JP7614991B2 - Superconducting wire connection structure and connection method - Google Patents

Description

An embodiment of the present invention relates to a connection technology for superconducting wires that carry persistent current.

In equipment that uses superconductivity, such as MRI devices and NMR magnets, the generation of a stable magnetic field over long periods of time is required. In such equipment, a superconducting closed circuit is generally formed by a superconducting coil (main coil) that generates the main magnetic field, and a persistent current switch (PCS) in which a superconducting wire is wound non-inductively.

By forming such a superconducting closed circuit, the decay of the persistent current is suppressed and the stability of the magnetic field is maintained for a long period of time. To achieve this, the superconducting filaments in the main coil, or in the main coil and PCS, must be connected with an extremely low resistance. There are two main types of connection methods for such superconducting wires: switchback connection and wrap connection. A switchback connection is a method in which two superconducting wires are connected alongside each other so that the current turns back at the connection. A wrap connection is a method in which two superconducting wires are connected in a straight line without changing the direction of the current at the connection.

NbTi wire, which is widely used as a practical superconducting wire, has a structure in which multiple thin superconducting filaments are embedded in a stabilizing (matrix) metal in order to stably pass a persistent current. When connecting two superconducting wires to each other, it is ideal to have the superconducting filaments embedded in each wire directly contact each other over a wide area without any intervening material.

Methods for exposing such superconducting filaments include dissolving the matrix with acid. When the NbTi filaments exposed in this way are exposed to the atmosphere, an oxide layer forms on their surfaces, which causes an increase in resistance at the connections. For this reason, a conventional method has been to apply an external force to the superconducting connections to connect the filaments together, break the oxide layer on the surfaces, and bring the clean surfaces of the NbTi into contact with each other.

JP 2013-62210 A Japanese Patent Application Publication No. 5-152045

However, with the above-mentioned method of connecting superconducting wires, if the contact of the filaments is too weak, the contact resistance becomes excessive, and if the contact is too strong, the filaments will be constricted or broken, resulting in excessive contact resistance or even inability to pass electricity, making it difficult to control the connection conditions. In other words, when connecting two bundles of filaments, it is difficult to reliably achieve optimal connection conditions with high reproducibility that avoid the risk of poor contact or breakage.

The embodiment of the present invention has been made taking these circumstances into consideration, and aims to provide a superconducting wire connection technology that reliably achieves optimal superconducting filament connection conditions that minimize the resistance at the connection.

In a superconducting wire connection structure according to an embodiment, two superconducting wires each having a bundle of superconducting filaments are arranged with their tips aligned, the superconducting filaments exposed from each of the superconducting wires at the tip are electrically connected to each other via their side surfaces, a sleeve is attached to the connection portion of the superconducting filaments at the tip, and when the connection portion is viewed in cross section in order toward the tip, the inner region of the sleeve is formed so that the space factor of the superconducting filaments increases in the order , and fine fragments having the same composition as the superconducting filaments are arranged, thereby increasing the space factor of the superconducting filaments in the inner region of the sleeve .

Embodiments of the present invention provide a superconducting wire connection technology that reliably achieves optimal superconducting filament connection conditions that minimize resistance at the connection.

1A to 1D are explanatory diagrams of a method for connecting superconducting wires according to an embodiment of the present invention. 1A and 1B are cross-sectional views of a superconducting wire connection structure according to a first embodiment of the present invention. FIG. 6 is an explanatory diagram of a connection structure of superconducting wires according to a second embodiment. FIG. 11 is an explanatory diagram of a connection structure of superconducting wires according to a third embodiment.

First Embodiment
Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Figures 1(A) to 1(D) are explanatory diagrams of a method for connecting superconducting wires 10 (10a, 10b) according to a first embodiment of the present invention. Figure 2 is a cross-sectional view of a connection structure 20 (20a) for superconducting wires 10 according to the first embodiment, with Figure 2(A) being a vertical cross-sectional view and Figure 2(B) being a horizontal cross-sectional view.

In the method of connecting superconducting wires 10 (10a, 10b) according to the embodiment, first, as shown in FIG. 1(A), two superconducting wires 10 (10a, 10b) each consisting of a bundle of superconducting filaments (hereafter abbreviated as filaments) 11 are arranged with their tips aligned. Next, as shown in FIG. 1(B), the filaments 11 exposed from each superconducting wire 10 at their tips are electrically connected via their respective side surfaces. Furthermore, a sleeve 15 is attached to the connection portion 17 of the filaments 11 at their tips as shown in FIG. 1(C).

As shown in FIG. 1(D), in the connection structure 20a of the superconducting wire 10, the sleeve 15 is compressed in one direction in cross section so that the compression ratio increases continuously toward the tip. As a result, when the connection portion 17 is cross-sectionally viewed in the order of (III), (II), and (I) toward the tip, the sleeve 15 is formed in the order of (III), (II), and (I) so that the void ratio in its internal region is low, i.e., the space factor of the filament 11 is high, as shown in FIG. 2(A). Note that an insulating coating (not shown) is applied to the outer periphery of the superconducting wire 10, but the superconducting wire 10 shown in the figure shows a state in which this insulating coating has already been removed.

In this way, the embodiment is suitably applied to a switchback connection in which two superconducting wires 10 (10a, 10b) are connected alongside each other so that the persistent current 16 turns back at the connection 17. In the first embodiment, an external force is applied from one direction in the radial direction of the sleeve 15 attached to the connection 17, and the sleeve 15 is compressed until the cross section of the connection 17 becomes flat at the tip side, and the compression rate increases toward the tip side.

The superconducting wire 10 (10a, 10b) has a large number of superconducting filaments 11, which have a relatively high electrical resistivity in the normal conducting state, embedded in a normal conducting matrix 12. The matrix 12 of the superconducting wire for the superconducting coil (main coil) that generates the main magnetic field is made of oxygen-free copper, which has a low electrical resistivity. The matrix 12 of the superconducting wire for the persistent current switch (PCS) is made of a copper alloy, which has a higher electrical resistivity than oxygen-free copper. The reason for increasing the electrical resistivity of the matrix 12 of the PCS wire in this way is that a sufficient electrical resistance value is required when the PCS is in the OFF state.

The end of the superconducting wire 10 (10a, 10b) has the matrix 12 removed or replaced with another superconducting (or low electrical resistance) metal at the connection 17. One method for removing the matrix 12 is to selectively dissolve the matrix 12 with nitric acid or the like. Another method for replacing the matrix 12 is to immerse the wire in a bath of molten metal (e.g., Sn) that dissolves Cu or a Cu alloy. There is also a secondary replacement method in which the Sn-substituted portion is immersed in a bath of another molten metal (e.g., an alloy containing Sn, Bi, Bi, etc. (so-called solder)).

In this way, by replacing it with Sn or another solder alloy, the surface of the filament 11 is not exposed to the atmosphere, and an oxide film is not formed. And if an oxide film is not formed on the surface of the filament 11 in this way, and it is simply covered with Sn or another solder alloy, it becomes easy to connect the filament 11 with a clean surface without setting the compression ratio of the sleeve 15 so high that the filament is excessively plastically deformed.

The connection portion 17 (FIG. 1B) may not only bring the strands of the filament 11 close to each other, but may also twist the bundles of filaments 11 together, or may divide the filament 11 into multiple bundles and twist them together. If the Cu or Cu alloy matrix 12 is replaced with another filler metal (solder), the connection portion 17 may be heated when the sleeve 15 is compressed to push out the filler metal from around the filament 11, thereby further increasing the space factor of the filament 11 at the tip side.

As shown in FIG. 2(A), when the compression rate of the sleeve 15 is at a low level (III), adjacent filaments 11 are in contact with each other on their lateral surfaces. However, the space factor of the filaments 11 in the internal region of the sleeve 15 is low, and the contact state of the filaments 11 in the longitudinal direction is not necessarily uniform. For this reason, the electrical resistance of the connection 17 located in the sleeve 15 at a low compression rate (III) is high, and it can be said to be a poor contact.

When the compression rate of the sleeve 15 progresses to the intermediate level (II), the space factor of the filaments 11 in the internal region of the sleeve 15 increases. Accordingly, adjacent filaments 11 undergo elastic or plastic deformation, expanding the contact area and reducing the electrical contact resistance. At this time, even if an oxide film is formed on the surface of the filament 11, this oxide film breaks, allowing the clean metal surfaces of the filaments 11 to come into contact with each other. Therefore, the electrical resistance is low and the contact is good at the connection part 17 located in the sleeve 15 with an intermediate level of compression (II).

Furthermore, when the compression rate of the sleeve 15 progresses to a high level (I) state, the space factor of the filaments 11 in the internal region of the sleeve 15 increases. Accordingly, the filaments 11, which are already in contact over the entire area, undergo further plastic deformation, and the contact interface stretches further, resulting in constriction or breakage. For this reason, the electrical resistance is high at the connection 17 located in the sleeve 15, which has a high compression rate (I), and it can be said to be a poor contact.

As a result, as shown in the horizontal cross section of FIG. 2B, in the connection portion 17, the compression ratio of the sleeve 15 increases continuously toward the tip, and accordingly, the space factor of the filament 11 also increases continuously. This ensures that the connection portion 17 has a location where the electrical resistivity between the pair of filaments 11, 11 is close to zero, and where the persistent current 16 is folded back and allowed to pass through without attenuation.

Second Embodiment
Next, a second embodiment of the present invention will be described with reference to Fig. 3. Fig. 3 is an explanatory diagram of a connection structure 20 (20b) of a superconducting wire 10 according to the second embodiment. In Fig. 3, parts having the same configuration or function as Fig. 1 are indicated by the same reference numerals, and duplicated explanations will be omitted.

In the connection structure 20b according to the second embodiment, two superconducting wires 10 are arranged with their tips aligned, and at these tips the filaments 11 are electrically connected via their side surfaces. A sleeve 15 is attached to the connection portion 17 of the filament 11, and when the connection portion 17 is cross-sectionally viewed in the order (III), (II), and (I) toward the tip, the internal region of the sleeve 15 is formed so that the space factor of the filament 11 increases in this order.

In the connection structure 20b of the second embodiment, the sleeve 15 is compressed from the entire circumference in a cross-sectional view, and the space factor of the filament 11 in the internal region of the sleeve 15 is increased in three stages toward the tip. However, this is not limited to this embodiment, and the sleeve 15 may be compressed from two or more directions in a cross-sectional view, and the space factor may be increased in three or more stages toward the tip.

It is also possible to use solder or the like as the filler 18 in the internal region of the sleeve 15, heat it to a molten state, and then attach it to the connection portion 17 of the filament 11. If molten Sn replacement or secondary replacement with another solder alloy is used as a method for removing the matrix 12, the solder alloy that has been previously applied so as to cover the surface of the filament 11 is configured as the filler 18. In the second embodiment, the compression of the sleeve 15 can be performed in stages, for example, by using a swaging machine. By using such a compression method, the filler 18, which is softer than the filament 11, undergoes plastic flow, and the space factor of the filament 11 in the internal region of the sleeve 15 increases.

Third Embodiment
Next, a third embodiment of the present invention will be described with reference to Fig. 4. Fig. 4 is an explanatory diagram of a connection structure 20 (20c) of a superconducting wire 10 according to the third embodiment. In Fig. 4, parts having the same configuration or function as Fig. 1 are indicated by the same reference numerals, and duplicated explanations will be omitted.

The connection structure 20c according to the third embodiment has two superconducting wires 10 arranged with their tips aligned, and the filaments 11 electrically connected to each other at their tips via their side surfaces. A sleeve 15 is attached to the connection portion 17 of the filament 11, and the internal region of the sleeve 15 is formed so that when the connection portion 17 is cross-sectionally viewed in the order (IV), (III), (II), and (I) toward the tip, the space factor of the filament 11 increases in this order.

In the connection structure 20c of the third embodiment, the sleeve 15 has fine fragments 19 of the same composition as the filament 11 arranged therein, thereby increasing the space factor of the filament 11 in the internal region of the sleeve 15. Note that while the illustration shows an example in which the sleeve 15 is not compressed in the radial direction, it may be compressed in the radial direction with the fine fragments 19 arranged in the sleeve 15. Alternatively, it may be compressed in one axial direction or in two or more directions.

In the above-mentioned embodiments, a filler 18 that has superconductivity at extremely low temperatures, such as Pb-Bi solder, can be used. As a result, the filler 18 interposed between the filaments 11 inside the sleeve 15 does not become an element of electrical resistance, and acts to reduce the overall resistance value of the connection part 17. Furthermore, when the filler 18 is compressed at a high compression ratio so that the space factor of the filament 11 at the tip side is increased, a transition is introduced into the solder structure, which acts to increase the pinning centers of the superconductor.

Furthermore, by rapidly cooling and solidifying the superconducting alloy composition filler 18 from a molten state, the Bi structure that melts and solidifies can be refined, and the magnetic field characteristics of the critical current density of Pb-Bi can be improved. This reduces the resistance value at the connection 17, and a connection structure for superconducting wires with a large current carrying capacity can be obtained.

According to at least one embodiment of the superconducting wire connection structure described above, the filament space factor in the inner sleeve region when viewed in cross section of the connection increases sequentially toward the tip, thereby reliably achieving optimal filament connection conditions with an extremely small resistance value at the connection.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. These embodiments and their modifications are within the scope of the invention and its equivalents as set forth in the claims, as well as the scope and spirit of the invention.

10...superconducting wire, 11...superconducting filament (filament), 12...matrix, 15...sleeve, 16...persistent current, 17...connection, 18...filler, 19...fine pieces, 20 (20a, 20b, 20c)...connection structure.

Claims (8)

Two superconducting wires, each consisting of a bundle of superconducting filaments, are arranged with their tips aligned.
the superconducting filaments exposed from each of the superconducting wires at the tip are electrically connected to each other via their side circumferential surfaces;
a sleeve is attached to the connection portion of the superconducting filament at the tip;
when the connection portion is viewed in cross section in order toward the tip, the inner region of the sleeve is formed so that the space factor of the superconducting filaments increases in the order ,
A connection structure for superconducting wires, in which fine pieces having the same composition as the superconducting filaments are arranged, thereby increasing the space factor of the superconducting filaments in the inner region of the sleeve . 2. The superconducting wire connection structure according to claim 1,
A connection structure for superconducting wires, wherein the sleeve is compressed in one direction in the cross-sectional view, and the compression ratio increases continuously toward the tip. 2. The superconducting wire connection structure according to claim 1,
The sleeve is compressed from multiple directions in the cross-sectional view, and the compression ratio increases stepwise toward the tip. The superconducting wire connection structure according to any one of claims 1 to 3 ,
The superconducting wire has a matrix in which the bundle of superconducting filaments is embedded and which contains Cu as a composition,
A connection structure of a superconducting wire, in which the matrix having a composition of Cu is removed at the connection portion. The superconducting wire connection structure according to claim 4 ,
A connection structure of a superconducting wire in which, by immersing in molten Sn, the matrix containing Cu in the connection portion is removed and replaced with a metal containing Sn in its composition, or the metal containing Sn in its composition is further replaced with another metal. The superconducting wire connection structure according to claim 4 or 5 ,
A connection structure for superconducting wires, wherein the sleeve is filled with a filler material having a superconducting alloy composition. 7. The superconducting wire connection structure according to claim 6 ,
A connection structure for superconducting wires, wherein the filler material is rapidly cooled from a molten state and solidified. A step of arranging two superconducting wires each having a bundle of superconducting filaments with their tips aligned;
a step of electrically connecting the superconducting filaments exposed from each of the superconducting wires at the tip end via their side circumferential surfaces;
attaching a sleeve to the connection of the superconducting filament at the tip;
forming an inner region of the sleeve such that, when the joint is viewed in cross section in order toward the tip, the space factor of the superconducting filaments increases in the order;
and increasing a space factor of the superconducting filaments in an inner region of the sleeve by arranging fine pieces having the same composition as the superconducting filaments .

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