High field properties of multifilamentary (Nb-4at%Ta)3Sn

A series of multifilamentary (Nb-4at%Ta)3Sn wires with varying Sn content was examined inorder to optimize the high field performance. Various Sn thicknesses were plated onto sections of 0.15 mm (.006") o.d. multifilamentary wire for external bronze processing. Long heat treatments achieved almost complete conversion of the filaments to A15. A systematic increase in Bc2, Jcand Tcwas measured for these wires with increasing Sn content. The highest measured values were Bc2(4.2K)=27.8T (resistive mid-point),Jc(4.2K)=1.5×10⁴A/cm²(overall) at 20T, and Tc=17.8K. These values appear to be the highest reported for multifilamentary, bronze processed, ternary (Nb-X)3Sn. The narrow (<1T) Bc2transition suggests a uniform homogeneous A15. When the precompression due to the bronze matrix was removed, the values of Jc(4.2K) increased by 40 to 50% in fields of 18 to 22T, but only a slight increase (<0.5T) in Bc2(4.2K) was detected. Jcincreases by a factor of about 3 as temperature is reduced from 4.2 to 1.5K.