The Effects of Ti Addition and High Cu/Sn Ratio on Tube Type $(\hbox {Nb}, \hbox {Ta})_{3}\hbox {Sn}$ Strands, and a New Type of Strand Designed to Reduce Unreacted Nb Rati

In this paper, we report the properties of two tube type Ta doped Nb₃Sn strands: one strand was additionally Ti doped by way of a Sn-Ti alloy core, and the other had high Cu/Sn ratio within the filaments. Higher irreversibility field (Birr) was obtained on the quaternary strand with respect to the (Nb-7.5 wt.%Ta)₃Sn strand. High Cu/Sn ratio decreased the amount of coarse grain formation, but also degraded the layer J_c of the tube type strand by depressing the Sn content in the fine grain (FG) layer. A new type of strand, the subelement of which is composed of seven bare Cu-Sn cored Nb tube filaments, was designed with the aim to reduce the unreacted Nb area fractions. The test results of the first experimental strand are reported. The unreacted Nb ratio is reduced relative to normal tube type strands and the FG area fraction is improved. The unique structure of this strand makes it also possible to improve the stoichiometry of FG and reduce the effective diameter (deff) .