Inter-strand current sharing and ac loss measurements in superconducting YBCO Roebel cables

A Roebel cable, one twist pitch long, was modified from its as-received state by soldering copper strips between the strands to provide inter-strand connections enabling current sharing. Various DC transport currents (representing different percentages of its critical current) were applied to a single strand of such a modified cable at 77 K in a liquid nitrogen bath. Simultaneous monitoring of I–V curves in different parts of the strand as well as in its interconnections with other strands was made using a number of sensitive Keithley nanovoltmeters in combination with a multi-channel high-speed data acquisition card, all controlled via LabView software. Current sharing onset was observed at about 1.02 of strand I_c. At a strand current of 1.3I_c about 5% of the current was shared through the copper strip interconnections. A finite element method modeling was performed to estimate the inter-strand resistivities required to enable different levels of current sharing. The relative contributions of coupling and hysteretic magnetization (and loss) were compared, and for our cable and tape geometry, and at dB/dt = 1 T s⁻¹, and our inter-strand resistance of 0.77 mΩ, (enabling a current sharing of 5% at 1.3I_c ) the coupling component was 0.32% of the hysteretic component. However, inter-strand contact resistance values of 100–1000 times smaller (close to those of NbTi and Nb₃Sn based accelerator cables) would make the coupling components comparable in size to the hysteretic components. (paper)