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The Large Helical Device (LHD) is a heliotron/torsatron-type confinement device (B = 4 T, R = 3.9 m) equipped with a helical divertor. In the LHD configuration, the plasma region is shifted inwards approximately a third of the plasma minor radius relative to the center of the two pairing helical coils, thus making the distance between the coil center and the edge plasma small, 328 mm on the small major radius side of the torus. Within this space, many components must be installed, such as the superconducting helical coil (163), its coil can (45), the thermal shield (30), the vacuum gap (35), the plasma vacuum vessel (15), the first wall (25) (the numbers in the parentheses are allocated radial space in mm for the respective components). Effective edge plasma control by the divertor requires a space of more than 15 mm between the plasma and the first wall. To meet the above space requirement, the thickness of the helical coil is designed to be as small as possible, yet the coil current density and maximum field strength are within the limits of reliable coil operation.

The Large Helical Device (LHD) is designed to have six pairs of superconducting coils. In the first phase of the project, these will be energized by six steady-power supplies. These power supplies are thyristor types, which generate positive and negative voltages and large currents (up to 30 kA). Each of them has a smoothing filter which reduces the voltage ripple to less than 0.2 V, and a protection circuit to transfer rectifier current to a dump resistor and drop it rapidly during coil quenches. These six steady power supplies are powered from the commercial line. In the second phase of the project, three more power supplies powered by a motor generator (120 MVA) or superconducting energy storage will be added to the poloidal coil circuits. These will be used in experiments which require rapid changes in the poloidal coil currents during a plasma discharge of 5 s. A protection circuit is now under development, composed of disconnectors, a vacuum circuit breaker, a power fuse and diodes. It has worked well up to 21 kA, and will be tested up to 30 kA.

A vacuum system for the LHD cryostat has been preliminary designed to guarantee reasonable operation of the system. The required working pressure for the cryostat had to be determined from the heat transfer. In order to complete the engineering design of the system, it is urgent to generate the outgassing data of the thermal insulation materials in the cryostat at low temperatures and establish a realistic operational scenario including conditioning and maintenance phases.

The Large Helical Device (LHD) will be operated under various loads, such as strong electromagnetic forces, atmospheric pressure, and thermal stress. The outer vessel of the cryostat has a toroidal shape and is the largest component of the LHD. The designed maximum pressure of the vessel is 0.1 MPa inward and 0.01 MPa outward. Since the cross section of the outer vessel is not circular, the presence of unbalanced atmospheric pressure requires carefully designed studies of the rigidity of the cryostat. The poloidal cross section of the vessel is designed as a bell-shape, of which the bottom is a flat plate of 150 mm and the top has a major radius of 4 m, the minor radius is 2 m, with a thickness of 50 mm, by the finite element method using ANSYS. In this case, the maximum deformation appears at the center of the bottom flat plate; its value is 1.7 mm when stainless steel is used. The poloidal cross section of the outer vessel of the cryostat is desirable to have a symmetric circular shape which increases the mechanical stiffness and reduces the unbalanced magnetic field due to the eddy current during experiments. However, from an engineering point of view, the bottom is required to have the shape of a flat plate to have working space for inner components. We analyzed the stress field and deformation of the outer vessel under atmospheric pressure by using a finite element analysis code to pursue the optimized shape and thickness of the body.

A new protection circuit for the superconducting coils in the Large Helical Device has been developed. The circuit transfers a high dc current from a power supply to a dump resistor at a coil quench. It is composed of a conventional ac vacuum circuit breaker, disconnectors, power fuse(s), diodes and a dump resistor. Its features include excellent cost performance and reliability and handling of currents up to 32kA in DC.