The effect of fuel oxidation on plasma decay in combustible mixtures excited by high-voltage nanosecond repetitively pulsed discharge

The results of the experimental study of high-voltage nanosecond repetitively pulsed discharge afterglow in propane:O2, ethane:O2 and H2:O2 mixtures were presented for room temperature and pressures from 2 to 6 Torr. Time-resolved electron density during the plasma decay was measured using a microwave interferometer for initial electron densities in the range between 3 × 1011 and 3 × 1012 cm−3 and the effective recombination coefficients were obtained. In hydrocarbon-containing mixtures, the rate of plasma decay varied nonmonotonously with the oxidation degree increase. The effective recombination coefficient (i) peaked when the amount of intermediate species was expected to be high and (ii) was independent of the number of voltage pulses (the effect of saturation) when oxidation was complete. In H2:O2 mixtures, the rate of plasma decay and the effective recombination coefficient increased monotonously with increasing oxidation degree. It was shown that plasma decay in completely oxidized fuel:O2 mixtures is close to that in water vapor. It was suggested that the nonmonotonous behavior of the effective recombination coefficient during plasma decay in hydrocarbon:O2 mixtures is explained by the production of some hydrocarbon intermediates in the oxidation processes that can favor plasma decay. The possible mechanisms of plasma decay acceleration in the oxidized mixtures were discussed.