Degradation of Bi-2223 tape after cooling with superfluid helium

Future superconducting magnets for fields of 25 T and above have to be composed of LTS-HTS hybrid coil systems. To obtain a higher field contribution and for reasons of stability, the outer low temperature superconducting (LTS) magnet section is cooled particularly with superfluid helium. In the classical set-up, the high temperature superconducting (HTS) insert is assembled together with the LTS outsert in a common bath, i.e. in our case it is cooled with superfluid helium. Our first 5 T Bi-2223 prototype insert coil was successfully operated and produced 5.4 T in a background field of 11.5 T. After warming up, ballooning was observed in the tape apparently caused by the penetration of superfluid helium. In this paper we investigate the impact of superfluid helium on the superconducting properties of the Bi-2223 tape used for our HTS insert. In particular, the voltage-current relation, U(I), is examined. It is shown that the resulting critical current and the n-value, which is a differential variable, are not adequate to describe the widely degraded U (I) - curves. In addition, we suggest the use of an integral method. The measurement results and the interpretation of the U(I)-curves are presented and discussed.