Weakly-Emergent Strain-Dependent Properties of High Field Superconductors

All superconductors in high field magnets operating above 12 T are brittle and subjected to large strains because of the differential thermal contraction between component parts on cool-down and the large Lorentz forces produced in operation. The continuous scientific requirement for higher magnetic fields in superconducting energy-efficient magnets means we must understand and control the high sensitivity of critical current density J c to strain ε . Here we present very detailed J c ( B , θ , T , ε ) measurements on a high temperature superconductor (HTS), a (Rare−Earth)Ba 2 Cu 3 O 7− δ (REBCO) coated conductor, and a low temperature superconductor (LTS), a Nb 3 Sn wire, that include the very widely observed inverted parabolic strain dependence for J c ( ε ). The canonical explanation for the parabolic strain dependence of J c in LTS wires attributes it to an angular average of an underlying intrinsic parabolic single crystal response. It assigns optimal superconducting critical parameters to the unstrained state which implies that J c ( ε ) should reach its peak value at a single strain ( ε = ε peak ), independent of field B , and temperature T . However, consistent with a new analysis, the high field measurements reported here provide a clear signature for weakly-emergent behaviour, namely ε peak is markedly B , (field angle θ for the HTS) and T dependent in both materials. The strain dependence of J c in these materials is termed weakly-emergent because it is not qualitatively similar to the strain dependence of J c of any of their underlying component parts, but is amenable to calculation. We conclude that J c ( ε ) is an emergent property in both REBCO and Nb 3 Sn conductors and that for the LTS Nb 3 Sn conductor, the emergent behaviour is not consistent with the long-standing canonical explanation for J c ( ε ).