In-situ measurement of surface modifications of tungsten exposed to pulsed high heat flux for divertor design in tokamak-type fusion nuclear reactors

Abstract A tungsten (W) divertor is exposed to pulsed high heat caused by disruptions and Type-I Edge Localized Modes (ELMs). Therefore, it is important to investigate surface morphology of W during phase change. Heretofore, W melting dynamics under disruption-like heat load has been researched and tried to modify the optical system of our apparatus for knowing more detailed surface behavior. In this paper, the two measurement methods are shown; the in-situ observation of the W surface modification by a High Speed Video (HSV) camera and 3-Dimensional (3D) measurement of melting W surface by stereoscopic photography technique (SPT). In the former, the detailed process of W melting was clarified by observing W surface modification and analyzing luminance at the center of laser-irradiated spot. It was confirmed that luminescence at the center area of heat load fluctuated violently due to the phase change of W. In the latter, 3D images of W surface shape during melting was obtained using SPT and the results indicated that the W surface fluctuated in melting and gathered to the center gradually in solidifying. The calibration accuracy was still scope for improvement but the 3D measurement of W surface behavior loaded by high heat flux was established successfully.