Experimental and 2D fluid simulation of a streamer discharge in air over a water surface

The high reactivity and attractive properties of streamer discharges make them useful in many applications based on plasma-surface interactions. Therefore, understanding the mechanisms governing the propagation of a streamer discharge as well as its properties is an essential task. This paper presents the development and application of a 2D fluid model to the simulation of discharges triggered at the air-water interface by a pulsed nanosecond high voltage. Experimental characterization using 1-ns-time-resolved imaging reveals rapid transitions from a homogeneous disc to a ring and finally to dots during the discharge process. The simulation enables the determination of the spatio-temporal dynamics of the E-field and electron density, highlighting that the discharge reaches the liquid surface in less than 1 ns, triggering a radial surface discharge. As the discharge propagates along/over the water surface, a sheath forms behind its head. Furthermore, the simulation elucidates the transitions from disc to ring and from ring to dots. The former transition arises from the ionization front's propagation speed, where an initial disc-like feature changes to a ring due to the decreasing E-field strength. The ring-to-dots transition results from the destabilization caused by radial electron avalanches as the discharge head reaches a radius of ~1. 5 mm. The simulation is further utilized to estimate a charge number and a charge content in the discharge head. This work contributes to a better understanding of discharge propagation in air near a dielectric surface, with the agreement between simulation and experiment validating the model in its present version.