Mechanism-guided control of high-spatial-frequency roughness in small-diameter plasma jet machining by suppressing near-surface vortex-induced transport

The demand for full-spatial-frequency, nanometer-precision machining of complex freeform surfaces across various materials has been rapidly increasing in fields such as extreme ultraviolet (EUV) optics, large synchrotron-radiation facilities, astronomy, and precision metrology. Compared with conventional contact machining, non-contact jet-based processes have attracted wide attention from their high processing flexibility and damage-free characteristics. Among them, atmospheric-pressure plasma chemical vaporization machining (AP-PCVM) has been widely applied for ultra-precision figuring of optical components, benefiting from its chamber-free operation and high etching efficiency. To enhance figuring accuracy in the low-spatial-frequency region, PCVM employs a small-diameter plasma jet to achieve higher spatial resolution; however, such configurations often cause surface roughness deterioration in the high-spatial-frequency region. The underlying mechanism remains unclear and severely limits its further advancement and applications. In this study, the mechanism behind surface roughness deterioration was clarified by combining experimental analysis with CFD simulation. It was found that a small-diameter jet intensifies the near-surface shear layer, triggering Kelvin-Helmholtz (K-H) instabilities that continuously generate vortices which rapidly evolve into unsteady three-dimensional structures. The resulting sweep, lateral refresh, and ejection regions locally enhance the convective–diffusive mass transfer of species toward and away from the surface, causing meso-to-micro-scale non-uniformity in material removal, and ultimate surface roughness deterioration. Multi-dimensional surface characterizations further confirmed that this deterioration originates from vortex-shaped etch pits, validating the proposed mechanism. Based on the revealed vortex-induced mechanism, suppressing the vortex strength and occurrence frequency effectively mitigates vortex-induced roughness deterioration of PCVM, providing a mechanism-driven optimization pathway for small-diameter jet figuring process design.