Examination of Nb3Sn Conductors for ITER Central Solenoids

The performance of two Nb3Sn conductors for the ITER central solenoids was tested. The current sharing temperatures (T-cs) were measured over 17 050 electromagnetic cycles, including four thermal cycles between 4.2 K and room temperature. T-cs declined almost linearly over the 10 000 rated electromagnetic cycles. T-cs was nearly constant for 70% of the rated electromagnetic cycles, which implies the existence of a fatigue limit in the conductors. For 85% of the rated cycles, a very sharp T-cs degradation of approximately 0.2 K occurred. Some type of large deformation of strands, such as buckling, may have caused this sharp degradation. The effective strain degraded linearly with the electromagnetic force on the cable. The gradient after 10 000 cycles was 1.5 times greater than that before cycling. After 10 000 cycles, the ac losses of both conductors considerably decreased to less than half of those before cycling. These ac losses before cycling were less than a fourth of those of toroidal field conductors. After the test campaign, destructive inspection of the conductor clarified that on average, the distribution of residual strain along the cable was almost uniform at -32 ppm. It was also clarified that some strands were visibly deformed under a high magnetic field, whereas strands under a low magnetic field did not appear to be deformed. The deformations of the central solenoid cable were larger and wavier in subcables than those observed in the toroidal field cable. This plastic deformation of the strands could be one of the major reasons for the T-cs degradation during cyclic operation.