Time-resolved characterization of plasma properties in a CH4/He nanosecond-pulsed dielectric barrier discharge

Non-equilibrium plasmas for plasma-assisted combustion, pollutant remediation, fuel reforming, and catalysis rely on the production of energetic electrons that ionize, dissociate, and excite the fuel and oxidizer molecules. Experimental characterization of the electron temperature, electron density, and vibrational temperature are necessary to validate and improve plasma kinetic models. An experimental apparatus capable of Thomson scattering and vibrational Raman scattering measurements in the same discharge with molecular admixtures was developed. Both diagnostics are necessary to study the induced vibrational non-equilibrium from electron impact. Thomson scattering spectra were resolved by placing a physical mask at the output of a single grating spectrometer. The electron temperature and density and the impact of hydrocarbon addition was measured for a 60 Torr CH4/He nanosecond pulsed plane-to-plane dielectric barrier discharge with 0%-2% CH4 addition. Electron densities as low as 1 x 10(12) cm(-3) and electron temperatures ranging from 0.5 eV to 9 eV were observed. A decrease in the electron temperature and density was observed even with 1% H-4 addition. Moreover, the addition of N-2 to the discharge enabled vibrational Raman scattering and quantification of the first level vibrational temperature starting from 75 ns after the voltage pulse. The electron temperature and density were also measured in this CH4/N-2/He mixture by Thomson scattering. Addition of N-2 led to a faster electron temperature decay than in the original CH4/He mixture. The advantages and disadvantages of this detection scheme for Thomson scattering over the triple grating spectrometer and the volume Bragg grating notch filter is discussed.