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

Abstract

The low-frequency susceptibility of six Bi-2223/Ag superconducting tapes with the same 61 filaments but different cross-sectionalaspect 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 necessarysample-length corrections. The measured results are compared with two-dimensional calculationsperformed 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 0.55for the filament-containing core, the measured low-field agrees well with the calculated critical state when the core is assumed to be completely superconducting, when the aspect ratio of thecore ranges from 13 to 32. This indicates a strong magnetostatic coupling among thefilaments. The same coupling is theoretically shown to reduce the field where maximum occurs, . However, the experimental is much higher than that even without coupling when the transport critical currentdensity is used for the model calculation. This suggests that ac magnetization arises fromall superconducting grains and the links between them, whereas the transport current islimited by the filament cross-section where grains are the worst linked, and thegreat discrepancy between magnetic and transport results is a consequence of thenon-uniformity of the superconducting structure of the tape. Therefore, transport criticalcurrent can be significantly improved by ensuring the uniformity of the grain links.