Effect of impurity content in stainless steel on resolidified surface condition after disruption load

Plasma disruptions in tokamaks may cause melting of the surface layer of first walls. If the layer resolidifies without any movement, the same layer repeatedly melts and protects the rest of the wall. A number of stainless steel samples were irradiated by a neutral beam injector. After irradiation tests with the same condition, some had wavy resolidification surfaces with dents, while some had smooth surfaces. In the former samples, the amplitude of the roughness increased by repetitive heat loads. The melt layer moved to the convex areas, which deepened the dents on each occasion. Sulphur was found to be one of the harmful elements which promotes the formation of the wavy surface. It is recommended to be kept below 0.005 wt% for obtaining a uniform resolidification surface. Even if samples have a low sulphur content, a high oxygen content also makes the surface rough. Other impurities have some effects, but they have not been clarified yet.