Critical current transition study on multifilamentary NbTi superconductors having a Cu, a CuNi, or a mixed matrix

Measurement of voltage-current characteristics in multifilamentary wires, showing the transition from the superconducting to normal state, provides information about the quality of the wires and their production techniques. Two methods of describing this transition are discussed. In general, the so-called n-power law turns out to be adequate in the critical current region. The dependence of n on the magnetic field yields information about the inner structure of the wire, especially whether the limitation of the current is caused by intrinsic or extrinsic effects. Measurements were carried out at 4.2 K on several wires in order to investigate the above mentioned aspects with respect to the critical current transition. The current range in which the n-power law was valid was tested for two wires with highly resistive matrices. Then the dependence of n on the applied field was investigated. A wire with very thin filaments and one with thicker filaments were used. When submicrovolt measurements were carried out, it was found that in several ranges of current other n-values can be distinguished. This is thought to be caused by a current diffusion process. In an alternate way of assessing the resistive transitions, the authors used a critical current distribution function found by calculating the second derivative of the voltage with respect to the current.