Coupling losses in cables for fusion application: influence of the strand

The coupling time constant of conductors for fusion magnets must be kept at a sufficiently low value to minimize the temperature increase related to the rapid field variations experienced by the pulsed coils during a run. Some parameters such as the twist pitches and the void fraction are well known to Influence the coupling time constant. Another parameter which can play a role is the strand itself through its electrical characteristics. Two identical conductors can have very different time constants and it is important to understand the strand parameters which govern this effect. To demonstrate that, several 36 strands subcables have been made with strands coming from different companies and associated to different manufacture processes (bronze Nb3Sn, internal tin Nb3Sn, NbTi). As far as possible the other parameters of these cables have been kept identical. Contact resistance and time constants have been measured and compared for the different samples.