Design and structural analysis of the JT-60 upgrade vacuum vessel

The tokamak, JT-60, was upgraded to double the plasma current to 6 MA by replacing the vacuum vessel and poloidal field coils with those of large D-shaped cross-sections. The vessel is unique since it employs a three-dimensionally curved double-skin construction. The vessel, as well as the poloidal field coil, are nestled very closely to the original toroidal field coil, and minimization of the wali thickness to below 40 mm is inherent in order to ensure that the plasma current exceeds 6 MA. In order to design a thin but strong vessel wali, detailed structural analyses were conducted, including dynamie effects, and a multi-arc-shaped vessel of double-skin construction with a midplane support was found to be the best and was adopted. The poloidal field coils support structures were used for the vessel support. Three-dimensional shell analysis results, including effects from the support condition, are described. Local stresses near the base of ports and support rods were highest, and were confirmed within the limit by three-dimensional shell analysis. A simple analysis of effects from nonrigid support structures is also described.