JP3330267B2 - Superconducting connection structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure for superconducting connection of a metal superconducting wire or an oxide superconducting wire with a low melting point alloy interposed therebetween. The present invention relates to a superconducting connection structure in which central magnetic field attenuation is not generated.

[0002]

2. Description of the Related Art The superconductivity phenomenon has been widely used in various fields such as large current transmission and high magnetic field generation equipment, taking advantage of the feature that a large current can flow with zero resistance. In particular, superconducting magnets used in high-resolution NMR analyzers generate a strong magnetic field due to high-current dielectric and perform permanent current operation using zero resistance. This is a typical application that can be realized for the first time by using superconductivity. is there.

Conventionally, metal superconducting wires such as NbTi, Nb ₃ Sn, and Nb ₃ Al have been known as superconducting wires used as materials for superconducting magnets. These metal superconducting wires are used in MRI medical diagnostic devices, It has been applied to various devices that require a high-precision stable magnetic field, such as a high-resolution NMR apparatus. In these cases, in order to effectively utilize the above-described features, the superconducting wires used in the device are connected to each other while maintaining the superconducting state, so that a permanent current flows in a loop. In recent years, a technique for forming a superconducting magnet using an oxide-based superconductor wire has also been attempted. In order to operate these superconducting magnets in the permanent current mode, it is necessary to connect metal superconducting wires or oxide superconducting wires, or connect a metal superconducting wire and an oxide superconducting wire in a superconducting state. Will be needed.

[0004] Regardless of the metal-based superconducting wire and the oxide-based superconducting wire, from the viewpoint of increasing the critical current density, the wire is generally configured to include a large number of filament groups inside the wire. However, methods for connecting these wires can be roughly classified into the following two methods. That is, (1) a method of directly connecting the filament of the superconducting wire to be connected to the filament by a technique such as welding; It is a method of making a connection.

The former method aims at perfect connection, but has few successes in connection and has drawbacks such as brittle connection parts. In particular, this method cannot be performed when the connected wire is a brittle material such as an oxide-based superconducting wire or a metal-based superconducting wire such as Nb ₃ Sn.

The latter method is disclosed, for example, in JP-A-6-2957.
As disclosed in Japanese Patent No. 53, there is an advantage that the connection can be made easily and in a short time by interposing a low melting point alloy containing Pb or the like as a main component, but there is a disadvantage that the critical current value is lowered. This is because the critical current density of the low melting point alloy is one of that of NbTi, which is a kind of metallic superconductor.
This is because it is extremely low at about / 100. Therefore, when a superconducting wire filament and another superconducting wire filament are brought into contact with a low-melting alloy in the same area as the cross-sectional area of the filament to mediate current, the low-melting alloy portion becomes a rule, The critical current density will be limited.
Therefore, from the viewpoint of avoiding such inconvenience, the length of the exposed filament group is made as long as possible,
In general, the critical current density is improved by increasing the contact area between the filament and the low melting point alloy as much as possible.

[0007]

When the superconducting magnet is operated in the permanent current mode, there is a problem that the central magnetic field of the magnet is attenuated (hereinafter sometimes referred to as "drift"). This is due to the fact that the current flowing in the closed circuit composed of the superconducting magnet and the superconducting wire is attenuated by various factors. One of such factors is the non-uniformity of the transport current distribution inside the superconducting wire. The non-uniformity of the transport current distribution occurs when direct current is applied to the superconducting wire. That is,
When direct current is applied, the transport current flows only outside the filament group due to the magnetic field generated by the superconducting wire itself,
It does not flow near the center. Such a phenomenon is generally called a self-magnetic field effect. When the permanent current mode is set while this phenomenon is occurring, the transport current becomes uniform, so the transport current distribution accompanied by magnetic flux creep progresses,
This causes the drift as described above.

[0008] When the superconducting magnet is operated in the permanent current mode, the quality of the superconducting connection is extremely important for the drift. The factor that determines the performance of the superconducting connection is not only the resistance of the superconducting connection part, but also the uniformity of the distribution of the transport current flowing into the superconducting filament group in the superconducting connection.

In the case where Nb ₃ Sn is superconductively connected using a low melting point alloy, for example, no current flows through the filaments inside the filaments exposed in the connection region, and almost no current flows through the filaments outside the filaments. Will flow in. In order to cope with such a phenomenon, the critical current density of the filament increases due to the small external magnetic field near the superconducting connection portion, and the filament can be sustained only by energizing only the filament outside the filament group. However, in such a state, even if the non-uniform current distribution due to the self-magnetic field effect described above has progressed toward a uniform transport current distribution due to magnetic flux creep, the uniformity of the transport current distribution near the connection part Will not progress.

In order to prevent the above-mentioned phenomenon, various measures have been taken to prevent the current distribution inside the superconducting connection from becoming uneven. For example, Japanese Patent Application Laid-Open No. 60-157167 discloses a technique in which NbTi filaments exposed to each other are twisted one by one and further impregnated with a low melting point alloy. Also, Japanese Patent Application Laid-Open No. 3-194874 proposes that the exposed length of the superconducting filament is inclined so that both short wires are joined in order. However, each of these methods is based on the assumption of a filament having good workability like NbTi filament, and is hard and brittle like Nb ₃ Sn, Nb ₃ Al, or oxide superconductor. It has the disadvantage that it cannot be applied to filaments.

The present invention has been made under such a circumstance, and has an object to be applied to a hard and brittle filament, to achieve a uniform distribution of transport current flowing into a superconducting wire in a superconducting connection, An object of the present invention is to provide a superconducting connection structure capable of effectively preventing current attenuation when applied to the operation of a superconducting magnet in a current mode.

[0012]

The present invention, which has achieved the above object, has a structure in which a metallic superconducting wire or an oxide superconducting wire is superconductively connected with a low melting point metal interposed therebetween. In addition to exposing the superconducting filament group at the end of the connected wire rod, the superconducting filament group is formed and connected in a mountain shape so that all or a part of the outer peripheral filaments of the superconducting filament group are shortened. It is a superconducting connection structure.

In the above-described superconducting connection structure, when the metal-based superconducting wire includes a superconducting filament group made of Nb ₃ Sn or Nb ₃ Al,
The effects of the present invention are most effectively exhibited. Specific examples of the mountain shape include a cone shape and a pyramid shape, and a double-edge shape.

When the metal-based superconducting wire has a diffusion barrier layer on the outer periphery of the superconducting filament group, when the exposed end of the superconducting filament group is formed into a double-edged mountain shape, the double-edged type is used. The two surfaces may be inclined surfaces, and the remaining two surfaces may leave the diffusion barrier layer. Further, as the low melting point alloy used in the present invention, a Pb-Bi alloy or a wood metal can be suitably used.

[0015]

DETAILED DESCRIPTION OF THE INVENTION As described above, the present invention relates to a superconducting structure in which a metal superconducting wire or an oxide superconducting wire is connected with a low melting point alloy interposed therebetween, and is also applicable to an oxide superconducting wire. However, the following mainly describes the metal-based superconducting wire.

The present invention is constructed as described above. In short, in order to obtain a good superconducting connection, Nb ₃ Sn or Nb _3
3 When exposing a brittle filament group such as Al or the like, if the whole outer filament of the exposed filament group is formed into a mountain shape so that all or a part of the outer filament is shortened, the problem as in the prior art can be solved. It has been found that a good superconducting connection can be achieved without causing such a problem, and the present invention has been completed.

A specific procedure for forming the filament group into a mountain shape is as follows, for example. First, Nb ₃ Sn
An end portion of the wire before heat treatment for producing Nb ₃ Al or the like is embedded with an embedding material such as a resin and polished at an angle.
At this time, the filament group is polished together with the outer matrix material and the embedding material, and a wide cross section is exposed. Next, it is further embedded with an embedding material and polished from another angle to obtain an oblique cross section.

For example, in the case of a flat wire, FIG.
As shown in (a), the filament group was exposed as it is. According to the connection structure of the present invention, for example, in the case of a wire rod of a type in which external stabilizing copper (diffusion barrier layer) is embedded on the outer peripheral side. As shown in FIG. 1 (b), leaving the diffusion barrier layer on the short side and polishing the two sides on the long side to expose the surface of the filament group (filament surface),
It is a double-edged mountain type. When there is no diffusion barrier layer, as shown in FIG. 1 (c), the entire outer peripheral filaments of the filament group are shortened, and the filament groups are polished so that all four planes are filament planes. It may be mountain-shaped.

On the other hand, similarly, in the case of the round wire, the filament group is conventionally exposed as shown in FIG. 2A, but as shown in FIGS. 2B to 2D, for example. Then, it is formed and exposed to various shapes. FIG. 2 (b)
FIG. 2 shows a case where two surfaces are polished while leaving a diffusion barrier layer.
FIG. 2C shows the case where all four surfaces are polished to form a pyramid-shaped mountain, and FIG. 2D shows the case where the filament group is polished in a conical shape.

After the end of the filament group exposed as described above is subjected to the formation heat treatment, for example, as shown in FIG. 3, wood metal (Pb-Sn-Bi-Cd-based alloy)
Immersed in a low melting point alloy bath together with the other wire ends, and connected.

The principle of the superconducting connection structure of the present invention will be described in more detail. Here, a superconducting connection between an externally stabilized copper-type Nb ₃ Sn wire having a filament region of 2 × 1 (mm) in cross section and an NbTi wire is assumed. The low melting point alloy used for the connection has a critical current density at 1 T (tesla) of 5 × 10 ⁶ A / m ² . It is also assumed that the rated current is 250 A. Further, the contact area between the NbTi filament and the low melting point alloy is sufficiently larger than the contact area between Nb ₃ Sn and the low melting point alloy.
In fact, this is because NbTi filaments are soft and can be unraveled. If the length of the exposed filament group is 50 mm,
When processed as shown in FIG. 1B, the contact area becomes about 200 mm ² .

According to the present invention, Nb ₃ Sn or Nb ₃ Al
By forming such filaments into a mountain shape, each filament in the filament group comes into contact with the low melting point alloy with a uniform contact area from the central part to the peripheral part. Therefore, the critical current density Jc on the straight line (a) of the wire whose cross section is shown in FIG. 4 is, for example, as shown by the line (i) in FIG. At this time, the critical current value of the superconducting connection itself is limited by the critical current density of the low melting point alloy and the contact area between Nb ₃ Sn and the low melting point alloy.
Is 1000 A.

On the other hand, in the case of the conventional shape (FIG. 1A) in which the mountain-shaped processing is not performed, the surface area of the filament group is about 300 mm ² . Therefore, the critical current value of the entire connection portion is 1500 A. However, Nb ₃
Assuming that the diameter of the Sn filament is 5 μm, current flows only into the filament at the outer peripheral portion (a region of about 1.5% in volume) of the filament group. Accordingly, the critical current density in the straight line (a) in FIG. 4 is as shown in line (j) in FIG.

As described above, the unevenness of the transport current distribution due to the self-magnetic field effect is gradually made uniform by the flux creep. However, in the conventional connection method, the distribution of the transport current J at the end of the magnet is extremely uneven, so that
For example, as shown in FIG. 6, a phenomenon occurs in which the transport current J flows into the center from the end of the magnet to a certain length. On the other hand, when the connection structure of the present invention is employed, a uniform distribution of the transport current J is obtained from the end as shown in FIG. The dotted arrows shown in FIGS. 6 and 7 indicate the flow of current in the superconducting magnet, and the flowing portion of the superconducting magnet is a straight line formed by spirally winding the superconducting wire in the superconducting magnet. It is assumed that the state is extended. That is, the end of the magnet indicates a portion close to the superconducting connection, and the center of the magnet indicates a portion which is actually spirally wound.

Calculation of the normal conduction resistance value of the matrix when the transport current flows into the central part when the conventional connection is adopted is as follows. For example, FIG. 8 shows a schematic diagram of a superconducting magnet, and FIG. 9 shows an equivalent circuit thereof. The electric resistance R of this equivalent circuit is generally described as in the following equation (1). R = ρ (l / S) (1) where ρ: electric resistivity of the matrix l: length of a conductor generating resistance S: cross-sectional area of a conductor generating resistance

As shown in FIG. 8, the cross-sectional width of the superconducting wire is w, the cross-sectional height of the superconducting wire is t, the length of the region where the inflow occurs in the wire is d, and the matrix of the superconducting wire is occupied. If the ratio is λ, l = w / 2, S =
Since d · t · λ, the above equation (1) can be transformed into the following equation (2). R = ρ {(w / 2) / (d · t · λ)} (2) where d = 1 m, t = 1
Assuming that mm and w = 2 mm, R is obtained by the following formula. R = (1.6 × 10 ⁻¹⁰ ) × {(1 × 10 ⁻³ ) / (1 × 1 × 10 ⁻³ × 0.5)} = 3.2 × 10 ⁻¹⁰ (Ω) That is, the present invention In the superconducting connection structure described above, the above-described electric resistance can be cut.

The superconducting wire of the present invention is not limited to a metal-based superconducting wire as described above, but may be an oxide-based superconducting wire. Superconducting wire, Y-based oxide superconducting wire, and the like. Examples of the low melting point alloy used in the present invention include a Pb-Bi alloy in addition to the above-mentioned wood metal.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention, and any design change in the spirit of the present invention will be described. Included in the technical scope.

[0029]

FIG. 10 is a circuit diagram of a superconducting magnet system to which the superconducting connection of the present invention is applied. At this time, the inductance of the superconducting magnet is 170H (Henry). Further, the superconducting wire is described in "Embodiments of the Invention"
What was shown by the item of was used.

First, a heater for a permanent current switch (PSC)
The heater was turned on, and a current was supplied from a power supply. Then, when the current reached the target current value of 250 A, the switch of the PSC heater was turned off. After the transport current distribution inside the wire settled down, the magnetic field attenuation of the superconducting magnet was measured with an NMR magnetometer.

An experiment using a conventional connection for the Nb ₃ Sn portion of the superconducting connection shown in FIG. 10 is shown as (A), and an experiment using the above-described superconducting connection of the present invention is shown as (B). In the experiment (B) of the present invention, both ends of Nb ₃ Sn are shown in FIG.
It processed into the shape shown to (b). Table 1 below shows the measurement results of the center magnetic field attenuation in each of the experiments (A) and (B). The experiment was performed three times each. As is evident from Table 1, the drift using the superconducting connection of the present invention can be reduced by about 40%.

[0032]

[Table 1]

[0033]

As described above, according to the present invention, the central magnetic field attenuation of the superconducting magnet in the permanent current mode can be greatly reduced as compared with the conventional superconducting connection structure. In devices such as NMR, it is common that 10 to 20 superconducting connections are used, but if all of these superconducting connections employ the superconducting connection structure of the present invention, the characteristics thereof will be significant. Improvement can be expected.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing various shapes of a wire end portion of a rectangular wire used for superconducting connection.

FIG. 2 is a schematic explanatory view showing various shapes of a wire end portion of a round wire used for superconducting connection.

FIG. 3 is a longitudinal sectional view showing a superconducting connection structure of the present invention.

FIG. 4 is a cross-sectional view of a superconducting wire.

FIG. 5 is a graph showing a current distribution example of a superconducting wire.

FIG. 6 is a schematic explanatory view showing a transport current distribution of a superconducting magnet whose both ends are connected by a conventional connection structure.

FIG. 7 is a schematic explanatory view showing a transport current distribution of a superconducting magnet whose both ends are connected by the connection structure of the present invention.

FIG. 8 shows a state in which a transport current flows into a central portion.
It is a schematic diagram of the superconducting magnet for calculating the normal conduction resistance value of a matrix.

FIG. 9 is an equivalent circuit of the superconducting magnet shown in FIG.

FIG. 10 is a circuit diagram of a superconducting magnet system to which the superconducting connection structure of the present invention is applied.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoichi Mizomata 1-5-5 Takatsukadai, Nishi-ku, Kobe City, Hyogo Prefecture Kobe Steel, Ltd. Kobe Research Institute (72) Inventor Ryuichi Hirose Kobe, Hyogo Prefecture 1-5-5 Takatsukadai, Nishi-ku, Kobe Examiner, Koto Steel Works, Ltd. Kobe Research Institute, Masato Matsunawa (56) References JP-A-3-194874 (JP, A) JP-A-59-18584 (JP, A) Japanese Utility Model 60-187464 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01R 4/68

Claims (7)

(57) [Claims] 1. A superconducting connection between a metal-based superconducting wire and an oxide-based superconducting wire with a low-melting-point alloy interposed therebetween, wherein at least one of the superconducting filaments at the end of the connected wire is exposed. At the same time, an end portion formed and exposed in a mountain shape such that all or a part of the outer peripheral side filament of the superconducting filament group is shortened.
Immersed in a low-melting alloy bath and connected. 2. The superconducting connection structure according to claim 1, wherein the metal superconducting wire includes a superconducting filament group made of Nb ₃ Sn or Nb ₃ Al. 3. The superconducting connection structure according to claim 1, wherein the mountain shape is formed in a conical shape or a pyramid shape. 4. The superconducting connection structure according to claim 1, wherein the mountain shape is formed as a double-edge shape. 5. When the metal-based superconducting wire has a diffusion barrier layer on the outer periphery of a superconducting filament group, when the end of the superconducting filament group to be exposed is formed into a mountain shape, a double-edged blade is used. 5. The superconducting connection structure according to claim 4, wherein the two surfaces are inclined surfaces, and the remaining two surfaces are left with a diffusion barrier layer. 6. The low melting point alloy is a Pb-Bi alloy
The superconducting connection structure according to any one of claims 1 to 5. 7. The low melting point alloy is wood metal
A superconducting connection structure according to claim 1.