The influence of magnetic field on current-voltage characteristics of fine-filament NbTi superconductors and the effect of a filament break

The current-voltage characteristics of fine-filament composite wires for AC applications are exponential in a wide range of the transport currents l and electric fields E. It is observed that the straight lines log E(l) measured at different external magnetic fields H intersect each other close to the single point extrapolated to the low-current and field E region. This fact and some additional experiments allow us to conclude that l-E characteristics of the fine-filament wires are controlled by strong defects at which the critical current value decreases substantially. These defects are assumed to be due to breaks in the superconducting filaments. A simple model is proposed to describe the E(l,H) dependence for fine-filament superconductors. (author)