The effect of electron heating on plasma decay in H2:O2 mixture excited by a repetitively pulsed nanosecond discharge

Abstract

Plasma decay was experimentally studied in the afterglow of a repetitively pulsed nanosecond discharge in a stoichiometric H2:O2 mixture. An additional bias DC electric field was applied to heat electrons during the discharge afterglow between the high-voltage pulses. The energy input per pulse was so small that the changes in both chemical composition and gas temperature were negligible. Using the microwave interferometer, the temporal evolution of the electron density in the discharge afterglow was measured when it decreased from 3 × 1012 to 5 × 1010 cm−3. Measurements were performed for various numbers of discharge pulses (various degrees of fuel oxidation) at room gas temperature and pressures from 1 to 2 Torr. The effect of electron heating on the rate of plasma decay was most profound for low oxidation degrees and practically disappeared after complete fuel oxidation. A kinetic model was suggested for describing ion-molecule processes and plasma decay in the discharge afterglow at various H2 oxidation degrees. The analysis of calculated results showed that plasma decay was governed by dissociative recombination of electrons with simple molecular ions at low oxidation degrees and with hydrated H3O+(H2O) k ions at sufficiently high oxidation degrees. An increase of H2O fraction due to H2 oxidation in the mixture led to a less efficient electron heating during plasma decay in the additional electric field. This was associated with extremely high cross-sections for the elastic and inelastic scattering of low-energy electrons by H2O molecules. As a result, the rate of electron-ion recombination increased with increasing number of discharge pulses, whereas the effect of electric field became less profound, in agreement with the observations.