Investigation of critical current distribution in composite superconductors

For the design of superconducting magnets, the E-field vs. current curve (E(I)-curve) of the composite superconductor is an important property. We studied a model which describes the E(I)-curve by means of a Gaussian distribution of local critical currents. Therefore, resistive measurements in magnetic fields up to B=15 T were performed on several niobium-titanium, niobium-tin conductors and Bi-2223-conductors. We calculated the critical current distribution by differentiating E(I) twice with respect to the current. For metallic superconductors we got only the lower portion of the distribution because of sample quenches. That means, no complete distribution could be seen, but only a fraction of the curve. We developed a new numerical method to estimate the parameters of these fragmented critical current distributions. The knowledge of the parameters enabled us to calculate the whole curves and to compare them with the results of the measurements. This comparison clearly showed that for NbTi and Nb/sub 3/Sn composite superconductors, which are not additionally stabilised, the quench of the sample occurs far below the mean critical current.