Parametric study of fiber-optic laser-induced breakdown spectroscopy for elemental analysis of Z3CN20-09M steel from nuclear power plants

Abstract

A compact laser probe of the fiber-optic laser-induced breakdown spectroscopy (FO-LIBS) system, based on a single-lens for energy focus and radiation collection, was developed in this paper. Elemental analysis of the Z3CN20-09 M steel sample, which is usually used as the structural material for main pipelines in nuclear power plants, was carried out using this laser probe. Most elements (e.g. Fe, Cr, Ni, Mn, Mo, Co, Cu, Nb, and Si) were successfully detected. The distance between the fiber output end face and the lens, which was one of the most important factors affecting the focused laser spot size and further inducing different degrees of self-absorption, was optimized. It was clear that the spectral intensities increased with the distance before decreasing. The optimal range of the distance was obtained by finding out the maximum spectral intensities and minimum spectral fluctuations. The effects of distances, which can be understood as the effects of spot sizes, were studied using plasma diagnostic methods. When the distance was shorter than the optimal range, the distinct self-absorption was observed, which was analyzed by the fast photography. Calculated changing trend of self-absorption effects was consistent with the observed spectral profiles. When the distance was longer than the optimal range, the plasma temperature and density became lower after ~1 μs. This demonstrates that it is possible for the single-lens laser probe to realize the accurate calibration and control of the focused spot size to achieve the best working conditions for a FO-LIBS system.