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

Abstract

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 ne and temperature Te 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 Te value in Gd LPPs increases with increasing laser intensity IL , with the dependence T e ∝ I L 0.37 , in the IL range of 1010– 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 Te was found to be in good agreement with the experimental data over the used IL range. The simulation indicates that higher overall maximum Te 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 Te profile. Our experimental findings, supported by the simulations, show that larger spots can create uniform Te profiles, higher maximum Te , and larger high-Te 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.