Development of high Cu ratio Nb3Al and Nb3Sn CICCs for superconducting toroidal field coils of JT-60

Two types of cable-in-conduit conductors (CICCs) for the JT-60 toroidal field coil (TFC) have been developed. The operational conditions of this conductor are 7.4 T in a maximum magnetic field and 19.4 kA in operational current. Each conductor consists of a stainless steel conduit (17 mm in inner diameter) containing 216 high performance superconducting strand (Nb₃Al or Nb₃Sn) and 108 copper wires. The diameter of strand is 0.74 mm including 2-μm Cr plating. In order to attain a compact and stable coil, strands with high Cu/non-Cu ratio and high critical current density (Jc) were developed. The Cu/non-Cu ratio of Nb₃Al and Nb₃Sn strands are 4.1 and 3.6, respectively. The noncopper J_c of developed Nb₃Al and Nb₃Sn strands were 1914 A/mm² and 1843 A/mm² at 7.4 T, 4.2 K, respectively. Measurements of critical current (I_c) of full size conductors showed that the I_c of Nb₃Al and Nb₃Sn CICCs were 98% and 79% of I_c estimated from I_c in strands. These values exceed the target values indicating that the developed conductors are quite appropriate for TFC of JT-60.