Layer-resolving ability and model analysis of LIBS for multilayer samples with a four-layer structure under different focusing conditions†

Abstract

The depth resolution of the deposited layer on the plasma-facing components (PFCs) is essential for understanding the plasma–wall interaction (PWI) processes. Laser-induced breakdown spectroscopy (LIBS) has been proven effective for the in situ diagnosis of the depth profile of the deposited layer on the first wall of a fusion device. However, the effect of focusing conditions on LIBS depth profiling requires further investigation. In this study, depth profiling of multilayer samples with a four-layer structure was performed in a vacuum by adjusting the distance from the focal point to the sample surface (ΔL). The impact of focusing conditions on LIBS depth resolution was investigated, and the resulting depth distribution profiles were modeled. The results indicated that the laser profile factor gradually decreases with increasing ΔL and the resolving ability of the Ni intermediate layer gradually decreases. According to the LPIR model, the depth profiles obtained under different focusing conditions were successfully simulated, and the interface positions between layers were quantitatively identified. The correlation coefficients between the modeling and experimental results for the depth distributions under different focusing conditions were greater than 0.99. Additionally, layer thicknesses were calculated and compared under different focusing conditions. Overall, the medium spot size obtained in the defocused state is beneficial for depth profiling of a complex multilayer sample, considering the layer-resolving ability of the Ni intermediate layer and the accuracy of the layer thickness. This study will be of great significance in optimizing the experimental conditions of the in situ LIBS system for fusion devices.