Probes for investigating the effect of magnetic field, field orientation, temperature and strain on the critical current density of anisotropic high-temperature superconducting tapes in a split-pair 15 T horizontal magnet

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 Jc to strain ε. Here we present very detailed Jc(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 for Jc(ε). The canonical explanation for the parabolic strain dependence of Jc 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 Jc(ε) should reach its peak value at a single strain (ε = εpeak), 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 εpeak is markedly B, (field angle θ for the HTS) and T dependent in both materials. The strain dependence of Jc in these materials is termed weakly-emergent because it is not qualitatively similar to the strain dependence of Jc of any of their underlying component parts, but is amenable to calculation. We conclude that Jc(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 for Jc(ε).

High current variable temperature electrical characterization system for superconducting wires and tapes with continuous sample rotation in a split coil magnet

A new state-of-the-art electrical transport measurement system was developed for the characterization of industrially produced coated conductors (CCs). The current leads are rated to a conduct current of up to 1000 A, which opens up the possibility of measuring the critical current I_c of tapes at a wide range of temperatures. The setup operates in a He-gas flow cryostat that provides stable temperatures between 1.8 and 200 K. The setup is equipped with a split-coil magnet that can apply fields of up to 6 T. A continuous rotation of the sample with respect to the magnetic field with an angular resolution of 0.5° enables characterization of anisotropic I_c of different tapes. In the measured voltage-current curves, weak sample heating mostly occurs from the dissipation in the tape during the I_c transition. It is demonstrated that the system can provide reliable data on the properties of CCs at temperatures lower than 77 K for a magnet design and other applications. The results allow the study of vortex pinning for further prospects of engineering the microstructure of the superconducting layer as well as to assess the performance of various tapes with different architectures to achieve optimum performance at different operating temperatures and magnetic fields.

A versatile facility for investigating field-dependent and mechanical properties of superconducting wires and tapes under cryogenic-electro-magnetic multifields

To investigate the field-dependent and mechanical properties of superconducting wires and tapes as a function of cryogenic temperature, transport current, and magnetic field, we designed and constructed a versatile facility capable of providing cryogenic-electro-magnetic multifields. The facility comprises several relatively independent systems to acquire multiple fields and explore various properties for superconductors. A superconducting racetrack magnet is manufactured to generate a transverse background field up to 3.5 T in a relatively large space of a homogeneous region of ∅200 mm × H 150 mm. A cryogenic system consisting of a vacuum Dewar vessel with a visible window cooled by two Gifford-McMahon (GM) cryocoolers for providing refrigeration was built to accommodate the background magnet and testing devices, in which one GM cryocooler cools the magnet at an operation temperature of about 4 K and the other maintains a cryogenic environment for specimens in conduction mode with the cryocooler head directly contacting the fixtures. The continuous variations of temperature (4-293 K) and transport current (0-1000 A) in the superconducting wires and tapes that were tested are, respectively, implemented by an integration differentiation temperature control with an optional temperature sweep rate and a DC high-power supply. Most prominently, the facility can measure the field-dependent and mechanical properties for superconducting wires and tapes, which is implemented by a mechanical loading and measuring system equipped with a universal testing machine possessing a specific design of widening and heightening size and a noncontact digital image correlation method with a high-speed, high-resolution CCD camera for real-time recording and full-field deformation of specimens. The preliminary results of tests verify the multifield functionalities of the versatile facility and illustrate the performance of the facility for studying the properties of superconducting wires and tapes as a function of magnetic field, cryogenic temperature, transport current, and mechanical loading.

A visualization instrument to investigate the mechanical-electro properties of high temperature superconducting tapes under multi-fields

We construct a visible instrument to study the mechanical-electro behaviors of high temperature superconducting tape as a function of magnetic field, strain, and temperature. This apparatus is directly cooled by a commercial Gifford-McMahon cryocooler. The minimum temperature of sample can be 8.75 K. A proportion integration differentiation temperature control is used, which is capable of producing continuous variation of specimen temperature from 8.75 K to 300 K with an optional temperature sweep rate. We use an external loading device to stretch the superconducting tape quasi-statically with the maximum tension strain of 20%. A superconducting magnet manufactured by the NbTi strand is applied to provide magnetic field up to 5 T with a homogeneous range of 110 mm. The maximum fluctuation of the magnetic field is less than 1%. We design a kind of superconducting lead composed of YBa2Cu3O7-x coated conductor and beryllium copper alloy (BeCu) to transfer DC to the superconducting sample with the maximum value of 600 A. Most notably, this apparatus allows in situ observation of the electromagnetic property of superconducting tape using the classical magnetic-optical imaging.