Study On Water Dissolution Ultra-precision Continuous Polishing Of Large Sized KDP

Potassium dihydrogen phosphate(KDP)crystal is a high-quality nonlinear optical material,which is the only material for electro-optic switches and laser-frequency conversion applications in the inertial confinement fusion(ICF).For the ICF project,the strict requirements of KDP crystal are large size,low surface roughness,and high accuracy of flatness.However,owing to its soft texture,anisotropic and easy deliquescence,KDP crystal is regarded as difficult-to-machine material.The processing of large sized and high-quality KDP crystal has become one of the bottlenecks of ICF and other related projects.Therefore,the water dissolution was combined with the continuous polishing technology on the basis of the principle of KDP crystal's deliquescence.The paper explored the feasibility and effectiveness of large sized KDP crystal's processing by water dissolution ultra-precision continuous polishing technology.The analysis of water droplets trajectory uniformity was performed in the polishing by using kinematics and computer simulation technology.Furthermore,to verify the reliability of simulation,some experimental tests were also carried out.First of all,based on the regional division and the regional statistical strategy,a new simulation analysis method of trajectory uniformity about water droplets was proposed.According to the movement relationship between the KDP crystal and the polishing pad,the motion trajectory equations of water droplets were established.Meanwhile,the coefficient of variation(CV)was utilized to evaluate the trajectory uniformity.The simulation of water droplets trajectory on the KDP crystal under certain eccentricity and uncertain eccentricity motion modes was then analyzed.Results show that the CV varies from 0.67 to 2.02 under the certain eccentricity motion mode,and varies from 0.48 to 0.65 under the uncertain eccentricity motion mode.Hence,the trajectory uniformity is better under the uncertain eccentricity.In addition,the trajectory uniformity under the circular and spiral distribution of water droplets were simulated.The coefficient of variation was always small under spiral distribution,which indicated the trajectory was more uniform.Secondly,the structure of the large sized water dissolution ultra-precision continuous polishing experimental system was optimized.The multi-point variable position automatic drop device was design to achieve the uniform distribution of polishing slurry on the large sized polishing pad.The wind shower device with multi-line distribution was used to quickly dry the polishing pad.It not only saved time but also improved efficiency.The gear-driven clamping device ensured the stable transmission of the rotation speed.What's more,the crystal holder is designed to clamp various sizes of KDP crystal.Last but not least,the simulation results were verified by using self-made large sized water dissolution ultra-precision continuous polishing test system,and the experimental results were consistent with the simulation analysis.The KDP crystal had a more high surface quality at a non-integer speed ratio by using the polishing pad with spiral grooves,which was processed under the uncertain eccentricity motion mode.Moreover,the processed surface was further protected by film for double-sided processing of 100 mm×100 mm×10 mm KDP crystal.A super-smooth and scratch-free surface with surface RMS roughness of less than 3 nm was obtained.