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

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 proportional to I-L(0.37), in the I-L range of 10(10)-10(11) W.cm(-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 6x10(11) W .cm(-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 mu m above the target and modifies the dependence as T-e proportional to I-L(0.44). Experiments reveal that a laser intensity of 1.9x10(12) W.cm(-2) is required to achieve the optimum condition (T-e similar to 100 eV and ion charge states similar to Gd18+, at n(e) similar to (2-3) x10(19) cm(-3)) for efficient beyond extreme ultraviolet (EUV) light source. In addition, two spot sizes (150 and 250 mu 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.