Effect of Laser Focusing on Laser-Induced Plasma Confined by Hemispherical Cavity
CHEN Xu-dong1, WANG Jing-ge1, 2*, FENG Di1, WEI Jia-wei1, WANG Li-ping1, WANG Hong1
1. School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, China
2. Henan Key Laboratory of Electromagnetic Transformation and Detection，Luoyang Normal University, Luoyang 471934, China
Abstract：Spectral enhancement is one of the key methods to improve Laser-Induced Breakdown Spectroscopy (LIBS) analysis performance. Spatial confinement of plasma is often used due to its simple device and better confinement effect. The characteristics of plasma will directly affect the spatial confinement. The properties of the plasma are closely related to the focusing of the laser in the experimental system. In order to study the effect of the laser focusing on the spectral enhancement of the plasma confined by a hemispherical cavity, the condition of the laser focusing was changed by adjusting the distance between the lens and the sample (Lens to Sample Distance, LTSD). Under the experimental configurations without and with confinement, the alloy steel sample was ablated to produce plasma, and the time evolution spectra at 15 different LTSD positions were collected. The two-dimensional spatial distributions of the spectral line intensity and enhancement factor with LTSD and acquisition delay were obtained. The results had shown that the spectral line intensity of the plasma without confinement peaks when the LTSD was 94 and 102 mm, respectively. When the acquisition delay was less than 8 μs, the maximum value of the spectral line intensity was at the LTSD of 94 mm. The maximum intensity appeared at the LTSD of 102 mm when the delay time was greater than 8 μs. Moreover, the line intensity has two sequential enhancements when the hemispherical cavity confined the plasma. The delay time ranges corresponding to these two enhancements were 4~10 and 12~15 μs. The main reason for the second enhancement is that the shockwave reflected by the inner wall of the hemispherical cavity will continue to propagate after interacting with the plasma and it will encounter the other side of the cavity wall and be reflected again secondary compress the plasma. The two-dimensional distribution of the enhancement factor with LTSD and delay time was analyzed. It is found that the maximum enhancement factor of the first enhancement has no obvious trend with the change of LTSD and the enhancement factor fluctuates from 2 to 6. The maximum enhancement factor of the second enhancement first increases and decreases as the LTSD changes and decreases after a small increase. The enhancement factor is relatively high. It reaches the maximum when the LTSD is 96 mm, and the maximum enhancement factor is about 6. The delay time corresponding to the maximum enhancement factor was defined as the optimal delay time. It is found that the optimal delay time for the first enhancement varies from 6 to 9 μs. When the LTSD is in the range of 85~93 mm, the optimal delay time remains unchanged. When the LTSD varies from 94 to 104 mm, the optimal delay time of the first enhancement first decreases and then increases. However, the optimal delay time of the second enhancement maintains at a range from 14 to 15 μs, and there is no obvious change with the change of LTSD.
Key words：Laser-induced plasma; Hemispherical cavity confinement; Spectral enhancement; Focusing position
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