抄録
A. C. superconducting machines such as superconducting generators, transformers or resistive current limiters need large current ca-pacity conductors. These conductors are generally fabricated as multi-strand cables with multifilamentaly NbTi superconductors whose current capacity is a few tens of Ampere. AC current degradation has been often observed in a. c. use of such multi-strand cables. Several reasons for this degradation have been pointed out; non-uniformity of strand currents, mechanical disturbances, thermomagnetic in-stability and a. c. losses. However, it has not been overcome yet.
Since the angle between the strand axis and the cable axis changes along the cable axis in multi-strand cables, the strands are exposed to a spatially distributed magnetic field, which has longitudinal and transverse components changing periodically due to multiple cabling. This paper discussed mainly the thermomagnetic instability due to the distributed transverse magnetic field, which was compared with the self-field instability and the longitudinal field instability experimentally and theoretically. It was confirmed that the a. c. current degradation due to the distributed transverse field could be induced for four reasons; non-linear E-j characteristic, strong dependence of E-j characteristic on magnetic field in low field region, uniformity of the axial current profile in the superconductor with the high resistive matrix and the poor thermal diffusion of CuNi/NbTi composites.
