Spatially and temporally resolved fs/ps CARS measurements of rotation-vibration non-equilibrium in a CH_4/N_2 nanosecond-pulsed discharge

摘要

Due to concerns about climate change, there is significant interest to establish CH_4 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 CH_4 fuel mixtures, it is necessary to obtain time and spatially resolved data of key parameters such as the CH_4 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% CH_4/60% N_2 nanosecond-pulsed discharge at 60 Torr, while the CH_4 concentration was measured by vibrational CARS. The measurement region was 2 mm along the electrode axis, within 150 urn of the cathode surface. Gradients in N_2 rotational and vibrational temperature and CH_4 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 CH_4 consumption occurred during the voltage pulse. Additional CH_4 consumption along with rapid heating occurred during the first 2 μs of the afterglow, indicating a role of electronically excited N_2 quenching in dissociation of CH_4.