Influence of longitudinal magnetic field on thermomagnetic instabilities in a.c. superconducting cables

Abstract

As the critical current of a single a.c. superconducting wire cannot be more than several tens of amperes, such single wires (strands) are bundled and twisted to make cables with a large current capacity. As the result of the twisting and bundling, a longitudinal magnetic field parallel to a strand axis is produced in the cables and windings by the transport current. It is shown theoretically and experimentally in this paper that the a.c. longitudinal magnetic field influences the current distribution in multifilamentary superconductors with an a.c. transport current and can induce thermomagnetic instabilities in them. A longitudinal magnetic field of the order of 10 −1 T is easily produced in these cables. It is shown that thermomagnetic instabilities induced by the longitudinal magnetic field in the cables cause a.c. quench current degradation in multiply-twisted a.c. superconducting cables. A way of twisting to decrease these instabilities is also proposed.