Repetitively pulsed nanosecond discharge plasma decay in propane–oxygen gas mixture in the presence of a heating electric field

摘要

Plasma decay in the afterglow of a repetitively pulsed nanosecond discharge in a stoichiometric propane–oxygen mixture was experimentally investigated when a weak heating DC electric field was applied andin its absence. The discharge was ignited at room gas temperature and a pressure of 1–2 Torr and wascharacterized by low specific energy inputs ( < 0.004 eV per molecule in one pulse). Using microwave interferometry,the temporal evolution of the electron density during plasma decay was studied, and theeffective recombination coefficients were obtained from data processing. It was shown that the rate ofplasma decay behaved in a non-monotonic manner with increasing degree of propane oxidation; at firstthe decay rate grew, then passed through a maximum, fell and saturated in the limit of a large (～2000)number of pulses. In this limit, the effect of the heating DC electric field on the plasma decay decreasedwith approaching chemical equilibrium. Numerical simulation of the observed effects was performed forlow and high oxidation degrees of propane taking into account changes in the composition of positiveions in the plasma. Good agreement was obtained between measurements and calculations of the electrondensity during plasma decay in these cases. It was shown that the formation of cluster ions in thedischarge afterglow plays a fundamental role. The plasma decay was controlled by electron recombinationwith hydrocarbon cluster ion at low oxidation degree of propane and with water cluster (hydrated)ions at high oxidation degree. A hypothesis was proposed to explain the observed nonmonotonic behaviorof the plasma decay rate with an increase in the propane oxidation in the discharge, based on theformation of hydrated hydrocarbon ions C_xH_y + (H_2O) k at moderate oxidation degrees.