Spatiotemporal Evolution of Aluminum-lithium Alloy Plasma Using a Coaxial LIBS System under Vacuum

As an important technique for in-situ wall diagnostics in tokamaks, Laser-Induced Breakdown Spectroscopy (LIBS) has been demonstrated to effectively detect fuel retention and element distribution on the wall surface of the Experimental Advanced Superconducting Tokamak (EAST). However, despite its potential, the accurate quantitative analysis of wall surface elements remains a critical challenge for LIBS technology. One of the primary factors contributing to this challenge is the significant spatiotemporal non-uniformity exhibited by the laser-induced plasma under vacuum conditions. Therefore, investigating the spatiotemporal evolution of the plasma holds great significance for optimizing signal quality and enabling qualitative and quantitative analysis in wall diagnostics using LIBS. In this work, the researchers employed a coaxial optical structure based on a linear array of optical fibers, this setup enabled them to perform spatiotemporal integration and spatiotemporal resolution measurements of aluminum-lithium alloy plasma emission spectra generated by pulsed laser ablation at a wavelength of 1 064 nm, a pulse width of 5 ns, and a power density of 6.3 GW/cm(2) under vacuum conditions. The emission time of the plasma was evaluated to be similar to 1 mu s, and the emission area size was similar to 1 cm. The spatiotemporal evolution behavior of continuous radiation, ionic lines, and atomic lines were analyzed to determine the emission time scale of different radiative mechanisms of laser-ablated plasma. Spatially resolved measurements revealed that the spatial distributions of aluminum (Al) and lithium (Li) atoms and ions were distinct from each other, revealing an element "spatial separation" phenomenon that occurred during the lateral expansion of the laser-ablated aluminum-lithium alloy plasma in a vacuum environment. The results of the spatiotemporal evolution of the species' lateral expansion velocity showed that the ion velocity of the same element was greater than its atomic velocity (Al III>Al II> Al I, Li II>Li I), this discrepancy in velocities can potentially be attributed to the acceleration provided by the formed transient sheath. Additionally, the study discussed the differences in velocity among different elements in the same charge state, attributing it to the "mass separation" effect and ions recombination. In conclusion, this work aimed to tackle the challenges associated with quantitative analysis of wall surface elements using LIBS in tokamaks. By employing a coaxial optical structure and conducting detailed spatiotemporal measurements, the researchers gained valuable insights into the plasma's behavior during laser ablation. The study shed light on the emission time scale of various radiative mechanisms, spatial distributions of elements, and the velocities of ions and atoms. These findings contribute to understanding plasma-wall interactions in tokamak devices, providing crucial information for the development of advanced wall diagnostics techniques in fusion research.