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. In this paper several 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. However, when submicrovolt measurements are carried out on short-sample specimens with high-resistivity matrices, in several ranges of current other n-values can be distinguished. This can be caused by a current diffusion process. A different way of describing the resistive transition is by means of a critical current distribution function. Such a function can be found by calculating the second derivative of the voltage with respect to the current