Development and quality control of the superconductors for the helical coils of LHD

A composite-type superconductor with NbTi/Cu compacted strands and aluminum/copper stabilizers has been developed for the pool-cooled helical coils of the Large Helical Device. The internal configuration of the conductor was determined from the viewpoint of high stability performance and fabrication reliability, by incorporating some newly developed techniques, such as a Cu–2%Ni-clad pure aluminum stabilizer and electron beam welding of the half-hard copper sheath. The conductor has been fabricated over the total length of 36 km under careful inspections both on the component materials and on the final full-conductors from the viewpoint of quality control. Short sample tests of the conductors showed that the measured critical currents and the recovery currents satisfied the specified values for the Phase I operation condition of LHD, although they showed some scattering around their mean values.