JP2018129408A - Permanent current switch, magnetic field generating device, and method of controlling magnetic field generating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a permanent current switch capable of fast and stably performing an opening operation when a superconducting coil is demagnetized; a magnetic field generating device including such a permanent current switch; and a method of controlling the same.SOLUTION: A permanent current switch includes: a first switch part which includes a first section of a superconducting wire; a second switch part including a second section of the superconducting wire and a thermal conductor having higher thermal capacity than the first switch part and in which the second section of the superconducting wire and the thermal conductor are thermally coupled; a heat resistor which is interposed between the first switch part and the second switch part and which thermally separates between the first switch part and the second switch part; a heater capable of heating the first section of the superconducting wire; and a magnet capable of applying a magnetic field to the second section of the superconducting wire.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a permanent current switch using a superconducting wire, a magnetic field generator provided with a permanent current switch, and a control method for the magnetic field generator.

  In a magnetic field generator using a superconducting coil, all of the closed circuit including the superconducting coil is transferred to the superconducting state, and a permanent current is passed through the circuit in the superconducting state (hereinafter referred to as “superconducting circuit”). Some generate a magnetic field. With such a configuration, a current source for driving the superconducting coil during generation of the magnetic field becomes unnecessary, and a constant magnetic field having extremely high temporal stability can be generated. In an NMR (Nuclear Magnetic Resonance) analyzer or an MRI (Magnetic Resonance Imaging) apparatus, the constant magnetic field generated by the magnetic field generator is required to have extremely high temporal stability.

  Even in a superconducting coil driven by a permanent current, a current source is used during excitation (a process of increasing the energization current to a predetermined current value) and demagnetization (a process of decreasing the energization current). During excitation, power is supplied from the current source to the superconducting coil with a part of the superconducting circuit being opened. When the current of the superconducting coil increases to a predetermined value, the superconducting circuit is closed and the current source is stopped. Thereby, the current flowing through the superconducting coil becomes a permanent current and flows through the superconducting circuit. When demagnetizing, the reverse of the procedure for excitation is performed. The permanent current switch (Persistent Current Switch 20) has a function of opening and closing a part of the superconducting circuit in this way.

  The permanent current switch includes a superconducting wire and means for switching the superconducting wire between a normal conducting state and a superconducting state. The closed state is when the superconducting wire is in the superconducting state and the resistance is zero, and the open state is when the superconducting wire is in the normal conducting state and electrical resistance is generated.

  At present, a superconducting coil and a permanent current switch of a magnetic field generator are usually composed of a low-temperature superconducting (LTS) wire. Hereinafter, the low-temperature superconducting wire is referred to as an LTS wire, and the high-temperature superconductor (HTS) wire is referred to as an HTS wire. The first reason for using the LTS wire is that, in the case of the HTS wire, it is difficult to join the superconductors, that is, so-called superconducting joining, and it is difficult to bring all the circuits into a superconducting state. The second reason is that the HTS wire has a high critical temperature at which superconductivity is lost, and it takes time to open and close the permanent current switch by raising and lowering the temperature. Regarding the first reason, many companies or research institutes are currently developing technology for superconducting joining of HTS wires, and it is expected that a simple superconducting joining technology will be established in the near future. Alternatively, even if a part of the circuit includes a wire connection that does not transition to the superconducting state, a pseudo permanent current with a slight current decay is passed through the superconducting coil to provide a constant magnetic field that is stable over a long period of time. Can be generated. Considering such a situation, it is considered important to solve the problem cited as the second reason, that is, the problem that the permanent current switch using the HTS wire can be opened and closed at high speed.

  By the way, an MRI apparatus used for human body inspection is required to have a function of degaussing in a short time in an emergency. Almost all current MRI apparatuses employ a superconducting coil of an LTS wire and immersion cooling of liquid helium. In the LTS coil, emergency demagnetization can be performed by heating the superconducting coil to cause artificial quenching. However, this method of emergency demagnetization has the disadvantage that a large amount of liquid helium is lost by gasification, and there is a risk of accumulation of damage to the superconducting coil due to a rapid temperature rise. On the other hand, when a technique for generating an artificial quench in a superconducting coil is applied to a magnetic field generator employing an HTS coil, the superconducting coil is almost certainly burned out. Therefore, for the purpose of urgently demagnetizing the superconducting coil, means for opening the permanent current switch at high speed is effective in either the configuration using the HTS wire or the configuration using the LTS wire.

  Patent Document 1 discloses a technique of a permanent current switch provided in a superconducting circuit that is conductively cooled as a conventional technique related to the present invention. In this technique, a thermal resistance is interposed between a winding portion of the permanent current switch and a heat bath for cooling the superconducting circuit including the permanent current switch. With this configuration, the permanent current switch can be opened by raising the temperature of the winding portion with a small amount of heat from the heater.

  Patent Document 2 proposes a technique for quickly opening a permanent current switch using an HTS wire as a conventional technique related to the present invention. Patent Document 2 discloses a technique for opening a permanent current switch in a short time by causing a heater and a magnetic field of the superconducting coil to act on the permanent current switch when the superconducting coil is degaussed urgently.

JP-A-10-189324 JP 2010-73856 A

  The temperature difference for opening and closing the permanent current switch of the HTS wire is larger than that of the LTS wire, and the amount of heat that must be exchanged by the opening and closing operation is also increased. On the other hand, Patent Document 1 discloses a technique for increasing the temperature of a winding portion of a permanent current switch using thermal resistance with a small amount of heat of a heater.

  However, in the configuration shown in Patent Document 1, if the opening operation is performed during energization, there is a problem that the permanent current switch is damaged by its own heat generation. This is due to the following reason. Usually, in a magnetic field generator using a superconducting coil, a protective resistor connected in parallel with a permanent current switch is provided outside the cooling device. If the permanent current switch is opened while the superconducting coil is energized, the current of the superconducting coil is passed through the protective resistor, and the current of the superconducting coil is reduced to zero. On the other hand, the superconducting wire constituting the permanent current switch is provided with a protective layer having electrical conductivity along the superconducting layer, and when the superconducting layer transitions to the normal conducting state, the current bypasses the protective layer and the superconducting layer To avoid damage. Therefore, even if the permanent current switch is opened, the resistance of the permanent current switch does not increase so much. For this reason, the current of the superconducting coil flows not only in the protective resistance but also in the permanent current switch at a large rate, and Joule heat is generated in the permanent current switch. Here, as shown in Patent Document 1, when a thermal resistance is provided between the superconducting wire of the permanent current switch and the heat bath, rapid discharge of Joule heat is hindered, and the temperature of the permanent current switch is reduced. Raise it to a level that will damage it. Improving the heating efficiency of the permanent current switch and improving the exhaust heat efficiency are conflicting issues.

  In the technique disclosed in Patent Document 2, as described above, a short-time opening operation of the permanent current switch is expected by applying a heater and a magnetic field to the permanent current switch. However, when a magnetic field is applied to the superconducting wire, the critical current of the superconducting wire is significantly reduced, but the critical temperature is not so lowered. In other words, the permanent current switch to which a magnetic field is applied generates a voltage when a current exceeding the critical current flows, but when the current decreases, the permanent current switch returns to a substantially closed state and completely demagnetizes the superconducting coil. I can't. In order to completely demagnetize, it is necessary to quickly heat the superconducting wire above the critical temperature so that the critical current of the superconducting wire becomes zero, but when the permanent current switch is opened by the action of the magnetic field, the permanent current Large Joule heat is generated in the switch. Therefore, in the permanent current switch of Patent Document 2, as in the permanent current switch that does not act on a magnetic field, the conflicting problems of improvement of exhaust heat efficiency and improvement of heating efficiency remain.

  An object of the present invention is to provide a permanent current switch that can be opened at high speed and stably when demagnetizing a superconducting coil, a magnetic field generator including such a permanent current switch, and a control method thereof.

The invention described in claim 1
A first switch portion including a first section of superconducting wire;
A second switch including a second section of the superconducting wire and a heat conductor having a larger heat capacity than the first switch unit, wherein the second section of the superconducting wire and the heat conductor are thermally coupled. And
A thermal resistor interposed between the first switch unit and the second switch unit to thermally separate the first switch unit and the second switch unit;
A heater capable of heating the first section of the superconducting wire;
A magnet capable of applying a magnetic field to the second section of the superconducting wire;
A permanent current switch comprising:

The invention according to claim 2 is the permanent current switch according to claim 1 or 2,
The superconducting wire is a rare earth-based superconducting wire or a bismuth-based superconducting wire.

According to a third aspect of the present invention, in the permanent current switch according to the first or second aspect,
The second switch unit is
It further includes a heat conducting wire that is electrically insulated from the second section of the superconducting wire, and the second section of the superconducting wire and the heat conducting wire are wound together on a reel. Yes.

According to a fourth aspect of the present invention, in the permanent current switch according to any one of the first to third aspects,
The first section and the second section of the superconducting wire are wound in opposite winding directions;
The combined inductance of the first section and the second section of the superconducting wire is smaller than the sum of the self-inductance of the first section of the superconducting wire and the self-inductance of the second section of the superconducting wire. It is said.

According to a fifth aspect of the present invention, in the permanent current switch according to any one of the first to fourth aspects,
The semiconductor device further includes a semiconductor switch electrically connected in parallel to at least a part of the superconducting wire.

According to a sixth aspect of the present invention, in the permanent current switch according to any one of the first to fifth aspects,
The first switch unit includes a first part and a second part having different heat capacities,
The heater includes a first heater capable of heating the first portion and a second heater capable of heating the second portion.

The invention described in claim 7
Superconducting coils connected in parallel to each other, the permanent current switch according to any one of claims 1 to 6, a protective resistor and a current source;
A cooling device for cooling the superconducting coil and the permanent current switch;
A control unit for controlling the permanent current switch;
With
The control unit operates the magnet and the heater based on a demagnetization request of the superconducting coil, switches the second switch unit to open by the action of the magnetic field of the magnet, and then heats the first. The magnetic field generator is characterized in that the switch unit is switched to open.

The invention according to claim 8 is the magnetic field generator according to claim 7,
The control unit causes the current source to output a current when the second switch unit is opened by the action of the magnetic field of the magnet.

The invention according to claim 9
A superconducting coil connected in parallel to each other, the permanent current switch according to any one of claims 1 to 6, a protective resistor and a current source, and a cooling device for cooling the superconducting coil and the permanent current switch. A method of controlling a magnetic field generator comprising:
The magnet and the heater are operated, the second switch part is switched to open by the action of the magnetic field of the magnet, and then the first switch part is switched to open by heating the heater to demagnetize the superconducting coil. It is characterized by that.

A tenth aspect of the present invention is the method for controlling a magnetic field generator according to the ninth aspect,
When the second switch unit is switched to open by the action of the magnetic field of the magnet, the current source is caused to output a current.

  According to the present invention, when demagnetizing the superconducting coil, the critical current in the second section of the superconducting wire is reduced by the action of the magnetic field of the magnet, and thereby the second switch portion can be quickly switched to open. At this time, since the Joule heat generated in the second section of the superconducting wire is discharged to the heat conductor having a large heat capacity, the second section of the superconducting wire can be prevented from becoming very high temperature. On the other hand, when the current falls below the critical current, the second switch unit is again switched to the closed state. However, in the present invention, the first switch unit has a smaller heat capacity than the second switch unit, and the first switch unit and the second switch unit are thermally separated by the thermal resistor. For this reason, while the second switch is opened and the demagnetization of the superconducting coil is proceeding, the temperature of the first section of the superconducting wire is raised to a temperature higher than the critical temperature by heating the heater, and the first switch portion is Can be switched to open. At this time, since the second switch part is opened before that, and the current of the superconducting coil is reduced, the current flowing through the first switch part is small. Therefore, the heat generation of the first switch portion due to this current does not increase, and damage due to an excessive temperature rise can be avoided. As a result, the open state of the permanent current switch can be stably maintained. Therefore, according to the present invention, the permanent current switch can be opened at high speed and stably when the superconducting coil is demagnetized.

It is a lineblock diagram showing the magnetic field generator concerning the embodiment of the present invention. It is a conceptual diagram which shows the structure of the permanent current switch which concerns on 1st Embodiment of this invention. It is a time chart explaining opening operation at the time of emergency of the permanent current switch which concerns on 1st Embodiment. It is a circuit diagram which shows the structure of the permanent current switch which concerns on 2nd Embodiment of this invention. It is a time chart explaining closing operation of the permanent current switch concerning a 2nd embodiment. It is a conceptual diagram which shows the structure of the permanent current switch which concerns on 3rd Embodiment of this invention. It is a side view which shows the permanent current switch which concerns on Example 1 of this invention. The cross section of a 2nd switch part is shown, (A) is the sectional view on the AA line of FIG. 7, (B) is sectional drawing to which the one part was expanded. The cross section of a 1st switch part is shown, (A) is the BB sectional drawing of FIG. 7, (B) is sectional drawing to which the one part was expanded. It is a disassembled perspective view which shows the junction part of a 1st switch part and a 2nd switch part. It is a perspective view which shows the permanent current switch which concerns on Example 2 of this invention. 6 is a diagram illustrating a laminated structure of a permanent current switch according to Embodiment 2. FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a configuration diagram showing a magnetic field generator according to an embodiment of the present invention.

  As shown in FIG. 1, the magnetic field generator 1 according to the embodiment of the present invention includes a superconducting coil 11, a permanent current switch 20, a protective resistor 12, a current source 13, a cooling device 14, and a control unit 15.

  The superconducting coil 11 generates a constant magnetic field by passing a permanent current or a pseudo permanent current. As a material for the superconducting coil 11, an HTS wire such as a REBCO wire (rare earth-based superconducting wire) or a BSCCO wire (bismuth-based superconducting wire) is used. Here, the permanent current means an unattenuated current flowing in a closed circuit whose entire circumference is in a superconducting state. The pseudo-permanent current means a current with a very small amount of attenuation that flows in a closed circuit in a superconducting state over most of the entire circumference, except for a part such as a connection part connecting superconducting wires. When the magnetic field generator 1 is applied to an NMR analyzer or an MRI apparatus, the superconducting coil 11 forms a magnetic field in a space where an analysis object or a subject is placed.

  The permanent current switch 20 has a superconducting wire extending from one end to the other end of the current path, and means for switching the superconducting wire between a superconducting state and a normal conducting state. The permanent current switch 20 is closed when the superconducting wire is transitioned to the superconducting state and the electric resistance becomes zero, and at least a part of the superconducting wire (except for a portion that is not superconducting such as a wire connecting portion) is in the normal conducting state. Thus, a state where an electrical resistance is generated between both ends of the current path is opened. As the material of the superconducting wire of the permanent current switch 20, an HTS wire such as a REBCO wire or a BSCCO wire is used. The permanent current switch 20 is connected in series with the superconducting coil 11 and forms a closed circuit together with the superconducting coil 11.

  The cooling device 14 is, for example, a conduction cooling type cryostat using a GM refrigerator, and can cool the closed circuit including the superconducting coil 11 and the permanent current switch 20 to a temperature at which the HTS wire transitions to the superconducting state.

  The current source 13 is connected in parallel with the superconducting coil 11 and the permanent current switch 20. The current source 13 supplies a predetermined current to a circuit including itself by adjusting the voltage output.

  The protective resistor 12 is connected in parallel with the superconducting coil 11 and the permanent current switch 20. The protective resistor 12 causes the current of the superconducting coil 11 to flow when the superconducting coil 11 is demagnetized, and consumes the energy accumulated in the superconducting coil 11.

  The control unit 15 performs opening / closing control of the permanent current switch 20 and output control of the current source 13.

(First embodiment)
FIG. 2 is a conceptual diagram showing the configuration of the permanent current switch according to the first embodiment of the present invention.

  The permanent current switch 20 includes a first switch portion 25A, a second switch portion 25B, a thermal resistor 26, a heater 28, and a magnet 29. The first switch portion 25 </ b> A includes a first section 21 a of the superconducting wire 21 and a frame 22 that holds the first section 21 a of the superconducting wire 21. The second switch portion 25 </ b> B has a second section 21 b of the superconducting wire 21 and a frame 23. The frame 23 corresponds to an example of a heat conductor according to the present invention. The superconducting wire 21 is formed continuously over the first section 21a and the second section 21b. The second section 21b of the superconducting wire 21 may be held by the frame 23 or may be held by another member. Superconducting wire 21 and frame 22 are arranged away from cooling unit 14A of cooling device 14.

  The superconducting wire 21 may be provided so as to be folded back halfway and reciprocate once or a plurality of times between the frame 22 and the frame 23. In this case, when the superconducting wire 21 is viewed in a straight line, the section held by the frame 22 or the section held by the frame 23 is divided into a plurality. However, even if the frame is divided into a plurality of sections, the section held in the frame 22 is defined as a first section 21a, and the section held in the frame 23 is defined as a second section 21b.

  The frame 23 has a larger heat capacity than the first switch portion 25 </ b> A, and is thermally coupled to the second section 21 b of the superconducting wire 21. Here, “thermally coupled” is described in detail later, but is not limited to the configuration in which the frame is directly contacted, and is connected via a member having low thermal resistance to the extent that the operation of the frame 23 described later is realized. Configuration is included. The frame 23 is connected to the cooling unit 14 </ b> A of the cooling device 14.

  The thermal resistor 26 thermally separates the first switch unit 25A and the second switch unit 25B, and physically connects the first switch unit 25A and the second switch unit 25B. With this connection, the connection between the first section 21a and the second section 21b of the superconducting wire 21 can be maintained. Here, “thermally separated” is described in detail later, but is not limited to a configuration in which the movement of heat is completely blocked, and the thermal resistance is high enough to achieve the action of the thermal resistor 26 described later. Includes configurations connected to the state.

  As a material of the thermal resistor 26, FRP (Fiber-Reinforced Plastics) can be applied. Thereby, high mechanical strength and stability in a cryogenic environment are obtained. In addition, as a material of the thermal resistor 26, an alloy such as stainless steel or brass can be applied when there is a restriction to use a metal in order to reduce a difference in thermal expansion from a member to be connected. In such an alloy, the temperature dependence of the thermal conductivity is small, and the property of low thermal conductivity can be obtained even in an extremely low temperature environment.

  The heater 28 can be switched between operation and stop and adjusted in output under the control of the control unit 15, and heats the first section 21 a of the superconducting wire 21 during operation.

  The magnet 29 is an electromagnet, for example, and can be switched between operation and stop by the control of the control unit 15, and applies a magnetic field to the second section 21 b of the superconducting wire 21 during operation.

<Emergency opening operation>
Next, an operation of opening the permanent current switch 20 and demagnetizing the superconducting coil 11 urgently will be described. Urgent demagnetization is required, for example, when some abnormality is detected in the superconducting coil 11.

  FIG. 3 is a time chart for explaining an emergency opening operation of the permanent current switch according to the first embodiment. 3A to 3E respectively show the output of the heater 28, the operation (ON) and stop (OFF) of the magnet 29, the output (ON) and stop (OFF) of the direct current from the current source 13, The opening and closing of the first switch unit 25A and the opening and closing of the second switch unit 25B are shown. FIG. 3F shows the current of the superconducting coil 11. “Open” and “closed” of the first switch part 25A are a state in which at least part of the current path of the first section 21a of the superconducting wire 21 is in a normal conducting state and a state in which all are in a superconducting state. Each is shown. “Open” and “closed” of the second switch portion 25B are a state in which at least a part of the current path of the second section 21b of the superconducting wire 21 is in a normal conducting state and a state in which all are in a superconducting state. Each is shown. Hereinafter, “open” and “closed” are used in the same meaning for a permanent current switch made of a superconducting wire or a portion thereof.

  Before the opening operation of the permanent current switch 20 (before timing t1), the superconducting coil 11 and the superconducting wire 21 of the permanent current switch 20 are cooled to below the critical temperature and kept in the superconducting state. The superconducting coil 11 is excited by a permanent current or a pseudo permanent current having a predetermined magnitude that flows through the permanent current switch 20.

  When there is an urgent demagnetization request, the control unit 15 activates the heater 28 and the magnet 29 (timing t1). When the magnet 29 is activated and a magnetic field is applied to the second section 21b of the superconducting wire 21, the critical current in the second section 21b falls below the current flowing through the permanent current switch 20, resulting in the voltage and the associated heat. Occurs. The second section 21b quickly transitions to the normal conducting state by being heated by the heat. Thereby, the 2nd switch part 25B of the permanent current switch 20 is opened (timing t2).

  Even if the magnet 29 is activated and the critical current of the second switch portion 25B is lowered, the second switch portion 25B is not opened if the current flowing through the permanent current switch 20 is smaller than the critical current. In such a case, the control unit 15 may use a control for outputting a direct current from the current source 13 as shown in FIG. The output of the current source 13 hardly changes the current flowing through the superconducting coil 11 having a large inductance, and increases the current of the permanent current switch 20 and the protective resistor 12. As a result, the current of the permanent current switch 20 is supplemented to exceed the critical current of the second switch unit 25B, and the second switch unit 25B can be sufficiently opened. If it is not necessary to supplement the current, the control of the current source 13 in FIG. 3C may be omitted. Moreover, if the 2nd switch part 25B is temporarily opened, the open state of the 2nd switch part 25B of the period T2 mentioned later will be obtained. For this reason, the control unit 15 may immediately stop the output of the current source 13 at the timing t2 when the second switch unit 25B is opened.

  When the second switch unit 25B is switched to open, the current flowing through the permanent current switch 20 is changed according to the ratio between the electrical resistance value of the second switch unit 25B and the electrical resistance value of the protective resistor 12 and The current is shunted to the protective resistor 12. With this current, the energy accumulated in the superconducting coil 11 is consumed as Joule heat, and the current in the superconducting coil 11 is attenuated.

  On the other hand, even if the second switch portion 25B is opened, the electrical resistance value of the second section 21b of the superconducting wire 21 does not increase so much. This is because the superconducting wire 21 has a protective layer having electrical conductivity in order to avoid damage to the superconducting layer, and when the superconducting layer transitions to the normal conducting state, the protective layer having a small electrical resistance bypasses the current. If the wire length of the second section 21b of the superconducting wire 21 is increased, the electrical resistance value can be increased. However, this increases the size of the permanent current switch 20 and increases the cost. Therefore, there is a limit to the electrical resistance value in the open state of the second switch portion 25B. For this reason, when the current value of the superconducting coil 11 is high, the current shunted to the second switch portion 25B is large, and a large Joule heat is also generated in the second section 21b of the superconducting wire 21.

  The Joule heat generated here quickly moves to the frame 23 having a large heat capacity that is thermally coupled to the second section 21 b of the superconducting wire 21. As a result, the second section 21b of the superconducting wire 21 does not rise to a temperature at which damage occurs. That is, the “thermal coupling” between the superconducting wire 21 and the frame 23 means that the Joule heat is transferred from the second section 21b of the superconducting wire 21 to the frame 23 so as to avoid a temperature rise that causes damage. It means a bond that can be quickly released. Note that Joule heat also moves to the first switch portion 25A via the thermal resistor 26, but the amount of heat that moves by the thermal resistor 26 remains small.

  In the second switch part 25B, Joule heat generated in the superconducting wire 21 quickly moves to the frame 23, and a part of this heat escapes to the cooling part 14A. By such heat transfer, the temperature of the superconducting wire 21 and the frame 23 of the second switch part 25B can be kept to such an extent that the second switch part 25B is not damaged. And the open state is continued in the 2nd switch part 25B by this temperature rise. Thereafter, when the current of the superconducting coil 11 decays, Joule heat generated in the second switch portion 25B decreases. Thereby, the temperature of the superconducting wire 21 and the frame 23 of the second switch portion 25B becomes a critical temperature or lower. Thereby, the 2nd switch part 25B switches to a close again (timing t4). As a result, an open state of the second switch part 25B in the period T2 is obtained. The control unit 15 may stop the magnet 29 at the timing t4. Alternatively, it is more preferable to stop the magnet 29 at the timing when the voltage is no longer generated in the second switch portion 25B.

  On the other hand, in the first switch portion 25A, after the timing t1, the heating by the heater 28 is continued, and the temperature of the first section 21a of the superconducting wire 21 gradually increases. As described above, the first switch unit 25A is thermally separated from the second switch unit 25B through the thermal resistor 26, and the first switch unit 25A is disposed away from the cooling unit 14A of the cooling device 14. ing. For this reason, the heat of the heater 28 is efficiently transferred to the first section 21a of the superconducting wire 21, and the temperature of the first section 21a of the superconducting wire 21 can be raised to a critical temperature or higher in a relatively short period T1. . As a result, the first switch unit 25A is switched to open (timing t3). When the first switch unit 25A is switched to the open state, the control unit 15 reduces the output of the heater 28 to such an extent that the temperature of the first section 21a of the superconducting wire 21 can be maintained above the critical temperature.

  The length of the period T1 from the start of heating of the heater 28 to the opening of the first switch part 25A is set shorter than the period T2 in which the second switch part 25B can be kept open. That is, “thermal separation” between the first switch portion 21A and the second switch portion 21B by the thermal resistor 26 means a state in which the heating efficiency of the heater 28 satisfying this setting is obtained. In other words, it means a state in which the discharge of heat from the first switch unit 25A to the second switch unit 25B is limited so as to satisfy the above setting.

  When the first switch portion 25A is opened, Joule heat is generated in the first switch portion 25A due to the electrical resistance of the first section 21a of the superconducting wire 21. However, at the timing t3 when the first switch portion 25A is opened, the current of the superconducting coil 11 has already been greatly attenuated. Furthermore, the resistance of the first switch section 25A and the resistance of the second switch section 25B are added together, and the total resistance of the permanent current switch 20 is increased, so that the current shunt ratio of the protection resistor 12 is increased. As a result, the current flowing through the permanent current switch 20 is reduced. Therefore, the Joule heat generated in the first switch portion 25A does not increase, and the first section 21a of the superconducting wire 21 does not become so high as to be damaged even if it is thermally separated.

  Note that it is not preferable that the timing at which the first switch unit 25A is opened is too early because the current flowing through the first switch unit 25A increases. In such a case, the first switch portion 25A can be opened at an appropriate timing by delaying the timing of operating the heater 28.

  When the first switch portion 25A is switched to open, the remaining current of the superconducting coil 11 is reduced to zero, and the superconducting coil 11 is completely demagnetized. Further, after the first switch portion 25A is opened, even if the current of the superconducting coil 11 is attenuated and the second switch portion 25B is closed again (timing t4), the permanent current switch 20 is opened as a whole. It becomes a state. For this reason, the superconducting coil 11 is demagnetized to the end.

  As described above, according to the permanent current switch 20 of the first embodiment, the permanent current switch 20 can be opened quickly and the superconducting coil 11 can be demagnetized while the superconducting coil 11 is excited. Further, during the demagnetization of the superconducting coil 11, the permanent current switch 20 is not damaged by an excessive temperature rise.

  Further, according to the magnetic field generator 1 and the control method thereof according to the first embodiment, when the permanent current switch 20 is opened, the current of the permanent current switch 20 does not exceed the critical current of the second section 21b of the superconducting wire 21. In some cases, the control unit 15 activates the current source 13. Thereby, the electric current which flows into the permanent current switch 20 increases, and the 2nd switch part 25B can be switched to open rapidly. Therefore, even in the above case, when the demagnetization request is made, the permanent current switch 20 can be quickly opened to demagnetize the superconducting coil 11.

(Second Embodiment)
FIG. 4 is a circuit diagram showing a configuration of a permanent current switch according to the second embodiment of the present invention.

  The permanent current switch 20A of the second embodiment includes a first switch unit 25A, a second switch unit 25B, a heater 28, a magnet 29, and a semiconductor switch 31. The first switch unit 25A, the second switch unit 25B, the heater 28, and the magnet 29 are the same as in the first embodiment, and a detailed description thereof will be omitted. The magnetic field generator of 2nd Embodiment is the structure which replaced the permanent current switch 20 of the magnetic field generator 1 of FIG. 1 with the permanent current switch 20A of 2nd Embodiment.

  The semiconductor switch 31 is, for example, a thyristor, and is connected in parallel with the first switch unit 25A and the second switch unit 25B. The semiconductor switch 31 is controlled to be opened and closed by the control unit 15. The semiconductor switch 31 is for speeding up the closing operation of the permanent current switch 20A.

  FIG. 5 is a time chart for explaining a closing operation of the permanent current switch according to the second embodiment when the superconducting coil 11 is excited (energized). 5A to 5E respectively show the current of the superconducting coil 11, the output (ON) and stop (OFF) of the current source 13, the opening and closing of the semiconductor switch 31, the opening and closing of the first switch unit 25A, and the second. The opening and closing of the switch unit 25B is shown.

  In FIG. 5, the superconducting coil 11 is excited by being supplied with a current from the current source 13 in a state where the permanent current switch 20 </ b> A is opened before the timing t <b> 11. Usually, the 2nd switch part 25B is a closed state. Therefore, the first switch portion 25A may be closed to transition the permanent current switch 20A from the open state to the closed state. However, since the first switch unit 25A is connected to the cooling unit 14A via the thermal resistor 26 and the second switch unit 25B, it takes a relatively long time to cool the first switch unit 25A.

  Therefore, in the second embodiment, the control unit 15 closes the semiconductor switch 31 at the timing t11 when the permanent current switch 20A is desired to be shifted to the closed state. Thereby, the permanent current switch 20A is substantially closed, and when the current output of the current source 13 is further stopped, the current flowing through the superconducting coil 11 passes through the permanent current switch 20A.

  However, the semiconductor switch 31 includes a PN junction, and a predetermined voltage drop occurs even in the closed state. For this reason, it is difficult to maintain a state where a large current flows through the semiconductor switch 31 for a long time from the viewpoint of cooling. Therefore, it is preferable to adjust the delay of the closing operation of the second switch portion 25B to be as short as possible. After that, as shown in timing t12 of FIG. 5, when the cooling of the permanent current switch 20A proceeds and the first switch unit 25A is switched to the closed state, the current flows between the first switch unit 25A and the second switch unit 25B in the superconducting state. Priority to flow. The control unit 15 may switch the semiconductor switch 31 to open after the first switch unit 25A is closed.

  Even if the superconducting coil 11 is not excited before the timing t11, the permanent current switch 20 can be changed to the closed state by the above-described operation. Even if the superconducting coil 11 is excited by a current circuit that does not include a current source or a current circuit that includes electrical elements other than the current source before the timing t11, the permanent current switch 20 is also closed by the above-described operation. It is made to transition to.

  As described above, according to the permanent current switch 20A of the second embodiment, a quick closing operation is possible in addition to a quick opening operation as in the first embodiment.

(Third embodiment)
FIG. 6 is a conceptual diagram showing a configuration of a permanent current switch according to the third embodiment of the present invention.

  The permanent current switch 20B of the third embodiment includes a second switch unit 25B, a magnet 29, a thermal resistor 26, a first switch unit 25C, and two heaters 28a and 28b. Among these, the 2nd switch part 25B, the thermal resistance body 26, and the magnet 29 are the same as that of 1st Embodiment, The detailed description about these is abbreviate | omitted.

  In the third embodiment, the first switch unit 25C includes a first small section 21a1 and a second small section 21a2 that bisect the first section 21a of the superconducting wire 21, a thermal conductor 25, a frame 24a, and a thermal resistor 24b. . The first small section 21a1 and the second small section 21a2 of the superconducting wire 21 are separately temperature controlled. The superconducting wire 21 is formed continuously over the first section (the first small section 21a1 and the second small section 21a2) and the second section 21b.

  The second small section of the superconducting wire 21 is held by the frame 24a via the thermal resistor 24b. The first small section 21a1 of the superconducting wire 21 may be held by the heat conductor 25 or may be held by another member. The first small section 21a1 and the heat conductor 25 of the superconducting wire 21 correspond to an example of the first portion of the first switch unit according to the present invention. The second small section 21a2, the thermal resistor 24b, and the frame 24a of the superconducting wire 21 correspond to an example of the second portion of the first switch unit according to the present invention.

  The heat conductor 25 has a smaller heat capacity than the frame 23 of the second switch portion 25B, and is thermally coupled to the first small section 21a1 of the superconducting wire 21.

  The thermal resistor 24b is a polymer sheet or the like, and thermally separates the frame 24a having a small amount of heat capacity and the second small section 21a2 of the superconducting wire 21.

  The two heaters 28 a and 28 b are arranged so that the first small section 21 a 1 and the second small section 21 a 2 of the superconducting wire 21 can be independently heated, and can be driven independently from each other by the control unit 15.

<Emergency opening operation>
Next, an opening operation when the demagnetization of the superconducting coil 11 is urgently described.

  In the third embodiment, when there is an urgent demagnetization request, the control unit 15 activates the magnet 29 and one heater 28a. Accordingly, first, the second switch portion 25B is quickly opened by the action of the magnet 29, and a large Joule heat is generated in the second switch portion 25B. Since this Joule heat quickly moves to the frame 23 having a large heat capacity, it is avoided that the second switch portion 25B including the second section 21b of the superconducting wire 21 is damaged due to an excessive temperature rise.

  Here, a case is assumed in which it is difficult to maintain the open state of the second switch portion 25B before the current of the superconducting coil 11 is sufficiently attenuated. In such a case, in the third embodiment, the first small section 21a1 of the superconducting wire 21 can be switched to open by heating of one heater 28a. When the first small section 21a1 is opened, the current of the superconducting coil 11 is not yet sufficiently attenuated, and thus relatively large Joule heat is generated in the first small section 21a1. However, in the first small section 21a1, by escaping this Joule heat to the heat conductor 25, damage due to an excessive temperature rise in the first small section 21a1 of the superconducting wire 21 and its peripheral portion can be avoided. If the heat capacity of the heat conductor 25 is too small, Joule heat cannot be absorbed, and if it is too large, the heating efficiency of the first small section will be significantly reduced. Therefore, the heat capacity of the heat conductor 25 is preferably set in consideration of these trade-offs.

  The open state of the first small section 21a1 of the superconducting wire 21 is continued until the current of the superconducting coil 11 becomes zero, whereby the superconducting coil 11 is completely demagnetized.

<High speed opening and closing operation>
Next, an operation for opening and closing the permanent current switch 20B at high speed when a large current does not flow through the permanent current switch 20B will be described. First, the operation of opening at high speed will be described.

  In the stage before opening the permanent current switch 20B, the first section 21a and the second section 21b of the permanent current switch 20B are cooled to a transition temperature or lower and transitioned to a superconducting state. When the permanent current switch 20B is opened at high speed without worrying that a large current flows through the permanent current switch 20B, the control unit 15 activates the second heater 28b. Here, the control unit 15 does not need to operate the magnet 29 and the first heater 28a.

  In 3rd Embodiment, since the heat conductor 25 was provided in the 1st switch part 25C, the heating efficiency of the 1st small section 21a1 of the superconducting wire 21 falls comparatively. On the other hand, the second small section 21a2 of the superconducting wire 21 is held by the frame 24a via the thermal resistor 24b, and is thermally separated from the heat capacities of the heat conductor 25 and the frame 24a. For this reason, the heating efficiency of the second small section 21a2 of the superconducting wire 21 is very high. Therefore, when the second heater 28b operates, the temperature of the second small section 21a2 of the superconducting wire 21 rises quickly, and the second small section 21a2 can be opened exceeding the critical temperature.

  At this time, a current may be supplied from the current source 13 to the circuit. The current supplied at this time flows through a circuit including the permanent current switch 20B having neither inductance nor electric resistance, and does not flow through other paths. The current flowing in the second small section 21a2 generates heat at a temperature lower than the critical temperature, and this helps to heat the second small section 21a2, thereby realizing a temperature increase in a shorter time. Thereby, the quick opening operation of the permanent current switch 20B is achieved.

  Next, a high-speed closing operation will be described. In order to close the permanent current switch 20B at high speed, as the state before the closing operation, the amount of heat output from the second heater 28b is the minimum amount of heat that maintains the second small section 21a2 of the superconducting wire 21 in the open state. Keep close to. That is, the control unit 15 performs control so that the amount of heat that maintains the open state is minimized. As a result, the temperature of the second small section 21a2 of the superconducting wire 21 is maintained at or above the critical temperature without being significantly separated from the critical temperature. Furthermore, before the closing operation, the second section 21b and the first small section 21a1 of the permanent current switch 20B are cooled to a critical temperature or lower.

  When closing at high speed, the control unit 15 stops the heating of the second heater 28b. As described above, the second small section 21a2 of the superconducting wire 21 is not thermally coupled to a small heat capacity. Furthermore, according to the state before the opening operation, the temperature of the second small section 21a2 of the superconducting wire 21 is maintained without significant separation from the critical temperature. Therefore, by stopping the heating of the second heater 28b, the temperature of the second small section 21a2 of the superconducting wire 21 can be lowered to a critical temperature or less by moving a small amount of heat. Although the heat transfer speed is reduced by the thermal resistors 24b and 26, a quick closing operation is realized because the amount of heat necessary for the closing operation is small.

  As shown in the second embodiment, the permanent current switch 20B of the third embodiment may adopt a configuration in which a semiconductor switch (for example, thyristor) 31 is connected in parallel to the superconducting wire 21. In this case, the closing operation of the semiconductor switch 31 can be used together with the closing operation of the semiconductor switch 31 similarly to the second embodiment.

  As described above, according to the permanent current switch 20B of the third embodiment, a quick opening operation is possible to demagnetize the superconducting coil 11 as in the first embodiment, and no large current flows. It is possible to perform a high-speed opening / closing operation in the state. In addition, since the output of the second heater 28b becomes very small during a high-speed opening / closing operation, the cooling capacity required for the cooling device 14 can be kept small.

  The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiment. For example, in the said embodiment, the example which used the HTS wire for the superconducting wire 21 of the superconducting coil 11 and the permanent current switch 20 was shown. However, these may be LTS wires. Moreover, in the magnetic field generator 1 of the said embodiment, the example which applied the conduction cooling type apparatus as the cooling device 14 was shown. However, an immersion cooling type device may be applied as the cooling device. In this case, the permanent current switches 20, 20 </ b> A, and 20 </ b> B may be arranged so that the frame 23 is immersed in the coolant and the first switch portions 25 </ b> A and 25 </ b> C are isolated from the coolant. With such an arrangement, the same action as in the embodiment can be obtained. In addition, the details shown in the embodiments can be appropriately changed without departing from the spirit of the invention.

  Next, a first embodiment in which the permanent current switch 20 is more specific will be described. Example 1 is an example in which the permanent current switch 20 of the first embodiment shown in FIG. 2 is embodied. FIG. 7 is a side view showing the permanent current switch according to the first embodiment of the present invention. 8A is a cross-sectional view taken along line AA in FIG. 7, and FIG. 8B is a cross-sectional view in which a part thereof is enlarged. 9A is a cross-sectional view taken along line BB in FIG. 7, and FIG. 9B is an enlarged cross-sectional view of a part thereof. FIG. 10 is an exploded perspective view showing a joint portion between the first switch portion 25A and the second switch portion 25B.

<Magnetic field generator>
In the magnetic field generator 1 (see FIG. 1) to which the permanent current switch 20 of the first embodiment is applied, the superconducting coil 11 is a coil having an inductance of 10H using a 1 km REBCO wire, and has a permanent current of 400A. Driven. The stored magnetic field energy of the superconducting coil 11 is 0.8 MJ. The superconducting circuit including the superconducting coil 11 and the permanent current switch 20 is housed in a vacuum chamber (cryostat), and the cooling device 14 cools the superconducting circuit to 40K by conduction cooling. The current source 13 and the protective resistor 12 are connected in parallel to the permanent current switch 20 via a normal conducting wire led out of the vacuum chamber. The electrical resistance of the protective resistor 12 is 1Ω, and the time constant of current decay is 10 seconds in the series circuit of the superconducting coil 11 and the protective resistor 12. It is assumed that the permanent current switch 20 is completely opened instantaneously (the electrical resistance is infinite) in a state where the superconducting coil 11 is driven at a constant current of 400 A. Then, a current flows through the protective resistor 12, a potential difference of 400V is generated as an induced voltage at both ends of the superconducting coil 11, and then the potential difference is attenuated along with the attenuation of the current. Actually, since the electric resistance of the permanent current switch 20 does not increase so much in the open state, the induced voltage generated in the superconducting coil 11 becomes smaller than that, and the electric resistance of the permanent current switch 20 is infinite. Slower than when it grows up.

<Overall structure of permanent current switch>
As shown in FIGS. 7 to 9, the permanent current switch 20 of the first embodiment is close to a first switch portion 25 </ b> A in which a heater 28 (see FIG. 9B) is integrated and a magnet 29. A second switch unit 25B and a thermal resistor 26 are provided. As shown in FIGS. 9A and 9B, the first switch portion 25 </ b> A has a cylindrical frame 22 and is configured by winding a strip-shaped superconducting wire 21 around the frame 22. A portion of the superconducting wire 21 wound around the frame 22 is referred to as a first section 21a. As shown in FIGS. 8A and 8B, the second switch portion 25 </ b> B includes a cylindrical frame 23, and a band-shaped superconducting wire 21 is wound around the frame 23. A portion of the superconducting wire 21 wound around the frame 23 is referred to as a second section 21b. A cooling unit 14 </ b> A of the cooling device 14 is connected to the cylindrical frame 23.

The superconducting wire 21 is a REBCO wire having a width of 6 mm, and has a silver protective layer having a thickness of 5 μm on the upper surface of the superconducting layer. The total length of the superconducting wire 21 is 100 m, of which 40 m is allocated to the second switch unit 25B and the remaining 60 m is allocated to the first switch unit 25A. The basic set temperature at which the permanent current switch 20 is opened is 100K obtained by adding a margin to 90K, which is the critical temperature of the REBCO wire. The specific resistance of silver at 100K is about 3 μΩ · mm. Since the cross-sectional area of silver in this REBCO wire is 0.03 mm ² , the electrical resistance per unit length at 100 K is 0.1 Ω / m, and the electrical resistance of the 100 m superconducting wire 21 is 10 Ω. The opening operation of the permanent current switch 20 at the normal time other than the emergency is performed only by the first switch portion 25 </ b> A heated by the heater 28. Therefore, the electrical resistance in the open state of the permanent current switch 20 in a normal state is 6Ω.

  The superconducting wire 21 is non-inductively wound in a solenoid shape with a cylindrical frame 22 having an outer diameter of φ16 cm and a frame 23 connected thereto as a winding frame. The inductance of the permanent current switch 20 can be reduced by non-inductive winding. The length of the frame 22 (the length along the axial direction of the cylindrical shape) is 84 cm, the length of the frame 23 in the same direction is 56 cm, and the length of the combined winding frame (22, 23) is 140 cm. The frame 22 and the frame 23 are pure copper (C1020), and are connected via a thermal resistor 26 made of FRP. The thickness of the cylindrical side wall of the frame 22 is 0.5 mm, and the thickness of the cylindrical side wall of the frame 23 is 10 mm. The terminal side surface of the frame 22 (the side surface opposite to the side surface connected to the frame 23) is not connected and is in a vacuum, and the terminal side surface of the frame 23 is connected to the cooling unit 14A of the cooling device 14. Hereinafter, the direction along the central axis of the cylindrical shape of the frame 22 and the frame 23 is defined as “axial direction”, the direction perpendicular to the central axis of the cylindrical shape is defined as “radial direction”, and both The direction that forms a right angle is defined as the “circumferential direction”.

  As shown in FIG. 10, the thermal resistor 26 connecting the frame 22 and the frame 23 has the same shape as the axial end surface of the frame 23, that is, an outer diameter of φ16 cm, an inner diameter of φ14 cm, and a thickness of 1.0 mm. It is a constant ring shape. Copper connecting members 61 having the same shape as the thermal resistor 26 are provided at the ends of the frame 22 and the frame 23 facing each other. The connection member 61 is joined to the end portion of the frame 22 by welding or the like, and the connection member 61 and the end portion of the frame 23 are joined by a bolt or the like with the thermal resistor 26 interposed therebetween. Bolts with low thermal conductivity are used. Thereby, the frame 22 and the frame 23 are integrally connected in a state of being thermally separated.

  On the outer periphery of the frame 22 and the frame 23, the superconducting wire 21 is wound in a single-layer non-inductive manner like a solenoid through an insulating polyimide sheet 71 having a thickness of 50 μm. The superconducting wire 21 has a thickness of about 0.06 mm and is wound with the superconducting layer facing the outer periphery. That is, when the superconducting wire 21 is configured by sequentially laminating a metal substrate, an intermediate layer, a semiconductor layer, and a silver protective layer, the metal substrate is wound toward the inner peripheral side and the silver protective layer is directed toward the outer peripheral side. ing.

  In the solenoid-shaped non-inductive single layer winding, the superconducting wire 21 is wound from one end (first end) in the axial direction of the winding frame (22, 23) to the opposite end (second end). It is folded back at the end of 2 and wound back to the first end again. More specifically, the superconducting wire 21 is wound by reciprocating between both ends of the winding frames (22, 23), and the superconducting wire 21 is wound with a gap in the outward path, and fills this gap in the return path. The superconducting wire 21 is wound around. The winding direction of the superconducting wire 21 is opposite between the forward path and the return path. Although not particularly limited, the end portions e1 and e2 of the superconducting wire 21 are disposed at one end of the second switch portion 25B, and the folded portion e3 is disposed at the opposite end of the first switch portion 25A. The wound superconducting wire 21 is fixed by an epoxy resin impregnation process and a curing process together with insulating polyimide sheets 71 and 72, a copper strip 73 and a heater 28 described later.

<Structure of the first switch part>
The volume of the first switch portion 25A is 211 cc when calculated based on the above-described dimensions. If the specific gravity of copper is 8.96 g / cc, the weight is about 1.9 kg. Further, when the heater 28 is added to the first switch portion 25A, the entire weight is about 2.1 kg. As shown in FIG. 9B, the heater 28 is a metal strip having the same shape (strip shape) as that of the superconducting wire 21 and is made of an alloy having a small temperature dependency of electrical resistance, such as stainless steel or Hastelloy (registered trademark). ) Can be adopted. The metal strip constituting the heater 28 is wound around the outer peripheral side of the superconducting wire 21 with a polyimide tape for electrical insulation interposed therebetween. Heat generated from the heater 28 by energization warms the superconducting wire 21 and the frame 22, and is transmitted to the frame 23 of the second switch unit 25 </ b> B and then to the cooling unit 14 </ b> A of the cooling device 14 via the thermal resistor 26. . Since the cooled pure copper has a high thermal conductivity, a large temperature difference is not formed inside the pure copper frame. The cross-sectional area perpendicular to the heat transfer direction of the thermal resistor 26 (cross-sectional area perpendicular to the axial direction) is 4.7 × 10 ⁻³ m ² , and the thickness is assumed to be 0.1 W / Km. The thermal resistance of the 1.0 mm thermal resistor 26 is 2.1 K / W. When the heater 28 is completely turned off, the entire temperature of the permanent current switch 20 is 40K. When the heater 28 generates heat, a temperature difference is formed between both ends of the thermal resistor 26 according to the amount of generated heat. In the normal open state of the permanent current switch 20, the first switch portion 25A only needs to be maintained at 100K or 100K or higher, so that a temperature difference of 60K should be generated at both ends of the thermal resistor 26. When converted using the thermal resistance value of the thermal resistor 26, this state is achieved if the heater 28 generates heat at 30W.

  Note that the above-described heat transfer method is in a thermally steady state, and the temperature difference cannot be generated immediately after the heater 28 is heated. When calculating in consideration of the temperature dependence of the specific heat of copper, the amount of heat required to heat the first switch 25A from 40K to 100K is about 18 kJ based on the weight of the first switch 25A. Therefore, heating up to 100K takes 600 seconds in a state where the output of the heater 28 is 30 W and there is no outflow of heat, and 1 minute is required even if the output of the heater 28 is 10 times 300 W. Thus, it takes a long time to open the permanent current switch 20 by heating. Similarly, it takes time to close the permanent current switch 20 by cooling. If the thickness of the frame 22 is reduced to reduce the heat capacity, the time required for heating or cooling can be shortened, but it is not preferable that the frame 22 is too thin from the viewpoint of mechanical strength.

<Magnet structure>
As shown in the first embodiment, when any abnormality is detected in the superconducting coil 11, the opening operation of the second switch portion 25 </ b> B is performed by the action of the magnet 29. It is difficult to apply a uniform external magnetic field to all of the second sections 21b of the superconducting wire 21 wound around the cylindrical frame 23. When the magnet 29 is applied, the second section 21b of the superconducting wire 21 is locally applied to the second section 21b. An external magnetic field having a different size and direction is applied. And only in the area | region where the big external magnetic field was applied, a critical current falls and a voltage and the heat accompanying it generate | occur | produce. The superconducting wire 21 wound around the frame 23 is 40 m. When all the parts are heated to 100K, a resistance of 4Ω is generated. With this resistance, the current flowing through the permanent current switch 20 decreases from 400 A to 80 A, and the rest flows through the protective resistor 12. At this time, the voltage generated in the second section 21b of the superconducting wire 21 is 320 V, and the Joule heat is 25.6 kW. However, it is assumed that the magnitude of the external magnetic field is locally different, and only half of the superconducting wire 21 wound around the frame 23 is transferred to the normal conducting state, and Joule heat is generated therefrom. At this time, the resistance of the second section 21b of the superconducting wire 21 is 2Ω, the current flowing through the permanent current switch 20 is 133A, the voltage generated therebetween is 267V, and the heat generation amount is increased to 35.5 kW. Furthermore, since the heat generation range is half of the second section 21b of the superconducting wire 21, when compared with the amount of heat generated per wire length, there is a portion that generates heat nearly three times as much as when the whole transitions to the normal conducting state. End up. Thus, since it is not preferable that the calorific value per wire length becomes large, it is preferable that the magnet 29 applies a magnetic field to many portions of the superconducting wire 21 wound around the frame 23. For this reason, as the magnet 29, a wide racetrack type electromagnetic coil in the axial direction of the second switch portion 25 </ b> B is adopted, and four or six magnets 29 are arranged in the circumferential direction of the frame 23. The racetrack type refers to a shape in which a planar shape has two semicircular curved portions facing each other and two parallel straight portions connecting these curved portions. Each magnet 29 is arranged so that the axial direction along the winding center of the electromagnetic coil faces the radial direction of the frame 23 and the two adjacent magnets 29 have opposite polarities. Such an arrangement configuration of the magnet 29 is called a quadrupole type or a hexapole type, and a magnetic field (in the radial direction of the frame 23) facing a wide range inside the magnet 29 in the radial direction (the radial direction of the frame 23). 3A can be formed). The strip-shaped superconducting wire 21 has a magnetic field dependency of the critical current in the magnetic field in the thickness direction of the wire (referred to as a vertical magnetic field) compared to the magnetic field in the width direction or longitudinal direction of the wire (referred to as a horizontal magnetic field). It has the property of becoming larger. For this reason, the arrangement configuration of the magnet 29 described above allows the small magnet 29 to sufficiently reduce the critical current of the second section 21 b of the superconducting wire 21. For example, in a superconducting wire whose magnetic field characteristics are improved by introducing an artificial pin into the superconducting layer, when a 1 T vertical magnetic field is applied at 40 K, the critical current is reduced to about half compared to the state without a magnetic field. On the other hand, even with the same superconducting wire, when a horizontal magnetic field of 1 T is applied at 40 K, the rate of decrease in critical current compared to the state of no magnetic field remains at about 30%.

<Structure of second switch part>
Even if the arrangement configuration of the magnet 29 described above is employed, a sufficient magnetic field cannot be applied to all the portions of the superconducting wire 21 wound around the frame 23. Therefore, immediately after the urgent opening operation, the second section 21b of the superconducting wire 21 includes a portion that generates a large amount of heat and other portions. Since excessive heat is generated in a part that generates a large amount of heat, if the heat is moved to a part having a small amount of heat generation, the temperature of the entire second section 21b of the superconducting wire 21 can be increased. For this reason, in the 2nd switch part 25B, the copper strip 74 (refer FIG. 8 (B)) which bypasses heat and diffuses heat to the whole 2nd area 21b of the superconducting wire 21 is wound. The copper strip 74 corresponds to an example of a heat conducting wire according to the present invention. The copper strip 74 is wound around the outer peripheral side of the superconducting wire 21 via the insulating polyimide sheet 72. The copper strip 74 may be wound over the entire region where the second section 21b of the superconducting wire 21 is wound. In addition to the function of bypassing heat, the copper strip 74 has a function as a reinforcing material that suppresses peeling of the superconducting wire 21 from the frame 23 due to thermal stress.

  As described above, the superconducting wire 21 is wound with the superconducting layer facing the outer peripheral side. This is because heat generated by the superconducting layer and the silver protective layer is easily transferred to the copper strip 74. If the superconducting layer faces the inner peripheral side, most of the heat escapes to the inner peripheral side frame 23 instead of the outer peripheral side, and the unheated portion of the superconducting wire 21 cannot be efficiently heated.

  On the other hand, even if the heat bypass by the copper strip 74 is used, it is difficult to diffuse the heat throughout the second section 21b of the superconducting wire 21 in a short time. For example, a sufficient magnetic field cannot be applied to both ends of the frame 23 in the axial direction, and heat is transferred from these portions where Joule heat is generated to a considerable time after the formation of the magnetic field. It becomes from. Accordingly, during an emergency opening operation, a part of the superconducting wire 21 wound around the frame 23 does not continuously change from the superconducting state. Therefore, hereinafter, a case will be described in which only 30 m of the 40 m superconducting wire 21 of the second switch section 25B is transferred to the normal conducting state and generates Joule heat.

When 30 m of the superconducting wire 21 is transferred to the normal conducting state in the second switch portion 25B, the electrical resistance at 100K becomes 3Ω, and the current flowing through the permanent current switch 20 decreases from 400A to 100A. The area where the 30 m superconducting wire 21 is in contact with the frame 23 which is a heat bath is 6 mm × 30 m = 0.18 m ² , and the electrically insulating polyimide sheet 71 (thickness 0.05 mm) inserted between them is 6 mm × 30 m = 0.18 m ² . The thermal conductivity per unit volume is 0.1 W / Km regardless of the temperature. Under this condition, the overall thermal conductivity considering the shape of the polyimide sheet 71 is 0.36 kW / K. Considering the decrease in thermal conductivity due to the interface, the overall thermal conductivity of the polyimide sheet 71 is about 0.25 kW / K. At this time, the thermal resistance represented by the reciprocal of the thermal conductivity is 4 K / kW. The weight of the frame 23 is about 24.4 kg, the weight of the superconducting wire 21 and the copper strip 74 wound around the outer periphery of the frame 23 is slightly larger than 0.4 kg, and the total mass of the second switch portion 25B is about 24.8 kg.

<Behavior during emergency opening operation>
Next, the behavior when the permanent current switch 20 is urgently opened under the above-described conditions will be described. When the quadrupole magnet 29 is excited and a magnetic field is applied to the second switch portion 25B, the critical current is less than 400A in most of the second section 21b of the superconducting wire 21, and a voltage and accompanying heat are generated. This heat spreads over most of the second section 21b of the superconducting wire 21 and its surroundings, and an electrical resistance of 3Ω is generated in the second switch portion 25B after a very short time. As a result, the current flowing through the permanent current switch 20 is reduced to 100A, the voltage across the second switch portion 25B is 300V, and the Joule heat of the second switch portion 25B is 30 kW. The remaining current 300A flows through the protective resistor 12, and the amount of heat generated here is 90 kW. Since there is a thermal resistance of 4 K / kW between the frame 23 and the second section 21 b of the superconducting wire 21, the temperature of the second section 21 b of the superconducting wire 21 is 30 kW × 4 K / kW = 120 K than the frame 23. Get higher. Since the frame 23 connected to the cooling unit 14A is 40K, the temperature of the second section 21b of the superconducting wire 21 rises to about 160K. Since the critical temperature of the superconducting wire 21 is around 90K, at this time, the superconducting layer of the second switch portion 25B has the superconducting layer in the part where heat is generated completely transferred to the normal conducting state. At the site where the superconducting layer has transitioned to the normal conducting state, current flows through the silver protective layer. The electrical conductivity of silver has temperature dependence, and the electrical resistance of 0.1 Ω / m per unit length of the superconducting wire 21 used for the calculation of the electrical resistance of 3Ω is a value at 100K. Therefore, when the temperature rises to 160K, the electrical resistance of the second switch portion 25B is higher than 3Ω. In this case, the current flowing through the permanent current switch 20 is smaller than 100A, and the temperature rise in the second section 21b of the superconducting wire 21 is stabilized at a value lower than 120K. Here, it is assumed that the electric resistance is stable at 4Ω. At this time, the electric resistance of the parallel circuit of the permanent current switch 20 and the protective resistor 12 is about 0.8Ω, and the current of the superconducting coil 11 is a ratio of the time constant 12.5 seconds (= 10H ÷ 0.8Ω) (12.5 After a second, it attenuates at the original 1 / e≈37%). As a result, the superconducting coil 11 is quickly demagnetized and shifts to a safe state.

  On the other hand, as the current of the superconducting coil 11 attenuates, the amount of heat generated by the second switch portion 25B decreases. However, Joule heat generated over time is accumulated in the frame 23, and the temperature of the frame 23 is high. Assume that 160 kJ corresponding to 20% of the 800 kJ energy accumulated in the superconducting coil 11 before the opening operation is consumed by the heating of the second switch portion 25B. Then, since the total weight of the second switch portion 25B is 24.8 kg, the temperature of the frame 23 exceeds 90K, which is the critical temperature of the superconducting wire 21. Here, although the thermal energy required to heat 24.8 kg of copper from 40K to 90K is 140 kJ, it is considered that a part of the heat escapes to the cooling unit 14A. At this stage, the second section 21b of the superconducting wire 21 can maintain the normal conducting state for a while even without the amount of heat generated by the second switch portion 25B. Alternatively, it is assumed that the heat generation amount of the second switch portion 25B is smaller than that in the above case, and the temperature of the frame 23 has not reached the critical temperature of the superconducting wire 21. In this case, the amount of heat generated by the superconducting wire 21 decreases due to the attenuation of the current of the superconducting coil 11, and the second switch portion 25B eventually falls into a substantially closed state. However, even in this case, the current flowing through the superconducting coil 11 only maintains the value at that time, and does not turn up.

  During an emergency opening operation, the heating of the first switch unit 25 </ b> A by the heater 28 is also performed in parallel with the driving of the magnet 29. Although it takes time to heat the superconducting wire 21 of the first switch part 25A to the transition temperature or higher, if there is a period during which the second switch part 25B is kept open, the time to heat it to the critical temperature or higher is required. can get. Or after the 2nd switch part 25B falls into a substantially closed state, the superconducting wire 21 of the 1st switch part 25A may become more than critical temperature. When the first switch portion 25A is switched to open by the heating of the heater 28, Joule heat is also generated in the first switch portion 25A. At this time, the current in the superconducting coil 11 has already been greatly attenuated, and the first switch portion 25A. An excessive temperature rise that damages the device does not occur. After that, even if the second switch portion 25B changes to a substantially closed state, the attenuation of the current of the superconducting coil 11 is continued because the first switch portion 25A is open, and demagnetization is completed. Can do.

  Next, a second embodiment in which the permanent current switch 20 is more specific will be described. FIG. 11 is a perspective view showing a permanent current switch according to Embodiment 2 of the present invention. FIG. 12 is a diagram for explaining a laminated structure of the permanent current switch of the second embodiment.

  The permanent current switch 20 of Example 2 is coaxially arranged so that the first switch portion 25A, the second switch portion 25B, the thermal resistor 26, the heater 28, and the magnet 29 are each in a cylindrical or solenoidal layer. It is constructed by stacking. FIG. 12 shows the layers of the permanent current switch 20 shifted.

  In the permanent current switch 20 of the second embodiment, the first coil constituting the frame 23, the electrical insulating sheet 81, the second section 21b of the superconducting wire 21, the thermal resistance layer 82, and the magnet 29 in this order from the center side to the outer peripheral side. 29a, the thermal resistor 26, the first section 21a of the superconducting wire 21, the heater 28, the thermal resistance layer 83, the second coil 29b constituting the magnet 29, and the thermal conduction layer 84 are provided. The thermal resistor 26 also functions as the frame 22 of the first switch portion 25A. The first section 21a and the second section 21b of the superconducting wire 21 are continuous, and the end portions e4 and e5 of the superconducting wire 21 are arranged at one end in the axial direction of the permanent current switch 20 and connected to the circuit. The part e6 is folded and held at the other axial end of the permanent current switch 20. The superconducting wire 21 has the same structure as that of Example 1, and is a REBCO wire having a width of 6 mm.

  The frame 23 arranged closest to the inner periphery also functions as one heat bath of the permanent current switch 20. The frame 23 is cylindrical pure copper (C1020) having an outer diameter of 16 cm, an inner diameter of 15.2 cm, a thickness of 0.4 cm, and a length of 70 cm. However, the frame 23 is not completely cylindrical, and a slit SL1 extending from one end to the other end in the axial direction is provided in a part of the circumferential direction. That is, the frame 23 has a C-shaped cross section perpendicular to the axial direction. The slit SL1 prevents an eddy current from circulating around the cavity when the magnetic field penetrates the cavity at the center of the frame 23 in the axial direction. The weight of the frame 23 is 12.3 kg. The frame 23 is connected to the cooling unit 14A of the cooling device 14 and is conductively cooled.

  The second section 21 b of the superconducting wire 21 is wound around the frame 23 by a single layer on the inner peripheral side via an electrical insulating sheet 81. The dimensions of the superconducting wire 21 are the same as those in the first embodiment. The second section 21b of the superconducting wire 21 has a winding diameter of 160 mm, which is the same as the outer diameter of the frame 23, and the length of one winding is 0.50 m. The second section 21b of the superconducting wire 21 is wound with the superconducting layer facing the inner peripheral side.

  The first coil 29a is a multi-layer solenoid coil (so-called layer-wound coil) in which a copper strip having an insulating layer is wound into 10 layers. The total thickness of the first coil 29a is 1.8 mm, and the average thickness per layer is 0.18 mm.

  Each of the thermal resistance layer 82 and the thermal resistance body 26 has a thickness of 1 mm, and has a structure in which a polyimide sheet having the same thickness of 50 μm as the electrical insulating sheet 81 is wound in multiple layers. The permanent current switch 20 according to the second embodiment is impregnated with paraffin during assembly, and the interface between the polyimide sheets wound in multiple layers is filled with paraffin. Unlike resins such as epoxy, paraffin cracks at the interface when cooled to a very low temperature. As a result, the thermal conductivity of the interface is lowered, and a high thermal resistance is realized without increasing the thickness. Further, the thermal resistance increases in proportion to the number of interfaces existing in the heat transfer path. In a structure in which sheets having a thickness of 50 μm are stacked to have a thickness of 1 mm, there are about 20 interfaces, so the thermal conductivity in the stacking direction is a small value of about 0.001 W / Km. In this case, the heat resistance of the heat transfer path in which heat is transferred to the next layer from the superconducting wire 21 having a width of 6 mm and a length of 1 m with the heat resistance layer 82 or the heat resistor 26 interposed therebetween is 167 K / W. Hereinafter, the thermal resistance of such a heat transfer path is also referred to as a thermal resistance for a wire length of 1 m of the superconducting wire 21. The total thickness of the second section 21b of the conductive wire 21, the thermal resistance layer 82, the first coil 29a, and the thermal resistor 26 described so far is approximately 4 mm.

  The first section 21a of the superconducting wire 21 and the heater 28 are laminated with an insulating tape (not shown in FIG. 12) interposed therebetween. As described above, the first section 21a and the second section 21b of the superconducting wire 21 are continuous and are folded at one end of the winding frame and wound around another layer. In each layer, the superconducting wire 21 is not inductively wound, but the winding direction of the first section 21a of the superconducting wire 21 and the winding direction of the second section 21b are reversed. With this configuration, the magnetic field generated by the current in the first section 21a of the superconducting wire 21 and the magnetic field generated by the current in the second section 21b of the superconducting wire 21 cancel each other, and substantially non-inductive winding of the superconducting wire 21 is achieved. Is realized. That is, the combined inductance of the first section 21a and the second section 21b of the superconducting wire 21 is significantly reduced. Ordinary non-inductive winding causes both work complexity and design complexity, but according to the substantially non-inductive winding of the second embodiment, such complexity can be reduced.

  The first section 21a of the superconducting wire 21 has a winding diameter larger than the diameter of the frame 23 and becomes φ168 mm. When the length of one winding is 0.53 m and the winding is shifted 98 times in the axial direction (70 cm), the total length is 52 m. It becomes. The electric resistance of the superconducting wire 21 is 0.1Ω / m at 100K, and an electric resistance of 5.2Ω is generated at 52m. In contrast to the second section 21b, the first section 21a of the superconducting wire 21 is wound with the superconducting layer facing the outer peripheral side. The superconducting wire 21 is turned upside down at an intermediate part e6 that is folded back.

  The heat resistance layer 83 near the outer periphery has the same configuration as the heat resistance layer 82 near the inner periphery, but has a thickness as thin as 0.6 mm. When calculated in the same manner as the heat resistance layer 82 closer to the inner periphery, the heat resistance of the superconducting wire 21 of the heat resistance layer 83 corresponding to the wire length of 1 m is 100 K / W.

  The second coil 29b has the same configuration as the first coil 29a except that the winding diameter is large and the number of stacked layers is small. The first coil 29a and the second coil 29b are wound with copper strips in the same direction, and the ends of the copper strips are electrically coupled at one end in the axial direction of the permanent current switch 20. That is, the first coil 29a and the second coil 29b are connected in series, and an equal amount of current always flows. Between the first coil 29a and the second coil 29b, the magnetic field of the first coil 29a and the magnetic field of the second coil 29b cancel each other, preventing the magnetic field from being applied to the first section 21a of the superconducting wire 21. To do. In a region other than the interlayer between the first coil 29a and the second coil 29b, the magnetic field of the first coil 29a and the magnetic field of the second coil 29b reinforce each other.

  The heat conductive layer 84 is formed by bending a copper plate having a thickness of 1 mm into a cylindrical shape and attaching it to the outer peripheral side of the second coil 29b. However, both sides of the copper plate in the circumferential direction of the permanent current switch 20 are not electrically joined, and are tightened by electrically insulated bolts. Thereby, the heat conductive layer 84 has a configuration in which a slit SL2 extending in the axial direction is formed in a part of the circumferential direction. The slit SL2 prevents a circumferential eddy current from being generated in the cylindrical heat conductive layer 84. The weight of the heat conductive layer 84 is 3.8 kg. The heat conductive layer 84 is connected to the cooling unit 14A of the cooling device 14 separately from the frame 23 and is conductively cooled.

  The permanent current switch 20 according to the second embodiment is manufactured by sequentially assembling layers from the radially inner periphery side to the outer periphery side. In the middle of the manufacturing process of the permanent current switch 20 and the stage in which the heat resistance layer 83 is assembled, the structure of this stage is impregnated with paraffin. The purpose of this impregnation treatment is not to increase the mechanical strength but to stabilize the thermal bonding. Subsequently, at the stage where the second coil 29b and the heat conductive layer 84 are assembled, the structure at this stage is fixed by the impregnation process and the curing process of the epoxy resin. The purpose of this impregnation treatment is to make the thermal resistance of the second coil 29b as small as possible. Epoxy resins, unlike paraffin, do not crack at the interface even when cooled to a very low temperature, and the thermal resistance at the interface does not increase. In the state impregnated with the epoxy resin, the thermal resistance of the second coil 29b is dominated by the thermal insulation layer formed around the copper strip. Assuming that the number of laminated copper strips of the second coil 29b is, for example, five laminates corresponding to half of the number of laminated copper strips of the first coil 29a, the total thickness of the electrical insulating layer of the second coil 29b is 0. 4 mm. According to this configuration, the heat resistance of the second coil 29b is less than 1 K / W in the heat transfer path perpendicular to the tape surface of the superconducting wire 21 having a length of 1 m. Since the epoxy resin does not attack the paraffin, the epoxy resin does not impregnate inside the heat resistance layer 83 previously impregnated with paraffin.

<Opening / closing operation of first switch 25A>
The opening / closing operation when a large current does not flow through the permanent current switch 20 is performed by temperature control of the first switch portion 25A. As described above, the frame 23 and the heat conductive layer 84 are individually connected to the cooling unit 14A and are normally cooled to 40K. In order to switch the first switch portion 25A to open, the temperature of the first section 21a of the superconducting wire 21 may be maintained at 100K or higher, and the required temperature difference is 60K. From the layer of the first section 21a of the superconducting wire 21 to the heat conducting layer 84 even if the heat resistance of the second coil 29b is set to 1 K / W in the heat transfer path orthogonal to the tape area of the wire length of 1 m of the superconducting wire 21. The thermal resistance is about 101 K / W. When converted by the total length 52 m of the first section 21 a of the superconducting wire 21, the thermal resistance is 2.0 K / W. On the other hand, the thermal resistance in the heat transfer path from the layer of the first section 21a of the superconducting wire 21 to the innermost frame 23 is very large because the thermal resistance layer 82 and the layer of the thermal resistor 26 are interposed. It becomes large and becomes about 404 K / W for the 1 m line length of the superconducting wire 21. In this case, the amount of heat generated by the heater 28 required to create a temperature difference of 60K is 38W. Of this, about 80% of 30 W flows to the heat conduction layer 84 on the outer peripheral side, and the remaining 20% of about 8 W flows to the frame 23 on the inner peripheral side. Here, the temperature of the superconducting wire 21 in the closed state is assumed to be 60K. This temperature is a temperature at which the critical current of the superconducting wire 21 sufficiently exceeds the normal energization current (400 A), and a higher one can speed up the opening operation at the normal time. Under the above conditions, the amount of heat required to raise the temperature of the first section 21a of the superconducting wire 21 from 60K to 100K is about 1.6 kJ. Therefore, if the output of the heater 28 is set to 300 W, the first time is about 5 seconds. 1 switch part 25A can be opened. Further, the heat removal rate from the layer of the first section 21a of the superconducting wire 21 to the frame 23 and the heat conduction layer 84 is 38 W at the maximum, and when the temperature difference becomes small, the heat removal rate decreases accordingly. For this reason, by stopping the output of the heater 28 from the open state, the first switch portion 25A can be closed in about 100 seconds, that is, in about 2 minutes.

<Emergency opening operation>
Next, an operation of urgently opening the permanent current switch 20 of the second embodiment and demagnetizing the superconducting coil 11 while the superconducting coil 11 is excited will be described. The emergency opening operation is started by energizing the first coil 29a and the second coil 29b. By energization, the magnetic field of the first coil 29a and the magnetic field of the second coil 29b overlap and strengthen each other and are applied to the layer of the second section 21b of the superconducting wire 21. On the other hand, these magnetic fields cancel each other in the layer of the first section 21a. Therefore, the critical current differs between the layer of the second section 21b and the layer of the first section 21a of the superconducting wire 21. However, the first coil 29a and the second coil 29b are made of copper strips, and a large current cannot flow as in the superconducting coil. Therefore, a strong magnetic field on the order of several T cannot be formed. Furthermore, the magnetic field formed by the first coil 29a and the second coil 29b faces the width direction of the superconducting wire 21, and the magnetic field dependence of the critical current of the superconducting wire 21 is not large with respect to the magnetic field in this direction. For this reason, the critical current of the superconducting wire 21 cannot be greatly reduced only by applying this magnetic field. That is, it does not lead to a large difference in critical current between the layer of the second section 21b and the layer of the first section 21a of the superconducting wire 21.

  Thus, in the second embodiment, the controller 15 activates the current source 13 along with the formation of the magnetic field during the emergency opening operation. While the superconducting coil 11 is excited by a permanent current, the current source 13 is stopped. When a current is output from the current source 13 in this state, the superconducting coil 11 has a large inductance, so that the flowing current does not change much, and the protective resistor 12 has an electric resistance, so that a large current does not flow. The output current mainly flows through the permanent current switch 20. And although it is not strong in the 2nd section 21b of the superconducting wire 21, a magnetic field is applied, and since the magnetic field is hardly applied to the 1st section 21a, the 2nd section 21b is more than the 1st section 21a. The generation of voltage and heat is started at a low current value. Thereby, the 2nd section 21b of the superconducting wire 21 transfers to a normal conducting state, and the 2nd switch part 25B is opened. Here, it is assumed that voltage and heat are generated in the second section 21b of the superconducting wire 21 when the current value increases from 400A to 480A. This heat is generated in the superconducting layer where most of the current flows and heats the superconducting layer first, so immediately after the heat is generated, the superconducting layer exceeds the critical temperature and transitions to the normal conducting state. The generation of larger Joule heat is maintained. Therefore, the control unit 15 may return the output of the current source 13 to zero after the voltage across the permanent current switch 20 becomes a voltage indicating the generation of Joule heat. The voltage at both ends can be detected by providing a voltage detection unit between the wirings connected to both ends of the permanent current switch 20 and inputting the detection result of the voltage detection unit to the control unit 15.

  In Example 2, a relatively uniform magnetic field is applied to each part of the second section 21 b of the superconducting wire 21. For this reason, the voltage and heat generation in the second section 21b described above occur almost simultaneously in the entire wire. Therefore, it is not necessary to provide a configuration such as a copper strip for bypassing heat as described in the first embodiment.

  The total length of the second section 21b of the superconducting wire 21 is 49 m, and the electrical resistance at 100K is equivalent to 4.9Ω. However, when the second switch portion 25B is switched to the open state, only the superconducting wire 21 of about 40 m transitions to the normal conducting state in consideration of the weak magnetic field formation at both axial ends of the permanent current switch 20. Suppose that Then, the electrical resistance of the second switch part 25B becomes about 4.0Ω. In this case, when the output of the current source 13 returns to zero, the current of the permanent current switch 20 decreases to 80 A, and the remaining 320 A is shunted to the protection resistor 12. At this time, the heat generation amount of the second section 21b of the superconducting wire 21 is 25.6 kW, and almost all of the heat generation amount flows to the frame 23 on the inner peripheral side. The thermal conductivity of the electrical insulating sheet 81 having a length of 40 m is 0.48 kW / K, and the thermal resistance is slightly larger than that obtained by adding the effect of the interface to 2.1 K / kW which is the reciprocal thereof, and about 2.5 K / kW It becomes. If heat of 25.6 kW is generated in the second section 21b of the superconducting wire 21, a temperature difference of 64K occurs between them. Since the frame 23 is initially 40K, the temperature of the second section 21b of the superconducting wire 21 rises to about 104K, and the second section 21b of the superconducting wire 21 can be reliably transferred to the normal conducting state.

  On the other hand, a part of the heat generated when the second switch portion 25B is opened flows toward the heat conducting layer 84, and the thermal resistance between them is 500 K / min for the 1 m line length of the superconducting wire 21. It is W or more, and the wire length of the wire 50m is 10 K / W or more. Even if a temperature difference of 64K occurs on both sides of this thermal resistance, the amount of heat that flows is only about 6 W, and the temperature rise in the first section 21a of the superconducting wire 21 due to this amount of heat is about 13K. Even if the temperature rises by 13K, if the critical current of the first section 21a of the superconducting wire 21 exceeds the energization current 80A, no voltage and accompanying heat will be generated in the first section 21a. Then, the completely closed state is maintained.

When the second switch portion 25B is opened and an electric resistance of 4Ω is generated, the time constant of current attenuation in the entire circuit including the resistance (1Ω) of the protective resistor 12 and the inductance (10H) of the superconducting coil 11 is 12 Seconds. The energy consumed by the resistance in 12 seconds is about 40% of the amount of the consumption speed at the beginning of the 12 seconds continued for 12 seconds (strictly, (1-e ⁻² ) / 2 = 0. 43). Since the energy consumption immediately after the second switch unit 25B is opened is 25.6 kW, the amount of heat generated for the subsequent 12 seconds is about 130 kJ. This amount of heat is sufficient to heat the frame 23 having a weight of 12.3 kg from 40K to 90K or more, even considering the heat removal to the cooling unit 14A. Accordingly, at the time point after 12 seconds, the second switch portion 25B is stably open in the second section 21b of the superconducting wire 21 even when no Joule heat is generated. In order for the second switch portion 25B to be closed again, it takes time until the entire frame 23 is cooled, during which the energization current of the superconducting coil 11 is sufficiently attenuated.

  On the other hand, in the first section 21a of the superconducting wire 21, the temperature rises to a critical temperature or higher due to the operation of the heater 28 while the second switch portion 25B is kept open. As a result, the first switch portion 25A is opened. At this time, since the current of the superconducting coil 11 is sufficiently attenuated, no large Joule heat is generated in the first section 21a of the superconducting wire 21, and there is no possibility of damaging the superconducting wire 21. Thereafter, even if the second switch part 25B is switched to the closed state, the superconducting coil 11 can be completely demagnetized by the open state of the first switch part 25A.

DESCRIPTION OF SYMBOLS 1 Magnetic field generator 11 Superconducting coil 12 Protection resistance 13 Current source 14 Cooling device 15 Control part 20, 20A, 20B Permanent current switch 21 Superconducting wire 21a 1st area 21a1 1st small area 21a2 2nd small area 21b 2nd area 22 Frame 23 Frame 25A 1st switch part 25B 2nd switch part 26 Thermal resistor 28, 28a, 28b Heater 29 Magnet 31 Semiconductor switch 74 Copper strip (heat conduction wire)

Claims (10)

A first switch portion including a first section of superconducting wire;
A second switch including a second section of the superconducting wire and a heat conductor having a larger heat capacity than the first switch unit, wherein the second section of the superconducting wire and the heat conductor are thermally coupled. And
A thermal resistor interposed between the first switch unit and the second switch unit to thermally separate the first switch unit and the second switch unit;
A heater capable of heating the first section of the superconducting wire;
A magnet capable of applying a magnetic field to the second section of the superconducting wire;
A permanent current switch comprising:   The permanent current switch according to claim 1, wherein the superconducting wire is a rare earth-based superconducting wire or a bismuth-based superconducting wire. The second switch unit is
It further includes a heat conducting wire that is electrically insulated from the second section of the superconducting wire, and the second section of the superconducting wire and the heat conducting wire are wound together on a reel. The permanent current switch according to claim 1 or 2. The first section and the second section of the superconducting wire are wound in opposite winding directions;
The combined inductance of the first section and the second section of the superconducting wire is smaller than the sum of the self-inductance of the first section of the superconducting wire and the self-inductance of the second section of the superconducting wire. The permanent current switch according to any one of claims 1 to 3.   5. The permanent current switch according to claim 1, further comprising a semiconductor switch electrically connected in parallel with at least a part of the superconducting wire. 6. The first switch unit includes a first part and a second part having different heat capacities,
6. The heater according to claim 1, wherein the heater includes a first heater capable of heating the first portion and a second heater capable of heating the second portion. Of permanent current switch. Superconducting coils connected in parallel to each other, the permanent current switch according to any one of claims 1 to 6, a protective resistor and a current source;
A cooling device for cooling the superconducting coil and the permanent current switch;
A control unit for controlling the permanent current switch;
With
The control unit operates the magnet and the heater based on a demagnetization request of the superconducting coil, switches the second switch unit to open by the action of the magnetic field of the magnet, and then heats the first. A magnetic field generator characterized in that the switch part is switched to open.   The magnetic field generator according to claim 7, wherein the control unit causes the current source to output a current when the second switch unit is opened by the action of the magnetic field of the magnet. A superconducting coil connected in parallel to each other, the permanent current switch according to any one of claims 1 to 6, a protective resistor and a current source, and a cooling device for cooling the superconducting coil and the permanent current switch. A method of controlling a magnetic field generator comprising:
The magnet and the heater are operated, the second switch part is switched to open by the action of the magnetic field of the magnet, and then the first switch part is switched to open by heating the heater to demagnetize the superconducting coil. A method for controlling a magnetic field generator.   10. The method of controlling a magnetic field generator according to claim 9, wherein when the second switch unit is switched to open by the action of the magnetic field of the magnet, the current source is caused to output a current.

Images