Combined enhancement of fiber-optic laser-induced breakdown spectroscopy coupling spatial confinement and double-pulse irradiation

The mechanism of double-pulse laser irradiation under spatial confinement remains unclear due to complex plasma plume dynamics and multiple shock wave interactions. In this study, coupling of spatial confinement and double-pulse irradiation was investigated using fast photography, laser shadowgraphy, and spectroscopy. The experimental results demonstrated unique dynamic processes of plasma plume and shock waves, confirming that the plasma plume was eventually reshaped into a jet-like structure, and the combined enhancement effect can be regulated through different parameters, including inter-pulse delay and plate spacing. The inter-pulse delay dictates the primary mechanism of the secondary pulse's interaction with the initial plasma and the target surface; as it increases, plasma front absorption decreases while target surface ablation intensifies. Plate spacing determines the delay from laser incidence to plasma plume compression; along with the inter-pulse delay, it influences whether the secondary pulse generates a second-generation shock wave that merges with the initial shock wave for compression or performs a secondary compression after the first. Compared to double-pulse irradiation alone, combined enhancement increased emission intensity of Fe, Ni, and C spectra by up to 4.1 times accompanied by the increase in plasma temperature. The R² of calibration curves was increased, and the sensitivity for trace components Cr and C was improved with limits of detection reaching 255 and 659 ppm.