Development of Nb3Sn cable-in-conduit conductors with stainless steel jackets for central solenoid of JT-60SC

Development of Nb₃Sn cable-in-conduit (CIC) conductors with stainless steel (SS) jackets for the central solenoid (CS) and divertor coil of the superconducting tokamak called JT-60SC, is presented. Full-size and subsize conductors were manufactured using two types of high-copper-ratio and high-J_c strands made by different processes (bronze and internal-tin). Use of a SS jacket causes decrease of critical current of the conductor because compression strain due to difference of the thermal contraction between the SS jacket and Nb₃Sn cable is generated. The strains are estimated from the results of critical current measurements using short full-size samples which were pool-cooled. In order to investigate J_c-B-T characteristics of a CIC conductor, a novel apparatus, which can control the temperature and magnetic field, has been developed. This apparatus makes it possible to measure directly the critical current and temperature of a subsize conductor, which is composed of two superconducting strands, one copper wire and a SS jacket. Strains of the subsize conductors are estimated from the results of the J_c-B-T characteristic measurements. By comparison of the estimated strains between the full-size and subsize samples, we concluded that the characteristics of the full-size conductor can be predicted by measuring of J_c-B-T characteristics of subsize conductors. Finally, the strain of the actual conductor under the operational condition is estimated. The result confirms that the developed conductors are applicable to CS and divertor coil of JT-60SC.