Theoretical analysis on ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands

The ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands were theoretically investigated. The constituent strands generally need to be insulated and transposed for the sake of uniform current distribution and low ac loss. In case the transposition points deviate from the optimum ones, shielding current is induced according to the interlinkage magnetic flux of the twisted loop enclosed by the insulated strands and the contact resistances at the terminals. It produces an additional ac loss. Supposing a simple situation where a two-strand parallel conductor with one-point transposition is exposed to a uniform ac magnetic field, the basic equations for the magnetic field were proposed and the theoretical expressions of the additional ac losses derived. As a result, the following features were shown. The additional ac loss in the non-saturation case, where the induced shielding current is less than the critical current of a strand, is proportional to the square of the magnetic field amplitude and the square of deviation length in transposition and has a Debye-type frequency dependence. On the other hand, in the saturation case, the additional ac loss is nearly proportional to the field amplitude and the deviation length in transposition. It has no frequency dependence at a higher frequency than the critical frequency.