We have developed HTS cables using Bi-2212 round wires, which have high mechanical strength and an average J, of 200kA/cm2 at 4.2K in self-field. Four designs of cables were fabricated in this work and those cables could carry I, values from 1kA to lOkA at 4.2K and self-field depending on the number of strands. A 70m-long 20-strand Rutherford cable was successfully manufactured. Optimization of heat treatment conditions was effective to reduce I, degradation observed in the case of heat treatment with a long length. I, vales of a Rutherford cable were measured under loading at 4.2K in 4T. The criticalvalues of the stress on face loading and edge loading were 60MPa and 100MPa, respectively. We successfully obtained high J, value of 500kA/cm2, which was almost double that o f the conventional wires capitalize "Rutherford".
We used ferromagnetic artificial pinning centers in superconducting NbTi wires to achieve a large critical current density (J c) in a magnetic field. Four wires were fabricated that contained nanometer-sized arrays of Ni or Fe pins inside micron-sized filaments of Nb 0.36 Ti 0.64 alloy. A ferromagnetic pin volume of only 2% Ni produced J c 's ͑e.g., 2500 A/mm 2 at 5 T, 4.2 K͒ that were comparable to those of commercial wires that have a pin volume of ϳ20% Ti. We conclude that ferromagnetic artificial pins are more effective than nonmagnetic pins for a given volume percent.
Low-temperature superconductors containing artificial pinning centers (APC) have produced record critical current densities in NbTi at magnetic fields below 4 T and promise further improvements at these and higher fields. Peak current densities are achieved when pinning center spacings are matched to spacings of the flux line lattice at the field of operation. The enhancement of Jc through inclusion of artificial pinning materials is accompanied by reduction in Tc and Hc2 through proximity effects. We find that the choice of the superconducting alloy as well as the pin material has marked effect on both the characteristic pinning force Fp and the critical magnetic field Hc2.
Scanning tunneling microscopy and spectroscopy are employed in studies of the proximity effect between normal metals and superconductors. The experimental configuration is unique, in that the tunneling current flows in parallel to the interfaces between different materials. The samples are superconducting wires consisting of ordered arrays of submicron diameter normal-metal filaments, either Cu or Ni ͑a ferromagnet͒, embedded in a NbTi superconducting matrix. By taking topographic images simultaneously with current-voltage curves, we map with nanometer resolution the local quasiparticle density of states. Two main issues are addressed in this work. The first is the spatial variation of the superconductor gap as a function of distance from the boundary between the normal-metal and the superconductor. We find that the healing length of the gap on the superconducting side is much larger than the superconductor coherence length. The second is the observation of pronounced Tomasch oscillations and bound states arising from multiple Andreev reflections of quasiparticles propagating in the plane perpendicular to the tunneling current.
We report on the development of multifilamentary Nb 3 Sn superconductors by a versatile powder-in-tube technique (PIT) that demonstrates a simple pathway to a strand with a higher density of flux-pinning sites that has the potential to increase critical current density beyond present levels. The approach uses internal oxidation of Zr-alloyed Nb tubes to produce Zr oxide particles within the Nb 3 Sn layer that act as a dispersion of artificial pinning centres (APCs). In this design, SnO 2 powder is mixed with Cu 5 Sn 4 powder within the PIT core that supplies the Sn for the A15 reaction with Nb1Zr filament tubes. Initial results show an average grain size of ∼38 nm in the A15 layer, compared to the 90-130 nm of typical APC-free high-J c strands made by conventional PIT or Internal Sn processing. There is a shift in the peak of the pinning force curve from H/H irr of ∼0.2 to ∼0.3 and the pinning force curves can be deconvoluted into grain boundary and point-pinning components, the point-pinning contribution dominating for the APC Nb-1wt%Zr strands.
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