THERMAL-SHOCK BEHAVIOR OF VARIOUS FIRST-WALL MATERIALS UNDER SIMULATION LOAD TESTS BY LASER-BEAM IRRADIATION

For materials selection of plasma-facing components in nuclear fusion devices it is necessary to determine threshold values of materials damage under short-time high heat fluxes in laboratory experiments. Plasma disruption loads were simulated with different nonmetallic candidate materials using Nd-YAG high power solid state lasers. The impinging energy density was varied between 0.2 and 20 MJ/m(2) with pulse lengths t(p) ranging from 0.1 to 10 ms. Special experiments were carried out in situ in a scanning electron microscope coupled with a pulsed Nd-YAG laser and various analytical equipments. Thermal shock crack formation and propagation response, erosion behaviour and distribution of elements after successive thermal shock loading can be studied in situ with high lateral resolution inside a SEM. The dependence of damage initiation and propagation on the laser beam parameters such as pulse energy and number, spot size and pulse length was studied for fine grain graphites, nitride and carbide ceramics, CFC compound and coated materials. The threshold values for these structural damages were quantified using a heat flux parameter phi(abs)root t(p). Detected damage parameters are 3D-erosion profiles, length and orientation distribution of cracks, redeposition rate, etc. Threshold values received by electron beam loads and by numerical calculations under comparable conditions are in approximate agreement with the laser load experimental results. It can be concluded that laser beam simulation represents a simple method for thermal shock testing.