Research On Combined Processing For Continuous Phase Plate Based On Atmospheric Pressure Plasma Processing And Bonnet Polishing

Continuous phase plate(CPP)is a kind of microstructural surface with micronlevel amplitude and millimeter-level spatial period.CPP can modulate far-field spot and improve uniformity of illumination on target in inertial confinement laser fusion facility(ICF),which is a key optical component for fusion ignition.In the meanwhile,the complex CPP form and the hard brittle optical silica-based glass bring a significant challenge to ultra precision optical fabrication techniques.Atmospheric pressure plasma processing(APPP),based on noncontact chemical etching process,has superiority of high removal rate and high spatial machining resolution,which show strong capability to fabricate complex surface optics.However,the pure chemical etching process can hardly obtain optical-level smooth surface,that restricts its application in CPP fabrication field.Therefore,a combined processing technique based on APPP and bonnet polishing(BP)is proposed to fabricate CPP.APPP plays a role in efficient material removal to generate CPP form structure rapidly,then BP is used to finish the plasma etched surface through minor material removal.The freeform generation theory using APPP and form-preserving finishing using BP are both investigated in this paper.This study will establish theoretical foundation and provide technological support for combined fabrication technique of CPP.Firstly,a six-axis APPP machined tool was developed.Based on plasma discharge theory,a multiple-physics time-dependent model of plasma discharge,integrating electron diffusion,transport of heavy particles and chemical reactions,was established.The energy coupling process in plasma discharge system is analyzed and the average heating power was calculated.The average heating power was coupled into the heat transfer process between plasma jet and substrate,thus,a substrate temperature predication model was established to analyze long-time machining process.Then,the characteristics of global and local temperature field in freeform surface fabrication were analyzed.It was found that the local temperature change induced by dwell time has the main influence on freeform surface fabrication.Considering the effect of local dwell time,a mathematical model of time-variant influence function was obtained,according to calibration experiment of machined trench at different feed rate.The limitation of linear dwell time calculation method for time-variant influence and the relative non-linear form error were illuminated.Through analyzing generation process of freeform surface,an idea of controlling volume removal to generate freeform surface was proposed,and a non-linear processing algorithm was established.As for characteristics of CPP form generation,the theorical calculation accuracy,applicability with different surface structure characteristics and anti-disturbance performance with calibration error of the algorithm were analyzed,which proves its feasibility.To reveal the influence on generated CPP form of BP,a BP analysis model on microstructural surface based on Preston equation was established.Then,the influence on contact pressure,relative speed between BP tool and substrate of freeform structure characteristics and polishing parameters were investigated using one-dimensional sinusoidal surface.The related polishing experiments were conducted as well.By summarizing the simulation and experimental results,it was found that the contact pressure difference is the main influencing factor on formpreserving capability.The influence law of freeform structure characteristics on formpreserving performance was obtained.In addition,BP was proved to be able to realize excellent form-preserving polishing on microstructural surface.To study characteristics of the combined processing technique,the surface texture,chemical component and roughness on substrate were investigated firstly.The texture roughening process together with the appearance and coalescence of etched pits after APPP,and the finishing process of the etched texture using BP were revealed.Then,the form error,including generation error of APPP and influence on the generated form of BP,was analyzed based on numerical simulation and experiments.Finally,a large aperture CPP was fabricated and tested in Shenguang ? protype system.The results prove that the combined processing technique can fabricate CPP with high efficiency and high quality.