Ionized plasmas in air

High-energy electron beams can create nonequilibrium plasmas with power budget 2-3 orders of magnitude lower than that of electric field-sustained discharges. In this paper, we analyze kinetics and dynamics of plasmas generated in dense gases by beams of keV-class electrons with beam current density on the order of 1-10 mA/cm2. Of specific interest is the case when the beam relaxation length is considerably less than the distance to the anode or other conducting objects. The paper focuses on a "fountain" regime where the space charge of the beam is removed by the back current of plasma electrons towards the injection foil. 1D modeling couples electron beam propagation, electrodynamics, charge particle kinetics, and kinetics of chemical species and molecular excited states. Kinetics of high-energy electrons is described in the 'forward-back' approximation developed by us earlier, while the drift-diffusion approximation is used for low-energy electrons. Temporal and spatial evolution of electron and ion densities, electric field, electron energy distribution function, and densities of chemical species and molecular excited states are computed. In the regime when attachment to oxygen is largely balanced by detachment processes, electric field in the plasma is found to be very low, so that electron temperature of plasma electrons is only about 0.1 eV or lower. Because of that, vibrational excitation is much weaker than in conventional nonequilibrium discharges where electron temperature is about 1-3 eV. The low electron temperature also results in excitation and dissociation processes being driven by beam electrons, with virtually no role of plasma electrons. The results of calculations are compared with recent experimental results obtained in the laboratory, and a good agreement is found.