| Fused silica have been widely used in high power laser facility,such as inertial confinement fusion system.Currently,the world's largest laser system is the national ignition facility of the United States,which aims to realize fusion ignition by means of192 high-flux laser beams.After years of construction and research,the system has not been successfully ignited.The investigation results show that the ultraviolet laser induced damage of fused silica elements is the main factor that restricts the success of fusion ignition.Currently,the damage threshold of the large-aperture fused silica element is only8J/cm~2,far from meeting the ignition demand.In order to further improve the output of the laser facility,it is urgent to solve the laser damage of optical elements.Calcium fluoride is a kind of traditional optical material.Compared with fused silica,it has strong resistance to laser damage.Replacing fused silica with calcium fluoride is expected to improve the output of high-power laser device.Calcium fluoride is a soft and brittle crystal material.In the process of grinding and polishing,the surface is prone to residual scratches,contaminating impurities and other damage precursors.The existence of these defects restricts the improvement of damage threshold.In order to solve this problem,the surface defect of calcium fluoride is taken as the research object in this paper,focusing on the damage characteristics of ultraviolet laser induced damage by scratch and impurity pollution,providing theoretical guidance for optimizing calcium fluoride process.According to the results of theoretical analysis,the combined process of magnetorheological polishing,dynamic acid etching and plasma cleaning is adopted in this paper,which greatly improves the calcium fluoride damage threshold.The main research contents of this paper include:(1)Atomic force microscope was used to measure the morphology of scratch and establish the geometric model of scratch.The modulation effect of micro-nano scale scratches with different geometrical morphologies on incident laser was analyzed by FDTD simulation.It was found that compared with radial and continuous pitted scratches,Hertz scratches had the strongest light field modulation effect.Based on Mie scattering theory and thermal steady state heat transfer equation,the heat absorption of nano-scale impurities under ultraviolet laser irradiation can be obtained.The typical pollution:iron and cerium oxide were calculated,the results show that the pollution particles steady temperature can reach 10000?above,far more than calcium fluoride crystal laser damage critical temperature.Then,the damage induced by the defects of fused quartz silica and calcium fluoride was compared.(2)The effect of chemical mechanical polishing and magnetorheological polishing on surface scratch defects was studied.Compared to conventional mechanical polishing,chemical mechanical polishing and magnetorheological polishing do not introduce new fracture defects when removing surface materials.Chemical mechanical polishing has low processing efficiency and cannot be applied to the processing of optical elements with complex surface.Therefore,magnetorheological polishing is selected in the combination process to remove the surface scratches.In the magnetorheological polishing experiment,the passivation effect of magnetorheological polishing on scratch defects was found,which can effectively reduce the light field modulation of scratches and improve the damage threshold of components.However,contaminant defects introduced by magnetorheological polishing limit the increase in damage threshold.(3)The acid etching process and the plasma cleaning process were optimized to form the combined process,which is magnetorheological polishing-dynamic acid etching-plasma cleaning,the process effectively removed scratches and surface pollution.The threshold test results showed that the damage threshold of calcium fluoride under the combined process reached 14.7 J/cm~2. |
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