K. Tagawa

Hitachi Cable, Ltd., Edo, Tōkyō, Japan

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Publications (35)39.42 Total impact

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    ABSTRACT: We have recently developed a thin-type Cu clad stabilized Nb<sub>3</sub>Al flat-wire which can be more densely wound into a coil. Using this flat-wire we have upgraded an existing 14 T Nb-Ti/Nb<sub>3</sub>Sn magnet with a ?? 30 mm clear bore, by replacing the inner coil, to a compact 15 T Nb-Ti/Nb<sub>3</sub>Al magnet. The Nb<sub>3</sub>Al inner coil was connected in series with the Nb-Ti outer coil and excited by a common power supply to generate 15 T in a ?? 40 mm clear bore, after a training of Nb-Ti coil. The resultant higher field and wider space will be used for critical current measurements of various superconducting wires being developed in NIMS.
    IEEE Transactions on Applied Superconductivity 07/2010; · 1.20 Impact Factor
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    ABSTRACT: Rapid heating, quenching and transformation (RHQT)-processed Nb<sub>3</sub>Al wires possess better strain tolerance than Nb<sub>3</sub>Sn wires. However, RHQT-processed Nb<sub>3</sub>Al wires so far developed show lower non-copper critical current density (J<sub>c</sub>) than recently developed high-J<sub>c</sub> Nb<sub>3</sub>Sn wires in the field range of 12-15 T. Therefore, the development of Nb<sub>3</sub>Al wires having enhancing J<sub>c</sub> in this field range is a major concern for accelerator magnet designers. Among a number of fabrication parameters that might affect J<sub>c</sub> properties, we analysed the effect of thickness of Al and Nb foils in the jelly-roll-shaped filaments embedded in the precursor wires. Mechanical and superconducting properties of these wires with various Al-foil thicknesses in the filaments were investigated.
    IEEE Transactions on Applied Superconductivity 07/2010; · 1.20 Impact Factor
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    ABSTRACT: We have been developing rapid heating, quenching and transformation (RHQT) processed Nb<sub>3</sub>Al wires for next-generation accelerator magnets that require magnetic fields between 12 and 17 T. However, it has recently been found that the conventional Nb-matrix wire shows a rather strong magnetic instability at low fields. Taking this into consideration, we have begun developing Ta-matrix Nb<sub>3</sub>Al wires. We have fabricated a precursor wire with a Ta-matrix volume fraction of 0.8, and the mechanical properties of the RHQ-processed wire were examined. The effects of the RHQ condition and the area reduction (AR) after RHQ on the critical current density ( J <sub>c</sub>) , and magnetization of the wire have also been examined. This paper describes the properties for the Ta-matrix wire in comparison with those of the Nb-matrix wire.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor
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    ABSTRACT: We have developed Nb3Al wires with a low Nb matrix ratio in order to attain a high critical current density. Test wires were prepared with different fabrication parameters, mainly the Nb matrix ratio, RHQ current, and the area reduction ratio of the wire after an RHQ heat treatment. The critical current and n-values of these wires were measured. Our newly developed wire with a Nb matrix ratio of 0.6 was almost the same RHQ heat-treatment effect and area reduction effect as those of samples with different Nb matrix ratios. However, the critical current density of our wire was lower than that of these samples. In order to determine the reason for this low critical current density, Tc measurements and tensile strain tests were also performed. In this paper, we report the characteristics of the sample with a Nb matrix ratio of 0.6 and compare them with those of the other samples.
    Journal of Physics Conference Series 03/2008; 97(1):012059.
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    ABSTRACT: We attempted to reduce the matrix ratio down to 0.5 and 0.25 for RHQT (rapid-heating, quenching and transformation) Nb3Al wires, which is directly related to improvement of the non-Cu Jc. The Ta matrix was chosen to improve the wire strength. It was found that the matrix ratio of 0.25 is too small to carry out RHQ treatment although drawing was possible by using cassette-roller dies (CRDs) as well as conical dies. The reduction of the matrix ratio down to 0.5 can also be achieved. However, the scale-up would make the drawing very difficult. It would be necessary for the scale-up to improve adhesion among the jelly-roll (JR) cores and also the outer Ta sheath in the multi-billet. The RHQ treatment was possible for the wire with a matrix ratio of 0.5. However, it was successful only when the wire diameter was less than 1.13 mm. It is necessary to suppress the experienced bend strain in the operation. The non-Cu Jc was found to be improved by about 20% by the reduction of the matrix ratio, compared to conventional Nb3Al wires.
    IEEE Transactions on Applied Superconductivity 01/2008; 18(2):1035-1038. · 1.20 Impact Factor
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    ABSTRACT: Nb3Al has advantages of better tolerance to strain/stress and a higher critical magnetic field (30T at 4.2K) for stoichiometric composition over Nb3Sn. The rapid-heating, quenching and transformation annealing (RHQT) process enables to form a stoichiometric Nb3Al with fine grain structures via metastable bcc supersaturated-solid-solution. As a result a large critical current density of Nb3Al is achieved over the whole range of magnetic fields without trading off the excellent strain tolerance. A long-length of RHQ processing has been established, and a rectangular but Cu stabilized Nb3Al strand is about be commercially available for NMR uses. Ag or Cu internal stabilization and Cu ion-plating/electroplating techniques have been also developed to enable the stabilized round wire for accelerator and fusion magnets. Successfully energized test coils that were manufactured with a wind-and-react technique have demonstrated that a long piece of Cu stabilized RHQT Nb3Al wire is really available for practical applications.
    Cryogenics 01/2008; 48(7):371-380. · 1.17 Impact Factor
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    ABSTRACT: Long Nb<sub>3</sub>Al strands with copper stabilizer are promising for future high field accelerator magnets. A 1.2 kilometer Nb<sub>3</sub>Al strand with Cu stabilizer was fabricated at the National Institute for Materials Science in Japan. Using this strand a 30 meter Cu stabilized Nb<sub>3</sub>Al Rutherford cable was made for the first time by a collaboration of NIMS and Fermilab. The Nb<sub>3</sub>Al strands extracted from cable with a relatively low packing factor showed almost no J<sub>c</sub> degradation. But the extracted strands from the highly compacted cable showed some degradation in both J<sub>c</sub> and n value, which may be caused by local separation of the copper stabilizer. Still, its J<sub>c</sub> degradation is lower than that of typical Nb3Sn strands. The current limit due to magnetic instability in low field is about 500 A at 4.2 K. The magnetization of the strands, which was measured with balanced coils at 4.2 K, showed large flux jumps, usually around 1.5 T. This value is much larger than the B<sub>c2</sub> (4.2 K) of the Nb matrix, which is around 0.4 Tesla. The magnetic instability of the Nb<sub>3</sub>Al strand at low field is not completely understood, but it might be explained by the superconducting coupling current through the Nb matrix.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: Recent studies of RHQ-processed Nb<sub>3</sub>Al wires for future accelerator magnets are presented and discussed. Test wires were prepared with different fabrication parameters, such as the Nb matrix ratio, RHQ current, area reduction ratio of the wire after an RHQ treatment and the 2nd heat treatment condition. Measurements of the critical current density (J<sub>c</sub>) and n-value have been performed during the past several years. Recently, we performed the critical temperature (T<sub>c</sub>) measurement, and the relationship between J<sub>c</sub> and T<sub>c</sub> was investigated by an RHQ treatment, a 2nd heat treatment, the Nb matrix ratio, and the area reduction effect.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: The authors have been developing the Nb<sub>3</sub>Al conductors through transformation process, where the Nb<sub>3</sub>Al phase forms from the BCC (body-centered cubic) Nb(Al) supersaturated solid solution phase; the BCC phase is quenched by rapid-cooling from a temperature around 2000degC. It was reported several times that deformation to the quenched BCC Nb(Al) phase influences the superconducting properties, usually improving . But, the results have not been discussed in detail so far from a metallurgical viewpoint. This work investigates the fundamental relationship between the BCC-deformation, transformation temperature, microstructure and superconducting properties. The transformation temperature is identified by DTA measurement, detecting the release of the free energy between the BCC and the phases. It was found that the transformation temperature has a tendency of monotonic decrease with the deformation. But, the transformation temperature does not seem to go down below a temperature of about 720 degC even by strong deformation. The microstructures observed by FESEM suggests that the grain size is reduced almost proportionally with the deformation. In case of reduction in area of 99%, the grain size is reduced down to about 250 nm from 1.2mum . The increases with deformation, not showing a peak. The J<sub>c</sub> of the wire (ME365) with reduction in area of 86% shows the best performance of the wire ever reported. But, at least high-field J<sub>c</sub> seems to saturate at a certain value.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: To demonstrate the RHQT Nb<sub>3</sub>Al conductor to be really a candidate for a high-field magnet, we have fabricated 1 km-class Cu stabilized RHQT Nb<sub>3</sub>Al flat-wires, manufactured two insert real-sized coils #A and #B, and tested in a superconducting back-up field of 15 T at 4.2 K. Using the coil #A, a thinning of insulator, a winding technique, a new transformation annealing profile in a vacuum with a fast temperature-ramp above 500degC, decarbonization, an impregnation technique used for the Nb<sub>3</sub>Sn coil were preliminarily checked to be no matter. The coil #B that was wound with a 764 m piece could produce an additional 4.5 T (total 19.5 T) at 4.2 K in a clear bore of 30 mm without any training, when the coil current achieved the I<sub>c</sub> of short samples that were simultaneously transformation annealed with the coil #B.
    IEEE Transactions on Applied Superconductivity 01/2007; 17:2684-2687. · 1.20 Impact Factor
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    ABSTRACT: We observed crack formation in transformation-processed Nb3Al wires at room temperature, the wire being bent with a small clamp fixture with a curvature. The polished cross-section parallel to the longitudinal axis was observed, using a high power optical microscope or a field-emission scanning electron microscope. The bend strain limit for microcrack formation is found, changing the radius of the curvature of the clamp. The bend strain limit was found to be around 0.3% for standard Nb3Al wires. This corresponds to the irreversible tensile strain limit of the Ic characteristics determined with a 0.1 µV cm−1 criterion. Reduction of the barrier thickness should be avoided to keep to the bend strain limit. A new configuration of the Nb3Al wire is demonstrated to improve the bend strain limit. The filament is divided into segments in the transverse cross-section. The wire is fabricated by a double-stacking method. The bend strain limit is enhanced to about 0.85% for the wire surface; the equivalent strain of the outermost filament location is about 0.66%. A simple react and wind test for this wire was performed, where the wire experienced 0.86% bend strain. The degradation of Jc was found to be very small.
    Superconductor Science and Technology 09/2006; 19(10):1057. · 2.76 Impact Factor
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    ABSTRACT: Recent developments in RHQ Nb<sub>3</sub>Al wire for accelerator magnets are presented and discussed. The main items of the development are to increase the critical current density (J<sub>c</sub>) and to find a good stabilization method. For the former item, test wires with different Nb-matrix/filament ratios were made, and the effect of the ratio on J<sub>c</sub> was investigated in correlation with the heat treatment and the area reduction after the RHQ process. For the latter item, a special copper electroplating technique was developed to deposit a thick Cu layer on the surface of the wire. The mechanical bonding strength and the electrical characteristics of the Cu layer were studied by bending and drawing the wire, and by measuring the resistance. Although the present piece length of the Cu stabilized Nb<sub>3</sub>Al wire is about 40 cm, we can draw and reduce the wire down to 60% of the original diameter, without breaking the wire or damaging the bonding of the Cu stabilizer. The highest noncopper J<sub>c</sub> achieved during this study was 2156 A/mm<sup>2</sup> at 10T and 4.2 K
    IEEE Transactions on Applied Superconductivity 07/2006; · 1.20 Impact Factor
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    ABSTRACT: An attempt was made to incorporate the Cu stabilizer internally to the Ta-matrix jelly-roll RHQT Nb<sub>3</sub>Al round-wire, based on less-reactivity of Cu with Ta at elevated temperatures during the RHQ operation of wire. Compared to the conventional internal stabilization technique using Ag filaments jacketed with Nb, the present round wire has several advantages; (1) less induced-radioactivity when irradiated with fast neutrons, (2) higher mechanical strength at both elevated and cryogenic temperatures, and (3) less reactivity with Ga-coolant. The overall residual resistance ratio RRR<sub>ov</sub> of the internally Cu stabilized Ta matrix RHQT Nb<sub>3</sub>Al round-wire is as much as 75 in spite of a smaller stabilizer-ratio, and magnetoresistance is comparable to that of the conventional Cu clad stabilized Nb matrix RHQT Nb<sub>3</sub>Al flat-wire. Core J<sub>c</sub> is also comparable to that of the Nb matrix wire
    IEEE Transactions on Applied Superconductivity 07/2006; · 1.20 Impact Factor
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    ABSTRACT: For the practical conductors with large current capacity, the Cu stabilizer is necessary in order to obtain their fully performance. The most important issue remaining for producing the RHQT (rapid-heating/quenching and transformation)-processed Nb<sub>3</sub>Al round wire was how to fabricate the Cu stabilizer after the RHQ treatment. We successfully solved this problem and developed the Cu ion-plating technique for fabricating the Cu stabilizer into the RHQT-processed Nb<sub>3</sub>Al round wires. Although stable Nb oxide layer is formed on the wire surface, the Cu ion-plating can continuously remove them and cover the Cu seed-layer of 1 mum in thickness. Then, the Cu stabilizer can be also fabricated by the Cu electroplating with an appropriate of Cu ratio. The reel-to-reel apparatus of both the Cu ion-plating and electroplating for km-class long-wire have been set up. It was possible to apply the wire drawing by conventional die after the Cu electroplating due to the good bonding between the Cu stabilizer and Nb<sub>3</sub>Al round wire. This is very attractive advantage for obtaining the severe homogeneity of the wire diameter in long-length. Moreover, in order to obtain large enough RRR, it may be suitable to carry out the following techniques before fabricating the Cu stabilizer, which are an anode-oxidization of wire surface, normal die drawing and easy chemical etching of Nb oxide layer. This additional process is very effective to get much clean and homogeneous surface of the Nb<sub>3</sub>Al round wires with Nb matrix, and then it has been fabricated the Cu stabilizer without degradation of RRR. RRR of about 200 and non-Cu Jc (4.2K, 15T) of about 1,000 A/mm<sup>2</sup> were obtained on the present Cu-stabilized RHQT-processed Nb<sub>3</sub>Al round wires having 1.0 mm in diameter and about 1.0 of Cu/non-Cu ratio. Nb<sub>3</sub>Al filament diameter is 30-50 mum at present. These results must be very attractive for application of the high energy physics con- - ductors
    IEEE Transactions on Applied Superconductivity 01/2006; 16(2):1224-1227. · 1.20 Impact Factor
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    ABSTRACT: We have produced so far about a 400 m length of Cu stabilized RHQT (rapid-heating, quenching and transformation) Nb3Al flat-wire using a small-sized multi-billet (10 kg). Although our goal is to make a 2.5 km unit length of Cu cladding flat-wire for high-field NMR uses, an attempt was made to fabricate a 1 km class unit-length of conductor as a milestone of commercialization. A long-length of precursor wire over 2.5 km was prepared by using a large-sized multi-billet (50 kg). A 1.3 km length RHQ operation by using a newly installed large-scale RHQ apparatus was performed and then a km-class length Cu cladded flat-wire was fabricated to evaluate the uniformity of resultant superconducting characteristics of wire
    IEEE Transactions on Applied Superconductivity - IEEE TRANS APPL SUPERCONDUCT. 01/2006; 16(2):1168-1171.
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    ABSTRACT: Three kinds of stabilized rapid-heating, quenching and transformation annealing (RHQT) Nb3Al round wires have been developed for nuclear fusion devices; the Ag internally stabilized Nb matrix, the Cu internally stabilized Ta matrix and Cu ion-plated/electroplated wires. The critical current densities Jc of Nb3Al filaments are almost the same as the conventional Cu clad stabilized flat-wire, as long as the Jc is optimized by mechanical deformation of wire after quenching but before the final transformation annealing. To demonstrate the applicability of RHQT Nb3Al wire to nuclear fusion devices, we have trial manufactured a cable-in-conduit conductor (CICC) using the Ag internally stabilized round strands and confirmed the critical current of CICC to be nearly the same with the strain-free designed one.
    Fusion Engineering and Design - FUSION ENG DES. 01/2006; 81(20):2443-2448.
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    ABSTRACT: The surface of RHQT-processed Nb<sub>3</sub>Al wires with pure Nb matrix are covered by a strong stable Nb oxide surface layer. In order to obtain good mechanical, electrical and thermal bonding between the Cu stabilizer and RHQT-processed Nb<sub>3</sub>Al wires, it is required to destroy the Nb oxide layer on the surface of the wire. We tried to fabricate a thin Cu layer on the surface of the RHQT-processed Nb<sub>3</sub>Al wires through the Cu ion-plating technique. Before electroplating of thick Cu stabilizers, Cu was ion-plated to a thickness of about 1 μm. The Cu ion-plated wire showed no folded projections, cracks, or exfoliation of the Cu stabilizer even when the wire was bent through 180 degrees, showing that Cu and the wire were tightly bonded mechanically. This tight bonding between Cu and the wires should be due to the removal of stable Nb oxide layers from the surface of the wire. The V-I characteristic of wires that were ion-plated and then applied with an appropriate amount of Cu by an electroplating showed no quenching, and the wires were able to carry current up to the normal critical current transition. The measured values of recovery current were almost equivalent to the calculated values, showing that the ion-plated Cu/Nb interface had a very tight bond, allowing good electrical and thermal conductivity. In addition, a reel to reel Cu-ion plating apparatus for the long length wires is demonstrated, and Cu ion plating has been successfully carried out on 100 m long RHQT-processed Nb<sub>3</sub>Al wire at present.
    IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
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    ABSTRACT: Since the RHQT-Nb<sub>3</sub>Al conductor shows excellent superconducting characteristics in high magnetic fields, its application to a 1 GHz NMR insert coil is in particular expected. For such an application, the development of a long-length of RHQT-Nb<sub>3</sub>Al wire is indispensable. However, the unit length of wire achieved was no more than 400 m so far, due to a restricted size of the multi-billet, while about a 2.5 km-length of wire is necessary for the NMR insert coil application. Thus, an attempt was made to produce a 2.5 km-length of precursor wire by using a large-sized billet, and we have succeeded in producing such a long-length of precursor wire (2.6 km) without any breaking of wire during hydrostatic extrusion and subsequent drawing processes.
    IEEE Transactions on Applied Superconductivity 07/2005; · 1.20 Impact Factor
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    ABSTRACT: We tried to enlarge a Nb/Al strand dimension to enhance a current capacity of RHQT- Nb<sub>3</sub>Al further. So far good fsRHQ operation could be made by using the strand dimension of less than φ 1.35 mm. This time we tried to make RHQ operation by using a Nb/Al rectangular strand of 1.45 × 2.2 mm <sup>2</sup> which is double cross-section area of φ 1.35 mm. Bcc phases could be formed even in the large cross-section rectangular strand by RHQ operation. The samples were post-annealed at 800°C for transforming into A15. The resultant I<sub>c</sub> and core J<sub>c</sub> were 232.5 A and 130 A/mm <sup>2</sup> at 4.2 K, 22 T, respectively. These values largely surpassed I<sub>c</sub> of 57.5 A and J<sub>c</sub> of 72.7 A/mm <sup>2</sup> at 4.2 K, 22 T for the sample which was made by using a conventional φ 1.35 mm strand.
    IEEE Transactions on Applied Superconductivity 07/2004; · 1.20 Impact Factor
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    ABSTRACT: Nb<sub>3</sub>Al superconductors have shown promising performance compared to Nb<sub>3</sub>Sn conductors when processed by rapid heating/quenching process. Therefore we have started an R&D program of Nb<sub>3</sub>Al conductors for future accelerator magnets. Several test wires of around 0.8 mm diameter, which have relatively small filament (∼50 micron diameter) and low matrix ratio (∼1.0), were fabricated, and the heat treatment and area reduction conditions after the rapid quenching process were studied. The highest noncopper J<sub>c</sub> achieved during this study was 1734 A/mm<sup>2</sup> at 10 T and 4.2 K.
    IEEE Transactions on Applied Superconductivity 07/2004; · 1.20 Impact Factor