AC loss in large-scale superconducting cables

The ac losses in superconducting composites have been a subject of study for a long time, but nevertheless several problems remain to be solved, especially with respect to large-scale cables where the geometry of the cable plays an important role. Some of the aspects that will be considered here, are: presence of fields parallel with the conductor, coupling loss in composite conductors having various regions of stabilizer and /or superconductor, interstrand coupling loss in cables, influence of the reinforcement jacket on the loss, combined effect of transport current and alternating magnetic fields in different directions. In order to calculate the overall loss in a complex superconducting coil system such as the NET fusion machine [1], the current scenario has to be divided in time steps during which /~ is almost constant. The magnetic field vectors B(t) and B(t + At) are calculated at regular positions in the coils and are decomposed in local components perpendicular and parallel with the cable. The key to this approach is to have suitable methods for determining the local energy loss from B(t), AB, At, the orientation of AB with respect to the cable and the geometry of the cable. Then, the losses can be integrated over time and over the volume of the coils. Tokamak fusion reactors are very interesting from the point of view of ac losses. The cabled conductors have a complicated geometry and they are subjected to alternating and rotating magnetic field components in transverse and longitudinal directions (especially in the toroidal coils). The conductors of the poloidal as well as the toroidal system should be able to withstand the large field rates (up to 50 T / s ) occurring during plasma disruptions, so the coupling loss has to be carefully considered. On the other hand, during regular operation of the fusion machine the fields rates are much lower (less than 1 T / s ) so the hysteresis loss becomes relatively important.