Power-law scaling of plasma pressure on laser-ablated tin microdroplets

Abstract

The measurement of the propulsion of metallic microdroplets exposed to nanosecond laser pulses provides an elegant method for probing the ablation pressure in dense laser-produced plasma. We present the measurements of the propulsion velocity over three decades in the driving Nd:YAG laser pulse energy, and observe a near-perfect power law dependence. Simulations performed with the RALEF-2D radiation-hydrodynamic code are shown to be in good agreement with the power law above a specific threshold energy. The simulations highlight the importance of radiative losses which significantly modify the power of the pressure scaling. Having found a good agreement between the experiment and the simulations, we investigate the analytic origins of the obtained power law and conclude that none of the available analytic theories is directly applicable for explaining our power exponent.

Mass Loss from a Stretching Semitransparent Sheet of Liquid Tin

“…3. These discrepancies reflect the different physical origins of the droplet deformation in the two cases: in our experiments it is the laser-produced plasma pressure that causes the deformation of the droplet into a sheet [10,11,23], while in the experiments by Ref. [17] it is the impact onto a solid surface.…”

“…To maintain cylindrical symmetry, the laser light entering the chamber has circular polarization, produced by employing a quarter-wave plate. The dynamics of the expanding droplets are captured by stroboscopic shadowgraphy imaging systems [10,11,23]. These systems are based on two broadband 560 nm wavelength (with a 12 nm FWHM bandwidth) 5-ns-long light pulses combined with three long-distance microscopes coupled with CCD cameras.…”

“…Plasma formation may also lead to small splashing events producing some microparticulate debris. Full radiation-hydrodynamics simulations have, however, shown that these mass-loss channels only contribute a few percent [11].…”

“…Such debris could limit the lifetime of nearby light-collection optics [4,[6][7][8][9]. Several recent studies have provided insight into the response of a tin microdroplet to a nanosecond laser pulse in which the propulsion [10,11] accompanies a deformation [10][11][12][13][14] of the droplet into a thin sheet. The eventual breakup and fragmentation of the sheet into much smaller droplets has also been studied [9,15].…”

“…[16,17]. Droplet deformation and fragmentation upon impact, be it onto a solid substrate or by a laser pulse, is a topic of particular current interest in fields ranging from epidemiology, governing the transport of pathogens from sneezes and coughs [17,18], through sprays relevant in, e.g., agriculture [18,19], to state-of-the-art nanolithography [7,[10][11][12][13][14]. Knowledge of the thickness profile of the thin liquid sheet formed upon impact is crucial for understanding the deformation and complex fragmentation processes involved.…”