One-dimensional simulation of Ar dielectric barrier discharge driven by combined rf/dc sources at atmospheric pressure

Abstract

We present the dielectric barrier discharge (DBD) mechanism of Argon (Ar) plasma driven by combined rf/dc voltage sources at atmospheric pressure, based on one-dimensional self-consistent coupled fluid model. Using finite element method (FEM) to numerically calculate the model, the average value of period average electron density varying with the average value of period average gas voltage in one rf period, and the variation of the minimum rf sustaining voltage were obtained under different dc voltages. In addition, the spatial-temporal distribution of the electron density and electron generation rate, the spatial distribution of electron temperature, and the time-domain variation of electron conduction current flowing to the dielectric were studied. Results show that the introduction of the dc voltage source has a significant effect on the rf discharge process of atmospheric pressure Ar, and the parameters of the plasma state are changed correspondingly. The discharge process is mainly controlled by the air gap voltage, and the dc voltage affects the gap voltage by changing the charge density on the dielectric surface. The minimum rf sustaining voltage <i>V</i><sub>rf,min<i> </i></sub>first increases and then decreases with the increase of dc voltage. The amplitude of rf minimum sustaining discharge voltage is changed by the dc voltage. And when the amplitude are reached or exceeded, the discharge is controlled by the rf power supply.<br>On one hand, at α mode, when the dc voltage is low, electrons are generated near the ground electrode. The electric field intensity in the ionization area is too small to maintain ionization. When the dc voltage is high, the sheath is formed, and electrons are generated at the rf sheath on both sides and the boundary of the plasma region. At γ mode, when the rf voltage amplitude is equal to or greater than the rf minimum sustain discharge voltage amplitude <i>V</i><sub>rf</sub> ≥ <i>V</i><sub>rf,min</sub>, the generation and distribution of electrons are almost unaffected by the dc voltage.<br>On the other hand, at α mode, the ionization cannot be sustained for the low dc voltage, resulting in the failure to form the main plasma area. Therefore, the electron temperature is generally high. Due to the high electron density near the ground electrode, the electron temperature is higher. The electron density near the dielectric is less than that near the electrode, so the temperature is lower. When the dc voltage is getting larger, the sheath and the main plasma region are formed. The dc voltage significantly affects the electron temperature by controlling the sheath voltage and the length of the main plasma region.<br>Finally, at α mode, the electron density near the medium is very low and the air gap voltage is negative for the low dc voltage. As a result, few electrons can reach the surface of the dielectric, and the conduction current of electrons flowing to the medium is very small. With the increase of the dc voltage, the electric field between air gaps increases, and electrons, being acted on by the electric field, flow from the dielectric surface. After the sheath formed, some speedy non-localization electrons that should have reached the dielectric surface are reflected back to the sheath, leading to a significant reduction in the number of electrons that can reach the dielectric surface.