Ultrathin boron layer (a few nm to 100 nm) diagnostics on tungsten substrates via spatiotemporally resolved picosecond LIBS in a vacuum

• Spatiotemporal dynamics of B emission validated by spectroscopy and fast imaging. • B presence rapidly identified via spatial separation of B II and W I emissions. • Detection limits for B layer thickness and content on W substrates determined. • Quantitative calibration curve established for B layer on W substrate. The impact of the thickness and uniformity of boron (B) layer deposited by the boronization process, as well as the understanding of the layer’s lifetime and oxygen gettering effectivity, remains uncertain and real time diagnosis is required in future. Building upon our previous work, the picosecond laser-induced breakdown spectroscopy (ps-LIBS) technology with spatial resolution capability was further employed to detect B elements in B films deposited on tungsten (W)-substrates samples. By adjusting experimental parameters such as laser energy and spot size, and studying the spatiotemporal evolution of W and B elements, the optimal experimental parameters were determined. Under a laser spot diameter of 488 μm, a laser fluence of 9.6 J/cm 2 , a delay time of 30 ns, and a gate width of 600 ns were used to improve the limit of detection (LOD) of B by ps-LIBS. A set of samples with different B layer thicknesses on W-substrates from 5.6 to 151.5 nm was obtained through exposure of samples by magnetron sputtering in a laboratory arrangement and exposure of samples on a manipulator during two boronizations in the stellarator W7-X. The thickness of the B layers was measured by Focused Ion Beam combined with Scanning Electron Microscopy (FIB-SEM) and Nuclear Reaction Analysis (NRA). After extracting the net B signal from the LIBS spectra, a quantitative calibration curve and LOD of the B II-703.2 nm for the B layers on W-substrates were established for the first time. These results confirm that ps-LIBS technology holds promise for in situ diagnostics of ultrathin B layers, ranging from a few nm to 100 nm, on W-substrates in fusion devices after boronization.