Design and construction of helical coils for LHD

Helical coils for LHD are pool-cooled superconducting coils. In order to produce a fine magnetic surface, highly accurate manufacturing tolerances within ±2 mm and high rigidity against large electro-magnetic forces are required. Additionally, high current densities of over 50 A mm−2 are necessary to keep enough distance between the helical coil and plasma. The helical coil is designed to enhance cryogenic stability by optimizing the wetted surface fraction of each conductor in considering both the magnetic field and stress in the insulator between conductors. For attaining highly accurate helical winding and cryogenic stability, composite conductors with pure aluminum stabilizers were developed and directly wound on highly accurate thick cases. The actual winding was carried out on-site from January 1995 to May 1996. After that, the top covers of the case were set on the coils and welded very carefully. The entire assembly was installed into an outer supporting shell.