Weakly-Emergent Strain-Dependent Properties of High Field Superconductors

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Abstract 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 densityJcto strainε. Here we present very detailedJc(B,θ,T,ε) measurements on a high temperature superconductor (HTS), a (Rare−Earth)Ba2Cu3O7−δ(REBCO) coated conductor, and a low temperature superconductor (LTS), a Nb3Sn wire, that include the very widely observed inverted parabolic strain dependence forJc(ε). The canonical explanation for the parabolic strain dependence ofJcin 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 thatJc(ε) should reach its peak value at a single strain (ε = εpeak), independent of fieldB, and temperatureT. However, consistent with a new analysis, the high field measurements reported here provide a clear signature for weakly-emergent behaviour, namelyεpeakis markedlyB, (field angleθfor the HTS) andTdependent in both materials. The strain dependence ofJcin these materials is termed weakly-emergent because it is not qualitatively similar to the strain dependence ofJcof any of their underlying component parts, but is amenable to calculation. We conclude thatJc(ε) is an emergent property in both REBCO and Nb3Sn conductors and that for the LTS Nb3Sn conductor, the emergent behaviour is not consistent with the long-standing canonical explanation forJc(ε).

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