New restrictions on the use of Pb alloys will soon become a serious concern for superconducting magnet manufacturers who often use Pb-Bi alloys as superconducting solders. We are studying the binary Sn-In and ternary Sn-In-Bi systems as potential replacements for these Pb-Bi alloys for superconducting joints between NbTi superconducting wires. In both systems, a range of compositions have been fabricated into 2-mm cylindrical bulk samples, and the influence of both chemistry and microstructure on the superconducting properties have been investigated using analytical scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and SQUID magnetometry.
All the alloys in both systems show transition temperatures (TC) above 4.2K. In the Sn-In system, TC and upper critical field (HC2) values can be enhanced by increasing the volume fraction of In-rich β-phase in the microstructure. The superconducting properties of the β-phase also improve with increasing Sn content in the solid solution. By quenching the samples, the phase chemistry and microstructure can be manipulated, leading to higher values of TC, HC2 and critical current density (JC). All the alloys in the ternary Sn-In-Bi system that we have studied show better superconducting properties than those of the binary Sn-In system. The best values we have achieved so far are 7.1 K, 0.18 T and 1.5 x 108 A/m2 for TC, HC2 and JC respectively for the Sn35In50Bi15 composition consisting of a majority phase β-phase in which fine fibers of γ and BiIn2 are embedded.

We have developed a nondestructive characterization method for local critical current distribution in MgB₂ wires based on magnetic microscopy. Critical current of an MgB₂ wire is generally evaluated by the conventional four-probe transport method. However, it has become difficult for such a method to measure very high critical current values of recent high-performance MgB₂ wires due to the heat generation and/or the capacity of a measurement system. Furthermore, only the overall critical current can be estimated by the method; it is difficult to find current blocking obstacles which should be cleared for the further performance improvement of MgB₂ wires. From these viewpoints, we have been developing a characterization method for local critical current distribution in MgB₂ wires based on magnetic microscopy without transport current. Our previous study was successful; however it was necessary to polish or remove their magnetic sheath materials such as iron and Monel which were all used for recent high-performance MgB₂ wires; otherwise the magnetic signal from the filament could not come out. In this study, we have developed a nondestructive characterization method for local critical current distribution in MgB₂ wires with magnetic sheath materials by using in-field magnetic microscopy over saturation fields of the magnetic sheath. As a result, it was found that the locally minimum critical current value estimated by this method agreed very well with the value estimated by the four-probe method. Furthermore, large critical current value such as J_e > 1000 A/mm², which would be difficult to measure by the four-probe method, was estimated for < 4 T; this should be important condition for promising MRI applications. We believe that this will become a fundamental technology for the quality control and the performance evaluation of MgB₂ wires.


This work is supported by Japan Science and Technology Agency (JST) as Advanced Low Carbon Technology Research and Development Program (ALCA).

The presence of porosity in the superconductors produced by powder-in-tube (PIT) technology is unavoidable. If the porosity is present inside the superconducting phase, like in MgB₂, it can block the supercurrent and reduce the critical current density. The visualisation of porosity and a quantitative description of the pore distribution can help to better understand the porosity formation and redistribution, and ultimately to optimise the processing procedures.
We compare the porosity in different ex situ and in situ MgB₂ Powder-in-Tube wires. The sub-micrometer structure was determined using Focused Ion Beam (FIB) nanotomography. The segmentation in the tomographic reconstructions provides a 3D rendering of the porosity. The pore size distributions are compared with experimental data about superconducting properties of the wires.

Recent developments in internal-tin/RRP Nb3Sn conductors, YBCO 2G coated conductors and Bi2212 round wires have significantly enhanced their prospects in next generation high field accelerator magnets. AC losses and filament (de)coupling are important aspects which are relevant to both conductor optimisation and coil design. Measurements of ac losses have been carried out on several samples of RRP Nb3Sn, 2G YBCO roebel cable and cable segments, in applied ac fields up to 0.2T over a wide range of frequencies between 5Hz and 5kHz. With simultaneous measurements of the fundamental and the 3rd harmonics, the total loss in the composites and flux pinning hysteresis loss in the superconducting filaments were independently determined. Therefore the coupling/eddy current losses were also isolated. The magnitude of coupling current losses and the critical (de)coupling fields, deduced from the frequency response and the change in the width of the super-current loop, are compared for different matrix resistivity and twist pitch. The dependence of coupling field/current on the critical current has also been studied in detail by measurements at different temperatures from 3K up to Tc

EU FP7 support for project EuCARD-2

The Nb3Sn cable in conduit conductors (CICCs) have a multi-stage twisted configuration. The inner structure of CICC such as the twist pitch and void fraction have a substantial impact on the superconductive cable performances. X-ray tomography is a powerful non-destructive technique which can provide 3D images of strands configuration. Therefore, this technique is used to determine the space structure which in turn can be used to verify the cabling process and even further for modelling purposes. Additionally, scanning electron microscopy (SEM) analysis has been carried out to show the interaction between strands and to characterize the filaments of superconductive compound. Through an energy-dispersive X-ray spectroscopy (EDX) analysis, a chemical characterization of the superconductive filaments was performed. It was found that the trajectory of some strands was irregular and different to the one defined during cabling. The contact between strands produces detachment between the central zone and the outer ring of pure copper. Regarding the superconductive filaments, the solid state reaction performed to obtain the superconductive compound was not fully completed and an inner central core composed of pure Nb was obtained in several filaments. Also, a tendency of the Nb filaments to bridge together can be observed and some cracks were seen, specially in those with an inner central core of pure Nb.

Single ReBCO coated conductor tapes must be assembled in a cable geometry to reach currents in the order of several tens of kA. For AC application, wire twist can be introduced to reduce coupling currents during magnetic field ramping but also the inter-tape contact resistance plays a crucial role. Two cable types were subjected to transverse applied alternating magnetic fields, CORC manufactured by Advanced Conductor Technologies and soldered stacked tape conductors assembled at CRPP Villigen and ENEA Frascati. The applied magnetic field is sinusoidal with amplitudes in the range of 5 to 400 mT and frequencies up to 0.5 Hz. In addition for some types, inter-tape contact resistances were measured under applied transverse stress. An overview of the results is presented.

The flux flow characteristic of HTS coated conductors is hard to observe experimentally due to the fast heating rate that occurs when an over-critical transport current is applied. In addition, critical current inhomogeneities along the tape’s length generate normal zones that propagate at finite rate and further bias the electric field measurements. The only way to get valuable information about the flux flow regime with transport current measurements in self or external field is to apply the current for very short period of times, which limits heating to small values. Correction of measurements is still required, but by applying the transport current incrementally and performing a cumulative backward correction, it is possible to extract valuable information. This contribution builds up on previous work in which a suitable microsecond-range pulsed current source was developed for this purpose [1], and which has been drastically improved afterwards. The experimental procedure is explained in details, as well as the correction method to apply in order to obtain intrinsic E-J curves of the flux flow regime in the superconducting layer. The sensitivity of the measurements to dynamical effects was also verified by performing experiments on samples with different thicknesses of metallic stabilizer and by varying the rising rate of the applied currents. The final results allow improving the validity range of the E-J curves to be used in numerical modeling codes for simulating quench processes in applications such as fault current limiters, magnets, etc.

[1] F. Sirois, J. Coulombe, F. Roy, and B. Dutoit, “Characterization of the electrical resistance of high temperature superconductor coated conductors at high currents using ultra-fast regulated current pulses,” Superconductor Science and Technology, vol. 23, no. 3, p. 034018 (6 pp.), 2010.

In order to elucidate the effect of uniaxial strain on the critical current of practical REBCO wires (REBa2Cu3O7-d, RE = Y and Gd), a springboard sample holder was employed that can smoothly and continuously apply both tensile and compressive strains to tape samples. The maxima of critical current appear in either compressive or tensile strain region for all four kinds of wires examined here. Their strain dependence of critical current was asymmetric around the maximum. By means of synchrotron radiation, the local strain exerted on the SC layer was measured by using the same type of springboard. Then the force free strain was estimated, where the local strain on the SC layer becomes zero along the strain axis. Comparing the force free strain with the maximum strain for critical current, both characteristic strains did not coincide with each other. On this issue, recently Hampshire et al have proposed a single mechanism—flux flow along channels— which can explain the functional form of the critical current in their LTS and HTS practical SC wires as a function of uniaxial strain in low and high magnetic fields. Here it is made clear that the relationship of strain dependence between critical current and local strain on SC layer can be well explained by the present theory mentioned above.