DESIGN STATUS OF SUPERCONDUCTING LARGE HELICAL DEVICE

Large Helical Device (LHD) is a superconducting heliotron/torsatron type device. The SC coil system is composed of l = 2 helical coils and 3 sets of poloidal coils with a total stored magnetic energy of 1.63 GJ. The main machine parameters, 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 our LHD program, and there is no danger from the major current disruptions. The material of the super-conductor 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 the maximum experienced magnetic field strength of 9.6 T, the refrigeration with the super-fluid helium is required. LHD has a divertor to control the steady particle recycling and to improve the confinement potentiality. The vacuum vessel has a dumbbell shaped poloidal cross-section making it possible to install the closed divertor chamber. The necessary R&D programs and detailed design are now in progress, and we start the construction of LHD from the next year. The construction of LHD will be completed in 1997.