Superconducting coil

Abstract

To provide a superconducting coil which has a structure for avoiding or suppressing thermal runaway caused by quenching.SOLUTION: In a superconducting coil 1, a superconducting coil body 2 is formed by winding a metallic tape 18 made of copper or the like around a superconducting wire 15 in such a manner that the metallic tape is fitted along the superconducting wire. The metallic tape 18 is not adhered or anything like that to the superconducting wire 15, but they are wound around in a manner slidably fitted along each other. Upper and lower surfaces of the superconducting coil body 2 are each provided with a cooling mechanism formed by a coolant made of liquid hydrogen or the like. With such a configuration, when local heat is generated in the superconducting wire 15 by degradation or the like, a current may bypass to the metallic tape 18 and sufficient cooling can be achieved via the metallic tape 18, thus enabling stabilization of the cooling. Further, because the superconducting wire 15 and the metallic tape 18 are slidably fitted along each other, it is possible to prevent heat stress caused by difference in thermal expansion between both parts.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a structure for avoiding or suppressing thermal runaway due to quenching in a superconducting coil.

Background Art

[0002] A superconductor has the property of losing superconductivity when the temperature exceeds a certain critical temperature. When an electric current is passed through a superconductor, a normal conducting state may locally occur due to some cause, exceeding the critical temperature. Such a phenomenon is called quenching. When quenching occurs, the resistance of the superconductor locally increases, resulting in local heating and a phenomenon called thermal runaway, where heat generation exceeds cooling, and in some cases, it may lead to damage or burnout due to thermal stress. Avoiding quenching or thermal runaway was particularly important in high-temperature superconductors. A high-temperature superconductor generally refers to a substance with a critical temperature higher than 77 Kelvin, and this property means that it has a large heat capacity until the critical temperature is exceeded. That is, even if a normal conducting part locally occurs, it takes time for it to spread to other parts, and by the time quenching can be detected, the normal conducting part may have expanded to the extent that burnout occurs. Therefore, in high-temperature superconductors, avoiding quenching or thermal runaway caused by it becomes an even more important issue.

[0003] Patent Document 1 discloses a technique in a so-called pancake-type superconducting coil wound with a high-temperature superconducting wire, in which a plate-shaped conductive member is brought into contact with the upper or lower surface thereof to form an electrical bypass circuit between wires at different positions in the radial direction. By doing so, when quenching occurs in the wire, the current flows through the bypass circuit, making it possible to avoid thermal runaway.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the prior art, the suppression effect of thermal runaway was not sufficient, and there was still room for improvement. The problem of thermal runaway is not limited to high-temperature superconductivity but is a problem common to all superconductors. In view of such problems, an object of the present invention is to suppress thermal runaway caused by quenching or the like in a superconducting coil.

Means for Solving the Problems

[0006] The present invention is a superconducting coil body, comprising a thin plate-shaped superconducting wire including a superconducting layer, and a thin plate-shaped metal tape prepared separately from the superconducting wire, having an electrical resistance in a range higher than the resistance value when the superconducting layer is in a superconducting state and lower than the resistance value when the superconducting layer is in a normal conducting state, and it can be configured as a superconducting coil body wound with the metal tape laminated on the superconducting wire.

[0007] The present invention is characterized in that a metal tape is prepared separately from the superconducting wire and wound in a state where both are laminated. Although a plating layer made of metal may be provided on the outer periphery of the superconducting wire, even in such a case, a metal tape is prepared separately from the superconducting wire. The thickness of the metal tape does not necessarily have to be constant and may be changed according to the position in the radial direction of the superconducting coil. Also, the metal tape does not necessarily have to be a single one, and a plurality of metal tapes may be arranged in the width direction of the superconducting wire or joined together in the longitudinal direction of the superconducting wire and used.

[0008] When a quench occurs in a superconducting wire, as a means of avoiding or suppressing thermal runaway, it is effective to suppress and bypass the current flowing through the quench portion. In the present invention, by using a metal tape, a bypass path can be secured as compared with the case of a single superconducting wire. Further, since the resistance value of the metal tape of the present invention is higher than the resistance value when the superconducting layer is in the superconducting state, in the superconducting state, the current flowing through the superconducting wire is not inhibited. On the other hand, when the superconducting layer is in the normal conducting state, the resistance value of the metal tape becomes lower, so the current will be bypassed to the metal tape. Further, since the metal tape is along the superconducting layer, it is also possible to effectively cool the superconducting layer through the metal tape. Thus, in the present invention, by using a metal tape, it is possible to effectively avoid or suppress thermal runaway due to quenching.

[0009] In Patent Document 1, which is a prior art, a technique of bringing a plate-shaped conductive member into contact with the upper surface or the lower surface of a pancake-type superconducting coil is disclosed. However, in this method, since the conductive member only contacts the side surface of the wire forming the superconducting coil, the bypass path becomes very narrow. In contrast, in the present invention, since the metal tape contacts the plane of the superconducting wire, the area of the bypass path can be sufficiently secured, and it becomes possible to effectively avoid or suppress thermal runaway.

[0010] The further significance of using the metal tape will be explained. For the purpose of securing the bypass path described above, methods such as thickening the metal plating layer on the outer periphery of the superconducting wire can also be considered. However, generally, thickening the plating layer has a problem that the cost increases very much. Further, when the plating layer of the superconducting wire is thickened, problems also occur in that the flexibility and flexibility of the superconducting wire itself are impaired, and it becomes difficult to wind it as a coil. In comparison, in the present invention, by separately providing a metal tape, these problems can be solved together.

[0011] In the present invention, The metal tape may be made of any one of copper, aluminum, and indium.

[0012] By doing so, it is possible to realize an electrical resistance in a range that is higher than the resistance value when the superconducting layer is in the superconducting state and lower than the resistance value when the superconducting layer is in the normal conducting state.

[0013] In the present invention, the superconducting wire and the metal tape may be laminated so as to be slidable.

[0014] By doing so, there is an advantage that it can be wound without impairing the flexibility and flexibility of the superconducting wire. Further, when heat is generated due to a quench, both the superconducting wire and the metal undergo thermal expansion. However, if the thermal expansion coefficients of the two are different, thermal stress is generated between the two, and there is also a concern that the superconducting coil may be damaged. On the other hand, as described above, if the metal tape is laminated slidably, the difference in thermal expansion coefficient is absorbed by the sliding of the superconducting coil and the metal tape, so that the generation of stress can be avoided or suppressed.

[0015] In addition, in order to effectively realize a bypass when a quench occurs, it is preferable that the superconducting wire and the metal tape are in sufficient contact. From this point of view, in the present invention, it is desired to wind them sufficiently tightly so as not to generate a gap between the two.

[0016] In the present invention, a metal plating layer is laminated on the superconducting wire in addition to the superconducting layer, and the thickness of the metal tape may be made thicker than the thickness of the plating layer.

[0017] By doing so, it becomes possible to sufficiently secure a bypass path by the metal tape.

[0018] In the present invention, the width of the metal tape may be equal to or greater than the width of the superconducting wire.

[0019] By doing so, the metal tape can be surely aligned along the entire width direction of the superconducting wire, so that thermal runaway can be suppressed even if a quench occurs at any position in the width direction. Further, by securing the cross-sectional area of the metal tape in the width direction, there is also an advantage that the resistance of the bypass path when a quench occurs can be suppressed.

[0020] In the present invention, The insulating layer may be formed around the metal tape and the superconducting wire after they are laminated.

[0021] By doing so, it is possible to avoid a short circuit when winding. According to Patent Document 1 which is a prior art, since it is in contact with a plate-shaped conductive member, the upper surface or the lower surface of the pancake coil cannot be insulated. On the other hand, according to the present invention, since the entire outer periphery of the superconducting wire can be insulated, it is possible to effectively suppress a short circuit or the like in the superconducting coil body.

[0022] In the present invention, The superconducting layer may be formed of a high-temperature superconducting material.

[0023] The high-temperature superconducting material generally refers to a substance having a critical temperature higher than 77 Kelvin. In a high-temperature superconducting material, generally, it is difficult to detect a crunch, and there is a problem that when a quench can be detected, the normal conducting portion expands to such an extent that burnout occurs. In the present invention, it is particularly useful in a high-temperature superconducting material in that when a quench occurs, it can be avoided or suppressed from spreading.

[0024] The present invention is composed of any one of the superconducting coil bodies described above, and a superconducting coil including a cooling mechanism for cooling one or both of the upper and lower surfaces of the superconducting coil body with a refrigerant.

[0025] By doing so, both the superconducting wire and the metal tape can be effectively cooled. In the present invention, when a quench occurs, the bypassed current flows through the metal tape, causing the metal tape to generate heat. However, by providing the cooling mechanism described above, such heat generation can be suppressed. The cooling mechanism can be structured, for example, to provide a refrigerant flow path on one or both of the upper and lower surfaces, i.e., the surfaces with the central axis of the superconducting coil as the normal. The flow path can be formed in various modes such as the radial direction and the circumferential direction. As the refrigerant, liquid helium, liquid hydrogen, liquid nitrogen, etc. can be considered.

[0026] In the superconducting coil of the present invention, the superconducting layer is formed of a high-temperature superconducting material, the refrigerant may be liquid hydrogen.

[0027] In the case of using liquid helium as the refrigerant for high-temperature superconductivity, when a quench occurs and the temperature rises, it is likely to easily reach so-called film boiling, and a sufficient cooling effect may not be obtained. In contrast, in the case of liquid hydrogen, it has the ability to sufficiently cool the heat generated in the superconducting wire and the metal tape due to a quench. Therefore, in high-temperature superconductivity, by using liquid hydrogen, thermal runaway can be suppressed or avoided, and the cooling stability of the superconducting coil can be realized.

[0028] The present invention does not necessarily have to include all of the above-described features, and may be configured by appropriately omitting or combining some of them. Further, the present invention can be configured in various modes such as a superconducting coil body or a manufacturing method of a superconducting coil.

Brief Description of the Drawings

[0029]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0030] Hereinafter, the superconducting coil as an embodiment of the present invention will be described. FIG. 1 is an explanatory diagram showing the overall configuration of the superconducting coil in the embodiment. The superconducting coil 1 includes a pancake-type superconducting coil body 2 around which a coil wire 10 is wound, cooling plates 3 sandwiching the upper and lower surfaces thereof, and a spacer 4 attached to the cooling plate 3 to form a flow path for the refrigerant. (For convenience of explanation, the upper and lower surfaces refer to the upper and lower surfaces in FIG. 1, that is, the surfaces having the central axis of the pancake-type coil as the normal line are referred to as the upper and lower surfaces.) The cooling plate 3 is provided to cool the superconducting coil body 2 by a refrigerant, and can be composed of a thin metal plate having a high thermal conductivity or the like. In this embodiment, as shown by the arrow in the figure, liquid hydrogen as a refrigerant flows from the outer peripheral side toward the center. The spacer 4 can be provided in various shapes so that cooling by the refrigerant is performed efficiently. In the figure, an example of providing a plurality of spacers 4 radially is shown, but the structure is not limited thereto, and the spacer 4 may be provided to form a flow path in the circumferential direction. The spacer 4 may be similarly provided on the lower cooling plate 3. Note that the cooling plate 3 may be omitted, and the spacer 4 may be directly arranged on the upper and lower surfaces of the superconducting coil body 2 to form a flow path for the refrigerant.

[0031] FIG. 2 is an explanatory diagram showing the configuration of the coil wire. Fig. 2(a) shows a cross-sectional view of the coil wire 10. The coil wire 10 has a structure in which a superconducting wire 15 and a metal tape 18 are laminated, and its outer periphery is covered with an insulating layer 19. The superconducting wire 15 has a superconducting layer 12 about 3 - 4 micrometers formed on a substrate 11 about 50 micrometers, and copper plating layers 13, 14 about 20 micrometers are formed on both sides thereof. The overall thickness of the superconducting wire 15 is about 0.1 mm, and the width is 11 - 12 mm, but the thickness and width of the wire can be arbitrarily determined. As the metal tape 18, copper, aluminum, indium, etc. can be used. In this embodiment, copper is used. The thickness of the metal tape 18 can be arbitrarily determined, but can be about 0.2 mm. It is preferably thicker than the plating layers 13, 14 formed on the superconducting wire 15. The superconducting wire 15 and the metal tape 18 may be adhered or the like, but preferably have a structure in which they are simply wound along each other so that they slide. Fig. 2(b) shows the structure of the superconducting wire 15. On the substrate 11, an alumina layer 12a, a yttria layer 12b, an MgO layer 12c formed by ion beam assisted deposition (IBAD method), a HOMO-epiMgO layer 12d, a LaMnO 3 layer 12e, a yttrium-based superconducting layer 12f, and a silver layer 12g are laminated. On its upper surface, a copper plating layer 14 about 20 micrometers is formed. A copper plating layer 13 of 20 micrometers is also formed on the lower side of the substrate 11. The structure of the superconducting wire 15 is not limited to that shown in Fig. 2. Also, as the superconducting material, various materials such as bismuth-based oxides and MgB 2 etc. can be used in addition to yttrium-based oxides.

[0032] According to the structure of the superconducting coil body 2 of this embodiment, there are the following advantages. First, since the metal tape 18 is arranged along the superconducting wire 15, even if the superconducting wire 15 deteriorates due to damage or the like and superconductivity is partially lost, the current will flow by bypassing the metal tape 18, and the expansion of the quench can be avoided or suppressed. From this perspective, it is preferable to use a material for the metal tape 18 that has a higher electrical resistance than the superconducting wire 15 in the superconducting state and a lower electrical resistance than the superconducting wire 15 that has transitioned to normal conduction. In this embodiment, copper is used from this perspective. Although plating layers 13 and 14 are formed on the superconducting wire 15, they are extremely thin layers. Therefore, as the above-described bypass, the cross-sectional area is insufficient, the resistance heating increases, and the cross-sectional area for cooling cannot be ensured, so thermal stability cannot be realized. On the other hand, in this embodiment, by arranging the metal tape 18 along it, such problems can be alleviated and thermal stability can be realized. There is also an advantage in that the superconducting wire 15 and the metal tape 18 are arranged in a slidable state. When heat is generated, since the superconducting wire 15 and the metal tape 18 have different coefficients of thermal expansion, if the two are adhered or the like, stress may be generated due to the difference in the coefficient of thermal expansion at the contact surface or the like between the two. On the other hand, if the two are in a slidable state, the generation of such stress can be avoided or suppressed.

[0033] FIG. 3 is an explanatory diagram showing the cooling capacity by a refrigerant. FIG. 3(a) shows the change in resistivity with temperature. Curve C1 shows the change in copper, and curve C2 shows the change in silver. In both cases, it is shown that the resistivity decreases as the temperature decreases and reaches a minimum value at about 20K. FIG. 3(b) shows the change in the critical heat flux of the refrigerant and the temperature. Curve C11 represents liquid helium, curve C12 represents liquid hydrogen, and curve C13 represents liquid nitrogen. Focusing on the temperature range Ta of about 20K, the critical heat flux of liquid hydrogen (C12) is about 10 times that of liquid helium (C11). That is, liquid helium (C11) relatively easily enters the film boiling state and effective cooling cannot be achieved, while liquid hydrogen (C12) can maintain a sufficient cooling effect. In other words, even if heat is generated due to deterioration or the like in the superconducting coil, using liquid hydrogen as the refrigerant increases the possibility of realizing a state where thermal runaway in which superconductivity is lost due to heat generation is avoided or suppressed, that is, thermal stability. On the other hand, focusing on the temperature range Tb of about 90K, liquid nitrogen (C13) has the highest critical heat flux. However, in such a temperature range Tb, since the resistivity of the metal is high, heat generation also becomes large, so the possibility of realizing thermal stability is low. Thus, in the superconducting coil of this embodiment, the possibility of realizing thermal stability can be increased by using liquid hydrogen as the refrigerant.

[0034] FIG. 4 is an explanatory diagram showing the experimental results regarding the avoidance effect of thermal runaway of the superconducting coil in the embodiment. FIG. 4(a) shows the results when the superconducting coil of the embodiment is energized while being cooled with liquid nitrogen as the refrigerant. FIG. 4(b) shows the results when the same energization is performed on a conventional superconducting coil without a metal tape. For both superconducting coils, partial deterioration is caused by bending processing with a radius of about 3 mm. The curve C21 in FIG. 4(a) is the voltage value of the superconducting wire, and the curve C22 is the voltage value of the metal tape. Each shows the results of multiple experiments. As shown by the results of the curve C21, at about 66 amperes, as shown in the region a1, the voltage value of the superconducting wire has risen sharply, indicating that the electrical resistance has increased rapidly, that is, a transition to normal conduction has occurred. Also, at the same current value, as shown by the region a2 of the curve C22, the voltage value of the metal tape has also increased rapidly, indicating an increase in the current flowing through the metal tape. This is a phenomenon caused by an increase in the resistance of the superconducting wire, and it can be considered that a transition to normal conduction has also occurred in locations other than the deteriorated part.

[0035] Figure 4(b) shows, as a comparative example, the energization results of a conventional superconducting coil without a metal tape. The results of three experiments are shown. When energized in a cycle of increasing the current and then reducing it, the voltage value changed as indicated by the arrow in the figure. From this result, it can be seen that at about 52 amperes, the voltage value increased rapidly as shown in region b, indicating that a transition to normal conduction has occurred.

[0036] From the above results, it was confirmed that according to the superconducting coil of this embodiment, by arranging the metal tape along the superconducting wire, the current value at which the transition to normal conduction occurs can be improved from about 52 amperes (Figure 4(b)) to about 66 amperes (Figure 4(a)). That is, in the superconducting coil of this embodiment, due to the effect of the metal tape, it can be seen that even when exceeding about 52 amperes, thermal stability is achieved and energization can be carried out without the spread of quench.

[0037] According to the superconducting coil of the embodiment described above, by arranging the metal tape along the superconducting wire, thermal runaway due to quench can be avoided or suppressed. Also, by using liquid hydrogen as the refrigerant, it is also possible to achieve thermal stability. What is described in the embodiment is only an example of the present invention, and the present invention can further constitute various modifications. For example, in the embodiment, a pancake-type superconducting coil is shown, but the present invention is also applicable to other superconducting coils.

Industrial Applicability

[0038] The present invention can be used to avoid or suppress thermal runaway due to quench in a superconducting coil.

Explanation of Reference Numerals

[0039] 1 Superconducting coil 2 Superconducting coil body 3 Cooling plate 4 Spacer 10 Coil wire 11 Base material 12 Superconducting layer 13 and 14 plating layers 15 superconducting wire 18 metal tape 19 insulating layer

Claims

1. A superconducting coil body, comprising: a thin plate-shaped superconducting wire including a superconducting layer; and a thin plate-shaped metal tape that is prepared separately from the superconducting wire and has an electrical resistance in a range higher than the resistance value when the superconducting layer is in a superconducting state and lower than the resistance value when the superconducting layer is in a normal conducting state; The superconducting coil body is wound with the metal tape laminated on the superconducting wire.

2. The superconducting coil body according to claim 1, wherein the metal tape is made of any one of copper, aluminum, and indium.

3. The superconducting coil body according to claim 1, wherein the superconducting wire and the metal tape are laminated slidably.

4. The superconducting coil body according to claim 1, wherein a metal plating layer is laminated on the superconducting wire in addition to the superconducting layer, and the thickness of the metal tape is thicker than the thickness of the plating layer.

5. The superconducting coil body according to claim 1, wherein the width of the metal tape is equal to or greater than the width of the superconducting wire.

6. The superconducting coil body according to claim 1, wherein an insulating layer is formed around the laminated metal tape and superconducting wire.

7. The superconducting coil body according to claim 1, wherein the superconducting layer is formed of a high-temperature superconducting material.

8. A superconducting coil comprising the superconducting coil body according to any one of claims 1 to 7, and a cooling mechanism for cooling one or both of the upper and lower surfaces of the superconducting coil body with a refrigerant.

9. The superconducting coil according to claim 8, wherein the superconducting layer is formed of a high-temperature superconducting material, and the refrigerant is liquid hydrogen.

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