Vibrational Excitation of Nitrogen in Atmospheric Pressure Plasma Jets Sustained by Ns Pulse and RF Waveforms*

Time-resolved N₂ vibrational temperature and translational-rotational temperature in quasi-two-dimensional atmospheric pressure plasma jets excited by ns pulse and RF discharges are measured by broadband vibrational CARS, in collinear phase matching geometry. As expected, the results indicate much stronger vibrational excitation in the RF plasma jet due to both lower reduced electric field and higher coupled power. In a ns pulse discharge in N₂/He, N₂ vibrational temperature is significantly lower compared to that in N₂/Ar, due to the more rapid V-T relaxation of nitrogen by helium atoms. In the RF plasma jets in N₂/Ne and N₂/Ar, the vibrational excitation increases considerably as the nitrogen fraction in the mixture is reduced. The experimental data in the RF plasma jet in N₂/Ar jet are compared with kinetic modeling predictions. The model solves the electron energy equation, the heavy species energy equation, and equations for the species concentrations. It incorporates electron impact ionization, dissociation, electronic excitation, and vibrational excitation processes; energy transfer among the excited electronic states of Ar and N₂; and N₂ vibrational relaxation (state-specific vibration-vibration energy transfer for N₂-N₂ and vibration-translation relaxation for N₂-N).