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The critical current of an internal tin Nb3Sn wire developed by Oxford Instruments, Superconducting Technology for International Thermonuclear Experimental Reactor (ITER) (OST type-I, billet No. 7567) has been studied under axial strain at fields between 12 and 19 T at 4.2 K. Simulating the situation in a cable in conduit, where thermally induced compressive strain is important, a single wire (strand) was jacketed with AISI 316L stainless steel. The reinforced wire shows an important increase in [variant_greek_epsilon]m, the applied strain where Ic reaches its maximum, from 0.25% to 0.57%. In addition the irreversibility limit, [variant_greek_epsilon]irr, is improved from 0.50% applied strain to >1.10%. It could also be shown that the Ic at zero intrinsic strain is almost identical. This demonstrates that jacketing does not influence the physical parameters of the original wire. Experimental data of the bare wire has been well fitted by different strain functions. However, it was not possible to model the data of the jacketed wire. There are indications that only models which take into account the multidimensional character of strain are able to describe the behavior but further development is required. [ABSTRACT FROM AUTHOR]

In modeling the properties of superconducting multifilamentary strands, transverse resistivity plays a crucial role in the definition of the coupling losses in ac regime, as well as of the current transfer length, that affects the transport properties of Nb3Sn wires subject to bending strain. We present the first direct measurement of the interfilament transverse resistance in superconducting strands from room temperature to 4.2 K. Results have been compared to the transverse resistance of a sample on which the outer copper stabilization layer has been removed by chemical etching, obtaining interesting indication on the preferential current paths within the wire cross section. An excellent agreement between experimental data and theoretical models has been found in describing the whole strand, while improvements are required in modeling the filamentary region alone. [ABSTRACT FROM AUTHOR]

We report on the effect of bending strain on the transport properties of multifilamentary Nb3Sn strands having twisted or untwisted superconducting filaments in order to clarify some aspects of the current transfer process within the wire cross section. Critical current measurements were performed on internal tin Nb3Sn strands, which were inserted and compacted inside thin stainless steel tubes before the heat treatment. With the application of a maximum bending strain of 0.5%, the strands with twisted filaments present a degradation of the critical current and of the n-index, while an enhancement of the performances is observed for the untwisted strands. The experimental results are interpreted in relation to the predictions of Ekin’s model. [ABSTRACT FROM AUTHOR]

We present the fabrication of FeSe0.5Te0.5 polycrystalline samples by self-flux method, showing the presence of the peak-effect in the vortex lattice configuration. To improve the performances at high magnetic fields for this iron-based superconductor of the 11-family, the two-step solid-state reaction process has been successfully modified by increasing the temperature of the heat treatment and by slowing down the cooling rate. The magnetic field-temperature phase diagram has been investigated by magneto-resistance, magnetization, and heat capacity measurements in applied magnetic fields up to 18 T. The magnetization curves exhibit an enhancement of the peak-effect whose position shifts by varying the temperature, following a similar dependence than that previously reported in high-temperature superconducting materials such as YBa2Cu3O7-δ. The presence of the peak-effect can be correlated to the sample manufacture, since by tuning a proper heat treatment it becomes observable ever more in the magnetic field-temperature phase diagram. This fabrication route paves the way to a systematic increase in the critical current density thus becoming relevant for applications. [ABSTRACT FROM AUTHOR]

In the framework of the European fusion program for energy, EUROfusion funds the studies for the future fusion power demonstrator reactor DEMOnstration fusion reactor (DEMO). Commissariat 'a l'energie atomique et aux energies alternatives (CEA) is involved in the conceptual design of the superconducting conductors for the toroidal field (TF) and central solenoid (CS) magnets. The CEA design proposal corresponds to wind and react Nb3Sn pancake-wound coils using cable-in-conduit conductors cooled at about 4.5 K by forced flow of supercritical helium. This paper presents the latest thermal-hydraulic analyses performed on both TF and CS conductors. Two TF conductor designs have been analyzed, with nominal currents of 111 kA and 88 kA, respectively. The analyses were performed in normal (burn) and off-normal (quench) conditions. Burn simulations focused on the central and lateral pancakes with heat load corresponding to neutron heating. The central pancake is the most critical one regarding the magnetic field, while the lateral one receives a more important heat load from the casing. The influence of case cooling on temperature margin (ΔTma) was analyzed by means of a dedicated 2D Cast3M model. TheΔTma sensitivity to driving parameters such as conductor friction factor correlations was analyzed. The impact of interturn thermal coupling on temperature margin was also assessed. Quench studies were performed on the TF conductor, considering a quench initiated either on the innermost turn or at the middle of hydraulic length. Regarding the CS conductor, burn scenario was led on the latest design featuring a nominal current of 53.7 kA, focusing on the impact of ac losses and of dwell duration. [ABSTRACT FROM AUTHOR]

The transverse resistivity among filaments is a crucial parameter of composite superconducting wires, since it affects the coupling losses in ac regime, the current transfer length of the wire subjected to bending strain, and the current redistribution among filaments in the case of a quench. A direct measurement of the transverse interfilament resistance as a function of temperature in several \Nb3 \Sn multifilamentary wires was performed by means of a 4-probe method. This paper deals with the numerical analysis of the measurements performed on a OCSI wire manufactured by Luvata, containing 82 superconducting subelements each made of 150 filaments. The measurements are analyzed by applying a 2-D FEM of the wire cross-section and a 3-D electrical circuit model of the wire sample. The results obtained have been compared with those found on two other \Nb3\Sn wires with different architecture and a NbTi wire taken as a reference. [ABSTRACT FROM PUBLISHER]

Superconducting strands are composite wires made of fine superconducting filaments embedded in a metallic matrix. The transverse resistivity among superconducting filaments affects the coupling losses during electromagnetic transients and the electro-thermal behavior of the wire in case of a quench. A direct measurement of the transverse interfilament resistance as a function of temperature in NbTi multi-filamentary wires was performed at the ENEA Frascati Superconductivity Division, Italy by means of a four-probe method. The complexity of these measurements is remarkable, due to the current distribution phenomena that occur among superconducting filaments during these tests. A two-dimensional finite element method model of the wire cross section and a three-dimensional electrical circuit model of the wire sample developed at the University of Bologna are applied here to derive qualitative and quantitative information about the transverse electrical resistance matrix. The experiment is aimed at verifying the qualitative behaviors and trends predicted by the numerical calculations, especially concerning the current redistribution length and consequent length effects of the sample under test. A fine tuning of the model parameters at the filament level allowed us to reproduce the experimental results and get quantitative information about the current distribution phenomena between filaments. [ABSTRACT FROM AUTHOR]

In this paper we compare two different experimental methods to estimate the transverse resistivity in superconducting NbTi and Nb3Sn multifilament wires. The first method is based on direct measurements of the resistance through micro wires bonded on the cross section of the strand. The second method exploits the physical meaning of the ac susceptibility of the multifilament wires. Both methods have been previously used: the first on Nb3Sn and NbTi strands, while the second has been applied only to NbTi wires. Here we first compare and analyze results in a NbTi system, then we extend the ac susceptibility method to Nb3Sn wires. The advantages and the limits of both techniques, when used on wires with different layouts, are analyzed. The results of the transverse resistivity obtained by using the two experimental methods are finally compared to the estimation based on a numerical approach. [ABSTRACT FROM AUTHOR]

Superconductors for the ITER Poloidal Field Coils are large cable-in-conduit conductors (CICC) made of NbTi strands encased in a round-in-square stainless steel jacket. Three prototype conductor sections for poloidal field coils PF1/6, PF2/3/4 and PF5 have been fabricated in collaboration of the domestic RF, CN and EU agencies and tested in SULTAN Test Facility at the nominal operating conditions. [ABSTRACT FROM PUBLISHER]

With the aim of clarifying the relationship between lattice deformations and superconducting properties of Nb3Sn technological wires we have carried out high-energy x-ray diffraction experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble on individual samples of multi-filamentary internal-tin-type Nb3Sn wires. In particular, a test probe developed at the University of Geneva allowed us to perform these experiments at 4.2 K, while applying an axial tensile load to the specimen. In this way, the lattice parameter values of all the constituents (Nb3Sn, Nb, Cu) were determined, in both the parallel and orthogonal directions with respect to the applied load axis, as a function of the applied strain. The experiments were performed on industrial wires, which were reinforced by a stainless steel outer tube, applied before the Nb3Sn reaction heat treatment, in order to evaluate the effect of an additional pre-compression strain. The relation between the microscopically determined crystalline lattice deformations and the measured applied strain is discussed as a basis for the analysis of the superconducting performances of Nb3Sn wires subject to mechanical loads. With the aim of clarifying the relationship between lattice deformations and superconducting properties of Nb3Sn technological wires we have carried out high-energy x-ray diffraction experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble on individual samples of multi-filamentary internal-tin-type Nb3Sn wires. In particular, a test probe developed at the University of Geneva allowed us to perform these experiments at 4.2 K, while applying an axial tensile load to the specimen. In this way, the lattice parameter values of all the constituents (Nb3Sn, Nb, Cu) were determined, in both the parallel and orthogonal directions with respect to the applied load axis, as a function of the applied strain. The experiments were performed on industrial wires, which were reinforced by a stainless steel outer tube, applied before the Nb3Sn reaction heat treatment, in order to evaluate the effect of an additional pre-compression strain. The relation between the microscopically determined crystalline lattice deformations and the measured applied strain is discussed as a basis for the analysis of the superconducting performances of Nb3Sn wires subject to mechanical loads. [ABSTRACT FROM AUTHOR]

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,ε), 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. [ABSTRACT FROM AUTHOR]

A two-components model has been recently developed, for describing the normalized bulk pinning force curves and the critical current density of NbTi strands over a wider B-T range with respect to conventional single-component models. The model was previously successfully applied to data collected on several NbTi commercial strands, with different size, Cu:nonCu ratio, filament diameter and layout, thus confirming the presence of two different pinning mechanisms in conventionally processed NbTi wires. For a further validation, we have extensively tested a strand recently produced by the Chinese Company Western Superconducting Technologies for the ITER Poloidal Field (PF) magnets PF2 to PF5, and applied the model to these data. In order to take into account the observed non-scaling with temperature of the reduced pinning force curves, the model has been updated, including the observed difference in the temperature dependences of the two components contributing to the overall bulk pinning force. The importance of testing wires over very wide temperature ranges is evidenced, and the good agreement between experimental and fit results validate the proposed formulation, which can be regarded as a reliable tool for the description of NbTi performances, to be used in the design of superconducting magnets. From the phenomenological point of view, it is shown that at low temperatures, the two pinning mechanisms contribute to the bulk pinning force, resulting in a pinning force peaking at a reduced field B/Birr \cong 0.5. As the temperature increases, the pinning force peak moves to lower fields, indicating that the low field component pinning mechanism becomes dominant. [ABSTRACT FROM AUTHOR]

The primary issue facing Nb3Sn cable-in-conduit conductors (CICCs) has been degradation, often observed during electromagnetic cycling, in the current sharing temperature, n-index, and critical current, in respect to measured strands values, due to strain effects. In the last years, work performed mostly relating to fusion magnet technology has led to a better understanding of the parameters required to improve the constraints imposed by the brittle nature of the Nb3Sn filaments, such as cables low void fraction and long twist pitch sequence. On the other hand, experimental campaigns on bent Nb3Sn wires, pre-compressed into a stainless-steel jacket, have shown that an appreciable decrease in the n-index values already occurs at the strand level, well below the irreversible mechanical load regime for filament breakage. This result is explained, with the support of simulation results, taking into account the broadening of the critical current distribution on wires cross section due to the presence of the jacket and to bending strain. [ABSTRACT FROM AUTHOR]

The superconducting wires are generally made of several hundreds or thousands of fine superconducting filaments embedded in a metallic matrix. Several relevant properties of the superconducting wires depend on the transverse resistances between filament bundles. In Nb3Sn wires realized with Bronze Route or Internal Tin technology, the presence of the bronze matrix can determine a significant increase of the transverse interfilament resistance with respect to wires with copper matrix. This increased resistivity in turn plays a role in determining the ac losses, thermal stability, and sensitivity to mechanical bending of the wire. The direct measurements of the transverse electrical resistances give useful information both for stability computations and to analyze the mechanical performance of the wire. The complexity of these measurements is however remarkable, due to the current distribution phenomena that occur among superconducting filaments during these tests. This paper presents the application of a 2D FEM model of the wire cross section and of a 3D electrical circuit model of the wire sample to derive qualitative and quantitative information about the transverse electrical resistance matrix. The paper shows that a detailed qualitative and quantitative description of the measurement results can only be obtained by means of a 3D model, that allows computing the current distribution along and across the sample length during the measurements. [ABSTRACT FROM AUTHOR]

In the framework of the JT-60SA design activities, EU home team has defined a reference layout for the Toroidal Field conductor: it is a slightly rectangular Cable-In-Conduit NbTi conductor, operating at 25.7 kA with a peak field of 5.65 T. ENEA has assigned LUVATA Fornaci di Barga the task to produce the strands and to perform cabling, whereas jacketing and compaction have been carried out in its own labs. The sample, successfully tested at the CRPP SULTAN facility, has been assembled in such a way as to avoid the bottom joint between the two legs, thus using a single conductor length (about 7 m). An ad-hoc developed solution to restrain the U-bent conductor section (where jacket is not present), consisting in a stainless steel He-leak tight box with an inner structure designed in order to completely block the cable, has been also developed and manufactured by ENEA, where the sample has been also assembled. Instrumentation installation and final assembly of the sample have been performed by the SULTAN team. The main aspects of the sample manufacturing and characterization are here presented and discussed. [ABSTRACT FROM AUTHOR]

We present the design and the performances of a vertical Josephson interferometer, which can be inserted within superconducting loops in order to obtain controllable Josephson devices. The most interesting feature of the vertical interferometer is that the Josephson current is precisely controlled by a small transversal magnetic field parallel to the superconducting loop plane and can be modulated from a maximum to zero with high precision. This system can be used to realize a switchable flux transformer to couple quantum bits when desired as well as to vary the critical current of a flux qubit in situ. In the latter application, the vertical interferometer is controlled by a magnetic field orthogonal to the one applied to tilt the qubit potential, thus minimizing the cross talking of the two magnetic fields. [ABSTRACT FROM AUTHOR]

We present different types of integrated planar SQUID devices in view of Macroscopic Quantum Coherence (MQC) experiments. These devices include an rf SQUID as a probe to test the MQC, coupled to a readout system based on a dc SQUID sensor. Both dc SQUID and flux transformer are in a gradiometric configuration to minimize the electromagnetic noise from environment. We report data on dc SQUID performances as well as preliminary results on the decay from the metastable flux state for rf SQUID-based devices above the crossover temeperature (classical limit). [ABSTRACT FROM AUTHOR]

We present our recent experiments on macroscopic tunneling in the quantum regime for underdamped Josephson junctions with different classical-quantum crossover temperatures. These experiments are performed towards the observation of macroscopic quantum coherence in Josephson systems. A new method to measure coherence in a two-level system based on the adiabatic inversion in an rf squid, is presented. This approach could open new perspectives in view of realization of an elementary quantum bit. [ABSTRACT FROM AUTHOR]

We present niobium-based Josephson devices as prototypes for applications to quantum computation with flux qubits. The most interesting feature of this device is the use of a Josephson vertical interferometer for a fine tuning of flux qubit and interquibit coupling. These devices allow a control of the off-diagonal Hamiltonian terms of flux qubit and of the flux transfer function of a superconducting transformer. In the vertical interferometer, the Josephson current is precisely modulated from a maximum to zero with fine control and precision by a small transversal magnetic field parallel to the radio-frequency superconducting loop plane and to the flux transformer. [ABSTRACT FROM AUTHOR]

We present the characterization of a fully integrated Josephson device consisting of a rf-superconducting quantum interference device (SQUID) coupled to a readout system based on a dc SQUID sensor. Particular care in the design is devoted to minimize the electromagnetic noise coming from the environment. We report data on dc SQUID performances as well as on the decay rate from metastable flux states of the rf SQUID in the classical regime. The low dissipation level and the good insulation of the probe from the external noise are encouraging in view of macroscopic quantum experiments. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present Nb/AlO[sub x]/Nb–Al/AlO[sub x]/Nb stacked Josephson junctions which we propose as three-terminal elements to control the quantum behavior of Josephson complex systems. A bias current can be independently injected in the “top” junction, providing a fine control of the critical current of the “bottom” junction. The reported characterization of the device refers to data in the classical limit including measurements of the switching dynamics between metastable states of the system at different temperatures and bias conditions. At low temperature, the effective dissipation of the bottom junction remains substantially unchanged when different injection currents flow in the top junction. These results are interesting to project new configurations of macroscopic quantum experiments. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A correction to the article "Performance enhancement under bending of Nb3Sn strands with untwisted filaments" that was published online on May 5, 2008.