13 research outputs found

    Toward a standard for critical current versus axial strain measurements of Nb<sub>3</sub>Sn

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    Knowledge of the critical current of Nb3Sn wires under mechanical loads is increasingly important. In this work, based on an investigation of the bronze Nb3Sn reference wire for the international ITER benchmarking, practical aspects of an Ic(ε,B) measurement are discussed and commented on. The use of a helical spring according to Walters et al (1986 Cryogenics 26 406) is favored. It is shown that Ic at zero applied strain can be measured precisely, thus contributing to an improvement in the determination of εm, the strain for the maximum of Ic. Further, a new approach for the evaluation of the irreversibility limit, εirr, is proposed by a detailed analysis of logV versus logI characteristics. Finally some recommendations for an eventual future standard are made

    Field and temperature scaling of the critical current density in commercial REBCO coated conductors

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    Scaling relations describing the electromagnetic behaviour of coated conductors (CCs) greatly simplify the design of REBCO-based devices. The performance of REBCO CCs is strongly influenced by fabrication route, conductor architecture and materials, and these parameters vary from one manufacturer to the others. In the present work we have examined the critical surface for the current density, Jc(T,B,θ), of coated conductors from six different manufacturers: American Superconductor Co. (US), Bruker HTS GmbH (Germany), Fujikura Ltd. (Japan), SuNAM Co. Ltd. (Korea), SuperOx ZAO (Russia) and SuperPower Inc. (US). Electrical transport and magnetic measurements were performed at temperatures between 4.2 K and 77 K and in magnetic field up to 19 T. Experiments were conducted at three different orientations of the field with respect to the crystallographic c-axis of the REBCO layer, θ = 0°, 45° and 90°, in order to probe the angular anisotropy of Jc. In spite of the large variability of CCs' performance, we show here that field and temperature dependences of Jc at a given angle can be reproduced over wide ranges using a scaling relation based only on three parameters. Furthermore, we present and validate a new approach combining magnetic and transport measurements for the determination of the scaling parameters with minimal experimental effort

    Electro-mechanical properties of PIT Nb<sub>3</sub> Sn wires under transverse stress: experimental results and FEM analysis

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    Nowadays there is a great deal of interest in the scientific community in developing next generation accelerator magnets based on high-Jc Nb3Sn Rutherford cables. Inside a cable the wires are subjected to the combined effect of axial and transverse load. Since Nb3Sn is a strain sensitive material, electromechanical characterization of cables is essential for magnet design. Testing a full-size Rutherford cable is an extremely complex and involved task. For this reason special Walters springs have been developed at the University of Geneva to test single wires under longitudinal and transverse load. In this work we analyze three PIT wires under transverse compressive load. To better understand the experimental results, a finite element model was developed. This model enabled better understanding of the mechanical behavior of the three samples and investigation of the mechanisms that determine wire performance degradation upon loading

    Reversible stress-induced anomalies in the strain function of Nb<sub>3</sub>Sn wires

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    The full-matrix set of combined temperature (4.2–14 K) and applied axial strain (εa) data for the bulk pinning force of a technological Nb3Sn wire (OST type-I) has been studied at fields up to 19 T by combining transport (variable εa) and magnetic (variable T) measurements. Some length of the wire was also jacketed with AISI 316L stainless steel, in order to apply a radial strain and to simulate the thermally induced axial compressive strain that the Nb3Sn wires experience in a cable-in-conduit-conductor (CICC). Within the framework of the unified scaling law, raw scaling data for the effective upper critical field B*c2(T, ɛ), have been used in order to experimentally determine the strain function, s(ε), of both the bare and the jacketed wires. A direct testing of the various proposed models for s(ε) has been carried out, including the power law, the deviatoric description and the polynomial form. All models adequately fit to the s(ε) of the bare wire, but in the jacketed wire none of them is able to describe the tensile strain region above the Ic maximum, where the enhanced radial compression cannot be neglected. The origin of the onset of a reduced Bc2 is also discussed

    Phase Formation, Composition and T<sub>c</sub> Distribution of Binary and Ta-allowed Nb<sub>3</sub>Sn Wires Produced by Various Techniques

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    Low temperature calorimetry was used to determine the distribution of the superconducting transition temperature in binary and Ta-doped wires fabricated by the Bronze route and by the Powder-In-Tube (PIT) method. From this analysis we were able to discern the effects of Sn and Ta compositions on the distribution of the superconducting parameters Tc and Bc2 in the different samples. The influence of the heat treatment conditions on the superconducting properties was investigated for the PIT wires. In particular we determined the field dependence of the distribution for a commonly used reaction schedule (675 /84 h) and for an optimized heat treatment (625 /320 h). For the first time, we show that the wire reacted at 625/320 h exhibits two separated contributions in the distribution directly related to the grain morphology of the A15 layer: a narrow peak determined by the large grains, with a lower Bc2, and a broad peak due to the fine grains, with a higher Bc2. The kinetics of the Sn diffusion in Nb and the growth rate of the A15 layer were experimentally studied. The influence of Ta doping on the A15 phase formation was analysed by electron microscopy, the growth rate and the grain morphology in binary and Ta-alloyed Bronze route wires with the same filament layout being compared at different stages of the heat treatment. At the end of reaction, the well known microstructure comprising equiaxed and columnar regions was observed in the filaments of both the binary and the Ta-alloyed wires. Based on these observations and from the growth rate analysis we conclude that Ta does not affect the Sn diffusion rate

    Extensive Characterization of the 1 mm PIT Nb<sub>3</sub>Sn Strand for the 13-T FRESCA2 Magnet

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    In the framework of the EuCARD program, CERN is participating in the development of a 13 T 100-mm-aperture dipole magnet to upgrade the superconducting cable test facility FRESCA at CERN. The conductor candidates for building this magnet are two 1-mm Nb3Sn strands: the Powder In Tube (PIT) produced by Bruker-EAS and the 132/169 RRP by Oxford Superconducting Technology (OST). Recently the PIT strand has been extensively characterized by CERN in collaboration with the University of Geneva (UniGe). The critical current dependence on the magnetic field and on the axial strain has been measured at different temperatures. Furthermore, the strand magnetization has been measured at different temperature using a vibrating sample magnetometer. Finally the magneto-thermal stability of this strand was studied by measuring the quench current between 0 T and 12 T at 1.9 K and 4.3 K. The experimental results are compared with an optimized scaling law for the critical current of Nb3Sn strands. In this paper the results obtained for the PIT strand are summarized and discussed