Development and Fabrication of Superconducting Helical Coils for LHD

Development and Fabrication of Helical Coils for LHD

Abstract

A pair of helical coils for LHD, the largest pool-cooled superconducting coil system with a magnetic energy of 1.6GJ. In order to produce a fine magnetic surface, highly accurate manufacture within ±2mm and small deformation below 3.4mm against electromagnetic forces at 4T operation are required. Besides high current density, more than 53A/mm² is necessary to keep sufficient distance between the coil and the plasma vacuum vessel. The helical coils are designed to satisfy the cryostable criterion by optimizing the wetted surface fraction of each conductor. Against large electromagnetic forces, the conductors were packed into thick cases supported by an outer shell structure. In order to suppress the maximum tensile stress in the conductors below the yield stress, the average compressive Young's modulus of the coils should be high. We used an insulator with a high compressive modulus of 30GPa, and we planned to maintain the average gap between layers within 65μm while winding. Whole conductors 36km long had been wound on-site by day-and nighttime work over 16 months. We have successfully managed average gap and position error within 50μm and ±1.5mm, respectively. After winding, the top cover of the coil cases were set up on the coils. By inserting machined spacers and controlling the shrinkage of welding, the top covers were successfully installed without any additional gaps and without deforming the accurate shape.