Losses in cable-in-conduit superconductors used for the poloidal coil system of the large helical device

The authors discuss the loss features of the NbTi cable-in-conduit (CIC) superconductor for the Large Helical Device (LHD) poloidal coil system. Eight conductors were prepared. The conductors differ in the matrix or in the surface condition of the strands. The interstrand coupling losses tend to increase during the pulsive operation of the poloidal coil. Short and straight samples of about 500 mm in length were provided for this experiment since bending may change the interstrand contact condition. The authors measured the loss-frequency characteristics of the short sample conductors with various types of strands to clarify their loss properties. The measurement was carried out in the transverse AC ripple field superposed on bias fields, in the frequency range from 0.1 Hz to 200 Hz. The measurements showed that the replacement of the CuNi matrix strands in the conductor by the Cu-matrix strands did not increase the intrinsic coupling loss significantly, but that noninsulation of the strands caused considerably higher interstrand coupling losses.<>