Study of the Combination of Nano and Femto Second Laser Pulses to Generate Gold Plasma for Water Window Emission

Abstract

In this study, we employed the one-dimensional hydrodynamic simulation code MED103 to theoretically investigate the generation and spatial–temporal evolution of highly charged gold ions induced by a double-pulse Ti:sapphire laser system. A pre-pulse of 10 ns duration and intensity of 8.5 x 10¹¹ W/cm² and a main pulse of 100 fs duration and  intensity  of  7 x 10¹⁶ W/cm²  were  used  to irradiate a 20 µm thick gold target. The temporal separation between the pulses was varied from 0 to 10 ns to explore its effect on the formation of ion charge states ranging from Au¹⁹⁺ to Au³⁴⁺, with particular emphasis on optimizing conditions for soft X-ray emission within the water window spectral range. Simulation results reveal that the most favorable ion fractions for ion stages (Au²⁵⁺–Au³⁰⁺), which are key contributors to unresolved transition arrays in the water window region,  are  predominantly  formed  at delay times between 5.8 and 7.5 ns. Additionally, both the electron temperature and average charge state were found to decrease with increasing delay time, affecting the ion distribution and emission characteristics. These results highlight the importance of precise delay time control in tailoring plasma conditions for efficient soft X-ray generation in high-Z laser-produced plasmas.