Development of Nb tube processed Nb3Al multifilamentary superconductor

The Nb tube process has recently been developed at NRIM (Japan) for fabricating Nb3Al multifilamentary superconductors containing more than one million continuous ultrafine filaments of less than 0.1 μm diameter. The adjustment of hardness of Al cores relative to Nb matrix by alloying Al cores with additives of Mg, Ag(-Ge), Cu(-Ge), Zn, etc. improved remarkably the cold workability of Nb/Al composites. Wires heat treated below 1000 degrees C show superior properties to a commercial multifilamentary (Nb,Ti)3Sn conductor: (1) higher Jc (4.2 K) (1.5 x 109 A/m2 at 10 T), (2) smaller degradation in Jc with mechanical strain, and (3) smaller ac loss (resulting from a smaller effective superconducting filament diameter: about 2 μm). Furthermore, sensitivity to irradiation is nearly the same as that of (Nb,Ti)3Sn. Other techniques to produce Nb3Al are unsuitable for developing a conductor with multifilamentary structure, and thereby the Nb tube processed Nb3Al multifilamentary conductors is the best available for fusion reactor magnets. 36 refs., 19 figs., 2 tabs