Kinetics of O and H radicals in a nanosecond pulsed He+H₂O pin-pin discharge

A nanosecond pulsed discharge generated in a pin–pin 2.2 mm gap geometry in He + H₂O was studied experimentally and by 1D fluid modelling. The density of O and H radicals were measured by ps-TALIF for varying concentrations of H₂O. Good agreement was obtained on the absolute density and decay rate of O. The density of O peaked about 1 µs after the end of the current pulse, reaching 1x10¹⁶cm^-3. It then remained about constant over 10 µs before decaying. Modelling suggested that O was predominantly produced by electron impact reactions: direct dissociation of O₂ during the discharge (7 ns to 200 ns) and dissociative recombination of O₂⁺ during the post-discharge. O₂⁺ was produced primarily by charge exchange collisions of O₂ with He₂⁺ and H⁺ ions, and the 3-body recombination of O⁺ and O with He. The main loss mechanism of O was ionisation by electron impact from 10 ns to 190 ns, and 3-body recombination with O⁺ and He, between 190 ns and 1 μs. According to numerical simulations, the production of O was mostly unchanged for a range of pulse shapes with different ratios of maximum voltage to the voltage pulses width, between 0.1 to 4.0 kV/μs, with constant pulse energy. At higher ratios, the efficiency of O production increased by maintaining higher reduced electric fields.