AC losses in multifilamentary Bi(2223) tapes

In this work the AC transport current losses, as well as the AC losses in external magnetic fields, in multifilamentary Bi(2223) tapes are investigated. For the measurements of the AC transport current losses a set-up was built to measure the losses in the frequency range from 10 Hz to 1000 Hz and for amplitudes from 50 mA to 50 A. The AC transport current losses show a linear frequency dependence due to hysteresis losses in the self field and no significant dependence on the twist length or the matrix material, as it was expected from the theory. Measurements at different 55 filament tapes with nearly the same critical current and filament geometry show different transport current losses which could be explained by different critical current densities in different shells of the filamentary region. The loss factors of tapes were found to not depend on the filament geometry, filament number, matrix material and the critical current. For tapes with only 7 filaments, or with only filaments in an outer ring, a small reduction of the loss factor was observed. This may be explained by a more homogeneous AC current distribution. AC loss measurements at untwisted tapes in external magnetic fields show a loss behaviour which could be described by the critical state model for coupled filaments. Twisted tapes could be described by the model of decoupled filaments, when measuring in parallel field direction. From the total losses the loss contributions from the hysteresis and coupling current losses were separated. The penetration fields are estimated from the hysteresis loss contribution. A comparison between these fields and calculated penetration fields gives significant differences which should be related to different critical current densities in the tapes. From the coupling current loss contribution the effective matrix resistivity was calculated