Rotating characteristics of glow discharge filament on liquid electrode surface

Atmospheric pressure glow discharge above liquid electrode has extensive application potentials in biomedicine, chemical degradation,environmental protection,etc.In this paper,such a kind of discharge excited by a direct current voltage is generated through using a metal rod above water surface.Results show that the discharge has a ring shape on the water surface when the current is low.With increasing the discharge current,its diameter first increases,and then decreases after reaching a maximum,and finally slightly increases.In this process,the discharge transits from a conical shape to a column.Fast photography indicates that the conical discharge actually originates from the rotation of a discharge filament,which can be attributed to the effect of electronegative particles generated in the discharge channel. These electronegative particles,mainly including NO,NO2,NO3,O,O3 and OH,can increase electron attachment coefficient β,resulting in extinguishment of the original discharge channel.Due to a similar field value and a normal β coefficient,the breakdown conditions can be satisfied in a region adjacent to the original channel.Therefore,the discharge will move into the new region.Further investigation indicates that both the conical discharge and the column discharge are in a normal glow regime.By optical emission spectroscopy,it is found that the vibrational temperature,the rotational temperature and the intensity ratio of I391.4/I337.1 increase with increasing the current.Electron mobility decreases in the conical discharge due to voltage decreasing with the current.Hence,electrons have an increased possibility with which they are attracted by the electronegative particles to form negative ions.Consequently,with increasing the discharge current,more negative ions will be accumulated not only near the conical center,but also in the vicinity of the discharge channel.Obviously,there is repulsive force between the negative ions in the two regions.The repulsive force increases with increasing the discharge current,which leads to the ring diameter increasing with the current.Besides the negative ions,gas temperature plays another important role in the discharge.It increases with current increasing,leading to the decrease of gas density in the discharge channel.Hence,electrons have a reduced probability with which they are attached by electronegative particles.This factor will lead to a reduced force between less negative ions in the two regions.Consequently,after reaching its maximum,the ring diameter decreases with current increasing.If the current is high enough,the discharge channel will have a sufficiently high temperature and an adequately lower gas density, resulting in an increased electron energy as well as an increased α(the first Townsend ionization coefficient).Therefore, the discharge will be self-sustained in the original region,other than move into an adjacent region.Consequently,the column discharge appears with the current increasing to some extent.In the column discharge,more negative ions will be accumulated above the water surface with increasing the current.These negative ions extend along the water surface,which contributes to the slight diameter increase of the luminous column.These experimental results are of great significance for theoretically studying liquid anode discharge.