Effect Of Surface Micro-Defects And Their Repairing On Laser Damage Resistance Of KDP Crystal

To alleviate the tough issues in fossil fuel shortage and environmental pollution,governments all over the world have been engaged in building the huge laser-driven Inertial Confinement Fusion(ICF)facilities for realizing the controllable and clean fusion energy.Due to the unique physical and elecro-optical properties,KH2PO4(potassium dihydrogen phosphate,KDP for short)crystals have been contemporarily regarded as the irreplaceable optics and serve as frequency converter and optoelectronic switch-Pockels cell in ICF projects.Since KDP crystal is soft,brittle,extremely water soluble,thermally sensitive,prone to fracture and scratch,it has been viewed as one of the most difficult-to-cut materials for a long time.However,under the irradiation of intense lasers,the ultra-precision manufactured KDP optics are susceptible to suffer from laser-induced damage,which would largely reduce their optical performances and lifetimes.Even though new techniques(e.g.,laser conditioning,precision repairing)have been developed worldwide to solve the issue of laser damage on KDP optics,the practical laser-induced damage threshold(LIDT)of KDP optics was still far below the theoretically calculated one.Currently,low LIDT of KDP optics has been the bottleneck issue limiting the energy output promotion of ICF projects.Aiming for the surface defects(e.g.,micro-pit,scratch and micro-crack)introduced during the process of ultraprecision machining,laser conditioning and precision micro-machining repairing of KDP optics,we thoroughly investigated the effect of surface micro-defects and their repairing on the laser damage resistance using the combination method of numerical analysis and laser damage experiments.Since the nonlinear ionization under intense laser irradiation is the underlying physical mechanism involved in the early stage of laser-induced damage,the evolution of nonlinearly excited free-electron density was firstly calculated based on synergetic considerations of Keldysh photo-ionization,electron decay and different prevailing impact-ionization models.By comparing the calculated results with previously reported experimental data on KDP materials,the ionization models,which are the most applicable to KDP crystals,were chosen and the theoretical model used for predicting the LIDT of ideal KDP crystals was correspondingly developed.Based on the finite difference time domain(FDTD)algorithm,the numerical model was also built to evaluate the impact of micro-defects on the properties of laser propagation and the feasibility of this model was validated finally.To theoretically investigate the influence of surface micro-defects on the laser damage resistance of KDP crystals,FDTD models for micro-pits,cracks and scratches on KDP surface were built according to the experimentally tested morphological information.The effects of shape,structural parameters,and spatial location of surface micro-defects on the laser propagation properties and the corresponding mechanisms were systematically analyzed.The results indicated that different types of surface micro-defects would produce light intensification inside KDP crystal to different extents through their modulation behaviors to incident lasers.Surface micro-pits with size range close to laser wavelength could induce the largest light intensification.The peak light intensity enhancement factor(LIIF)caused by conical cracks would reach up to 500 times due to the effects of convex-lens focusing and double internal total reflections.The rates of nonlinear photo-ionization,impact ionization and electron decay were strongly light intensity dependent.Hence,we proposed to develop a theoretical LIDT model for flawed KDP surfaces with the bridging role of light intensification caused by surface micro-defects.The LIDT model was then adopted to quantitatively analyze the mapping relationships between LIDTs and micro-defects.In order to experimentally study defect-induced laser damage and its growth behaviors,we firstly measured the single-shot LIDT of ideal KDP surface by means of various testing protocols.The experimental results agreed well with those predicted by our developed LIDT model,which validated its feasibility.Then,micro-defects with ductile,brittle and ductile-brittle blending features were introduced on KDP surfaces using micro-indentation and milling methods.The artificially flawed crystal surfaces were tested in the laser damage experiments and the results indicated that the ductile defects would not greatly affect the laser damage resistance,while the brittle defects could lower the LIDT to 28.9% of that for defect-free surface.The morphology and location of laser damage sites further confirmed that brittle fracture site was the primary factor lowering the laser damage resistance of crystal surface.The laser damage growth behaviors and underlying mechanisms were also investigated under the irradiation of multiple-pulse lasers.By comparing the laser damage morphology and growth behavior with the theoretical analysis,it was concluded that light intensification caused by circumambient micro-cracks and energy-accumulation effect under multi-pulse laser irradiations were the two physical mechanisms governing the laser damage growth behaviors for long-and short-pulse lasers.To improve the laser damage resistance and extend the service life of KDP optics,micro-milling with ball-end tool was proposed to repair the micro-defects and preexisting damage sites on KDP surfaces by holding their growth behaviors.The repairing mechanism for improving the laser damage resistance was explored by comparing the effects of repaired and unrepaired KDP surfaces on the laser propagation properties.The strong width-depth ratio effect was found existing in the light intensification caused by repaired crystal surfaces.It was suggested that the width-depth ratio of repaired contour must be devised to be larger than 5.0 for mitigating the local light intensification by avoiding the occurrence of double internal total reflections.The repaired smooth surfaces with spherical and Gaussian contours were achieved using micro-milling cutters with surface roughness Ra smaller than 50 nm.Laser damage test indicated that the LIDT of repaired KDP surface was improved to be 3 times as high as that of predamaged surfaces.Further,the repaired crystal surfaces showed extremely strong stability under intense laser exposure.The tool marks,especially multiple tool marks introduced inside the repaired surfaces could badly decrease the repairing quality of KDP crystals that the LIEFs induced by front-and rear-surface tool marks were 5.9 and 2.1 times as high as those of ideally repaired surface.The LIDT of repaired KDP surface with tool marks was just 83.6% of that for ideally repaired surface.The tested damage morphology and LIDT on repaired KDP surface were reasonably consistent with the FDTD simulation results.