Completion of Helical Coils for LHD

Abstract

Helical coils of LHD are large scale pool-cooled superconducting coils. In order to produce a fine magnetic surface, the accuracy of the coil position is very important. The required tolerance for manufacturing is ± 2 mm. Against large electromagnetic forces, the conductors were designed to be packed into thick cases which are supported by an outer shell structure. In order to suppress the maximum tensile stress of the conductors below the yield stress, the average compressive Youngs’ modulus of the coils should be high. We developed insulator with a high compressive modulus, and we have managed the average gaps between layers within the aimed value while winding. After winding, the top cover of the coil case should be installed without leaving the residual gaps on the coils and without deforming the accurate shape. In order to reduce the assembling gap, we have machined adjusting spacers that were inserted between the coils and the top covers. After that, we tried to diminish the assembling gaps by utilizing the shrinkage by welding.