Temporal evolution of the electron density produced by nanosecond repetitively pulsed discharges in water vapor at atmospheric pressure

A study of plasma discharges produced by nanosecond repetitive pulses (NRP) in water vapor at 450~K and 1~atm is presented. The plasma was generated between two point electrodes with 20-ns duration, high-voltage (0-20~kV) pulses, at a repetition frequency of 10~kHz, in the spark regime (2~mJ/pulse). Atomic lines measured by optical emission spectroscopy were used to determine the electron number density in this non-equilibrium water-vapor plasma. The broadenings and shifts of the H$_\alpha$ and H$_\beta$ lines of the hydrogen Balmer series and of the atomic oxygen triplet at 777 nm were analyzed. For a maximum reduced electric field of about 200\,Td, a maximum electron density of 2~x~$10^{18}$~cm$^{-3}$ was measured, corresponding to an ionization level of about 10$\%$. This ionization level is two orders of magnitude higher than the one obtained for similar NRP discharges in air at atmospheric pressure.