INFLUENCE OF LONGITUDINAL MAGNETIC-FIELD ON THERMOMAGNETIC INSTABILITIES IN AC SUPERCONDUCTING CABLES

As the critical current of a single a.c. superconducting wire cannot b e more than several tens of amperes, such single wires (strands) are b undled and twisted to make cables with a large current capacity. As th e result of the twisting and bundling, a longitudinal magnetic field p arallel to a strand axis is produced in the cables and windings by the transport current. It is shown theoretically and experimentally in th is paper that the a.c. longitudinal magnetic field influences the curr ent distribution in multifilamentary superconductors with an a.c. tran sport current and can induce thermomagnetic instabilities in them. A l ongitudinal magnetic field of the order of 10(-1) T is easily produced in these cables. It is shown that thermomagnetic instabilities induce d by the longitudinal magnetic field in the cables cause a.c. quench c urrent degradation in multiply-twisted a.c. superconducting cables. A way of twisting to decrease these instabilities is also proposed.