Optimization of CFETR CSMC cabling based on numerical modeling and experiments

The China Fusion Engineering Test Reactor (CFETR) is a new tokamak device, whose magnet system includes toroidal field (TF), central solenoid (CS) and poloidal field (PF) coils. The main goal is to build a fusion engineering tokamak reactor with 50–200 MW fusion power and self-sufficiency by blanket, which means that the deuterium–tritium reaction in the plasma produces neutrons and alpha particles, and the neutrons react with the lithium-containing blanket surrounding the plasma, breeding the tritium by lithium–neutron reaction. To develop the manufacturing technique for the full-size CS coil, the Central Solenoid Model Coil (CSMC) project for CFETR was launched first. A Nb3Sn conductor is to be used in the CFETR CSMC, whose design refers to the ITER CS conductor with the same short-twist-pitch cable pattern. Due to the short twist pitch and relatively low void fraction, a high compaction ratio is required during cabling and the risk of strand damage is increased significantly. Although it is impossible to avoid strand deformation for this design, it is crucial to find a way to reduce strand damage as much as possible. A numerical model was used to analyze the causes of strand damage, including variation in twist pitch length as well as different mechanical properties for copper and Nb3Sn strands. Several experiments have been performed to verify the numerical results, including cabling trials for different conditions and critical current (I c) tests on strands with/without deformation. The results show that the numerical analysis is consistent with the experiments and provides the optimal cabling conditions for the CFETR CSMC.