Design status of superconducting Large Helical Device

The Large Helical Device (LHD) is a superconducting heliotron/torsatron device. The SC coil system is composed of l=2 helical coils and three sets of poloidal coils with a total stored magnetic energy of 1.63 GJ. The m-number, l-number, major radius, coil minor radius magnetic field, plasma minor radius, and plasma volume are 10, 2, 3.9 m, 0.975 m, 4 T, 0.65 m, and 30 m³, respectively. This is an alternative toroidal device which aims at producing plasmas extrapolatable to the reactor regime. The currentless steady operation is the final goal of the LHD program, and there is no danger from the major current disruptions. The material of the superconductor is NbTi, and the cooling systems are pool boiling for helical coils and forced flow for poloidal coils. Since the current density of the helical coils is as high as 53.3 A/mm² with a maximum experienced magnetic field strength of 9.6 T, refrigeration with superfluid helium is required. The LHD has a divertor to control the steady particle recycling and to improve the confinement potential. The vacuum vessel has a dumbbell-shaped poloidal cross section, which makes it possible to install the closed divertor chamber. The necessary R&D programs and detailed design are described