We elaborate a reduced kinetics model to study humid air plasmas at atmospheric pressure generated by X-ray irradiation. The originality of the present approach is to use the experimental results of the transmission measurements, in the case of a microwave signal by the X-ray-induced plasma filled waveguide, to fit the calculated time evolutions of some plasma parameters such as average electron energies and an effective loss coefficient. The reduced kinetics model used to restitute the transmission measurements is based on the solution of a one-dimensional transport of a guided microwave signal coupled to the calculation of the complex electric conductivity of the plasma. The conductivity is calculated using a simplified kinetics scheme based on three species (electrons, positive ions, and negative ions) and coupled to the electron energy balance equation. The input parameters of the model are the collision cross sections of the electrons impact with air molecules (N2, O2, and H2O) and the electron energy distribution functions pre-tabulated for a large set of average electron energies. The latter takes into account the main processes leading to the decrease of average electron energies. This model is more generally usable for the modelling of weakly ionized atmospheric air plasmas during, for instance, the streamer development in corona or dielectric barrier discharges.

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