Strain dependence of the critical current of Nb 3Sn superconductors doped with Hf and Ga

Abstract

Strain dependence of I c of bronze-processed NbHf/CuSnGa superconductors (namely Nb 3Sn superconductors fabricated with starting composite consisting of a NbHf alloy core and a CuSnGa sheath) and that of CuSnGa/NbHf superconductors (Nb 3Sn superconductors fabricated with starting composite consisting of a CuSnGa alloy core and a NbHf alloy sheath) were studied. It has been revealed that the additions of Hf and Ga improve strain tolerance of I c for both kinds of Nb 3Sn superconductors. I c of Nb 3Sn superconductors to which Hf and Ga are added scarcely depends on strain at 6.5 T in contrast to I c of pure Nb 3Sn superconductors. ε irr (the value of strain up to which the reversibility of the I c - strain curve is preserved) decreases with increasing thickness of the Nb 3Sn layer. The existence of the negative and positive residual strains in the Nb(Hf)/CuSn(Ga) and CuSn(Ga)/Nb(Hf) specimens, respectively, is considered to be the reason for the difference of I c − ε (strain) curves of these specimens. Only fine cracks which distribute homogeneously throughout the Nb 3Sn layer were observed in the specimen to which ε irr- Δε had been applied. These fine cracks are considered to enable the reversible elongation and compression of the Nb 3Sn layer up to ε≈1%