Nitric oxide (NO) production in a kHz pulsed Ar plasma jet operated in ambient air

Abstract

The characteristics of nitric oxide (NO) production in a kHz pulsed atmospheric pressure Ar plasma jet, operated in ambient air, are explored by measuring the absolute density n gnd of the ground state NO ( X 2 Π ) using laser-induced fluorescence, and the emission of the excited state NO ( A 2 Σ + ) using optical emission spectroscopy. The quenching rates of NO ( A 2 Σ + ) are determined and the rate constants by air and Ar are evaluated as k q , air = 4.7 ( ± 0.2 ) × 10 − 17 m 3 s − 1 and k q , Ar = 1.4 × 10 − 19 m 3 s − 1 . The time-resolved NO measurement is carried out within one discharge cycle. It is found that NO generation shows fast rise accompanied by the propagation of the plasma ionization wave and a constant n gnd remains in the afterglow time. n gnd is in the order of magnitude of 10¹⁹ m⁻³ in our plasma. Spatially resolved n gnd is mapped for the plasma plume and the influences of voltage, flow rate, gas admixture and pulse frequency on NO production are discussed. NO generation is revealed to benefit from increased injected power and a flow rate that leads to an appropriate amount of mixing with ambient air (e.g. 4 slm in this work). The addition of 1% N 2 is optimal for a higher n gnd and air continuously contributes to the NO ( X 2 Π ) formation below 3%. The NO excitation proportion per voltage pulse is found to be almost the same. Through analysis, it is confirmed that NO is produced mostly from the reactions involving atomic N and metastable N 2 ( A ) .