Study on melting and solidification behaviors of tungsten loaded by high heat flux for divertor in tokamak fusion reactor

Tungsten is a candidate as a surface material of divertor in Tokamak fusion reactor. A pulsed high heat flux emitted in plasma disruption and Type I Edge Localized Mode (ELM) is considered to melt the tungsten surface repeatedly. The repetitive phase change can transmute surface characteristics of divertor and then reduce the thermal resistance. For the safety design and the development acceleration of fusion demonstration reactor, investigation and understanding of surface melting and solidification procedure are important In previous studies, experiments have been conducted by Ueda et al. and the surface after the laser irradiation had the geometry as a crater with a center cone. However, the formation mechanism of the crater was not fully understood. In this paper, two measurement methods are studied. One is an in-situ observation method of tungsten and molybdenum surface during the laser irradiation using high speed video camera, and another is a 3-dimensional measurement of tungsten surface shape after the laser irradiation experiments using a stereoscopic photography technique. In the former, it was confirmed that a molten pool had wavy surface pattern until solidification, and the applicability of the stereoscopic photography technique for the in-situ observation during the laser irradiation was indicated in the latter.