AC losses in multifilamentary HTS tapes can be classified into hysteresis loss, coupling loss, and eddy current loss from the viewpoint of their generation mechanism. Operating electromagnetic conditions significantly influence their AC loss characteristics. From the viewpoint of the major magnetic field component generating them, they can be classified into magnetization loss, transport loss, and total loss. Dividing a superconductor into fine filaments, twisting the filament bundle and increasing transverse resistivity effectively reduce magnetization loss and total loss when the external magnetic field is relatively large, while they do not effectively reduce transport loss. General analytical expressions for the total loss in HTS tapes carrying AC current in an AC magnetic field have not been derived. Numerical electromagnetic field analysis based on the finite element method is a powerful tool to study total loss theoretically. In the magnetic field parallel to the tape's wide face, twisting can reduce the AC loss in Bi2223 tapes with pure silver matrix, while increasing transverse resistivity is required essentially for the AC loss reduction in a perpendicular magnetic field. Recently, twisted multifilamentary Bi2223 tapes with pure silver matrix were fabricated, and the reduction of magnetization loss was proved experimentally in a parallel magnetic field. Furthermore, transverse resistivity was increased successfully by the introduction of a resistive barrier between filaments.
The AC loss in high-Tc superconductors near the glass-liquid transition temperature, Tg, is studied numerically and theoretically. It is shown that the AC loss in a slab sample under a DC bias magnetic field depends noticeably on the angular frequency, ω, and hence, cannot be described by the usual critical state model using critical current density, Jc, defined with the aid of the electric field criterion. However, the AC loss including the flux flow loss is shown to be describable quantitatively by present handy theoretical expression, which has the same form as the well-known expression based on Bean's critical state model except that Jc is replaced by the effective critical current density, Jce(ω).
The possibility of reducing AC losses in superconducting Bi-2223 tapes with the aid of the reversible motion of flux lines is investigated. It was found that the AC loss was reduced by reducing the size of superconducting filaments, and the behavior was explained by the theoretical model of Campbell. To get a sufficient reduction of AC losses at 77.3K from the prediction using the critical state model, it is necessary to reduce the filament thickness below 2.5μm.