Impurity desorption behavior from low activation ferritic steel installed in the JFT-2M tokamak

Compatibility of a low activation ferritic steel, F82H, with plasma has been investigated in the JFT-2M tokamak with three stages. The tokamak experiments in the third stage have just started, where the vacuum vessel is fully covered with the ferritic steel. Prior to the tokamak experiment, vacuum properties of F82H were investigated using a test-stand. The outgas rate is less than 6×10−8 Pa m3/sm2 after baking at 120 °C for 10 days. As is predicted from the results, the base pressure of 6×10−6 Pa, which is the same as the previous level, was obtained in JFT-2M. In spite of the magnetic effect, the Taylor discharge was obtained without any difficulties and removal of oxygen was clearly observed. Tokamak discharges were obtained with the ferritic steel wall. The improved confinement mode (H-mode) was obtained with a neutral beam of 800 kW with a single-null divertor configuration. Within the preliminary results, the radiation loss did not increase due to the ferritic steel wall.