Energy Balance and Gas Thermalization in a High Power Microwave Discharge in Mixtures

The dynamics of fast gas heating in a high power microwave discharge in air, is investigated in the framework of FDTD simulations of the Maxwell equations coupled with the fluid simulations of the plasma. It is shown that, an ultra-fast gas heating of the order of several 100 Kelvins occurs in less than 100 ns. The main role in the heating is played by the electron impact dissociation of , dissociation via quenching of metastable states of , as well as,  quenching by nitrogen molecules. Among the electronically excited metastable states,  are the most important species. Slow heating of the gas above 1 is attributed to the vibrational relaxation processes of , among them vibrational-translational relaxation of demonstrates the highest heating rate. The heating rate and thus the gas temperature are significantly increased with increasing of the microwave pulse amplitude, pulse width, and the gas pressure. In all cases, enhanced  dissociation is the main factor behind the enhanced gas heating. The same effects are observed for increasing of the initial gas temperature, and percentage in a  mixture.