Coupling Loss Time Constants in Full-Size Nb₃Sn CIC Model Conductors for Fusion Magnets

Abstract

The cable-in-conduit conductor for the ITER coils have to perform at magnetic fields up to 13 T under the conditions of normal high ramp rates as well as extreme magnetic pulses during a plasma disruption. Modelling, ac loss computations and design optimisations require to understand and identify the coupling loss time constants in multistage cables. For this AC loss measurements are performed on jacketed full size Nb₃Sn cable-in-conduit conductors. A transverse sinusoidal magnetic field is applied on the conductor to determine the coupling loss time constants with a calorimetric method. Moreover the decay of the coupling currents after a linear ramp is monitored with compensated pick-up coils. A comparison is made between the results obtained with both measuring methods. It appears that the n.τ value taken from the slope of the loss versus frequency curve in the low frequency limit has only a meaning at these low frequencies. At higher rates of magnetic field which are relevant to describe a plasma disruption, internal shielding effects are not negligible and a different approach has to be used. The experimental results and a straightforward model are presented to find the coupling current time constants of this type of conductors. It is shown that several dominant time constants can exist that are associated with relatively small volume fractions of a cable.