THE LENGTH DEPENDENCE OF LOCALLY VARIABLE CRITICAL CURRENT

The design and use of long superconductors requires critical current dependences on length estimated from tests on short pieces. A simple method has been experimentally proven for estimating the critical currents of long superconductors with locally variable critical currents from measured short sample critical currents. Critical current at 1-mu-V and 5-mu-V/cm equivalent intercepts on I-V plots were determined for superconducting oxide-silver composite ribbons with integer multiple lengths of about 0.2 cm. Averages and standard deviations of the experimental shortest length critical currents were used to calculate most probable critical current dependences on length with the normal distribution assumed for short length critical current variability. Calculated and averaged experimental length dependences coincided and standard deviations scaled well. Averaged 1-mu-V based critical currents decreased to near constant values as length increased in earth's field at 77 K and 4.2 K. Averaged length dependences in 130 G fields at 77 K approached (length)-1 relations yielded by Ohm's law for a 1-mu-V criterion. Averaged critical current length dependences based on a 5-mu-V/cm criterion deviated from the model. Microscopy and image analysis showed that sample cross-sectional area variability caused some of the shortest tested length critical current variability.