Development and analyses of high heat flux components for divertors of future fusion devices

In order to test the plasma wall interaction in a stellarator machine, the project Wendelstein7-X is in the beginning of the building up phase. As thermally highest loaded part of the machine, the target plates of the of the divertor will be exposed to maximum heat loads of 10 MW/m2 up to 1000 s. Due to safety reasons the target elements should sustain heat loads up to 12 MW/m2 and short time failure loads of 15 MW/m2 without damage. The position of the cryopumps behind the target plates will lead to special elements including a cooling flow reversal element. Under this boundary conditions an alternative concept of target elements has been developed, analyzed and tested. Thermomechanical analyzes at 10 MW/m2 using the method of Finite Elements are performed to select the optimum cross-section geometry and a proper material combination. Due to the lowest surface temperature of 830 C, the lowest peaking factor and the lowest residual stresses in the carbon fiber-reinforced carbon (CFC-material), the monoblock concept using the CFC material DMS704 or N11 brazed onto two parallel copper tubes of 26 mm distance is chosen as best solution for the realization of the target elements. The bonding of the copper tubes with the CFC will be done by active brazing with a partial liquid phase. The liquid brase phase is formed by the interaction of a deposited thin layer of titanium with the surface of the copper tubes. Heat load experiments in the electron beam facility TSEFEY and the ion beam facility MARION, nondestructive and metallographic inspections show that the main reason for defects in the braze interface are due to a lack of liquid phase. In order to avoid these defects, the braze gap at room temperature must be adjusted as small as possible. In addition a large amount of braze depots are placed on the rear side of the target elements to overcome the problem of manufacturing tolerances. One target element, manufactured under these conditions, is successfully tested up to power densities of 10.5 MW/m2, using a coolant flow velocity of 10 m/s. At higher power densities a failure of the cooling is observed and large cracks occur in the CFC material parallel to the fiber planes. (orig.)