Reduction of the a.c. losses in Ag sheathed PbBi2223 tapes with twisted filaments

Abstract

Measurements of the total a.c. losses in an applied parallel a.c. magnetic field have been measured for Bi2223/Ag tapes with twisted and untwisted filaments. The measurements were undertaken using suitable pickup loops and a lock-in amplifier. For all frequencies (37–200 Hz) and field amplitudes used (<45 mT) the tape with untwisted filaments behaved as if its filaments were fully coupled. For this situation, the total losses were accurately described using the Bean model for a 1D slab (assuming the tape acts as a monoblock). In contrast, the losses for the tape with twisted filaments (10-mm pitch) were considerably less than for the tape with untwisted filaments over the entire frequency and field amplitude range. The loss behaviour indicated that the filaments were partially decoupled, and the total losses in this tape were best described by adding the filament hysteresis losses to the coupling current losses. The filament losses were calculated using the Bean model for a slab with dimensions of an individual filament and the coupling current losses predicted from existing theory for a tape geometry in parallel field.