Critical current distributions in superconducting composites

The transition from the flux pinning state to the full flux flow state occurs over a range of current in composite superconductors. The critical current, therefore, does not have a single, unique value, there being a distribution of critical currents throughout the composite. This Paper describes a technique for deriving the critical current distribution from the resistive critical current transition. The analysis has been applied to Nb---Ti and Nb₃Sn composites of different types. The technique has been used to analyse good quality conductors, where the l_c is largely determined by the intrinsic fluxoid-microstructure interactions, At the other extreme, composites with irregular filaments in which the transport critical current is significantly less than the intrinsic critical current, have also been examined. The analysis shows that good conductors have narrow resistive transitions with average l_c values within 5–10% of a high sensitivity measurement of l_c. This difference broadens to ≈ 35% for a badly sausaged composite. A relationship between the n value of the resistive transition and the relative width of the l_c distribution is presented. A simple but accurate method of deriving the average l_c from the resistive transition is also presented.