The transition from short- to long-timescale pre-pulses: Laser-pulse impact on tin microdroplets

We experimentally study the interaction of intense laser pulses with metallic microdroplets and the resulting deformation. Two main droplet deformation regimes have previously been established: that of sheet-type expansion after impact of long (typically > 10 ns) pulses governed by incompressible flow and that of spherical expansion by internal cavitation after impact of short (typically < 100 ps) pulses governed by shock waves, i.e., strongly compressible flow. In this work, we study the transition between these regimes by scanning pulse durations from 0.5 to 7.5 ns, where the boundaries of this range correspond to the limiting cases for the employed droplet diameter of 45 mu m. We qualitatively describe the observed deformation types and find scaling laws for the propulsion, expansion, and spall-debris velocities as a function of pulse duration and energy. We identify the ratio of the pulse duration to the acoustic timescale of the droplet as the critical parameter determining the type of deformation. Additionally, we study the influence of fast rise times by comparing square- and Gaussian-shaped laser pulses. These findings extend our understanding of laser-droplet interaction and enlarge the spectrum of controllable target shapes that can be made available for future tin-droplet-based extreme ultraviolet sources. (c) 2022 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0082352

Automated summary

We experimentally investigate the interaction between intense laser pulses and metallic microdroplets and the resulting deformation. Two distinct droplet deformation regimes, governed by incompressible flow or shock waves, have been identified. By scanning pulse durations, we study the transition between these regimes and find the key parameter determining the type of deformation to be the ratio of pulse duration to the acoustic timescale of the droplet. We also examine the influence of fast rise times on the results.