Low activation vacuum vessel for R-Tokamak

The vacuum vessel design of R-Tokamak, which is a moderate-size DT experimental device, is presented. To reduce the induced radioactivity due to DT neutrons, the vacuum vessel is made of aluminum alloy. Polyimide insulation is adopted to obtain high toroidal one-turn resistance, and double vacuum seal is applied using both polyimide and EP rubber O-rings to prevent tritium leakage. Electromagnetic forces, which act on the vacuum vessel, are due to (a) magnetization of the toroidal field at each discharge, (b) trip of the TF power supply and (c) plasma current disruption. In the cases (a) and (b) forces for the Al-alloy vacuum vessel become very large compared with those for AISI 304 SS, because the characteristic time in the magnetic field change is longer than the skin time of the vacuum vessel. In order to sustain the load of the case (a), which acts on the vacuum vessel as a hoop force, lots of insulated bolts are installed. In order to support the electromagnetic force due to plasma current disruption, flanges at the break section are fastened with insulated pins and insulated reamer bolts. The static, buckling and thermal stress analyses have been made with FEM code. The result of the static analysis shows that maximum stress intensities are lower than the allowable values for A1-alloys. It is found that the critical buckling mode is the deformation of the inner straight section of the torus with n=4 mode. The thermal stress is relatively low, because the maximum temperature is limited below about 1200C and the heat conductivity of Al-alloy is high