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

Abstract

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 μm. 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.