The interplay between laser focusing conditions, expansion dynamics, ablation mechanisms, and emission intensity in ultrafast laser-produced plasmas

The interplay between ultrafast laser focusing conditions, emission intensity, expansion dynamics, and ablation mechanisms is critical to the detection of light isotopes relevant to nuclear energy, forensics, and geochemistry applications. Here, we study deuterium ( 2 H alpha) emission in plasmas generated from femtosecond laser ablation of a Zircaloy-4 target with a deuterium concentration of & AP;37 at. %. Changes in emission intensity, plume morphology, crater dimensions, and surface modifications were investigated for varying focusing lens positions, where the laser was focused behind, at, and in front of the target. Spatially resolved optical emission spectroscopy and spectrally integrated plasma imaging were performed to investigate emission spectral features and plume morphology. Laser ablation crater dimensions and morphology were analyzed via optical profilometry and scanning electron microscopy. The 2 H alpha emission intensity showed significant reduction at the geometrical focal point or when the focal point is in front of the target. For all laser spot sizes, a two-component plume was observed but with different temporal histories. At the best focal point, the plume was spherical. When the laser was focused behind the target, the plume was elongated and propagated to farther distances than for the best focal position. In contrast, when the laser was focused in front of the target, filaments were generated in the beam path, and filament-plasma coupling occurred. By focusing the laser behind the target, the amount of material removal in the laser ablation process can be significantly reduced while still generating a plasma with a sufficient 2 H alpha emission signal for analysis.

Automated summary

The study focused on the emission of deuterium (2 H alpha) in plasmas generated from femtosecond laser ablation of a Zircaloy-4 target with varying focusing lens positions. Changes in emission intensity, plume morphology, and crater dimensions were observed. The results showed that adjusting the laser focal point position can significantly impact the emission intensity and plasma characteristics.