Modelling the vibrational non-equilibrium dynamics in a pure N₂ discharge: interplay of transport and power deposition

Vibrational excitation of N₂ beyond thermodynamic equilibrium enhances the reactivity of this molecule and the production of radicals. Strong vibrational non-equilibrium has been found experimentally in microwave (MW) discharges in pure N₂ either outside the plasma core for continuous wave discharges or as an effect of power pulsing for pulsed discharges.

A 1D radial time-resolved self-consistent model has been developed to study the mechanism of formation of vibrationally excited N₂. Temporal and spatial profiles of gas and vibrational temperature, spontaneous optical emission, electron density and electron temperature are compared to validate the model and the choice of input power density against experimental measurements in a pulsed N₂ discharge.

The model reveals two regions in the plasma: a core where chemistry is dominated by power deposition and where vibrational excitation proceeds on a time scale of ~10 μs and an outer region reliant on radial transport, where vibrational excitation happens slowly during the whole length of the pulse (200 μs). The two regions are separated by a sharp gradient in the estimated deposited power density. The latter is revealed to not be in a direct proportionality relation with the emission intensity.

The low concentration of excited species outside the prevents the gas from heating and the reduced quenching rates prevent the destruction of vibrationally excited N₂ and thereby maintain the observed high non-equilibrium.