JP2016092152A - Protection device and method for protecting superconducting coil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a protection device and a method for protecting a superconducting coil capable of simplifying the configuration, and capable of stopping the operation smoothly while protecting a superconducting coil at the time of normal conducting transition.SOLUTION: A single pancake coil 17 as a superconducting coil is formed by winding superconducting wires 11 without insulating therebetween, and a plurality of pickup coils 23, as magnetic field variation detection elements, are arranged in the vicinity of the inner peripheral part of the single pancake coil 17. An abnormality detection mechanism 27 interrupts a current supplied from a power supply 20 to the single pancake coil 17 by opening an on/off switch 22, when any pickup coil 23 detects magnetic field variation due to normal conducting transition.SELECTED DRAWING: Figure 1

Description

  In the present invention, for example, when an abnormality such as quench (normal conduction transition) occurs when using a superconducting coil formed by winding a superconducting wire formed of a rare earth oxide superconductor, while exhibiting a sufficient protection function, The present invention relates to a protection device and a protection method for a superconducting coil capable of stopping operation.

  In general, in superconducting coils using superconducting wires made of rare earth oxide superconductors, even if an abnormality such as a quench occurs during use, it is difficult to detect the abnormality, so it is difficult to detect the abnormality. Therefore, a high temperature part (heat spot) is locally generated and the characteristics of the superconducting coil are deteriorated. As this superconducting wire, for example, a superconducting layer made of a rare earth oxide superconductor is formed on a substrate via an intermediate layer, and a stabilizing layer made of copper or the like is formed on the superconducting layer via a protective layer such as silver. Things are known. When normal conduction transition occurs, it can be considered that current is caused to flow from the superconducting layer to the stabilizing layer, thereby suppressing heat generation and preventing damage to the superconducting wire.

  However, since electric power is stored in the superconducting coil, it is necessary to quickly stop energization and consume the stored electric power when an abnormality occurs. For this reason, a separate protection circuit is provided for the superconducting coil. When an abnormality occurs, a current is induced in the protection circuit, and the stored power of the superconducting coil is consumed by the resistive load in the protection circuit, and the operation is stopped.

  This type of superconducting coil device is described in Patent Document 1, for example. This superconducting coil device includes a protection circuit for consuming energy (stored power) when the superconducting coil is in a normal conduction transition, and a resistance connected to the superconducting coil in parallel and separate from the protection circuit. Device. And, when the superconducting coil makes a normal conducting transition, the energy of the superconducting coil is consumed by the protection circuit, and the energy is consumed by a resistance different from the protection circuit to diffuse the normal conducting transition part. .

JP-A-5-67524

  In the conventional superconducting coil device described in Patent Document 1 described above, a protection circuit that consumes stored power of the superconducting coil must be provided separately from the superconducting coil, and the protection circuit includes a superconducting coil. A resistance that matches the inductance of the superconducting coil must be set. For this reason, there are disadvantages that the number of parts of the superconducting coil device increases and the configuration becomes complicated.

  Furthermore, this superconducting coil device is configured to detect a voltage generated by a resistance different from that of the protection circuit, and to open the open / close switch provided between the superconducting coil and the power source to cut off the energization to the superconducting coil. Has been. The voltage fluctuation generated in the resistor is very small when the abnormality due to the normal conduction transition is very small, and there is a problem that detection of the voltage fluctuation is easily affected by noise and detection accuracy is low.

  Therefore, an object of the present invention is to provide a protection device and a protection method for a superconducting coil that can be simplified in configuration and can be smoothly stopped while protecting the superconducting coil at the time of normal conduction transition. .

  In order to achieve the above object, the superconducting coil protection device according to the first aspect of the present invention winds a superconducting wire in a coil shape, attaches electrodes to both ends of the superconducting wire, and A superconducting coil connected to a power source and energized, wherein the superconducting coil is formed without being wound between the superconducting wires, and the superconducting coil has a magnetic field fluctuation detecting element in a range covered by the magnetic field. And an abnormality detection mechanism for cutting off the current supplied from the power supply when the magnetic field fluctuation detection element detects a magnetic field fluctuation.

A superconducting coil protection device according to a second aspect of the present invention is the superconducting coil protection device according to the first aspect, wherein the magnetic field fluctuation detecting element is a pickup coil.
A protection device for a superconducting coil according to a third aspect of the present invention is the invention according to the first or second aspect, wherein the magnetic field fluctuation detecting element is disposed on an inner peripheral portion of the superconducting coil.

A superconducting coil protection device according to a fourth aspect of the present invention is the superconducting coil protection device according to any one of the first to third aspects, wherein a plurality of the magnetic field fluctuation detecting elements are arranged.
According to a fifth aspect of the present invention, there is provided the superconducting coil protection device according to any one of the first to fourth aspects, wherein the superconducting wire is a rare earth oxide superconducting material via an intermediate layer on a substrate. A superconducting layer is formed by a body, a protective layer is formed on the superconducting layer, and a stabilizing layer is formed on the outer peripheral portion thereof.

  According to a sixth aspect of the present invention, there is provided a superconducting coil protection method using the superconducting coil protection device according to any one of the first to fifth aspects, wherein a current is supplied from a power source to the superconducting coil. When the coil is in an operating state, when the magnetic field fluctuation detecting element detects a magnetic field fluctuation, the current supplied from the power supply is cut off, and a current based on the stored power of the superconducting coil is passed between the superconducting wires to make the entire superconducting coil The storage power is consumed and the operation is stopped.

  According to a seventh aspect of the present invention, there is provided the superconducting coil protection method according to the sixth aspect of the invention, wherein the magnetic field fluctuation detecting element is disposed on an inner periphery of the superconducting coil, and the magnetic field fluctuation detecting element detects a magnetic field fluctuation. When the current supplied from the power supply is cut off.

  The superconducting coil protection method according to an eighth aspect of the present invention is the invention according to the sixth or seventh aspect, wherein a plurality of the magnetic field fluctuation detection elements are arranged, and any one of the magnetic field fluctuation detection elements is arranged. When the fluctuation detecting element detects a magnetic field fluctuation, the current supplied from the power supply is cut off.

  According to the protection device for a superconducting coil of the present invention, the configuration can be simplified, and the operation can be smoothly stopped while protecting the superconducting coil at the time of normal conduction transition.

Schematic which shows typically the protection apparatus of the superconducting coil provided with the abnormality detection mechanism in embodiment. The schematic perspective view which shows the state which has arrange | positioned four pick-up coils to the single pancake coil. Sectional drawing which shows a superconducting wire. The schematic plan view which shows the flow of the electric current in the state which supplied with electricity to the superconducting coil. The schematic plan view which shows the flow of the electric current in a superconducting coil when the abnormality detection mechanism of a superconducting coil detects abnormality. The graph which shows the relationship between time and the magnetic field of a coil, the voltage of a coil, or the current of a coil. The schematic perspective view which shows the other example of this invention and shows the state which has arrange | positioned the three pickup coils to the double pancake coil. The schematic top view which shows the further another example of this invention, and shows the state which has arrange | positioned the two pick-up coils to the irregular shaped single pancake coil.

Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS.
As shown in FIG. 3, a superconducting wire 11 in the form of a tape has a superconducting layer 14 formed on a substrate 12 via an intermediate layer 13, and a protective layer 15 is provided on the superconducting layer 14, and the outer peripheral portions thereof. The copper stabilization layer 16 is coated on the surface.

The substrate 12 is formed of a metal such as nickel alloy (Hastelloy), silver, or silver alloy, for example, to a thickness of 50 to 100 μm and a width of 4 to 10 mm. The intermediate layer 13 is made of gadolinium / zirconium oxide (Gd / Zr oxide), lanthanum (La) / manganese (Mn) oxide, magnesium oxide (MgO), yttrium stabilized zirconium (YSZ), barium / zirconium oxide. For example, it is formed to a thickness of 500 nm by a compound such as (Ba · Zr oxide) or cerium oxide (CeO ₂ ).

  The superconducting layer 14 is formed to a thickness of, for example, about 1 μm by a rare earth oxide superconductor CVD method (chemical vapor deposition method). As rare earth elements, lanthanum (La), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), dysprosium (Dy), holmium (Ho), erbium (Er), yttrium (Y), And ytterbium (Yb). Examples of rare earth oxides include RE, Ba, Cu, and O. However, RE represents a rare earth element. Specific examples of the superconducting layer 14 include yttrium / barium / copper oxide (Y / Ba / Cu oxide) and lanthanum / barium / copper oxide (La / Ba / Cu oxide).

  The protective layer 15 is formed of a metal such as silver or a silver alloy, and is configured to cover and protect the superconducting layer 14. The protective layer 15 is formed to a thickness of about 2 to 8 μm by sputtering of a metal such as silver.

  The stabilization layer 16 is made of copper, and even when the superconducting state becomes unstable and resistance is generated, the current flowing in the superconducting layer 14 is diverted from the protective layer 15 to the stabilizing layer 16 to be described later. It has the function of stabilizing the superconducting characteristics of the coil. The copper forming the stabilizing layer 16 includes a copper alloy. The thickness of the stabilization layer 16 increases depending on the magnitude of the energization current, but is practically preferably 5 to 30 μm. When the thickness of the stabilization layer 16 is less than 5 μm, the amount of current flowing through the stabilization layer 16 is lowered, and the function as the stabilization layer 16 cannot be sufficiently exhibited, which is not preferable. On the other hand, when the thickness of the stabilization layer 16 is greater than 30 μm, the superconducting wire 11 has a high rigidity and tends to be difficult to handle, which is not preferable because it makes it difficult to form a superconducting coil.

  This stabilization layer 16 is preferably formed by copper plating. Copper plating is performed according to a conventional method of electroplating, and is performed by using a plating solution such as copper sulfate and passing a required current between the electrodes. By this copper plating, the stabilization layer 16 can be formed easily and stably.

  As shown in FIG. 2, a single pancake coil (also simply referred to as a coil) 17 as a superconducting coil is formed by winding the tape-shaped superconducting wire 11 in a coil shape (spiral shape) in one step. When the superconducting wire 11 is wound in a coil shape, no electrical insulating material is interposed between the superconducting wires 11 and the superconducting wires 11 are in direct contact with each other, that is, between the stabilization layers 16 of the superconducting wire 11. It is wound in direct contact. A pair of electrodes 18 and 19 made of a metal having good conductivity such as copper are connected to the end of the superconducting wire 11 on the inner peripheral side of the coil 17 and the end of the superconducting wire 11 on the outer peripheral side. . 1 to 5, the coil 17 is schematically drawn for easy understanding, and the thickness of each layer and the thickness of the superconducting wire 11 are drawn thicker than actual.

  As shown in FIG. 1, a power source 20 is connected between both electrodes 18 and 19 of the single pancake coil 17 by a connection line 21, and a current from the power source 20 is supplied to the coil 17. The connection line 21 is provided with an open / close switch 22 so that when an abnormal situation (normal conduction transition) occurs due to a quench, a power failure, a chiller stop, or the like, the current supply to the coil 17 can be cut off. Yes.

  In the vicinity of the inner periphery of the single pancake coil 17, four pickup coils (single coils) 23 as magnetic field fluctuation detection elements are arranged at intervals in the circumferential direction. These pickup coils 23 detect the magnetic field fluctuation when the coil 17 causes a magnetic field fluctuation based on the normal conducting transition. Since the inner peripheral portion of the coil 17 has a stronger magnetic field than the outer peripheral portion, the detection accuracy of the magnetic field fluctuation by the pickup coil 23 can be increased.

  Each of the four pickup coils 23 is connected to a control device 25 by an input line 24, and the control device 25 is connected to the open / close switch 22 by an output line 26. When any of the four pickup coils 23 detects a magnetic field variation, the signal is input to the control device 25 via the input line 24, and the control signal is transmitted from the control device 25 via the output line 26 to the open / close switch 22. The open / close switch 22 is opened and the current supplied from the power source 20 to the coil 17 is cut off.

  The abnormality detection mechanism 27 includes the four pickup coils 23, the control device 25, the open / close switch 22, and the like. The protection device 10 for the single pancake coil 17 includes a single pancake coil 17, both electrodes 18, 19, a power source 20, an abnormality detection mechanism 27, and the like.

Next, an effect | action is demonstrated about the protection apparatus 10 of the single pancake coil 17 comprised as mentioned above.
When the single pancake coil 17 of the present embodiment is cooled according to a conventional method to be in a superconducting state and a predetermined current is supplied from the power source 20 between the electrodes 18 and 19 of the coil 17 in the superconducting state, As indicated by the dotted line arrow, current flows from one electrode 19 to the other electrode 18 through the superconducting layer 14 in the superconducting wire 11. At this time, energization can be continued in a superconducting state without electrical resistance, a large current can be energized with a high current density, electric power can be stored, and a strong magnetic field can be generated around the coil 17.

  In this superconducting state, a part of the superconducting wire 11 constituting the single pancake coil 17 may deviate from the superconducting state and shift to the normal conducting state for some reason. At this time, the current flowing in the superconducting layer 14 flows to the stabilizing layer 16 and further flows between the superconducting wires 11, and the number of turns of the coil 17 is apparently reduced, so that the magnetic field generated from the coil 17 rapidly decreases. To fluctuate. Any of the four pickup coils 23 detects this magnetic field fluctuation. An input signal based on the magnetic field fluctuation detected by any one of the pickup coils 23 is sent to the control device 25, and the control signal is sent from the control device 25 to the open / close switch 22, and the open / close switch 22 immediately changes to the two-dot chain line in FIG. Open as shown. Therefore, the current supplied from the power source 20 to the coil 17 is interrupted, and at the same time, as indicated by the two-dot chain line arrow in FIG. After flowing from the superconducting layer 14 to the stabilizing layer 16, it flows to the superconducting wire 11 adjacent to the inner peripheral side, and further flows instantaneously toward the center of the coil 17.

  That is, an induced voltage is generated in the single pancake coil 17 due to an abrupt current change, and a current flows between the superconducting wires 11 so that the stabilization layer 16 of the entire coil 17 is coupled, which looks like a copper bulk. The entire coil 17 generates heat and the stored power is consumed. As a result, local heat generation of the superconducting wire 11 can be suppressed and damage to the coil 17 can be avoided without providing a protection circuit separately from the coil 17 and consuming stored power.

The effects obtained by the embodiment described in detail above are collectively described below.
(1) In the protection device 10 for the single pancake coil 17 of the present embodiment, the coil 17 is formed by winding the superconducting wire 11 without being insulated, and the coil 17 has an abnormality detection having a magnetic field fluctuation detection element. A mechanism 27 is provided. Therefore, when this magnetic field fluctuation detecting element detects a magnetic field fluctuation accompanying an abnormality such as a quench, the current supplied from the power source 20 can be cut off to suppress the inflow of overcurrent to the power source 20 and the like. Without using a circuit, the operation of the coil 17 can be stopped while suppressing a local temperature rise in the coil 17.

Therefore, according to the protection device 10 of the present embodiment, the configuration can be simplified, and the operation can be stopped smoothly while protecting the single pancake coil 17 at the time of normal conduction transition.
(2) The magnetic field fluctuation detecting element is a pickup coil 23. For this reason, magnetic field fluctuations can be detected with high accuracy by a general-purpose detection element having a simple structure.

  (3) The pickup coil 23 is disposed on the inner periphery of the single pancake coil 17. In the inner peripheral portion of the coil 17, a strong magnetic field is generated when energized, and the detection accuracy of the magnetic field fluctuation can be further improved.

  (4) A plurality of the pickup coils 23 are arranged. Therefore, when any of the plurality of pickup coils 23 detects a magnetic field variation, the operation of the coil 17 can be stopped quickly.

  (5) In the superconducting wire 11, a superconducting layer 14 made of a rare earth oxide superconductor is formed on a substrate 12 via an intermediate layer 13, and a protective layer 15 is provided on the superconducting layer 14, and the outer peripheral portions thereof. In addition, a stabilization layer 16 is formed. For this reason, the coil 17 can exhibit a superconducting characteristic favorably, and can stop the operation | movement rapidly at the time of a normal conduction transition.

  (6) The method for protecting the single pancake coil 17 is that when the single pancake coil 17 is in an operating state, when the magnetic field fluctuation detecting element detects a magnetic field fluctuation, the current supplied from the power source 20 is cut off, A current is passed between the superconducting wires 11 being rotated, the stored power is consumed in the entire coil 17, and the operation is stopped. Accordingly, it is possible to smoothly stop the operation while protecting the coil 17 without damaging it during the normal conducting transition.

Next, the embodiment will be described more specifically with reference to examples.
Example 1
First, in Example 1, a superconducting layer made of a yttrium-based superconducting material via a lanthanum / manganese-based oxide and magnesium oxide intermediate layer 13 on a nickel alloy (Hastelloy) substrate 12 having a width of 4 mm and a thickness of 50 μm. 14 was formed. Next, a protective layer 15 by sputtering of silver is formed thereon, a stabilization layer 16 by copper plating is formed on the outer periphery thereof, and a yttrium-based superconducting wire 11 having a thickness of about 100 μm (in liquid nitrogen in a self-magnetic field). A critical current 130A) was produced. Then, this superconducting wire 11 was wound 30 times in one stage to produce a single pancake coil 17 having an inner diameter of 60 mm and an outer diameter of 66 mm.

And the change of the magnetic field (maximum magnetic field, mT) of the coil when energizing the obtained single pancake coil 17 in liquid nitrogen at a speed of 0.5 A / s up to 125 A energizing current (energizing current) The change in voltage (V) between both electrodes 18 and 19 of the coil was measured, and the result is shown in FIG. In FIG. 6, a solid line indicates a change in the magnetic field (mT) of the coil 17, a broken line indicates a change in the current (10 ² A) of the coil 17, and a two-dot chain line indicates a change in the voltage (V) of the coil 17.

  From the results shown in FIG. 6, the increasing tendency of the magnetic field of the single pancake coil 17 slowed down when the coil current reached near the critical current. When the coil current was further increased, the magnetic field suddenly decreased and at the same time the coil voltage rose rapidly. Therefore, in the single pancake coil 17 in which the superconducting wire 11 is not insulated, it was confirmed that the magnetic field of the coil 17 rapidly decreased when the energized state collapsed due to the normal conducting transition.

  Next, as shown in FIG. 1, four pickup coils 23 are arranged in the vicinity of the inner peripheral surface of the coil 17, and the opening / closing switch 22 is opened as soon as the pickup coil 23 detects a magnetic field variation. The pickup coil 23 was produced by winding a copper wire having a diameter of 0.25 mm with an inner diameter of 5 mm.

  Then, when the single pancake coil 17 was operated in the superconducting state, it was detected by the pickup coil 23 that the magnetic field suddenly decreased with the coil current 87.5A, and as a result, the open / close switch 22 was opened and the coil 17 Operation stopped. Then, when the state of the coil 17 was confirmed by visual observation, no change was seen in the appearance. Furthermore, when the coil 17 was energized and excited, as in the case of FIG. 6, a result that the magnetic field increased as the current increased was obtained. Therefore, in Example 1, it was found that, without providing a protection circuit separately from the coil 17, damage to the coil 17 due to heat generation based on the stored power of the coil 17 can be suppressed, and the superconducting characteristics can be maintained.

  Here, the calorific value of the coil 17 was analyzed and evaluated using an analysis program in which PEEC (Partial Element Equivalent Circuit) analysis and thermal analysis were coupled. The PEEC analysis is one of electromagnetic field analysis methods based on integral equations, and is a three-dimensional full-wave modeling method suitable for analysis combining an electromagnetic field and a circuit. Is omitted.

First, the temperature of the coil 17 when the inner diameter of the coil 17 is 60 mm, the number of turns of the superconducting wire 11 is 60, and the contact resistance between the wound superconducting wire 11 is set to three types of 70, 350, and 700 μΩ · cm ^2. The rise was analyzed. The inductance of the coil 17 was 432 μH, the energization current was 30 A, the stored energy was 194.5 mJ, and the temperature rise was analyzed as being in an adiabatic state. As a result, the temperature rise of the coil 17 was 0.017K regardless of the contact resistance. Therefore, the temperature rise of the coil 17 is slight, and the thermal stability of the single pancake coil 17 in which the superconducting wire 11 is not insulated is shown.

Next, the temperature rise was analyzed when the inner diameter of the coil 17 was 60 mm, the contact resistance between the superconducting wires 11 was 70 μΩ · cm ^2, and the number of turns of the superconducting wire 11 was 120, 240, and 360. The analysis conditions and temperature rise are shown in Table 1.

From the results shown in Table 1, when the number of turns of the superconducting wire 11 is increased, the energy consumption due to contact resistance shows a certain increase, but the temperature rise of the coil 17 is small. Therefore, it becomes clear that the single pancake coil 17 in which the superconducting wire 11 is not insulated has high thermal stability against an abnormal situation due to normal conduction transition, and the coil 17 can be connected without using protective resistance. It became possible to stop the operation while protecting.

It should be noted that the embodiment described above can be modified and embodied as follows.
-A double pancake coil may be used as the superconducting coil. That is, as shown in FIG. 7, the double pancake coil 28 has coils 28a and 28b formed by winding a tape-like superconducting wire 11 around an inner peripheral frame (not shown). It is formed in two layers. For example, three pickup coils 23 are arranged on the inner peripheral portion of the double pancake coil 28.

A Hall element, a Faraday element, or the like may be used as the magnetic field fluctuation detection element.
As shown in FIG. 8, a single pancake coil 17 having a deformed shape may be used. In this case, it is preferable to arrange the pickup coil 23 on the inner peripheral portion of the bent portion of the coil 17 that generates a magnetic field stronger than the surroundings and can improve the detection accuracy of the magnetic field fluctuation.

As the superconducting wire 11, a bismuth oxide superconducting wire may be used. The superconductor of this bismuth-based oxide is a superconducting material formed of an oxide containing bismuth (Bi). For example, Bi2223, that is, Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _10-α (α is 0 to 0. 15), Bi2212, that is, Bi ₂ Sr ₂ CaCu ₂ O _8-α (α is 0 to 0.15) or the like is used. This bismuth-based oxide superconductor is configured to be dispersed in a sheath layer of a metal material having stretchability.

  DESCRIPTION OF SYMBOLS 10 ... Protection apparatus, 11 ... Superconducting wire, 12 ... Substrate, 13 ... Intermediate layer, 14 ... Superconducting layer, 15 ... Protective layer, 16 ... Stabilization layer, 17 ... Single pancake coil as superconducting coil, 19 ... Electrode, DESCRIPTION OF SYMBOLS 20 ... Power supply, 23 ... Pick-up coil as a magnetic field fluctuation detection element, 27 ... Abnormality detection mechanism, 28 ... Double pancake coil as a superconducting coil.

Claims (8)

A superconducting coil in which a superconducting wire is wound in a coil shape, electrodes are attached to both ends of the superconducting wire, and a power source is connected between both electrodes to energize,
The superconducting coil is formed by winding the superconducting wires without insulation, and the superconducting coil is provided with a magnetic field fluctuation detecting element in a range covered by the magnetic field, and when the magnetic field fluctuation detecting element detects a magnetic field fluctuation, a power source is provided. Superconducting coil protection device equipped with an abnormality detection mechanism that cuts off the current supplied from the coil.   The superconducting coil protection device according to claim 1, wherein the magnetic field fluctuation detection element is a pickup coil.   The protection device for a superconducting coil according to claim 1, wherein the magnetic field fluctuation detection element is disposed on an inner periphery of the superconducting coil.   The protection device for a superconducting coil according to any one of claims 1 to 3, wherein a plurality of the magnetic field fluctuation detection elements are arranged.   The superconducting wire is composed of a superconducting layer formed of a rare earth oxide superconductor through an intermediate layer on a substrate, a protective layer formed on the superconducting layer, and a stabilization layer formed on the outer periphery thereof. The superconducting coil protection device according to any one of claims 1 to 4, wherein the protection device is a superconducting coil.   When the superconducting coil protection device according to any one of claims 1 to 5 is used and a current is supplied from a power source to the superconducting coil and the superconducting coil is in an operating state, the magnetic field fluctuation detection element is a magnetic field. When the fluctuation is detected, the current supplied from the power supply is cut off, and the current based on the stored power of the superconducting coil is passed between the superconducting wires to consume the stored power in the entire superconducting coil and stop the operation. To protect the superconducting coil.   7. The method of protecting a superconducting coil according to claim 6, wherein the magnetic field fluctuation detecting element is disposed on an inner periphery of the superconducting coil, and when the magnetic field fluctuation detecting element detects a magnetic field fluctuation, the current supplied from the power source is cut off.   A plurality of the magnetic field fluctuation detection elements are arranged, and when any of the magnetic field fluctuation detection elements detects a magnetic field fluctuation, the current supplied from the power source is cut off. 2. A method for protecting a superconducting coil according to 1.