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The pinning mechanism for transformed \Nb3\Al is still an open question: the pinning force does not increase monotonically with reciprocal grain size in the case of \Nb3\Al, which is remarkably different from the behavior seen in diffusion-processed \Nb3\Sn. One of the possible pinning centers is the planer defects stacked in \langle 100\rangle direction in A15 \Nb3 \Al phase that are recognized only after massive transformation from BCC Nb–Al to A15 \Nb3\Al phases. In this paper, we focus on the stacking faults as possible primary pinning centers for the transformed \Nb3\Al phase and discuss the correlation between the pinning characteristics and the stacking faults. Al segregation on the stacking faults was observed using 3-D atom probe tomography. The spacing of the stacking faults and the areal fraction of the stacking fault region were analyzed statistically for the microstructures observed using transmission electron microscopy. Obvious shrinkage of the stacking faults was found in low-pinning-force samples. [ABSTRACT FROM PUBLISHER]

Transmission electron microscopy and electron backscatter diffraction were used to observe the grain morphologies of transformation-processed Nb3Al superconductors to identify pinning centers. Their grain sizes and stacking fault densities were analysed to determine whether they are correlated with the critical current density (Jc). The stacking faults can be seen as planar defects stacked in three orthogonal directions, when the electron beam was parallel with the \langle 100\rangle direction. Jc was not inversely proportional to the grain size, whereas Jc and Bc2 appear to be correlated with the stacking fault density. [ABSTRACT FROM PUBLISHER]

To suppress the low-magnetic-field instability (flux jumps in low magnetic fields) of a rapid-heating, quenching and transformation (RHQT) processed Nb3Al superconductor, we had previously modified the cross-sectional design of an RHQT Nb3Al by adopting a Ta filament-barrier structure. Unlike Nb barriers, Ta barriers are not superconducting in magnetic fields at 4.2 K so that they electromagnetically decouple filaments. However, small flux jumps still occurred at 1.8 K, which is a typical operating temperature for the magnets used in high-energy particle accelerators. Furthermore, poor bonding at the Ta/Ta interface between neighboring Ta-coated jelly-roll (JR) filaments frequently caused precursor wires to break during drawing. To overcome these problems, we fabricated a new RHQT Nb3Al wire with a Ta/Cu/Ta three-layer filament-barrier structure for which an internal stabilization technique (Cu rods encased in Ta are dispersed in the wire cross section) was extended. Removing the Ta/Ta interface in the interfilamentary barrier (JR filament/Ta/Cu/Ta/JR filament) allowed precursor wires to be drawn without breaking. Furthermore, the Cu filament barrier electromagnetically decoupled filaments to suppress flux jumps at 1.8 K. The ductile Cu layer also improved the bending strain tolerance of RHQT Nb3Al. [ABSTRACT FROM AUTHOR]

We have recently developed a thin-type Cu clad stabilized Nb3Al flat-wire which can be more densely wound into a coil. Using this flat-wire we have upgraded an existing 14 T Nb-Ti/Nb3Sn magnet with a Φ 30 mm clear bore, by replacing the inner coil, to a compact 15 T Nb-Ti/Nb3Al magnet. The Nb3Al 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 Φ 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. [ABSTRACT FROM AUTHOR]

A Nb3Al strand, produced by the "rapid-heating, quenching and transformation annealing (RHQT)" process, has been developed in collaboration between JAEA and NIMS aiming at application to the conductor in a fusion DEMO reactor. A detailed description of critical current characteristics of the Nb3 Al RHQT strand may assist in predicting accurate conductor performance. Calculations of the critical current density are adjusted to S scaling laws. Advantages of the newly developed strand compared to the Nb3 Sn ITER strand and the Nb3 Al Insert strand include improved strain tolerance and a higher current density at around 16 1. Performance parameters of the Nb3Al RHQT strand include 450 A/mm2 at 15.9 T, 5.7 K and -0.4%. These performance parameters fulfill the requirements of the TF conductor of the fusion DEMO reactor. Therefore, the Nb3 Al RHQT strand represents an improved performance with prospective application in the fusion DEMO reactor. In addition, the Nb3 Al RHQT strand has the potential to enhance further the critical current density. [ABSTRACT FROM AUTHOR]

The surface of RHQT-processed Nb3Al 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 Nb3Al 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 RHQTprocessed Nb3Al 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 Nb3Al wire at present. [ABSTRACT FROM AUTHOR]

Nb3(Al, Ge) conductors were prepared with a diffusion reaction process at a temperature around 1400°C, from multifilamentary Nb/Al-25 at % Ge precursors. Critical current densities of 300 A/mm² at 21 T and 220 A/mm² at 23 T at 4.2 K have been obtained after a diffusion reaction at 1400° C for 7 h followed by 10 h/800° C heat treatment. The Hc2 in excess of 28.5 T is about 2 T larger than that of the so-called RHQT Nb3Al. Large Al-Ge filament size in the precursors over a few microns might be acceptable to improve high-field Jc performance, significantly reducing fabrication difficulty of the Nb/Al-Ge precursors. Small matrix ratio of 0.8 and piece-length of 30 m was achievable in the precursor preparation at our laboratory workshop without intermediate annealing. In addition, the current capacity up to values higher than 300 A at 21 T, 4.2 K has been given by an enlarged cross-section sample. [ABSTRACT FROM AUTHOR]

The magnetization property is one of the key issues when considering the accelerator-magnet applications of Bi-2212 wires. In this work, first, the critical current densities of several types of Bi-2212 round wires were measured by the transport cur- rent method at various temperatures. Then, the magnetization of the wires was measured at various temperatures and applied field amplitudes using the SQUID magnetometer. Hysteresis loops were plotted, and the time decay of the magnetization was studied. It was observed that magnetization had various values, but the hysteresis curves and time decays of all samples had the same tendency. The equivalent filament diameter can be determined from the measured magnetization and the measured critical current density. The equivalent filament diameters are several times larger as compared with the actual filament size. [ABSTRACT FROM AUTHOR]

In a Nb/Al-Cu precursor wire, Nb3 Al filaments are formed directly through a proper rapid heating and quenching (RHQ) treatment, and show the excellent superconducting properties at two different regions in the RHQ-treatment conditions. The region at heating energy density, W, of 4.4-4.6 J/mm3 is one of the best RHQ treatments to obtain the excellent superconducting properties, when the Cu content in Al-Cu alloy is 1.2 to 5.2at%. The other optimized region was observed at W of 5.2-5.6 J/mm3, which exists clearly for the 2.5 and the 5.2at%Cu added wires, but unclearly for the 1.2at%Cu added wire. The best improvement in .4 is obtained for 2.5at%Cu addition, such as Jc (4.2 K) values of 800 A/mm2 in 20 T and 1200 A/mm2 in 18 T, which are the highest values among the metallic multifilamentary superconductors. The best Tc of 18.34 K is obtained for the 2.5at%Cu added wire. The wires with 1.2at% Cu addition show intermediate Jc vs. B curves between those of the 2.Sat%Cu added and non-Cu added wires. [ABSTRACT FROM AUTHOR]

Thermomagnetic instability in superconducting wires composing multistrand cables is a problem in the development of cables with large current capacity. This paper elucidates the quenching properties of ac superconducting wires in a distributed magnetic field applied to the strands in the cable, and the stabilization of the ac superconducting wires considering the effect of the longitudinal magnetic field or the fraction of copper embedded in each strand. First, the degradation of the quench current of CuNi/NbTi superconducting wires in a distributed magnetic field is exhibited with simple test samples. Second, the quench properties of the strand in a (6 + 1)3 cable and the optimal twist pitch of the cable for high stabilization are discussed. Last, the effect of copper on the quench properties of the strand and the appropriate fraction of copper for suppression of quench current degradation in a distributed magnetic field are discussed. © 2001 Scripta Technica, Electr Eng Jpn, 135(4): 26–34, 2001 [ABSTRACT FROM AUTHOR]

Abstract: When the quenching occurs in a superconducting coil, excessive joule heating in normal area may damage the coil. It is necessary to detect quenching in the coil as soon as possible and discharge the magnetic energy stored in the coil. Therefore, we propose a superconducting coil protection system based on an active power method. The system is highly resistant to the noise and does not require cancel voltage taps, so it is useful for both AC and DC coils. We have presented the effectiveness of the system using some test coils cooled in LN2 or LHe. However, we have not discussed the effectiveness of the proposed system for helium-free cryocooled magnets, in which a larger temperature rise occurs after quenching than in liquid-cooled magnets. In this paper, we verify the effectiveness and practicality of the proposed system through coil protection tests for a cryocooled Nb3Al LTS coil. [Copyright &y& Elsevier]