Early-time hydrodynamic response of a tin droplet driven by laser-produced plasma

We experimentally and numerically investigate the early-time hydrodynamic response of tin microdroplets driven by a ns-laser-induced plasma. Experimentally, we use stroboscopic microscopy to record the laser-induced dynamics of liquid tin droplets and determine the propulsion speed (U) and initial radial expansion rate (˙R0). The ratio of these two quantities is a key parameter to be optimized for applications in nanolithography, where laser-impacted tin droplets serve as targets for generating extreme ultraviolet light. We explore a large parameter space to investigate the influence of the tin droplet diameter, laser beam diameter, and laser energy on the ˙R0/U ratio. We find good agreement when comparing the experimentally obtained U and ˙R0 values to those obtained by detailed radiation-hydrodynamic simulations using RALEF-2D. From the validated simulations, we extract the spatial distribution of the plasma-driven pressure impulse at the droplet-plasma interface to quantify its influence on the partitioning of kinetic energy channeled into propulsion or deformation. Our findings demonstrate that the width of the pressure impulse is the sole pertinent parameter for extracting the kinetic energy partitioning, which ultimately determines the late-time target morphology. We find good agreement between our full radiation-hydrodynamic modeling and a generalized analytical fluid-dynamics model [H. Gelderblom et al., J. Fluid Mech. 794, 676 (2016)]. These findings can be used to optimize the kinetic energy partition and tailor the features of tin targets for nanolithography.

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