Critical heat flux loading experiments on CVD-W coating in the TEXTOR tokamak

A copper block coated with 0.2 mm layer of CVD-W was exposed to high heat fluxes in the edge plasma of the TEXTOR tokamak. The coating showed good performance in general, however, the coating was destroyed during the critical heat flux loading at an estimated peak power density of 28 MW/m2. This destruction process was first initiated by the cracking of the CVD-W layer due to the large difference in thermal expansion between W and Cu. Molten Cu emerged through the cracks leaving cavities beneath the CVD-W layer. The formed cavities reduced the heat conduction from the CVD-W layer to the Cu block, and the CVD-W layer was damaged further due to the overheating during plasma loads. To avoid this coating failure, it is necessary to keep the temperature at the interface reasonably low and to release thermal stresses by castellation and/or adopting a functionally graded interlayer.