Observation of surface deformation of tungsten exposed to single pulsed high heat flux and magnetic field for divertor design

Abstract

Tungsten (W) is one of candidate surface materials for a plasma facing component (PFC) in a magnetically confinement fusion reactor such as a divertor from the perspective of its high melting point, high thermal conductivity, low tritium retention and low sputtering yield. In the fusion reactor in operating, the divertor surface is exposed by the transient heat load generated by plasma disruption or ELM. The heat load is sufficiently high to melt the W surface. Because of the repetition of melting and solidification, it is concerned that W surface deformation occurs, the droplets scatter and its heat resistance degrades. These are critical issues for developing the fusion reactor. In previous studies, the in-situ observations of the W specimen surface during irradiating by a pulsed laser simulating the heat load were conducted using a HSV camera. However, PFCs lies under intense magnetic field in the actual fusion-reactor condition. Magnetic field affects the motion of fluid by the effect of generated Lorentz force. Therefore, this study aims to clarify the melting and solidification behavior of W under the intense magnetic field. In order to simulate the heat load, the Nd/YAG laser, which has almost flatten spatial profile and rectangular temporal profile, was irradiated on a W specimen. The power density of the laser was 3.12 GW/m2 and 3.9 GW/m2 and the pulse width was 3 [ms]. Helmholtz-like electromagnetic coils were applied for generating pulsed magnetic field. The magnetic density was also changed in the range from 0 to 4 [T] in this study. From observation of the light emitted from the heated W surface using a high-speed video camera and measurement of irradiation traces, the behavior of molten W and the effect of the magnetic field will be reported and discussed.