Stress Effects in Nb₃Sn-Nb-Bronze Composites

Abstract

Since the effect of mechanical loading on the superconducting properties of Nb₃Sn will be crucial for the development of large Nb₃Sn magnets, much work has been done on this subject [^1–4]. Although most results show an initial increase of the critical current upon uniaxial loading before stress-induced degradation, there also have been experiments indicating degradation only [^1–4]. The initial increase of the critical current, I_c, upon loading is due to the relief of a compressive stress on the superconducting layer, which is generated upon cooldown by differences of thermal contraction between the components of the conductor. At maximum I _c, the applied tensile stress just compensates the compressive stress. So the appearance of a maximum and the strain at the maximum will be determined by the internal stresses between the components. The mechanical properties at 4.2 K and the behavior of I _c under applied strain have been investigated for five commercial conductors, all stabilized by additional copper at the outside of the conductor. Since it is generally accepted that the reversible degradation of I _c is related to the degradation of the intrinsic parameters, the behavior of the critical temperature, T _c upon straining was also studied.