NH₃ dissociation along the gas flow lines of Ar–NH₃ dielectric barrier discharges and its effects on global and local properties

Abstract This paper examines the effects of NH 3 dissociation on the physics driving low frequency dielectric barrier discharges operated in Penning Ar–NH 3 gas mixtures. Optical emission and absorption diagnostics in combination with current–voltage characteristics are used to probe the global (averaged over the whole discharge zone) and local (at a given position along the gas flow lines) properties as a function of the gas flow rate. When the gas flow rate is sufficiently high, NH 3 does not remain in the discharge zone long enough to be significantly dissociated and thus to have a significant impact on the discharge; hence, the characteristics are those of a typical glow discharge operated in a Penning mixture along the entire electrode length. When the gas flow rate decreases, the gas residence time in the discharge region increases such that NH 3 becomes strongly converted to reaction products other than those leading to Penning ionization. This leads to a glow discharge only over a limited length of the electrode zone near the entrance, with no further breakdown towards the exit. The perceived effects on the discharge properties become significant if the overall behavior is considered, but on a local scale, the populations of Ar(1s 5 ), Ar(2p), and H 2 (a) states deduced from optical diagnostics are only weakly modified. Using Ar(1s 5 ) measurements and the predictions of a 1D fluid model, values of the degree of NH 3 dissociation at two gas residence times are estimated. From such analysis, it is highlighted that the discharge can remain in a diffuse mode even when the quantity of NH 3 is much lower than the one in the injected gas mixture.