Current stability boundary of liquid helium cooled YBCO tape

To make an YBa₂Cu₃O₇ superconducting current-carrying element, it is important to determine the boundary of the limiting currents flowing stably in a superconductor. Therefore, the current stability analysis of Cu/Ag/YBa₂Cu₃O₇/Hastelloy coated tape cooled by liquid helium for coil quench design was performed. It was assumed that an external magnetic field is parallel to its surface and varies in the range from 10 T to 30 T. Under these conditions, the stable current modes of a superconducting tape are limited by the transition from the nucleate to the film boiling regimes. This current instability problem was studied using static zero-dimensional model when the thickness of a copper layer was varied in the wide range. It is shown that the instability currents increase practically linear with increasing the copper thickness. Current-sharing analysis depicts that this current-carrying capacity variation is due to the corresponding increase of the current stably flowing in the copper layer. Namely, the calculations indicate the existence of the stable sharing current, which is about 0.7 A per 1 μm copper thick at 30 T. As a result, the stable current sharing may occur at currents that exceed noticeably the critical currents of the tape in the wide variation range of the external magnetic field.