Damage and melting of ITER-like flat-type tungsten castellated blocks exposed to long pulse H-mode plasmas

Damage and melting of ITER-like tungsten castellated blocks made of tungsten top layer, copper inter-layer, copper alloy base (CuCrZr) - exposed to long pulse H-mode plasmas were investigated. Since the blocks were not actively cooled, the temperature of blocks was increased rapidly up to melting point of tungsten during H-mode plasmas resulting in severe damage such as cracks and melting of tungsten layer. It is observed that the copper inter-layer located at 5 mm below the surface of the tungsten was also melted indicating that the temperature of the interface had reached the melting point of copper, at least 1200 degrees C. Melted tungsten material was moved towards j x B direction (high filed side). The melting event was reproduced by 3D transient modeling using COMSOL with 2 MW/m(2) heat load and explains the observation quantitatively well. This indicates that failure of cooling system during a long pulse operation with metal walls, especially melting of copper inter-layer will be critical.