Influence of gas flow on discharge mode in coaxial argon DBD under atmospheric pressure

Abstract

Dielectric barrier discharge (DBD) under atmospheric pressure is usually characterized with a large number of streamer discharge filaments. So its applications are greatly limited due to the non-uniform discharge mode. Nowadays, most research efforts are being focused on how to achieve the uniform DBD, and results show that there are many ways to improve the non-uniformity of the discharge, one of which is the introduction of flowing gas into the discharge gap at controllable flow rates. However, the mechanism of gas flow on discharge mode on DBD is still not understood clearly. This paper presents a coaxial DBD plasma jet, in which a powered electrode covered by polytetrafluorethylene (PTFE) is fastened in the center of a quartz glass tube, and a copper strip as grounded electrode is wrapped outside the tube. The device is operated under atmospheric pressure with an intermediate frequency sinusoidal resonant power supply. Argon (99.999%) is employed as the working gas, and its flow rate is measured and controlled through a flow meter within the range of 0–25 L/min. The influence of different gas flow rates on the discharge mode is investigated and the mechanism is analyzed. From the discharge photos, it can be seen that the discharge is generated inside the tube and the plasma is blown out by the gas flow to form a plasma jet. When the flow rate is relatively slower, there are obvious discharge filaments near the tube nozzle, which can also be confirmed through the waveform of the discharge current characterized by many current pulses per half cycle of the applied voltage. The faster the gas flows, the fewer the discharge filaments are, and the more homogeneous discharge is obtained. At a certain gas flow rate, together with the lowest breakdown voltage value, the discharge mode presents a glow like state. It can be concluded that the faster flowing argon can take more heat generated in the discharge away, which can induce the thermal stability of discharges and prevent the transition the Townsend discharge to a filamentary discharge effectively. Besides, there is the distortion of applied electric field caused by the wall charge accumulated on the surface of dielectric and the space charge moving towards electrodes in the discharge gap. When the argon flows through the gap at different rates, the spatial distribution of wall charges and space charge is influenced. The faster the argon flows, the less wall charges and space charge accumulates, and the more homogeneous the discharge presents.