density increases and a high critical current density can be obtained. The wire is wound around a bobbin, and the gap between the wires is impregnated with an insulating resin.

There are a wind-and-react method and a reactor-and-wind method as a method for producing a superconducting wire wound in a coil shape. The wind-and-react method is a method in which a wire rod is wound into a coil and then heat-treated to form a superconducting filament. The reactor-and-wind method is a method in which a wire rod is wound into a coil after being heat-treated to form a superconducting filament.

As a method for producing the superconducting wire 100 for a permanent current switch, it is preferable to use a wind-and-react method. In the case of the reactor-and-wind method, distortion occurs during winding after heat treatment, and the superconducting filament may be damaged. Therefore, great care must be taken and workability is poor. The wind-and-react method can reduce the distortion including the boundary of the small diameter portion 120. In the case of the wind-and-react method, it is preferable to use a glass braid as the insulating coating because it undergoes high-temperature heat treatment.

FIG. 9 is a diagram schematically showing an example of a permanent current switch.
As shown in FIG. 9, the permanent current switch 3 includes a bobbin 301 and a superconducting wire 302 wound around the bobbin 301. As the superconducting wire 302 wound around the bobbin 301, the superconducting wire 100 for the permanent current switch can be used.

The bobbin 301 is preferably formed of copper, and particularly preferably of oxygen-free copper. Since copper has a high thermal conductivity, it is possible to maintain a uniform temperature distribution when cooling the superconducting wire 302 wound around the bobbin 301.

The superconducting wire 302 wound around the bobbin 301 is non-inductively wound. The non-inductive winding can be formed by forming a folded portion 303 in the middle of the superconducting wire rod 302 and winding the folded portion 303 around the bobbin 301. In the non-inductive winding, both ends of the superconducting wire 302 are on the same side to form the lead wire 304.

FIG. 10 is an enlarged cross-sectional view showing a main part of the permanent current switch. FIG. 10 schematically shows the A region of the permanent current switch 3 in FIG. 9 in an enlarged manner.
As shown in FIG. 10, the permanent current switch 3 is provided with a heater 305 for switching the permanent current switch 3 to the off state. Further, the superconducting wire 302 wound around the bobbin 301 is impregnated with the insulating resin 306.

In the non-inductive winding permanent current switch 3, the superconducting wire 302 is wound around the bobbin 301 in pairs. According to the non-inductive winding, the inductance is reduced because the current flows in the opposite direction to each of the superconducting wires 302 adjacent to each other.

A film heater is inserted as a heater 305 for each layer of the winding around the superconducting wire 302 wound around the bobbin 301. According to the film heater, the space between the ends of the superconducting wire 302 can be heated as a whole along the longitudinal direction of the wire. That is, since the small diameter portion 120 of the superconducting wire 100 for the permanent current switch can be heated as a whole along the longitudinal direction of the wire, the amount of heat input can be minimized and local heating can be prevented. be able to.

In FIG. 10, a film heater is used as the heater 305 for switching the permanent current switch 3 to the off state, but a nichrome wire may be used instead of the film heater. The nichrome wire can be wound around the bobbin 301 together with the superconducting wire 302. When the nichrome wire is heat-treated by the wind-and-react method, it undergoes high-temperature heat treatment. Therefore, it is preferable to use a glass braid as the insulating coating of the nichrome wire.

According to the above-mentioned superconducting wire rod for permanent current switch and its manufacturing method, a small diameter portion having a relatively small wire diameter and a relatively small cross-sectional area of the superconducting filament between the ends of the superconducting wire rod. Therefore, it is possible to reduce the volume and heat capacity of the wire as a whole while ensuring the mechanical strength and the connectivity of the superconducting connection at the end portion. Since the amount of heat input from the heater is small when the permanent current switch is switched off, the temperature rise inside the cryostat is suppressed, the risk of quenching increases, and the time required for recovery after emergency demagnetization is long. It is possible to prevent time.

Further, according to the above-mentioned superconducting wire for a permanent current switch and the manufacturing method thereof, an end having a relatively large wire diameter and a relatively large cross-sectional area of the superconducting filament is provided, so that the end machine is provided. The strength of the target and the connectivity of the superconducting connection are improved. In superconducting connection of the permanent current switch, breakage and damage to the superconducting filament are suppressed at the end of the superconducting wire constituting the permanent current switch. In addition, the workability of superconducting connection using a connection container or the like can be improved.

In addition, according to the above-mentioned superconducting wire for permanent current switch and its manufacturing method, the material that affects the composite resistivity of the superconducting wire and the wire length of the superconducting wire are appropriately selected for emergency demagnetization. It is possible to prevent the permanent current switch from burning. In providing a small diameter portion with a relatively small wire diameter and a relatively small cross-sectional area of the superconducting filament, the wire diameter of the superconducting wire is considered to be irrelevant to the likelihood of burning, so the heat capacity of the entire wire is It can be said that the degree of freedom in selecting the line length increases with respect to the goal of reducing the size.

Therefore, according to the above-mentioned superconducting wire rod for a permanent current switch and the manufacturing method thereof, it is possible to provide a superconducting magnet device equipped with a permanent current switch having a small heat capacity under good workability of a superconducting connection. can. By using MgB ₂ with a large quenching margin and high-temperature superconductors, it is possible to use emergency demagnetization mechanisms such as protection resistance and power supply, and the superconducting wires that make up the permanent current switch can be significantly thinned. It will be possible.

It is highly possible that the use of such an emergency demagnetization mechanism and the drastic thinning of wires are measures limited to MgB ₂ having a large quenching margin and high-temperature superconductors. Conventional niobium-titanium has low thermal stability and always has a risk of quenching, so that it may be burnt out by a slight heat generation due to cracks in the impregnated resin or the like. If MgB ₂ or a high-temperature superconductor with a large quenching margin, sufficient thinning is possible, the effect of the small diameter part with a relatively small wire diameter and a relatively small cross-sectional area of the superconducting filament is appropriate. Can be obtained.

FIG. 11 is a diagram schematically showing a superconducting wire for a permanent current switch according to a modified example of the present invention.
As shown in FIG. 11, the superconducting wire constituting the permanent current switch can also be provided as a form in which the wire is gradually thinned from the end portion (superconducting wire 500 for the permanent current switch according to the modified example).

The superconducting wire 500 for a permanent current switch according to a modified example has an end portion 510 having a relatively large wire diameter and a relatively large cross-sectional area of the superconducting filament 10, and a superconducting wire having a relatively small wire diameter. The filament 10 has a small diameter portion 520 having a relatively small cross-sectional area. In FIG. 11, the small diameter portion 520 is gradually reduced in diameter from the end portion 510 as described later. Such a small diameter portion 520 can be formed by aging processing.

For example, when one end of the wire is passed through the die by stopping the processing device and reaches the part to be thinned, the device is started to form a small diameter part, and when it reaches the other end, the device is stopped again. When the die is passed through the die, a small diameter portion 520 can be formed on the central side in the longitudinal direction of the wire rod. Alternatively, a method may be used in which the end side of the wire is processed with a small compressive force and a large drawing, and the center side is processed with a large compressive force and a small drawing.

In FIG. 11, the small diameter portion 520 of the superconducting wire 500 for the permanent current switch is reduced in diameter from the end portion 510 in one step, but may be reduced in two or more steps. For example, a portion having a smaller wire diameter can be formed on the central side in the longitudinal direction from the end portion of the wire rod, and a portion having a smaller wire diameter can be formed on the central side in the longitudinal direction than the portion.

According to the superconducting wire 500 for such a permanent current switch, since the shape is gradually thinned from the end 510, it can be manufactured by a simple operation of a processing device. When the end portion 510 is thinned in a plurality of steps, the raw material powder forming the superconducting filament of the small diameter portion 520 is not rapidly compressed, so that a superconducting filament having high uniformity in the longitudinal direction can be formed.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the present invention is not necessarily limited to those having all the configurations included in the above embodiment. Replacing part of the configuration of one embodiment with another, adding part of the configuration of one embodiment to another, or omitting part of the configuration of one embodiment. Can be done.

For example, in the superconducting wire rods 100 and 500 for the permanent current switch, the small diameter portions 120 and 520 are continuously provided along the longitudinal direction of the superconducting wire rods 100 and 500, but the wire diameters are relative to each other. Small diameter portions that are relatively small and have a relatively small cross-sectional area of the superconducting filament may be provided intermittently along the longitudinal direction of the superconducting wire. That is, they may be provided at a plurality of locations along the longitudinal direction of the wire so as to sandwich the portions provided with a relatively large diameter. The plurality of small diameter portions may have the same wire diameter or different wire diameters.

Further, the superconducting wires 100 and 500 for the permanent current switch are single-core wires having one superconducting filament, but may be multi-core wires having a plurality of superconducting filaments. If the superconducting wire is a multi-core wire, a high operating current can be passed while reducing the AC loss. The multi-core wire can be produced by bundling a plurality of metal tubes filled with raw material powder, inserting them into another metal tube, and performing wire drawing, aging, and heat treatment.

Further, the superconducting wires 100 and 500 for the permanent current switch are provided with ends 110 and 510 in order to appropriately perform superconducting connection, but the method of superconducting connection is not particularly limited. .. The positional relationship of the superconducting wires to be superconductingly connected to each other, the type of the precursor, the shape and structure of the container used for the superconducting connection, the compression method of the precursor, and the like can be set as appropriate conditions. As the precursor, in addition to magnesium and boron, MgB ₂ powder can also be used.

1 Superconducting coil 2 Connection part 3 Permanent current switch 4 Cryostat 5 Current lead 6 Excited power supply 7 Protective resistance 10 Superconducting filament 11 Sheath 100 Superconducting wire 110 End 120 Small diameter 200 Superconducting magnet device 301 Bobin 302 Superconducting wire 303 Folded part 304 Outlet wire 305 Heater 306 Resin

Claims (9)

A superconducting wire for a permanent current switch that constitutes a permanent current switch of a superconducting magnet.
A superconducting wire having a small diameter portion between the ends of the superconducting wire whose cross-sectional area of the superconducting filament is smaller than the cross-sectional area at the end. The superconducting wire for the permanent current switch according to claim 1.
The small diameter portion is provided between the ends of the superconducting wire along the longitudinal direction of the superconducting wire.
A superconducting wire whose cross-sectional area of the superconducting filament at the center in the longitudinal direction of the superconducting wire is smaller than the cross-sectional area of the superconducting filament at the end of the superconducting wire. The superconducting wire for the permanent current switch according to claim 1.
A superconducting wire whose cross-sectional area of the superconducting filament in the small diameter portion is ½ or less of the cross-sectional area of the superconducting filament at the end portion. The superconducting wire for the permanent current switch according to claim 1.
A superconducting filament and a barrier material that covers the periphery of the superconducting filament are provided.
The superconducting filament is made of magnesium diboride.
The barrier material is a superconducting wire made of niobium-titanium. A method for manufacturing superconducting wires for permanent current switches.
The process of filling a metal tube with magnesium and boron powder,
The step of wire drawing the metal tube filled with the powder, and
A step of performing aging processing between the ends of the wire-drawn metal pipes to form a small diameter portion in which the cross-sectional area of the superconducting filament is smaller than the cross-sectional area at the ends.
A method for producing a superconducting wire, which comprises a step of heat-treating the powder filled in the metal tube to produce magnesium diboride. The method for manufacturing a superconducting wire according to claim 5.
The metal tube is a double tube having an inner layer and an outer layer.
The inner layer is made of niobium-titanium.
The method for manufacturing a superconducting wire whose outer layer is made of copper. The method for manufacturing a superconducting wire according to claim 6.
A method for producing a superconducting wire, wherein the outer layer is removed from the double tube after the aging process and before the heat treatment. With a superconducting coil
Permanent current switch composed of superconducting wire for permanent current switch,
A cryostat containing the superconducting coil and the permanent current switch,
It is equipped with an emergency demagnetization mechanism that attenuates the energy stored in the superconducting coil when the superconducting coil is demagnetized.
The superconducting wire for the permanent current switch is a superconducting wire having a small diameter portion between the ends of the superconducting wire whose cross-sectional area of the superconducting filament is smaller than the cross-sectional area at the end.
The emergency demagnetization mechanism is a superconducting magnet device that attenuates the energy stored in the superconducting coil outside the cryostat. The superconducting magnet device according to claim 8.
The emergency demagnetization mechanism is a superconducting magnet device that is a protective resistance that attenuates a current or a power source that applies a negative voltage.

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