A device to investigate the axial strain dependence of the critical current density in superconductors

We have developed an instrument to study the behavior of the critical current density (J_c) in superconducting wires and tapes as a function of field (μ₀H), temperature (T), and axial applied strain (ε_a). The apparatus is an improvement of similar devices that have been successfully used in our institute for over a decade. It encompasses specific advantages such as a simple sample layout, a well defined and homogeneous strain application, the possibility of investigating large compressive strains and the option of simple temperature variation, while improving the main drawback in our previous systems by increasing the investigated sample length by approximately a factor of 10. The increase in length is achieved via a design change from a straight beam section to an initially curved beam, placed perpendicular to the applied field axis in the limited diameter of a high field magnet bore. This article describes in detail the mechanical design of the device and its calibrations. Additionally initial J_c(ε_a) data, measured at liquid helium temperature, are presented for a bronze processed and for a powder-in-tube Nb₃Sn superconducting wire. Comparisons are made with earlier characterizations, indicating consistent behavior of the instrument. The improved voltage resolution, resulting from the increased sample length, enables J_c determinations at an electric field criterion E_c=10 μV/m, which is substantially lower than a criterion of E_c=100 μV/m which was possible in our previous systems.