AbstractThe 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.