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 ≤ a ≤ 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.