JPH0690244B2 - Superconductor critical current measurement method - Google Patents

Description

The present invention relates to a method for measuring a critical current of a superconductor.

[Prior Art] One of the typical applications of high critical temperature superconductors is the realization of superconducting magnets that operate at liquid nitrogen temperatures. For its application, it is necessary to put into practical use a wire that can carry a large current under a strong magnetic field of several T (tesla) or more.

Conventionally, evaluation of superconducting materials for this purpose has been performed by critical current measurement. A DC four-terminal method, in which a large current is directly applied to the wire, or a magnetization measurement method, in which the magnetization in a magnetic field is measured and an assumption is made with respect to the critical current to calculate the critical current, has been adopted as this method. However, it is known that a high critical temperature superconductor currently under development has a characteristic that it is a material having a strong two-dimensional property, and that a voltage is generated from a minute current, that is, loss and heat generation. This loss means that the magnet always generates a certain amount of heat even in a steady state, and the evaluation of this characteristic is extremely important in the operation of the magnet. However, for this evaluation, it was necessary to measure the voltage drop in the superconductor to a very small current, and the conventional method was difficult to measure due to sensitivity problems.

In the DC four-terminal method, thermoelectromotive force generally affects the DC voltage measurement at low temperatures, and it is difficult to increase the accuracy to 0.1 μV or more. Therefore, it is very difficult to measure the non-linearity in the minute current region when the practical wire has a small electric resistance during normal conduction.

In the normal AC four-terminal method, the voltage output of the superconductor 2
When there is a common mode in which the average potential of the terminals fluctuates at the same frequency as the frequency F of the input current, it is very difficult to reduce the effect to zero. That is, it is known that a false offset often appears in the measured resistance value. In addition, there are often couplings due to stray capacitance etc. at the input and output during AC measurement, but the pickup voltage due to the coupling also gives a false offset to the resistance value,
Its evaluation is difficult.

In the magnetization measurement method, there is no theory that gives the relationship between the magnetization and the critical current for such a small current, and it is completely useless.

[Problems to be Solved by the Invention] The present invention is to measure the current-voltage characteristics up to a current region much smaller than the conventional one.

[Means for Solving the Problems] In order to solve the problems, the present invention applies a sinusoidal alternating current of frequency F to a superconductor, and a voltage output from the superconductor is used as a reference wave in synchronization with the frequency F. A means of measuring the critical current of the superconductor by phase detection was used.

[Operation] While the present invention has the advantage of the AC method that the sensitivity is sufficiently improved without the influence of thermoelectromotive force because AC is used,
The effects of the common mode potential and the pickup found in the conventional AC method have the advantage that they are not affected because they all fluctuate at the frequency F and differ in frequency from the harmonic component detected in the present invention.

[Embodiment] As an embodiment, as shown in the principle diagram of FIG.
A method in which a sinusoidal alternating current of frequency F is applied to the phase detector 4 and the phase output of the superconductor 3 is detected by the phase detector 4 using a harmonic synchronized with the frequency F as a reference wave is given. In the superconducting state, for example, as shown in FIG. 2, a large non-linearity characteristic of the output voltage shown by the solid line appears with respect to the input current. Due to this non-linearity, the output voltage is distorted with respect to the input sine wave current as shown by the solid line in FIG. 3, but the present invention uses that the distortion is selectively detected by the harmonic detection.

FIG. 5 shows a concrete example thereof. Any harmonic frequency can be applied to the present invention as long as it is an odd multiple of the frequency of the input current, that is, 3F, 5F, 7F, etc., but 3F was used in this embodiment. In the embodiment shown in FIG. 5, an oscillator for supplying 3F synchronized with F, a filter section 2 for producing a sine wave output from an output F component, a superconductor 3, and a phase detector 4 for phase-detecting the voltage output of the superconductor 3 with a 3F component are provided. And a current monitor terminal 5. As shown in Fig. 4, the 3F harmonic component, which is three times F, in the output voltage has a maximum value when the critical current is slightly smaller than the maximum amplitude of the input sine wave current. That is, the determination of the critical current can be given from the peak of the output.

An example of measurement is shown in FIG. In the measurement, the temperature of the superconductor was changed while applying an alternating current of frequency F to the superconductor with a constant amplitude. Changing the temperature changes the critical current,
At the temperature where the current amplitude and the critical current are almost equal
The critical current at the temperature at which the 3F component has a negative maximum can be determined.

DC measurements taken at the same time are given for comparison.
In this 3F measurement, if the resolution is evaluated by the noise, it can be measured if there is an abnormality of 0.2 nV or more in the current-voltage characteristics.
Compared with the measured value of 0.1 μV, the resolution is finer to several hundredths.

The particularly important points in this embodiment will be described below.

The current flowing in the superconductor needs to be a distortion-free sine wave of frequency F. Therefore, it will generally be necessary to remove frequency components above 3F in the filter system.

This measurement method uses two synchronized frequencies, F and 3F, and it is important to determine the mutual phase relationship. Here, the correct output is obtained by temporarily guiding the output from the current monitor terminal to the phase detector and determining the phase of the phase detector 4 so that the output after the phase detection is as shown in FIG.

[Effects of the Invention] The present invention having these features, as in the examples, has a high level that cannot be achieved by any conventional method that can resolve anomalies to a voltage conversion of 0.2 nV in the current-voltage characteristics of a superconductor. Sensitivity can be obtained. This method can be measured even in a magnetic field, and processing of superconductors is no different from the conventional four-terminal method and is simple. Therefore, it can be easily replaced by the DC four-terminal method as a material evaluation method, and the sensitivity can be dramatically improved. Further, since the present invention provides an extremely high-sensitivity measurement method for current-voltage characteristics, if there is a material or the like whose current-voltage characteristics depend on temperature, magnetic field, etc. It is also possible to manufacture a sensitive device or sensor.

[Brief description of drawings]

FIG. 1 is a block diagram showing the principle of the present invention. FIG. 2 is a graph showing an abnormal current-voltage characteristic expected in a superconductor. FIG. 3 is a graph showing the output voltage which is distorted by the input current (broken line) due to the characteristics of FIG. FIG. 4 is a graph showing the critical current dependence of the 3F harmonic component. FIG. 5 is a block diagram showing an embodiment. FIG. 6 is a graph showing the measurement results according to the example. FIG. 7 is a graph showing an output waveform used for phase determination in the embodiment. 1 …… Oscillator that supplies 3F synchronized with F 2 …… Filter section 3 …… Superconductor 4 …… Phase detector 5 …… Current monitor terminal 6 …… Output

Claims (1)

[Claims] 1. A superconducting device for measuring a critical current of a superconductor by applying a sinusoidal alternating current of frequency F to the superconductor and phase-detecting a voltage output from the superconductor using a harmonic synchronized with the frequency F as a reference wave. Body critical current measurement method