Properties and characteristics of the nanosecond discharge developing at the water-air interface: tracking evolution from a diffused streamer to a spark filament

The characteristics of nanosecond discharge propagating along the water-air interface in a unique DBD-like configuration with coplanar electrodes submerged in deionized/tap water are studied by combining ultrafast imaging and emission spectra with electrical characteristics. Time-resolved images provide a clear signature of diffusive plasma excited on the water surface at either side of the blade (insulated plastic separating the anode/cathode) called streamer phase and propagating perpendicularly away from it towards the anode /cathode with different velocities. Later on, the diffusive plasma converts into a few discrete and bright plasma filaments due to ionization instability (spark phase). There is no distinctive dependence in the streamer phase on water conductivity, but in the spark phase, more numerous, brighter, and thicker filaments form in tap water. The time-resolved emission spectra reveal the dominance of the first and second positive system of $\mathrm{N_2}$ molecular bands in the streamer phase, followed by the appearance of atomic lines of hydrogen, nitrogen, and oxygen in the spark phase. The emission spectra are utilized to estimate plasma parameters (gas temperature ($T_d$), electric field ($E/N$), and electron density ($n_e$)) where a more dominant spark phase is formed in tap water with $T_d\sim1100$ K, $E/N$ $\sim800$ Td, and $n_e\sim10^{18}$ /$\rm cm^{-3}$.