Thomson scattering measurements of electron temperature and electron density in laser-driven Gd plasmas

Abstract In this study, we demonstrate the laser intensity scaling of electron temperature in Gd laser-produced plasmas (LPPs) through experiment and simulation. The spatial and temporal profiles of electron density n e and temperature T e in Gd LPPs were directly measured during a drive laser pulse duration of 7 ns at a laser wavelength of 1064 nm using collective Thomson scattering. We found that the measured maximum T e value in Gd LPPs increases with increasing laser intensity I L , with the dependence T e ∝ I L 0.37 , in the I L range of 10 10 – 10 11 W ⋅ c m − 2 . Radiation-hydrodynamic simulation code of the STAR-2D was performed under identical conditions to those used in the experiment, and extended further to higher laser intensities of up to 6 × 10 11 W ⋅ c m − 2 ; the simulated T e was found to be in good agreement with the experimental data over the used I L range. The simulation indicates that higher overall maximum T e typically exists 50–70 μ m above the target and modifies the dependence as T e ∝ I L 0.44 . Experiments reveal that a laser intensity of 1.9 × 10 12 W ⋅ c m − 2 is required to achieve the optimum condition ( T e ∼ 100 eV and ion charge states ∼ G d 18 + , at n e ∼ ( 2 – 3 ) × 10 19 c m − 3 ) for efficient beyond extreme ultraviolet (EUV) light source. In addition, two spot sizes (150 and 250 μ m in diameter) were used to study the effect of the spot size on the T e profile. Our experimental findings, supported by the simulations, show that larger spots can create uniform T e profiles, higher maximum T e , and larger high- T e regions in LPPs. The results and scaling laws presented in this study provide important information for developing more powerful and energy-efficient EUV light sources.