Characteristics of the different plasma regimes of discharges with water cathodes

Summary form only given. In recent years, there has been considerable interest in discharges in and above liquids for environmental applications. The discharge geometry described in this contribution is a stainless steel metal pin electrode with a tip curvature radius of 0.5 mm and a water reservoir which is used as cathode in ambient air. With increasing voltage a corona discharge is generated before spark formation occurs. The ionic wind causes a depression of the water surface underneath the metal pin electrode. The properties of the corona are similar to those of a corona discharge in a pin-metal plate geometry. Breakdown is investigated by fast CCD-imaging and current-voltage waveforms. The breakdown streamer is always branched near the water electrode. This breakdown streamer develops into a spark. An intense filamentary discharge regime is formed after breakdown with typical currents of 10 mA and burning voltages of the order of 1 kV. Although the current density is independent of the current in the case of a distilled water cathode, fast framing images show a filamentary structure of the discharge at the water cathode, that seems to dance over the water surface. The optical emission spectra are dominated by the second positive system of nitrogen, OH, H, O, NO, NH and N₂ ⁺. The rotational temperature of OH is in the range of 3500-4400 K and the rotational temperature of nitrogen 2500- 750 K in the bulk of the plasma. These temperatures are smaller than the atomic excitation temperatures of hydrogen which is in the range of 5700-7250 K. Electron densities are determined by the Stark broadening of the hydrogen beta Balmer line and are in the range of 5-8 times 10¹⁴ cm^-3.