A comparative study on the spectral characteristics of nanosecond pulsed discharges in atmospheric He and a He+2.3%H₂O mixture

Abstract

Nanosecond pulsed discharges at atmospheric pressure in a pin-to-pin electrode configuration are well reproducible in time and space, which is beneficial to fundamentals and applications of low temperature plasmas. In this experiment, the discharges in helium (He) and He with 2.3% water vapor (H2O) are driven by a series of 10 ns overvoltage pulses (~13 kV). Special attention is paid on the spectral characteristics obtained in the center of discharges by time resolved optical emission spectroscopy. It is found that in the case of helium the emission of atomic and molecular helium during the afterglow is more intense than that in the active discharge, while in the case of He+2.3%H2O mixture the helium emission is only observed during the discharge pulse and the molecular helium emission disappears. In addition, the emissions of OH(A-X) and Hα present a similar behavior that increases sharply during the falling edge of voltage pulse as the electrons cool down rapidly. The gas temperature is determined to remain low at 540 K by fitting the OH(A-X) band. A comparative study on the emission of radiative species (He, He2, OH and H) is performed between these two cases of discharges to derive their main production mechanisms. In both cases, the dominant primary ion is He+ at the onset of discharges but their charge transfer processes of He+ are quite different. Based on these experimental data and qualitative discussion on the discharge kinetics, as for the present discharge conditions, it is shown that the electron-assisted three-body recombination processes appear to be the significant sources of radiative OH and H species in high-density plasmas.