NO production in an RF plasma jet at atmospheric pressure

A time modulated RF atmospheric pressure plasma jet, operated in ambient air with a flow of argon with a few per cent of air, N₂ or O ₂, was characterized by measuring the gas temperature with Rayleigh scattering, the absolute NO density with laser-induced fluorescence, and the emission of NO A and N₂ C with time resolved optical emission spectroscopy. The gas temperature, NO density and the emission measurements are carried out both time and spatially resolved. The atmospheric pressure plasma jet has the advantage that the plasma dissipated power can be measured, and it was found that the gas temperature depends on the power, rather than the gas mixture. The NO density increases with increasing plasma power, and was found to have a maximum around 1.5 × 10²¹ m^-3 at an air admixture of 2%. The N₂ C emission is modulated by the 13.9 MHz RF frequency, while the NO A emission front increases with much slower velocity during the 20 kHz duty cycle, which gives an insight into the excitation mechanisms in the plasma. Through the addition of either N₂ or O ₂ to the plasma it was experimentally confirmed that the production of atomic N radicals are of key importance for the NO production in this atmospheric pressure plasma jet.