Drop Charging as a Space Charge Indicator in a Tri-electrode Atmospheric Discharge

R SOSA; J MARTINEZ; D GRONDONA; A MÁRQUEZ; H KELLY; G ARTANA

Mar del Plata

Congreso; 14th Latin American Workshop on Plasma Physics; 2011

INFIP

In this work we present measurements of the charge acquired by water droplets passing through the gap of a three electrode discharge (tri-electrode). The tri-electrode discharge is based on the combination of a dielectric barrier discharge (DBD) with a positive or negative corona discharge (CD) in atmospheric air. The DBD is established applying an ac voltage between two flat aluminum foils electrodes placed at the opposite sides of an dielectric plate, and the CD discharge is generated applying a dc voltage either to a third flat aluminum foil electrode or to one of the DBD electrodes. Between the DBD electrodes and the third electrode, there is a 20 mm air gap.  A train of droplets passes through the center of the inter-electrode gap. This train is produced by the breakup of a water jet generated with a small capillary vibrating at a fixed frequency to produce mono sized droplets. It has been found that when the DBD or the CD are ignited alone  the drop charge is not appreciated, but when both discharges are ignited together, the drop charge becomes of the same order of magnitude than the charge of Rayleigh limit. This limit indicates the maximum charge a given droplet can support without breaking into smaller droplets as a consequence of the repulsive electrostatic forces. Four discharge configurations have been studied varying the dc polarity and the electrode to which the dc voltage was applied. Two of them correspond to the generation of a plasma curtain discharge (composed of a train of streamers crossing the inter-electrode gap). The other two correspond to an atmospheric discharge in which no plasma curtain is developed. It has been found that the presence of the plasma curtain discharge does not enhance significantly the drop charge. Our experimental results can be explained considering that the droplets are mainly charged by the impact of negative or positive ions drifting through the inter-electrode gap under the influence of the electric field. The study of the acquired drop charge allows us to infer the prevailing space charge in the gap for different discharge configuration.