Time-resolved electron temperature and electron density measurements in a nanosecond pulse filament discharge in H₂-He and O₂-He mixtures

Time evolution of electron density and electron temperature in a nanosecond pulse, diffuse filament electric discharge in H₂-He and O₂-He mixtures at a pressure of 100 Torr is studied by Thomson/pure rotational Raman scattering and kinetic modeling. The discharge is sustained between two spherical electrodes separated by a 1 cm gap and powered by high voltage pulses ∼150 ns duration. Discharge energy coupled to the plasma filament 2-3 mm in diameter is 4-5 mJ/pulse, with specific energy loading of up to ∼0.3 eV/molecule. At all experimental conditions, a rapid initial rise of electron temperature and electron density during the discharge pulse is observed, followed by the decay in the afterglow, over ∼100 ns-1 s. Electron density in the afterglow decays more rapidly as H₂ or O₂ fraction in the mixture is increased. In He/H₂ mixtures, this is likely due to more rapid recombination of electrons in collisions with and ions, compared to recombination with ions. In O₂/He mixtures, electron density decay in the afterglow is affected by recombination with and ions, while the effect of three-body attachment is relatively minor. Peak electron number densities and electron temperatures are n _e = (1.7-3.1) • 10¹⁴ cm^-3 and T _e = 2.9-5.5 eV, depending on gas mixture composition. Electron temperature in the afterglow decays to approximately T _e ≈ 0.3 eV, considerably higher compared to the gas temperature of T = 300-380 K, inferred from O₂ pure rotational Raman scattering spectra, due to superelastic collisions. The experimental results in helium and O₂-He mixtures are compared with kinetic modeling predictions, showing good agreement.