Non-contact measurement of electric charges on water surface supplied with plasma

Transferred charges in water associated with discharges are measured using an electrostatic voltmeter, without any physical contact between the electrode and water. Unlike the method that uses a voltage probe, this method allows for the estimation of charges in water under an electrically floating state. In this paper we introduce a needle-water discharge system which uses the needle electrode over the ultrapure water surface. First, we evaluated our needle-water discharge system with various applied voltages by analysing the current waveforms. Thereafter, by synchronising an oscilloscope and a data logger, the surface potential and discharge waveforms were obtained simultaneously at different time scales. It was found that the measured surface potential of water is proportional to the transferred charges. Two types of materials, namely, glass and polystyrene, were used as the water container material, and it was observed that when the glass container is used, the characteristic decay time of the potential is 2.02 s, which is much shorter than in the case of a polystyrene container. This is due to a much lower surface resistivity of polystyrene. This finding is important because when a DC voltage is applied to the needle electrode, discharging is terminated due to the charges accumulated in water. The termination tendency is, however, varies based on the container material used, even when the same voltage conditions are applied.