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

Abstract

The experimental study of damage to tungsten (W), molybdenum (Mo), and silicon carbide (SiC) surfaces induced by focused extreme ultraviolet laser radiation (λ ~ 47 nm/~1.5 ns/21–40 µJ) 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.

Surface Morphological Features of Molybdenum Irradiated by a Single Laser Pulse

“…Such cracks are due to thermal stresses occurring in the metal after fast heating and successive cooling; as shown in Figure 9c, they depart from the bubbles to then propagate through the material. Similar results have been recently published by Straus et al [37], who studied the damage of W and Mo surfaces induced by an ultraviolet laser radiation (λ ∼ 47 nm). Figure 10a shows a cross-section of the crater: the morphology and size of the grains differs remarkably from that of the unaffected material (Figure 1c), since elongated substructures substantially disappeared, and grains with a size of a few microns cover only a small part of the observed area.…”

Subthreshold Erosion of an Organic Polymer Induced by Multiple Shots of an X-Ray Free-Electron Laser

“…Of course, there are more areas of a possible application of the above-reported results, e.g., durability of the first walls of prospective inertial fusion reactors [51], radiation damage to samples probed by FEL pulses [52,53], and high-dose-rate radiation biophysics [54], but their detailed discussion lies behind the scope of this paper.…”