Chemically reactive Plasma Jet Machining (PJM) is a contactless and efficient surface machining technique increasinglyapplied to the surface shape generation and error correction of various optical elements. However, the application offluorine-based PJM to surface machining of N-BK7? is challenging since the chemical interaction between reactive plasmaspecies and metal components of N-BK7 induces a residual layer in the contact zone and surrounding of the plasma-treatedarea. It was noticed that a residual layer degrades the ability of obtaining the prerequisite surface profile and causes anonlinear and hardly predictable removal behavior with respect to the etching time. In this paper, extensive studies areconducted for relaxing constraints in applying the fluorine-based PJM to the surface machining of N-BK7, particularlyregarding to the manufacture of freeform optical elements. In this regard, the chemical composition of residual layer isevaluated by using SEM/EDX analysis aiming at clarifying the chemical kinetics between plasma generated active particlesand the N-BK7 surface atoms. Furthermore, the etching behavior of N-BK7 is compared with Fused Silica to verify theoptimality of obtained results. Finally, the area machining is tested at different plasma dwell times to evaluate thepredictability and regularity of results.
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