Very high critical current densities (Jc=8×105 A/cm2 at 4.2 K and 6 T) have been achieved in commercial Nb3Sn tape and more recently in multifilamentary Nb3Sn superconductors. The relative contributions of two types of pinning centers, grain boundaries and second phase particles, to flux pinning have been determined by correlating Jc with microstructure. It was found that grain boundaries were the only defect present in sufficient density to account for flux pinning in the multifilamentary Nb3Sn material. The dependence of Jc on grain size was measured, and the relationship between pinning force and grain size is discussed in terms of current theories for flux pinning in type-II superconductors.

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Nb 3 Al strands with four different designs have been processed and tested. Strand specifications and some generic aspects of strand fabrication, processing, and testing are described. One strand type was long-length RHQT processed and transport tested at various lengths. Transport results for short samples and small solenoid coils are presented and discussed for various RHQT protocols. Critical current measurements on various strands have been performed in fields from 18 to 26 T. Various strand stabilization techniques have been investigated including electroplating, plasma spray coating, and direct Nb-Cu high temperature diffusion bonding; these results are described. Fifty meters of strand was RHQ processed and then cabled into a short length of experimental 12-strand Nb 3 Al Rutherford cable in preparation for transformation ( ) and testing. Cabling properties are described.