Tungsten melt layer motion and splashing on castellated tungsten surfaces at the tokamak TEXTOR

Dedicated melt experiments of castellated tungsten (W) surfaces under high heat-loads were carried out in TEXTOR. Several 2 mm thick tungsten plates were used in single and multiple exposure to steady state heat loads of ∼20 MW/m2 up to 70 MW/m2. The evolution of the melt layer, its motion under tokamak conditions as well as melt ejection are shown. The motion is dominated by the Lorentz-force, driven via the thermionic emission current of the heated W-plate and the toroidal magnetic field while plasma pressure can be neglected under TEXTOR conditions. In theses experiments no significant material losses are observed (<100 μg), while melt layer ejection in form of μm sized droplets is evident. The castellated structure can hinder the motion of thin melt while thick layers and multiple exposures lead to filling of gaps. Power-handling capabilities are significantly influenced by the changes in surface topology.