Resuts of interaction of XUV laser pulses of nanosecond duration with difficult-ablated-materials

It is well known that each photon of extreme ultraviolet (XUV) radiation carries energy higher than 20 eV, what is more than any binding energy in a solid state. Provided that energy of these photons is deposited in some localized volume (for example in a surface layer — which is true for XUV and soft X-rays) a non-thermal melting can appear1. This contribution presents experimental results of interaction of focused pulsed XUV laser (λ ∼ 47 nm/ ∼1.5 ns/150–350 μJ) radiation with tungsten (W), molybdenum (Mo), and silicon carbide (SiC) — three materials considered as perspective armour for plasma facing components in future thermonuclear reactors. It was found that W and Mo behave similarly: during the first shot the laser footprint is covered by melted and re-solidified material, in which circular holes appear — residua of just opened pores, from which explosively escaped pressurized (up to atmospheric pressure) air. The W has tendency to peel off its surface layer: semidetached chip is then more intensely heated (due to locally reduced thermal conductivity) and rounded. The SiC has negligible porosity, and at melting point it de-composes to elements; therefore, the crater morphology can be related to local laser-energy-density above ablation threshold. When more shots are superimposed, in all three investigated materials the crater depth remarkably increases up to ∼10 accumulated shots, between 10 and 20 accumulated shots this increase is slowed down, and above 20 it is very small.