Microwave air plasmas in capillaries at low pressure I. Self-consistent modeling

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This work presents the self-consistent modeling of micro-plasmas generated in dry air using microwaves (2.45 GHz excitation frequency), within capillaries (<1 mm inner radius) at low pressure (300 Pa). The model couples the system of rate balance equations for the most relevant neutral and charged species of the plasma to the homogeneous electron Boltzmann equation. The maintenance electric field is self-consistently calculated adopting a transport theory for low to intermediate pressures, taking into account the presence of O− ions in addition to several positive ions, the dominant species being O, NO+ and O+ . The low-pressure small-radius conditions considered yield very-intense reduced electric fields (∼600–1500 Td), coherent with species losses controlled by transport and wall recombination, and kinetic mechanisms strongly dependent on electron-impact collisions. The charged-particle transport losses are strongly influenced by the presence of the negative ion, despite its low-density (∼10% of the electron density). For electron densities in the range (1– cm−3, the system exhibits high dissociation degrees for O2 (∼20–70%, depending on the working conditions, in contrast with the ∼0.1% dissociation obtained for N2), a high concentration of O2(a) (∼1014 cm−3) and NO(X) ( cm−3) and low ozone production (<).