Response of fusion plasma-facing materials to nanosecond pulses of extreme ultraviolet radiation

The experimental study of damage to tungsten (W), molybdenum (Mo), and silicon carbide (SiC) surfaces induced by focused extreme ultraviolet laser radiation (lambda similar to 47 nm/similar to 1.5 ns/21-40 mu J) is presented. It was found that W and Mo behaved similarly: during the first shot, the damaged area is covered by melted and re-solidified material, in which circular holes appear residua of just opened pores/bubbles, from which pressurized gas/vapors escaped. Next cracks and ruptures appear and the W has a tendency to delaminate its surface layer. Contrary, single-crystalline SiC has negligible porosity and sublimates; therefore, no escape of "pressurized" gas and no accompanying effects take place. Moreover, SiC at sublimating temperature decomposes to elements; therefore, the smooth crater morphology can be related to local laser energy density above ablation threshold. When more shots are accumulated, in all three investigated materials, the crater depth increases non-linearly with number of these shots. The surface morphology was investigated by an atomic force microscope, the surface structure was imaged by a scanning electron microscope (SEM), and the structure below the surface was visualized by SEM directed into a trench that is milled by focused ion beam. Additionally, structural changes in SiC were revealed by Raman spectroscopy.