Investigation of laser ablation features of molybdenum bulk for picosecond laser-based techniques in fusion devices

Laser-based techniques, such as laser-induced ablation spectroscopy and laser-induced breakdown spectroscopy, have successfully demonstrated both in-situ and postmortem first wall analysis capabilities for fusion applications. In this study, the ablation features of molybdenum (Mo) bulk by a 35-picosecond laser at 355-nm wavelength with laser fluences up to 70 J/cm(2) were investigated. The ablation depth and morphology of the ablated craters were determined using confocal microscopy. The structural features of the craters were characterized by scanning electron microscopy in combination with the focused ion beam technique. By analyses of the ablation depth and the surface morphology modification, the ablation threshold, depth resolution, and minimum damage of Mo bulk were determined. Moreover, three ablation regimes as a function of laser fluences were identified for both the ablation depth per pulse (ablation rate) and the structural features of the craters resulting from the ablation process. Ablation rate equations for the three identified ablation regimes were proposed and the corresponding ablation mechanisms discussed. In addition, the implications for the application of picosecond laser-based techniques to monitor the first wall in current and upcoming fusion devices, such as W7-X, EAST, ITER, and CFETR, are discussed.