Self-consistent modelling of atmospheric micro-plasmas produced by a microwave source

This paper presents the self-consistent modelling of argon micro-plasmas, produced by a microwave source (2.45 GHz) at atmospheric pressure. The source is a microstrip-like transmission line, with a 50–200 µm final gap where the micro-plasmas are created. Simulations use a one-dimensional, stationary code that solves the fluid-type transport equations for electrons, positive ions Ar + and , and the electron mean energy; the rate balance equations for the main neutral species; Poisson's equation for the space-charge electrostatic field; Maxwell's equations for the electromagnetic excitation field; the gas energy balance equation for its temperature distribution; and the kinetic electron Boltzmann equation considering several direct and stepwize electron collision processes. The model uses a kinetic scheme that considers the atomic excited states Ar(4s) and Ar(4p), two excimer states and , and two ionization states associated with the atomic and the molecular ions. The model predicts average gas temperatures of ∼600 K (similar to the measured rotational temperatures), power densities of 1–10 kW cm −3 (which overestimate measurements at low gap sizes), and excitation temperatures of ∼0.5–0.8 eV (which, at short gap sizes, are slightly above the experimental value of ∼0.5 eV, obtained by optical emission spectroscopy measurements), for on-axis electron densities of 5 × 10 13 , 10 14 , 5 × 10 14 cm −3 and wall gas temperatures of 500, 550, 600 K, at gap sizes of 150, 100, 50 µm.