Corona Discharges in Asymmetric Electrode Configurations

The dual corona electrode, consisting in an ionizing wire in parallel with a metallic cylindrical support, at same high voltage supply, has been extensively studied in relation with various electrostatic applications. In practical situations, the dual corona electrode may be installed in the proximity of metallic objects that will affect the electric field repartition and, hence, the development of the discharge. The aim of the present work is to analyze the operating conditions of such electrodes in the presence of metallic rods or plates connected at fixed or floating potential. The paper reports the results of current-voltage characteristics and current density repartition measurements for various configurations. The proximity of metallic objects leads to the increase of inception voltage and shifts the I-V characteristics to higher voltages. The objects at floating potential may reduce discharge current to very low values, while those energized at the same voltage as the ionizing wire may simply anneal the discharge. INTRODUCTION Various corona electrode arrangements have been developed and extensively studied in relation with such electrostatic applications as separation of granular materials, charging of electret air filters, or neutralization of charged insulating materials [1-9]. The efficiency of corona charging or neutralization processes depends on a multitude of factors, including the level, polarity or frequency of the high-voltage, as well as on the geometry of the electrode system [10-15]. Many electrode arrangements, using needles, pins or wires as corona emitting elements, are commonly employed in industrial applications [16, 17]. In such configurations, the electric field distribution and hence the corona discharge are symmetrical with respect to the electrode axis. In pin-plane or wire-plane electrode system, the current density distribution can be described by the empirical Warburg law [18]. Other symmetrical configurations have also been characterized in relation with various industry applications [19-21]. Recently, the modification of corona electrode by connecting a metallic cylinder in parallel with the energized wire was suggested as a solution to enhance the charge process of non-woven filter media [22, 23]. Proc. ESA Annual Meeting on Electrostatics 2013 2 Fig. 1. “Standard” wire-type symmetric electrode arrangement (a) and asymmetric electrode arrangements, with cylindrical rod energized at the same voltage (b), with floating cylindrical rod (c) and floating plate (d). In practical situations, the corona electrode may be installed in the close vicinity of metallic objects, such as metallic rods or plates connected at fixed or floating potential. This will modify the symmetry of the electric field repartition and, hence, the characteristics of the corona discharge. The objective is to quantify the effect of asymmetry on the current-voltage characteristics of the electrode system and on the repartition of current density at the surface of the non-ionizing (collecting) electrode EXPERIMENTAL PROCEDURE A. Electrode arrangements The “standard” wire-type dual electrode consisted in a tungsten wire (0.2 mm in diameter) attached to a metallic cylinder (25 mm in diameter) and distanced at 34 mm from its axis (Fig. 1, a). The wire was located at 50 mm above the grounded electrode (aluminum plate, dimensions: 120 mm x 90 mm) and connected to an adjustable DC high voltage supply (positive polarity, 100 kV, 3 mA, Model SL 300, Spellman Inc.). Two asymmetrical electrode arrangements were obtained by attaching a copper cylindrical rod (diameter: 10 mm) to the dual electrode. The rod, the axis of which was located at 40 mm from the wire, was either energized at the same voltage as the “standard” symmetric electrode (Fig. 1, b), or left at a floating potential (Fig. 1, c). For the third asymmetrical electrode, the rod was replaced by a profiled aluminum plate, at floating potential (Fig. 1, d). B. Current density repartition measurements The repartition of corona current was measured using a custom-designed printed circuit board (PCB) as collecting electrode (Fig. 2). Three voltage levels were considered, at both positive and negative polarity: 18 kV, 21 kV, and 24 kV. The current probe consisted in a rectangular sector (19 mm x 14 mm), insulated from the rest of the conductive plate, and connected to an electrometer (Keitheley Instruments, model 6514). (a) (b)