Structural Optimization of an Inductively Coupled Plasma Torch for Enhanced Jet Stability in Optical Fabrication

Abstract Plasma jet machining has emerged as a promising noncontact technique for high-precision surface processing, advantageously avoiding subsurface damage. However, jet instability significantly limits its effectiveness. To address this limitation, this study presents a systematic structural optimization of an inductively coupled plasma torch. A novel inclined inlet geometry is proposed to improve axial flow guidance, and the effects of inlet angle and excitation-to-nozzle distance on flow dynamics are investigated using three-dimensional multiphysics simulations. An image-processing method for quantifying jet stability is developed using three key metrics: centroid displacement, area fluctuation, and temporal superposition. A 5° inlet angle and a 5-mm excitation-to-nozzle distance significantly improved jet radial uniformity and thermal symmetry. Plasma polishing experiments further validated the optimized structure. Surface roughness measurements confirmed a substantial reduction in Sa roughness from 5.589 to 1.033 nm, achieving near-Gaussian removal functions with reduced variance and improved rotational uniformity. The material removal rate also exhibited enhanced stability, with the coefficient of variation decreasing to 3.92% from 6.43% for the original design. Furthermore, parametric analysis showed consistent performance across varying flow rates and power levels. These highlight the effectiveness of structural optimization in stabilizing plasma jets and improving material removal quality, providing practical insights for the advancement of plasma-based processing technologies.