Influence of the RF voltage amplitude on a RF-LF discharge in α-γ mode

The aim of this study is to understand the influence of the RF voltage amplitude on a RF (5MHz) - LF (50kHz) double frequency discharge in α-γ mode in an Ar-NH3 DBD at atmospheric pressure. This is achieved through argon (at 750 nm) and continuum (at 500 nm) emission intensity measurements performed by time and space resolved spectroscopy, combined with measurements of the population of Ar metastable atoms. It is found that a rise in the RF voltage leads to an increase in the continuum emission, while both Ar emission and Ar metastable population decrease. A 1D fluid model is used to reveal the origin of this behaviour. It turns out that, in relation with the RF voltage increase, the electron density increases, explaining the rise of continuum emission. This higher electron density induces a decrease in the gas voltage applied between the electrodes. The gas voltage drop then leads to a lower electric field in the cathodic sheath, causing a decrease of the reactions involving highly energetic electrons, i.e. the emission of Ar and the formation of metastable Ar atoms.