The effect of inter-pulse delay on the spectral emission and expansion dynamics of plasma in dual-pulse fiber-optic laser-induced breakdown spectroscopy

Abstract

The role of inter-pulse delay on plasma dynamics and spectral emission in dual-pulse fiber-delivery plasma has been investigated using fast imaging, optical emission spectroscopy, and laser shadowgraphy. The detection on the return spectrum in dual-pulse fiber-optic laser-induced breakdown spectroscopy showed that the self-reversal and self-absorption were reduced as the inter-pulse delay increased from 50 to 1000 ns. Using scanning electron microscopy, the ablation depth showed a changing trend of increasing first and then decreasing, and a maximum of ∼2.8 μm was achieved at 250-ns inter-pulse delay. Experimental results confirmed that the improvement was due to the reduction of the plasma thickness from 1.161 mm to 0.964 mm, and the calculation of electronic excitation temperature along the photon collection path showed that the excited atom densities became more spread, which both contributed to the self-absorption reduction. At a long inter-pulse delay, the plasma trailing edge would gradually separate from the target surface with a little further expansion distance of the leading edge, resulting in the reduction of plasma thickness after a time delay of several hundred nanoseconds. The heating of the peripheral cold particles by the second-generation plasma in the later expansion led to the rapid reduction of the peak temperature. Also, the early expansion trajectory of the second-generation plasma was tracked by laser shadowgraphy, which started to appear at an inter-pulse delay of 100 ns, and the average expansion velocity reached its maximum of ∼3.8 km/s.