Investigation of critical current disitribution 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 stu died a model which describes the E(I)curve by means of a Gaussian distribut ion of local critical currents. Therefore, resistive measurements in magnet ic fields up to B = 15T 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 distributi ons. The knowledge of the parameters enabled us to calculate the whole curv es and to compare them with the results of the measurements. This compariso n clearly showed that for NbTi and Nb3Sn composite superconductors, which a re not additionally stabilized, the quench of the sample occurs far below t he mean critical current.