Evaluation of solders for superconducting magnetic shield

A magnetic shield with a superconducting winding has been studied aiming at the practical application of the shield in a magnetic field of higher than 3 T. Tubular magnetic shields have been fabricated with superconducting windings using NbTi multifilamentary composite wires. The shields were impregnated with two kinds of solders (Wood's metal and indium) to make the electric joint between the wires. In spite of the fact that the superconducting wire provides enough performance to shield the external field of 4 T (I/sub c/=225 A at 4 T), the maximum shielded field of the actual shield was 3.42 T. The superconducting magnetic shield inevitably induces a high-field region near the surface of the shield, and hence the performance of the solder under high field determines the shielding ability. After testing several alloy mixtures to see which provides improved joint properties (joint resistance and therefore I/sub c/), a high-performance shield was fabricated.