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Boronization is used in present-devices as wall conditioning technique to getter oxygen, reduce other intrinsic impurities in the vessel, and control the hydrogen recycling as well as improving plasma performance. The technique is also foreseen as reference wall conditioning method for the new ITER baseline with full-tungsten (W) wall. However, the effectiveness of the deposited Boron (B) layer thickness and its homogeneity after the boronization process is uncertain as well as the exact knowledge about the interlink of boron layer lifetime with improved wall conditions.
In this study, an approach of the picosecond-laser-induced breakdown spectroscopy (ps-LIBS) is investigated to analyze the depth distribution of B-layers on W-substrates in a vacuum environment mimicking in-vessel deposited B-layers in devices like ITER. At first, based on the dynamic behaviour of the LIBS-plasma in a vacuum, the appropriate spectral lines and acquisition settings for B and W were determined. Sequentially, the depth distribution of two types of B-films on W-substrates with the thicknesses of 130 nm and 260 nm were measured under different laser spot sizes (diameter:142-1518 μm)[1]. The measured average ablation rate of ps-LIBS shows a notable decrease with increasing laser spot size. The spectral lines of B II and W I exhibit distinct intensity distributions under different spot sizes. The interface between B-films and W-substrates, as well as the thickness of the B-films, were determined using the normalized intensity and intensity ratio method, respectively. The results from ps-LIBS measurements regarding the depth are in good agreement with those obtained through the FIB-SEM and EDS. Finally, the B-layers with a range of different thickness (10-150 nm) on the W-substrates were used to establish quantitative curve and obtain the limit of detection [2]. This calibration or quantitation is consistent with results of LIBS detection on ultrathin B-layers of two W samples exposed in W7-X during boronization.
These initial findings demonstrate the feasibility of using ps-LIBS to characterize, in situ within fusion devices, the thickness and uniformity of thin boron films on W substrates down to the 100-nm scale and below [3].
[1] H. Wu, R. Yi, A. Houben, et al., Nucl. Mater. Energy 101812, 41 (2024)
[2] H. Wu, R. Yi, S. Brezinsek, et al., Nucl. Mater. Energy 102018, 45 (2025)
[3] S. Mittelmann, J. Oelmann, S. Brezinsek, et al., Appl. Phys. A-Mater. Sci. Process 672, 126 (2020)