Analysis of tungsten melt-layer motion and splashing under tokamak conditions at TEXTOR

Abstract

Behaviour and characteristics of W plasma-facing components under impinging high heat fluxes are investigated in view of the material choices for the divertor in future devices such as ITER and DEMO. Experiments have been carried out in the plasma edge of the TEXTOR tokamak to study melt-layer motion, macroscopic tungsten erosion from the melt layer as well as the changes in material properties such as grain size and abundance of voids or bubbles. The parallel heat flux at the radial position of the plasma-facing components (PFCs) in the plasma ranges around q‖ ∼ 45 MW m−2 allowing samples to be exposed at an impact angle of 35° to 20–30 MW m−2. Melt-layer motion perpendicular to the magnetic field is observed following a Lorentz force originating from thermoelectric emission of the hot sample. Up to 3 g of molten W are redistributed forming mountain-like structures at the edge of the sample. The typical melt-layer thickness is 1–1.5 mm. Those hills are, due to the changes in the local geometry, particularly susceptible to even higher heat fluxes of up to the full q‖. Locally the temperature can reach up to 6000 K, high levels of evaporation are causing significant erosion in the form of continuous fine-spray (∼1 × 1024 atoms m−2 s−1). Strong evaporation cooling is observed hindering the further heating of the samples. In addition, the formation of ligaments and splashes occurs several times during the melt phase ejecting droplets in the order of several 10 µm up to 100 µm probably caused by an instability evolving in the melt. In terms of material degradation several aspects are considered: formation of leading edges by redistributed melt, bubble formation and recrystallization. Bubbles are occurring in sizes between 1 and 200 µm while recrystallization increases the grain size up to 1.5 mm. The power-handling capabilities are thus severely degraded. Melting of tungsten (W) in future devices is highly unfavourable and needs to be avoided especially in light of uncontrolled transients and possible unshaped PFCs