Strain analysis by neutron diffraction on Nb3Sn strands in ITER central solenoid conductors of short and long twist pitch

The ITER central solenoid (CS) conductors are composed of Nb3Sn superconducting cables and stainless steel jackets. Approximately 60 000 electromagnetic loading cycles will be applied to the Nb3Sn strands in the CS conductor over the course of ITER tokamak operation, and the CS conductor is required to maintain the current sharing temperature (Tcs) for these electromagnetic loading cycles. However, in CS conductor prototypes, degradation of Tcs was observed after electromagnetic loading cycles. Visual inspections of the tested CS conductors revealed large bending and buckling of the Nb3Sn strands. These strand deformations were considered to be the cause of Tcs degradation because even a small amount of strain markedly affects the critical current of Nb3Sn strands. To prevent the strands from bending and buckling, the twist pitch of the cable was shortened to improve stiffness. The stiffer cables did prevent Tcs from degrading after electromagnetic loading cycles owing to the shortened twist pitch of the conductors, but a slight increase of Tcs was observed. Visual inspections revealed no significant bent or buckled strands, but small deformations cannot be investigated visually. Thus, internal strain was measured by neutron diffraction and the internal strain states of the prototype CS conductors having long twist pitches and the improved short-twist-pitch (STP) CS conductors were evaluated. The results indicated that, after electromagnetic loading cycles, bending of the strands in the STP CS conductor was limited and compressive strain was reduced. Therefore, we determined that this STP is not only effective to prevent degradation caused by bent strands but it also increases Tcs by reducing compressive strain.