Current Measurements and Diagnosis on Dielectric Barrier Discharge in a Coaxial Geometry at

Atmospheric Pressure

Xuechen Li, Ning Yuan, Dongying Niu, Qian Song, Yuanyuan Li College of Physics Science and Technology

Hebei University Baoding 071002, China

yuanning0321@163.com

Abstract—Discharge current has been measured to diagnose the discharge modes of dielectric barrier discharge in a coaxial geometry through a mixture flows of argon and trace of air at one atmospheric pressure. Experimental results show that there are two discharge modes in the dielectric barrier discharge, corona discharge mode and glow discharge mode. The corona discharge only occurs in the tip of needle when the applied voltage not reaches the critical value for breakdown of the whole gas gap, and the current intensity is very low. The glow discharge is generated from the vicinity of needle tip to the entire tube gradually and turns uniform at last when the applied voltage exceeds the breakdown voltage for the whole gap, and the current intensity becomes higher. Under the same experimental parameters, the current intensity decreases with increasing the content of air. Experimental results also show that the critical value for breakdown of the whole gas gap becomes higher with increasing the content of air. However, it will decreases with increasing the gas velocity and the frequency of the applied voltage.

Keywords-current measurement; breakdown voltage; corona discharge; glow discharge

I. INTRODUCTION Plasmas generated by gas discharge have many important

application fields such as thin film deposition [1], material modification [2], removal or degeneration of harmful gas and plasma display panels [3]. Interest has grown over the past few years in generating atmospheric pressure plasmas for industrial application because low pressure requires expensive vacuum systems and batch processing of workpieces is hard to be realized. An atmospheric pressure glow discharge in a coaxial electrode configuration in argon have been investigated before[4]. Discharge characteristics are studied by the electrical method. The plasma parameters such as electron temperature and electron density are estimated from the Einstein relation and discharge current [5]. Different gas discharge has very important practical significance in the industrial processing. For example, argon and nitrogen gas plasma have potential applications in the synthesis of nitride thin films and material modification. Discharge current is usually used as a method to diagnoze the discharge modes. In this paper, the process of

plasma generation has been investigated by measuring the current waveforms, and the relationship between the breakdown voltage and the mixed amount of air is given. The data have great referential value to find suitable experimental conditions to improve the plasma applications.

II. EXPERIMENTAL SETUP

A schematic drawing of the experimental setup is shown in Fig 1. The discharge device is composed of two coaxial electrodes, a tungsten cylindrical electrode surrounded by a water electrode. The diameter of the tungsten electrode is 1mm. The tungsten electrode is coaxially surrounded by two glass tubes with a thickness of 1mm. Water filled between the two tubes is used as the electrode. Water is circulated by a pump to take away the heat. The active length of the discharge electrode is about 12.8 cm. The mixed gases feed to the gap between the tungsten electrode and the inner glass tube, and the gas gap width is 3mm. During the experiments, the tungsten electrode is connected to a high voltage output, and water electrode is grounded through a small resistor (50Ω). The applied voltage can be measured by a high voltage probe (Tektronix P6015A, 1000X) and recorded with an oscilloscope (Agilent DPO6054A 1GMHz). The discharge current is

This project is supported by the National Natural Science Foundation ofChina (Grant Nos. 10805013， 10647123), the National Natural ScienceFoundation of Hebei Province, China (Grant Nos. A2009000149,A2007000134), and the Research Foundation of Education Bureau of Hebei Province(Grant No. 2006106).

Figure 1 Schematic diagram of the experimental setup.

2010 International Conference on Electrical and Control Engineering

978-0-7695-4031-3/10 $26.00 © 2010 IEEE

DOI 10.1109/iCECE.2010.166

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2010 International Conference on Electrical and Control Engineering

978-0-7695-4031-3/10 $26.00 © 2010 IEEE

DOI 10.1109/iCECE.2010.166

652

measured with the small resistor (R=50Ω) in series with the grounded electrode.

III. RESULTS AND DISCUSSIONS Increasing the voltage of the power source, corona

discharges appear at the tip of needle when the peek of voltage (Up) is below the critical value for the whole gas gap breakdown. The discharge is a small light spot with a diameter of 2mm, as shown in Fig2.(a). Because the tip of needle is very sharp (radius of curvature is small), it is very easy to form a strong local electric field. The electric field can lead strong excitation and ionization of gas, so corona discharge, which is self-sustaining discharge but not entirely breakdown discharge, occurs firstly at the tip of needle. The light emission becomes stronger and the region of discharge increases slightly when the applied voltage exceeds the critical value for the whole gas gap breakdown. Glow discharge appears at the vicinity of the needle tip, as shown in Fig2.(b).Because the tip of needle exists a strong local electric field, the glow discharge appears firstly at the vicinity of the needle tip. Continue to increase the applied voltage, the discharge fills the entire tube uniformly, and the light emission emitted from the nozzle forms a stable plasma jet, as shown in Fig2.(c).

Fig. 3 shows the waveforms of the applied voltage and light emission at the corona discharge mode, which corresponds to the image of Fig. 2(a). For the discharge showed in the current waveform is not obvious (The current intensity is 6mA), the waveform of the light emission has been chosen. It can be found that there is only one discharge pulse for several cycles of the applied voltage, and the discharge pulse lasts for three cycles of the applied voltage. The discharge pulse rises at every positive half cycle, and the rising time of the light emission is 6μs. That is to say, the light emission contains three rising edges and three smooth areas. All the three rising edges occur in the rising edge of each positive half cycle of the applied voltage. In addition, the extent of the rising edge decreases gradually. The whole shape is showed as a ladder. The whole rising time of the pulse is about 67μs. The light emission begins at the maximum of the negative half-cycle of the applied voltage, which means the discharge begins at the maximum of the applied voltage.

Fig 4 shows the waveforms of the applied voltage and the current at the glow discharge mode, which corresponds to the image of Fig. 2(b). With the applied voltage increasing, some discharge pulses appear at the rising edge of each half cycle of

the applied voltage, the width from the first pulse to the last pulse is 6μs for each positive or negative half cycle and the current intensity increases to 40mA, as shown in Fig.4.

Figure 2 Discharge images under different voltages: (a) 1.8kV， (b) 3kV， (c) 3.1kV

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Figure 4 The waveforms of the applied voltage and the current when the discharge glow discharge appear at the vicinity of the needle tip.

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Fig.5 shows the waveforms of the applied voltage and current in the glow discharge mode which corresponds to the image of Fig. 2(c). With a further increase in the applied voltage, more and more discharge pulses occur at the rising edge of each half cycle of the applied voltage, the current intensity also increases, and some discharge current pulse is 120mA.

It can be found that this is just a rise at each rising edge of positive half cycle in Fig 3 but there are much discharge pulses occur at the rising edge of each half cycle in Fig. 5.

It can be clearly seen that a large number of pulses appear at the rising edge of the positive half-cycle, while little pulse can be discerned at the negative half-cycle, as shown in Fig.6(a), however, the pulses occur at the rising edge of both the positive and the negative half-cycle as shown in Fig.6(b) and Fig.6(c). The intensity of pulses at the rising edge of the negative half-cycle increases as the content of air increasing. In addition, the intensity of pulses in the positive half-cycle is bigger than that in the negative half-cycle. In the corona discharge, the positive ions move to the inner electrode in the negative half-cycle of applied voltage. After acceleration of the strong electric field near the electrodes, the bombardment of inner electrode leads to the generation of secondary electron.

However, the positive ions move to the outer electrode at the positive half-cycle of applied voltage, and there is no secondary electron generation. This reason causes a higher critical electric field for the positive half-cycle than the negative half-cycle. Based on the analysis above, it can be clearly deduced that the breakdown of the entire gas gap can only occur under a high breakdown electric field. The discharge increases rapidly and a strong pulse will be shown under high electric field. Therefore, the intensity of pulses for the positive half-cycle is bigger than that at the negative half-cycle.

Under the same experimental parameters, the current intensity will decrease as the content of air increasing, as shown in Fig.7. The oxygen absorbs the electrons because oxygen molecular is electronegative. The critical value for the whole gas gap breakdown increase as the number of electrons decreasing. Under the same applied voltage, the current intensity decrease as the content of air increasing.

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The critical value for the whole gas gap breakdown increases as the content of air increasing, however, it decrease as the argon flow rate increasing, as shown in Fig.8. Based on the analysis mentioned above, it can be deduced that the critical value for the whole gas gap breakdown increases as the content of air increasing. When the air flow rate keeps constant, the air content decreases with the argon flow rate increasing. Therefore, the critical value for the whole gas gap breakdown decreases as the argon flow rate increasing.

IV. CONCLUSION By measuring the current intensity it can be concluded that

there are two discharge modes in the dielectric barrier discharge, corona discharge mode and glow discharge mode. The corona discharge only occurs in the tip of needle when the applied voltage is below the critical value for the whole gas gap breakdown, the current intensity is low. The glow discharge will be generated from the vicinity of needle tip to entire tube gradually and change uniform at last when the applied voltage exceed the critical value for the whole gas gap breakdown, the

Figure 6 The waveforms of the applied voltage and the current under different content of air, (a)Air:1sccm (b)Air:5sccm (c)Air:9sccm

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Figure 7 Comparison of the waveform of current discharge

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Figure 8 The breakdown voltage as a function of the incorporation of different levels of air with different argon gas flow rate, f=40kHz

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current intensity become high suddenly. The critical value for the whole gas gap breakdown becomes higher with increasing the content of air. However, it will decrease as increase the gas velocity and the frequency of the applied voltage.

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[3] B. Eliasson and U. Kogelschatz “Nonequilibrium volume plasma chemical processing,” IEEE Trans. Plasma Sci. vol. 19, pp. 1063-1077, December 1991.

[4] L F. Dong, F C. Liu, S H. Liu, Y F. He and W L. Fan, “Observation of spiral pattern and spiral defect chaos in dielectric barrier discharge in argon/air at atmospheric pressure,” Phys.Rev. E vol. 72 046215, 2005

[5] X C. Li, N Zhao, T Z, Fang, Z H. Liu, L C. Li and L F.Dong “Characteristics of an atmospheric pressure argon glow discharge in a coaxial electrode geometry,” Plasma Sources Sci.Technol. vol. 17 015017(6pp) January 2008.

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