WO2006059497A1

WO2006059497A1 - Method and device for measuring critical current density of superconductor

Info

Publication number: WO2006059497A1
Application number: PCT/JP2005/021221
Authority: WO
Inventors: Yousuke Fukumoto; Teruo Matsushita; Edmund Soji Otabe; Masaru Kiuchi
Original assignee: Kyushu Institute Of Technology
Priority date: 2004-12-01
Filing date: 2005-11-18
Publication date: 2006-06-08
Also published as: JPWO2006059497A1

Abstract

Simple and highly accurate method and device for measuring critical current density of a superconductor are provided by improving the conventional third harmonic voltage analyzing method. In the method, the critical current density of the superconductor is measured by permitting an alternating current to flow in a coil arranged in a vicinity of the superconductor, and detecting the alternating current and a third harmonic induced voltage induced to the coil by the alternating current. The method is characterized in that independent two coils are used, and application of alternating current magnetic field and measurement of the induced voltage are performed by the separate coils. The device employing such method is also provided.

Description

Specification

Method and apparatus for measuring critical current density of superconductor

Technical field

The present invention relates to a method of measuring the critical current density of a superconductor, and more particularly to an improved method of third harmonic voltage analysis and an apparatus used therefor.

Background art

[0002] Since superconductors have the property of becoming in a superconducting state with zero electrical resistance when cooled below the critical temperature, various applications are conceivable in terms of engineering. As the critical temperature, so-called high temperature superconductors have also been found that can obtain a superconducting state under liquid nitrogen, which is as high as possible. And, as a superconducting applied device, for example, there are a superconducting magnet, a large capacity power transmission cable, a transformer, a current limiting device, a high sensitivity magnetic sensor, and they are already in practical use.

[0003] Since a superconductor has zero electrical resistance at a temperature below the critical temperature, it has a force that can flow a much larger current than commonly used copper and gold wires. Depending on the material, there is a limit to the current that can flow, and the maximum value is called the critical current. And, in general, superconductors with high critical current are more useful in engineering.

[0004] When applying a superconductor in practice, it is necessary to know the critical current characteristics in various temperatures and magnetic fields. As a method of measuring the critical current of a superconductor, the force by which various methods such as the four-terminal method, the magnetic field relaxation method, the Campbell method and the like are known is one of the most used methods. It is a four-terminal method in which four electrodes with a voltage terminal and a voltage terminal are attached and a current is supplied to measure the voltage. In this method, when the current flowing increases and exceeds the critical current, a voltage is generated from the superconductor, and by measuring this, it is possible to evaluate the critical current density. However, with this method, it is necessary to process the superconductor, and the deterioration of the superconducting characteristics at that time becomes a problem.

The critical current density is the critical current per unit area obtained by dividing the critical current by the cross-sectional area of the superconductor. [0005] A third harmonic voltage analysis method is considered promising as a method for nondestructively and noncontactly evaluating the superconducting characteristics of a superconductor. This method is a method in which a coil is placed on a sample made of a superconductor, an alternating current is supplied to it, and a third harmonic component of a voltage induced in the coil is measured and analyzed to obtain a critical current density. . In this method, since it is not necessary to apply a current directly to the sample, it is not necessary to carry the sample, and since it is non-contact, it is possible to measure one after another over a large area, which is a convenient method. However, in the third harmonic voltage analysis method which has been proposed so far (Patent Document 1), the application of the AC magnetic field and the measurement of the induction voltage are performed with one coil, so the sensitivity point and the noise point I have a problem with you.

In order to solve the problem of noise in the third harmonic voltage analysis method, a method using a cancel coil has also been proposed separately from the application of an alternating magnetic field and the coil for measurement of the induction voltage (patented) Literature 2). However, in this method, since the cancel coil is used, the measurement apparatus becomes complicated, and a measurement procedure for cancellation is also required, which is inconvenient. Furthermore, since the application of the AC magnetic field and the measurement of the induction voltage are performed with one coil, there is a problem that the sensitivity is low and noise is likely to enter.

Patent Document 1: Japanese Patent Application Laid-Open No. 2003-207526

Patent Document 2: Japanese Patent Application Laid-Open No. 2004-69674

Disclosure of the invention

Problem that invention tries to solve

When manufacturing a superconducting product, accurate critical current density measurement is required. Also, from the standpoint of improving the productivity of the product and managing the quality, a simple and accurate method of measuring critical current density and accuracy are indispensable, and development of better methods and equipment is desired. It was Therefore, an object of the present invention is to provide a simple and highly accurate method and apparatus for measuring critical current density of a superconductor, which is an improvement of the conventionally proposed third harmonic voltage analysis method.

Means to solve the problem

According to the present invention, an alternating current is supplied to a coil disposed in the vicinity of a superconductor, and the alternating current and the third harmonic induction voltage induced in the coil by the alternating current are detected. In a method of measuring the critical current density of the superconductor, two independent coils are used, and the application of the alternating magnetic field and the measurement of the induction voltage are performed by different coils. It is a current density measurement method.

[0009] Another aspect of the present invention is a coil disposed in the vicinity of a superconductor, a means for passing an alternating current thereto, and a third harmonic induction voltage induced in the coil by the alternating current and the alternating current. In a device for measuring the critical current density of the superconductor, which comprises means for detecting a coil, a coil for applying an alternating magnetic field and a coil for measuring an induction voltage are provided separately. It is a current density measuring device.

Effect of the invention

According to the present invention, there is provided a simple and highly accurate method of measuring critical current density of a superconductor, and an apparatus therefor, which is an improvement of the conventionally proposed third harmonic voltage analysis method. Moreover, since it is possible to apply a larger alternating current magnetic field than in the conventional method 'product, it is possible to measure a larger critical current density, and it is also possible to increase the distance between the sample and the coil. It becomes. In addition, since the measurement of the induced voltage is performed more effectively, the sensitivity is improved and the measurement of a small critical current density is also possible. Then, by using such a method and apparatus, for example, the productivity of the superconducting wire can be improved and the quality can be ensured.

Brief description of the drawings

FIG. 1 is an explanatory view of a conventional method of applying an AC magnetic field and measuring an induced voltage.

FIG. 2 is an explanatory view of a method of applying an AC magnetic field and measuring an induced voltage according to the present invention.

FIG. 3 is a view showing the relationship between the third harmonic induction voltage and the current obtained by the conventional measurement method.

FIG. 4 is a view showing the relationship between the third harmonic induction voltage and the current obtained by the measurement method of the present invention.

Explanation of sign

[0012] 1 Superconductor

2 coils

3 Pickup coil for inductive voltage measurement

4 Coil for AC magnetic field application BEST MODE FOR CARRYING OUT THE INVENTION

The present invention measures the critical current density of a superconductor by the third harmonic voltage analysis method. I cos (co t) current in small coils on the surface of a sufficiently wide superconductor film

0

And apply a magnetic field perpendicular to the surface, the coil has a third high V cos (3 co t +))

It is known that 3 3 harmonic voltages are induced. Then, specifically, a small coil is placed in the vicinity of the superconductor, for example, above the superconductor, and an alternating current is supplied to it, and the third harmonic component of the voltage induced in the coil is measured. Since the voltage is generated from the threshold current, the critical current density of the superconductor can be evaluated (see, for example, Patent Document 1).

By passing an alternating current through the coil, an alternating magnetic field is generated from the coil and applied to the superconductor. When the ac magnetic field is small, the superconductor completely reflects the magnetic field, but when the ac magnetic field becomes large, the shielding is not perfect and the opposite side ac magnetic field force is also generated. If this is observed with the voltage at both ends of the same coil, the third harmonic component will generate a signal when the shielding is broken when the AC magnetic field is shielded. The magnitude of the AC magnetic field and the critical current density have a direct proportional relationship, and the magnitude of the AC magnetic field can be evaluated.

[0015] This technique is superior to the four-terminal method in that measurement is relatively easy and that it is possible to measure nondestructively because terminals do not need to be connected to a superconductor. In addition, since it can be measured in a non-contact manner, for example, it is easy to use in the field of manufacturing a superconducting wire. Further, since local information about the size of the coil can be obtained, it is possible to identify, for example, a portion where the critical current density of the superconducting wire is degraded.

In the present invention, as means for detecting and measuring the third harmonic induction voltage, a coil, which is disposed in the vicinity of the superconductor, is a means for flowing an alternating current and applying a magnetic field to the superconductor. It is characterized in that a coil (coil for measuring an induction voltage) independent of (a coil for applying an alternating magnetic field) is used. The coil for applying the alternating magnetic field and the coil for measuring the induction voltage may be disposed in the vicinity of each other, but the coil for applying the alternating magnetic field is arranged inside the coaxial, and the coil for measuring the induction voltage is provided outside thereof. It is preferable that the configuration is arranged. Alternatively, or alternatively, a coil for applying an alternating magnetic field is disposed outside the coaxial, and A configuration in which a coil for measuring the induction voltage is disposed on the side is also preferable.

In general, a coil (magnet) with a small number of turns using a thick wire is used as a coil for applying an alternating magnetic field, and a coil using a thin wire is used as a coil for measuring an induction voltage. A coil with a large frequency (pickup coil) is used. Then, since the voltage induced to the pickup coil is composed of the fundamental wave component and the third harmonic component force, only the third harmonic component is separated and measured using, for example, a lock-in amplifier.

Hereinafter, description will be made using the drawings. Conventionally, as shown in FIG. 1 (cross-sectional view for explanation), a coil 2 is disposed on a superconductor 1, and application of an alternating magnetic field and measurement of an induced voltage are performed by one coil. In addition, in order to reduce the fundamental wave component from the induced voltage and to reduce noise, a cancel coil is separately provided (not shown), and the structure is complicated. Furthermore, in the prior art, there was a complication that a cancellation procedure was required to make the fundamental wave component differ from the induced voltage. When actually making measurements, it is directly affected by the noise of the power supply for energizing the alternating current, and therefore there is a problem that the noise is large.

In the present invention, conventionally, a coil for generating an alternating magnetic field and a coil for measuring an induction voltage are separately prepared. For example, as shown in FIG. 2 (cross-sectional view for explanation), a coil group is disposed above the superconductor 1. By devising the shape of the coil, the pickup coil 3 for measuring the induction voltage is arranged outside the coaxial, and the coil 4 for applying an alternating magnetic field is arranged inside. By doing this, the device becomes simple and the sensitivity is improved. In addition, the method 'device of the present invention simplifies the structure because it eliminates the need for the conventional cancel coil. Furthermore, no measurement procedure for cancellation is required. Also, since the sensitivity is improved, the distance between the superconductor and the measurement coil can be increased as compared with the conventional one, and the characteristics of the wire can be evaluated at high speed.

Example

[0020] As shown in Fig. 1, an experiment using a conventional measurement method apparatus in which a coil 2 is disposed on a superconductor 1 and application of an AC magnetic field and measurement of an induction voltage are performed by the coil 2 As shown in FIG. 2, according to the present invention, a coil group in which a pickup coil 3 for measuring an induction voltage is disposed outside the coaxial and a coil 4 for applying an alternating magnetic field is disposed above the superconductor 1 as shown in FIG. Measurement of Method · We conducted an experiment using the device.

[0021] The measurement was performed as follows. The signal from the function synthesizer is amplified by the bipolar power supply to generate an alternating current. The AC current is applied to the coil 4 for applying an inner AC magnetic field shown in FIG. 2 to generate an AC magnetic field. At this time, the lock-in amplifier measures the voltage induced in the pickup coil 3 for measuring the induction voltage, which is disposed outside. In addition, in order to measure the third harmonic induction voltage, 3f, which is three times the frequency f of the function synthesizer, is input to the lock-in amplifier as a reference signal. This enables the lock-in amplifier to measure the component of the third harmonic of the applied AC magnetic field. Some recent function synthesizers can simultaneously output two frequencies so that harmonic components can be measured in this way, and measurement can be performed easily.

This measurement was performed to the place where the alternating magnetic field was small and the force was also large. FIG. 3 shows the relationship between the third harmonic induction voltage and the current obtained by the conventional measurement method (the method and apparatus of FIG. 1), and in principle, a voltage can be generated at a low magnetic field. In order to be affected by the force noise that should not rise, the voltage is measured at a small level, and it is not constant. On the other hand, Fig. 4 is based on the method of the present invention, and it is confirmed that the accuracy is significantly increased compared to Fig. 3 which is not affected by noise when the voltage is not measured in principle. it can. Also, it can be seen that the voltage rises smoothly even in the place where the voltage suddenly increases when the alternating magnetic field becomes large and the magnetic field reaches the opposite side of the superconductor. From this, it can be understood that the threshold can be accurately determined by actually performing measurement using the method of the present invention.

As a result of the above, in the case of the present invention, it has been confirmed that the noise at the time of measurement is reduced as compared with the conventional type, and the accuracy in the measurement of critical current density is increased.

Industrial applicability

According to the present invention, there is provided a simple and highly accurate method of measuring critical current density of a superconductor and an apparatus therefor, which is an improvement of the conventionally proposed third harmonic voltage analysis method. Powerful method · The device can be effectively used to improve the productivity and control the quality of various products using superconductors, such as superconducting wires.

Claims

The scope of the claims

[1] A critical current density of the superconductor is obtained by supplying an alternating current to a coil disposed in the vicinity of the superconductor and detecting a third harmonic induction voltage induced in the coil by the alternating current and the alternating current. What is claimed is: 1. A method of measuring critical current density of a superconductor, comprising using two independent coils and performing application of an alternating current magnetic field and measurement of an induced voltage with separate coils.

[2] The superconductor according to claim 1, wherein the coil comprises an independent coil for applying an alternating magnetic field disposed on the inner side of the coaxial and a coil for measuring the induced voltage disposed on the outer side. Critical current density measurement method.

[3] The superconducting conductor according to claim 1, which comprises two independent coil forces, a coil for applying an alternating magnetic field, which is disposed on the outside of the coaxial, and a coil for measuring the induced voltage, which is disposed on the inside. Critical current density measurement method.

[4] A coil disposed in the vicinity of a superconductor, a means for passing an alternating current thereto, and a means for detecting a third harmonic induction voltage induced in the coil by the alternating current and the alternating current. An apparatus for measuring a critical current density of a superconductor, wherein a coil for applying an alternating magnetic field and a coil for measuring an induction voltage are separately provided.

[5] The apparatus for measuring a critical current density of a superconductor according to [4], wherein a coil for applying an alternating magnetic field is disposed on the inner side of a coaxial, and a coil for measuring an induction voltage is disposed on the outer side of the same axis.

[6] The apparatus for measuring a critical current density of a superconductor according to [4], wherein a coil for applying an alternating magnetic field is disposed on the outside of the coaxial, and a coil for measuring the induction voltage is disposed on the inside of the same axis!