Research On Surface Formation Mechanism In Removal Of Subsurface Damage On Fused Silica By Atmospheric Pressure Plasma Processing

In the field of inertial confinement fusion (ICF) large optical systemapplications, there is an increasing demand for damage-free optics, which thuspresents a number of formidable challenges to the manufacturing process. Theperformance of optics mainly depends on the surface quality. For instance, thesub-surface damage in fused silica optics--the weakest components in the whole ICFoptical system will lead to the decline of anti-laser induced damage threshold, andthus readily result in the damage of the optics themselves in high-powered lasersystem, which negatively impacts the stability, the performance, and the sevice lifeof the ICF system. Conventional mehcanical manufacturing techniques have atrade-off between speed and quality. Therefore, the developing of an opticprocessing technology based on subsurface-damage rapid removal is criticallyneeded to improve the optical damage threshold and service life.Atmospheric Pressure Plasma Processing(APPP) technology, a non-contactchemical etching processing method without any mechanical pressure applied to theoptics during processing the optic surface, can fullfill the above requirement with anadditional merit--rapid removal of the optic subsurface damage. Thereby, the APPPis a very promising and powerful technique that can be employed in the opticalmanufacturing field. This paper focuses on the fused silica sub-surface damageremoval mechanism of the APPP and the formation mechanism in removal ofsubsurface damage. Moreover, the deposition formation mechanism in the APPP andthe corresponding deposition inhibition method are also studied.To analyze the removal mechanism of sub-surface damage in the APPP, a sharpindenter was used to intrude into the surface to artificially produce sub-surfacedamage. The as-indented surface was then processed layer by layer by the APPP.Based on the procedure, the characteristics of the removal of sub-surface damageduring the APPP are revealed in this paper. By analyzing the dimension andmicro-scale morphology of damaged areas, the geometric model of the relationshipbetween surface quality and the sub-surface damage is established, and it can beused to predict the surface quality variation with the subsurface damage duringAPPP.The machnical and chemical properties of the deposition after APPP on thesurface are studied by means of the charaterizaitons of micro-scale morphology,mechanical properties, the element composition, and the coupling state of chemicalelements of surface deposits. Plasma excitation, the ionization of reaction gases, andreaction between plasma and the workpiece surface were examined to reveal the formation mechanism of the surface deposition during APPP.Emission spectroscopy technology was employed to analyze the functions of Fatoms and CF2groups ionized from reaction gas CF4. The effects of differentatmospheric plasma processing parameters on the ionization of reaction gas CF4arestudied. The flow distribution of the machining region is calculated with fluidsimulation function using FLUENT, which establishs the relationship between gasflow rate and surface deposition distribution.Based on effects of plasma processing parameters and flow characteristics ofon deposition, a new plasma torch is designed and the surface deposition issuppressed by adjusting process parameters. Furthermore, solid features arecharacterized by comparing the surface free energy between mechanically groundand polished suface. The impacts of the solid surface properties on chemicalreactions of the atmospheric plasma are studied. Finally, subsurface damage-freeremoval characteristic of the APPP is validated in terms of the micro-scalemorphology and mechanical properties of as-processed surface.