Research On Shock Wave And Ejection Behavior During Laser Damage On The Back Surface Of KDP Crystal Component

As a frequency doubling element in the optical path of Inertial Confinement Fusion(ICF)laser driver,potassium dihydrogen phosphate crystal(KDP crystal)needs to withstand high flux ultraviolet laser irradiation.However,KDP crystal is vulnerable to laser-induced damage and becomes a weak link in restricting the flux of load in practical applications.The damage of KDP crystals has become a problem that people pay close attention to and urgently solve.At present,the research on the damage of KDP crystals focuses more on the static morphology analysis of component damage,or on the number of damage points and morphologies during the damage process.There is no clear and complete study of the KDP crystal damage kinetics process.The experimental data is elaborated,so it is of great significance to carry out the research on the damage mechanical response behavior of laser induced by KDP crystal.A time-resolved imaging system with nanosecond time resolution combining pump-probe method with interferometry is designed and built in this paper to obtain images of laser damage on the exit surface of KDP crystal.Experimental study on shock wave and material ejection caused by 355 nm nanosecond pulsed laser on the exit surface of KDP crystal is executed.And then,laser-induced plasma shock wave image,free electron density,the spatial distribution of material ejection and the space-time evolution behavior of the shock wave are given.The laser flux in this experiment is set to 4-8 J/cm2 that is in the safe running flux range in practical engineering application and the diameter of the focusing spot on the surface of the crystal is 2 mm.The results show that when the rear surface damage of KDP crystal is induced by ultraviolet laser,the shock wave with rapidly decreasing velocity is generated.The point explosion model can be applied to the development of defect induced shock waves,and acquire the longitudinal velocity and the transverse velocity of the shock wave based on the above theory.The refractive index distribution and electron density of plasma are calculated through the measurement of interference fringes shifting.The results show that the change in the refractive index is about 10-4,the plasma electron density is approximately 1018/cm3.Time-resolved image reveals clearly the evolution stage of material ejection with time.During the first several hundreds ns,the ejection is diffused along with the shock wave and its main form is tiny powder.After 10 ?s,large particle ejection appears on the rear surface until it stops.At the same time,it concludes that the rules of development of ejection behavior on the back surface of crystals under multiple laser pulses,affected by defect induced mechanism mainly.In this thesis,the ejection distance caused by multiple damage is observed and analyzed,and the size of material ejection and the growth rule of its distance are summed up.In this paper,the spatial evolution and time evolution of the dynamic behavior of the mechanical response of the KDP crystal during the laser damage process are given.This provides an experimental reference for further establishing the UV damage kinetics model and reveals the UV damage mechanism of the KDP crystal for practical engineering purposes.