Research On Characteristics Of The Plasma Jets And Surface Evolution Mechanism Of Atmospheric Inductively Coupled Plasma Processing

With the development of science and technology,optical fabrication is driven forward in recent years.Demands for large aperture optical components are dramatically increased in the fields of aerospace,defense,new energy and astronomical research.The requirements for production efficiency and surface quality of the optics are more stringent.Hence,how to fabricate these optical components quickly,efficiently and without surface damage becomes an important issue in optical fabrication field.The emergence of Inductively Coupled Plasma(ICP)processing at atmospheric pressure which based on chemical etching provides an effective solution to this issue.Because this processing technology has a relatively high material removal rate,and it is believed to be able to mitigate or remove surface/subsurface damage without introducing new one.However,research on characteristics and the surface/subsurface damage removal mechanism of the plasma processing used in the field of optical fabrication are barely reported.Therefore,characteristics of ICP used in optical fabrication are studied theoretically and experimentally in this paper.This study will provide a solid theoretical basis and technical support for engineering application of the ICP processing in the field of optical fabrication.Firstly,a five-axis hybrid ICP machine tool which aims at fabricating large aperture optics is developed.The energy coupling mechanism of the ICP is analyzed after completing development of the machine tool.Flow and temperature fileds of the plasma with various radio frequency are analyzed based on a two temperature plasma model.Temperature dependence of composition of plasma at atmospheric pressure is computed and combined with the results obtained by using Optical Emission Spectroscopy to understand the characteristics of the plasma spectra.This lays foundation for analyzing the damage mechanism of plasma generators and for computing temperature of the plasma jet.For the observed damages of the plasma generator,experiments are combined with simulation analyses to verify the damage is caused by chemical erosion of the plasma.According to the characteristics of the flow field inside of the generator,the dominant factor for damaging the generator is elucidated that the damage is caused by electromagnetic pumping effect which is resulted from the high frequency magnetic field.This pumping effect could result in recirculation and backflow of the plasma.A fully demountable plasma generator is developed.The developed generator not only reduces the damage,but also prolongs the continual work time of a generator.Considering the high temperature,low velocity and weak stiffness of the plasma jets,it is difficult to stably,securely and reliably fabricate optical components.Additional constrained nozzles are proposed to solve the problem and recude the plasma jet temperature.Based on computational fluid dynamics,models for plasma jets passing through De-laval nozzles and conical nozzles are proposed.The effects of geometry parameters of the nozzles on characteristics of the plasma jets are studied.Conical nozzle with optimal structure are manufactured.Comparison analyses are conducted on characteristics of the plasma jets obtained with experiments and simulations to verify the reliablity and accuracy of the built models.The aim of reducing the temperature and improving the stiffness of the plasma jets is achieved.To investigate the evolution of micro-cracks under the effects of plasma processing,firstly,the formation mechanism of the surface/subsurface damage of fused silica glasse is analyzed.Depth and characteristics of surface/subsurface damage of fused silica substrate are measured and characterized.Then,evolution of micro-cracks which are processed by plasma processing is experimentally examined,and the corresponding physical and chemical process is studied accordingly.Finally,surface evolution and tracking method of Level Set Methods is introduced.Model for simulating the evolution of cracks under plasma processing is then developed.Simulation results are obtained and compared with experimental data to verify the reliability and accuracy of the built model.To reveal the mechanism of surface formation and evolution during surface/subsurface damage removal of fused silica using plasma processing,the model for simulating the evolution of cracks is firstly extended to two-dimensional surface evolution model.The surface formation mechanism and roughness evolution are investigated under the effects of characteristics of different surface/subsurface damages.Two-dimensional surface evolution model is then extended to three-dimensional,and three-dimensional morphologies with various plasma processing time are analyzed and verified experimentally.Four procedures in obtaining a planarized surface during damage removal with plasma processing are achieved.The evolution mechanism of the plasma processed surface is revealed.These results provide theoretical supports for predicting quality of processed surface and fabricating optical components with high efficiency and without damage.