Influence of laser energy on spectral properties of annular laser-induced plasma

Laser energy is a key experimental parameter affecting the analytical performance of LIBS. In order to investigate the characteristics of the plasma formed by an annular beam, an axicon was used to convert the Gaussian laser beam emitted from a Q-switched Nd:YAG laser into an annular laser beam. The annular beam was used to ablate the alloy steel sample to produce plasma. The influence of laser energy on the spectral emission and spatial evolution characteristics of the annular plasma was investigated. The results show that the melting of the material in the region where the annular beam interacts with the sample is more homogeneous than the melting material ablated by the Gaussian beam. The spectral line intensity, plasma temperature and electron density increase with the increase of the laser energy. The Signal-to-Background Ratio (SBR) increasing rapidly first, and then slowing down when the energy exceeds 145 mJ. The two-dimensional spatial distribution of the spectral intensity shows a flat shape, with two peaks along the transverse direction of the plasma, and the distance between the peaks corresponds to the size of the annular ablation crater. The increase in laser energy causes the faster expansion and the smaller gradient of the spectral line intensity change in the central region of the annular plasma. The positions of the two peaks along the transverse direction of the annular plasma are shifted away from the center with the increase of the laser energy. The two-dimensional spatial distribution of the plasma temperature and electron density also illustrates the coupling between the various regions of the annular plasma resulting in a more homogeneous region inside the plasma.