Experimental Study On Dynamic Characteristics Of Grain Ejection During Laser-driven Granular Cratering

The process of impact cratering widely exists in various natural phenomena,among which a typical example is the formation of meteorite craters on the surfaces of the moon and planets.Laboratory research on the dynamic process of granular impact cratering caused by low-energy projectiles such as solid spheres and liquid drops is helpful to reveal the formation mechanism of meteorite craters under extreme conditions.Besides,granular cratering driven by pulsed laser is a new way to study the cratering process in laboratory.To the best of our knowledge,only one article involving laser-driven granular cratering has been reported up to now.As a result,it is still unclear about the mechanisms corresponding to such a new cratering method and the influences of laser parameters and granular parameters on the dynamic characteristics of resulting grain ejection.In this thesis,taking size-selected dry glass beads with median diameters ranging from 84μm to 234μm as examples,we experimentally investigated the dynamics of grain ejection driven by nanosecond laser pulses(1064 nm,7 ns,10-100 m J)at different grain sizes and laser pulse energies.By using a 300,000 frame-rate camera to record the whole process of laser-driven grain ejection(LDGE),the effects of grain size and laser pulse energy on the LDGE dynamic characteristics,and the main experimental results were obtained as follows:1.The measured video sequences clearly exhibited that the LDGE process can be separated into two regimes:early-stage fast ejecting process and later-stage slow ejecting process.2.At given grain sizes,the kinetic energy of grains related to the early-stage fast ejecting process increases with increasing laser pulse energy with a power-law behavior.Furthermore,the temporal evolution of ejecta curtain diameter()related to later-stage slow ejecting process also obeys a power-law behavior,()=~,and thevalue is independent of laser pulse energy.3.When the laser energy is given,the kinetic energy of grains related to the early-stage fast ejecting process increases with increasing grain size.Thevalue in the temporal evolution of ejecta curtain diameter()=~,related to the later-stage slow ejecting process,also shows an increase with increasing grain size.Under the framework of point source model,the experimental observations mentioned above can be reasonably explained by considering the grain size dependent efficiency of impulse coupling between grain and plasma flow and laser-energy dependent plasma features generated by interaction of laser pulse with granular targets.This work improves the understanding of the LDGE mechanism.