JPH04186806A - Superconducting device and permanent current switch applicable to same - Google Patents

Abstract

PURPOSE:To enable the high permanent current to be applied stably by a method wherein the superconducting member parts exposed out of the matrix layers of permanent current switches are connected to low resistors so that the connection resistance value at the connection parts between the permanent current switches and the low resistors intermediating between a superconducting coil and the permanent coil and the permanent current switches for the parallel connection thereof may be lessened. CONSTITUTION:Within the title superconducting device provided with a conductive coil 3, a superconducting wire 8 with a matrix layer 8d in high specific resistance formed on the outer periphery of a superconducting member 8c, the permanent circuit switches 5a-5c with a heating means to heat the superconducting wire 8 and low resistors 9, 10 intermediating between the superconducting coil 3 and the permanent current switches 5a-5c for the parallel connection thereof, the permanent current switches 5a-5c are arranged so that the superconducting members 8c exposed out of the matrix layer 8d may be connected to the low resistors 9, 10. For example, the matrix layer 8d in the connection parts of respective leading wires 8a, 8b is removed to expose the fine multiple cores 8c which are covered with Cu sleeves 13, 14 to be pressure-fixed for connection and then the Cu sleeves 13, 14 are connected to the low resistors 9, 10 by soldering step.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a superconducting device and a persistent current switch used in the superconducting device, and particularly relates to a connection between a superconducting coil and a persistent current switch.

[Conventional technology]

In a superconducting coil device used in a nuclear magnetic resonance apparatus (MRI) or the like, a persistent current switch is connected in parallel with the superconducting coil in order to use the superconducting coil in a persistent current (circulating current) mode. This persistent current switch is equipped with a superconducting wire in which a matrix layer with a high specific resistance is formed around the outer periphery of a superconducting material and a heater that heats the superconducting wire, from the viewpoint of shape 2 weight and operability (controllability). This is a configuration in which the temperature of the superconducting wire is controlled to put the superconducting wire into a superconducting state (on state) or a normal conducting state (off state).

Figure 7 shows the electrical connection (arrangement) of such a superconducting device.
It shows the configuration. An excitation current is supplied from an excitation power source 4 to the superconducting coil 3 immersed in liquid helium 2 filled in the cooling container 1 . At the time of power supply, in order to turn off the persistent current switch S connected in parallel with the superconducting coil 3, current is supplied from the control power source 7 to the heater 6 that controls the persistent current switch 5 to heat the superconducting wire 8. to bring it into a normal conducting state. In order to make the excitation current flowing through the superconducting coil 3 into a persistent current circulating through the persistent current switch 5, the power supply to the heater 6 is stopped and the superconducting wire 8 is cooled with the surrounding liquid helium 2. A superconducting state is established, and the persistent current switch 5 is turned on.

In order to realize a preferable on/off function, the persistent current switch 5 is made of a superconductor, which constitutes the persistent current switch 5.
[8] NbTi alloy wire is used as a superconducting material, and in order to obtain a large resistance value in the off state, for example, CuN
An ultra-fine multi-core wire whose matrix is a metal with high specific resistance such as i-alloy is used.

However, superconducting wires using a matrix with a high specific resistance are electromagnetically unstable compared to superconducting wires for superconducting coils whose matrix is a metal with a low specific resistance, such as Cu or AI. The allowable current is small. For this purpose, a plurality of persistent current switches 5 are connected in parallel to the superconducting coil 3 to ensure a large persistent current. In order to evenly divide the current into the plurality of persistent current switches 5, it has been proposed to connect them to the excitation current line by interposing a low resistance element.

This type of superconducting device and persistent current switch was developed in Japanese Patent Application Laid-open No. 6
It is disclosed in JP-A No. 1-269301 and JP-A-1-102905.

[Problem to be solved by the invention]

However, in this type of conventional superconducting device and persistent current switch, the superconducting wire is often damaged due to Joule heat generation due to the resistance of the CuNi alloy matrix layer at the connection between the superconducting wire and the low-resistance element of the persistent current switch. There was a problem with the transition from a superconducting state to a normal conducting state.

Therefore, an object of the present invention is to provide a superconductor that can stably flow a large persistent current by reducing the connection resistance value of the connection between the persistent current switch and the low resistance body that mediates the parallel connection between the superconducting coil and the persistent current switch. The purpose of the present invention is to provide a permanent current switch with a permanent current switch.

[Means to solve the problem]

In order to achieve this object, the first invention provides a persistent current comprising a superconducting coil, a superconducting wire in which a matrix layer having a high specific resistance is 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 body that mediates parallel connection between the superconducting coil and the persistent current switch, the persistent current switch connects a superconducting material portion exposed outside the matrix layer to the low resistance body. A second invention is characterized in that a superconducting wire in which a matrix layer with a high specific resistance is formed on the outer periphery of a superconducting material, a heater for heating the superconducting wire, and an extension of the superconducting wire to form a lead wire portion. The persistent current switch is characterized in that the superconducting material of the connecting portion of the lead wire portion is exposed outside the matrix layer.

[Effect]

Since a matrix layer with a high specific resistance is not interposed in the connection portion between the persistent current switch and the low resistance element, the connection resistance is reduced, and transition to a normal conductive state due to Joule heat generation of the connection resistance is eliminated.

〔Example〕

In a superconducting device in which a plurality of persistent current switches are connected in parallel to a superconducting coil, in order to distribute current evenly to each persistent current switch, a low resistance body is used as an intermediary as described above, and the resistance value is 1/ Since the resistance is about 107Ω, the permanent current mode with zero circuit resistance does not occur. The decay time constant of the persistent current is determined by the ratio L/R of the inductance L of the superconducting coil and the resistance R of the circulation circuit. About 20 hours is sufficient for the persistent current mode to last in the superconducting coil, and the influence of the resistance value of the low resistance body on the decay time constant is not a problem.

However, as a result of various experiments conducted by the inventors, it was found that the resistance is concentrated at the connection part of the superconducting wire of the persistent current switch. It has been found that a situation occurs in which the superconducting wire of the switch transitions from the superconducting state to the normal conducting state and is no longer able to maintain the persistent current mode.

The superconducting wire of the persistent current switch is CuN, which has a relatively high resistance.
Since it is an NbTi ultra-fine multi-core wire with an i-alloy matrix, the stability margin is extremely small, and the Cu-matrix NbTi wire is used as a superconducting wire in a superconducting coil.
It was found that the superconducting state can be transitioned to the normal conducting state with less than 1/10 the thermal energy of an ultra-fine multi-core wire.

Therefore, each embodiment of the present invention is designed to minimize the connection resistance of the connection portion of the superconducting wire of the persistent current switch to the low resistance body.

From the viewpoint of connection resistance, the ideal connection between superconducting wires is a superconducting connection with zero connection resistance.
A configuration in which ultra-fine multi-filament wires are connected by crimping or spot welding results in an electromagnetically and mechanically unstable connection. Therefore, in the present invention, a low-resistance element is interposed at the connection portion of the superconducting wire of the persistent current switch to the superconducting coil, and a high-resistance matrix of the superconducting wire of the persistent current switch is interposed, so that the connection resistance value does not increase. In this way, the high-resistance matrix is removed to expose the NbTi ultrafine multifilamentary wire of the superconducting wire and connect it to a low-resistance body. However, since it is difficult to directly connect a bare NbTi ultrafine multi-core wire to a low-resistance element in good condition, a low-resistance sleeve or a Cu matrix superconducting wire with a low resistivity is interposed. ing. A preferable maximum allowable connection resistance value of the superconducting wire to the low resistance body is about 1/10 gΩ 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, which improves the electromagnetic stability of the connection and provides mechanical reinforcement against electromagnetic force. Furthermore, if the filling rate of the superconducting wires in the sleeve is low, the connection (pressure welding) state will become unstable, so it is preferable to fill the sleeve with Ag or Cu powder or fiber to obtain a firm connection state.

Furthermore, transition of the superconducting wire to the normal conductive state in a persistent current switch often occurs at the lead wire portion thereof. Even in persistent current switches in which superconducting wires are non-inductively wound, this problem occurs when the fixation against electromagnetic force is insufficient.

Although the winding portion inside the persistent current switch is firmly fixed by impregnating with epoxy resin, the lead wire portion is in a free state, and therefore this portion is likely to transition to a normal conduction state. In order to eliminate such parts and maintain a stable superconducting state,
It is preferable to provide a support that integrally supports the low resistance element, the persistent current switch (lead wire), and the connection part, and to perform fixation by impregnating (filling) with epoxy resin, tying, or the like.

Next, the results of measuring and evaluating the quench current value of a superconducting wire for a persistent current switch by changing the connection configuration of the superconducting wire will be described. The superconducting wires measured were 1171 NbTi ultrafine wires with an outer diameter of 0.4 scale and a diameter of about IQ μm, which were embedded in a Cu-30%Ni alloy matrix.

For measurement, the superconducting wire was made into a hairpin shape, both ends of which were connected to a low-resistance material (Cu wire), and quenched by immersing it in liquid helium at a temperature of 4.2 degrees while changing the magnitude of the external magnetic field. The current was measured. The measurement sample (1) is a superconducting wire for a persistent current switch that is directly connected to a Cu wire by soldering, and the measurement sample (n) is a superconducting wire for a persistent current switch that has the matrix removed from both ends and is made of an NbTi ultrafine multicore wire. is exposed, a Cu sleeve is caulked and fixed (connected) to the exposed part, and the Cu sleeve is connected to the Cu wire by soldering. Ten measurement samples (I) and (11) were each measured and evaluated using the same method, and the state of variation was also measured.

FIG. 8 shows the measurement results. Area (I) is the maximum and minimum values (distribution range) of the measured values of the 10 superconducting wires of the measurement sample (I), and area (II) is the measurement sample (Ilr
) are the maximum and minimum values (distribution range) of the measured values of 10 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 thought that the normal conduction transition of measurement sample (1) occurs at the connection, and the superconducting wire is heated by Joule heat generation due to the connection resistance at the connection, reducing the quench current. Looking at these measurement results, it seems that by devising the connection structure, it is possible to improve the conventional 1T in an external magnetic field of 1T.
．． It can be seen that it is possible to stably flow seven times the current.

Embodiments of the present invention will be specifically described below with reference to the drawings.

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

Lead wires 3-a and 3b at both ends of the superconducting coil 3 are connected to low resistance elements 9 and 10 by soldering.

Excitation current lines 11 and 12 for supplying excitation current from the excitation power source 4 to the superconducting coil 3 are also connected to these low resistance bodies 9 and 10.
connected by soldering. Three persistent current switches 5a, 5 are used to configure a parallel circuit for the superconducting coil 3.
b and 5c are connected in parallel between the low resistance bodies 9 and 10.

Each of the persistent current switches 58 to 5c has the same configuration as the persistent current switch 5 described above, and each has a superconducting wire 8 that is an ultrafine multi-core wire using a superconductor of NbTi alloy wire and a matrix of CuNi alloy; Each persistent current switch 58~ is equipped with a heater (not shown) for heating the wire 8.
Reference numeral 5c has a configuration in which the respective lead wire portions 8a and 8b are connected to the low resistance bodies 9 and 1°. Each connection part has a similar connection configuration, and the matrix of the connection part of each sunrise wire part 8a, 8b is dissolved and removed with nitric acid to expose an ultrafine multi-core wire of NbTi alloy, and Cu is applied to the exposed part. Sleeve 13.
The Cu sleeves 13 and 14 are connected to the low resistance elements 9 and 1o by soldering.

FIGS. 2(a) and 2(b) show a part of the connection structure of the connection portion in detail. The lead wire portion 8a, which is an extension of the superconducting wire 8 of the persistent current switch 5, consists of an ultra-fine multi-core wire 8c made of NbTi alloy and a matrix 8d made of CuNi alloy in which it is buried.
The i-alloy matrix 8d is removed to expose the ultrafine multifilamentary wires 8c. The Cu sleeve 13 is placed over the exposed portion of the ultra-fine multi-core wire 8c and crimped and connected, and the Cu sleeve 13 is connected to the low resistance element 9 by soldering 19.

In a conventional superconducting coil device in which five persistent current switches 5 are connected in parallel, the combined resistance when the five switches 5 are in the off state is 100Ω (the resistance value of one piece is 500Ω), and when the switches are in the on state In this embodiment, the characteristic that the allowable current of 686 A (the rated current of one is 137.5 A) could be obtained with the three persistent current switches 58 to 5c. In other words, one persistent current switch 5 generates 228.7A.
A rated current of , and a resistance value of 300Ω in the off state were obtained. This is achieved by reducing the connection resistance between the lead wire portions 8a, 8b of the superconducting wire 8 of each persistent current switch 5a to 5c and the low resistance elements 9, 10, and increasing the allowable current.

According to the superconducting coil device having such a configuration, the number of persistent current switches 5 is reduced and the super@ags in the persistent current switches 5 can be shortened, so that the space required for arrangement within the superconducting coil device, In addition to reducing weight and manufacturing costs, it is also possible to reduce the amount of evaporation of liquid helium for cooling, thereby reducing operating costs.

FIG. 3 shows each sunrise line portion 8 of the persistent current switches 5a to 5c.
A and 8b are connected to low resistance elements 9 and 10 by soldering via low resistance superconducting wires 20 and 21 in which ultrafine multi-core wires of N b Ti alloy are embedded in a Cu matrix. ing.

In this embodiment, each solar wire portion 8a, 8b of the superconducting gs in the persistent current switches 5a to 5c and the intermediate superconducting wire 2
The connection with 0.21 is made by removing the matrix of the connection part to expose the NbTi alloy ultra-fine multi-filament wire, and with the exposed parts overlapped, a Cu sleeve 13 and 14 is covered and the connection is made by crimping. .

FIGS. 4(a) to 4(e) show the connection process of the connection portion in detail. The CuNi alloy matrix 8d at the tip of the lead wire portion 8a, which is an extension of the superconducting wire 8 of the persistent current switch 5, is removed to expose the ultrafine multi-core wire 8c, and similarly, the intermediary superconducting wire 20 is removed. The Cu matrix 20d at the tip, which is the connecting part of
A Cu sleeve 13 is covered with the exposed portions of the two parts overlapped, and the two are crimped and connected. The other end of the intermediary superconducting wire 20 is soldered to the low resistance element 9 with the Cu matrix 20d still attached.

In this embodiment, the filling rate of the ultra-fine multi-core wires in the Cu sleeve 13 is significantly improved, the superconducting connection between the ultra-fine multi-core wires is stabilized, and the variation in quench current between the persistent current switches 5a to 50 is reduced. There are advantages.

FIGS. 5(a) and 5(b) show another embodiment of the connection between the persistent current switch superconducting wire and the intermediary superconducting wire.

The superconducting wire 8 of the persistent current switch 5 usually has a NbTi ultrafine multi-core wire 8c with Cu-30%Ni alloy as a matrix, and Cu is placed around it due to restrictions in the wire manufacturing method.
-30%N1 alloy layer 8d is present. For example, in a superconducting wire with an outer diameter of 0.46 mm, Cu on the outer periphery
The thickness of the -30% Ni alloy layer 8C is approximately 0.03 mm.

An intermediary superconducting wire in which ultrafine multi-core NbTi alloy wires were embedded in a Cu matrix was connected to the lead wire portion 8a of the superconducting wire 8 over a length of 300 mm by soldering, and the wire was cooled by immersing it in liquid helium. When measuring the connection resistance value in this state, there was a connection resistance of 8/10''Ω, and most of this connection resistance value was due to the Cu-3 on the outer periphery.
This is due to the 0% Ni alloy layer 8d.

In this example, such a Cu-30%Ni alloy layer 8
This suppresses the increase in connection resistance due to
For soldering of 30 mm, the Cu-30%Ni alloy layer 8d at the connection end is dissolved and removed with nitric acid, and two superconducting superconducting wires 20a, 2 with a Cu matrix ultrafine multi-filament structure are attached to the connection end.
0b were connected by soldering over a length of 450 mm.

When the electrical resistance of this connection was measured at a temperature of 4.2, it was found to be 9/1010Ω, a value that does not affect the quench current of the persistent current switch 8. Although this connection resistance value is inferior to the connection resistance values in other embodiments, it has the 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 persistent 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 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 quenching due to vibration. The connection structure between the low resistance element and the persistent current switch is the same as the embodiment described above with reference to FIG. Persistent current switch 58 arranged between two low resistance elements 9 and 10
5c, each of its exposed wire portions 8a, 8b, and the intermediate superconducting wire 20° 21, as well as the Cu sleeve 1 for crimping and connecting the drawing wires.
3.degree. 14 is integrally fixed in an insulated state by binding or fixing material 23 such as resin impregnation or filling to a stainless steel support plate 22 arranged in an insulated state along these parts.

〔Effect of the invention〕

In the present invention, since a matrix layer with high specific resistance is not interposed in the connection part between the persistent current switch and the low resistance element, the connection resistance is reduced, the influence of Joule heat generation of the connection resistance is reduced, and a large persistent current is stabilized. It is possible to provide a superconducting device and a persistent current switch that can conduct current.

[Brief explanation of the drawing]

Figures 1 to 6 show embodiments of the present invention. Figure 1 is a layout diagram of the main components of a superconducting coil device, which is one embodiment, and Figures 2 (a) and (b) are A vertical side view showing details of the connection between the resistor and the persistent current switch, and its BB sectional view; FIG. 3 is a layout diagram of the main components of a superconducting coil device according to another embodiment; FIG. 4(a) - (e) are connection process diagrams of the persistent current switch roller part and the intermediate superconducting wire, and Figures 5 (a) and (b) are other connections between the persistent current switch superconducting wire and the intermediate superconducting wire. FIG. 6 is a longitudinal cross-sectional view showing the embodiment, and is a layout diagram of the main components of a superconducting coil device according to yet another embodiment. FIG. 7 is a layout diagram of the main components of a conventional superconducting coil device. FIG. 8 is a connection resistance characteristic diagram. 3...Superconducting coil, 3a, 3b...
Lead wire, 58~5c...persistent current switch,
8a, 8b... Lead line part, 8c...
・・NbTi alloy ultra-fine multi-core wire, 8d・・・・・・
CuNi alloy matrix, 9,10...
・Low resistance element, 13, 1'4...Cu sleeve,
19.Yu soldering. Fig. 1 3 ---- Mi conductive coils 50 to 5C-1 - Persistent current switch 8 - Superconducting wires 8a, 8b - Lead wire portion 8 cm ---- NbTi alloy thin Multi-core wire 8d --
---CuNi alloy matrix 9.10----
Low resistance element 13.14 -- - Cu sleeve Fig. 2 Fig. 3 Fig. 4 d Fig. 5 Fig. 6 Fig. 7

Claims (9)

[Claims] 1. A persistent current switch comprising a superconducting coil, a superconducting wire in which a matrix layer with a high specific resistance is formed on the outer periphery of a superconducting material, and heating means for heating the superconducting wire, and a parallel connection of the superconducting coil and the persistent current switch. A superconducting device comprising a low-resistance element that mediates the persistent current switch, wherein a portion of the superconducting material exposed outside the matrix layer is connected to the low-resistance element. 2. 2. A superconducting device according to claim 1, wherein the persistent current switch has an exposed portion of the superconducting material connected to the low resistance body through a low resistance material sleeve. 3. 3. The superconducting device according to claim 2, wherein the low resistance sleeve is crimped and connected to the superconducting material exposed portion and is soldered to the low resistance body. 4. 4. The superconducting device according to claim 3, wherein the low-resistance sleeve is connected by caulking with a filler filled between the exposed portion of the superconducting material and the low-resistance sleeve. 5. 2. The superconducting device according to claim 1, wherein the persistent current switch is connected to the low resistance body through a low resistance matrix superconducting wire in which the exposed portion of the superconducting material is superconductingly connected. 6. 2. A superconducting device according to claim 1, wherein the persistent current switch is connected to the low resistance body through a low resistance matrix superconducting wire soldered to the exposed portion of the superconducting material. 7. 2. The superconducting device according to claim 1, wherein the low resistance element, the persistent current switch, and the connecting portion thereof are integrally fixed. 8. 8. The superconducting device according to claim 1, wherein the connection resistance value of the connection portion is set to 1/10^9Ω or less at 4.2K. 9. A persistent current switch including a superconducting wire in which a matrix layer with a high specific resistance is formed on the outer periphery of a superconducting material, a heater for heating the superconducting wire, and an extension of the superconducting wire as a lead wire part, the connecting part of the lead wire part. A persistent current switch characterized in that a superconducting material is exposed from the matrix layer.