Determination of the thermal shock limits of various non metallic materials using a laser beam of variable power density and pulse length

For the selection of materials for plasma facing components (PFC) in future nuclear fusion devices subjected to short-time high heat fluxes it is desireable to determine the threshold values of the occuring damages in laboratory experiments. In order to determine the response of candidate materials to thermal shock-loading different nonmetallic materials were exposed to the beam of a 10 kW NdYAG solid state laser situated in the Institute fuer Lasertechnik under the Fraunhofer Gesellschaft in Aachen. The impinging power density was varied between 200 and 2000 MW/m2 for pulse lengths tp ranging from 0.1 to 10 ms. Similar conditions are expected to occur under plasma disruptions in Tokamak fusion devices. The absorbed power density Φabs was determined by measuring reflexion losses from the sample surfaces. The main damages on nitride and carbide ceramics were cracking and chemical decomposition. Sublimation was the main structural change observed on all carbon materials though cracking could be detected in pyrolytic grafite and melting in a coatmix material. Two coated materials showed cracking and melting of the coating layer. The threshold values for these structural damages were quantified by using a heat flux parameter Φabs√tp. (orig.)