Performance of Japanese NbSn conductors for ITER toroidal field coils

Takahashi, Yoshikazu; Isono, Takaaki; Koizumi, Norikiyo; Nunoya, Yoshihiko; Matsui, Kunihiro; Nabara, Yoshihiro; Hemmi, Tsutomu; Oshikiri, Masayuki; Uno, Yasuhiro*; Okuno, Kiyoshi; Stepanov, B.*; Bruzzone, P.*

The ITER TF coil system consists of 18 D-shape coils. The operating current and the maximum field are 68 kA and 11.8 T, respectively. A NbSn cable-in-conduit conductor with a central channel is used, with a unit length of about 380 m. A cable consists of 900 NbSn strands and 522 Cu strands with a diameter of 0.82 mm. Superconducting performance of full-size conductors manufactured was measured at the operating condition of the TF coils with the maximum field. The strands made by bronze and internal-Sn methods were used for the sample conductors with a void fraction of 29% and 33%, respectively. The measured current sharing temperatures Tcs are 6.5-6.7K for the bronze route method and 5.7-5.9K for the internal-Sn method. The Tcs of the conductor with small void fraction is relatively higher with 0.1-0.2K than that with large void fraction. It is confirmed that Tcs of both strands is higher than the design value (5.7K). It is shown from the results that the strain on the conductor, estimated by the strand data, is about -0.7%. This value seems to be reasonable.