Study On The Damage Of Fused Silica Element Induced By Contamination Particles In The High Power Laser System

The high-power laser system is often applied in the research with extreme physical conditions,such as laser inertial fusion and high energy density physics.The serious pollution problems in the system limit the improvement of the load capacity and maximum laser energy of the system.Fused silica optics are significant working elements in the final optical assembly of the laser system.Contamination particles can adhere to and damage the optical elements far below their laser-induced damage threshold under the high-fluence laser,thereby seriously affecting their optical properties.The study on the damage mechanism of optical elements induced by pollution particles can help to understand the damage process of large optical elements employed in the high-power laser system and explore the evolution law of particles and effective methods of pollution removal.Although some methods have been proposed to explain the mechanisms of contamination particles induced damage,there is a lack of systematic research on different types of contamination particles.In this work,the damage behaviours of three typical pollution particles（Fe metal particle,Al₂O₃ceramic particle,and SiO₂glass particle,respectively）in the high-power laser system were studied,and their different damage mechanisms were investigated by a combination of experimental and simulation methods.（1）The mechanism of the laser-induced damage was studied for the Fe metal particles.Fe is the constituent material of stainless steel enclosures.The laser damage test,damage morphology characterization,and light field distribution,absorptivity,temperature evolution simulations of Fe particles were carried out respectively.At the same time,the plasma pressure distribution was simulated to explain the formation of small etching pit with the“shoulder”profile created by small Fe particles on the output optical surface.It was found that the Fe particles melted after laser irradiation and generated secondary pollution on the input surface of the element,while there was only a smooth etching pit and a circle of arc etching grooves left on the output surface.In addition,the plasma pressure distribution showed that the laser wavelength,laser energy,particle size,and the distance between particles and optics will also affect the morphology of small etching pit.（2）The Al₂O₃ceramic particles originate from the coatings of aluminum structural hardware.Their laser-induced damage mechanism was studied using methods the same as those of Fe particles.The results indicated that there is light distribution inside Al₂O₃particles.The small Al₂O₃particles on the input surface usually create a concentric circular small etching pit on the substrate,but large particles will explode and the semi-molten debris generated will attack optical elements to form fracture damage pits.However,for the Al₂O₃particles on the output surface after laser irradiation,large etching pits and radial etching grooves will be formed on the substrate.（3）The SiO₂glass particles resulting from the damage of fused silica will contaminate the optical elements when dropping on their surfaces.These laser-induced damages were investigated for different damage morphologies of the particles on input and output surfaces.The damage morphologies of SiO₂glass particles on different optical surfaces under a series of laser energy densities were statistically studied.The absorptivity of silica particles under355 nm laser was worked out through spectrophotometer analysis.It was found that SiO₂particles created a large number of brittle fragments during the explosion;At lower laser fluence,the particles on the input surface tended to splash slightly,and the particles on the output surface tended to splash partially.However,at high laser fluence,the input surface was more prone to splash severely,but there was almost no splashing on the output surface.This paper studies the damage behaviour of contamination particles with different optical properties on the input and output surfaces of fused silica elements under the high-power laser,revealing their different damage mechanisms.This work is of great significance to the prevention of pollution-induced damage in the high-power laser system.