Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2021.3136801

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Publishing type：Research paper (international conference proceedings)  

MgB2 superconducting bulk materials are characterized as simple and uniform metallic compounds, and capable of trapping field of non-distorted conical shapes. Although pulsed-field magnetization technique (PFM) is expected to be a cheap and an easy way to activate them, the heat generation due to the magnetic flux motion causes serious degradation of captured fields. The authors precisely estimated the flux trapping property of the bulk samples, found that the flux-shielding effect closely attributed to the sample dimensions. The magnetic field capturing of Ti-5.0wt% sample reached the highest value of 0.76 T. The applied field which reached the centre of the sample surface shifted from 1.0 T to 1.2 T with increasing sample thickness from 3.67 mm to 5.80 mm. This means that the shielding effect was enhanced with increasing the sample thickness. Moreover, Ti-addition affected the frequency of flux jump happenings. The occurrence of flux jumps was suppressed in 5.0wt%Ti-added sample. This means that the heat capacity of the compounds was promoted by Ti addition.

DOI： 10.1088/1742-6596/1975/1/012019

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1088/1361-6668/ab9543

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Other Link： https://iopscience.iop.org/article/10.1088/1361-6668/ab9543/pdf

Publishing type：Research paper (scientific journal)  

© 2002-2011 IEEE. A unique activation technique for permanent Nd-Fe-B magnets which were embedded in the rotors of interior permanent magnet (IPM) motors has been developed as a magnetizing tool with use of high temperature superconducting (HTS) bulk magnet. The experimental and numerical simulation studies were conducted to evaluate the magnetic field-trapping performances of the rotor of air-conditioner compressor in the hybrid-type automobiles. The sample rotor containing six demagnetized permanent magnet plates were exposed in the intense static magnetic fields above the magnetic pole which contained the bulk magnet generating over 4 T. As a result, the sample magnets were perfectly magnetized in the static magnetic field. The precise simulation on the flux distribution in the rotor clarified that it is important to make the flux direction in the rotor core and the easy magnetization axis of PM identical. We convinced this activation technique should enable us to promote the degrees of freedom of motor designing and its processing.

DOI： 10.1109/TASC.2020.2989181

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Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2020.2966421

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

<title>Abstract</title>
The magnetic flux capturing of MgB₂ bulk magnets made by spark plasma sintering process has been precisely investigated to clarify the mechanism of flux motions during the pulse-field magnetization processes. The field trapping ratio <italic>B</italic>
_T/<italic>B</italic>
_P was evaluated as a key parameter of field trapping ability which strongly relates to the heat generation due to the rapid flux motion in the samples. The time dependence of magnetic flux density revealed the actual flux motion which penetrated the samples. The trapped fields <italic>B</italic>
_T and field trapping ratios <italic>B</italic>
_T/<italic>B</italic>
_P of various samples were classified into three regions of ‘no flux flow’, ‘fast flux flow’ and ‘flux jump’ according to the generation of heat and its propagation. A flux jump was observed late at 280 ms from the beginning of PFM process, while the field penetration <italic>B</italic>
_P showed its peak at 10 ms. Considering the heat propagation speed, the long-delayed flux jump should be attributed to the macroscopic barriers against the heat propagation to the surface centre of bulk magnet.

DOI： 10.1088/1742-6596/1559/1/012024

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Other Link： https://iopscience.iop.org/article/10.1088/1742-6596/1559/1/012024

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2019.2963307

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Publishing type：Research paper (scientific journal)  

© 2020 The Author(s). Published by IOP Publishing Ltd. Nuclear magnetic resonance (NMR) magnets generally need an extremely uniform magnetic field in the space where the specimens will be placed. Although high temperature superconducting bulk magnets can obtain an intense magnetic field of more than 3 T, the field distribution is intrinsically characterized to be inhomogeneous, showing steep gradient. Aiming at the practical application of compact and portable NMR magnets, the authors have developed magnet systems capable of generating uniform magnetic fields in a narrow space between face-to-face settled magnetic poles, targeting to obtain a uniform magnetic field between the poles. Here, the authors modified the shape of magnetic field distribution from convex to concave by attaching a ferromagnetic iron plate on one of the pole surfaces, and then settled them face to face with a gap of 70 mm. The uniformity of the magnetic field in the x-y plane was experimentally measured and estimated along various z-axis directions. The best uniformity of 463 ppm at 1.25 T was obtained in the experimental evaluation in the x-y plane of 4 4 mm2 at z = 0.5 mm from the iron plate surface. The value of uniformity along the z-direction range was extended from 0.8 mm to a depth of 1.1 mm. These results show that the possible space of NMR signal detection or the available sample size can be enlarged.

DOI： 10.1088/2053-1591/ab8e6d

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Publishing type：Research paper (international conference proceedings)  

© 2019 Published under licence by IOP Publishing Ltd. Aiming to develop the compact nuclear magnetic resonance devices, the authors have been developing the uniform magnetic fields in the space between the face-to-face magnetic poles which contain HTS bulk magnets. The authors deformed the original convex shape of the magnetic field distribution to concave by attaching a ferromagnetic iron plate of 20 mm and 100 mm in diameter on one of the pole surfaces. The magnetic field distribution precisely measured along x-axis changed from concave to convex with increasing distance from the magnetic pole surface, which suggested the presence of magnetically flat region. From the results of mapping on the field uniformity data, we have succeeded in obtaining the field uniformity under 500ppm in the space of 4 x 4 x 0.8 mm3 when the magnetic poles emitting concave and convex field distributions were settled face-to-face with various gaps 30-70 mm. This performance exceeded the target value of 1,500ppm which is necessary to detect the NMR signals.

DOI： 10.1088/1742-6596/1293/1/012044

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2019.2898237

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tasc.2019.2902692

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TASC.2019.2901213

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The magnetic flux dissipation from the MgB2 bulk magnets during pulsed field magnetization (PFM) processes has been estimated for the samples fabricated by hot-pressing (HP) and spark plasma sintering (SPS). We defined the parameters as field penetration ratio B-P/B-a and the field trapping ratio B-T/B-P to evaluate the flux motion and performance. The data plots of trapping ratio B-T/B-P showed us two regions which originate from the fast flux flow and flux jump. The field trapping performance was drastically degraded when the flux jump happened. Flux jump happened at various times after the peak field. A very late flux jump happened at 420 ms which severely degraded the trapped field. This time delay, much larger than the heat propagation time in the MgB2 sample, remains to be explained.

DOI： 10.1109/TASC.2019.2908752

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Feasible applications in the industrial regions for the hulk high-temperature superconducting (HTS) magnets have been investigated in two categories of the strong magnetic field generators and the magnetic levitation devices by active and passive field trapping, respectively. When we adopt the system to the active field generators, magnetization procedures are necessary. The authors have studied the field-capturing abilities of the HTS bulk magnets on two ways of the activation procedure and the material designing. In the former, we clarified the magnetic flux behavior that invades into bulk magnets, and that the invading flux motion is affected by the previously existing flux distribution. Therefore, the magnetic flux can be accumulated with successive magnetic fields applications with descending heat generation in order. In the latter, the selective magnetic field invasion was observed between wide- and narrow-growth sector regions. Since the bulk magnets with steep magnetic field distributions deform their own field distribution shape to be uniform by the shielding effect when the ferromagnetic plate was attached on the magnetic pole, we have succeeded in obtaining the uniform magnetic field of several-hundred parts per million in the space between the magnetic poles.

DOI： 10.1109/TASC.2018.2867784

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Publishing type：Research paper (international conference proceedings)  

© Published under licence by IOP Publishing Ltd. The authors have developed a useful technique for extracting the Ni-sulfate compound from the "processed waste/fluid" of the electroless Ni-plating processes. After forming the fine NiHPO3 precipitate from the waste, the coarse NiSO4 crystals were synthesized through the reaction with the concentrated sulfuric acid. In the experiment, the open-gradient magnetic separation was employed to collect the NiSO4 crystals from the muddy mixture composed of these compounds. The experiments were practically conducted with use of the Gd123-based HTS bulk magnets generating up to 3.99 T, which were activated by the field cooling magnetizing method operated at 35 K. Intense magnetic field emitted from the bulk magnet attracted the paramagnetic fluid and coarse NiSO4 crystals on the bottom of the water channel, forming a giant drop of NiSO4-saturated clear fluid and the slurry of nickel sulfate hexahydrate. This preferential collection suggests a feasible recycling process of Ni resource as a raw material in the plating processes.

DOI： 10.1088/1742-6596/1054/1/012047

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The pulsed-field magnetization experiments have been carried out for the layered MgB2 bulk magnets to clarify the magnetic flux motion during the activation processes. Since MgB2 is characterized as homogeneous, lightweight, and inexpensive HTS materialin comparison to RE123 compounds (RE = rare earth), the material may have an advantage of possibly uniform magnetic field generation. The values of applied field B-a which begin to reach the sample center were responsible for the total sample thickness, reflecting the shielding effect to suppress the flux invasion. As well, the estimation on the ratios of trapped field B-T over penetration field B-p showed us the reluctant field invasion with increasing sample thickness. The field trapping ratios B-T/B-p decreased with increasing applied field due to heat generation. In the region over 1.4 T, the drastic flux flow and sudden flux jumps were often observed, which lowered the field-capturing ability. The arriving time which corresponds to the flux increasing speed showed us that the rapid flux invasions which occur in the low-field ranges resulted in the highest field-capturing. This suggests us the possible promotion of field-capturing without significant heat generation.

DOI： 10.1109/TASC.2018.2792530

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Language：English   Publishing type：Research paper (scientific journal)  

© 2017 IEEE. In this paper, we suggest a new magnetization loss model for all magnetic field directions. We measured three types of HTS tapes, two different critical current Bi2223/Ag tapes, and an YBCO tape to compare the measurement with the magnetization loss model. Our new model is based on the measured magnetization losses, because the loss is affected by the intrinsic critical current distribution and the cross-section. The studied conditions of the base magnetization losses are: the magnetic field applied perpendicular to the tape axis and perpendicular to the tape face, perpendicular to the tape axis and parallel to the tape face, and parallel to the tape axis. This model enables us to describe magnetization losses in response to a change in the amplitude of the magnetic field, the angle of the tape axis, and the angle of the tape face.

DOI： 10.1109/TASC.2017.2786708

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Language：English   Publishing type：Research paper (scientific journal)  

© 2017 IEEE. We measured the influence of the twist pitch, twist direction, and transport current balance of the inner and outer layers on the ac loss characteristics in a two-layer twisted high temperature superconducting (HTS) cable. To measure the ac loss in each layer, we placed the thermo-couples on each tape face with thermal insulation. In the HTS cable, the inner layer is exposed to a longitudinal magnetic field generated by the twisted outer layer, and the outer layer is exposed to a circumferential direction magnetic field generated by the inner layer. When the inner and outer layers connect in series, the ac loss characteristics in the inner layer exhibit the influence of the outer layer arrangement. When the transport current balance of the inner and outer layer changes, the lowest loss condition exists when the inner transport current exceeds the outer transport current. These phenomena are possible explanations for the amplitude and direction of the magnetic field in each layer.

DOI： 10.1109/TASC.2017.2776923

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The authors have conducted the mutual exchanges of the students and professors between the universities in Japan and Germany, discussing the topics on the engineering education. On the platform, the international collaboration on the engineering education researches in assistance of the educational reform program have been carried on by the grants-in-aid of Japanese government. Since the current issues on engineering education are generally common in every country, it is beneficial for us to discuss the educational information with foreign faculty. Since the experiences of collaboration with foreign universities are profitable for students, we have been conducting the individual student exchanges on the scientific research areas with each other by the mutual contracts of MOU with German institutes. The individually-dispatched students showed us that the experiences of collaborations were will beneficial for their career-design as well as the enhancement to motivate their scientific researches. The individual internships thus processed have resulted in some academic paper publications on the collaborative researches. 21 Japanese students have studied abroad and 10 German students have studied in Niigata. 13 scientific papers co-authored by the students have been published in a decade.

DOI： 10.1109/WEEF-GEDC.2018.8629681

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Language：English   Publisher：Japanese Society for Engineering Education  

We have been conducting the international collaboration on the engineering education researches in assistance of the educational reform program conducted by the grants-in-aid from Japanese government. Beside the announcements of educational results worldwide, we have been conducting the mutual exchanges of the students and faculty members between the universities, discussing the topics on the educational reforms, project-based learning programs, engineering ethics, and so on. Since the current issues on the engineering education are common to those of every country, it is beneficial for us to discuss about the educational information with foreign faculty members. In addition, since the experiences of collaboration with foreign universities are also profitable for student beside the faculties, we have been conducting the student exchanges by the mutual contracts with foreign universities on the academic research areas. The scientific research collaborations driven by the dispatched students showed us that the experiences of collaborations were beneficial for students' career-design as well as the enhancement of motivation on their scientific researches, which resulted in the academic paper publications on the collaborative academic researches. 21 Japanese students have studied abroad and 10 German students have studied in Niigata. 13 scientific papers co-authored by the students who contributed to the collaborative researches have been published in nine years.

DOI： 10.20549/jseeen.2018.0_28

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CiNii Books

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Measurements of AC losses in a HTS-tape placed in between two bulk magnetic shields of high permeability were performed by applying calorimetric techniques for various asymmetrical shielding arrangements. The experiment was supported by analytical calculations and finite-element simulations of the field and current distributions, based on the Bean model of the critical state. The simulated current and field profiles perfectly reproduce the analytic solutions known for certain shielding geometries. The evaluation of the consequent AC losses exhibits good agreement with measurements for the central position of the tape between the magnets but have increasing discrepancy when the tape is approaching the shields. This can be explained by the increasing contribution of the eddy currents and magnetic hysteresis losses in the conducting shields.

DOI： 10.1088/1361-6668/aa73bc

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

The motion of magnetic flux invading into the HTS bulk magnets were experimentally studied in their pulsed field magnetization processes. The authors paid attention to the effects of the shapes of the pre-magnetized trapped flux distributions before the successive field applications by means of varying the M-shaped distribution. We estimated the differences of the magnetic flux motions between the Dy123 and Gd123 systems, which might have different Jc properties of each sample. As for the Dy123 system, the increase of remaining flux in the periphery region of the M-shaped distribution resulted in the decrease of flux-trapping according to the promotion of flux invasion. On the other hand, trapped flux density has been raised to 3.4 T owing to the effective suppression of flux invasion for the Gd123 bulk magnet. The experiments showed that the peak heights and the positions of the formerly trapped M-shaped fluxes precisely affect the heat generation and the trapped field performance.

DOI： 10.1088/1742-6596/871/1/012049

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

MgB2 bulk magnets are one of the attractive bulk superconductors because they have homogeneous trapped field distribution, low material costs, lightweight, and so on. Pulse-field magnetization (PFM) also has some effective advantages in the point of view of compact setup, shorter magnetizing time, and an inexpensive way to magnetize the bulk samples. However, the trapped field by PFM is inferior to those by field cooling magnetization due to the heat generation caused by sudden changes of the magnetic flux motion. Especially, the temperature rise is remarkable for MgB2 bulk magnets because of its low specific heat. In this study, we have carried out the PFM technique for five-stacked or three-stacked MgB2 bulk samples to analyze the flux-invasion behavior. These samples were fabricated by ball-milling and hotpressing. As a result, maximum trapped field of BT = 0.64 T was obtained at 14.6 K at the center of the topmost sample when the external magnetic field B-ex of 0.90 T was applied. As B-ex increases further, BT were dramatically decreased because of heat generation. Furthermore, flux jumps frequently took place in the applied field higher than 2.0 T.

DOI： 10.1109/TASC.2016.2639298

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In this work, we exposed a high-temperature superconducting (HTS) tape to a rotating magnetic field, and measured ac losses using a calorimetricmethod based on the temperature rise caused by the ac loss. To compare the characteristics of the other ac loss conditions, we measured the magnetization losses in the uniform ac magnetic field perpendicular and parallel to the HTS tape face. The losses in the rotating magnetic field are the same as in the perpendicular magnetic field. In addition, we measured the influence of the rotating magnetic field waviness on the ac loss characteristics.

DOI： 10.1109/TASC.2017.2651099

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In this paper, we applied a magnetic field in multiple directions to a Bi2223/Ag tape and measured the transport current and magnetization losses in each position using the four terminal and pickup coil methods. We rotated the tape face and the tape axis to the ac magnetic field by 15 degrees segments. We report the magnetic field direction dependence of ac losses. When the magnetic field direction is parallel to the tape face and the tape axis is rotated in the magnetic field, each magnetization loss shows a crossing point. When the magnetic field is not parallel, the magnetization losses are increased as the angle of the tape axis to the magnetic field increases. We compared the total losses with additions of the transport current loss without the magnetic field and the magnetization loss without transport current in multiple magnetic field directions, and these results did not correspond.

DOI： 10.1109/TASC.2016.2516349

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The fundamental study on the ac loss reduction in high-temperature superconducting (HTS) coils for armature windings of ac rotational machines is presented in this paper. In this paper, a simplified model that simulates a part of the distributed armature winding of ac rotational machines is used. For the ac loss reduction, we study a method to reduce the perpendicular magnetic field applied to the tape surface in the coil by arranging a magnetic material around the coil winding. The numerical magnetic field calculation shows that the perpendicular magnetic field clearly decreases by arranging the magnetic material around the coil winding in the slot of the iron core. To investigate the effect of the insertion of the magnetic material on the ac loss reduction, we fabricate the measurement setup using the test HTS coils wound by a Bi2223 tape and a YBCO tape, combined with the laminated iron core. The transport ac losses in the test coils are measured by changing the arrangement of the magnetic material. The measurement results demonstrate well the ac loss reduction by use of the magnetic material around the coil winding in the slot of the iron core.

DOI： 10.1109/TASC.2016.2535208

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We report the ac loss characteristics of stacked HTS tapes in an iron core. To estimate the loss distribution, we measured each stacked HTS tape using a calorimetric method. To clarify the influence of an iron core, we measured the ac losses of the stacked HTS tapes without the iron core. The ac transport current losses of the stacked HTS tapes positioned in the iron core were greater than those without the iron core. The distribution of the magnetization losses by the leakage magnetic field was that the loss at the outside of the HTS tape was highest because shielding current prevented the magnetic field from penetrating to the inside of the stacked HTS tapes.

DOI： 10.1109/TASC.2016.2543504

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The magnetic flux invasion into the Dy123-based HTS bulk magnets were experimentally evaluated during the pulsed field magnetization processes. Since the invasion of the magnetic flux into the sample would be expected to be affected by the J(c) distribution, we estimated the differences of the magnetic flux motions between the growth sector boundary (GSB) and the growth sector region (GSR) by measuring the changes of magnetic flux density and temperature during the pulsed field application of 5 T at 30 K. The experimental results on the temperature measurement showed that the magnetic flux invades into the sample through the path in the GSR prior to that in the GRB region. Furthermore, the changing rate of magnetic flux gradually changed with the position on the GSR, while they irregularly changed and did not exhibited any tendency on the GSB. This means that the magnetic flux straightly invades the sample from GSR heading for the center portion, while it does not uniformly flow on the GRB. As a result, the heat generation occurs in the GSR area prior to the GSB region. The magnetic flux invasion behavior exhibits selective and nonuniform motion, reflecting the difference of the macrostructure on the GSR and the GSB.

DOI： 10.1109/TASC.2016.2529425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Melt-textured Y-Ba-Cu-O high temperature superconducting bulk magnets were fabricated by the cold seeding method with using single or twin-seed crystals composed of Nd-Ba-Cu-O thin films on MgO substrates. The behavior of the magnetic flux penetration into anisotropic-grown bulk magnets thus fabricated was precisely evaluated during and after the pulsed field magnetization operated at 35 K. These seed crystals were put on the top surfaces of the precursors to grow large grains during the melt-processes. Although we know the magnetic flux motion is restricted by the enhanced pinning effect in temperature ranges lower than 77 K, we observed that flux invasion occurred at applied fields of 3.3 T when the twin seeds were used. This is definitely lower than those of 3.7 T when the single-seeds were employed. This means that the magnetic fluxes are capable of invading into twin-seeded bulk magnets more easily than single-seeded ones. The twin seeds form the different grain growth regions, the narrow-GSR (growth sector region) and wide-GSR, according to the different grain growth directions which are parallel and normal to the rows of seed crystals, respectively. The invading flux measurements revealed that the magnetic flux invades the sample from the wide-GSR prior to the narrow-GSR. It suggests that such anisotropic grain growth leads to different distributions of pinning centers, variations of J(c) values, and the formation of preferential paths for the invading magnetic fluxes. Using lower applied fields definitely contributed to lowering the heat generation during the PFM process, which, in turn, led to enhanced trapped magnetic fluxes.

DOI： 10.1088/0953-2048/29/2/024003

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

The authors have been attempting to obtain the uniform magnetic field distribution in the space between the face-to-face HTS bulk magnets. The magnetic poles containing the HTS bulk magnets are usually characterized as non -uniform magnetic field distribution. Since the distributions show the conical or convex shapes, it is difficult to obtain the uniform magnetic field spaces even when the magnetic poles would be placed face-to-face. The authors have modified the shape of the distribution of one-side magnetic pole by attaching an iron plate on the surface, and formed the concave magnetic field distribution on the pole surface. The steep concave or convex distributions at each pole surface change to be flat with increasing distance from the pole surface. After the experimental result recording the best uniformity of 358 ppm by combining the concave and convex field distributions face-to-face, we attempted to simulate the feasible performance in this configuration. In the numerical simulation, the concave field distribution modified by attaching an imaginary spiral coil on the pole surface was coupled with the original convex field. We succeeded in obtaining the best uniformity of 30 ppm at 1.1 T in 4 x 4 mm(2) x-y plane at 7 mm distant from the pole surface in the gap of 30 mm. This result suggests that the concave and convex magnetic field distributions compensate the field uniformity with each other with keeping the magnetic field strength in the gap, and also suggests the novel compact NMR/MRI devices in the future. (C) 2016 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.phpro.2016.04.019

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In this paper, we constructed test equipment for a rotating machine that uses a superconducting bulk rotor and performed a rotation test. When a superconducting bulk rotor is used, a fluctuating magnetic field generates ac loss, it is possible to decrease its trapped magnetic flux. Performance of the rotating machine under test was influenced by the ac loss generated by a fluctuating ac magnetic field. We modeled the motor used in this paper on an axial-type synchronous motor, where the magnetic field generated by the stator coils causes the rotor to rotate. We then measured the influence of the stator coils on the superconducting bulk rotor. First, we confirmed that the temperature of the bulk rotor modules was properly controlled and could generate a trapped magnetic flux equal to that during zero-field-cooled magnetization. A technique for magnetizing two or more bulks concurrently was used. Next, we performed the rotation test at low speed. The maximum rotational speed during the test was 300 r/min. The operation time at a steady-state speed was 600 s. The maximum value of the generated magnetic field of the stator coils was 50 mT. Finally, we confirmed that a shielding ring made of superconducting wire decreased the influence of a fluctuating magnetic field.

DOI： 10.1109/TASC.2014.2379703

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In order to clarify the behavior of the magnetic flux that invade into themelt-processed Dy123-based high-temperature superconducting bulk magnet, the motions of magnetic fluxes and the phenomena of heat generation were precisely measured during the pulsed field magnetization processes. In the experiments, we estimated the time evolutional profiles that were measured at two portions in the growth sector region and the growth sector boundary. They are characterized as the areas of lower and higher critical current densities than each other, respectively. The serial pulsed magnetic fields of 5 T were applied to the sample at 30 K, and the data were obtained in turn by the Hall sensor and thermometer that were put at several positions along the radial rows on the growth sector boundary and the growth sector region. The profiles of magnetic flux during the pulsed field application actually proved that the magnetic flux invades the sample through the growth sector regions in shorter time and with stronger magnitude than through the growth sector boundaries. Since the low magnetic field application leads to low heat generation, it is expected to achieve the higher trapped field performances than ever.

DOI： 10.1109/TASC.2014.2373653

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Pulse field magnetization (PFM) for HTS bulk magnets has the advantage of a field cooling method, because it does not require a superconducting magnet. To reach the largest trapped magnetic field in the HTS bulk using the PFM method requires repeated applications of the pulse magnetic fields to the HTS bulk with controlled temperature and amplitude of the pulse magnetic field. We previously reported the influence of HTS bulk properties on the PFM method, and our numerical results corresponded with the experimental results when a single pulse magnetic field was applied. In this study, we applied multi pulse magnetic fields to the HTS bulk using this numerical model. The results show the outer and inner shielding currents in the HTS bulk exist in opposite directions, while the surface magnetic field is reduced by the magnetic field produced by the negative direction current. We suggest a new method to reduce the outer shielding current using a heater. Our numerical results demonstrate the possibility of increasing the peak of the trapped magnetic field in the heating method.

DOI： 10.1109/TASC.2014.2366236

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

A unique experimental attempt aiming to obtain a uniform magnetic field space as required for NMR has been carried out with use of HTS bulk magnets. The magnetic poles were activated as 1.8 T ( North) and 1.4 T ( South) at 30 K by applying a pulsed magnetic field up to 7 T, and positioned face-to-face with gaps less than 70 mm. The uniformity of the magnetic field required for detecting the NMR signals is less than 1,500 ppm at more than 0.3 T in the cross sectional plane of 2 x 2 mm(2). After the preliminary trials which revealed a uniformity of 5,421 ppm at 0.44 T in a 70 mm gap, we attached a ferromagnetic iron plate to a magnetic pole surface to change the magnetic field distribution to be concave. The best uniformity of 358 ppm at 1.11 T was obtained at 9 mm distance from the iron plate surface in a gap of 30 mm. It is stated that the concave magnetic field distribution was compensated by the counter conical-shape field, resulting in the uniform field plane. (C) 2015 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.phpro.2015.06.187

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

To optimize pulse field magnetization processes for high-temperature superconducting (HTS) bulk magnets, it is important to control the magnetic field amplitude and the driving temperature. Many publications have reported on the experimental and numerical results of the pulsed-field magnetization method. In the numerical method, the HTS hulk properties are modeled to simplify the calculations, examples being the Bean model and the Kim model, etc. During pulsed-field magnetization the critical current density in the HTS bulk changes dramatically due to the magnetic field and temperature rise induced by the ac losses. For these reasons these properties are key to the analysis of the pulsed-field magnetization process, which is calculated using the measured practical HTS bulk properties, and these results show different temperature rise and trapped magnetic field characteristics. We assumed that Furs bulk has intrinsic deterioration in practice. Therefore, we recalculated using the adjusted parameters, and these analytic results correspond with the experimental results.

DOI： 10.1109/TASC.2013.2282174

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The magnetic field distributions of an HTS bulk magnet system were precisely measured between the face-to-face 3.4-T magnetic poles. After the activation by 5-T field cooling magnetization, the magnetic flux density has reached the value of 2.26 T at the center of the 34-mm gap. The sum of the local magnetic fields generated from each magnetic pole coincides with the local field strength in the gap. We estimated the homogeneity of the magnetic flux densities of B-z and B-y, which were measured at various positions along z and y-axes in the center portion of the 60-mm gap. Since the magnetic flux density becomes smooth with increasing distance, we obtained the smallest distribution range of 123 ppm of B-z = 1.62 T at the center of the gap as for B-z. On the other, as for B-y, the smallest distribution range of 1125 ppm of B-z = 1.20 T was obtained at z = 1.7 mm distant from the center point. Since there must exist a certain plane which has no vector components along the radius direction at the center of the gap, the closer the z-positions come to the null point, the more homogeneous the field distribution becomes.

DOI： 10.1109/TASC.2013.2284859

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

For economic reasons high-temperature superconductor (HTS) machines, such as HTS motors and generators, are designed to use an iron core to reduce the HTS tape length. In these situations, the HTS tapes are positioned in the iron core and transport the ac current within the ac leakage magnetic field. We investigated the influence of the iron core arrangement on ac loss characteristics. We modified the iron core of a transformer by removing a rectangular segment and then positioned an HTS tape in the rectangular recess created. The iron core was affected by the ac magnetic field generated by the ac coil, and the HTS tape transporting the ac current. We placed the HTS tapes in different rectangular positions and varied their shape. In all conditions, the ac transport current losses were greater than losses experienced when the HTS tapes were situated without the iron core, and the ac leakage magnetic fields produce ac magnetization losses. Therefore, how the HTS tape is arranged in the iron core affects the ac loss characteristics and merits further study.

DOI： 10.1109/TASC.2013.2280730

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The magnetic separation experiment to collect the Ni compounds from the waste liquid of electroless plating processes was conducted in the open-gradient magnetic separation process with the high temperature superconducting bulk magnet system. The magnetic pole containing Gd-based bulk superconductors was activated to 3.45 T at 35 K in the static magnetic field of 5 T with use of a superconducting solenoid magnet. The coarse Ni-sulfate crystals were formed by adding the concentrated sulfuric acid to the Ni-phosphite precipitates which yielded from the plating waste liquid by controlling the temperature and the pH value. The open-gradient magnetic separation technique was employed to separate the Ni-sulfate crystals from the mixture of the Ni-sulfate and Ni-phosphite compounds by the difference between their magnetic properties. And we succeeded in collecting Ni-sulfate crystals preferentially to the Ni-phosphite by attracting them to the magnetic pole soon after the Ni-sulfate crystals began to grow. (c) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2013.01.018

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

The flux-invasion behavior into the melt-processed Y-Ba-Cu-O bulk magnets were precisely measured and analyzed during and after their pulsed-field magnetization processes operated at 30.6 K. The materials were fabricated as the bulk monoliths grown by adopting two seed-crystals, or shifting the seed-crystal positions from the centre of the sample surface, which exhibited the magnetically single-domain distributions. Although the performances of the trapped flux density after activations showed no obvious differences, the flux started invading into the sample bearing two seeds obviously at lower fields than those of normally-grown isotropic crystal. Since the flux penetration behavior were thus clearly different between the samples with the structure grown from two seeds and uniformly grown samples with a seed crystal, it is suggested that the structure results in an effective magnetizing method with less heating than those of conventional samples, which results in the higher performance of field trapping in the bulk magnets than usual. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.

DOI： 10.1016/j.phpro.2014.09.021

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We study the effect of a fluctuating magnetic field, which is one of the technical problems for trapped magnetic fields in a bulk superconductor, to realize a practical bulk superconductor rotating machine. Previous research and other's research has shown that fluctuating magnetic fields reduce the strength of trapped magnetic fields in superconducting bulk modules [1, 2]. This deters development of applications of AC rotating machines because superconducting bulk modules are always exposed to a fluctuating magnetic field. Therefore, it is necessary to develop a method to control decrease of the trapped magnetic field. We propose a method to use the shielding ring of a superconducting wire to achieve this goal and the effects are confirmed experimentally [3]. We are now building test equipment for examining the performance of a shielding ring in a bulk rotating machine. This paper reports the test result for the shielding ring applied to the bulk superconducting rotor that is a part of the test equipment.

DOI： 10.1088/1742-6596/507/3/032060

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

In order to enhance the field-trapping ability of high T (c) superconducting melt-textured bulk materials which act as quasi-permanent magnets when they capture the external magnetic fields, it is important to enhance the mechanical toughness of the materials to stand the stress induced by the magnetic repulsive force and thermal expansion. We adopted a dense Dy123-based bulk material with reduced void concentration in the experiment. Since the heat generations due to the flux motions in the samples results in the degradation of J (c), the time evolutions of the trapped magnetic fields and the temperature rises during and after the pulsed-field magnetizing processes were precisely measured at the same time to evaluate the penetrating flux motions and the heat generations in the sample. A single and couple of the magnetic pulsed fields with various intensities were successively applied to the sample at 30 K. The single magnetic field application exhibited a peak effect in the trapped-field behavior and tended to decline due to the heat generation. In the iterative pulsed-field application, the behaviors of the trapped fields and the temperature changes were found to be inverse between the first and the second pulsed-field applications. This implies that the flux penetration behavior into the sample magnet at the second field application is strongly restricted by the presence of former trapped fields which were formed by the first field applications.

DOI： 10.1007/s10948-012-2048-5

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In semiconductor devices, miniaturization of circuit patterning on wafers is required for higher integrations of circuit elements. Therefore, very high tolerance and quality are also required for patterning of microstructures of photo masks. The deposition of particulate dusts generated from mechanical bearings of the spin processor in the patterns of the photo mask is one of main causes of the deterioration of pattern preciseness. In our R&D, application of magnetic bearing utilizing bulk high temperature superconductors to the spin processors has been proposed. In this study, we develop a test spinner for the non-contact spinning process in the photo mask production system. The rotation test by using this test spinner shows that this test spinner accomplishes the improvement of the spinning ability compared with the test spinner developed in our previous study. This paper describes the rotation test results of the new test spinner applying the magnetic bearing with bulk high temperature superconductors. (C) 2012 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.physc.2012.03.008

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The magnetic separation experiment for recycling the nickel-bearing precipitates in the waste liquid from the electroless plating processes has been practically conducted under the high gradient magnetic separation technique with use of the face-to-face HTS bulk magnet system. A couple of facing magnetic poles containing Sm123 bulk superconductors were activated through the pulsed field magnetization process to 1.86 T at 38 K and 2.00 T at 37 K, respectively. The weakly magnetized metallic precipitates of Ni crystals and Ni-P compounds deposited from the waste solution after heating it and pH controlling. The high gradient magnetic separation technique was employed with the separation channels filled with the stainless steel balls with dimension of 1 and 3 mm in diameter, which periodically moved between and out of the facing magnetic poles. The Ni-bearing precipitates were effectively attracted to the magnetized ferromagnetic balls. We have succeeded in obtaining the separation ratios over 90% under the flow rates less than 1.35 L/min. (C) 2012 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.physc.2012.03.010

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Language：English   Publisher：The Materials Research Society of Japan  

The magnetic flux invasion into the Y-Ba-Cu-O-based high temperature superconducting bulk magnets were precisely evaluated during the pulsed field magnetization processes. Various bulk magnets with different contents of Y₂BaCuO₅ (Y211) particles were fabricated so as to examine how the Y211 concentrations affect the flux invasion behaviors. The flux motions and the resultant trapped field densities were monitored at the surface of each bulk sample at 30.6 K. As a result, the applied fields at which the magnetic flux began to invade the sample increased with increasing Y211 contents. This means that the pinning centers are promoted with increasing nominal Y211 addition. According to the evolutional profiles during the pulsed-field applications, the flux-penetration ratio gradually promoted with increasing applied fields, whereas the flux-trapping ratio decreased. This implies that the heat generation due to the flux motion seriously affects the field-trapping ability. The anomalous behaviors for the sample, which has displaced position of seed crystal, showed us more effective flux-trapping than usual, exhibiting the higher flux-penetration and flux-trapping ratios at lower applied-field than those of other samples. This suggests a possible high-field activation method with preferential flux invasion due to the different microstructures in the bulk magnet.

DOI： 10.14723/tmrsj.38.273

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This paper presents the experimental and numerical evaluations of the ac loss characteristics of one- and two-layer polygonal conductors assembled by YBCO tapes. The ac losses in the one-layer pentagonal and hexagonal conductors with various tape-to-tape gaps are measured, and the numerical simulation of the ac losses in the test conductors is performed by a previously developed numerical model. It is shown that the measured dependence of the ac loss in the one-layer polygonal conductors on the tape-to-tape gap is explained well by the numerical model. In the two-layer hexagonal conductor, the numerical simulation results also are in good agreement with the measurement results. It is considered that the experimental and numerical results obtained in this study support the validity of our previous studies on the ac loss in the polygonal assembled conductors made of thin high-temperature superconducting tapes. © 2002-2011 IEEE.

DOI： 10.1109/TASC.2013.2238982

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The flux invasion behaviors into the Dy123 based on HTS bulk magnet were precisely measured during the iterative pulsed field magnetizations operated at 31 K. The values of magnetic flux densities were measured at various positions on the bulk magnet sample during and after six successive applications of a peak applied field of 5T. The temperature changes were measured on the sample surface to compare them to the magnetic flux motions in the same manner. The invading flux were apparently repelled by the remaining trapped flux due to the former field application, and the domain where the magnetic flux reached was pushed back outward the sample with increasing the pulse number. The heat generation decreased with the iteration of field applications as well as the magnetic flux invasion expressing the same behavior, and saturated by the sixth field application. The time evolution of temperature measured at various positions on the sample surface showed that the heat generation occurs at the periphery of the sample and propagates toward the center of the sample. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2013.04.022

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Demagnetized rare earth magnets (Nd-Fe-B) can be fully magnetized by scanning them in the intense static fields over 3 T of a HTS bulk magnet which was cooled to the temperature range lower than 77K with use of cryo-coolers and activated by the field of 5 T. We precisely examined the magnetic field distributions of magnetized permanent magnets. The magnetic poles were 'in the narrow space on the surface of the permanent magnet with varying gap distances between the tracks of the HTS magnetic pole. Steep magnetic transitions between N and S were observed in the magnetic field distribution maps after the HTS magnetic poles were alternatively scanned for several times. The magnetizing method has been proposed for designing novel electromagnetic devices including magnetically excellent rare earth magnets which require intense magnetic fields over 3 T.

DOI： 10.1109/TASC.2011.2179510

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

A demagnetized Nd-Fe-B permanent magnet was scanned just above the magnetic pole which contains the HTS bulk magnet generating a magnetic field of 3.27 T. The magnet sample was subsequently found to be fully magnetized in the open space of the static magnetic fields. We examined the magnetic field distributions when the magnetic poles were scanned twice to activate the magnet plate inversely with various overlap distances between the tracks of the bulk magnet. The magnetic field of the "rewritten" magnet reached the values of the magnetically saturated region of the material, showing steep gradients at the border of each magnetic pole. As a replacement for conventional pulse field magnetizing methods, this technique is proposed to expand the degree of freedom in the design of electromagnetic devices, and is proposed as a novel practical method for magnetizing rare-earth magnets, which have excellent magnetic performance and require intense fields of more than 3 T to be activated. (C) 2011 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.cryogenics.2011.10.005

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

The field trapping ability of melt-textured bulk superconductors is usually restricted by the mechanical properties because of the magnetic stress induced in the samples during the activation processes which give them intense magnetic fields of several T. Therefore it is of crucial importance to enhance the toughness of the materials by means of reinforcing techniques. In this experiment, we adopted a dense Dy-based HTS bulk sample fabricated by means of controlling the concentration of voids inside during the heat treatment. A single magnetic pulse fields up to 7.66 T have been applied in the temperature range down to 30.6 K with use of GM refrigerator, and the profiles of pulsed fields and the resultant trapped fields are discussed with respect to their performances. It was found that the trapped field grows as elongating rise time of the pulsed field and lowering initial temperature. The densely fabricated Dy123 sample was activated up to 1.98 T at 31 K in the same manner with no substantial difference from the porous Gd123 samples when the magnetic pulse field of 7.66 T was applied. The temperature rises up to 20 K are not substancial in comparison to those of the Gd123 compounds which exhibit the temperature rises more than 30 K at 40 K by applying 4.64 T. This is attributed to the fact that the J(c) value of the dense Dy123 are inferior to that of Gd123, which reflectes the field trapping ability.

DOI： 10.1088/1742-6596/400/2/022089

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The paper reports on experimental and numerical studies of AC loss in a high temperature superconducting coil wound with a Bi2223/Ag tape. A measurement method of the AC loss in the HTS coil by using a power meter is proposed. The AC loss is measured on the test HTS coil made by stacking pancake coils wound with the Bi2223/Ag tape. The AC loss is also calculated using the numerical method developed in our previous work. The measured AC loss is compared with the calculated one. The agreement of measured and calculated data is reasonable and this shows the validity of the proposed measurement method. © 2011 IEEE.

DOI： 10.1109/TASC.2011.2175684

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

A demagnetized Nd-Fe-B permanent magnet was scanned in the strong magnetic field space just above the magnetic pole containing a HTS bulk magnet which generates the magnetic field 3.4 T. The magnet sample was subsequently found to be fully magnetized in the open space of the static magnetic fields. The finite element method was carried out for the static field magnetization of a permanent magnet using a HTS bulk magnet. Previously, our research group experimentally demonstrated the possibility of full magnetization of rare earth permanent magnets with high-performance magnetic properties with use of the static field of HTS bulk magnets. In the present study, however, we succeeded for the first time in visualizing the behavior of the magnetizing field of the bulk magnet during the magnetization process and the shape of the magnetic field inside the body being magnetized. By applying this kind of numerical analysis to the magnetization for planned motor rotors which incorporate rare-earth permanent magnets, we hope to study the fully magnetized regions for the new magnetizing method using bulk magnets and to give motor designing a high degree of freedom. (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee

DOI： 10.1016/j.phpro.2012.03.442

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Many papers report the AC transport current loss characteristics in HTS coils, which occur because the HTS tapes in the coil transport a current in a magnetic field generated by the currents in adjacent tapes. We previously reported our experimental results on the loss from HTS tape in an AC and DC mixed electro-magnetic condition which consists of the DC offset transport current, AC transport current and AC magnetic field. Losses were measured in the same segment by the electric and calorimetric methods, and we established the possibility of measuring the loss using the electric method in the AC and DC mixed electro-magnetic condition. © 2011 IEEE.

DOI： 10.1109/TASC.2011.2174580

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In high temperature superconducting (HTS) AC applications, HTS tapes are assembled into high current capacity conductors. We previously reported our experimental results showing the AC loss reduction in a polygonal arranged single layer YBCO assembled conductor. The AC loss reduction is greatest in round shape arrangements. © 2011 IEEE.

DOI： 10.1109/TASC.2011.2177792

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This paper presents the optimization study of the field winding structure of the 20 MW-class HTS motor for the electric ship propulsion based on the minimization of the flux flow loss. The examples of the optimized shape of the HTS field pole coils obtained by the numerical analysis are presented. By optimizing the cross section of the field pole coil, it is found that the magnetic field perpendicular to the tape surface in the optimized shape coil significantly decreases compared with that in the rectangular shape base coil. As the results of the reduction of the perpendicular magnetic field, it is shown that the flux flow loss remarkably decreases. It is also pointed that the necessary length of the HTS tape for the optimized shape coil increases. © 2002-2011 IEEE.

DOI： 10.1109/TASC.2012.2183331

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A magnetic separation study for groundwater purification has been practically conducted by using the multi-pole magnet system. The magnetic pole was composed of 10 open magnetic spaces by arranging five HTS melt-processed bulk magnets in a line in a vacuum sheath. The individual bulk magnets were activated by feeding intense pulsed magnetic fields up to 6 T. The magnetic field distribution was estimated with respect to various pole arrangements. The actual groundwater samples of Sanjo City were processed so as to form large precipitates by adding the coagulant and pH controlling. The maximum separation ratio of the iron-bearing precipitates has exceeded over 70% when slurry water was exposed to 10 magnetic poles of up to 2.5 T at a flowing rate of less than 4.8 l/mm. An obvious attraction of flocks to the magnetic poles was observed even when the water contains no magnetite powder at the flow rate of 1.01 l/min. This implies the validity of the multi-pole magnet system with respect to the actual application to water purification. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2011.05.226

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We study the electro-magnetic design of 10 MW-class wind turbine generator with high temperature superconducting field winding by using the FEM analysis. The design examples are presented and the generator characteristics are investigated. The 10 MW-class HTS wind turbine generator is considered to be feasible from the stand point of the electro-magnetic design. In this study, 8-pole and 12-pole generators are investigated. From the results, it is considered that the 8-pole design is preferable compared with the 12-pole design. The analysis also reveals the small synchronous reactance of the HTS wind turbine generator. Therefore, proper protection is necessary for the large short circuit current.

DOI： 10.1109/TASC.2010.2090115

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It is considered that particle deposition on the photo mask, such as oil dusts which are generated from mechanical bearings of the spin processors, is one of main causes of the deterioration of pattern preciseness. In our R & D, application of magnetic levitation utilizing bulk HTSs to the spin processors for the photo mask production has been proposed. In this study, we try to develop a new model apparatus demonstrating non-contact spinning clean-up processor in photo mask production system. The levitation and rotation test by using this model apparatus showed that this new model apparatus accomplished the remarkable improvement of the levitation and the rotation ability. This paper describes the levitation and rotation test results of the new model spinner comparing with those obtained in our previous study.

DOI： 10.1109/TASC.2010.2081655

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Trapped magnetic fluxes in HTS bulks are decayed when the bulks are subjected to perturbation of external magnetic fields. In HTS bulk apparatuses such as HTS bulk motors and magnetic levitation bearings, there is a possibility that the trapped magnetic fluxes in the bulks disappear because the bulks are inevitably subjected to perturbations of the external magnetic fields. For those apparatuses to work stably and reliably, it is necessary to develop methods to suppress the decay. It was shown in our previous experiments that the decay is caused by temperature rises of the bulks due to AC losses in the bulks. The authors proposed a method to put a shorted shielding coil wound of HTS wire around the bulk aiming at reduction of the AC losses in the bulk. It was experimentally shown by use of a shielding coil wound of Bi/Ag sheathed wire that the decay of the trapped magnetic flux was effectively suppressed. In this paper, shielding performance of the coil wound of GdBCO wire was experimentally investigated comparing with that of the coil wound of Bi/Ag sheathed wire. AC losses in the bulks and shielding coils are calculated for various arrangements of the shielding coil to find a preferable shielding coil arrangement from stand points of shielding performance and reduction of AC losses.

DOI： 10.1109/TASC.2010.2093111

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Melt-textured large grain high temperature superconducting materials have a characteristic feature so as to trap the magnetic fields applied from outside, and act as quasi-permanent magnets yielding intense magnetic fields which reach values of several T in the open space above the sample surface. The performance of the magnets has been precisely investigated when intense pulsed magnetic fields generated by feeding currents up to 9.12 kA to the pulse coil settled around a cylindrical bulk magnet. In the experiment a condenser bank with large capacitances of 40-120 mF has been adapted to apply the magnetic fields up to 8.44 T to the Gd-Ba-Cu-O-based bulk magnet with the size of 30 mm in diameter. The trapped field has reached 3.28 T on the sample surface. The magnetic fields successively penetrating the sample in the process called the IMRA method have been investigated with respect to the behavior of the magnetic flux in the sample.

DOI： 10.1109/TASC.2010.2091236

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The AC transport current loss in a YBCO polygonal arranged assembled conductor depends on its arrangement. The azimuthal magnetic field produced by the currents in neighboring tapes affects the current distribution in each tape, and it is anticipated that the AC loss reduction effect is greatest with a round form. To clarify the AC loss characteristics in a polygonal arranged YBCO assembled conductor, we investigated the influence of tape distances and the angle number in the polygon. To evaluate distance, we measured the AC losses in a YBCO hexagonally assembled conductor which was arranged with distances ranging from 0.2 mm-4.0 mm. To evaluate polygonal shapes, we compared YBCO polygonal assembled conductors which were arranged as a square, pentagon, hexagon, octagon or decagon at a distance of 0.5 mm. The experimental results show that the AC loss reduction effect is greatest when the polygon is arranged as a round shape.

DOI： 10.1109/TASC.2010.2080659

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In this paper we present the experimental results of the loss of the high temperature superconducting (HTS) tape in AC and DC mixed electro-magnetic condition. The DC magnetic field is not of concern, because its influence parallels the critical current variation. The electro-magnetic environment consists of the DC offset transport current, the AC transport current and the AC magnetic field. The experimental results described were measured by the electric and calorimetric methods in the same section, and we establish the feasibility of measuring the loss using the electric method in AC and DC mixed electro-magnetic conditions. The experimental results show the loss components of the joule loss, AC transport current loss and magnetization loss. DC offset current affects the total loss characteristic in a DC and AC mixed electro-magnetic condition. Especially, when the peak transport current exceeds the critical current, the losses are increased, because there is a period when the current density is greater than the critical current density.

DOI： 10.1109/TASC.2010.2088099

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The magnetic separation technique in combination with high temperature superconducting bulk magnets has been investigated to purify the ground water which has been used in the coolant system for the incinerator furnace to cool the burning gas. The experiment has been operated by means of the newly-built alternating channel type magnetic separating device. The separation ratios of ferromagnetic flocks including fine magnetite powder have been estimated by means of the high gradient magnetic separation method with small iron balls filled in the water channels. As the magnetic force acting on the magnetic particle is given by the product of a magnetization of the material and a gradient of magnetic field, and as the ferromagnetic stainless steel balls yield the steep gradient of magnetic field around them in a strong magnetic field, the system has exhibited a quite excellent performance with respect to the separation ratios. The separation ratios of the flocks which contain the magnetite powder with the values more than 50 ppm have remained over 80% for under the flow rates less than 5 L/min. (c) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.05.210

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We study the optimal design of 2-and 4-pole coils wound by YBCO tapes as the case studies of the optimal design of the multiple-coil system based on the AC loss minimization. The winding cross sections of the element coils of the 2-and 4-pole coils were optimized starting from the conventional rectangular cross section to minimize the AC loss. The result shows that the AC loss in the optimized coil is significantly reduced and the critical current of the optimized coil is increased compared with the base coil. According to the results, the validity of our optimal design method is verified to effectively reduce the AC loss in the multi-pole coils wound by the YBCO tapes. Some disadvantageous points of the optimized coil are also discussed. © 2006 IEEE.

DOI： 10.1109/TASC.2010.2044161

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Minimization of circuit patterning on wafers is required for higher integrations of circuit elements in semiconductor devices and, therefore, extremely high preciseness and quality are strictly required for patterning of microstructures on photo masks. Particle deposition on the photo mask, such as oil dusts which are generated from mechanical bearings of the spin processors, is one of main causes of the deterioration of pattern preciseness. In our joint R & D, application of magnetic levitation utilizing HTS bulks to the spin processors for the photo mask production has been proposed. In our previous work, a small size test spinner utilizing the magnetic levitation with the HTS bulks has been developed to investigate its applicability to the spin processors for the photo mask production. In this study, we developed a new test apparatus to investigate the improvement of the levitation and rotation ability. The levitation and rotation test by using this test apparatus showed that the levitation and the rotation ability remarkably improved. This paper describes the design and the fabrication of the test apparatus together with the levitation and rotation test results using the test apparatus.

DOI： 10.1109/TASC.2010.2041766

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This paper presents numerical studies on influences of geometrical parameters on AC loss in 2-layer cable conductors assembled by YBCO tapes in polygonal arrangement. By using the numerical model developed in our previous works, the AC loss characteristics of the 2-layer polygonal cable assembled by the YBCO tapes are analyzed with changing the cable geometrical parameters such as the tape width and the tape-to-tape gap. The numerical results show the strong influences of these parameters on the AC loss characteristics. Based on the numerical data, the effects of the decrease in the tape width and the tape-to-tape gap on AC loss reduction were evaluated. The numerical results also give the important information for the design of the multi-layer YBCO cable conductor.

DOI： 10.1109/TASC.2010.2043661

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We investigated the validity of AC transport current loss measurement in a high temperature superconducting (HTS) assembled conductor using the four-terminal method. We tested the effect of different arrangements, transport current direction, and transport current distribution in the assembled conductor. Neither the arrangement nor direction of the transport current affects the accuracy of the AC transport current measurement. When transport current distribution in an assembled conductor is unbalanced, it is possible to deduce the loss by the addition of the multiple of the current and voltage from the potential lead loops in each tape. If the transport currents in each HTS tape of the assembled conductor are not uniform and only transport current of the assembled conductor is measured, the AC transport current loss is not estimated exactly. © 2006 IEEE.

DOI： 10.1109/TASC.2010.2041641

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We have investigated the feasibility of strong magnetic field generators composed of the high temperature superconducting (HTS) bulk magnet systems to the magnetic separation techniques for the waste water including thin emulsion bearing the cutting oil. Two types of the strong field generators were prepared by the face-to-face HTS bulk magnet systems, which emit the magnetic field density of 1 and 2 T in the open spaces between the magnetic poles activated by the pulsed field magnetization and the field cooling methods, respectively. A couple of water channels containing iron balls were settled in the strong field to trap the magnetized flocks in the waste water. The separation ratios of flocks containing 200 ppm magnetite powder were evaluated with respect to the flow rates of the waste water. The performances of bulk magnet system have kept showing values of around 100% until the flowing rate reached up to 18 1/min. This suggests that the magnetic separation by using bulk magnets is effective for the practical water purification systems. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.123

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Trapped magnetic fields in high temperature superconductor (HTS) bulks are decayed when the bulks are subjected to perturbation of magnetic fields. This phenomenon is inconvenient for the applications of HTS bulks and it is necessary to develop methods to suppress the decay. It was found in a previous study that the decay is caused by temperature rises of the bulks due to AC losses in the bulks. Therefore, it is necessary to develop a method to reduce the AC losses. In this work, a method is proposed to reduce the AC losses by putting HTS shield rings around the bulk and shielding performance of the rings were investigated. The experimental result shows that the shielding rings can drastically suppress the decay, even if the bulk is used in the same cooling condition and the effectiveness of the method was demonstrated.

DOI： 10.1109/TASC.2009.2019015

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In this paper, effects of geometrical configuration on AC loss characteristics of cable conductors assembled by multiple YBCO tape in polygonal arrangement were numerically studied. The previously developed numerical model was modified to analyze the AC loss in the HTS tape in the polygonally assembled multi-layer cable. By using the numerical model, the influences of the cable geometrical parameters, especially the effects of the tape width and the tape-to-tape gap, on AC loss properties were evaluated.

DOI： 10.1109/TASC.2009.2018050

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AC transport current loss in high temperature superconducting (HTS) tapes can be measured without a direct electrical connection by using a pick-up loop method. This method requires a correction factor, which is related to the ratio of the linkage magnetic flux determined by the distribution of the transport current in the HTS tape and pick-up loop. In this study, to determine the accuracy of the AC transport current loss measurement using a pick-up loop method, we measured the critical current and AC transport current loss distribution in the tape axis. We found the pick-up loop voltage of the deteriorated section of the HTS tape to be higher than in the normal section with the same transport current. We concluded that the pick-up loop method accuracy is sufficient to measure the AC transport current loss in HTS tapes. © 2009 IEEE.

DOI： 10.1109/TASC.2009.2019546

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Influence of copper layer and mechanical stress on AC losses in copper stabilized YBCO coated conductor (CC) was investigated. The uni-axial tensile stress was applied to the sample CC in liquid nitrogen at atmospheric pressure, and the AC losses (transport, magnetization and total AC losses) were measured by an electric method. Influence of the copper layer was investigated by investigating frequency dependence of the AC losses. The experimental results show that there are some influences of the copper layer. However, the influence of the copper layer is not significant from the stand point of practical applications. As to the stress dependences, experimental results show that, when stress dependences of the transport current and magnetization losses are expressed by properly normalized values using stress dependent critical currents, the loss curves fall close to unified curves, respectively and that the stress dependence of the AC losses can be estimated by knowing stress dependent critical current and AC losses in CCs without stresses.

DOI： 10.1109/TASC.2009.2019286

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

We present numerical simulation of separating magnetic particles with different magnetic susceptibilities by magnetic chromatography using a high-temperature superconducting bulk magnet. The transient transport is numerically simulated for two kinds of particles having different magnetic susceptibilities. The time evolutions were calculated for the particle concentration in the narrow channel of the spiral arrangement placed in the magnetic field. The field is produced by the highly magnetized high-temperature superconducting bulk magnet. The numerical results show the flow velocity difference of the particle transport corresponding to the difference in the magnetic susceptibility, as well as the possible separation of paramagnetic particles of 20 nm diameter.

DOI： 10.1088/1468-6996/10/1/014610

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

In designing high temperature superconducting (HTS) coils, it is important to secure large magnetic fields and stored energy using shorter tape length. Thus, it is necessary to improve the transport current performance of the coils. The critical current and n-value of an HTS tape depend on magnetic fields and flux angles under constant temperature. Considering these dependencies, we established a model to analyze coil critical current. This model clarifies that relatively large electric fields are generated at the coil edges. This adversely affects the transport current performance. In this study, the coil edge is separated into several coils, keeping the total tape length constant. This increases the coil critical current, stored energy, central magnetic field, and also the coil volume, which contains vacancies created by the separation. To estimate coil performance. we calculated the stored energy density, whose denominator is the increased Coil Volume. This stored energy density reaches its maximum value when the number of the separated coils is eight. At this optimum separation, the central magnetic field increases by 13%, and the stored energy improves by 43%, compared to a rectangular coil wound with the same tape length. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2008.05.206

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have investigated the application of the HTS bulk magnets to the magnetic separation techniques for the waste water drained from the university laboratories. The study has been conducted in comparison with the electromagnets, and the cryo-cooled superconducting solenoid magnet. A superconducting bulk magnet has the highest value of the trapped magnetic fields at the centre of the sample surface, showing a sharp gradient of the magnetic field on the surface. Magnetic force acting on magnetic particles in magnetic fields is given by the product of a magnetization of particles and a gradient of magnetic field. The HTS bulk is superior to a solenoid magnet which his a poor gradient in magnetic fields in a bore. The separation ratios of ferrite precipitates in the waste slurry were estimated by means of the high gradient magnetic separation method which requires iron filters in the water channel and open gradient magnetic separation without any filters. The magnetic separation using HTS bulk magnet is substantially effective for the practical water purification. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2008.05.234

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For designing inexpensive high-temperature superconducting (HTS) coils, it is essential to obtain large magnetic fields and stored energy with shorter lengths of HTS tape. To improve the performance of a coil, it is necessary to improve its transport-current performance. The critical current and n-value of an HTS tape were measured at various magnetic field magnitudes and angles at 77 K. The HTS tape employed in the coil was Bi-2223/Ag tape. From this measured data, fitting equations of the critical current and n-values were obtained. With these fitting equations, coil critical currents were analyzed according to our analytical model. The analysis showed that relatively large electric fields are generated at coil edges, inhibiting improvement of the transport-current performance of the coil. To solve this problem, we propose an HTS coil, which is produced by cutting and displacing the central portion of the rectangular cross section. By this rearrangement, the magnetic field distribution changes, resulting in an improvement in the coil critical current. We calculated performances of the proposed coil by varying the magnitude of displaced coils while maintaining a constant total HTS tape length. We found that there is an optimum cross section shape of the proposed coil, which provides improvements in the stored energy and the central magnetic field. In particular, the stored energy improves by approximately 43% compared with a rectangular cross section coil employing the same HTS tape length. © 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2008.05.148

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Language：English   Publishing type：Research paper (international conference proceedings)  

In high temperature superconducting (HTS) AC applications, HTS tapes are assembled into a conductor with a high current capacity and transport AC current in an AC magnetic field generated by currents in neighboring tapes. It is important to clarify the AC loss characteristics of each HTS tape in the assembled conductor. In this work, we compared measured and numerical analysis of AC transport current loss characteristics in three HTS tapes with edge-to-edge or face-to-face configuration and parallel or anti-parallel transport currents. Losses were measured by the calorimetric method, which gives actual losses regardless of the electro-magnetic environment around the sampled tape. The numerical analysis is based on the relationship between the current distribution and the inductive voltage. Our results show that the AC transport current loss in HTS tapes in an assembled conductor depends on the tape arrangement and the direction of the transport current. © 2008 IEEE.

DOI： 10.1109/TASC.2008.920696

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We suggested a pick-up loop method for measuring AC transport current loss in high temperature superconducting (HTS) tapes, which avoids a direct electrical connection with the HTS tape. This method requires a correction factor, which is related to the ratio of the linkage magnetic flux determined by the distribution of the transport current in the HTS tape and pick-up loop. We reported previously that the correction factor for an HTS tape obtained experimentally corresponded with the numerical analysis result. In this study, we numerically analyzed the correction factor for rectangular and spiral pick-up loops. The correction factor for an ideal pick-up loop does not depend on the amplitude of the transport current and is not affected by the position of the HTS tape, even if slightly altered. Our numerical analysis suggests the ideal shape for a pick-up loop to measure AC transport current loss without an electrical connection. © 2008 IEEE.

DOI： 10.1109/TASC.2008.920817

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper presents an optimal design method of winding cross section of high temperature superconducting coils wound by Bi2223/Ag tape based on AC loss minimization. We try to optimize the winding cross section starting from the conventional HTS coil with the rectangular cross section to minimize the AC loss. An example of the optimal cross section coil is presented. The result shows that the AC loss in the optimized coil is significantly reduced and the critical current of the optimized coil is increased compared with the rectangle cross section coil. It is verified that our optimal design method is effective to reduce the AC loss in the HTS coil. The disadvantageous points of the optimized coil are also discussed.

DOI： 10.1109/TASC.2008.920823

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Numerical simulation results for characteristics of separating multiple kinds of magnetic particles with different magnetic susceptibility by magnetic chromatography using high temperature superconducting bulk magnet are presented in this paper. The transient particle transport process of two kinds of particles with different magnetic susceptibility are numerically simulated. The calculated time evolutions of the particle concentration in the narrow channel of the spiral arrangement placed in the magnetic field produced by the highly magnetized high temperature superconducting bulk magnet show the flow velocity difference of the particle transport corresponding to the difference of the magnetic susceptibility. The numerical results also show that the separation of paramagnetic particles of 20 nm diameter is possible.

DOI： 10.1109/TASC.2008.921248

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATL INST OPTOELECTRONICS  

It was found in our previous experiment that trapped magnetic field in high temperature superconductor (HTS) bulks are decayed when the bulks are subjected to perturbation of magnetic fields. This phenomenon is inconvenient for the applications of HTS bulks and it is necessary to develop methods to suppress the decay. It was proven also in our previous experiments that the decay is caused by temperature rises of the bulks due to AC losses in the bulks. In the paper, studies on mechanism of the decay are reviewed. Methods to suppress the decay are proposed based on those studies and their effectiveness is experimentally investigated.

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The authors have constructed various types of strong magnetic field generators using superconducting bulk magnets in conjunction with compact refrigerators. The magnetic field in the open space outside the vacuum chamber that contains a bulk magnet has been estimated as over 3 T when activated by the static fields of a 5 T superconducting solenoid magnet. The authors have tried to extend the variation of such strong field generators which are promised to develop in the future. In the study, a novel and compact bulk magnet system with use of a compact pulse tube cryocooler has recorded the maximum trapped field of 2.78 T on the magnetic pole surface. The characteristic feature of superconducting bulk magnet is defined as a compact and strong magnetic field generator. This implies that various kinds of equipments must be successively proposed even in the very early stage of industrialization of strong magnetic field generators. © 2008 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/97/1/012102

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Two types of strong magnetic field generator have been newly constructed by using superconducting bulk magnets that mainly consist of rare earth 123 single domain compounds ( REBa2Cu3Oy) in conjunction with different kinds of compact cryocoolers. The magnetic flux densities in the open space outside the vacuum chambers that contain bulk magnets have reached 3.37 T and 2.78 T when a Gifford-McMahon (GM) cryocooler and a Stirling-cycle (ST) pulse tube cryocooler cooled them to 30.8 K and 62.0 K, respectively. In this study, the activation of a bulk magnet was accomplished by field cooling magnetization using the static field of a 5 T superconducting solenoid magnet. It was clarified that the trapped field distributions strongly reflect the temperature dependence of superconducting properties of the bulk magnet. The conditions for supplying intense fields in the open space are discussed to utilize them in practical applications. The most characteristic feature of superconducting bulk magnets is defined as a compact and strong magnetic field generator, which is discussed on the map from the industrial point of view.

DOI： 10.1088/0953-2048/20/12/025

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

In this study, we investigated AC transport current measurement using a pick-up loop. In a single HTS tape the correction factors for different pick-up loop shapes correspond with the experimental and numerical results. In an assembled conductor, we experimentally investigated the difference in the correction factors for the serial and parallel connected transport current in each HTS tape. In parallel mode, the correction factor has a constant value against the total transport current, while in serial mode the correction factor varies. These results can be explained by the relation between the transport current distribution in each HTS tape and the position of the pick-up loops. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.02.022

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Analytical study on the design of the superconducting magnet for the magnetic force assisted drug delivery system is presented in this paper. The necessary magnetic field condition to reside the magnetic drug particle in the blood vessels is determined by analyzing the particle motion in the blood vessel. The design procedure of the superconducting magnet for the M-DDS is presented and some case studies are conducted. The analytical results show that the superconducting magnet to satisfy the magnetic field conduction for the M-DDS is practically feasible. (C) 2007 Published by Elsevier B.V.

DOI： 10.1016/j.physc.2007.04.306

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Ultra-fine patterning of several ten nanometer pitch for semiconductor devices is required for higher integrations of circuit elements and, therefore, extremely high preciseness and quality are also required for patterning of microstructures on photo masks. A serious problem that causes the deterioration of pattern preciseness of photo mask is the deposition of micron or sub-micron particles, such as oil dusts, that generate from mechanical bearings of the spin processors in the grooves of the micro patterns. In this work, application of magnetic levitation utilizing high temperature superconducting (HTS) bulks to the spin processors for the photo mask production is proposed. To address this issue, a small size test spinner utilizing the magnetic levitation with the HTS bulks is developed to investigate its applicability to the spin processors for the photo mask production. The structure of the test spinner is presented and the results of levitation and rotation tests are reported in this paper. The test results show the basic compatibility of the magnetic levitation with the HTS bulks to the spin processor. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.04.300

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In designing an HTS coil, it is necessary to secure the large magnetic field and/or the stored energy with shorter tape length. From this point of view, the transport current performance of the HTS coil needs to be improved. The critical current and n value depends on the magnitude and direction of the traversing magnetic field against the Bi-2223/Ag tapes. The critical current and n value of the tape was measured with various magnitude and direction of the traversing magnetic field. From this, the fitting equations of the critical current and n value for any magnitude and direction of the traversing magnetic field were obtained. Considering this anisotropic property of the tape, we considered the analytical model to obtain the coil critical current. As an initial coil shape, the minimum volume coil derived from the Fabry Factor constant curve is taken, which is often employed in low temperature superconducting coils. The electric field distribution within the coil cross-section was calculated to examine its effect on a current carrying capability. It was clear that high electric field portions appear at the coil edges due to the inclined magnetic flux against the tapes. In this study, the external corner portions of the coil are displaced under the condition that the total coil volume is kept constant. As a result of this, the coil critical current increased and the stored energy was improved by 20%.

DOI： 10.1109/TASC.2007.898426

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Numerical studies on a AC transport loss measurement method for assembled conductors of multiple HTS tapes in multi-layer polygonal arrangement by using a spiral pick-up loop without electrical contacts are presented in this paper. The previously developed numerical model to analyze the electro-magnetic field distribution and the AC loss in the HTS tape in the polygonally assembled conductor was modified. The AC loss measurement using the spiral pick-up loop for single and multi layered polygonal conductors assembled by this HTS tapes was numerically simulated. The detection properties of the AC transport loss by the spiral pick-up loop were evaluated and the validity of this measurement method was verified.

DOI： 10.1109/TASC.2007.897249

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper presents numerical studies on structures to minimize AC losses in high temperature superconducting tri-axial multi-layer three-phase cable conductors. The Simulations were performed for tri-axial cables of 2-layer/phase configuration under 3-phase AC operation. From the numerical results, the relation between the AC loss and the layer-to-layer current sharing showed that the AC loss can be minimized by adjusting the layer-to-layer current sharing. The reason was clarified from the numerical data of the current density profiles across the superconducting layers. Based on the numerical analysis, the twist pitch formulation for the AC loss minimization was obtained.

DOI： 10.1109/TASC.2007.897243

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Dispersed generators are being introduced to power systems to secure the electric power supply. However, a short-circuit capacity of the power system increases with the introduction of dispersed generators. As a result, there is concern that a fault current will increase further, and instantaneous voltage sag might be caused. It is proposed to apply a superconducting fault current limiter in the distribution system with dispersed generators to solve these issues. In this paper, the effect of superconducting fault current limiter in this power system is studied by the analysis with EMTP and the confirmation test is conducted with a simulator.

DOI： 10.1109/TASC.2007.899884

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

As for the stored energy density, the solenoid coil has the larger value. On the other hand, the toroidal field coil has the least leakage flux. The primary concern of HTS SMES coil is the reduction of the cost, while securing superconducting coil performance. For this purpose, the required HTS tape length needs to be as short as possible. The critical current and n values of HTS tapes depends on the magnetic fields as well as its field angle to the tapes. Therefore, the performance of magnets wound with HTS tapes needs to be analyzed based on above factors. The current-voltage characteristics of the HTS coil were analyzed by means of fitting equations of the critical current and n value obtained from the measured values of the Bi2223/Ag tape. The stored energy of four kinds of coil arrangement was analyzed, thereby pursuing the shortest length of HTS tapes. The toroidal coil requires 58% longer HTS tapes than the solenoidal coil to store the energy of 100 Q, but 0.01 T leakage field region at the mid-plane is about a half of the solenoid coil.

DOI： 10.1109/TASC.2007.898425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The trapped magnetic field in an HTS bulk is decayed and even erased by the application of an AC magnetic field whose amplitude is much smaller than the maximum trapped magnetic field. In previous experiments, the decay of the trapped magnetic field was shown to be due to the temperature rise of the bulk caused by the AC losses. Especially, in a high frequency magnetic field, AC losses are larger and the decay of the trapped magnetic field is accelerated by this AC loss increase. We investigated the time relationships of the AC loss of the HTS bulk in a high frequency magnetic field using the thermal equilibrium equation.

DOI： 10.1109/TASC.2007.899000

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

To apply high temperature superconductors (HTS) to AC power apparatuses, R&Ds for AC loss reduction and quench protection of HTS wires are as important as R&Ds for high performance HTS wires. Perspectives of elementary technologies for the AC loss reduction and quench protection are presented in the paper. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmatprotec.2006.03.004

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It is in particular of importance for HTS coils to secure a larger central magnetic field and/or a large stored energy with shorter length of HTS tapes. The critical current of an HTS tape depends on both the flux density and the flux angle against tapes. From this point, the performance improvement of HTS coils is taken into account with an analytical model. The minimum volume coil derived from the Fabry Factor constant curve is taken concerning the original coil shape, which is often employed in low temperature superconducting coils. The coil critical current was analyzed in consideration of the anisotropic properties of the tape.
The electric field of HTS tapes in the coil was calculated at the coil critical current and the high electric field portion were cut out. The optimal coil cross section is obtained by iterating this calculation process. As a result, the critical current and the stored energy density of the coil were improved. The stored energy density increased about 17% and the central magnetic field was almost kept constant regardless of 19% reduction of HTS tapes, as compared with the original coil with the rectangular cross section. (c) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.cryogenics.2006.09.003

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HTS bulk can trap higher magnetic field than a permanent magnet, and non-contact magnetic bearings, electric motors and magnetic levitations are promising applications. The HTS bulks in those electric machines are exposed to magnetic field perturbations, which cause AC losses in the bulks. The AC losses deteriorate not only the efficiency of the apparatuses but also the performance by decaying trapped magnetic field. Therefore, it is important to understand the AC loss characteristics. We developed an electric method to measure the AC losses in a bulk using a double pick-up coil method. In this paper, correction factors are formulated to improve the accuracy of the method and the validity of the method is investigated by comparing the data measured by the electric and calorimetric methods. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2006.04.025

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In this work we present the validity of the measurement result and potential lead loops arrangement of the transport current losses in three superconducting tapes arranged parallel on a flat plane with tape faces on the plane. The experimental and numerical results show that the center of the superconducting tapes is smaller than other tapes. The numerical analysis result of the potential lead loop arrangement for the three superconducting tapes shows good agreement with the true value, even if the critical currents are not equal in each tape. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2006.05.038

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Recent advances in the manufacturing of multifilamentary high temperature superconducting composite wires allow for wider practical applications of the conductors, e. g. in power transmission cables, transformers and motors. The wires, based mainly on BSCCO-2223 and YBCO-123 cuprates, are used in the forms of variously shaped coils; therefore they are subjected to different kinds of mechanical stresses and strains. These, in turn, lead to some changes in the physical parameters of the superconducting material, mainly in the critical current density, and thus in the dissipated electromagnetic energy, when subjected to changing magnetic fields and transport currents. In this work we report some experimental results related to the AC loss characteristics of Bi-2223/Ag multifilamentary tapes and their dependences on bending strains. These losses are compared to the losses of virgin, straight tapes. The total AC losses, i.e. transport current and magnetization losses, in the Bi-2223/Ag tapes, were measured by means of the electrical and calorimetric methods. The experimental data obtained are compared with the critical state model predictions for AC loss behaviour in the experimental conditions presented here.

DOI： 10.1088/0953-2048/19/8/017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In this work we present numerical analysis results of the magnetization loss in a perpendicular AC magnetic field. The numerical analysis is based on the relation of the linkage magnetic flux to current distribution, and takes into account the J(c)-B characteristic, and n value-B characteristic. We discuss the relation between the magnetization losses with striated superconducting tape and the semicylindrical intrinsic critical current density distribution across the width of the superconducting tape. The result shows that the magnetization loss of monocore superconducting tape in a low magnetic field is affected by the intrinsic semicylindrical critical current density distribution. With a striated structure, the influence of the intrinsic critical current density distribution is reduced.

DOI： 10.1109/TASC.2005.869673

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper presented numerical studies on layer-to-layer current distributions and AC losses in triaxial multi-layer three-phase cable conductors. The simulations were performed for triaxial cables of 2-layer/phase configurations with several combinations of twist pitch to investigate the influence of the cable twist configuration on the layer-to-layer current distribution and the AC loss under 3-phase AC operation. From the numerical results, it was shown that the twist pitch combinations for uniform layer-to-layer current distributions could be found. The numerical results also showed, however, that the AC loss can not be reduced by equalizing the layer-to-layer current distribution. The reason is clarified from the numerical data of the current density profiles across the superconducting layers. For the cable design, it is necessary to select proper twist configuration considering not only the equalization of current distribution but also the minimization of AC loss.

DOI： 10.1109/TASC.2006.870817

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Numerical studies on AC loss characteristic of a assembled conductors of multiple HTS tapes with high aspect ratio simulating YBCO tape in polygonal arrangement were presented in this paper. We developed a numerical model to analyze the AC loss in the HTS tape in the polygonally assembled conductor based on our previously developed model. The relations between the geometrical configuration and the AC loss properties of the conductor were made clear by the numerical simulation. The numerical results give the important information for the design of the cable conductor assembled by the HTS tapes with high aspect ratio such as YBCO tapes.

DOI： 10.1109/TASC.2006.870818

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

AC loss characteristics of high temperature superconducting coils wound by Bi2223/Ag tape with various cross sections are numerically investigated. The numerical results show that the smaller coil aspect ratio, the smaller AC losses in the HTS coil. It 1 as considered from the numerical results that the coil aspect ratio giving the minimum coil volume is the better shape judging from the many aspects such as the required length of tape, the stored energy, the central magnetic field and the critical current together with the AC loss. It was also shown by the numerical study that the optimal coil design giving the reduced AC loss can be done by employing the stepped cross section coil and the turn number graded coil.

DOI： 10.1109/TASC.2006.870018

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The axial tensile stress dependences of AC transport current losses and magnetization losses in YBCO conductors, made by the IBAD/PLD ( ion beam assisted deposition/pulse laser deposition) process, were investigated in the stress range 0-770 MPa. Measured transport current losses showed a slight stress dependence, which can be explained by the stress dependence of the critical current of the conductor I-c. The magnetization losses were less sensitive to the stress than the transport current losses. The tensile stress dependences of those losses can be estimated from the data for those losses at zero stress and stress dependent I-c.

DOI： 10.1088/0953-2048/19/6/004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper presents numerical simulation results for characteristics of separating multiple kinds of magnetic particles with different magnetic susceptibility by magnetic chromatography using quadrupole magnetic field. The dependencies of separation of two kinds of particles with different magnetic susceptibility on the system parameters are evaluated by the model to analyze the transient particle transport process developed in our previous study. The calculated time evolutions of the particle concentration in the narrow cylindrical channel placed in the quadrupole magnetic field show the flow velocity difference of the particle transport corresponding to the difference of the magnetic susceptibility. The numerical results also show that the separation of paramagnetic particles of 50 nm is possible.

DOI： 10.1109/TASC.2006.871336

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We experimentally and numerically investigated the characteristics of the AC losses in Bi2223/Ag sheathed tape by the phase difference between the transport current and external magnetic field perpendicular and parallel to the tape face. The characteristics can be explained by the time evolution of the current density distributions of the cross section of superconducting tape.

DOI： 10.1109/TASC.2006.871213

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Stress dependencies of AC transport current losses, magnetization losses and total losses were investigated applying tensile stresses to an YBCO (IBAD/PLD) conductor. Measured transport current losses showed slight stress dependence in the range of 0 similar to 1 GPa. That can be explained by stress dependence of the critical current of the conductor. The magnetization and total losses are less sensitive to the stress than the transport current losses.

DOI： 10.1109/TASC.2005.869700

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

We measured AC transport current losses in three YBCO tapes placed in parallel with the wide faces of the tapes on a flat plane using an electric method which was specially developed to properly measure the losses in tapes arranged in such a way. The measurement results showed that the transport current losses in an YBCO tape were significantly affected by the magnetic fields produced by the currents in the adjacent tapes. In the paper, the experimental results are explained by considering the magnetic field distribution around the tapes.

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HTS bulk can trap higher magnetic field than a permanent magnet and electric motor and actuators are promising applications. The HTS bulks in those electric machines are exposed to AC magnetic field perturbations, which cause AC losses in the bulks. Therefore, it is important to understand the AC loss characteristics. In the previous works, we measured the AC losses by an electric method using the double pick-up coils method. This time, we measured the AC losses by a calorimetric method using a cryo-cooler and heater at different temperature. In this paper, the AC losses measured by the electric and calorimetric methods are compared with each other to investigate the validity of the measurement methods.

DOI： 10.1088/1742-6596/43/1/146

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

A numerical model to calculate inter-layer current distributions and AC losses in tri-axial multi-layer three-phase cable conductors was developed. The numerical model was applied to the two-layer/phase tri-axial cables with several combinations of twist pitch and twist direction to investigate the influence of the cable twist configuration on the interlayer current distribution and the AC loss under three-phase AC operation. From the numerical results, the twist pitch combinations to equalize the inter-layer current distribution could be found. The numerical results also showed that the AC loss can be much reduced by equalizing the inter-layer current distribution. It was also notable that the AC losses in the cables with different twist configurations were different, although the inter-layer current distribution was equalized. For the cable design, it is necessary to select proper twist configuration considering both equalization of current distribution and minimization of AC loss together. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2005.03.055

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It is in particular of importance for HTS coils to secure a larger central magnetic field, a large stored energy, etc. with shorter length of HTS tapes. The critical current of an HTS tape depends on both the flux density and the flux angle with respect to tapes. In view of this, the performance improvement of HTS coils is taken into account with an analytical model. As a coil shape, the minimum volume coil derived from the Fabry Factor constant curve is taken up, which is often employed at low temperature coils. The electric field distribution within a coil cross-section is calculated to examine effects on a current carrying capability. It is clear that high electric field portions appear at the coil edge region due primarily to inclined magnetic fluxes against HTS tapes. Considering this, a grade winding method of a coil is proposed, where the winding density of conductors is reduced at coil edge portions. With this coil winding structure, the critical current of an HTS coil is improved since the magnetic field is reduced at edge portions. The stored energy per HTS tape length and the central magnetic field of the coil can be remarkably increased by this kind of grade winding method. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2005.02.108

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Electrical power devices, e.g. motors, power transmission cables, transformers, etc. are the most promising, and nowadays the most technologically advanced, applications of high temperature superconductors (HTS). Multifilamentary HTS tapes (usually Bi-2223/Ag composites) are used in forms of various in shape coils, therefore are subjected to different kinds of mechanical stresses. This leads to some changes in physical parameters of superconducting material, mainly in the critical current density, and thus in the dissipated electromagnetic energy. In general, when a superconductor is carrying an AC transport current and simultaneously subjected to an external AC magnetic field, the total dissipated energy can be considered as a sum of components, the transport loss and the magnetization loss. In this work we report some experimental results related to the total AC loss characteristics of Bi-2223 tapes and their dependencies on bending strains. An external magnetic field, in phase with an AC transport current, was directed parallel to the wider surfaces of the samples. The total AC losses in the Bi-2223/Ag tapes were measured by means of a calorimetric method. Obtained experimental data are compared with the critical state model predictions for total losses behavior in presented here experimental conditions. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2005.01.068

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An analytical study of AC loss characteristics of a high temperature superconducting coil wound by a Bi2223/Ag tape is presented in this paper. The AC loss in the high temperature superconducting coil wound by Bi2223/Ag tape was numerically calculated by using the measured DC data of the tape. The calculated AC loss was compared with the measured AC loss in the Bi2223 HTS coil obtained in the previous study. For the comparison of the measured and calculated AC losses, the mechanical loss was subtracted from the measured total loss data. The calculated AC losses were compared with the measured AC losses in the coil, producing rather good agreements. The relation between the AC loss characteristics in the HTS coil and shape of coil cross-section was numerically studied. A possible way to reduce the AC loss in the HTS coil was also presented and discussed. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2005.03.052

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For the purpose of protecting electric power system, many researches and developments of fault current limiters are being performed. The authors studied a dc hybrid three-phase fault current limiting interrupter (FCLI) composed of a superconducting reactor and an S/N transition element, connected in series each other. The dc hybrid type fault current limiting interrupter can limit a fault current by means of the inductance of high temperature superconducting (HTS) coil together with the normal transition of HTS bulk material (HTSB). In the case of an accident, the normal transition of the bulk material can be accelerated by the magnetic field of the HTS coil. In this paper, the dc hybrid type fault current limiting interrupter for 5.5 km long 6.6 kV-600 A power distribution system is analyzed, and performances of fault current limitation and interruption are confirmed. Moreover, a reclosing operation is discussed for this power distribution system. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2005.01.070

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

HTS bulks applied to electric machines are exposed to AC magnetic field perturbations which cause AC losses in the bulks. AC losses affect the efficiency of the machines and decay the trapped magnetic field in an HTS bulk. Critical current density and the maximum trapped magnetic field of a bulk are enhanced as temperature is decreased. Thereby, the lower temperature operation of the bulk is preferable. The AC losses in the bulk and their influence on the trapped magnetic field are dependent on the temperature. Therefore, to estimate the efficiency and decay of the trapped magnetic field at various operation temperatures, it is necessary to know the temperature dependence of the AC losses. We investigated AC loss characteristics of a bulk at various temperatures. In the work, AC losses were measured for the amplitude of the AC external magnetic field up to 0.08 T and frequency 17.3 Hz similar to 62.2 Hz at the temperature 65 K similar to 100 K. It is also shown that the temperature dependences of the AC losses are well estimated by a theoretical model based on the Bean model.

DOI： 10.1109/TASC.2005.848253

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We developed an electric method to measure AC transport current losses in HTS tapes placed parallel on a flat plane to study on influence of magnetic field produced by the neighboring tapes on AC loss characteristics of a tape in an assembled conductor. To measure the AC losses in a tape in such arrangement electrically, proper cares for potential leads arrangements are necessary to avoid spurious loss components mixed in the measured data. In our method a combination of spiral and rectangular loops was used. Configuration of the loop arrangements was determined by a study of energy flows among the tapes. The validity of the method was verified by a calorimetric measurement conducted with the same arrangement of the tapes. The experimental data showed that the AC losses in the tape were significantly affected by the magnetic fields produced by the neighboring tapes.

DOI： 10.1109/TASC.2005.848257

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper reports an analytical study on AC loss characteristics of a high temperature superconducting coil wound with a Bi2223/Ag tape. The electromagnetic AC loss in the high temperature superconducting coil made by stacking pancake coils wound with the Bi2223/Ag tape was numerically calculated by using the measured DC data of the HTS tape. The calculated AC loss was compared with measured AC losses in Bi2223 HTS coils obtained in a previous study. It had been found that the measured AC losses in the coils contained large mechanical loss. For the comparison of the measured and calculated losses, the mechanical loss was subtracted from the measured total loss data to estimate the electromagnetic loss. The calculated AC losses were compared with the measured electromagnetic losses in the coils, producing rather good agreement. A possible way to reduce the AC loss in the HTS coil was also presented and discussed.

DOI： 10.1109/TASC.2005.849173

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A method is proposed to reduce AC magnetization losses in YBCO tape-shaped coated conductors subject to external AC magnetic fields. To reduce the magnetization losses, it is well known that the YBCO film should be subdivided into multiple filaments. In this multifilamentary conductor, the electromagnetic coupling between the filaments should be suppressed to reduce the AC losses by twisting the conductor somehow. The proposed method is that a YBCO layer on a substrate of a coated conductor is subdivided into multiple filaments by slanted striations and that a quasi-twisted assembled conductor is composed by soldering a pair of those striated tapes. The effect of the twisting is obtained by this composition without twisting the conductor itself. A numerical simulation showed that magnetization losses in a quasi-twisted conductor were greatly reduced at a sacrifice of a slight increase of the transport current losses.

DOI： 10.1109/TASC.2005.848235

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Numerical study on the open gradient magnetic separation using the quadrupole magnetic field is presented in this paper. The characteristics of separating multiple kinds of magnetic particles with different magnetic susceptibility are numerically evaluated by the model to analyze the transient diffusion process developed in our previous study. We consider two methods for the separation of multiple kind particles. In the first method, the numerical results show that the two kinds of particles with different magnetic susceptibility can be selectively separated. The second method is one of the field flow fractionation which is called the magnetic chromatography. The calculated time evolutions of the particle concentration in the thin cylindrical channel show the flow velocity difference of the particle transport corresponding to the difference of the magnetic susceptibility. It is considered that the open gradient quadrupole magnetic field can be applied for the magnetic chromatography.

DOI： 10.1109/TASC.2005.849661

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Fault current limiting and interrupting performances of three types of fault current limiting interrupter (FCLI) are compared; a DC reactor type, a DC S/N transition type and a DC hybrid type, which are made of the Bi-2223 tape and bulk. The DC hybrid type FCLI can limit a fault current by means of the inductance of HTS coil and the normal resistance of high temperature superconducting bulk (HTSB). In the case of an accident, the normal transition of the bulk can be accelerated by the magnetic field of the HTS coil. In this paper, performances of the DC hybrid type three-phase FCLI are compared analytically with the DC reactor type and the DC S/N transition type. Features derived from the combination of a reactor coil and an S/N transition element for the DC hybrid type are discussed and as an example it is applied to 6.6 kV-2000 A power distribution system.

DOI： 10.1109/TASC.2005.849469

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

In our previous work, it was observed that trapped magnetic field in an high-temperature superconductor (HTS) bulk was decayed and even erased by application of AC external field whose amplitude was much smaller than the peak value of the trapped magnetic field. Therefore, knowledge on the mechanism of the decay of the trapped magnetic field is important to design the machines and to develop a method to suppress the decay. This work studies a mechanism of the decay due to application of AC magnetic field by a numerical analysis and experiments. An analytical model is proposed to explain the mechanism based on the thermal effect due to AC losses in the bulk. In the model it is assumed that the AC loss characteristic follows the Bean model, which was experimentally demonstrated also in our previous work. We conducted experiments in which temperature rise of the bulk was measured by thermocouples and the decay of the trapped magnetic was observed by a hall probe. Results numerically calculated from the analytical model well agreed with those obtained from the experiments and we consider it was clarified that the decay of the trapped field was caused by the thermal effect due to AC loss. Once the mechanism has been clarified, methods to suppress the decay can be easily developed. (c) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmatprotec.2004.07.009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

In this work we present dependence of AC losses in a YBCO tape conductor carrying an AC transport current in an external AC magnetic field on the angle between the direction of the external field and the tape face of the conductor. The losses were measured by a calorimetric method whose validity was proven by simultaneous electric and calorimetric measurements of the losses in the same part of the conductor. The experiment was conducted at 77 K. The measured data show that the AC losses are dominated by the normal component of the external magnetic field to the tape surface. It is also shown that the AC losses sharply drop when the external magnetic field is parallel to the tape surface, which is due to extremely high aspect ratio of the YBCO conductor. (C) 2004 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.cryogenics.2004.07.004

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A trapped magnetic field in a bulk is decayed and even erased by the application of an AC external magnetic field whose amplitude is much smaller than the trapped magnetic field. In the paper an analytic model to describe behavior of the trapped magnetic field is proposed assuming that its decay is caused by thermal effect due to AC losses in the bulk. The proposed model well explains AC loss characteristics, temperature rise and the decay of the trapped magnetic field of the bulk. Based on the proposed model, methods to suppress the decay of the trapped are shown and the effectiveness of the methods is shown. (C) 2004 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.cryogenics.2004.06.007

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE LTD  

Dependence of AC loss characteristics of Bi2223/Ag-sheatled tapes on bending strains was experimentally investigated. AC transport current losses in the tapes subject to various bending strains were measured in the zero external magnetic field. These losses were compared with the losses of straight tapes. The measurement results show that values of the AC losses are affected by the degradations of the critical currents caused by the bending strain. Based on these results it can be concluded that the AC transport current loss characteristics of Bi2223/Ag-sheathed tapes subject to bending strain can be estimated by knowing values of critical currents of bended tapes and the loss characteristics of straight tapes.

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The trapped magnetic field in a bulk is decayed and even erased by the application of the AC external magnetic field whose amplitude is much smaller than the trapped magnetic field. In the previous work, an experimental result showed that the decay of the trapped magnetic field was due to temperature rise of the bulk caused by the AC losses. Based on this result, it is considered that the decay can be suppressed by (a) improvement of the cooling of the bulk and (b) reduction of the AC losses. We verified the effectiveness of these methods by conducting an experiment using a recently developed metal impregnated bulk which has high critical current density and improved thermal conductivity. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2004.01.094

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Compared with LTS materials, HTS materials have some peculiarities affecting AC loss characteristics of the conductors. We measured the AC transport current losses in YBCO thin film coated conductors and a Bi2223/Ag sheathed tape. Comparing the measured data with analytical calculations, the dependence of the AC transport current losses on the n value and critical current density distributions are studied. It is shown that, considering the n values and J(c) distributions, the peculiarities in the HTS materials can be taken into consideration and the transport current losses in HTS conductors can be calculated by the same analytical method used for LTS. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2003.09.031

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：CRYOGENICS AND SUPERCONDUCTIVITY SOCIETY OF JAPAN  

We measured total AC losses in a YBCO tape conductor using a calorimetric method for which the validity was proven by simultaneous electric and calorimetric measurements of the losses in the same part of the conductor. Our data is accurate to 5x10^-5&sim;5x10^-2 J/m/cycle at 64.1 Hz. The data can be reduced to one curve plotted against a corrected abscissa of the external magnetic field plus the internal magnetic field. The AC loss in different phases of transport current and external magnetic field varies with the phase. Our finding demonstrates the possibility of reducing loss in the case of tri-axial superconducting cables.

DOI： 10.2221/jcsj.39.593

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In our previous work it was observed that trapped magnetic field in a YBCO bulk was decayed and even erased by application of AC external field whose amplitude was much smaller than the peak value of the trapped magnetic field. To study a reason for decay we investigated temperature rise of a YBCO bulk subject to the AC external magnetic field by use of thermocouples. A case was observed that the temperature rise of the bulk reached almost the critical temperature. This result shows that the decay of the trapped magnetic field was related to the temperature rise due to AC losses in the bulk. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0921-4534(03)00988-2

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In the paper we present experimental results of power losses measurements in a YBCO-123 coated conductor carrying a DC (or AC) transport current when subjected simultaneously to an external AC magnetic field. Two kinds of measurements were carried out; the first was AC transport current losses as a function of the current amplitude and the AC external magnetic field ("AC-AC" case); the second was dynamic resistance losses versus DC transport current and the AC external magnetic field ("DC-AC' case). Additionally, for both cases the magnetization losses were measured. The increase of the AC transport current loss due to the AC external magnetic field in the full saturation regime is described well with the dynamic resistance effect. This makes it possible to replace the AC transport current measurement in AC applied magnetic field with the much more simple dynamic resistance loss measurement when the total AC loss has to be determined. (C) 2003 Published by Elsevier Science B.V.

DOI： 10.1016/S0921-4534(03)00676-2

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Dependence of AC magnetization losses in a YBCO bulk on the trapped magnetic field is experimentally investigated. The experimental results showed that the losses were hysteretic and that the losses in a bulk with trapped magnetic field were larger than those without trapped magnetic field. Distributions and profiles of the trapped magnetic field in the bulk were also measured before and after applying AC external magnetic field. It was observed that the trapped magnetic field was decreased by applying AC external magnetic field. From the trapped magnetic field profile, the full penetration magnetic field is estimated and the AC losses are calculated based on the Bean model. The calculated results well agree with the measured data. (C) 2002 Published by Elsevier Science B.V.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

In the work we present experimental results for AC losses and critical currents of biaxially aligned YBCO-123 coated conductors, with Hastell as a substrate material. The critical current density of the tapes was of the order of 4V.10(9) A/m(2) at the liquid nitrogen temperature. Magnetization losses, in sinusoidally varying external AC fields directed perpendicular to the plane of the tapes, were measured using a standard Lock-in technique. The frequency of the magnetic field was in a range of 7-64 Hz. The losses in YBCO are pure hysteretic for amplitudes up to 40 mT. Magnetization losses in the tape when carrying DC transport current are also reported. From obtained experimental data of dynamic resistance and the magnetization losses the total losses dissipated in the tape have been evaluated and compared with the critical state model predictions.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

We measured AC transport current losses in twisted multifilamentary Bi2223/Ag sheathed tapes subject to AC external longitudinal magnetic fields. The tapes specially made for the experiment have wider space between superconductor filaments than usual multifilamentary Bi/Ag sheathed tapes. The wide space suppresses superconductive bridgings between the superconductor filaments. The AC transport current losses were electrically measured. The losses almost followed the Norris elliptical model when tile direction of the external longitudinal field was in the opposite direction to that of the self longitudinal field which was produced by the transport current in the twisted multifilament tape. On the other hand, when the direction of the external longitudinal field was in the same direction as the self longitudinal field, the losses became much smaller than the Norris model if the amplitudes of the longitudinal field was optimized for that of the transport current.

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Power dissipation in multifilamentary Ag/Bi-2223 composite tapes was measured using AC magnetization loss measurement method. In order to decrease the interfilamentary electromagnetic coupling currents in the tapes the filaments were separated by resistive ceramic barriers, made of Bi-2201 phase or Al2O3 powder. The tape samples used in this study were with twisted and untwisted filaments. The magnetization loss measurements were performed in a sinusoidally varying external magnetic field (parallel to the plane of a tape) and with a DC transport current flowing along a tape. The frequency range of the sinusoidal varying field was 11-115 Hz. Some anomalies in magnetic losses due to DC current flowing distributions are observed. The obtained results are compared with the critical state model predictions. (C) 2001 Elsevier Science B.V. All rights reserved.

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We developed an ex situ method for magnetization of a bulk array with alternate polarities for a full HTS linear actuator by one magnetization process. The linear actuator is composed of primary windings of Bi/Ag sheathed wires and a secondary made of YBCO bulks. The linear actuator is a synchronous type and the secondary is a bulk array with alternate polarities. For practical applications, a simple method for magnetizing the bulk array needs to develop. We demonstrated that the bulk array was alternately polarized by our method. In the paper, the method for the magnetization is explained and the results of the magnetization experiments are presented. (C) 2001 Elsevier Science B.V. All rights reserved.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Power dissipation in a set of multifilamentary Bi-2223 silver sheathed composite tapes (with 61 filaments) was measured using AC magnetization and transport loss measurement techniques. In order to decrease the inter-filamentary bridging and thus electromagnetic coupling, the space between filaments was considerably increased. The tape samples used in this study were with twisted (twist pitch 15 mm) and untwisted filaments. Critical currents were of the order of 55 Amps, as measured by a standard four-point technique, and corresponds to a critical current density of approximately 9 kAcm(-2), The AC transport current losses were measured by a standard four-point Lock-in method, in the range of the transport current frequency of 40-160 Hz The magnetization loss measurements were performed in a sinusoidally varying external magnetic field, at the frequency range of 11 to 155 Hz. The external magnetic field was oriented parallel to the plane of a tape. The obtained results are compared with theoretical predictions of the critical state model.

DOI： 10.1109/77.919633

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

AC transport losses in a biaxially aligned YBCO-123 tape (10 cm in length) were measured (at 77K) as a function of frequency and amplitude of a sinusoidally varying transport current. In the frequency range of 30-320 Hz the transport current losses are hysteretic in nature. The measurements of the self-field losses caused by the AC transport current as a function of the magnitude and orientation of the applied low external DC magnetic field (up to 150 mT) are also reported. The external magnetic field was rotated in the range of +120 to -120 degrees with respect to the plane of the tapes, and was in all cases perpendicular to the transport current flow. Strong anomaly in angular critical current dependence on external magnetic field was observed.

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DOI： 10.1109/77.920297

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© 2018 Nova Science Publishers, Inc. Various types of strong magnetic field generators with use of HTS bulk magnets and the small-scale refrigerators have been actually proposed and experimentally estimated for the feasible applications. The most characteristic feature of bulk magnets is regarded as compact and strong field generators with respect to their magnetic field spaces or their machine structures of themselves. Among them, the magnetic flux density of the face-toface magnetic poles reached 3.2 T and 4.4 T when they were activated by the pulsed field magnetization and the field cooling method, respectively. Furthermore, a very small bulk magnet generating 2.8 T and the system featured by the wide magnetic poles containing seven magnets have been constructed with use of ST pulse tube coolers in order to expand the feasible application areas. On the other hand, although the bulk magnets are characterized by their steep field distributions, we attempted to gain the uniform magnetic field space, and successfully achieved the uniformity of 385 ppm in 4 mm range by combining the deformed magnetic field profiles which generated between the face-to-face magnetic poles. The performances of these unique systems have exceeded those of the conventional electromagnet or any permanent magnets by far, demonstrating the superiority to other strong magnetic field generators.

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© Published under licence by IOP Publishing Ltd. A peculiar magnetic separation technique has been examined in order to remove the Cs-bearing Fe precipitates formed of the waste ash from the withdrawn incinerator furnaces in Fukushima. The separation system was constructed in combination with high temperature superconducting bulk magnets which generates the intensive magnetic field over 2 T, which was activated by the pulsed field magnetization process. The separation experiment has been operated with use of the newly-built alternating channel type magnetic separating device, which followed the high-gradient magnetic separation technique. The magnetic stainless steel filters installed in the water channels are magnetized by the applied magnetic fields, and are capable of attracting the precipitates bearing the Fe compound and thin Cs contamination. The experimental results clearly exhibited the positive feasibility of HTS bulk magnets.

DOI： 10.1088/1742-6596/897/1/012024

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A characteristic magnetic separation technique has been developed to recycle the Ni resource from the waste fluid of electroless Ni-plating processes. Since the plating waste still contains high concentration of Ni ions after several cycles of plating processes, the magnetic separation experiments were conducted to collect the precipitates containing Ni compounds. After extracting the fine NiHPO3 precipitate from the waste liquid, we synthesized the NiSO4 crystals from the NiHPO3 precipitates through the reaction with the concentrated sulfuric acid. The open-gradient magnetic separation was employed to separate the NiSO4 crystals from the mixed slurry composed of the NiSO4 and the NiHPO3 due to the difference between their magnetic properties. The experiments were practically conducted with use of the Gd123-based HTS bulk magnets with very steep gradient of magnetic field of over 100 T/m, which were activated to the magnetic fields of 1.53 and 1.95 T by the pulsed field magnetization process operated at 35 K. The ratio of NiSO4 content in the collected slurry attracted to the magnetic poles reached up to 98%, which suggests that the paramagnetic NiSO4 crystals are preferentially collected by the magnetic force from the mixed slurry in the strong magnetic field generated by the HTS bulk magnet.

DOI： 10.1109/TASC.2016.2517923

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The authors have been attempting to obtain the uniform magnetic field distribution in the space between the face-to-face HTS bulk magnets. The magnetic poles containing the HTS bulk magnets are usually characterized as non-uniform magnetic field distribution. Since the distributions show the conical or convex shapes, it is difficult to obtain the uniform magnetic field spaces even when the magnetic poles would be placed face-to-face. The authors have modified the shape of the distribution of one-side magnetic pole by attaching an iron plate on the surface, and formed the concave magnetic field distribution on the pole surface. The steep concave or convex distributions at each pole surface change to be flat with increasing distance from the pole surface. After the experimental result recording the best uniformity of 358 ppm by combining the concave and convex field distributions face-to-face, we attempted to simulate the feasible performance in this configuration. In the numerical simulation, the concave field distribution modified by attaching an imaginary spiral coil on the pole surface was coupled with the original convex field. We succeeded in obtaining the best uniformity of 30 ppm at 1.1 T in 4 x 4 mm2 x-y plane at 7 mm distant from the pole surface in the gap of 30 mm. This result suggests that the concave and convex magnetic field distributions compensate the field uniformity with each other with keeping the magnetic field strength in the gap, and also suggests the novel compact NMR/MRI devices in the future.

DOI： 10.1016/j.phpro.2016.04.019

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© 2015 The Authors. A unique experimental attempt aiming to obtain a uniform magnetic field space as required for NMR has been carried out with use of HTS bulk magnets. The magnetic poles were activated as 1.8 T (North) and 1.4 T (South) at 30 K by applyinga pulsed magnetic field up to 7 T, and positioned face-to-face with gaps less than 70 mm. The uniformity of the magnetic field required for detecting the NMR signals isless than 1,500 ppm at more than 0.3 T in the cross sectional plane of 2 x 2 mm2. After thepreliminary trials whichrevealed auniformity of 5,421 ppm at 0.44 T in a70 mm gap, we attached a ferromagnetic iron plate to a magnetic pole surface to change the magnetic field distribution to be concave. The best uniformity of 358 ppm at 1.11 T was obtained at 9 mm distance from the iron plate surface in a gap of 30 mm. It is stated that the concave magnetic field distribution was compensated by the counter conical-shape field, resulting in the uniform field plane.

DOI： 10.1016/j.phpro.2015.06.187

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A magnetic separation technique has been conducted to recycle the nickel (Ni) element from the waste fluid of electroless Ni plating processes. The experiment was operated using the face-to-face type electromagnet generating 1.96 T and the data was compared to those in the case of the high temperature superconducting bulk magnet which generates 3.45 T. In the electroless plating processes, the plating fluid containing high concentration of Ni ions is disposed after several turns of the plating cycles when the concentration of phosphite ions exceeds a certain level. In order to extract the Ni element at first, we formed the coarse Ni-sulfate crystals with the sizes over 1 mm from the Ni-phosphite precipitates by controlling the temperature and the pH value. Next, the magnetic separation experiments were operated to collect the Ni-bearing precipitate which carried the weak magnetism. Although the performance of high temperature superconducting bulk magnet is superior to that of electromagnets, as a result, we succeeded in collecting Ni-sulfate crystals preferentially to the Ni-phosphite precipitates soon after the crystal growth began.

DOI： 10.2497/jjspm.61.S164

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The magnetic flux penetration into the melt-textured Y-Ba-Cu-O high temperature superconducting bulk magnets were precisely evaluated during and after the pulsed field magnetization processes operated at 30 K. The bulk magnets were carefully fabricated by the cold seeding method with use of a single and a pair of seed crystals composed of the Nd-BaCu- O thin films. These seed crystals were put on the top surfaces of the precursors to let the large grains grow during the heat treatments. We observed the flux penetrations which occurred in the lower applied-field regions at around 3.1 T for the samples bearing the twin seeds than those of the single-seeded crystals at around 3.8 T. This means that the magnetic fluxes are capable of invading into the twin-seeded samples more easily than the single-seeds. It suggests that the anisotropic grain growths of parallel and normal to the rows of seed crystals affects the variations of J(c) values with different distributions of the pinning centers, results in the preferential paths for the invading magnetic fluxes.

DOI： 10.1088/1742-6596/507/1/012039

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© 2014 The Authors. The flux-invasion behavior into the melt-processed Y-Ba-Cu-O bulk magnets were precisely measured and analyzed during and after their pulsed-field magnetization processes operated at 30.6 K. The materials were fabricated as the bulk monoliths grown by adopting two seed-crystals, or shifting the seed-crystal positions from the centre of the sample surface, which exhibited the magnetically single-domain distributions. Although the performances of the trapped flux density after activations showed no obvious differences, the flux started invading into the sample bearing two seeds obviously at lower fields than those of normally-grown isotropic crystal. Since the flux penetration behavior were thus clearly different between the samples with the structure grown from two seeds and uniformly grown samples with a seed crystal, it is suggested that the structure results in an effective magnetizing method with less heating than those of conventional samples, which results in the higher performance of field trapping in the bulk magnets than usual.

DOI： 10.1016/j.phpro.2014.09.021

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Language：Japanese   Publisher：CRYOGENICS AND SUPERCONDUCTIVITY SOCIETY OF JAPAN  

Intense magnetic fields exceeding 2 T are necessary to magnetize rare-earth magnets to extract the full performance of magnet materials. In this report, we deal with the experimental measurements and numerical analysis of the magnetic flux after scans of the 3 T HTS bulk magnet on the surface of the rare-earth magnet. The distributions of the magnetic flux density between measured and simulated data on the magnet slab showed good agreement after the scanning. The study is conducted to evaluate the magnetization performances of bulk magnets for the interior permanent magnet (IPM) motors. According to the numerical analysis, the magnetic field of 3 T reached the point of 22 mm from the surface of the rotor. The experimental data clarified that the magnet slab (26 × 20 × 5 mm³) in the rotor was fully magnetized by scanning the bulk magnet on it, while the neighboring magnet was barely magnetized in the rotor because of its anisotropic magnetizing property. This shows that the magnets in IPM motors are capable of being activated from outside the rotors with use of HTS bulk magnets, which suggests an improved degree of freedom in designing the motor.

DOI： 10.2221/jcsj.47.568

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A magnetic separation experiment to recycle nickel from the waste fluid of electroless plating processes was conducted using the open-gradient magnetic separation technique and a HTS bulk magnet system. A magnetic pole containing Gd123-based bulk superconductors was activated to 3.44 T at 34.9 K using a 5 T superconducting solenoid and the field cooling method. The coarse precipitates of nickel sulfate are composed of phosphite ions, which are yielded during the plating reaction by controlling the temperature and pH. Next, the open-gradient magnetic separation technique was employed, with use of water channels to separate the nickel-sulfate crystals from the mixture of the nickel sulfate and phosphite compounds based on the difference in magnetic properties. From the concentrations of each precipitate attracted to the magnetic pole, we succeeded in collecting nickel-sulfate crystals preferentially to the phosphite ions soon after crystal growth began.

DOI： 10.2221/jcsj.46.655

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A demagnetized Nd-Fe-B permanent magnet was scanned just above the magnetic pole containing the HTS bulk magnet, generating a magnetic field of 3.27 T. The magnet sample was subsequently found to be fully magnetized in the open space of the static magnetic fields. We examined the magnetic field distributions when the magnetic poles were scanned twice to activate the magnetic plates inversely with various overlap distances between the tracks of the HTS bulk magnet. The magnetic field of the "rewritten" magnet reached the values of the magnetically saturated region of the material, showing steep gradients at the border of each magnetic pole. As a replacement for conventional pulse field magnetizing methods, this technique is proposed to expand the degree of freedom in the design of electromagnetic devices, and is proposed as a novel practical method for magnetizing rare-earth magnets, which have excellent magnetic performance and require intense fields of more than 3 T to be activated.

DOI： 10.2221/jcsj.46.111

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The application of superconducting bulk magnets to the magnetic separation techniques has been investigated for the Mn-bearing waste water drained from the university laboratories. The research has been conducted in comparison with the electromagnets, and the cryo-cooled superconducting solenoid magnet. The separation ratios of ferrite precipitates including Mn element in the waste slurry were estimated by means of the high gradient magnetic separation method with ferromagnetic iron filters in the water channel and open gradient magnetic separation without them. As the magnetic force acting on the particles is given by the product of a magnetization of particles and a gradient of magnetic field, and a superconducting bulk magnet shows a sharp gradient of the magnetic field on the surface, the performances of the bulk magnet system were almost equivalent to those of the superconducting solenoid magnet with wide bore with respect to the magnetic separation ratios. The separation ratios for Mn have reached over 80 % for HGMS and 10 % for OGMS under the flow rates less than 3 liter/min. © 2009 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/156/1/012031

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