Melt-layer ejection and material changes of three different tungsten materials under high heat-flux conditions in the tokamak edge plasma of TEXTOR

The behaviour of tungsten plasma-facing components has been investigated in the plasma edge of the TEXTOR tokamak to study melt-layer ejection, macroscopic tungsten erosion from the melt layer as well as the changes of material properties such as grain-size and abundance of voids or bubbles. The parallel heat flux at the radial position of the exposed tungsten tile in the plasma ranges around 45MWm(-2) causing samples to be exposed at an impact angle of 35° to 20-30MWm(-2). Locally the temperature reached up to 6000 K, high levels of evaporation and boiling are causing significant erosion in the form of continuous fine-spray or droplet ejection. The amount of fine-spray tungsten emission depends strongly on the material properties (in case of W-Ta the effect of spraying and droplet emission is significantly higher at lower temperatures as compared with regular W or UHP-W). Differences in the material composition, grain structure and size may be related to the different evolution of macroscopic erosion. In addition the re-solidified material is studied and strong differences in terms of recrystallized grain size and evolution of the grain structure and orientation are observed. The build up of large voids has been observed.