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

Abstract

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 Nb 3 Sn 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 ε 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.

Superconducting Properties of Titanium Alloys (Ti-64 and Ti-6242) for Critical Current Barrels

“…The thermal coefficient of linear expansion for Ti alloys is similar to the intermetallic superconductor Nb3Sn (i.e. 7.6 10 -6 K -1 ), but of course in practice, the thermal properties of a Ti alloy and a composite Nb3Sn strand, which includes softer materials such as copper that bring higher thermal contraction coefficients, are not perfectly matched [17]. We sort a barrel material with similar thermal and mechanical properties to Ti-64 but with a lower critical temperature.…”

Effect of Barrel Material on Critical Current Measurements of High-${\rm J}_{\rm c}$ RRP ${\rm Nb}_{3}{\rm Sn}$ Wires

“…The is taken from the susceptibility-temperature measurement of this wire that was made in a magnetometer. Using the same parameters as that used to fit the Ti-alloy barrel measurements, the measured on other barrels are fit by only changing the strain coefficient [1]. In fitting the data, the self-field generated by the transport current is taken into account.…”

“…Various materials have been tried in the past including copper, stainless steel, Inconel 600, and fiberglass reinforced plastics like G-10 or G-11. In 2005, Taylor of ITER-type wires using helical springs of different materials [1]. They used Ti-6Al-4V alloy, Cu-Be (copper-beryllium), brass and 316 L stainless steel as spring material, and showed that the change in behavior of the wire in the different springs is solely due to a change in the thermal pre-strain in the wire which depends on the thermal contraction of the spring material.…”

A New Experimental Setup to Measure Critical Current in Superconducting Strands Under Periodic Bending

“…Many facilities already offer precise and thorough measurement of performances versus axial strain (helical spring [1], pacman spring [2]), bending strain (tarsis probe [3]) or contact stresses (tarsis contact stresses [4]). Our idea was to develop a setup able to test a strand in bending conditions in the limited space required for a VAMAS sampleholder.…”