Energy partitioning in N₂ microwave discharges: integrated Fokker–Planck approach to vibrational kinetics and comparison with experiments

Abstract

This work investigates energy transfers between electrons, vibrational and translational degrees of freedom and their effect on dissociation mechanisms in a N2 microwave plasma in the pressure range between 50 and 400 mbar. A novel self-consistent 0D plasma chemistry model describing vibrational kinetics via the vibrational energy equation and the Fokker-Planck approach is developed. It is used to simulate conditions achieved experimentally, providing good agreement with measured values of vibrational and gas temperature and electron density. Above 100 mbar energy efficiency of dissociation increases with power density, due to the significant contribution of collisions between vibrationally excited N2 and electronically excited molecules. Energy transfer to vibrations is maximum at low power density and low pressure due to reduced gas heating.

Spatio-temporal analysis of power deposition and vibrational excitation in pulsed N2 microwave discharges from 1D fluid modelling and experiments

“…A detailed list of all the exothermic reactions considered can be found in a previous work by the authors of this paper [17].…”

“…Lastly, term Q axial takes into account gas cooling due to convective transport and Q rad is the heat loss due to radiation, both are describes in previous publications [16,17].…”

“…They showed a non-concave electric field and reduced electric field profile, which, depending on the profile of electron temperature and density, might result in a non-concave profile of deposited power density. The large uncertainties linked to the estimation of P d lead to large variations in model results in 0D and one dimensional (1D) models [8,[15][16][17].…”

“…Uncertainties are present also on the chemistry set to be used for intermediate pressure (10-1000 mbar) discharges. Extensive work has been dedicated to the modelling of lowpressure discharges in pure nitrogen [23], resulting in a selfconsistent chemistry set that was recently employed to simulate CW experiments for p > 50 mbar [17]. Given the role of N atoms in the heating of the gas in N 2 discharges [10,11], rate coefficients for dissociation mechanisms are often carefully treated in literature.…”

“…Finally, due to the role of electronically excited species in ionisation, dissociation and heating [4,17], their creation and destruction mechanisms are also under investigation in literature, with particular attention to state-specific impact excitation from vibrational levels [27][28][29].…”