Layered structure around an extended gliding discharge column in a methane-nitrogen mixture at high pressure

The current work aims at investigating the detailed spatial structure of the thin plasma column of a gliding arc (GA) discharge extended in N2-CH4 gas mixtures, using visualization techniques. The GA discharge was operated at up to 5 atm in a high-pressure vessel with extensive optical access. The results show that the emission intensity from the plasma column increased tenfold with the addition of 0.1% CH4 in nitrogen, compared to that in pure N2. Furthermore, an additional layer located around the GA discharge column is detected. Imaging through spectral filters and spectral analysis of the emitted signal indicate that the emissions of this outer layer are mostly from the CN A-X and CH A-X transitions. This outer layer can propagate and extinguish dynamically, similar to the flame front in combustion. Besides, the separation of this outer layer to the plasma core decreases with pressure. The layered structure and its dynamical behaviors can be explained by a plasma-sustained radical propagation mechanism. The high-power plasma column can produce a high-temperature zone with rich atomic species, surrounded by the relatively cold N2-CH4 mixture. At the mixing layer between the high-temperature zone and the N2-CH4 mixture, some highly exothermic reactions occur to produce excited CN and CH species, which emit their specific spectra. As the high-temperature zone expands with time, the outer layer propagates outward. However, with the propagation continuing, the radical species involved in the outer layer formation are rapidly consumed, and thus, this layer disappears when it propagates too far away from the plasma column