On the different regimes of gas heating in air plasmas

Simulations of the gas temperature in air (N₂–20%O₂) plasma discharges are presented for different values of the reduced electric field, E/N _g, electron density n _e, pressure and tube radius. This study is based on the solutions to the time-dependent gas thermal balance in a cylindrical geometry coupled to the electron, vibrational and chemical kinetics, for $E/N_{\text{g}}=50$ and 100 Td (1 Td = 10⁻¹⁷ V cm²), 10⁹  ⩽  n _e  ⩽  10¹¹ cm⁻³, pressure in the range 1–20 Torr, and also considering different tube radius, 0.5, 1 and 1.5 cm. The competing role of different gas heating mechanisms is discussed in detail within the time range 0.01–100 ms. For times below 1 ms, gas heating occurs from O₂ dissociation by electron impact through pre-dissociative excited states, e + O₂  →  e + ${\text{O}}_2^{\ast }$  →  e + 2O(³P) and …  →  e + O(³P) + O(¹D), as well as through the quenching of N₂ electronically excited states by O₂. For longer times, simulation results show that gas heating comes from processes N(⁴S) + NO(X)  →  N₂(X, v ∼ 3) + O, N₂(A) + O  →  NO(X) + N(²D), V–T N₂–O collisions and the recombination of oxygen atoms at the wall. Depending on the given E/N _g and n _e values, each one of these processes can be an important gas-heating channel. The contribution of V–T N₂–O exchanges to gas heating is important in the analysis of the gas temperature for different pressures and values of the tube radius. A global picture of these effects is given by the study of the fraction of the discharge power spent on gas heating, which is always ∼15%. The values for the fractional power transferred to gas heating from vibrational and electronic excitation are also presented and discussed. (paper)