On the different regimes of gas heating in air plasmas

Abstract

Simulations of the gas temperature in air (N2–20%O2) plasma discharges are presented for different values of the reduced electric field, E/Ng, electron density ne, 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 and 100 Td (1 Td = 10−17 V cm2), 109 ⩽ ne ⩽ 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 + → 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/Ng and ne 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.