福岡 克弘

2020年度日本AEM学会技術賞 受賞

© 2019 IOS Press and the authors. All rights reserved. In this study, we developed and characterised a method for detecting microcracks in steel materials by eddy current testing (ECT). Magnetic saturation ECT is applied because magnetic noise is generated by steel materials during ECT. Although direct-current (DC) magnetisation is generally used for magnetic saturation ECT, the use of alternate-current (AC) magnetisation offers several advantages. When using AC magnetisation for surface flaw detection, the magnetisation is not influenced by the thickness of the test object owing to the skin effect; therefore, demagnetisation is unnecessary. In this study, we evaluated microcrack detection results obtained using AC magnetisation ECT with different synchronous positions (conditions) of the magnetisation and ECT; and identified the optimum synchronous position. In addition, the magnetisation distribution in a test object was evaluated by the finite element method analysis and the flaw detection results were verified.

© 2019 IOS Press and the authors. All rights reserved. Non-uniform permeability distributions in steel materials reduce signal-to-noise (S/N) ratios in eddy current testing (ECT). Therefore, steel materials must be magnetised to reduce magnetic noise. In this study, microcrack detection in spring steel materials using ECT was considered. An ECT system that was able to detect 40-μm-deep crack with S/N ratio of 3 or more was developed. In addition, the depth profile of microcracks using the detection signal was accurately estimated.

© 2018 Wiley Periodicals, Inc. Mechanical parts, plants, and cross-linkages inspected with magnetic-particle testing (MT) are typically complex 3D shapes. In complex 3D shape portions, because a magnetizer often cannot be configured to inspection portions and the test object cannot be appropriately magnetized, there is a possibility of overlooking a crack in such an instance. Thus, MT system development that was successfully able to detect omnidirectional cracks in 3D shape portions was considered in this study's trials. Two multi-coil type magnetizers were hence arranged face-to-face, and the magnetization of omnidirectional scenarios for all surfaces of 3D shape test object was evaluated.

<p>Mechanical parts, plants and cross-linkages inspected with magnetic-particle testing (MT) are typically complex 3D shapes. In complex 3D shape portions, because a magnetizer often cannot be configured to inspection portions and the test object cannot be appropriately magnetized, there is a possibility of overlooking a crack in such an instance. Thus, MT system development that was successfully able to detect omnidirectional cracks in 3D shape portions was considered in this study's trials. Two multi-coil type magnetizers were hence arranged face-to-face, and the magnetization of omnidirectional scenarios for all surfaces of 3D shape test object was evaluated.</p>

<p>Eddy current testing (ECT) system for the spring steel materials that is able to estimate crack shape was developed. Inspection signal of spring steel material is affected by the permeability distribution about its surface. Then the magnetizer that reduce variability of permeability and the pancake probe that grasps regional states of spring steel material were introduced to the system. It was confirmed thereby that this probe is able to detect depth 50 µm crack with a sufficient S/N ratio. Exciting coil that generates two-way direction eddy current was developed and average depth and outline of cross-sectional shape were estimated by its inspection signals. Then the method of estimating crack shape was developed, which has feature that integrates two results by different direction eddy current. In addition, the validity of the method was confirmed by evaluating relationship between crack shapes and calculated inspection results with finite element method (FEM) analysis.</p>

<p>A high-speed and high-sensitive eddy current testing (ECT) system was developed for the inspection of spring steel materials in manufacturing lines. The permeability distribution of the steel materials affects inspection results and the signal-to-noise (S/N) ratio decreases. Therefore, magnetizing the materials is required to reduce the magnetic noise. In this research, the spring steel materials that have artificial microcracks (29∼114 µm depth) were magnetized and inspected to evaluate detection performance of a developed ECT probe. It was confirmed that the probe is able to detect the 40 µm depth crack with the S/N ratio of 3 or more. The detectivity of pitting corrosions was evaluated for the consideration about applicability of the probe in natural flaws. In addition, the permeability distribution around the crack in the magnetic saturation ECT was clarified with a finite element method (FEM) analysis.</p>

<p>Mechanical parts, plants and cross-linkages inspected with MT are typically complex 3D shapes. In the complex 3D shape portion, because a magnetizer often cannot be configured to the inspection portion and the test object cannot be appropriately magnetized, there is a possibility of overlooking a crack in such an instance. Thus, MT system development that was successfully able to detect omnidirectional cracks in 3D shape portions was considered in this study's trials. Two magnetizers were hence arranged face-to-face, and the magnetization of omnidirectional scenarios for all surfaces of 3D shape test object (arranged in between both magnetizers) was evaluated.</p>

As magnetic particle testing (MT) detects microcracks using a simplified method, it is used for non-destructive inspection of ferromagnetic materials in various industrial fields. However, MT can provide only a rough distribution shape of a crack; a quantitative technique for evaluating the crack shape has not been established in conventional MT. In the present study, the relationship between the crack shape and the shape of the adhered magnetic particles was revealed by image measurements using a high-speed video camera. We developed a technique for quantitatively evaluating the crack shape from the results of magnetic particle indication in MT.

Nonuniform permeability distribution in ferromagnetic materials creates noise in eddy current testing (ECT). To overcome this problem, ferromagnetic materials are magnetized and inspected in the magnetic saturation state to reduce noise. In this study, I targeted a spring steel material that is not easily magnetized and developed an ECT system with an encircling coil probe that was able to detect microcracks. Clear signals for 70 μm deep microcracks were obtained with a signal-to-noise ratio of 3.5 by magnetizing a test object using the optimal exciting current.

In a rotating-magnetic-field-type magnetizer using a three-pole (three-phase) coil, omnidirectional cracks can be detected via a single test. However, the rotating magnetic field ultimately experiences non-homogeneous distribution in an area far from the centre of the magnetizer. In this study, a multi-coil-type magnetizer that widely obtains uniform rotating magnetic fields was hence developed. Distributions of rotating magnetic flux densities in test object were evaluated using finite-element analysis, and the utility of the multi-coil was confirmed. In addition, the technique for omnidirectional magnetization on whole surfaces of 3D-shaped test object was considered via implementation of the subject multi-coil magnetizer.

The maximum magnetic flux leakage from a crack is obtained when the magnetic flux is distributed to the longitudinal direction of the crack in the orthogonal direction. In magnetic-particle testing with a yoke method, since we usually cannot predict the direction of the crack to be detected, it is necessary to perform the testing at least two times by changing the direction of magnetization. In a rotating magnetic field type magnetizer using a three-pole coil (a three-phase alternating current), omnidirectional crack can be detected by a single testing. However, the rotating magnetic field becomes no homogeneous distribution at a position far from the center of the magnetizer, and the strong magnetic field is generated in a specific direction. In this research, the multi coil magnetizer that widely obtained uniform rotating field distribution was developed. The distribution of the rotating magnetic flux density on the steel plate was evaluated with a finite element method analysis. In addition, a prototype magnetizer was produced based on the analytical results, and the characteristic was evaluated.

The maximum magnetic flux leakage from a crack is obtained when the direction of the magnetic flux is orthogonal to the longitudinal direction of the crack. In magnetic-particle testing with a yoke method, since we usually cannot predict the direction of the crack to be detected, it is necessary to perform the testing at least two times by changing the direction of magnetization. In a rotating magnetic field type magnetizer using three-pole coils (three-phase alternating current), omnidirectional crack can be detected by a single testing. However, directions of the weak magnetic flux density appear at positions far from the center of the magnetizer, and the rotating magnetic field becomes no homogeneous distribution. In this research, it was considered to split each magnetic pole in the magnetizer to generate the uniform rotating magnetic field. The distribution of the rotating magnetic flux density was evaluated with a finite element method analysis, and an optimal disposition angle of the split coil was discussed. In addition, a multi coil magnetizer was developed to generate the uniform rotating magnetic field more widely.

Because magnetic particle testing (MT) can detect microcracks by a simplified method, it is applied in non-destructive inspections of ferromagnetic materials in various industrial fields. Recently, establishing a technology that quantitatively evaluates crack shapes as well as detects cracks with high-precision has become an important topic in the non-destructive inspection. We consider developing such a quantitative evaluation technique that employs magnetic particle pattern of a crack in MT. In this research, the process of magnetic particle adherence to a crack was observed with a high-speed video camera and the change in the magnetic particle amount was evaluated at each instant. In addition, the technique for evaluating small magnetic flux leakage (MFL) density accurately was discussed with the measurement and the numerical analysis. From these results, the relationship between the MFL density and the magnetic particle amount was evaluated.

We designed exciting coils of uniform eddy current multiprobes with considering the shape of structures in order to obtain a uniform eddy current flow. A probe for a flat plate could obtain clear signals for an electrical discharge machining (EDM) slit of depth 0.5 mm and distinguish EDM slits arranged at 2 mm intervals. We confirmed that stress corrosion cracking (SCC) is detectable with sufficient sensitivity using the developed multiprobe. The probe for a curved portion obtained clear signals for the EDM slits on a curved surface of radius 25 mm. Moreover, we examined the technique for estimating crack shape from detection signals. © 2012 - IOS Press and the authors. All rights reserved.

Quantitative investigation of a standard test shim for magnetic particle testing has been carried out. The current values of the magnetizing yoke to obtain magnetic particle indication were measured experimentally. An analytical model for the experimental conditions was then formulated to obtain the amplitude of the leakage magnetic flux from an artificial groove in a type A standard test shim. The magnetic flux densities in the type A standard test shim and the test object were obtained using finite element analysis. In a test object with high permeability, the magnetic flux density was almost in the saturation regime when the magnetic particle indication was obtained. On the other hand, in a test object not having high permeability, the magnetic flux density was small when magnetic particle indication was obtained. © 2011 Elsevier Ltd. All rights reserved.

In a conventional transformer, secondary voltage and power transfer efficiency decrease when the magnetic flux leaks from the magnetic circuit of the transformer and the leakage reactance increases. Magnetic shielding of the magnetic circuit of the transformer with plural bulk high temperature superconductors (HTSs) was evaluated as a method of improving transformer characteristics. In our previous research, transformer characteristics were evaluated with a constant secondary load at each frequency. In this research, appropriate secondary loads were connected for each frequency to evaluate the characteristics in conformity with the actual transformer operation. The frequency characteristics of transformers with and without bulk HTSs shielding were evaluated with a constant magnetic flux density in the magnetic circuit. An increase in the secondary voltage and power transfer efficiency was confirmed for a transformer with bulk HTSs shielding at all frequencies, as dispersion of the magnetic flux was controlled by bulk HTSs shielding. © 2009 Elsevier B.V. All rights reserved.

The establishment of non-destructive inspection technology for plant structures is necessary, since the occurrence of cracks has been reported in some nuclear power plants. In this research, a uniform eddy current multi-probe to inspect cracks on a curved structure was developed. We designed exciting coils of this probe, considering the shape of the curved structure, so that the eddy current flows uniformly. Pick-up coils were arranged on a flexible printed circuit board to fit on the curved surface shape portion. The detection characteristics for EDM (electrical discharge machining) slits provided on the curved surface shape portion of the specimen were evaluated. The clear signals for the EDM slits provided on the curved surface which had a curvature radius of 25 mm were obtained by this probe. We confirmed that the crack shape could be estimated by detecting the signals from the developed probe.

In magnetic-particle testing, the maximum leakage flux from a flaw is obtained when the direction of the magnetic flux is normal to the longitudinal direction of the flaw. Since we usually cannot predict the direction of the flaw to be detected, it is necessary to perform the testing at least two times by changing the direction of magnetization. In a rotating field type magnetizer using a three-phase alternating current (AC), any directional flaw can be detected by a single testing. However, until now, the details of the rotating magnetic flux density in a specimen and its change with time have not been understood. In this research, the distributions of the rotating magnetic flux density were evaluated by measuring this density in three directions using Hall elements and by numerical analysis. The distributions of the magnetic flux density around the flaw were also evaluated by numerical analysis. The distribution of the rotating magnetic flux density near the central part of the magnetizer was made clear, and it was confirmed that the distribution became more uniform by equalizing the impedance of each coil of the magnetizer.

Some bulk high temperature superconductors (HTSs) are inhomogeneous due to weak links. Thus, we have measured distributions of the magnetic flux density on the HTSs surface using a Hall element, and evaluated its magnetic characteristics with a magnetic field visualization technique. However, the evaluation of the magnetic flux density at the actual surface layer of the HTS was difficult in the measurement using the Hall element. In this research, we examined the quantitative evaluation for the magnetic characteristics of the HTS including weak links with a three-dimensional finite element method (FEM) analysis in static magnetic fields. The distribution of the magnetic flux density at the actual surface layer of the HTS specimen and the diffusion of the magnetic flux in the space above the HTS surface were evaluated with FEM analysis. The validity of the proposed analytical method was confirmed by comparison with the experimental results and the verification of the analysis accuracy. © 2008 Elsevier B.V. All rights reserved.

&nbsp;&nbsp;The objectives of this study are to develop a new inspection probe for complex structures using eddy current testing method (ECT), to establish the recognition technique of real flaw shape such as stress corrosion cracking (SCC), and to develop a rapid nondestructive inspection system. An ECT system consists of measurement and computational modules that are connected via a network circuit. The measurement module includes a flexible ECT sensor, which could fit on the curvature surface of radius larger than 25 mm with a mechanical manipulator. The computational module is used for SCC sizing. It includes a database of ECT signals that are used to execute numerical calculations. SCC sizing can be evaluated by comparing the measured ECT signals with the calculated ECT signals within the database. We have demonstrated that the system could be used to estimate the SCC length and depth sizing in 140 s. The accuracy of length sizing showed an RMS error of 3.7 mm and the depth sizing was 0.63 mm in this work.<br>

In a transformer using a ferromagnetic core, the magnetic flux comes to leak easily from the core at the higher frequency range. Thus, we have examined the magnetic shielding of a magnetic circuit of the transformer with plural bulk high temperature superconductors (HTSs) in order to improve characteristics of the transformer at the higher frequency range. From previous researches, it was confirmed that the secondary voltage and the power transfer efficiency of the transformer with the bulk HTSs shielding increased. In this research, it was examined to make surrounding the transformer with the bulk HTSs double in order to strengthen the magnetic shielding with the HTSs, and characteristics of the transformer were evaluated at the further high frequency range. From experimental results, it was confirmed that the characteristics of the transformer were excellent at 100 kHz or more. © 2006 Elsevier B.V. All rights reserved.

The establishment of the nondestructive inspection technology with plant structures has been stimulated by the recent occurrence of cracks in the nuclear power plant structures. In this research, a uniform eddy current multi-probe to apply to the complex structure and inspect the cracks at high-speed data acquisition was developed. Pick-up coils of the developed probe were arranged on a flexible printed circuit board. This probe was able to obtain clear signal for an EDM (electro-discharge machining) slit with 0.5 mm depth and distinguish EDM slits arranged at 2 mm intervals. It was confirmed that the SCC (stress corrosion cracking) of real flaw was able to be detected with developed uniform eddy current multi-probe by using the ferrite core for the exciting coil and considering the impedance matching of the exciting coil and the flaw detection device.

AC magnetic properties of bulk high-temperature superconductor were evaluated with three-dimensional finite element method (FEM) analyses. When an external magnetic field strength increased, the heat was generated in weak link parts and the critical current density decreased greatly in these parts. The critical current densities in the weak link parts were estimated by experimental and analytical results under various external magnetic field strengths. The relation among the external magnetic field strength, the critical current density, and the heat generation were evaluated overall. © 2005 Elsevier B.V. All rights reserved.

Since bulk high-Tc, superconductor (HTS) characteristics are not homogeneous in some specimens due to grain boundaries and cracks, magnetic characteristics should be assessed under the static and AC magnetic field. Thus, static magnetic characteristics of HTS have been evaluated by measurements of the magnetic flux density on the specimen surface using a Hall element and numerical analyses using a finite element method. AC magnetic characteristics have been experimentally evaluated by measurements of the magnetic field using a Hall element and pickup coils. In this research, AC magnetic characteristics of HTS are evaluated by numerical analyses using a three-dimensional finite element method. Critical current densities of HTS under various external magnetic field strength are estimated by analysis and experiment results. Moreover, the heat generation of the superconductor is considered. © 2005 Wiley Periodicals, Inc.

As for a general transformer using a ferromagnetic core, the magnetic flux may leak by using the high frequency and the overload. When the magnetic flux leaks from the core, the secondary voltage and its power transfer efficiency decrease. Bulk high Tc superconductors (HTS's) with the high critical current density have the strong magnetic shielding characteristic at the static magnetic field and the AC magnetic field. Thus, we examined to surround a magnetic circuit of a transformer with bulk HTS's. From this research, when the magnetic circuit of the transformer was surrounded with plural HTS's, it was confirmed that the secondary voltage and the power transfer efficiency of the transformer increased especially in the high frequency range.

Recently, bulk-type high-Tc superconductors (HTSs) having a characteristic of critical current density over 104 A/cm2 in liquid nitrogen (77 K) and at 1 T have been produced. They are promising for many practical applications such as a magnetic bearing, a magnetic levitation, a flywheel, a magnetic shielding, and others. In this research, we propose a magnetic circuit that can be used as a magnetic shield for several superconductors as an application of the HTS. It is a closed magnetic circuit by means of a toroidal core. Characteristics of the magnetic circuit surrounded with HTSs are evaluated and the possibility is examined. Because the magnetic circuit of the ferrite core is surrounded with HTSs, the magnetic flux is shielded even if it leaks from the ferrite core. © 2004 Wiley Periodicals, Inc.

High-Tc superconductors (HTS), which have the characteristics of critical current density over 3 × 104 A/cm2 at liquid nitrogen temperature (77 K) and 1 T, can be produced. Thus, they are promising for many practical applications such as a magnetic bearing, magnetic levitation, flywheel, and magnetic shielding. Since the HTS characteristics are not homogeneous in some specimens due to grain boundaries and cracks, the distribution of magnetic characteristics should be assessed. Thus, we have measured the distribution of the magnetic flux density on the surface of the HTS using a Hall element, and have evaluated its magnetic characteristics. The measurement of magnetic characteristics using a Hall element is difficult regarding the distribution of the magnetic flux density on the actual surface and inside of the HTS sample. In this research, we conducted a quantitative evaluation of the magnetic shielding characteristics of the HTS including weak links under a static magnetic field with the three-dimensional finite element method analysis.

Since bulk high T_c superconductor (HTS) characteristics are not homogeneous in some specimens due to grain boundaries and cracks, magnetic characteristics should be assessed under the static and AC magnetic field. Thus static magnetic characteristics of the HTS have been evaluated by measurements of the magnetic flux density on the specimen surface using a hall element and numerical analyses using a finite element method. AC magnetic characteristics have been experimentally evaluated by measurements of the magnetic field using a hall element and pick up coils.<br>&nbsp;&nbsp;In this research, AC magnetic characteristics of HTS are evaluated by numerical analyses using a three-dimensional finite element method. Critical current densities of the HTS under various external magnetic field strength are estimated by analysis and experiment results. Moreover, the heat generation of the superconductor is considered.

Recently, bulk-type high T_c superconductors having a characteristic of critical current density over 10⁴ A/cm² in liquid nitrogen temperature (77K) on 1T, can be produced. They are promising for many practical applications such as a magnetic bearing, a magnetic levitation, a flywheel, a magnetic shielding and others. In this research, we propose a magnetic circuit that is able to use for the magnetic shield of plural superconductors as an application of bulk-type high T_c superconductors. It is a closed magnetic circuit by means of a toroidal core. Characteristics of the magnetic circuit surrounded with superconductors are evaluated and the possibility is examined. As the magnetic circuit of the ferrite core is surrounded with superconductors, the magnetic flux is shielded even if it leaked from the ferrite core.

High-T_c superconductors (HTS), which have a characteristic of the critical current density over 3&times;10⁴ A/cm² in liquid nitrogen temperature (77K) and 1T, can be produced. Thus, they are promising for many practical applications such as a magnetic bearing, a magnetic levitation, a flywheel, a magnetic shielding and etc.. Since the HTS characteristics are not homogeneous in some specimens due to grain boundaries and cracks, the distribution of magnetic characteristics should be assessed. Thus, we have measured the distribution of the magnetic flux density on the surface of the HTS using a hall element, and have evaluated its magnetic characteristics. The measurement of magnetic characteristics using a hall element is difficult for measuring the distribution of the magnetic flux density on the actual surface and the inside of the HTS sample. In this research, we examined a quantitative evaluation of the magnetic shielding characteristics of the HTS including weak links under the static magnetic field with the three-dimensional finite element method analysis.

東京大学 学位記番号：第15158号

The magnetic shielding is used as one of applications of the high-Tc superconductor (HTS). Magnetic characteristics of the HTS are not uniform, since it has grain boundaries and cracks (weak links). Thus, the evaluation of the magnetic flux invasion by the flux flow into the HTS is important. In this research, magnetic characteristics of the HTS on AC magnetic field from 1 to 250 Hz were evaluated. From these experiments, we were able to understand that the polycrystal HTS had high magnetic shielding performance at the higher frequency, even if it contains a lot of weak links.

Strong magnetic fields can be generated by magnetization of the high-T_c superconductor(HTS)with pulsed magnetic fields using a small exciting coil. The magnetization using the pulsed magnetic field is performed on the zero field cooled state for HTS bulk samples. It is necessary to evaluate the property of the pulsed field magnetization of it on the pulsed field magnetization process. Thus, we measured the magnetic flux density on the pulsed field magnetization process and evaluated the property of the magnetization of single crystal YBCO bulk superconductor. The optimum conditions were studied for the pulsed field magnetization of the HTS. In addition, we examined the improvement of the magnetization characteristic on repeatedly impressing the pulsed magnetic field and deterioration in the material because of the pulsed magnetization.

YBCO superconductors, which have the characteristic of critical current density over 1×10^4A/cm^2 in liquid nitrogen temperature (77K) on 1T, can be produced. Thus, they are promising for many practical applications such as the magnetic shielding. The magnetic property of the high-Tc superconductors (HTSCs) don't uniform in the specimen, they have discontinuous distribution by grain boundaries and cracks. It is important in order to design applied HTSC to develop an evaluation method of the magnetic property distribution. Thus, we had developed the magnetic field visualization system, which enables the measurement of the magnetic field using a Hall device and produced the visual images of the magnetic field on a computer display. The magnetic characteristics of the HTSC on the DC magnetic field and the AC magnetic field with the frequency of 50 Hz have been evaluated using this visualization system. However, the magnetic characteristic evaluations of various frequency elements are necessary to use the HTSC by actual applications.Thus, the magnetic properties of the HTSC on the AC magnetic field from 1 to 250 Hz were evaluated in this study. From the experimental results, we are able to understand that the polycrystal HTSC has good magnetic shielding performance on higher frequency, even if it contains many weak links.

A pulsed magnetic field using a small coil can be applied generating strong magnetic field for magnetization of the high-T-c superconductor (HTS). The magnetization using the pulsed magnetic field is done on the state of the zero field cooling. Wt examined optimum conditions for the pulsed field magnetization of the HTS. An optimum value of the pulsed field magnetization was confirmed. To predict the characteristics of the pulsed magnetic field to produce enough magnetization of the HTS, it is necessary to consider the time constant for the magnetic flux invasion and the flux flow at the decreasing process of the applied magnetic field. Moreover the relation between the pulsed magnetic field and the time constant of the magnetic flux invasion was explained by measurement of the pulsed field magnetization process.

Strong magnetic fields can be generated for magnetization of the high-T_c superconductor (HTS) with pulsed magnetic fields using a small exciting coil. In the present study, magnetization using the pulsed magnetic field is performed on HTS bulk samples in the zero-field cooled state. In order to saturate the magnetization, it was necessary to optimize the time constant of the magnetic flux invasion and the flux flow during the increasing and decreasing processes of the pulsed magnetic field. Therefore, we measured the magnetic flux density on the pulsed field magnetization process and explained the relation between the pulsed magnetic field and the time constant of the magnetic flux invasion and flux flow. An empirical model of the pulsed field magnetization process was also proposed and optimum conditions were studied for pulsed field magnetization of the HTS. From this study, it was confirmed that an optimum pulsed waveform existed for pulsed field magnetization.

Bulk-type YBCO superconductors prepared by the MPMG process exhibit the high critical current density Jc, and are promising for many practical applications such as magnetic levitation. A magnetic levitation system is to use as a strong superconducting bulk magnet by magnetizing the high-Tc superconductor (HTSC). When superconducting bulk magnets are activated using the field cooling procedure under a DC magnetic field, a large electromagnet system or a superconducting coil is needed. In contrast, a pulsed magnetic field using a small coil is applied a strong magnetic field for magnetizing the HTSC. Therefore, it is important to evaluate the pulsed field magnetization properties of the HTSC. Since superconducting properties are not homogeneous in some specimens due to grain boundaries and cracks, spatial distribution of magnetization properties should be assessed. In the present research, the distribution of the pulsed field magnetization was measured with a magnetic field visualization system.<br>The magnetization with the pulsed magnetic field is done in the state of the zero field cooling. However, it is necessary for enough magnetization of the HTSC to consider the time constant on the magnetic flux invasion and the flux flow, when the applied magnetic field decreases. Therefore, we examined the optimum pulsed magnetization for the HTSC. The optimum value of the pulsed magnetization existed.

The bulk-type YBCO superconductor prepared by the MPMG process characterizes high critical current density Jc over 3&times;104A/cm2 at 77K and 1T. Therefore it is expected to be applied to various fields such as magnetic bearing, magnetic levitation, flywheel for electricity storage, and etc.. In these applications, evaluation of magnetic characteristics of the high-Tc superconductor is an important problem. It is necessary to value not only DC magnetic properties but also AC magnetic properties. Especially, in real applications it is important to value the AC quench properties of it. So we had developed the magnetic field visualization system, which enables measurement of magnetic field using a Hall device and produced the visual images of magnetic field on a computer display. We measured the AC quench properties of the YBCO ring specimen under the AC magnetic field with the frequency of 50Hz. From the experimental results we can understand that the superconductor results in the quench by the heat under the some AC magnetic fields, and that the quench properties of the ring specimen are ruled by its weak links.

Because the bulk type YBCO superconductor prepared by the MPMG process characterizes high critical current density J(c), it is expected to be applied to various fields. It is important to develop measurement methods of magnetic properties of the high T-c superconductor (HTSC). It is necessary to value not only DC magnetic properties but also AC magnetic properties. So we determined the magnetic properties of the HTSC under an AC magnetic field (5-500Hz). From the experimental results we can understand that there is the first order lag element caused by the flux flow effect, and the magnetic properties in low frequency are ruled by the flux flow.

Because the bulk-type YBCO superconductor prepared by the MPMG process has high critical current density J_c. It is expected to be applied to various fields such as magnetic bearing, magnetic levitation, magnetic shielding, and etc. In such applications it is important to develop measurement methods of magnetic property to high-T_c superconductor. It is necessary to value not only DC magnetic property but also AC magnetic property. So we measured magnetic pmperty of the high-T_c superconductor under the AC magnetic field (5-500Hz).<br>From the experimental results we can understand that there are two different kinds of elements in AC magnetic property: the one is the first order lag element by the flux flow, and the other is the applied magnetic flux that go around the ring specimen into that inside. The magnetic property in low frequency is ruled by the flux flow.

学位授与機構 学位記番号：424番