Effects of pulse energy ratios on plasma characteristics of dual-pulse fiber-optic laser-induced breakdown spectroscopy

Laser-induced plasmas of dual-pulse fiber-optic laser-induced breakdown spectroscopy with different pulse energy ratios are studied using optical emission spectroscopy (OES) and fast imaging. The energy of the two laser pulses is independently adjusted within 0-30 mJ with the total energy fixed at 30 mJ. The inter-pulse delay remains 450 ns constant. As the energy share of the first pulse increases, a similar bimodal variation trend of line intensities is observed. The two peaks are obtained at the point where the first pulse is half or twice the second one, where the maximum spectral enhancement is at the first peak. The bimodal variation trend is induced by the change in the dominated mechanism of dual-pulse excitation with the trough between the two peaks caused by weak coupling between the two mechanisms. With increasing the first pulse energy, there is a transition from the ablation enhancement dominance near the first peak to the plasma reheating dominance near the second peak. The calculations of plasma temperature and electron number density are consistent with the bimodal trend, which have the values of 17024.47 K, 2.75×1017 cm-3 and 12215.93 K, 1.17×1017 cm-3 at a time delay of 550 ns. In addition, the difference between the two peaks decreases with time delay. As the increase in the first pulse energy share, the plasma morphology undergoes a transformation from hemispherical to shiny-dot and to oblate-cylinder structure during the second laser irradiation from the recorded images using an Intensifed Charge-Coupled Device (ICCD) camera. Correspondingly, the peak expansion distance of the plasma front first decreases significantly from 1.99 mm in the single-pulse case to 1.34 mm at 12/18 (dominated by ablation enhancement) and then increases slightly with increasing the plasma reheating effect. The variations in plasma dynamics verify that the change of pulse energy ratios leads to a transformation in dual-pulse excitation mechanisms.