Testing for Compatibility of Reduced Activation Ferritic Steel with Plasma on JFT-2M Partial Coverage of the VacuumVessel with Ferritic Steel

The compatibility of reduced activation ferritic steel (F82H), which is a leading candidate material for the demo reactor (e.g. SSTR), with plasma has been investigated in the JFT-2M tokamak with 3 steps in an AMTEX (Advanced Material Tokamak EXperiment). In the first step, the reduction of fast ion losses was well demonstrated with the ferritic steel outside the vacuum vessel. In the second step, the ferritic steel was installed inside the vacuum vessel in order to perform a preliminary investigation of the effect of the ferro-magnetism on plasma stability and control, and impurity release. For this purpose, ferritic steels of 7 mm thickness were installed to form 2 sets of toroidally uniform belts, which cover 20% of the vacuum vessel. No deteriorative effects were observed regarding mode locking, plasma control, and impurity desorption. The initial boron coating was applied in order to modify the surface of the ferritic steel. The impurity is remarkably reduced and high normalized-beta plasma was obtained. Thus encouraging results were obtained for the third step, where whole vacuum vessel wall will be covered with ferritic steel.