| Optical parts with high precision and no sub-surface-damage are widely applied in major projects such as Laser-driven inertial confinement fusion system, and becomes one of the determining factors of promoting these projects. Particularly, fabrication of complex freeform surface with high efficiency is much difficult, and it poses a grand challenge for advanced optical fabrication. Atmospheric Pressure Plasma Processing(APPP) is a controllable chemical etching processing method with sub-millimeter spatial resolution, which offers a feasible way for manufacturing complex freeform surface without sub-surface-damage. However, the material removal rate and long term processing stability can not satisfy the requirements of fabricating large demand optical components. In the APPP system, reactive etching species are produced through exciting reactant gas by RF power which is coupled into plasma by a torch. Therefore, the characteristics of material removal rate and processing stability of a torch are largely restricted by power coupling efficiency and plasma torch performance. Thus, there is an urgent need for a newly developed plasma torch with high efficiency and stability for application of the APPP.In this paper, a novel atmospheric pressure plasma jet torch is designed and established for high efficiency and stability. The torch discharge performance is studied by theoretical analyses and experimental investigations. A set of processing experiments are then conducted to validate its high material removal efficiency. Based on the experimental result, a processing method, intermittent processing, is proposed to accomplish the aim of long term stability.In order to fabricate complex freeform surface with high spatial resolution, two kinds of atmospheric plasma jet torches are designed based on the principle of gas discharge. The voltage and current are measured during discharge test process to analyze the discharge mode, the needle-hole electrode torch is then selected for its high performance. Using single factor experiment method, a set of processing experiments are conducted to study the relationships between the material removal function and gas flow rate, and standoff distance. The results show that appropriate oxygen ratio can improve material removal rate largely, which can meet the requirement of high efficiency machining.Later, theoretical analysis and simulation study of the plasma torch are conducted from outside to inside of the torch. A power matching network outside of the torch is developed, and the condition for maximum coupled power is obtained. The electrical characteristics variation of plasma torch is investigated based on experiment. For discharge process inside the torch, the flow field and the detailed species distribution are calculated using COMSOL multiphysics software. Based on the results, simplified capacitive coupled plasma discharge models are established to compare the reactive species distribution and density for different discharge modes.Finally, the processing stability of the the needle-hole electrode torch is conducted by means of experimental method. The torch temperature variation law and the impact of workpiece temperature on removal rate are studied. Based on this, the processing stability experiment is then conducted. And the evolvement of electrode surface characteristic is investigated during the machining process, which proves that the generation of Al F3 film on the electrode surface is an essential factor for influencing the processing stability. Then, electrodes made of different kinds of material and with forced water cooling are tested to obtain processing stability. Thus an intermittent machining method is proposed, which is able to accomplish the aim of long term processing stability. |
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