Thermal damage by erosion and cracking under pulsed irradiation.
Thermische Schädigung durch Erosion und Rißbildung unter kurzzeitiger Bestrahlung
Thermal damage involves a broad range of physical phenomena as thermally induced structural degradation, crack and propagation, and the ablation of material. Their complex superposition is analyzed for the case of first wall materials in fusion devices, especially carbon materials. In addition to the continuous heat flux from the plasma, the wall is attacked by power peaks from plasma disruptions. For optimization of wall materials, the energy spikes are simulated by pulsed irradiation with intense laser, electron or plasma beams. Careful analysis of experimental data of the pulse induced ablation and comparison with theoretical estimates show that the usual assumption of thermal erosion by evaporation is not adequate. It is suggested that thermal shock damage is a main cause of erosion: Contrary of the thermal quenching shock where transient tensile stresses appear, transient compressional stresses arise in the case of heating shock. They do not cause crack propagation immediately but give rise to creep resulting in tensile residual stresses after decay of the thermal peak. Numerical calculations show that the tension arising in this way can cause crack propagation. This supports the view that thermally induced crack propagation essentially contributes to erosion.