JP2859953B2 - Superconducting device and permanent current switch used for the superconducting device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting device and a permanent current switch used for the superconducting device, and more particularly to a connection between a superconducting coil and a permanent current switch.

[Conventional technology]

In a superconducting coil device used for a nuclear magnetic resonance apparatus (MRI) or the like, a permanent current switch is connected in parallel with the superconducting coil to use the superconducting coil in a permanent current (circulating current) mode. This permanent current switch is provided with a superconducting wire having a matrix layer having a large specific resistance formed on the outer periphery of a superconducting material and a heater for heating the superconducting wire from the viewpoints of shape, weight and operability (control). Thus, the temperature of the superconducting wire is controlled to bring the superconducting wire into a superconducting state (on state) or a normal conducting state (off state).

FIG. 7 shows an electrical connection (arrangement) configuration of such a superconducting device. An excitation current is supplied from an excitation power supply 4 to the superconducting coil 3 immersed in the liquid helium 2 filled in the cooling container 1. When power is supplied, the superconducting coil 3
A current is supplied from a control power supply 7 to a heater 6 for controlling the permanent current switch 5, which is connected in parallel with the superconducting current switch 5, and the superconducting wire 8 is heated to a normal conduction state. . Then, in order to make the exciting current flowing in the superconducting coil 3 a permanent current circulating through the permanent current switch 5, the power supply to the heater 6 is stopped and the superconducting wire 8 is cooled by the surrounding liquid helium 2. Into superconducting state,
The permanent current switch 5 is turned on.

The permanent current switch 5 uses a NbTi alloy wire as a superconducting material for the superconducting wire 8 constituting the permanent current switch 5 in order to realize a preferable on / off function. For example, an ultrafine multifilamentary wire having a matrix made of a metal having a large specific resistance such as a CuNi alloy is used.

However, a superconducting wire using a matrix having a large specific resistance is electromagnetically unstable compared to a superconducting wire for a superconducting coil using a metal having a small specific resistance, such as Cu or Al, in the superconducting state. Is small. For this reason, a large permanent current is secured by connecting a plurality of permanent current switches 5 to the superconducting coil 3 in parallel. Then, in order to distribute the current evenly to the plurality of permanent current switches 5, it has been proposed to make a connection with a low resistance element between the excitation current line and the excitation current line.

This type of superconducting device and permanent current switch is disclosed in
It is disclosed in JP-A-61-269301 and JP-A-1-102905.

[Problems to be solved by the invention]

However, this type of conventional superconducting device and the persistent current switch often have a problem in that the superconducting wire is often connected to the superconducting wire of the persistent current switch and the low-resistance body due to the Joule heat generated by the resistance of the CuNi alloy matrix layer. There was a problem that a transition from a superconducting state to a normal conducting state occurred.

Accordingly, an object of the present invention is to provide a superconducting coil that can flow a large permanent current stably by reducing the connection resistance value of the connection portion between the low-resistance body and the permanent current switch that mediates the parallel connection of the superconducting coil and the permanent current switch. It is to provide a device and a permanent current switch.

[Means for solving the problem]

In order to achieve this object, a first invention provides a permanent current comprising a superconducting coil, a superconducting wire having a matrix layer having a large specific resistance formed on the outer periphery of a superconducting material, and a heating means for heating the superconducting wire. In a superconducting device including a switch and a low-resistance element that mediates a parallel connection of the superconducting coil and the persistent current switch, the permanent current switch connects a superconducting material portion exposed outside the matrix layer to the low-resistance body. The second invention is characterized in that a superconducting wire in which a matrix layer having a large specific resistance is formed on the outer periphery of a superconducting material, a heater for heating the superconducting wire, and the superconducting wire are extended to form a lead wire portion. In the permanent current switch, a superconducting material at a connection portion of the lead wire portion is exposed outside the matrix layer.

[Action]

Since a matrix layer having a large specific resistance is not interposed at the connection portion between the permanent current switch and the low-resistance body, the connection resistance is reduced, and the transition of the normal conduction state due to the double heat generation of the connection resistance is eliminated.

〔Example〕

In a superconducting device in which a plurality of permanent current switches are arranged in parallel with a superconducting coil, as described above, a low-resistance element is interposed as described above in order to distribute current evenly to each of the permanent current switches. Since it is about 10 ⁷ Ω, it does not become the permanent current mode with zero circuit resistance. The decay time constant of the permanent current is determined by the ratio L / R of the inductance L of the superconducting coil and the resistance R of the circulation circuit. The duration required for the permanent current mode in the superconducting coil is about 20 hours, which is sufficient, and the effect of the resistance value of the low-resistance body on the decay time constant is not significant.

However, as a result of various experiments performed by the inventors,
Since the resistance is concentrated at the connection portion of the superconducting wire of the permanent current switch, when a large permanent current is applied, the superconducting wire of the permanent current switch changes from the superconducting state to the normal conducting state due to the heat generated by the module. It has been found that a situation in which the permanent current mode cannot be maintained occurs.

The superconducting wire of the persistent current switch is made of CuNi with relatively high resistance.
Since it is an NbTi extra-fine multi-core wire using an alloy as a matrix,
The margin of stability is extremely small, and it has been found that the transition from the superconducting state to the normal conducting state is made with less than 1/10 of the heat energy of the Cu matrix NbTi ultra-fine multi-core wire used as the superconducting wire of the superconducting coil. .

Therefore, each embodiment of the present invention is devised so that the connection resistance of the connection portion of the superconducting wire of the permanent current switch to the low resistance body is minimized.

Ideally, superconducting connections with a connection resistance value of zero are ideal for the connection of superconducting wires.However, many NbTi ultra-fine multi-core wires are connected by crimping or spot welding. Is a connection that is electromagnetically and mechanically unstable. Therefore, according to the present invention, the connection portion of the superconducting wire of the permanent current switch to the superconducting coil is provided with a low-resistance body, and the connection resistance value is increased due to the high resistance matrix of the superconducting wire of the permanent current switch. To avoid this, the high-resistance matrix is removed to expose the ultra-fine NbTi multifilamentary wire of the superconducting wire and connect it to a low-resistance body. However, it is difficult to directly connect bare NbTi ultrafine multifilamentary wires to a low-resistance body in good condition.Therefore, a low-resistance sleeve is interposed, or a Cu matrix superconducting wire with low specific resistance is interposed. ing. The allowable maximum value of the preferable connection resistance value of the superconducting wire with respect to the low-resistance body is about 1/10 ⁹ Ω at a temperature of 4.2 K when the superconducting coil is in an operating state.

Oxygen-free copper is used for the low-resistance sleeve to improve the electromagnetic stability of the connection and provide mechanical reinforcement against electromagnetic forces. If the filling rate of the superconducting wire in the sleeve is low, the connection (pressure welding) state becomes unstable. Therefore, it is preferable that a solid connection state is obtained by filling with Ag or Cu powder or fiber.

Further, the transition of the superconducting wire to the normal conducting state in the permanent current switch often occurs at the lead wire portion.
Even if the superconducting wire is a permanent current switch that is non-inductively wound, it occurs when the fixation to the electromagnetic force is insufficient. The winding inside the permanent current switch is firmly fixed by epoxy resin impregnation or the like, but since the lead wire is in a free state, the part easily transitions to a normal conduction state. In order to eliminate such parts and maintain a stable superconducting state, a low-resistance body, a persistent current switch (lead wire) and a support that integrally supports the connection portion are provided, and epoxy resin impregnation (filling) or the like is performed. It is preferable to perform fixing by binding or the like.

Next, the results of measurement and evaluation of the quench current value of the superconducting wire for a permanent current switch by changing the connection configuration of the superconducting wire will be described. The measured superconducting wire is one in which 1171 ultrafine NbTi wires having an outer diameter of 0.4 mm and a diameter of about 10 μm are bundled together and embedded in a Cu-30% Ni alloy matrix.

The superconducting wire is made into a hairpin shape for measurement, and both ends are connected to a low-resistance body (Cu wire), immersed in liquid helium and changing the magnitude of the external magnetic field at a temperature of 4.2 K to reduce the quench current. It was measured. For the measurement sample (I), the superconducting wire for the permanent current switch was soldered and connected to the Cu wire as it was. For the measurement sample (II), the matrix at both ends of the superconducting wire for the permanent current switch was removed to form an NbTi ultrafine multi-core wire. It is exposed, a Cu sleeve is crimped and fixed (connected) to the exposed portion, and the Cu sleeve is connected by soldering to a Cu wire. Each of the measurement samples (I) and (II) was measured and evaluated by the same method for 10 samples each, and the variation was also measured.

FIG. 8 shows the measurement results. The region (I) is the maximum and minimum values (distribution range) of the measured values of the ten superconducting wires of the measurement sample (I), and the region (II) is the 10th of the measurement sample (II).
These are the maximum value and the minimum value (distribution range) of the measured values of the superconducting wires. It can be seen that the value of the quench current of the measurement sample (II) is larger than the value of the quench current of the measurement sample (I).
It is considered that the normal conduction transition of the measurement sample (I) occurred from the connection part, and the superconducting wire was heated by the Joule heat generated by the connection resistance of the connection part, thereby reducing the quench current. From these measurement results, it can be seen that by devising the connection structure, a current 1.7 times larger than that of the conventional device can be stably passed in an external magnetic field of 1T.

Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings.

FIG. 1 shows an arrangement of main components of a superconducting coil device according to the present invention.

The lead wires 3a, 3b at both ends of the superconducting coil 3 are low-resistance bodies 9, 1
Connected to 0 by soldering. An exciting current line 1 for supplying an exciting current of the superconducting coil 3 from an exciting power supply 4
1, 12 are also connected to the low-resistance elements 9, 10 by soldering. In order to form a parallel circuit with the superconducting coil 3, three permanent current switches 5a, 5b, 5c are connected in parallel between the low-resistance elements 9, 10.

Each of the permanent current switches 5a to 5c has the same configuration as the above-described permanent current switch 5, and includes a superconducting wire 8 of an ultrafine multicore wire using a superconductor of an NbTi alloy wire and a matrix of a CuNi alloy. A heater (not shown) for heating the wire 8 is provided, and each of the permanent current switches 5a to 5c connects the lead wire portions 8a and 8b to the low resistance elements 9 and 10, respectively. Each connection part has the same connection configuration, and the matrix at the connection part of each lead wire part 8a, 8b is dissolved and removed with nitric acid to expose the ultrafine multifilamentary wire of the NbTi alloy, and the Cu sleeves 13, 14 are exposed on the exposed part. And the Cu sleeves 13 and 14 are connected to the low-resistance elements 9 and 10 by soldering.

2 (a) and 2 (b) show a part of the connection structure of the connection part in detail. The lead wire portion 8a, which is an extension of the superconducting wire 8 of the persistent current switch 5, is composed of an NbTi alloy ultra-fine multi-core wire 8c and a CuNi alloy matrix 8d in which it is embedded.
, And the CuNi alloy matrix 8d at the distal end serving as a connection portion is removed, and the ultrafine multi-core wire 8c is exposed. The Cu sleeve 13 is press-fitted over this exposed portion of the ultrafine multi-core wire 8c, and the Cu sleeve 13 is connected to the low-resistance body 9 by soldering 19.

In a conventional superconducting coil device in which five permanent current switches 5 are connected in parallel, the combined resistance when the five switches 5 are in the off state is 100Ω (one resistance value is 500Ω), and when the five switches 5 are in the on state. Current of 686A (one rated current
In this example, the characteristic of 137.5 A) could be obtained with three permanent current switches 5a to 5c. That is, a rated current of 228.7 A was obtained with one permanent current switch 5, and a resistance value of 300Ω was obtained in the off state. This is realized by reducing the connection resistance between the lead wires 8a, 8b of the superconducting wire 8 of each of the permanent current switches 5a to 5c and the low-resistance bodies 9, 10 and increasing the allowable current.

And according to the superconducting coil device having such a configuration,
Since the number of the permanent current switches 5 can be reduced and the superconducting wires 8 in the permanent current switches 5 can be shortened, the required space, weight, and weight in the superconducting coil device can be reduced.
The manufacturing cost can be reduced, and the amount of evaporation of the liquid helium for cooling can be reduced to reduce the operating cost.

FIG. 3 shows the respective lead wires 8a, 8 of the permanent current switches 5a to 5c.
FIG. 1B shows a superconducting coil device in which b is connected by soldering to low-resistance members 9 and 10 via low-resistance superconducting wires 20 and 21 in which ultrafine multi-core wires of an NbTi alloy are embedded in Cu matrix.

In this embodiment, the connection between the lead wires 8a, 8b of the superconducting wire 8 and the intermediary superconducting wires 20, 21 in the permanent current switches 5a to 5c is performed by removing the matrix at the connection portion and making the NbTi alloy extra fine multi-core. The configuration is such that the wires are exposed, the Cu sleeves 13 and 14 are covered with the exposed portions overlapped, and pressure-bonded.

4 (a) to 4 (e) show the connection process of the connection section in detail. The leading end portion serving as a connection portion at the lead wire portion 8a which is an extension portion of the superconducting wire 8 of the permanent current switch 5
The CuNi alloy matrix 8d is removed to expose the ultrafine multifilamentary wire 8c.Similarly, the Cu matrix 20d at the distal end serving as a connection portion of the intermediary superconducting wire 20 is also removed to expose the ultrafine multifilamentary wire 20c. With the exposed portions of the multicore wires 8c and 20c overlapped with each other, a Cu sleeve 13 is put on and connected by crimping. The other end of the intermediary superconducting wire 20 is a Cu matrix 20d.
Is connected to the low-resistance body 9 by soldering.

In this embodiment, the filling rate of the ultrafine multifilamentary wires in the Cu sleeve 13 is greatly improved, the superconducting connection between the ultrafine multifilamentary wires is stabilized, and the variation of the quench current between the permanent current switches 5a to 5c is reduced. There are advantages.

FIGS. 5 (a) and 5 (b) show another embodiment of the connection between the superconducting wire for permanent current switch and the superconducting wire for mediation.

The superconducting wire 8 of the permanent current switch 5 is usually made of Nb with a matrix of Cu-30% Ni alloy due to restrictions on the wire rod manufacturing method.
The structure is such that a Cu-30% Ni alloy layer 8d exists around the Ti ultrafine multi-core wire 8c. For example, for a superconducting wire having an outer diameter of 0.46 mm, the thickness of the Cu-30% Ni alloy
0.03 mm.

A superconducting superconducting wire in which an NbTi alloy ultrafine multifilamentary wire is buried in Cu matrix in the lead wire portion 8a of such a superconducting wire 8 is directly connected by soldering over a length of 300 mm and immersed in liquid helium for cooling. When the connection resistance is measured in this state, the connection resistance is as high as 8/10 ⁸ Ω, and most of the connection resistance is due to the Cu-30% Ni alloy layer 8d in the outer peripheral portion.

In this embodiment, an increase in connection resistance due to such a Cu-30% Ni alloy layer 8d is suppressed, and a 450 m long soldering connection end portion at a lead wire portion 8a of a superconducting wire 8 for permanent current switch 8 is used. The Cu-30% Ni alloy layer 8d is dissolved and removed with nitric acid, and two Cu matrix superconducting superconducting wires 20a, 20b having a length of 450 mm are soldered to the connection end portions over a length of 450 mm. It was done.

When the electrical resistance of this connection was measured at a temperature of 4.2K,
9/10 ¹⁰ Ω, which is a value that does not affect the quench current of the permanent current switch 8. Although this connection resistance value is inferior to the connection resistance values in the other embodiments, there is an advantage that the connection structure is extremely simple and a highly reliable connection portion can be obtained.

As described above, in order to stabilize the characteristics of the permanent current switch in such a superconducting coil device, it is preferable to prevent the lead wire portion or the intermediate superconducting wire from vibrating due to the electromagnetic force.

FIG. 6 shows an embodiment in which the lead wire portion and the intermediary superconducting wire are firmly held to prevent the occurrence of quench due to vibration. The connection configuration of the low resistance element and the permanent current switch is
This is the same as the embodiment described above with reference to FIG. Permanent current switches 5a to 5c arranged between two low-resistance elements 9 and 10
And the lead wires 8a and 8b and the intermediate superconducting wires 20 and 21 and the Cu sleeves 13 and 14 for crimp-connecting both wires are tied to a stainless steel support plate 22 arranged in an insulated state along these lines. Or by a fixing material 23 such as resin impregnation or filling,
They are integrally fixed in an insulating state.

〔The invention's effect〕

According to the present invention, since a matrix layer having a large specific resistance is not interposed at a connection portion between the permanent current switch and the low-resistance body,
It is possible to provide a superconducting device and a permanent current switch capable of stably supplying a large permanent current by reducing the connection resistance and reducing the influence of the connection resistance due to the joule heat.

[Brief description of the drawings]

1 to 6 show an embodiment of the present invention. FIG. 1 is a layout diagram of main components of a superconducting coil device as one embodiment of the present invention, and FIGS. FIG. 3 (a) is a longitudinal sectional side view showing details of a connection portion between a resistor and a permanent current switch and a BB sectional view thereof. FIG. 3 is a layout diagram of main components of a superconducting coil device as another embodiment, 5 (e) to 5 (e) are diagrams showing a process of connecting a permanent current switch lead wire portion and an intermediate superconducting wire, and FIGS. 5 (a) and 5 (b) show another embodiment of connection between a permanent current switch superconducting wire and an intermediate superconducting wire. FIG. 6 is a layout view of main components of a superconducting coil device according to still another embodiment. FIG. 7 is an arrangement diagram of main components of a conventional superconducting coil device. FIG. 8 is a connection resistance characteristic diagram. 3. Superconducting coil, 3a, 3b Lead wire, 5a to 5c
Permanent current switch, 8a, 8b ... Lead wire, 8c ... NbTi
Ultra fine multi-core wire of alloy, 8d …… Matrix of CuNi alloy, 9,
10 ... Low resistance, 13,14 ... Cu sleeve, 19 ... Soldering.

Claims (7)

(57) [Claims] 1. A permanent current switch comprising a superconducting coil, a superconducting wire having a matrix layer having a large specific resistance formed on the outer periphery of a superconducting material, and heating means for heating the superconducting wire. A superconducting device including a low-resistance element that mediates a parallel connection with a switch, wherein the permanent current switch is configured such that a superconducting material portion exposed outside the matrix layer is the low-resistance element. 2. The superconducting device according to claim 1, wherein the permanent current switch has the exposed portion of the superconducting material connected to the low-resistance body via a low-resistance material sleeve. 3. The superconducting device according to claim 2, wherein the low-resistance sleeve is connected to the exposed portion of the superconducting material by crimping and soldered to the low-resistance body. 4. The superconducting device according to claim 3, wherein said low-resistance sleeve is crimped in a state where a filler is filled between said superconducting material exposed portion and said low-resistance sleeve. 5. The superconducting device according to claim 1, wherein the low resistance element, the permanent current switch, and a connection portion thereof are integrally fixed. 6. The semiconductor device according to claim 1, wherein the connection resistance of said connection portion is 1/10 ⁹ Ω at 4.2 K.
A superconducting device characterized by the following. 7. A permanent current switch comprising a superconducting wire in which a matrix layer having a large specific resistance is formed on the outer periphery of a superconducting material, and a heater for heating the superconducting wire, wherein the superconducting wire is extended to form a lead wire portion. A permanent current switch wherein a superconducting material at a connection portion of the lead wire portion is exposed from the matrix layer.