Spatially and temporally resolved fs/ps cars measurements of rotation-vibration non-equilibrium in a ch4/n2 nanosecond-pulsed discharge

Due to concerns about climate change, there is significant interest to establish CH4 lean burn engines or convert it to valuable industrial chemicals using non-equilibrium plasmas. To quantitatively understand the dynamics and chemistry of plasma discharge in CH4 fuel mixtures, it is necessary to obtain time and spatially resolved data of key parameters such as the CH4 concentration and degree of rotation-vibration non-equilibrium. Rotational fs/ps CARS was used to simultaneously measure rotational and vibrational temperatures of a pin-to-pin 40% CH4/60% N2 nanosecond-pulsed discharge at 60 Torr, while the CH4 concentration was measured by vibrational CARS. The measurement region was 2 mm along the electrode axis, within 150 μm of the cathode surface. Gradients in N2 rotational and vibrational temperature and CH4 number density were observed to evolve in time and space. The vibrational temperature peaked above 6000 K, 100 μs after the voltage pulse, and the majority of CH4 consumption occurred during the voltage pulse. Additional CH4 consumption along with rapid heating occurred during the first 2 μs of the afterglow, indicating a role of electronically excited N2 quenching in dissociation of CH4.