Research On Dynamic Behaviors Of Nanosecond UV Laser Induced Damage On The Exit Surface Of Fused Silica Component

In high power laser engineering such as inertial confinement fusion facilities,laser induced damage(LID)is the most important factor that hinders the improvement of laser output energy.Among all the damage problems,LID on the exit surface of fused silica happened most frequently and seriously.The exit surface damage usually appears as volcanic crater-shape.Once appeared,it expands and extends into the body rapidly under shot sequences,resulting in the totally failure of component.Aiming at the LID problem on the exit surface of fused silica components,it is of great scientific value and engineering significance to study the behavior rule of its occurrence and development,clarify its physical mechanism and recognize the key factors that determine the formation and growth of the damage.Based on experiment,during UV LID process on exit-surface of silica from local initiated explosion to final damage crater,several dynamic behaviors and stage,including evaporation and explosion of material,liquid and solid ejection,generation and propagation of stress wave are included.Several processes like material ionization,state change,fluid movement and mechanical damage are involved.Their production are complex and diverse while their evolution is rapid.Up to now,the mechanism,rule,the characteristics of them and their relation to damage morphology have not been grasped totally.The damage dynamic behaviors are not fully collected and captured experimentally,so it also fails to support a complete and unified damage dynamic theory.Around these problems about UV LID on exit surface of fused silica,the process from surface explosion to stress induced damage is studied in this dissertation.Time-resolved imaging combining with several measuring methods is designed and established according to the temporal-spatial characteristics of behaviors.The evolution images and parameters of behaviors were obtained experimentally.The evolution rule and mechanism were revealed.The content of this dissertation is divided into the following parts:(1)According to the dynamic behaviors and product characteristics of damage stages,a time-resolved microscopic system combining several measuring methods was designed and established to obtain the images and parameter of these stages.A two-color interferometry system was used to obtain the property and distribution inside plume.The ionized and gaseous material with high transparency and complex composition was clarified based on their refractive index;A double-delay probes imaging method was used to record the unrepeatable motion of a single particle during ejection;A stress induced birefringence imaging method was used to analyze the property and intensity evolvement of stress during the damage process.Intensity was calculated according to the phase retardance of stress induced birefringence.(2)For the structure and evolution of explosion fluid field during exit-surface damage of fused silica,the evolvement and distribution of material inside the plume was studied.The relaxation of damage dominated by ionization and evaporation was revealed.When the damage area breaks the surface,ionized and gaseous compounds of the overheat material form explosion plume in the air.In this work,two-color interferometry was used to obtain the distribution and dynamic information of plume.The interferometric image showed a two-layer structure of plume,suggesting the two phases involved in this stage.In the first stage,surface material was ionized and severely vaporized,after several 10 ns,overheat subsurface material erupted and formed a secondary ejection.The ejection of evaporated material lasted over 100 ns.(3)For the morphology and motion of ejection during damage process,the evolvement of particle flight was analyzed.Combining with the distribution and morphology of particles,the physical state evolvement during the establishing of thermal equilibrium of damage was studyed.Ejection occured and changed continuously,resulting in the cavity and the decay of energy in damage site.A time-resolved imaging system with double-delay probes was used to obtain the size and speed of particles ejected at different delays,which were related to its morphology.Within 1500 ns,ejection transited from gaseous material to large fragment.Their velocity attenuated from 1 km/s scale to 10m/s scale,suggesting t he pressure in damage region decayed from over 1GPa to several MPa.These results indicated the relaxation process and response behaviors of material in this stage.(4)For the propagation characteristics of stress and mechanical damage,the evolvement of stress was studied.The property and distribution of stress during damage were revealed.The effect of stress on the fracture and growth of damage was discussed.The cracks at the periphery of damage were generated by stress and extended rapidly along and beneath the surface under shot sequences,leading to the failure of component.Time-resolved imaging with multi-polariscopic method was used to study transient stress behaviors.The distribution and eovlution of stress were showed based on stress induced birefringence.These results were related to the distribution of cracks.It showed that except body wave,Rayleigh wave was generated on surface due to the explosion impact on silica-air boundary.Damage on surface shows circumferential cracks consistent with the distribution of Rayleigh wave.Cracks showed a rapid extending under shot sequences,indicating the effect of Rayleigh wave on damage growth.The results reveal the stress property during the exit surface damage of silica and its relation with mechanical damage expansion.