FEM analysis of AC loss in twisted Bi-2223 multifilamentary tapes carrying AC transport current in AC transverse magnetic field with arbitrary orientation

A new FEM code for electromagnetic field analysis of twisted multifilamentary superconductors was developed based on triangular element system and tensor equivalent conductivity. Finite element models of twisted Bi-2223 tapes exposed to an AC transverse magnetic field with various orientations were made, and the current distribution in these tapes was calculated. The calculated current distribution in a twisted tape carrying AC transport current in a parallel or nearly parallel AC transverse magnetic field shows that filaments are decoupled effectively to reduce the total AC loss, even if the transverse resistivity equals the resistivity of pure silver. However, in the perpendicular transverse magnetic field, only a little reduction of the total AC loss can be attained by twisting even if the transverse resistivity is increased to 100 times larger than the resistivity of pure silver.

I. Introduction

AC loss reduction is one of the important issues for realization of AC superconducting electrical devices. In case of NbTi multifilamentary wires, the combination of twisting and the use of resistive CuNi matrix is effective to reduce the magnetization loss in an AC transverse magnetic field [1]. However, even if Bi-2223 multifilamentary tapes with pure silver matrix are twisted, the magnetization loss can be reduced only in the transverse magnetic field parallel to the tape wide-face [2]. For reduction of the magnetization loss in the perpendicular transverse magnetic field, increase in the transverse resistivity together with twisting is required [3], [4]. Since superconductors must carry AC transport current in an AC transverse magnetic field with arbitrary orientation in electrical devices, the total AC loss during simultaneous application of an AC transverse magnetic field with an arbitrary orientation and AC transport current must be reduced from the practical point of view.