Glow discharges observed in capacitive radio-frequency atmospheric-pressure plasma jets

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 alpha mode of a RF discharge can be sustained over a wide range of gap spacings and RF powers. Maximum electron densities of 0.22 x 10(12) cm(-3) were assessed for helium and of 2.75 x 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 alpha sheath occurred, and a transition to either a pure gamma mode covers only a small fraction of the electrode surface. In helium it was easy to ignite directly a pure alpha discharge, but not in argon. Due to the high over-voltage needed, the ignition in argon mostly led to a coexisting alpha and gamma mode. However, through RF power reduction a transition to a pure discharge can be initiated.