Properties of helical springs used to measure the axial strain dependence of the critical current density in superconducting wires

The use of helical (Walters) springs is an effective technique for measuring the critical current density (J(C)) of superconducting wires in high fields as a function of both compressive and tensile axial strains. We report J(C) versus strain measurements for Nb3Sn wires on a number of helical springs of different materials and geometries, together with results from finite element analysis (FEA) of these systems. The critical current density, n-value and effective upper critical field data are universal functions of intrinsic strain (to within +/- 5%) for measurements on four different spring materials. The strains on the wire due to the differential thermal contraction of the spring are equivalent to the applied mechanical strains and hence only produce a change in the parameter epsilon(M) (the applied strain at the peak in J(C)). Variable-strain J(C) data for springs having turns with rectangular and tee-shaped cross-sections (and hence different transverse strain gradients across the wire) show good agreement when the strain-gauge calibration data are corrected to give the strain at the midpoint of the wire. The correction factors can be obtained from FEA or analytical calculations. Experimental and FEA results show that the applied strain varies periodically along the wire with an amplitude that depends on the spring material and geometry. We suggest that Ti-6Al-4V springs with an integer number of turns and optimized tee-shaped cross-sections enable highly accurate measurements of the intrinsic properties of superconducting wires.