Spall-Velocity Reduction in Double-Pulse Impact on Tin Microdroplets

We explore the deformation of tin microdroplets of various diameters induced by two consecutive laser pulses having pulse durations of 0.4 ns. Impact of laser pulses with this duration mainly leads to shock-wave-induced cavitation and spallation. The main result obtained in this work is the observation of a strong reduction of the spall velocity that depends on the time delay between the two pulses. This reduction reveals a complex interplay between plasma recoil pressure and shock-wave-driven deformation, and enables an estimation of the moment of spall formation and the average shock-wave propagation velocity. We find that the shock wave traverses the droplet with an average velocity ranging from 1.2 to 1.6 times the speed of sound. We study the effects of the energy of the second pulse on the deformation and qualitatively discuss the formation of microjets. Crucially, we demonstrate the ability to manipulate the microdroplet expansion and spallation with double-pulse sequences, thereby increasing the portfolio of obtainable target shapes for droplet-based extreme ultraviolet light sources.