Glow Discharges Observed in Capacitive Radio‐Frequency Atmospheric‐Pressure Plasma Jets

Summary: The APPJ is the subject of intensive recent research. Basically, the APPJ consists of an atmospheric‐pressure RF discharge between two bare metallic electrodes and of a superimposed gas flow consisting predominantly of a rare gas. In its usual operating regime, a uniform glow discharge, which was identified as the α mode of a RF discharge, can be sustained over a wide range of gap spacings and RF powers. Maximum electron densities of 0.22 × 10 12 cm −3 were assessed for helium and of 2.75 × 10 12 cm −3 for argon by evaluation of electric measurements using a simple equivalent circuit model. At low RF powers, only partial coverage of the electrodes was observed. At high RF powers, a breakdown of the α sheath occurred, and a transition to either a pure γ mode or a coexisting α and γ mode took place. The γ mode covers only a small fraction of the electrode surface. In helium, it was easy to ignite directly a pure α discharge, but not in argon. Due to the high over‐voltage needed, the ignition in argon mostly led to a coexisting α and γ mode. However, through RF power reduction a transition to a pure α discharge can be initiated. Photograph of the APPJ in operation. magnified image Photograph of the APPJ in operation.