Observation of melting tungsten surface under disruption-like thermal load

In the plasma wall interaction, there is a concern about surface melting of tungsten(W) by transient thermal loads such as ELM(pulse width :0.1~1.0ms, heat fluxes :0.1~10GW/m²) and disruption(~several ms, 1.0～10GW/m²). If the molten layer becomes unstable, droplet ejection occurs, which generates high-Z dusts and accelerates erosions. W has a high melting point (3695K), and there is almost no situation other than nuclear fusion assuming melting. Therefore it is necessary to study molten layer behavior of W. In this study, surface stability conditions of melting W under the thermal loads equivalent to the reactor transient events have been investigated. We simulated disruption-like thermal loads using a Nd:YAG laser and evaluated the threshold over which droplets are generated with a square-shaped and a triangular-shaped pulsed heat fluxes. Since in actual disruptions, various shapes of pulsed influxes influence the surface stabilization condition, we have tried to correctly simulate these shapes by changing the pulse shapes. Surface melting of W, instability of molten layer, and ejection of droplets were observed by a high-speed camera and a long-distance microscope. These thresholds are summarized as a function of the average values of the pulsed heat flux and the time profile.