Development of Nb₃Sn and NbTi CIC conductors for superconducting poloidal field coils of JT-60

The full-size samples of 20-kA Nb₃Sn and NbTi cable-in-conduit conductors for the superconducting poloidal field coils of the fusion experimental tokamak JT-60 were manufactured. Since these coils operate in pulse mode, reduction of ac losses is one of the key issues. For the Nb₃Sn conductor, in order to separate the strands sintered with Cr coat after heat treatment, bending strains were applied on it. As a result, ac loss was effectively reduced, and it was found that 0.2% bending strain is enough to realize the design value level of 60 ms. Its critical current was also measured, and it was estimated that the compression strain due to the difference between thermal contractions of the stranded wire and stainless steel jacket was -0.6 %. For the NbTi conductor, comparison between the ac losses of the SnAg- and Cr-coated conductors was carried out. The measured coupling time constant of the Cr-coated conductor satisfied the design requirement of 50 ms, however the SnAg-coated conductor was more than five times that of the Cr-coated one.