Enhancement of LIBS plasma and efficient collection of emitted light by open smooth triangular cavities

Laser-induced breakdown spectroscopy (LIBS) has the disadvantages of weak signal, unreliability, and low sensitivity. Spatial confinement can enhance the spectra to overcome these shortcomings. The current related research is based on the observation that shock wave confines the plasma. Considering that the plasma is a light source and most of its emitted light is hindered by a cavity, in this work we designed an open smooth triangular cavity to enhance the LIBS spectra via plasma confinement and light collection. The spatial and temporal evolution of the plasma and optical traces in the cavity were simulated respectively. We researched the optimal parameters of the designed cavity and its influence on LIBS detection and then compared its enhancement effects with that of an open rough triangular cavity and a smooth parallel plate. The results indicate that the optimal angle of the cavity is 40° and the optimal diffusion distance of the shock wave is 2 mm. The plasma is confined at about 2.2 μs. The spectra confined by the designed cavity have the best enhancement factor, spectral stability, and sensitivity compared with the other two cavities. The enhancement factors of Fe I 500.5703 nm and Cr I 513.8852 nm with the designed cavity are 2.35 and 2.08, respectively. The contributions of improving light collection efficiency and plasma confinement effect to the enhancement factors are 31.2% and 68.8%, respectively. The plasma temperature shows that the plasma confinement effects of the three cavities are close. The designed cavity can improve the quantitative detection accuracy of Fe and Cr in stainless steel and reduce the limit of detection. This study provides theoretical and data support for the development of spatial confinement technology. Inside the smooth triangular cavity, the plasma is compressed by shock waves and more emitted light is reflected into the collecting system.