Comparative optical emission study of atmospheric-pressure plasma jets in Ar, He, and O₂ mixtures driven by a high-frequency sinusoidal source

Research on atmospheric-pressure plasma jets (APPJs) generated using argon (Ar), helium (He), and their oxygen (O₂) mixtures has been widely reported; however, direct comparisons of optical emission characteristics under identical operating conditions remain scarce, particularly for sinusoidal AC excitation. Most previous studies have focused on radio-frequency or pulsed discharges, which do not fully represent excitation modes relevant to stable AC-driven jets. In this work, the optical emission characteristics of APPJs operated in Ar, He, and their O₂ mixtures are systematically investigated using a dielectric barrier discharge configuration driven by a high-frequency sinusoidal power source. The relative intensities of emissive species, including Ar I, He I, O I, OH, and N₂(SPS)/N₂⁺(FNS), were analyzed in the ambient region downstream of the plasma plume using optical emission spectroscopy (OES). Electrical measurements were simultaneously performed to support the interpretation of the optical emission trends. Notable intensity variations were identified for Ar I (650–850 nm), He I (725 nm), O I (777 nm), OH (307 nm, A²Σ⁺→X²Π), and N₂(SPS)/N₂⁺(FNS) bands (335–405 nm), indicating that O₂ addition significantly modifies excited-species populations and plasma chemistry. The addition of O₂ to Ar or He favors the excitation of the N₂(SPS) band. In the Ar plasma, a small amount of O₂ slightly increases the Ar I emission, whereas higher O₂ concentrations progressively suppress Ar I, OH(A–X), and O I emissions. In the He plasma, a small O₂ addition slightly decreases OH emission while enhancing atomic oxygen excitation, resulting in a moderate increase in the O I (777 nm) line at low O₂ flow rates, whereas He I emission remains relatively stable. The results represent time-integrated optical emission from the plasma effluent under ambient conditions and are interpreted in a comparative manner under identical operating parameters.