Study of gas heating mechanisms in millisecond pulsed discharges and afterglows in air at low pressures
Résumé
A self-consistent model is developed to study the temporal variation of the gas temperature in millisecond single dc pulsed discharges and their afterglows in air-like mixtures (N220%O2) at low pressures. The model is based on the solutions to the time-dependent gas thermal balance equation, under the assumption of a parabolic gas temperature profile across the discharge tube, coupled to the electron, vibrational and chemical kinetics. Modelling results provide a satisfactory explanation for recently published time-resolved experimental data for the gas temperature in a 5 ms pulsed air plasma with a current of 150mA and the corresponding afterglow at a pressure of 133 Pa (1 Torr). It is shown that the main heating mechanisms during the first millisecond of the pulse come predominantly from O2 dissociation by electron impact through the pre-dissociative excited state O2(B 3− u ) and the quenching of nitrogen electronically excited states N2(A 3 u , B 3 g, a 1− u , a 1 g, w 1u) by O2, agreeing with other studies on fast gas heating in air plasmas. As the pulse duration increases, other gas heating sources become important, namely VT N2O energy exchanges, recombination of oxygen atoms at the wall, N2(A) quenching by O(3P) and reaction N(4S) NO(X) → N2(X, v ∼3) O, contributing altogether to an additional smooth increase in the gas temperature until the end of the pulse. In the first instants of the early afterglow, the gas temperature decreases very rapidly as a consequence of the minor role played by electronic collisions and due to a fast decay of N2 electronic states. For afterglow times up to 10 ms, the gas temperature continues to decrease, following the time-dependent kinetics of [N2(X,v)], [N(4S)], [O(3P)] and [NO(X)]. Sensitivity of the model to different input parameters such as thermal accommodation coefficient and probabilities for atomic recombination at the wall are reported.