The perpendicular low-frequency susceptibility of Bi-2223/Ag tapes

The low-frequency susceptibility χ=χ'-jχ'' of six Bi-2223/Ag superconducting tapes with the same 61 filaments but different cross-sectional aspect ratios is measured at 77 K as a function of the amplitude of the perpendicularly applied field, Hm, using a carefully designed and calibrated high-field ac susceptometer with necessary sample-length corrections. The measured results are compared with two-dimensional calculations performed based on the critical state model with constant critical current density or the -1 internal susceptibility at low fields. It is shown that although the fill fraction is about 0.55 for the filament-containing core, the measured low-field χ' agrees well with the calculated critical state χ' when the core is assumed to be completely superconducting, when the aspect ratio of the core ranges from 13 to 32. This indicates a strong magnetostatic coupling among the filaments. The same coupling is theoretically shown to reduce the field where maximum χ'' occurs, Hm(χm'). However, the experimental Hm(χm') is much higher than that even without coupling when the transport critical current density is used for the model calculation. This suggests that ac magnetization arises from all superconducting grains and the links between them, whereas the transport current is limited by the filament cross-section where grains are the worst linked, and the great discrepancy between magnetic and transport results is a consequence of the non-uniformity of the superconducting structure of the tape. Therefore, transport critical current can be significantly improved by ensuring the uniformity of the grain links