2005 年 40 巻 3 号 p. 86-92
An internal-tin route Nb3Sn superconducting wire that has both remarkably low hysteresis loss (Qh) and high critical -current density (Jc) was developed with a new design idea. The wire was constructed by arranging the filaments in a radial layout, enlarging the outer filaments along the radial direction, intentionally narrowing the filament spacing in the radial direction and enlarging the filament spacing in tangential direction. Thus, the electromagnetic combination among the filaments in tangential direction due to the bridging and/or proximity effect was suppressed without decreasing the volume fraction of Nb. As a result, excellent properties such as Jc(12 T)= 1.15×103 A/mm2 and Qh=301 mJ/cm3 (for 1 cycle of B=±3 T) were obtained. We also evaluated the transition temperature (Tc) and upper critical field (Bc2) of the wire. The values for Tc and Bc2 were 17.3 K and 24.1 T, respectively, which were much better than those of usual internal-tin route wires. Electron probe micro-analyses confirmed that the good Tc and Bc2 were the result of the qualitative improvement in the Nb3Sn compound based on the effects of arranging the Nb filaments radially, increasing the ratio of Sn-to-Nb and shortening the diffusion length for Sn due to the increase of the module number. This wire is promising for use with conduction-cooled high-field magnets, in which there is a need to decrease the load of the cryocooler, and also for the strands of fusion coils.