JP6877165B2 - Permanent current switch, magnetic field generator and control method of magnetic field generator - Google Patents

Description

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

In a magnetic field generator using a superconducting coil, all closed circuits including the superconducting coil are 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 is not required during the generation of the magnetic field, and a constant magnetic field having extremely high stability in time 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 stability over time.

Even in a superconducting coil driven by a permanent current, a current source is used during excitation (process of increasing the energizing current to a predetermined current value) and degaussing (process of reducing the energizing current). At the time of excitation, electric power is supplied from the current source to the superconducting coil with a part of the superconducting circuit open. Then, when the current of the superconducting coil increases to a predetermined value, the superconducting circuit is closed and the current source is stopped. As a result, the current flowing through the superconducting coil becomes a permanent current and flows through the superconducting circuit. When degaussing, the procedure is the reverse of that for excitation. The permanent current switch (permanent current switch 20: Persistent Current Switch) is responsible for opening and closing a part of the superconducting circuit in this way.

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

At present, the superconducting coil and permanent current switch of the magnetic field generator are usually composed of low-temperature superconductor (LTS) wire rods. 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 rod is that it is difficult to join the superconductors to each other, that is, the so-called superconducting joint, and it is difficult to put all the circuits in the 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 raise and lower the temperature to open and close the permanent current switch. Regarding the first reason, many companies or research institutes are currently developing superconducting bonding technology for HTS wire rods, and it is expected that a simple superconducting bonding technology will be established in the near future. Alternatively, even if a part of the circuit includes a wire connection part that does not shift to the superconducting state, a pseudo permanent current with slight current attenuation is passed through the superconducting coil to maintain a stable constant magnetic field for a long period of time. Can occur. In consideration of such a situation, it is considered important to solve the problem of the second reason, that is, the problem of enabling the permanent current switch using the HTS wire to be opened and closed at high speed.

By the way, the MRI apparatus used for the inspection of the human body is required to have a function of degaussing in a short time in an emergency. In the current MRI apparatus, a superconducting coil of LTS wire is adopted in almost all of them, and immersion cooling of liquid helium is adopted. In the LTS coil, emergency degaussing can be performed by heating the superconducting coil to cause an artificial quench. However, this emergency degaussing method has a drawback that a large amount of liquid helium is gasified and lost, and there is a risk of accumulation of damage to the superconducting coil due to a rapid temperature rise. On the other hand, if the method of artificially generating a quench in the superconducting coil is applied to a magnetic field generator that employs an HTS coil, the superconducting coil will almost certainly burn out. Therefore, for the purpose of urgently degaussing the superconducting coil, a 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 to be conducted and cooled as a conventional technique related to the present invention. In this technique, a thermal resistance is interposed between the winding portion of the permanent current switch and the heat bath that cools 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 of the heater.

Patent Document 2 proposes, as a prior art related to the present invention, a technique for opening a permanent current switch using an HTS wire rod faster. 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 urgently degaussed.

Japanese Unexamined Patent Publication No. 10-189324 Japanese Unexamined Patent Publication No. 2010-73856

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 in the opening and closing operation is also large. On the other hand, Patent Document 1 discloses a technique of using thermal resistance to heat the winding portion of a permanent current switch to a high temperature 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 arises a problem that the permanent current switch is damaged by its own heat generation. This is due to the following reasons. 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 excited, 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 material constituting the permanent current switch is provided with a protective layer having electrical conductivity along the superconducting layer, and when the superconducting layer is transferred to the normal conductive state, the current bypasses the protective layer and the superconducting layer. Damage is avoided. Therefore, even if the permanent current switch is opened, the resistance of the permanent current switch does not increase so much. Therefore, the current of the superconducting coil flows not only in the protection resistor 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, if a thermal resistance is provided between the superconducting wire of the permanent current switch and the hot bath, the rapid discharge of Joule heat is hindered, and the temperature of the permanent current switch is increased. Raise it to the level of damage. Improving the heating efficiency of the permanent current switch and improving the exhaust heat efficiency are contradictory issues.

In the technique shown in Patent Document 2, as described above, by acting a heater and a magnetic field on the permanent current switch, it is expected that the permanent current switch can be opened in a short time. However, when a magnetic field is applied to the superconducting wire, the critical current of the superconducting wire drops remarkably, but the critical temperature does not drop so much. In other words, the permanent current switch to which a magnetic field is applied generates a voltage when a current higher than the critical current is flowing, but when the current drops, it returns to a substantially closed state and completely degausses the superconducting coil. Can't. In order to completely demagnetize, it is necessary to quickly heat the superconducting wire above the critical temperature to reduce the critical current of the superconducting wire to zero, but when the permanent current switch is opened by the action of a magnetic field, the permanent current Large Joule heat is generated in the switch. Therefore, the permanent current switch of Patent Document 2 also has the contradictory problems of improving the exhaust heat efficiency and the heating efficiency, as in the permanent current switch that does not act on the magnetic field.

An object of the present invention is to provide a permanent current switch capable of high-speed and stable opening operation when degaussing a superconducting coil, a magnetic field generator provided with such a permanent current switch, and a control method thereof.

The invention according to claim 1
The first switch section including the first section of the superconducting wire, and
A second switch that includes a second section of the superconducting wire and a heat conductor having a heat capacity larger than that of the first switch portion, and the second section of the superconducting wire and the heat conductor are thermally coupled to each other. Department and
A thermal resistor that is interposed between the first switch portion and the second switch portion and thermally separates the first switch portion and the second switch portion.
With 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, and
It is a permanent current switch characterized by being provided with.

The invention according to claim 2 is the permanent current switch according to claim 1 or 2.
The superconducting wire is characterized by being a rare earth superconducting wire or a bismuth superconducting wire.

The invention according to claim 3 is the permanent current switch according to claim 1 or 2.
The second switch unit is
It further includes a heat conductive wire that is electrically insulated from the second section of the superconducting wire, and is characterized in that the second section of the superconducting wire and the heat conductive wire are both wound around a winding frame. There is.

The invention according to claim 4 is the permanent current switch according to any one of claims 1 to 3.
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.

The invention according to claim 5 is the permanent current switch according to any one of claims 1 to 4.
It is characterized by further including a semiconductor switch electrically connected in parallel with at least a part of the section of the superconducting wire.

The invention according to claim 6 is the permanent current switch according to any one of claims 1 to 5.
The first switch portion has a first portion and a second portion having different heat capacities from each other.
The heater is characterized by including a first heater capable of heating the first portion and a second heater capable of heating the second portion.

The invention according to claim 7
A superconducting coil connected in parallel with each other, a permanent current switch according to any one of claims 1 to 6, a protective resistor and a current source.
A cooling device that cools the superconducting coil and the permanent current switch,
A control unit that controls the permanent current switch,
With
The control unit operates the magnet and the heater based on the degaussing 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 heater to perform the first. It is a magnetic field generator characterized by switching the switch unit to open.

The invention according to claim 8 is the magnetic field generator according to claim 7.
The control unit is characterized in that when the second switch unit is switched to open by the action of the magnetic field of the magnet, a current is output to the current source.

The invention according to claim 9
A superconducting coil connected in parallel with each other, a permanent current switch according to any one of claims 1 to 6, a protection resistor and a current source, and a cooling device for cooling the superconducting coil and the permanent current switch. It is a control method of a magnetic field generator equipped with
After operating the magnet and the heater and switching the second switch portion to open by the action of the magnetic field of the magnet, the first switch portion is switched to open by heating the heater to degauss the superconducting coil. It is characterized by that.

The invention according to claim 10 is the control method for the magnetic field generator according to claim 9.
When the second switch portion is switched to open by the action of the magnetic field of the magnet, the current source is characterized by outputting a current.

According to the present invention, when degaussing the superconducting coil, the critical current in the second section of the superconducting wire is lowered by the action of the magnetic field of the magnet, whereby the second switch portion can be quickly switched to open. At this time, the Joule heat generated in the second section of the superconducting wire is discharged to the heat conductor having a large heat capacity, so that it is possible to prevent the second section of the superconducting wire from becoming extremely hot. On the other hand, when the current falls below the critical current, the second switch section is switched to closed again. However, in the present invention, the first switch portion has a smaller heat capacity than the second switch portion, and the first switch portion and the second switch portion are thermally separated by a thermal resistor. Therefore, while the second switch is opened and the degaussing 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 operated. You can switch to open. At this time, since the second switch portion is opened before that to reduce the current of the superconducting coil, the current flowing through the first switch portion is small. Therefore, the heat generated by 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 degaussed.

It is a block diagram which shows the magnetic field generator which concerns on embodiment of this 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 the opening operation of the permanent current switch which concerns on 1st Embodiment in an emergency. 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 the closing operation of the permanent current switch which concerns on 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 the second switch portion is shown, FIG. 7A is a sectional view taken along line AA of FIG. 7, and FIG. 7B is an enlarged sectional view thereof. The cross section of the first switch portion is shown, FIG. 7A is a sectional view taken along line BB of FIG. 7, and FIG. 7B is an enlarged sectional view thereof. It is an exploded perspective view which shows the joint part of the 1st switch part and the 2nd switch part. It is a perspective view which shows the permanent current switch which concerns on Example 2 of this invention. It is a figure explaining the laminated structure of the permanent current switch which concerns on Example 2. FIG.

Hereinafter, each embodiment 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 protection resistor 12, a current source 13, a cooling device 14, and a control unit 15.

The superconducting coil 11 is subjected to a permanent current or a pseudo permanent current to generate a constant magnetic field. As the material of the superconducting coil 11, HTS wires such as REBCO wire (rare earth superconducting wire) or BSCCO wire (bismuth superconducting wire) are used. Here, the permanent current means an unattenuated current that flows through a closed circuit in a superconducting state all around. The pseudo-permanent current means a current having a very small amount of attenuation flowing through a closed circuit in a superconducting state in most of the entire circumference except for a part such as a connection portion 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 in which an analysis target or a subject is arranged.

The permanent current switch 20 has a superconducting wire material extending from one end to the other end of the current path, and a means for switching the superconducting wire material between a superconducting state and a normal conducting state. The permanent current switch 20 is closed when the superconducting wire is transferred to the superconducting state and the electric resistance becomes zero, and at least a part of the superconducting wire (excluding the non-superconducting part such as the wire connection) is in the normal conducting state. The state in which 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, HTS wire such as REBCO wire or 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 a closed circuit including a superconducting coil 11 and a permanent current switch 20 to a temperature at which the HTS wire is transferred to a 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 the circuit including itself by adjusting the voltage output.

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

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

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

The permanent current switch 20 includes a first switch unit 25A, a second switch unit 25B, a thermal resistor body 26, a heater 28, and a magnet 29. The first switch portion 25A has a first section 21a of the superconducting wire member 21 and a frame 22 holding the first section 21a of the superconducting wire member 21. The second switch portion 25B has a second section 21b of the superconducting wire 21 and a frame 23. The frame 23 corresponds to an example of the thermal conductor according to the present invention. The superconducting wire 21 is continuously formed 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. The superconducting wire 21 and the frame 22 are arranged apart from the cooling unit 14A of the cooling device 14.

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

The frame 23 has a heat capacity larger than that of the first switch portion 25A, and is thermally coupled to the second section 21b of the superconducting wire 21. Here, although the details will be described later, the “thermally coupled” is not limited to the configuration in which they are in direct contact with each other, and is connected via a member having a low thermal resistance to the extent that the action of the frame 23 described later is realized. Configuration is included. The frame 23 is connected to the cooling unit 14A of the cooling device 14.

The thermal resistance body 26 thermally separates the first switch portion 25A and the second switch portion 25B, and physically connects the first switch portion 25A and the second switch portion 25B. By this connection, the connection between the first section 21a and the second section 21b of the superconducting wire 21 can be maintained. Here, the term "thermally separated" is described in detail later, but is not limited to a configuration in which heat transfer is completely blocked, and the thermal resistance is high enough to realize the action of the thermal resistor 26 described later. Includes the configuration connected to the state.

As the material of the thermal resistor 26, FRP (Fiber-Reinforced Plastics) can be applied. As a result, high mechanical strength and stability in an extremely low temperature environment can be obtained. Further, as the material of the thermal resistor body 26, an alloy such as stainless steel or brass can be applied when there is a restriction that the thermal resistor 26 is made of metal in order to reduce the difference in thermal expansion with the connecting member. In such an alloy, the temperature dependence of 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 the output can be adjusted by the control of the control unit 15, and heats the first section 21a of the superconducting wire 21 at the time of operation.

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

<Open operation in an emergency>
Subsequently, an operation of opening the permanent current switch 20 to urgently degauss the superconducting coil 11 will be described. Urgent degaussing is required, for example, when some abnormality is detected in the superconducting coil 11.

FIG. 3 is a time chart illustrating an emergency opening operation of the permanent current switch according to the first embodiment. 3 (A) to 3 (E) 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, respectively. The opening / closing of the first switch section 25A and the opening / closing of the second switch section 25B are shown. FIG. 3F shows the current of the superconducting coil 11. “Open” and “closed” of the first switch unit 25A refer to a state in which at least a 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 the entire current path is in a superconducting state. Each is shown. The "open" and "closed" of the second switch unit 25B refer to 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 the entire current path is in a superconducting state. Each is shown. Hereinafter, "open" and "closed" are used interchangeably with respect to a permanent current switch made of a superconducting wire or a portion thereof.

Before the opening operation of the permanent current switch 20 (before the timing t1), the superconducting coil 11 and the superconducting wire 21 of the permanent current switch 20 are cooled to a critical temperature or lower and kept in a superconducting state. Further, the superconducting coil 11 is excited by a permanent current or a pseudo permanent current of a predetermined size flowing through the permanent current switch 20.

When there is an urgent degaussing request, the control unit 15 operates the heater 28 and the magnet 29 (timing t1). When the magnet 29 operates 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 in the permanent current switch 20, resulting in a voltage and associated heat. Occurs. By heating itself with the heat, the second section 21b quickly shifts to the normal conduction state. As a result, the second switch portion 25B of the permanent current switch 20 is opened (timing t2).

Even if the magnet 29 operates and the critical current of the second switch section 25B decreases, the second switch section 25B will not open if the current flowing through the permanent current switch 20 is smaller than the critical current. In such a case, as shown in FIG. 3C, the control unit 15 may also use a control for outputting a direct current from the current source 13. 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 protection resistor 12. As a result, the current of the permanent current switch 20 is supplemented and exceeds the critical current of the second switch section 25B, so that the second switch section 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. Further, if the second switch portion 25B is temporarily opened, the open state of the second switch portion 25B during the period T2, which will be described later, can be obtained. Therefore, 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 becomes the permanent current switch 20 according to the ratio of the electric resistance value of the second switch unit 25B to the electric resistance value of the protection resistance 12. The current is divided into the protection resistor 12. Due to this current, the energy stored in the superconducting coil 11 is consumed as Joule heat, and the current of the superconducting coil 11 is attenuated.

On the other hand, even if the second switch portion 25B is opened, the electric 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 shifts to the normal conductive state, the protective layer having a small electric resistance bypasses the current. If the wire length of the second section 21b of the superconducting wire 21 is increased, the electric resistance value can be increased, but that would increase the size of the permanent current switch 20 and increase the cost. Therefore, there is a limit to the electric resistance value in the open state of the second switch portion 25B. Therefore, when the current value of the superconducting coil 11 is high, the current shunted to the second switch section 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 is rapidly transferred to the frame 23 having a large heat capacity, which is thermally coupled to the second section 21b 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. The Joule heat is also transferred to the first switch portion 25A via the thermal resistor 26, but the amount of heat transferred by the thermal resistor 26 is limited to a small amount.

In the second switch section 25B, the Joule heat generated in the superconducting wire 21 quickly moves to the frame 23, and a part of this heat escapes to the cooling section 14A. Due to such heat transfer, the temperatures of the superconducting wire 21 and the frame 23 of the second switch portion 25B can be kept to such an extent that the second switch portion 25B is not damaged. Then, due to this temperature rise, the second switch unit 25B continues to be in the open state. After that, as the current of the superconducting coil 11 is attenuated, the Joule heat generated in the second switch portion 25B decreases. As a result, the temperatures of the superconducting wire 21 and the frame 23 of the second switch section 25B become lower than the critical temperature. As a result, the second switch unit 25B is switched to close again (timing t4). As a result, the open state of the second switch portion 25B of 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 unit 25B.

On the other hand, in the first switch unit 25A, heating by the heater 28 is continued after the timing t1, and the temperature of the first section 21a of the superconducting wire 21 gradually rises. As described above, the first switch section 25A is thermally separated from the second switch section 25B via the thermal resistor 26, and the first switch section 25A is arranged away from the cooling section 14A of the cooling device 14. ing. Therefore, 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 above the critical temperature 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 open, 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 unit 25A is set shorter than the period T2 during which the second switch unit 25B can be maintained open. That is, the "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 can be obtained. In other words, it means a state in which heat discharge from the first switch unit 25A to the second switch unit 25B is restricted 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 electric resistance of the first section 21a of the superconducting wire member 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. Further, the resistance of the first switch portion 25A and the resistance of the second switch portion 25B are added up to increase the overall resistance of the permanent current switch 20, so that the shunt ratio of the current flowing through the protection resistor 12 becomes large. As a result, the current flowing through the permanent current switch 20 becomes smaller. 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 hot enough to cause damage even if it is thermally separated.

If the timing at which the first switch section 25A is opened is too early, the current flowing through the first switch section 25A increases, which is not preferable. In such a case, the first switch unit 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 degaussed. 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. Therefore, the superconducting coil 11 is degaussed to the end.

As described above, according to the permanent current switch 20 of the first embodiment, the superconducting coil 11 can be demagnetized by quickly opening the permanent current switch 20 in a state where the superconducting coil 11 is excited. Further, during the degaussing 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 of the first embodiment and its control method, 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 this case, the control unit 15 operates the current source 13. As a result, the current flowing through the permanent current switch 20 increases, and the second switch unit 25B can be quickly switched to open. Therefore, even in the above case, the superconducting coil 11 can be degaussed by promptly opening the permanent current switch 20 when the degaussing request is made.

(Second Embodiment)
FIG. 4 is a circuit diagram showing a configuration of a permanent current switch according to a 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 those in the first embodiment, and detailed description thereof will be omitted. The magnetic field generator of the second embodiment has a configuration in which the permanent current switch 20 of the magnetic field generator 1 of FIG. 1 is replaced with the permanent current switch 20A of the second 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 opened and closed controlled 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 illustrating a closing operation of the permanent current switch according to the second embodiment in an excited (energized) state of the superconducting coil 11. 5 (A) to 5 (E) show the current of the superconducting coil 11, the output (ON) and stop (OFF) of the current source 13, the opening / closing of the semiconductor switch 31, the opening / closing of the first switch section 25A, and the second. The opening and closing of the switch unit 25B is shown.

FIG. 5 shows a state in which the permanent current switch 20A is open before the timing t11, and the superconducting coil 11 is excited by being supplied with a current from the current source 13. Normally, the second switch unit 25B is in the closed state. Therefore, in order to shift the permanent current switch 20A from the open state to the closed state, the first switch unit 25A may be closed. However, since the first switch section 25A is connected to the cooling section 14A via the thermal resistance body 26 and the second switch section 25B, it takes a relatively long time to cool the first switch section 25A.

Therefore, in the second embodiment, when the timing t11 for transitioning the permanent current switch 20A to the closed state is reached, the control unit 15 closes the semiconductor switch 31. As a result, the permanent current switch 20A is substantially closed, and when the current output of the current source 13 is 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. Therefore, it is difficult to maintain a state in which a large current flows through the semiconductor switch 31 for a long time from the viewpoint of cooling. Therefore, it is preferable that the delay of the closing operation of the second switch unit 25B is adjusted to be as short as possible. After that, as shown in the timing t12 of FIG. 5, when the cooling of the permanent current switch 20A progresses and the first switch section 25A is switched to closed, the current is transferred to the first switch section 25A and the second switch section 25B in the superconducting state. Priority is given to the 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 transitioned to the closed state by the above operation. Further, 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 an electric element other than the current source before the timing t11, the permanent current switch 20 is still closed by the above operation. It is made to transition to.

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

(Third Embodiment)
FIG. 6 is a conceptual diagram showing a configuration of a permanent current switch according to a 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. Of these, the second switch unit 25B, the thermal resistance body 26, and the magnet 29 are the same as those in the first embodiment, and detailed description of these will be omitted.

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

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

The thermal conductor 25 has a smaller heat capacity than the frame 23 of the second switch portion 25B, and is thermally coupled to the first subsection 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 28a and 28b are arranged so that the first subsection 21a1 and the second subsection 21a2 of the superconducting wire 21 can be heated independently, and can be driven independently of each other by the control unit 15.

<Open operation in an emergency>
Subsequently, the opening operation when the superconducting coil 11 is urgently degaussed will be described.

In the third embodiment, when there is an urgent degaussing request, the control unit 15 operates the magnet 29 and one of the heaters 28a. As a result, 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 is rapidly transferred to the frame 23 having a large heat capacity, it is possible to prevent the second switch portion 25B including the second section 21b of the superconducting wire 21 from being damaged by an excessive temperature rise.

Here, it is assumed that it becomes 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 subsection 21a1 of the superconducting wire 21 can be switched to open by heating one of the heaters 28a. When the first subsection 21a1 is opened, the current of the superconducting coil 11 is not yet sufficiently attenuated, so that a relatively large Joule heat is generated in the first subsection 21a1. However, in the first subsection 21a1, by releasing this Joule heat to the heat conductor 25, damage due to an excessive temperature rise in the first subsection 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 completely absorbed, and if it is too large, the heating efficiency of the first small section is significantly lowered. Therefore, the heat capacity of the heat conductor 25 may be set in consideration of these trade-offs.

The open state of the first subsection 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>
Subsequently, an operation of opening and closing the permanent current switch 20B at high speed when a large current is not flowing through the permanent current switch 20B will be described. First, the operation of opening the operation at high speed will be described.

In the stage before the permanent current switch 20B is opened, the first section 21a and the second section 21b of the permanent current switch 20B are cooled to the transition temperature or lower and transferred to the 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 operates the second heater 28b. Here, the control unit 15 does not need to operate the magnet 29 and the first heater 28a.

In the third embodiment, since the heat conductor 25 is provided in the first switch portion 25C, the heating efficiency of the first subsection 21a1 of the superconducting wire 21 is relatively low. On the other hand, the second subsection 21a2 of the superconducting wire 21 is held by the frame 24a via the thermal resistor 24b, and is thermally separated from the heat capacity of the thermal conductor 25 and the frame 24a. Therefore, the heating efficiency of the second small section 21a2 of the superconducting wire 21 is very high. Therefore, when the second heater 28b is activated, the temperature of the second subsection 21a2 of the superconducting wire 21 rises rapidly, and the second subsection 21a2 can be opened beyond the critical temperature.

At this time, the current may be supplied to the circuit from the current source 13. The current supplied at this time flows through the 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 subsection 21a2 causes heat generation at a temperature below the critical temperature, which assists the heating of the second subsection 21a2 to realize a temperature rise in a shorter time. As a result, a 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, the amount of heat output from the second heater 28b is the minimum amount of heat for maintaining the second small section 21a2 of the superconducting wire 21 in the open state before the closing operation. Keep close to. That is, the control unit 15 controls so that the margin of the amount of heat for maintaining the open state is minimized. As a result, the temperature of the second subsection 21a2 of the superconducting wire 21 is maintained above the critical temperature without being significantly separated from the critical temperature. Further, in the stage before the closing operation, the second section 21b and the first subsection 21a1 of the permanent current switch 20B are kept cooled to the critical temperature or lower.

When the closing operation is performed at high speed, the control unit 15 stops heating of the second heater 28b. As described above, the second subsection 21a2 of the superconducting wire 21 is not thermally coupled to the small heat capacity. Further, the temperature of the second subsection 21a2 of the superconducting wire 21 is maintained without a large gap from the critical temperature due to the state before the opening operation. Therefore, by stopping the heating of the second heater 28b, the temperature of the second subsection 21a2 of the superconducting wire 21 can be lowered to the critical temperature or lower by transferring 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 required 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, a thyristor) 31 is connected in parallel to the superconducting wire 21. In this case, by using the closing operation of the semiconductor switch 31 together as in the second embodiment, a faster closing operation can be performed.

As described above, according to the permanent current switch 20B of the third embodiment, a quick opening operation is possible to degauss the superconducting coil 11 as in the first embodiment, and a large current does not flow. It is possible to perform high-speed opening and closing operations in the state. Further, since the output of the second heater 28b becomes very small during the high-speed opening / closing operation, the cooling capacity required for the cooling device 14 can be suppressed to a small value.

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

Next, the first embodiment in which the permanent current switch 20 is more embodied will be described. The first embodiment is an example embodying the permanent current switch 20 of the first embodiment shown in FIG. FIG. 7 is a side view showing a permanent current switch according to the first embodiment of the present invention. 8 (A) is a sectional view taken along line AA of FIG. 7, and FIG. 8 (B) is an enlarged sectional view thereof. 9 (A) is a sectional view taken along line BB of FIG. 7, and FIG. 9 (B) is an enlarged sectional view 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 REBCO wire for 1 km, and has a permanent current of 400 A. 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 protection resistor 12 are connected in parallel with the permanent current switch 20 via a normal conducting wire led out of the vacuum chamber. The electric resistance of the protection resistor 12 is 1Ω, and the time constant of the current attenuation in the series circuit of the superconducting coil 11 and the protection resistor 12 is 10 seconds. It is assumed that the permanent current switch 20 is instantly completely opened (electrical resistance is infinite) while the superconducting coil 11 is driven by a constant current of 400 A. Then, a current flows through the protection resistor 12 to generate a potential difference of 400 V as an induced voltage across the superconducting coil 11, and then the potential difference is attenuated as the current is attenuated. 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 as the current decay rate. It will be slower than when it becomes large.

<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 the first switch portion 25A into which the heater 28 (see FIG. 9B) is integrated and the magnet 29. A second switch portion 25B and a thermal resistor body 26 are provided. As shown in FIGS. 9A and 9B, the first switch portion 25A has a cylindrical frame 22, and a strip-shaped superconducting wire 21 is wound around the frame 22. The 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 25B has a cylindrical frame 23, and the band-shaped superconducting wire 21 is wound around the frame 23. The portion of the superconducting wire 21 wound around the frame 23 is referred to as a second section 21b. The cooling unit 14A 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 assigned to the second switch section 25B and the remaining 60 m is assigned to the first switch section 25A. The basic set temperature at which the permanent current switch 20 is opened is 100 K, which is 90 K, which is the critical temperature of the REBCO wire, plus a margin. The specific resistance of silver at 100 K is about 3 μΩ · mm. Since the cross-sectional area of silver in this REBCO wire is 0.03 mm ² , the electric resistance per unit length at 100 K is 0.1 Ω / m, and the electric resistance of the 100 m superconducting wire 21 is 10 Ω. The opening operation of the permanent current switch 20 in a normal time other than an emergency is performed only by the first switch unit 25A heated by the heater 28. Therefore, the electric resistance of the permanent current switch 20 in the open state in the normal state is 6Ω.

The superconducting wire 21 is electromagnetically wound around a cylindrical frame 22 having an outer diameter of φ16 cm and a frame 23 connected to the cylindrical frame 22 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 total length of the wound frames (22, 23) is It is 140 cm. The frame 22 and the frame 23 are made of pure copper (C1020) and are connected to each other via a thermal resistor 26 made of FRP. The wall thickness of the cylindrical side wall of the frame 22 is 0.5 mm, and the wall 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 in vacuum without being connected, 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 the "axial direction", and the direction at right angles to the central axis of the cylindrical shape is defined as the "radial direction". The direction forming 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, has an outer diameter of φ16 cm, an inner diameter of φ14 cm, and a thickness of 1.0 mm. It has a constant ring shape. A copper connecting member 61 having the same shape as the thermal resistor 26 is provided at the end of the frame 22 and the frame 23 facing each other. The connecting member 61 is joined to the end of the frame 22 by welding or the like, and further, the connecting member 61 and the end of the frame 23 are joined by bolts or the like with the thermal resistance body 26 sandwiched between them. Bolts with low thermal conductivity are used. As a result, the frame 22 and the frame 23 are integrally connected in a state of being thermally separated.

A superconducting wire 21 is wound around the outer periphery of the frame 22 and the frame 23 in a solenoid shape via a polyimide sheet 71 for insulation having a thickness of 50 μm. The superconducting wire 21 has a thickness of about 0.06 mm, and the superconducting layer is wound toward the outer peripheral side. That is, when the superconducting wire 21 is configured by laminating a metal substrate, an intermediate layer, a semiconductor layer, and a silver protective layer in this order, the metal substrate is wound toward the inner peripheral side and the silver protective layer is wound toward the outer peripheral side. ing.

In the solenoid-like 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 second end and wound back to the first end. More specifically, the superconducting wire 21 is wound back and forth between both ends of the winding frame (22, 23), and the superconducting wire 21 is wound with a gap on the outward route to fill this gap on the return route. The superconducting wire 21 is wound around. The winding direction of the superconducting wire 21 is opposite between the outward path and the return path. Although not particularly limited, the ends e1 and e2 of the superconducting wire 21 are arranged at one end of the second switch portion 25B, and the folded-back portion e3 is arranged at the opposite end of the first switch portion 25A. The wound superconducting wire 21 is fixed together with the insulating polyimide sheets 71 and 72, the copper strip 73 described later, and the heater 28 by an epoxy resin impregnation treatment and a curing treatment.

<Structure of the first switch part>
The volume of the first switch portion 25A is 211 cc when calculated based on the above-mentioned dimensions. Assuming that 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 total weight becomes about 2.1 kg. As shown in FIG. 9B, the heater 28 is a metal strip having the same shape (strip shape) as the superconducting wire 21, and is made of an alloy having a small temperature dependence of electrical resistance, such as stainless steel or Hastelloy (registered trademark). ) Can be adopted. The metal strips constituting the heater 28 are wound around the outer peripheral side of the superconducting wire 21 with a polyimide tape for electrical insulation sandwiched between them. The heat generated from the heater 28 by energization heats the superconducting wire 21 and the frame 22, and is transmitted to the frame 23 of the second switch portion 25B and then to the cooling portion 14A of the cooling device 14 via the thermal resistor 26. .. Due to the high thermal conductivity of the cooled pure copper, no large temperature difference is formed inside the pure copper frame. The cross-sectional area of the thermal resistor 26 perpendicular to the heat transfer direction (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 / K m when the thermal conductivity is 0.1 W / K m. The thermal resistance of the 1.0 mm thermal resistor 26 is 2.1 K / W. When the heater 28 is completely off, the overall temperature of the permanent current switch 20 is 40K, and when the heater 28 generates heat, a temperature difference is formed at both ends of the thermal resistor 26 according to the amount of heat generated. In the normally open state of the permanent current switch 20, the first switch portion 25A may be maintained at 100K or 100K or more, so that a temperature difference of 60K may occur at both ends of the thermal resistor 26. Converting using the thermal resistance value of the thermal resistor 26, this state is achieved when the heater 28 generates heat at 30 W.

The above-mentioned method of heat transfer is that in a thermally steady state, and the above-mentioned temperature difference cannot be generated immediately after the heater 28 is heated. When calculated in consideration of the temperature dependence of the specific heat of copper, the amount of heat required to heat the first switch section 25A from 40K to 100K is about 18kJ from the weight of the first switch section 25A. Therefore, to heat up to 100 K, it takes 600 seconds when the output of the heater 28 is 30 W and there is no heat outflow, and 1 minute even if the output of the heater 28 is 300 W, which is 10 times as large. It takes a long time to open the permanent current switch 20 by heating in this way. Similarly, it takes time to close the permanent current switch 20 by cooling. If the plate 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 some abnormality is detected in the superconducting coil 11, the second switch portion 25B is opened by the action of the magnet 29. It is difficult to apply a uniform external magnetic field to all of the second section 21b of the superconducting wire 21 wound around the cylindrical frame 23, and when the magnet 29 is acted on, it is locally applied to the second section 21b of the superconducting wire 21. An external magnetic field of different magnitude and direction will be applied to the magnet. Then, the critical current decreases only in the region where a large external magnetic field is applied, and a voltage and heat associated therewith are generated. The superconducting wire 21 wound around the frame 23 is 40 m long, and when all the parts are heated to 100 K, a resistance of 4 Ω is generated. This resistor reduces the current flowing through the permanent current switch 20 from 400A to 80A, with the rest flowing through the protection 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 from this. 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 during this period is 267V, and the calorific value increases to 35.5kW. Furthermore, since the heat generation range is half of the second section 21b of the superconducting wire 21, when compared in terms of the amount of heat generated per wire length, there are parts that generate heat nearly three times as much as when the whole is transferred to the normal conduction state. It ends up. Since it is not preferable that the amount of heat generated per wire length is increased in this way, it is preferable that the magnet 29 applies a magnetic field to many parts of the superconducting wire 21 wound around the frame 23. Therefore, as the magnet 29, a racetrack type electromagnetic coil wide in the axial direction of the second switch portion 25B is adopted, and four or six magnets 29 are arranged side by side in the circumferential direction of the frame 23. The race track type refers to a shape in which a planar shape has two opposing semicircular curved portions 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 hex pole type, and is a magnetic field (in the radial direction of the frame 23) directed in the axial direction of the electromagnetic coil over a wide range inside the radial direction of the magnet 29. (Indicated by the arrow line in FIG. 3A) can be formed. In the strip-shaped superconducting wire 21, the magnetic field in the thickness direction of the wire (called the vertical magnetic field) is more dependent on the magnetic field of the critical current than the magnetic field in the width or longitudinal direction of the wire (called the horizontal magnetic field). It has the property of becoming larger. Therefore, due to the arrangement of the magnets 29, the small magnets 29 can sufficiently reduce the critical current in the second section 21b 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 vertical magnetic field of 1 T is applied at 40 K, the critical current is reduced to about half as compared with the state of no magnetic field. On the other hand, even with the same superconducting wire, when a horizontal magnetic field of 1T is applied at 40K, the reduction rate of the critical current is only about 30% as compared with the state of no magnetic field.

<Structure of the second switch>
Even if the above-described arrangement of the magnets 29 is adopted, a sufficient magnetic field cannot be applied to all the parts of the superconducting wire 21 wound around the frame 23. Therefore, immediately after the emergency opening operation, a portion that generates a large amount of heat and a portion other than that are mixed in the second section 21b of the superconducting wire 21. Since excessive heat is generated in the portion that generates a large amount of heat, if this heat is transferred to the surrounding portion where the amount of heat generated is small, the temperature of the entire second section 21b of the superconducting wire 21 can be raised. Therefore, in the second switch portion 25B, a copper strip 74 (see FIG. 8B) that bypasses the heat and diffuses the heat to the entire second section 21b of the superconducting wire 21 is wound. Copper strip 74 corresponds to an example of a heat conductive wire rod 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 area around which 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 for suppressing 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 the 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 frame 23 on the inner peripheral side 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 to the entire 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 to these parts from the parts where Joule heat is generated after a considerable time has passed since the formation of the magnetic field. Becomes from. Therefore, at the time of 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, in the following, a case where only 30 m of the 40 m superconducting wire 21 of the second switch unit 25B transitions to the normal conducting state to generate Joule heat will be described.

When 30 m of the superconducting wire 21 is transferred to the normal conducting state in the second switch portion 25B, the electric resistance at 100K becomes 3Ω, and the current flowing through the permanent current switch 20 decreases from 400A to 100A. The area of the 30 m superconducting wire 21 in contact with the frame 23, which is a heat bath, is 6 mm × 30 m = 0.18 m ² , and the polyimide sheet 71 (thickness 0.05 mm) for electrical insulation inserted between them has an area of 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. Further, 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 circumference of the frame 23 is slightly heavier than 0.4 kg, and the total mass of the second switch portion 25B is about. It weighs 24.8 kg.

<Behavior during emergency opening operation>
Next, the behavior when the permanent current switch 20 is urgently opened under the above-mentioned 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 falls below 400 A in most of the second section 21b of the superconducting wire 21, and a voltage and heat associated therewith are generated. This heat spreads to most of the second section 21b of the superconducting wire 21 and its surroundings, and after a very short time, an electric resistance of 3Ω is generated in the second switch portion 25B. As a result, the current flowing through the permanent current switch 20 is reduced to 100A, the voltage between both ends of the second switch section 25B becomes 300V, and the Joule heat of the second switch section 25B becomes 30kW. The remaining current of 300 A flows through the protection resistor 12, and the calorific value here is 90 kW. Since there is a thermal resistance of 4K / kW between the frame 23 and the second section 21b of the superconducting wire 21, the temperature of the second section 21b of the superconducting wire 21 is 30kW × 4K / kW = 120K higher than that of the frame 23. It gets 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 90 K, at this point in the superconducting wire 21 of the second switch portion 25B, the superconducting layer of the heat-generating portion completely shifts 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 is temperature-dependent, and the electrical resistance of 0.1 Ω / m per unit length of the superconducting wire 21 used in the calculation of the electrical resistance of 3 Ω is a value at 100 K. Therefore, when the temperature rises to 160K, the electric resistance of the second switch portion 25B is higher than 3Ω. In this case, the current flowing through the permanent current switch 20 becomes smaller than 100A, and the temperature rise in the second section 21b of the superconducting wire 21 is stable 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 protection resistor 12 is about 0.8Ω, and the current of the superconducting coil 11 has a time constant of 12.5 seconds (= 10H ÷ 0.8Ω) ratio (12.5). After a second, it decays at the original 1 / e≈37%). As a result, the superconducting coil 11 is quickly degaussed and shifts to a safe state.

On the other hand, as the current of the superconducting coil 11 is attenuated, the calorific value of the second switch portion 25B decreases. However, the Joule heat generated with the passage of time is accumulated in the frame 23, and the temperature of the frame 23 is high. It is assumed that 160 kJ, which corresponds to 20% of the 800 kJ of energy stored in the superconducting coil 11 before the opening operation, is consumed by heating 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 the critical temperature of 90 K of the superconducting wire 21. Here, the heat energy required to heat 24.8 kg of copper from 40 K to 90 K is 140 kJ, but 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 conduction state for a while even if the second switch portion 25B does not generate heat. Alternatively, it is assumed that the calorific value of the second switch unit 25B is smaller than the above case, and the temperature of the frame 23 does not reach the critical temperature of the superconducting wire 21. In this case, the amount of heat generated by the superconducting wire 21 is reduced due to the attenuation of the current of the superconducting coil 11, and finally the second switch portion 25B is substantially closed. However, even in this case, the current flowing through the superconducting coil 11 only maintains the value at that time and does not start to rise.

At the time of an emergency opening operation, heating of the first switch portion 25A by the heater 28 is also executed in parallel with the driving of the magnet 29. It takes time to heat the superconducting wire 21 of the first switch section 25A above the transition temperature, but if there is a period during which the above-mentioned second switch section 25B is maintained in the open state, it takes time to heat above the critical temperature. can get. Alternatively, the superconducting wire 21 of the first switch section 25A may reach a critical temperature or higher after the second switch section 25B has fallen into a substantially closed state. When the first switch section 25A is switched to open by heating the heater 28, Joule heat is also generated in the first switch section 25A, but at this time, the current of the superconducting coil 11 has already been significantly attenuated, and the first switch section 25A There is no excessive temperature rise that would damage the. After that, even if the second switch portion 25B changes to a substantially closed state, the current of the superconducting coil 11 continues to be attenuated because the first switch portion 25A is open, and degaussing is completed. Can be done.

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

In the permanent current switch 20 of the second embodiment, the first switch portion 25A, the second switch portion 25B, the thermal resistor body 26, the heater 28, and the magnet 29 are coaxially formed as a cylindrical or solenoid-shaped layer, respectively. 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 frame 23, the electrically insulating sheet 81, the second section 21b of the superconducting wire 21, the thermal resistance layer 82, and the first coil constituting the magnet 29 are formed in this order from the center side to the outer peripheral side. 29a, a thermal resistor 26, a first section 21a of a superconducting wire 21, a heater 28, a thermal resistance layer 83, a second coil 29b constituting a magnet 29, and a thermal conductive layer 84 are provided. The thermal resistance body 26 also functions as a 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 ends 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, and are intermediate. The part e6 is folded back and held at the other end of the permanent current switch 20 in the axial direction. The superconducting wire 21 has the same structure as that of the first embodiment, and is a REBCO wire having a width of 6 mm.

The frame 23 arranged closest to the inner circumference also functions as one heat bath of the permanent current switch 20. The frame 23 is a cylindrical pure copper (C1020) having an outer diameter of φ16 cm, an inner diameter of φ15.2 cm, a wall 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 in 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 that orbits the cavity from being generated when a magnetic field penetrates the central cavity 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 conducted and cooled.

The second section 21b of the superconducting wire 21 is wound in a single layer around the frame 23 via an electrically insulating sheet 81 on the inner peripheral side. The dimensions of the superconducting wire 21 are the same as in the first embodiment. The second section 21b of the superconducting wire 21 has a winding diameter of φ160 mm, which is the same as the outer diameter of the frame 23, the length of one winding is 0.50 m, and the total length is 49 m after being wound for 98 turns in total. 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 provided with an insulating layer is wound in 10 layers. The total thickness of the first coil 29a is 1.8 mm, and the average thickness per layer is 0.18 mm.

The thermal resistance layer 82 and the thermal resistance body 26 each have a thickness of 1 mm, and have a structure in which a polyimide sheet having a thickness of 50 μm, which is the same as that of the electrically insulating sheet 81, is wound in multiple layers. The permanent current switch 20 of 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 an extremely low temperature. As a result, the thermal conductivity of the interface is lowered, and high thermal resistance is realized without increasing the thickness. Also, thermal resistance increases in proportion to the number of interfaces present in the heat transfer path. In the structure in which sheets having a thickness of 50 μm are laminated to have a thickness of 1 mm, there are about 20 interfaces, so that the thermal conductivity in the stacking direction is as small as about 0.001 W / Km. In this case, the thermal resistance of the heat transfer path from the superconducting wire 21 having a width of 6 mm and a length of 1 m to the next layer with the layer of the thermal resistance layer 82 or the thermal resistor 26 sandwiched between them is 167 K / It becomes W. Hereinafter, the thermal resistance of such a heat transfer path is also referred to as the thermal resistance of the superconducting wire 21 having a wire length of 1 m. The total thickness of the second section 21b of the conductive wire member 21, the thermal resistance layer 82, the first coil 29a, and the thermal resistance body 26 described so far is about 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) sandwiched between them. As described above, the first section 21a and the second section 21b of the superconducting wire 21 are continuous, and are folded back 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 opposite to each other. 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 out, and the superconducting wire 21 is wound substantially without induction. 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 complexity in work and complexity in design, 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 has a diameter of φ168 mm. It becomes. The electric resistance of the superconducting wire 21 is 0.1 Ω / m at 100 K, and 5.2 Ω is generated at 52 m. The first section 21a of the superconducting wire 21 is wound with the superconducting layer directed toward the outer periphery, contrary to the second section 21b. The front and back sides of the superconducting wire 21 are reversed at the folded intermediate portion e6.

The thermal resistance layer 83 near the outer circumference has the same structure as the thermal resistance layer 82 near the inner circumference, but has a thickness of 0.6 mm, which is rather thin. When calculated in the same manner as the thermal resistance layer 82 near the inner circumference, the thermal resistance of the superconducting wire 21 of the thermal resistance layer 83 for a 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 layers is small. Copper strips are wound around the first coil 29a and the second coil 29b in the same direction, and the ends of the copper strips are electrically coupled to each other 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 layers 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 out to prevent the magnetic field from being applied to the first section 21a of the superconducting wire 21. To do. In the 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 strengthen 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. As a result, the heat conductive layer 84 has a structure in which slits SL2 extending in the axial direction are 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 conducted and cooled.

The permanent current switch 20 of the second embodiment is manufactured by assembling each layer from the inner peripheral side to the outer peripheral side in the radial direction in order. In the middle of the manufacturing process of the permanent current switch 20, at the stage where the thermal resistance layer 83 is assembled, the structure of this stage is impregnated with paraffin. The purpose of this impregnation process is not to increase the mechanical strength, but to stabilize the thermal bond. Subsequently, at the stage where the second coil 29b and the heat conductive layer 84 are assembled, the configuration at this stage is fixed by the epoxy resin impregnation treatment and the curing treatment. The purpose of this impregnation treatment is to reduce the thermal resistance of the second coil 29b as much as possible. Unlike paraffin, epoxy resin does not crack at the interface even when cooled to an extremely low temperature, and the thermal resistance at the interface does not increase. In the state of being impregnated with the epoxy resin, the thermal resistance of the second coil 29b is dominated by the thermal resistance of the electrically insulating layer formed around the copper strip. Assuming that the number of copper strips of the second coil 29b is five, which corresponds to half the number of copper strips of the first coil 29a, the total thickness of the electrical insulating layer of the second coil 29b is 0. It becomes 4 mm. According to this configuration, the thermal resistance of the second coil 29b is less than 1 K / W in the heat transfer path orthogonal to the tape surface of the superconducting wire 21 having a wire length of 1 m. Since the epoxy resin does not attack paraffin, the epoxy resin does not impregnate the inside of the thermal resistance layer 83 previously impregnated with paraffin.

<Opening / closing operation of the first switch section 25A>
The opening / closing operation when a large current does not flow through the permanent current switch 20 is performed by the temperature control of the first switch unit 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 100 K or higher, and the required temperature difference is 60 K. In the heat transfer path orthogonal to the tape area of the superconducting wire 21 for a wire length of 1 m, even if the thermal resistance of the second coil 29b is 1 K / W, from the layer of the first section 21a of the superconducting wire 21 to the heat conductive layer 84. The thermal resistance of is about 101 K / W. When converted with the total length of the first section 21a of the superconducting wire 21 at 52 m, 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 high because the thermal resistance layer 82 and the layer of the thermal resistor 26 intervene. It becomes large and becomes about 404 K / W for the wire length of 1 m of the superconducting wire 21. In this case, the calorific value of the heater 28 required to create a temperature difference of 60K is 38W. Of this, about 80% of about 30 W flows to the heat conductive layer 84 on the outer peripheral side, and the remaining about 20% of about 8 W flows to the frame 23 on the inner peripheral side. Here, it is assumed that the temperature of the superconducting wire 21 in the closed state is 60 K. This temperature is a temperature at which the critical current of the superconducting wire 21 is sufficiently higher than the normal energizing current (400 A), and the higher the temperature, the faster the opening operation in the normal time can be performed. 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 1.6kJ. Therefore, if the output of the heater 28 is 300W, it takes about 5 seconds. 1 The switch unit 25A can be opened. Further, the maximum heat extraction 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, and as the temperature difference becomes smaller, the heat extraction rate decreases accordingly. Therefore, by stopping the output of the heater 28 from the open state, the first switch unit 25A can be closed in about 100 seconds, that is, about 2 minutes.

<Open operation in an emergency>
Next, an operation of urgently opening the permanent current switch 20 of the second embodiment to degauss the superconducting coil 11 while the superconducting coil 11 is excited will be described. The opening operation in an emergency 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, in the layer of the first section 21a, these magnetic fields cancel each other out. Therefore, there is a difference in the critical current 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 unlike a superconducting coil, a large current cannot flow. Therefore, it is not possible to form a strong magnetic field on the scale of several T. Further, 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 on the magnetic field in this direction is not large. Therefore, the critical current of the superconducting wire 21 cannot be significantly reduced only by applying this magnetic field. That is, it is not possible to make a large difference in the critical current between the layer of the second section 21b and the layer of the first section 21a of the superconducting wire 21.

Therefore, in the second embodiment, the control unit 15 operates the current source 13 together with the formation of the magnetic field at the time of the opening operation in an emergency. The current source 13 is stopped while the superconducting coil 11 is excited by the permanent current. 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 so much, and the protection resistor 12 has an electric resistance, so that a large current does not flow, and the current source 13 has a large current. The output current will mainly flow to the permanent current switch 20. A magnetic field is applied to the second section 21b of the superconducting wire 21 although it is not strong, and a magnetic field is hardly applied to the first section 21a. Therefore, the second section 21b is more than the first section 21a. Will also start generating voltage and heat at low current values. As a result, the second section 21b of the superconducting wire 21 is transferred to the normal conducting state, and the second switch portion 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. Since this heat is generated in the superconducting layer through which most of the current flows and heats the superconducting layer first, the superconducting layer exceeds the critical temperature and shifts to the normal conducting state immediately after the heat is generated. Maintains the generation of greater Joule heat. 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 across the 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 the second embodiment, a relatively uniform magnetic field is applied to each portion of the second section 21b of the superconducting wire 21. Therefore, the above-mentioned voltage and heat generation in the second section 21b occur almost at the same time in the entire wire rod. Therefore, it is not necessary to provide a structure such as a copper strip for bypassing the 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 electric resistance at 100 K is equivalent to 4.9 Ω. However, when the second switch portion 25B is switched to open, only the superconducting wire 21 of about 40 m is transferred to the normal conducting state in consideration of the weakening of the formation of the magnetic field at both ends in the axial direction of the permanent current switch 20. Suppose you did. Then, the electric resistance of the second switch portion 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 drops to 80A, and the remaining 320A is shunted to the protection resistor 12. At this time, the calorific value of the second section 21b of the superconducting wire 21 is 25.6 kW, and almost all of this calorific value flows to the frame 23 on the inner peripheral side. The thermal conductivity of the electrically insulating sheet 81 having a length of 40 m is 0.48 kW / K, and the thermal resistance is slightly larger than that by adding the effect of the interface to the reciprocal of 2.1 K / kW, which is about 2.5 K / kW. It becomes. If there is a heat generation of 25.6 kW in the second section 21b of the superconducting wire 21, a temperature difference of 64 K 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 amount of heat generated by opening the second switch portion 25B flows toward the heat conductive layer 84, but the thermal resistance between them is 500 K / for the wire length of 1 m of the superconducting wire 21. It is W or more, and the wire length of the wire rod 50 m 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 flowing is only about 6W, and the temperature rise of 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 energizing current 80A, no voltage and heat associated therewith will be generated in the first section 21a, and the first switch section 25A Will maintain a completely closed state.

When the second switch section 25B is opened and an electric resistance of 4Ω is generated, the time constant of the current attenuation in the entire circuit including the resistance of the protection resistor 12 (1Ω) and the inductance of the superconducting coil 11 (10H) is 12. Seconds. The energy consumed by the resistor during 12 seconds is about 40% of the amount that the consumption speed at the beginning of 12 seconds continues for 12 seconds (strictly speaking, (1-e- ² ) / 2 = 0. 43). Since the energy consumption immediately after the second switch portion 25B is opened is 25.6 kW, the calorific value 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 40 K to 90 K or more, even considering the deheat to the cooling unit 14A. Therefore, at the time after 12 seconds, the second switch portion 25B is stably opened in the second section 21b of the superconducting wire 21 even if Joule heat is not generated. It takes time for the entire frame 23 to be cooled before the second switch portion 25B is switched to the closed state again, during which the energizing 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 above the critical temperature due to the operation of the heater 28 while the second switch portion 25B is maintained in the open state. As a result, the first switch portion 25A is opened. At this time, since the current of the superconducting coil 11 is sufficiently attenuated, a large Joule heat is not generated in the first section 21a of the superconducting wire 21, and there is no risk of damaging the superconducting wire 21. After that, even if the second switch portion 25B is switched to closed, the superconducting coil 11 can be completely degaussed by the open state of the first switch portion 25A.

1 Magnetic field generator 11 Superconducting coil 12 Protective resistance 13 Current source 14 Cooling device 15 Control unit 20, 20A, 20B Permanent current switch 21 Superconducting wire 21a 1st section 21a1 1st subsection 21a2 2nd subsection 21b 2nd section 22 frame 23 Frame 25A 1st switch part 25B 2nd switch part 26 Thermal resistors 28, 28a, 28b Heater 29 Magnet 31 Semiconductor switch 74 Copper strip (heat conductive wire)

Claims (10)

The first switch section including the first section of the superconducting wire, and
A second switch that includes a second section of the superconducting wire and a heat conductor having a heat capacity larger than that of the first switch portion, and the second section of the superconducting wire and the heat conductor are thermally coupled to each other. Department and
A thermal resistor that is interposed between the first switch portion and the second switch portion and thermally separates the first switch portion and the second switch portion.
With 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, and
A permanent current switch characterized by being equipped with. The permanent current switch according to claim 1, wherein the superconducting wire is a rare earth superconducting wire or a bismuth superconducting wire. The second switch unit is
It further includes a heat conductive wire that is electrically insulated from the second section of the superconducting wire, and the second section of the superconducting wire and the heat conductive wire are both wound around a winding frame. 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. The permanent current switch according to any one of claims 1 to 4, further comprising a semiconductor switch electrically connected in parallel with at least a part of the superconducting wire material. The first switch portion has a first portion and a second portion having different heat capacities from each other.
The heater according to any one of claims 1 to 5, wherein the heater includes a first heater capable of heating the first portion and a second heater capable of heating the second portion. Permanent current switch. A superconducting coil connected in parallel with each other, a permanent current switch according to any one of claims 1 to 6, a protective resistor and a current source.
A cooling device that cools the superconducting coil and the permanent current switch,
A control unit that controls the permanent current switch,
With
The control unit operates the magnet and the heater based on the degaussing 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 heater to perform the first. A magnetic field generator characterized by switching the switch unit to open. The magnetic field generator according to claim 7, wherein the control unit outputs a current to the current source when the second switch unit is switched to open by the action of the magnetic field of the magnet. A superconducting coil connected in parallel with each other, a permanent current switch according to any one of claims 1 to 6, a protection resistor and a current source, and a cooling device for cooling the superconducting coil and the permanent current switch. It is a control method of a magnetic field generator equipped with
After operating the magnet and the heater and switching the second switch portion to open by the action of the magnetic field of the magnet, the first switch portion is switched to open by heating the heater to demagnetize the superconducting coil. A method for controlling a magnetic field generator. The control method for a magnetic field generator according to claim 9, wherein when the second switch portion is switched to open by the action of the magnetic field of the magnet, a current is output to the current source.

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