Temporal and spatial study of differently charged ions emitted by ns-laser-produced tungsten plasmas using time-of-flight mass spectroscopy

Tungsten (W) is an important material in tokamak walls and divertors. The W ion charge state distribution and the dynamic behavior of ions play important roles in the investigation of plasma–wall interactions using laser-ablation-based diagnostics such as laser-induced breakdown spectroscopy and laser-induced ablation spectroscopy. In this work, we investigate the temporal and spatial evolutions of differently charged ions in a nanosecond-laser-produced W plasma in vacuum using time-of-flight mass spectroscopy. Ions with different charge states from 1 to 7 (W⁺ to W⁷⁺) are all observed. The temporal evolutions of the differently charged ions show that ions with higher charge states have higher velocities, indicating that space separation occurs between the differently charged ion groups. Spatially-resolved mass spectroscopy measurements further demonstrate the separation phenomenon. The temporal profile can be accurately fitted by a shifted Maxwell–Boltzmann distribution, and the velocities of the differently charged ions are also obtained from the fittings. It is found that the ion velocities increase continuously from the measured position of 0.75 cm to 2.25 cm away from the target surface, which indicates that the acceleration process lasts through the period of plasma expansion. The acceleration and space separation of the differently charged ions confirm that there is a dynamic plasma sheath in the laser-produced plasma, which provides essential information for the theoretical laser-ablation model with plasma formation and expansion.