Effect of lens-to-sample distance on spatial uniformity and emission spectrum of flat-top laser-induced plasma

The optimal spectral excitation and acquisition scheme is explored by studying the effect of the lens-to-sample distance (LTSD) on the spatial homogeneity and emission spectra of flat-top laser converging spot induced plasma. The energy distribution characteristics before and after the convergence of the laser beam with quasi flat-top intensity profile used in this study are theoretically simulated and experimentally measured. For an aspheric converging mirror with a focal length of 100 mm, the LTSD (106 mm≥LTSD≥96 mm) was changed by raising the stainless-steel sample height. The plasma images acquired by ICCD show that there is air breakdown when the sample is below the focal point, and a ring-like plasma is produced when the sample is above the focal point. When the sample is located near the focal point, the plasma shape resembles a hemisphere. Since the spectral acquisition region is confined to the plasma core and the image contains all the optical information of the plasma, it has a lower relative standard deviation (RSD) than the spectral lines. When the sample surface is slightly higher than the focal plane of the lens, the converging spot has a quasi flat-top distribution, the spatial distribution of the plasma is more uniform, and the spectral signal is more stable. Simultaneously, there is little difference between the RSD of the plasma image and the laser energy. In order to further improve the stability of the spectral signal, it is necessary to expand the spectral acquisition area.