The procedure for mass producing Nb-Ti alloy has been established, where the development of high-homogeneity ingots improves the reliability of the alloy. The manufacturing process of fine filamentary wires of long length has also been optimized. The Nb-Ti wires can be used up to 9 T at 4.2 K. The non-Cu Jc of the wires at 6 T and 4.2 K reaches 2,500 A/mm2 through the combination of cold drawing and heat treatment. The microstructure of resulting Nb-Ti wires is very complicated, containing dislocation sub-bands and α-Ti precipitations. The structure and flux pinning in fine filamentary wires have been studied in detail. The introduction of artificial pinning centers, for example Nb, yields an appreciable increase in Jc. Ultra-thin filamentary Nb-Ti wires with low AC loss have also been successfully fabricated. Nb-Ti based ternary alloys, for example Nb-Ti-Ta and Nb-Ti-Hf, have been studied, aiming for the enhancement of Bc2. The microstructure and performance of alloys other than Nb-Ti, for example Nb-Zr, V-Ti and Mo-Re, are also described in this article.
We have fabricated a small test coil with an AlN former by employing Cu-Ni sheathed Ta barrier MgB2 multifilamentary wire made in a wind-and-react process. An overcurrent was applied to the coil conduction-cooled in an initial temperature range between 10 K and 30 K to investigate its thermal stability by measuring the temperature distribution in the winding and the terminal voltage after application of the overcurrent. The experimental results show that the permissive temperature rise without thermal runaway decreases with the initial temperature, while total heat generation at the time of thermal runaway is at a maximum when the initial temperature is approximately 14 K. We also numerically calculated the responses of the test coil to the overcurrent by simulating the electrical and thermal processes using the finite element method and V-I characteristics of the coil. The comparisons with the experimental results show that the electrical and thermal responses are reproduced well using the numerical model.
Dy-Ba-Cu-O is an ideal material for current leads because it has a low thermal conductivity and a high critical current density of 77 K in high magnetic fields. In this study, current carrying properties of Dy-Ba-Cu-O bulk current leads under the cryocooler-cooled condition have been investigated. The current leads consisted of a Dy-Ba-Cu-O bulk superconductor 40 mm x 3 mm x 0.8 mm, copper electrodes and a protective glass-fiber-reinforced-plastic sheet. Large currents of more than 400 A could be transported through the current leads at 77 K and the current-carrying capacity of the current leads increased with decreasing cooling temperature.