6 research outputs found

    Magnetization and Inter-Filament Contact in HEP and ITER Bronze-Route Nb<sub>3</sub>Sn Wires

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    Magnetization measurements are relevant tests for the characterization of superconductors. Practically they are the only measurements that allow estimating the critical current density at low fields of low temperature superconductors, the effective filament size and the hysteresis losses. For this purpose CERN, in collaboration with the University of Geneva, has carried out magnetization measurements on five types of Nb3Sn wires: three bronze route strands used in the ITER project; one Powder In Tube (PIT) and one Internal Tin (IT) wires used for developing next generation accelerator magnets. The field dependent magnetization has been determined using three setups: a Vibrating Sample Magnetometer (VSM), a Superconducting Quantum Interference Device (SQUID) and a special system used for the production control of LHC strands. Samples of different lengths have been tested to check the different coupling between the filaments. Unexpectedly, it was found that the magnetization of the tested bronze wires was strongly dependent on the sample length. In this paper, the results, which were obtained for different type of strands and sample lengths, are reported and compared

    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

    Effect of quasi-hydrostatical radial pressure on I<sub>c</sub> of Nb<sub>3</sub>Sn wires

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    High-performance Nb3Sn conductors are intended to be used in large-scale magnets like the International Thermonuclear Experimental Reactor (ITER) and in the upgrade of the Large Hadron Collider (LHC). Due to the occurrence of high electromagnetic forces, a detailed knowledge of the response of the critical current to the three-dimensional mechanical loads acting on the wires inside the cables is required. A detailed analysis of transverse stress effects on the critical current for powder-in-tube and bronze route Nb3Sn wires is presented. In an earlier publication, we have described the effect of transverse stress exerted on a Nb3Sn wire by means of two parallel plates. In the present paper, we analyse the effect of transverse stress exerted simultaneously by four walls on a wire being confined in a U-shaped groove. In order to get a more realistic picture of the situation of wires embedded in a Rutherford cable, the compression by four walls was also performed after impregnating the wire with epoxy in the same U-shaped groove. The result is very different from the case of pressing by means of two walls: the effect of pressure on Ic is now strongly reduced, which is attributed to the almost hydrostatic pressure in the U-shaped groove. This is further confirmed by the comparison between the effects of axial and transverse loads on the upper critical field and the pinning force. The present data are also compared against the effects of mechanical load on the critical current of cables in large-scale magnet

    Variation of <em>(Jc/Jc0)</em><sub>max</sub> of Binary and Ternary Alloyed RRP and PIT Nb<sub>3</sub>Sn Wires Exposed to Fast Neutron Irradiation at Ambient Reactor Temperature

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    The variation of Tc and Jc for a series of binary and ternary alloyed Nb3Sn wires of the RRP and PIT type has been investigated after fast neutron irradiation (E>;0.1 MeV) at the TRIGA reactor in Vienna up to a fluence of φt = 1.4 × 1022 m-2. In contrast to Ti alloyed bronze route Nb3Sn wires, where a maximum is observed at around 0.4 ×1022 m-2, it was found that the maximum of (Jc/Jc0) versus φt in the present ternary alloyed wires was still not reached. A comparison with an earlier study on bronze route wires at RTNS-II shows that the variations of Tc versus φt and Jc versus φt do not scale: a substantially higher Jc value is now observed for the same Tc value. The degree of atomic ordering being comparable, the shift of the maximum of Jc/Jc0 with fluence is attributed to radiation induced “defect clusters.” Two observations are used for explaining the observed effect: (a) a shift of (Jc/Jc0)max toward higher fields, observed for binary bronze wires with higher Sn contents and (b) by calorimetry, the average of the Tc distribution in the filaments of RRP and PIT wires is found to occur at values up to 1.5 K higher than in bronze wires. The present data suggest that the effect of “defect clusters” is higher for wires with higher average Sn contents

    Temperature induced degradation of Nb-Ti/Cu composite superconductors

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    The degradation mechanisms of state-of-the-art Nb-Ti/Cu superconductors are described, based on in-situ synchrotron X-ray diffraction measurements during heat treatment. A quantitative description of the Nb-Ti/Cu degradation in terms of critical current density, Cu stabiliser resistivity and mechanical composite strength is presented. In an applied magnetic field a significant critical current degradation is already observed after a 5-minute 400 °C heat treatment, due to variations of á -Ti precipitate size and distribution within the Nb-Ti alloy filaments. A strong degradation of the strand mechanical properties is observed after several minutes heating above 550 °C, which is also the temperature at which the formation of Cu-Ti intermetallic phases is detected. Several minutes heating at 250 °C are sufficient to increase the RRR of the strongly cold work strands inside a Rutherford type cable from about 80 to about 240. Heating for several minutes at 400 °C does not cause a significant conductor degradation in self-field and, thus, leaves enough temperature margin for the electrical interconnection of Nb-Ti/Cu conductors with common low temperature solder