Time-resolved optical emission profiles of RF atmospheric pressure plasmas: α and γ modes

Large-area atmospheric pressure glow discharges have received growing attention in recent years for their potential application in many scientific disciplines. One possible source configuration is a conventional parallel plate reactor. In this work, the stability of the discharge and the dynamic evolution of the optical emission are investigated by means of high speed (5 ns exposure time) images, electrical measurements and computer simulations. For a given driving frequency and as current increases, the emission profile evolves from a bell-shaped profile with a maximum emission at the centre of the discharge to a doubl-humped profile with two bright emission layers one above each of the electrodes. Although bright layers near the electrodes are often associated with the gamma mode, these layers can be first observed in the alphamode and only after a significant increase in current, the transition into the gamma mode takes place. During the mode, the bright emission layers get closer to the electrode and the discharge contracts radially. When the discharge transitions into the gamma mode, the dynamic impedance of the discharge becomes negative and unless a stabilizing mechanism is in place (e.g. a dielectric barrier on one of the electrodes or a current controlled power source) the discharge current runs away and the discharge becomes eventually an arc. The time-resolved study of the emission profiles reveals differences in the underlying physical principle governing the formation of the bright emission layers above the electrodes.The experimental observations are well reproduced by computer simulations which present a clear picture of the difference between the two modes and the transition between them.