Fabrication of helical coils. Design of helical coil and on-site winding

A pair of helical coil of LHD is the largest superconducting coil system with the magnetic energy of which reaches 1.6 GJ. In order to produce fine magnetic surfaces, high accurate manufacture within 2 mm and small deformation less than 3.4 mm against electromagnetic force at 4 T operation are required. Besides, high current density over 40 A/mm2 is necessary to keep enough distance between the coil and the plasma vacuum vessel. The helical coil is designed to satisfy cryostable criterion by optimizing the wetted surface fraction of each conductor. Whole conductors of 36 km length has been wound on-site all around day for 16 months. We have successfully managed the errors of the position within ± 1.5 mm, and the average gaps between layers were kept within 65 μm. (author)