Investigating The Mechanisms Of Laser-driven Granular Jetting And Cratering

Most of the circular depressions on the surface of celestial bodies were formed by high-speed meteorite impacts,which are called meteorite craters.The origin of these craters involves extremely complex physical processes.Studying the impact events that form these craters is of great significance for exploring the internal structures and evolution of planets,and promoting the development of geological exploration,aerospace technology and other engineering fields.There is a long history of searching for simple and effective approaches to simulate the origin of the meteorite craters in ground-based laboratories.However,some established simulations depending on a low-speed intruder to impact the granular bed don’t act as a typical hypervelocity impact cratering(HVIC),as strong shockwave and discretely aggregated intruders are simultaneously necessary in HVIC events.As a result,some characteristics of HVIC cannot be reproduced in such low-speed experiments.Regarding this situation,this dissertation demonstrates that the plasma generated by a focused low-energy laser pulse irradiating a granular bed can effectively drive a cratering event during its formation and expansion processes.The dynamics and resulting features of the laser-driven granular cratering are remarkably similar to those produced by a HVIC event.The main achievements are following:1.With a high-speed camera,the cratering processes caused by the focused pulsed laser(1064 nm,7 ns,5-150 m J,80 mm focal length lens)irradiating the granular bed surfaces were recorded.The experiment discovers a novel early stage and "high-speed" granular jetting phenomenon.By carefully studying the evolution behavior of jet characteristics as a function of laser parameters(pulse energy,spot size)and bed parameters(grain size),the physical mechanism of the observed jetting phenomenon was explained as follows: Due to some primary plasma generated inside the granular surface with high porosity,following the rapid expansion of the plasma controlled by bed surface,an upward and strong shockwave comes into being,which directly accelerates the grains wrapped by the plasma flow,and ultimately leads to the formation of an early-stage granular jet.Similarity analysis showed that the observed granular jetting phenomenon has good analogies to those occurrences in HVIC events on the formation mechanisms and the jetting features,thus confirming that the laser-driven granular cratering event satisfies the main criterion,that strong shockwave plays an important role in a HVIC event,when simulating the meteorite impacts in laboratory.2.Using a 3D laser profile scanner,the morphologies of the final craters driven by laser pulses were recorded.The geometry of laser-driven craters is bowl-shaped.By carefully studying the evolution behavior of the final crater morphologies(diameter,depth,and depth-to-diameter ratio)as a function of laser energy,a physical model for the main cratering process driven by a laser pulse was proposed: strong shockwaves created by the downward rapid expansion of the laser-induced plasma directly accelerates this part of grains located at the interface between the bottom of the plasma flow field and the granular bed.As the transient plasma expands,the grains acquire velocities vertically downward the target surface and initiate the main excavation process in the form of a group of discrete intruders with the same grain size and density as those of the granular bed.Similarity analysis showed that the laser-driven cratering process has good analogical characteristics with a HVIC event in the physical picture(a group of discrete intruders analogous to the fragments of meteorites and the ground)and the final cater features(bowl-shaped geometry,depth-diameter ratio,energy scaling),thus confirming that the laser-driven granular cratering events satisfy multiple criteria for simulating the meteorite impacts in laboratory.3.Using the discrete element method(DEM)simulation of the main excavation process of laser-driven granular cratering events,we found that the energy scaling of final crater diameter depends on the size and density of the incident sphere.The results showed that the similar or even smaller size and density of intruders compared to the grains of targets results in the energy scaling law of laser-driven craters being0.16.Furthermore,it implied that the morphologies of meteorite craters in real impact events may attribute to the size and density of the fragmented impactors,which are similar to or even smaller than the earth’s surface that can be considered as a discrete system.The research results of this dissertation provide basic physical pictures for the laser-driven granular cratering process,and some convincible evidences that laser-driven granular cratering experiments could probably be served as a low energy laboratory approach to satisfy some typical characteristics of the real meteorite impacts.It is expected that such a laboratory approach with similar simplicity as low-speed impact cratering experiments will provide certain potential perspectives for studying important scientific issues such as cratering dynamics and energy transfer rules in large impact events.