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

Abstract

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 µs. 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 ne = (1.7–3.1) · 1014 cm−3 and Te = 2.9–5.5 eV, depending on gas mixture composition. Electron temperature in the afterglow decays to approximately Te ≈ 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.