A large, magnetic-field-dependent, reversible reduction in critical current density with axial strain in YBa 2 Cu 3 O 7−δ coated conductors at 75.9 K has been measured. This effect may have important implications for the performance of YBa 2 Cu 3 O 7−δ coated conductors in applications where the conductor experiences large stresses in the presence of a magnetic field. Previous studies have been performed only under tensile strain and could provide only a limited understanding of the in-field strain effect. We now have constructed a device for measuring the critical current density as a function of axial compressive and tensile strain and applied magnetic field as well as magnetic field angle, in order to determine the magnitude of this effect and to create a better understanding of its origin. The reversible reduction in critical current density with strain becomes larger with increasing magnetic field at all field angles. At 76 K the critical current density is reduced by about 30% at −0.5% strain when a magnetic field of 5 T is applied parallel to the c-axis of the conductor or 8 T is applied in the ab-plane, compared to a reduction of only 13% in self-field. Differences in the strain response of the critical current density at various magnetic field angles indicate that the pinning mechanisms in YBa 2 Cu 3 O 7−δ coated conductors are uniquely affected by strain.

Bundling high-temperature superconductors together to form high-current cables is required in, for instance, power transmission and low-inductance magnet applications. Cabling techniques that have been applied so far have not resulted in compact, mechanically robust, high-current cables that remain flexible. Here, we demonstrate that the cabling technique that we have introduced only recently enables the construction of cables from high-temperature superconducting coated conductors that meet these requirements. We present details of a cable, wound from GdBa 2 Cu 3 O 7-δ coated conductors, that has an outer diameter of 7.5 mm and a critical current of about 2800 A at 76 K and self-field. The compact size and flexibility make the cable suitable for Navy and Air Force power transmission, and would allow superconducting power transmission lines that have been installed in the electric power grid to be reduced in diameter. The potential of increasing the engineering current density of the cable, while maintaining flexibility, makes them also suitable for high-field magnet applications.

A reversible strain effect on transport critical current I c was found in Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) high-temperature superconducting round wires. I c showed unambiguous reversibility at 4 K and 16 T up to an irreversible strain limit of about 0.3 % in longitudinal tension, prompting hope that the Bi-2212 conductor has the potential to sustain mechanical strains generated in high-field magnets. However, I c was not reversible under longitudinal compression and buckling of Bi-2212 grain colonies was identified as the main reason. A two-component model was proposed, which suggests the presence of mechanically weak and strong Bi-2212 components within the wire filaments. Porosity embedded in the weak component renders it structurally unsupported and, therefore, makes it prone to cracking under strain ε. I c (ε) is irreversible in tension if the weak component contributes to the transport critical current but becomes reversible once connectivity of the weak component is broken through strain increase or cycling. A modified descriptive strain model was also developed, which illustrates the effect of strain in the Bi-2212 conductor and supersedes the existing descriptive model. Unlike the latter, the new model suggests that higher pre-compressive strains should improve I c if buckling of Bi-2212 grains does not occur, and should result in a wider I c (ε) plateau in the applied tensile regime without degradation of the initial I c. The new model postulates that a reversible strain effect should exist even in the applied compressive strain regime if buckling of Bi-2212 grains could be prevented through elimination of porosity and mechanical reinforcement of the wire.

Nb 3 Sn superconducting wires made by the restacked-rod process (RRP ®) were found to have a dramatically improved resilience to axial tensile strain when alloyed with Ti as compared to Ta. Whereas Ta-alloyed Nb 3 Sn in RRP wires showed permanent damage to its current-carrying capacity (I c) when tensioned beyond an intrinsic strain as small as 0.04%, Ti-doped Nb 3 Sn in RRP strands exhibits a remarkable reversibility up to a tensile strain of about 0.25%, conceivably making Ti-doped RRP wires more suitable for the high field magnets used in particle accelerators and nuclear magnetic resonance applications where mechanical forces are intense. A strain cycling experiment at room temperature caused a significant drop of I c in Ta-alloyed wires, but induced an increase of I c in the case of Ti-doped strands. Whereas either Ti or Ta doping yield a similar enhancement of the upper critical field of Nb 3 Sn, the much improved mechanical behavior of Ti-alloyed wires possibly makes Ti a better choice over Ta, at least for the RRP wire processing technique. * Contribution of NIST, an agency of the US government, not subjected to copyright.

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Conductor on round core (CORC ®) cables wound from RE-Ba 2 Cu 3 O 7−δ coated conductors are currently being developed for the next generation of accelerator magnets because of their high flexibility and potential for high engineering current densities J E. CORC ® cables previously reached J E of 114 A mm −2 at 4.2 K and 20 T in a 7.5 mm diameter cable. Accelerator magnets require a current density of at least 300 A mm −2 and a cable-bending diameter as small as 40 mm, which has so far not been possible with superconducting tapes made on 50 μm thick substrates. CORC ® cables made from thinner substrates could have significantly increased J E with greater flexibility as we here demonstrate with a CORC ® cable made of tapes with 38 μm thick substrates. A custom cable machine produced higher cable quality and better retention of tape performance compared to previous cables that were wound by hand. The thinner substrate showed an almost twofold increase in engineering current density from 114 A mm −2 to 216.8 A mm −2 at 4.2 K and 20 T, at a reduction in cable diameter from 7.5 mm to 6.0 mm. The results clearly demonstrate that winding CORC ® cables from tapes with thinner substrates is a straightforward method for raising their current density and one that shows great promise for use in accelerator magnets.

A benchmarking experiment was conducted to compare strain measurement facilities at the National Institute of Standards and Technology (NIST) and the University of Twente. The critical current of a bronze-route wire, which was fabricated for the International Thermonuclear Experimental Reactor (ITER), was measured as a function of axial strain and magnetic field in liquid helium at both institutes. NIST used a Walters' spring strain device and University of Twente used a bending beam ("Pacman") apparatus. The ITER bronze-route wire investigated had a very high irreversible strain limit that allowed comparing data over a wide range of applied strain between 1% and 1%. Similarities of the data obtained by use of the two apparatuses were remarkable, despite the many differences in their design and techniques.