Loss contributions in superconducting magnets caused by transient magnetic fields

The mechanical relaxation processes of the support structure at energizing the W7-X stellarator with cable-in-conduit superconductor have been analyzed, employing finite element calculations for one coil with adapted boundary conditions from a global static model. In this way, the duration and the amplitude of the field change at the conductor was determined. We calculate the effect of such transient on all relevant contributions to ac losses: hysteresis losses in filaments, coupling losses in different substructures of the cable, eddy current losses in normal parts (casing, jacket). Especially, the partial screening of the cable and of the individual filaments by the normal jacket is evaluated. Then, the results are applied to the W7-X system. We emphasize that the procedure can be used in other systems consisting of more magnets, or for the mechanical movement of individual parts of the winding. The possible consequences for the heat generation and the stability of the superconducting windings are given. In addition, we show that the procedure can be used to minimize the total losses in superconducting cables.