Power Handling of the Tungsten Divertor in JET
With incident power densities of 5-10MW/m2 or higher, divertor target plates of large tokamaks are driven close to the material limits. The power handling performance is essential to their characterisation. The thermal performance of tungsten is critical to the decision as to whether to start ITER with an all tungsten divertor. In the new ITER-like Wall in JET, the divertor tiles consist of carbon-fibre composites coated with tungsten except for a specific row within the range of the outer strike point (horizontal target) where bulk tungsten is used. The multilayer coating (Mo/W) is 12­25µm thick depending on the poloidal position. Ion beam tests have shown that the risk of delamination is low (<1% area) as long as the coating is maintained below 1200°C and carbidisation is negligible. The tiles are coated by combined magnetron sputtering and ion implantation (CMSII). The bulk tungsten row consists of 48 tile assemblies. Each one is made of two sets of four stacks of 24 lamellae. The stacks are aligned with the toroidal direction. To minimise the risk of fracture, the brittle tungsten is subjected by the clamping to compression forces only. The surface temperature was initially limited to 1200°C to avoid appreciable grain growth and possible thermo-mechanical fatigue (no higher temperature was recorded, by the infrared systems up to now: Tsurf<1200°C). Densities of deposited energy in the order of 20­30 MJ/m2 were reached so far. The modelled temperature rise of the plasma-facing tungsten components shows a fair agreement with the records of thermocouples and of infrared cameras. The experimental behaviour is close to the design values in a wide range of operational parameters. Special protruding lamellae were designed to expose leading edges with a vertical step 0.3mm with the aim of deliberate melting. These future investigations are intended to support ITER since melting is the highest risk for a solid tungsten divertor.