基于Abel逆变换的激光诱导等离子体辐射特性研究

doi:10.3964/j.issn.1000-0593(2019)01-0250-07

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Abstract Laser-Induced Breakdown Spectroscopy (LIBS) has been widely used in more and more fields as a new measurement method of material composition. However, compared with the traditional analysis methods, the analytical performance of LIBS needs to be further improved. The basis of LIBS is the laser-induced plasma. It is helpful to optimize the experimental parameters of LIBS system and lays the theoretical foundation for improving the detection capability of LIBS. The laser induced plasma is a non-steady radiation source associated with the space. Spatial-resolved spectroscopy is one of the most important ways to explore the physical properties of plasma. In order to study the characteristics of laser induced plasma, a Q-switched Nd∶YAG laser operating at the wavelength of 1 064 nm was used to ablate the alloy steel and the plasma was generated. The two-dimensional distribution of plasma emission was measured by the spatial resolution device. It is analyzed that the spectral signal collected in the experiment is the integrated intensity of the spectrum along the line of sight of the signal detector. So the plasma parameter calculated by the integrated intensity is the average of the observed path. In order to investigate the emission characteristics from the inner layer to the outer layer of the plasma, we measured the transverse spatial distribution of integrated intensity firstly. Then, assuming that the plasma is optically thin and cylindrically symmetrical, a method of Abel inversion based on cubic spline functions was performed on the integrated intensity. And the radial spatial distribution of the emissivity of the plasma from the inner layer to the outer layer was obtained. The atomic emission lines of Fe Ⅰ: 374.55 nm and Mn Ⅰ: 403.08 nm were selected to analyze the spatial distribution characteristics of the plasma emission. It has shown that the distribution of the integrated intensity presents a greater intensity value in the central location and smaller intensity at the edge of the plasma. This is due to the expansion of the plasma. The radial distribution of the spectral emissivity of the plasma was obtained by Abel inverse transformation. It has shown that the emissivity increased and then decreased from the inner to the outer of the plasma. A minimum value of emissivity appears at the center of the plasma as a result of the lower electron density in the central region of the plasma source. Eleven atomic lines of Fe in the plasma emission spectra were selected to calculate the plasma temperature by Boltzman method. The corresponding integral spectral intensities and emissivity were used respectively. The two-dimensional distributions of the transverse and radial spatial distributions of the plasma temperature were obtained. They have the similar variation rule. It can be seen that transverse spatial distributions of the plasma temperature decreases monotonously with the increase of the distance from the sample surface. And the plasma temperature gradually decreases from the center of the plasma to the edge which is the result of the expansion of the plasma and the interaction with the ambient gas. From the radial spatial distribution of the plasma temperature, it can be seen that the temperature of the plasma gradually decreases from the inner layer to the outer layer due to the expansion and cooling of plasma. Therefore, the radiation characteristics of the plasma can be obtained by using the Abel inverse. It provides an experimental basis for further understanding of the physical mechanism of the laser induced plasma. It also lays a theoretical foundation for improving the analytical performance of laser-induced breakdown spectroscopy.

Key words：Laser-induced plasma; Spatial distribution; Abel inversion

Received: 2017-12-25 Accepted: 2018-04-30

ZTFLH:

O433.4

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2019)01-0250-07 OR https://www.gpxygpfx.com/EN/Y2019/V39/I01/250