Time and space-resolved imaging of an AC air discharge in contact with water

Numerous physical and chemical phenomena are involved in liquid plasmas. These phenomena are highly sensitive to the plasma source and to experimental conditions. In this study, we investigate the temporal dynamics of an AC-sustained plasma produced in air by a discharge between a pin electrode and water. Depending on the gap distance (d), a discharge transition is observed at d = 4.5 mm. At d < 4.5 mm, voltage drop is observed during both positive and negative half-periods (i.e. when water is cathode and anode, respectively). Meanwhile, at d > 4.5 mm, voltage drop is observed only during the negative half-period (i.e. when water is anode). Before transition and when water is cathode, the plasma emission in the gap space consists of two components: a conical-like emission at the water surface and a cylindrical-like emission in the gap. When water is anode, strong emission is observed at the electrode pin, along with a disc-like emission at the water surface. Increasing d beyond 4.5 mm (i.e. after transition) modifies the emission structure at the water surface, forming an inner homogeneous spot at the center of an outer ring. The temporal evolution of the emission in the gap and at the water surface is also discussed, and its relationship with the injected power is assessed.