Study On Thermodynamic Effects Of Fused Silica Laser Irradiation

Laser-induced damage in optical elements remains one of the key issues hindering the development of high-power solid laser facilities.As the preferred material of choice in the ultraviolet region for inertial confinement fusion laser drivers,fused silica suffers from vairous thermodynamical effects with respect to the changes of temperature or pressure upon laser irradiation,resulting in irreversible modifications in the structural and physical properties.Such modification is of great significance as it corresponds to the knowledge of damage sites,the mechanism of laser-induced damage,development of damage mitigation and techniques in lifetime prediction for silica optics.Based on the mechanism of laser-silica interaction,this thesis investigates the evolution of the microscopic structure as well as the thermal and mechanical properties of fused silica with molecular dymics(MD)simulations.The main contents and results of the thesis are as follows.Three MD force fields,including the BKS,Teter and ReaxFF are compared using different simulation parameters to explore their respective effects on characterization of the structure,elastic modulus and thermal conductivity of fused silica.While all these empirical potentials can reproduce realistic silica structures,they fail to offer complete description of the vibrational density of states.Compared with the Teter and ReaxFF potentials,the BKS provides the most accurate evaluation of room-temperature elastic modulus.While the BKS and Teter potential both overestimated the thermal conductivity of fused silica,simulations using the ReaxFF can reproduce best agreement with experiments at 300 K.The plastic compaction of fused silica due to the change of external pressure is explored with the Teter potential to gain insights into the structure and property of the compaction layer in the laser-induced damage craters.The results show that fused silica exhibits phase transition from elastic to plastic compaction with increased pressure,resulting in a maximum compaction ratio of 120%.After plastic compaction,the Si-O bond length remains unchanged while the Si-O-Si bond angle decreases.Meanwhile,the content of 3-membered,4-membered and 8?12-membered rings increases.The elastic modulus of densified silica is larger than that in the low-density phase while the thermal conductivity becomes smaller with a narrowed vibrational spectrum.In the energy representation,the atomic potential energy of Si and O in silica do not change in average value while their respective distribution turns out to be more discrete after compaction.The effect of rapid quenching due to the change of temperature is investigated with the BKS potential to reproduce the structure and thermodynamical properties of the molten part of silica during laser damage and mitigation.The results show that the density and defect concentration tend to increase if the silica is rapidly quenched from the temperature above the transition point,which will also induce 2-membered and 12-membered rings accompanied by a decrease of the Si-O-Si angle.The intermediate-range order of the rapidly quenched silica decreases with increased fictive temperature while the vibrational density of states is not significantly affected.Compared with the silica prepared with a normal cooling rate,the silica exhibits rapid quenching has larger elastic modulus and a comparable thermal conductivity.In the energy representation,rapid quenching does not significantly change the atomic potential of Si and O in fused silica.The low-fluence U V radiation effects on fused silica are studied with the ReaxFF potential,which can successfully mimic the breakage of Si-O bonds upon multi-photon excitation with a simplified knock-on model.The results show that the concentration of coordination defects increases with increased number of excitations.The average potential energy and vibrational properties of the identified coordination defects both differ strikingly with those normally coordinated atoms.The non-bridging O tend to induce shorter Si-O bonds while 5-coordinated Si give rise to a few 2-membered rings in the lattice.The elastic modulus of silica increases after multiple excitations while the thermal conductivity decrease slightly due to the scattering of defects.