Current-Sharing Properties in Parallel Conductors Composed of REBCO Superconducting Tapes

For the application of Re
₁
Ba
₂
Cu
₃
O
_7-δ
(REBCO: RE such as Y, Gd, and Eu) superconducting tapes to large-scale magnets, an enhancement of current capacity is required for magnet protection. In this study, a configuration of parallel conductors were introduced, and the current-sharing properties of three-strand parallel conductors wound into a pancake coil were investigated. An equivalent resistance exists even for a much smaller current than the critical current I
_c
. This should influence the current sharing among the tapes even if inductances are balanced by transposition. Uneven current sharing could lead to a fatal problem in the uniformity of a produced magnetic field, which is usually on the order of ppm. In this paper, every branch current was evaluated by numerically solving the circuit equation. As a result, it is shown that, in the case where the constituent tapes are different in Ic and n values, this should influence the current sharing among the tapes even if both the inductances and joint resistances are balanced among the tapes. An unbalance is serious when the difference in Ic and the n value is large. In addition, an unbalance increases as frequency decreases. The results suggest that uniformity in Ic and the n value of the REBCO tapes is important for the development of MRI magnets.