Quantitative analysis of elemental depth on Wendelstein 7-X divertor baffle screws by picosecond laser-induced breakdown spectroscopy

Analysis of elemental distributions in plasma-facing components (PFCs) is vital for the study of material erosion, deposition, and fuel retention in Wendelstein 7-X (W7-X) stellarator operating in full 3D geometry. In this work, we report the results on the application of picosecond laser-induced breakdown spectroscopy (ps-LIBS) combined with laser-induced ablation quadrupole mass spectrometry (LIA-QMS) to quantitatively investigate the elemental depth distribution and fuel retention on the screws PFCs from the divertor baffle of W7-X. Depth profiles of H, C, Ti and Mo intensities on the screws with specific multilayer structures have been achieved by LIBS combined with LIA-QMS. A linear correlation approach has been applied for identifying the interface between the C layer and the Ti layer and shows that a laser ablation rate of (82.9 ± 0.9) nm/shot can be achieved on C layer thickness measurement by a laser energy density of 6.5 J/cm2. Moreover, quantitative H retention content on the surface of the screws with stellarator discharge plasma exposure is approximately 1.1 × 1021 atoms/m2 which is higher than approximately 3.5 × 1020 atoms/m2 on the surface of the new screw without plasma exposure. The studies well demonstrate the ability to determine elementary resolved layer thicknesses and H retention measurements by depth analysis of LIBS on PFCs exposed in W7-X.