JPWO2006059497A1 - Method and device for measuring critical current density of superconductor - Google Patents

Abstract

  An object of the present invention is to provide a simple and highly accurate method and apparatus for measuring a critical current density of a superconductor, which is an improvement over the conventionally proposed third harmonic voltage analysis method. The present invention measures the critical current density of a superconductor by applying an alternating current to a coil arranged in the vicinity of the superconductor and detecting an alternating current and a third harmonic induction voltage induced in the coil by the alternating current. In the method, there are provided a method for measuring a critical current density of a superconductor, characterized in that two independent coils are used, and the application of an alternating magnetic field and the measurement of an induced voltage are performed by different coils, and a measuring device therefor.

Description

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

  Superconductors have the property of becoming superconducting with zero electric resistance when cooled to a critical temperature or lower, so various applications can be considered in terms of science and engineering. A so-called high-temperature superconductor has also been found, which has a critical temperature that is as high as possible and is easy to use, and a superconducting state can be obtained under liquid nitrogen. The superconducting applied devices include, for example, a superconducting magnet, a large-capacity power transmission cable, a transformer, a current limiter, and a high-sensitivity magnetic sensor, and some of them have already been put to practical use.

  Since the electric resistance of superconductors becomes zero at a temperature below the critical temperature, it is possible to pass a much larger current than the commonly used copper wire or gold wire, but depending on the material of the superconductor, There is a limit to the current that can flow, and the maximum value is called the critical current. In general, a superconductor having a high critical current is technically more useful.

When actually applying a superconductor, it is necessary to know the critical current characteristics in various temperatures and magnetic fields. Various methods such as four-terminal method, magnetization relaxation method and Campbell method are known as methods for measuring the critical current of a superconductor. This is a four-terminal method in which four electrodes of voltage terminals are attached and a voltage is measured by passing an energizing current. In this method, when the flowing current becomes large and exceeds the critical current, a voltage is generated from the superconductor, so that the critical current density can be evaluated by measuring the voltage. However, this method requires processing of the superconductor, which causes a problem of deterioration of superconducting characteristics.
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.

  The third harmonic voltage analysis method is regarded as a promising method for nondestructively and noncontactly evaluating the superconducting characteristics of superconductors. In this method, a coil is placed on a sample made of a superconductor, an alternating current is passed through it, the third harmonic component of the voltage induced in the coil is measured and analyzed, and the critical current density is determined. .. This method is a convenient method because it is not necessary to directly apply a current to the sample, so that it is not necessary to process the sample, and since it is non-contact, it is also possible to measure successively over a wide area. However, in the third harmonic voltage analysis method that has been proposed so far (Patent Document 1), since an AC magnetic field is applied and an induced voltage is measured with one coil, there is a problem in terms of sensitivity and noise. was there.

In order to solve the problem of noise in the third harmonic voltage analysis method, a method of using a cancel coil in addition to a coil for applying an AC magnetic field and measuring an induced voltage has been proposed (Patent Document 2). However, this method is inconvenient because the measuring device is complicated because a cancel coil is used and a measuring procedure for canceling is required. Further, since the AC magnetic field is applied and the induced voltage is measured with one coil, there is a problem that sensitivity is low and noise is likely to enter.
JP, 2003-207526, A JP, 2004-69674, A

  When manufacturing superconducting products, it is necessary to accurately measure the critical current density. Further, from the viewpoint of improving the productivity and controlling the quality of the product, a simple and highly accurate method and apparatus for measuring the critical current density are indispensable, and the development of a better method and apparatus is desired. It was Therefore, an object of the present invention is to provide a simple and highly accurate method and apparatus for measuring the critical current density of a superconductor, which is an improvement over the conventionally proposed third harmonic voltage analysis method.

  According to the present invention, an alternating current is passed through a coil arranged in the vicinity of a superconductor, and the alternating current and a third harmonic induction voltage induced in the coil by the alternating current are detected to detect a critical current of the superconductor. In the method for measuring the density, a method for measuring the critical current density of a superconductor is characterized in that two independent coils are used, and the application of an alternating magnetic field and the measurement of the induced voltage are performed by separate coils.

  Another aspect of the present invention is a coil arranged in the vicinity of a superconductor, a means for supplying an alternating current to the coil, and a means for detecting the alternating current and a third harmonic induction voltage induced in the coil by the alternating current. In a device for measuring the critical current density of the superconductor, the device for measuring the critical current density of a superconductor, characterized in that a coil for applying an alternating magnetic field and a coil for measuring an induced voltage are separately provided. is there.

  According to the present invention, there is provided a simple and highly accurate method for measuring a critical current density of a superconductor, which is an improvement over the conventionally proposed third harmonic voltage analysis method, and an apparatus therefor. Since a larger AC magnetic field can be applied as compared with the conventional methods and products, a larger critical current density can be measured, and a larger distance between the sample and the coil can be obtained. Further, since the induced voltage is measured more effectively, the sensitivity is improved and a small critical current density can be measured. By using such a method/apparatus, for example, the productivity and the quality of the superconducting wire can be improved.

It is explanatory drawing of the conventional application method of an alternating magnetic field and the measuring method of an induced voltage. It is explanatory drawing of the application method of the alternating magnetic field of this invention, and the measuring method of an induced voltage. It is a figure which shows the relationship between a 3rd harmonic induced voltage and a current obtained by the conventional measuring method. It is a figure which shows the relationship between a 3rd harmonic induced voltage and a current obtained by the measuring method of this invention.

Explanation of symbols

1 superconductor 2 coil 3 pickup coil for measuring induced voltage 4 coil for applying AC magnetic field

The present invention is to measure the critical current density of a superconductor by the third harmonic voltage analysis method. When a current of I ₀ cos(ωt) is applied to a small coil on the surface of a sufficiently wide superconducting thin film and a magnetic field perpendicular to the surface is applied, the third harmonic voltage of V ₃ cos(3ωt+θ ₃ ) is applied to the coil. Is known to be induced. Then, specifically, in the vicinity of the superconductor, for example, a small coil is arranged on the upper side of the superconductor, an alternating current is passed through the coil, 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 this superconductor can be evaluated (see, for example, Patent Document 1 above).

  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 is large, the shielding is not perfect and the AC magnetic field leaks from the opposite side. Observing this at the voltage across the same coil, the third harmonic component has almost no signal when the AC magnetic field is shielded, and a signal is generated when the shield is broken. The magnitude of this alternating magnetic field and the critical current density have a direct proportional relationship, and the critical current density can be evaluated from the magnitude of the alternating magnetic field.

  This technique is superior to the four-terminal method in that the measurement is relatively simple and that it is not necessary to connect the terminal to the superconductor, so that the measurement can be performed nondestructively. In addition, since the measurement can be performed in a non-contact manner, it can be easily used, for example, in the field of manufacturing a superconducting wire. Furthermore, since local information about the size of the coil can be obtained, for example, a portion where the critical current density of the superconducting wire is deteriorated can be specified.

  In the present invention, as a means for detecting and measuring the third harmonic induced voltage, a coil (AC magnetic field application) which is arranged in the vicinity of the superconductor and is a means for flowing an alternating current and applying a magnetic field to the superconductor. The coil is characterized by using a coil (coil for measuring the induced voltage) independent of the coil (for measurement). The coil for applying an alternating magnetic field and the coil for measuring an induced voltage may be arranged in the vicinity of each other, but the coil for applying an alternating magnetic field is arranged inside the coaxial and the coil for measuring an induced voltage is outside thereof. A configuration in which is arranged is preferable. Alternatively, it is also preferable that the coil for applying an alternating magnetic field is arranged on the outer side of the same axis, and the coil for measuring the induced voltage is arranged on the inner side thereof.

  Generally, a coil using a thick electric wire and having a small number of turns is used as a coil for applying an alternating magnetic field, and a coil having a large number of turns using a thin wire is used as a coil for measuring an induced voltage ( Pickup coil) is used. Since the voltage induced in the pickup coil is composed of the fundamental wave component and the third harmonic component, only the third harmonic component is separated and measured using, for example, a lock-in amplifier.

  Hereinafter, description will be given with reference to the drawings. Conventionally, as shown in FIG. 1 (a cross-sectional view for explanation), a coil 2 is arranged on a superconductor 1, and one coil is used to apply an AC magnetic field and measure an induced voltage. Further, in order to reduce the fundamental wave component from the induced voltage and reduce the noise, a cancel coil is separately provided (not shown), which makes the structure complicated. Further, the conventional device has a complicated procedure that requires a canceling procedure in which the fundamental wave component is differentiated from the induced voltage. In actual measurement, there is a problem that the noise is directly affected by the noise of the power supply for passing the alternating current, and thus the noise is large.

  According to the present invention, conventionally, a coil for generating an alternating magnetic field and a coil for measuring an induced voltage are provided separately, respectively. For example, as shown in FIG. 2 (cross-sectional view for explanation), a coil group is arranged above the superconductor 1. By devising the shape of the coil, the pickup coil 3 for measuring the induced voltage is arranged on the outer side of the coaxial, and the coil 4 for applying the alternating magnetic field is arranged on the inner side. By doing so, the device becomes simple and the sensitivity is improved. Further, the method and apparatus of the present invention do not require the canceling coil which has been conventionally used, so that the structure is simple. Furthermore, the measurement procedure for canceling is unnecessary. Further, since the sensitivity is improved, the distance between the superconductor and the measuring coil can be increased as compared with the conventional one, and the characteristic evaluation of the wire can be performed at high speed.

  As shown in FIG. 1, a coil 2 is arranged on a superconductor 1, and an experiment using a conventional measuring method/apparatus for applying an alternating magnetic field and measuring an induced voltage with the coil 2 is performed. As shown in FIG. 3, a measuring method and apparatus of the present invention using a coil group in which a pickup coil 3 for measuring an induced voltage is coaxially arranged outside the superconductor 1 and a coil 4 for applying an alternating magnetic field is arranged inside the superconductor 1. Experiments using

  The measurement was performed as follows. A signal from the function synthesizer is amplified by a bipolar power supply to generate an alternating current. This alternating current is passed through the coil 4 for applying the inner alternating magnetic field shown in FIG. 2 to generate an alternating magnetic field. At this time, the voltage induced in the pickup coil 3 for measuring the induced voltage arranged outside is measured by the lock-in amplifier. 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 allows the lock-in amplifier to measure the third harmonic component of the applied AC magnetic field. Some recent function synthesizers are capable of outputting two frequencies at the same time so that the harmonic components can be measured, and the measurement can be performed easily.

  This measurement was performed from a place with a small alternating magnetic field to a place with a large alternating magnetic field. FIG. 3 shows the relationship between the third harmonic induction voltage and the current obtained by the conventional measurement method (method/apparatus of FIG. 1). In principle, the voltage is not generated at a low magnetic field. It should not come, but because it is affected by noise, the voltage is measured though it is minute, and it is not constant. On the other hand, FIG. 4 is based on the method of the present invention, and it can be confirmed that, in principle, in a place where the voltage is not measured, it is less susceptible to noise and the accuracy is remarkably increased as compared with FIG. it can. Also, it can be seen that the voltage rises smoothly even in a place where the voltage suddenly increases when the alternating magnetic field increases and the magnetic field reaches the opposite side of the superconductor. From this, it can be understood that the threshold value can be accurately determined by actually performing the measurement using the method of the present invention.

  As a result of the above, it was confirmed that the present invention reduces noise during measurement and improves accuracy in measuring the critical current density as compared with the conventional type.

  According to the present invention, there is provided a simple and highly accurate method for measuring a critical current density of a superconductor, which is an improvement over the conventionally proposed third harmonic voltage analysis method, and an apparatus therefor. The method/apparatus can be effectively used for improving the productivity and controlling the quality of various products using superconductors, for example, superconducting wire.

Claims (6)

  The critical current density of the superconductor is measured by passing an alternating current through a coil arranged in the vicinity of the superconductor and detecting the alternating current and the third harmonic induced voltage induced in the coil by the alternating current. In the method, two independent coils are used, and the application of an alternating magnetic field and the measurement of the induced voltage are performed by separate coils. A method for measuring a critical current density of a superconductor.   2. The critical current density of a superconductor according to claim 1, wherein the two independent coils are composed of a coil for applying an AC magnetic field arranged inside the coaxial and a coil for measuring an induced voltage arranged outside. Measuring method.   2. The critical current density of a superconductor according to claim 1, wherein the two independent coils are composed of a coil for applying an alternating magnetic field arranged on the outer side of the coaxial and a coil for measuring an induced voltage arranged on the inner side. Measuring method.   A coil disposed in the vicinity of the superconductor, means for passing an alternating current through the coil, and means for detecting the alternating current and a third harmonic induction voltage induced in the coil by the alternating current, A device for measuring a critical current density, wherein a coil for applying an alternating magnetic field and a coil for measuring an induced voltage are separately provided, and a device for measuring a critical current density of a superconductor.   The critical current density measuring device for a superconductor according to claim 4, wherein the coil for applying the alternating magnetic field is arranged inside the coaxial, and the coil for measuring the induced voltage is arranged outside the coaxial.   The critical current density measuring device for a superconductor according to claim 4, wherein the coil for applying the alternating magnetic field is arranged on the outer side of the coaxial, and the coil for measuring the induced voltage is arranged on the inner side of the coaxial.

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