Design of a $\hbox {Nb}_{3}\hbox {Sn}$ Cable-in-Conduit Conductor to Withstand the 60 000 Electromagnetic Cycles of the ITER Central Solenoid

The performance of the ITER Nb₃Sn cable-in-conduit conductor tested in Sultan for the toroidal field (TF) coils have shown some limited degradation with cycling, which is consistent with the conductor performance requirement, as the TF coils are designed for an upper limit of 1000 electromagnetic cycles. The first samples of the central solenoid (CS) Nb3Sn conductor tested in Sultan have also shown some degradation with electromagnetic and thermal cycling. However, this is not considered appropriate as the CS conductor has to sustain 60 000 electromagnetic cycles during the lifetime of the tokamak. A sample of a CS conductor based on a locked cable configuration including segregated copper strands and designed to achieve saturation versus cycling has been fabricated and tested successfully in the Sultan test facility. Additional samples of CS conductor fabricated on the same principle have confirmed the observed very low sensitivity to cycling. The Sultan test results show that a locked cable configuration achieved by a short twist pitch cable pattern can cure the problem of degradation versus cycling for both types of strand, internal Sn and bronze route.