Manufacturing and high heat flux loading of tungsten coatings on fine grain graphite for the ASDEX-Upgrade divertor

Fine grain graphite tiles coated with tungsten layers by plasma spray (PS, thickness 100-550 μm) and physical vapour deposition (PVD, 30-200 μm), respectively, were subjected to thermal loads up to 17 MW/m2 and 2 s pulse duration. The damage limit was evaluated by increasing the heat flux and the pulse length stepwise. The results proved that PS coatings are capable of withstanding heat loads up to 15 MW/m2 at 2 s pulse length without any structural changes, and cyclic loading with 1000 cycles at 10 MW/m2. The highly dense PVD coatings suffered damage by crack formation at slightly lower heat loads, and thin PVD layers failed under cyclic loading with 1000 cycles at 10 MW/m2 due to thermal fatigue and melting. The good performance of PS coatings is related to their porosity, which provides a crack arresting mechanism, and to their mechanical strength, depending on the density of the PS layer. (orig.)