Critical persistent current for a loop formed by a Bi-2223 Ag-sheathed superconducting tape

A persistent current loop formed by a Bi-2223 Ag-sheathed superconducting tape has been fabricated by tape joint technique. The current decay measurement was used to determine the persistent current of the loop at 77 K. A dc current exceeding the critical current I_c of the loop defined by 1 μV/cm criterion was transmitted into the loop. The persistent current is found to about 25% of its I_c value of the tape. Based on detailed measurements of the voltage-current curves, the low level resistances down to very low voltage (<1 nanovolt) at the jointed and unjointed parts were tested, respectively. The jointed section has a larger I_c value of 1 μV/cm criterion than the unjointed one. However, it is found that the persistent current of this test loop is limited by the jointed section, because the residual resistance of the jointed section is larger. These results illustrate that in the jointed section the flux motion is more serious and the weaker grain boundaries or jointing interfaces limits the persistent transport currents