Temporal evolution of OH density and gas temperature of nanosecond repetitively pulsed discharges in water vapor at atmospheric pressure

We present a study of plasma discharges produced by nanosecond repetitively pulses in water vapor at 450~K and 1~atm. The plasma was generated in water vapor with 20-ns duration high-voltage (0-20~kV) pulses, at a repetition frequency of 10~kHz, in the spark regime (2~mJ/pulse). To investigate plasma kinetics we focused on intermediate products of the discharge, in particular the hydroxyl radical. Between two discharges, the time-resolved density of OH was measured by Planar Laser Induced Fluorescence~(OH-PLIF). The temperature during the discharge was determined by optical emission spectroscopy, and between two pulses by two-color OH-PLIF. A 150~K preheating effect from the previous pulses is measured, with a maximum temperature elevation of 950 K during the first 10~$\mu$s following each pulse. The OH density measurements were compared with chemical kinetics simulations. The numerical results obtained with an initial OH density of 500~ppm show good agreement with the experimental data, thus providing a quantification of the OH density produced by the pulse. The electron number density is also measured via stark broadened H$_{\beta}$ lines. A kinetics model is proposed to interpret the measures.