Study of the layer thickness of multilayer sample by the LIBS method based on ablation rate correction†

As a remote and in situ diagnostic technique for the first wall of tokamaks, laser-induced breakdown spectroscopy (LIBS) has shown promising potential for depth profile analysis of deposition layers on plasma-facing components (PFCs). However, due to the complexity of the interface of deposition layers and the limitations of laser profiles, achieving an accurate deposition layer thickness is often more difficult for an in situ LIBS system in tokamaks. In previous studies, a Laser Profile & Interface Roughness model (LPIR model), which considers the laser beam profile and interface roughness factors, has been developed to identify the interface of deposition layers. In this study, the effect of ablation rates from different materials in the deposited layers on the accuracy of their thickness has been investigated. The depth profiling of a Ni–Cu–Ni–Cu multilayer sample, which has a four-layer structure, has been carried out using the LIBS technique under different focusing conditions as well as various laser pulse energies, with the pressure maintained at 10⁻⁵ mbar. The LPIR model was used to reconstruct the depth distribution profile of the Ni–Cu–Ni–Cu multilayer sample and quantify the interfacial positions of the deposited layers. A layer thickness correction method for multilayer sample is proposed based on the dependence of the ablation rates of different layers on laser fluence. The correction ability has been evaluated based on the relative errors between the calculated and the scanning electron microscope (SEM) values for different layer thicknesses. The relative errors of the corrected layer thicknesses are all significantly improved, and the accuracy of the layer thicknesses has been substantially improved. The proposed method will not only help us better understand the LIBS depth profiling of multilayer samples under different laser fluence conditions, but it will also further improve the accuracy of the layer thickness analysis of multilayer samples. This result is of positive significance for the application of in situ LIBS diagnostics in plasma–wall interaction (PWI) studies.