Critical currents through strong links in Ag/Bi-Sr-Ca-Cu-O superconducting tapes

Transport critical currents were measured on Ag-sheathed Bi-based superconducting tapes at 77 K, with the field H parallel and perpendicular to the tape plane. A transition field from flux creep to flux flow regime of about 350 mT was obtained from the change of the curvature of the V-I characteristics. The critical current conducted via strong links (I-c(s)) was extracted from the total I-c. An empirical expression for I-c(s) versus H in the flux creep regime was found: I-c(s) = I(c0)(s)exp[-(H/H-0)(a+1)], with 0 less than or equal to a less than or equal to 1. log(I-c) versus H has a downward (negative) curvature in this regime, which is often mistakenly ascribed to the transition to flux flow. This curvature is defined by the values of a and H-0. In the flux flow regime, I-c decreases with positive curvature. Using the measured values of I-c0(s), we estimate J(c) would increase 2-3 orders of magnitude by turning all the weak links into strong ones. This shows there is still a great capacity for improvement of J(c) by still bettering the links between the grains.