Characteristics of a Normal Glow Discharge Excited by DC Voltage in Atmospheric Pressure Air

Transcription

1 Plasma Science and Technology, Vol.15, No.11, Nov Characteristics of a Normal Glow Discharge Excited by DC Voltage in Atmospheric Pressure Air LI Xuechen ( ) 1,2, ZHAO Huanhuan ( ) 1,2, JIA Pengying ( ) 1,2 1 College of Physics Science and Technology, Hebei University, Baoding , China 2 Key Laboratory of Photo-Electronics Information Materials of Hebei Province, Baoding , China Abstract Atmospheric pressure glow discharges were generated in an air gap between a needle cathode and a water anode. Through changing the ballast resistor and gas gap width between the electrodes, it has been found that the discharges are in normal glow regime judged from the currentvoltage characteristics and visualization of the discharges. Results indicate that the diameter of the positive column increases with increasing discharge current or increasing gap width. Optical emission spectroscopy is used to calculate the electron temperature and vibrational temperature. Both the electron temperature and the vibrational temperature increases with increasing discharge current or increasing gap width. Spatially resolved measurements show that the maxima of electron temperature and vibrational temperature appeared in the vicinity of the needle cathode. Keywords: atmospheric pressure glow discharge, positive column, optical emission spectroscopy, electron temperature, vibrational temperature PACS: Mg, Hc, Tn DOI: / /15/11/13 1 Introduction Atmospheric pressure glow discharges (APGDs) have been of great interest in the last few decades because of lower costs and simplified operation in comparison with low pressure glow discharges, and also because of potential applications to non-vacuum compatible materials and plasma processes [1]. Since APGD has many excellent properties such as non-thermal equilibration, proper electron energy and good uniformity, it has extensive application potentials, such as surface modification [2], ozone generation, biological sterilization [3] and pollution control [4], etc. APGD was first reported by KANAZAWA and collaborators in atmospheric pressure helium with a discharge device in dielectric barrier discharge configurations [5]. Then, in dielectric barrier discharge devices, APGDs were investigated numerically and experimentally in different gases such as helium, argon, nitrogen, etc [6 8]. Usually, dielectric barrier discharges excited by sinusoidal voltage operated in a pulsed mode. A diffuse dielectric barrier discharge in air was excited by a bi-directional nanosecond pulsed voltage using needleplate electrode configurations, and results showed that a pulsed light was emitted from the discharge [9]. Obviously, the temporal duty ratio was usually rather low in dielectric barrier discharges which operated in a pulsed mode. Although a high duty-ratio APGD excited by a saw-tooth voltage was reported in Ref. [10], very stringent conditions were needed, otherwise, the discharge would transit to a pulsed discharge. Apparently, APGD with a constant discharge current had a maximum of temporal duty cycle. This kind of APGD was usually excited by DC voltage. Although it was probably a pulsed discharge even under DC excitation, for example, LU et al. proposed a portable plasma jet powered by a 12 V DC battery, in which the jet operated in a pulsed mode with pulse duration of about 100 ns [11]. A DC excited APGD with constant discharge current in air was ignited between two needle electrodes with a gap width of 5 mm through blowing the air vertically to the electric field into the discharge region. The flowing air was preheated to be about 2000 K to realize the stable discharge [12]. Using an appropriate ballast resistor, atmospheric pressure discharges were investigated in stagnant air (no need to preheat the air) in a needle-to-plate device [13]. It was found that, besides streamer corona and a transient spark, a DC glow discharge was generated in a 6 mm gap. However, the visualized image of the glow discharge was more like a discharge filament and no current-voltage characteristics were given to validate the discharge mechanism. Atmospheric pressure DC glow discharges were investigated experimentally with a gap width of 0.4 mm in helium, argon, hydrogen, nitrogen and air, through analyzing the visualization of the discharges and the voltage-current characteristics [14]. Christophe LEYS found that atmospheric pressure glow discharge has characteristic regions such as a cathode fall, a negative glow, a Faraday dark space, a positive column and an anode glow [15,16]. A constant ballast resistor was used and only rotational temperature was investigated supported by National Natural Science Foundation of China (Nos and ), Funds for Distinguished Young Scientists of Hebei Province, China (No. A ), the Key Project of Chinese Ministry of Education (No ), the Natural Science Foundation of Hebei Province (No. A ), Hebei Province Department of Education for Outstanding Youth Project of China (Y )

2 Plasma Science and Technology, Vol.15, No.11, Nov in their research. A detailed investigation on this kind of DC excited glow discharge is conducted in air in this paper. Through changing the ballast resistor and the gas gap width, the discharge characteristics and plasma parameters (the electron temperature and vibrational temperature) are investigated experimentally in atmospheric pressure air with a needle cathode and a water anode. 2 Experimental setup Fig. 1 is a schematic diagram of the experimental setup used for creating APGD in air. A cone shaped tungsten needle cathode with a tip radius of curvature of approximately 100 µm is placed above a water reservoir. The reservoir with a diameter of about 7.6 cm and a height of 10 cm is full of tap water. A metal ring immersed in the water is placed at the bottom of the reservoir to conduct discharge current. The gas gap width between the tungsten needle cathode and the water anode is adjustable in the range of 10 mm. A DC power supply (Glassman 15R40) with maximal voltage of 15 kv and maximal current of 40 ma is connected in series with a ballast resistor, a shunt resistor and the discharge device. Various values of ballast resistor are tried to obtain APGD and it has been found that a stable APGD can be realized with a ballast resistor of the order of several hundred kω. The applied voltage between the two electrodes can be detected by a high voltage probe (Tektronix P6015A). The discharge current is measured by the voltage across the shunt resistor (50 Ω) in series with the discharge device. Both the applied voltage and the discharge current are recorded with a digital oscilloscope (Tektronix DPO4054). A digital camera (Canon EOS 7D) with a macro lens and an ICCD (hsfc pro) are used to record the discharge images with exposure times of 8 ms and 100 ns, respectively. The light emitted from the discharge is focused by a lens into a fiber which is connected to the entrance slit of a spectrometer (ACTON SP2758). The spectrometer has a focal length of 750 mm and a grating of 2400 grooves/mm. The optical emission spectrum can be acquired with a CCD detector ( pixels) equipped on the spectrometer. Fig.1 Schematic diagram of the experimental setup (color online) Results and discussions Corona discharges are firstly ignited in the vicinity of the needle tip by increasing the applied voltage to about 6.8 kv. The corona discharge was in a spherical shape. When the applied voltage increased to some threshold (about 12.8 kv), the two electrodes are bridged by APGD, then, the applied voltage decreased to about 2.2 kv. The brightness and the shape of the discharge vary with the change of the applied voltage. At the same time, the discharge current also changed. Fig. 2 shows the images of APGD under different voltages with a gas gap width of 6 mm. From Fig. 2, it can be found that APGD consisted of a bright cathode spot (negative glow) in the vicinity of the needle electrode, an anode spot (anode glow) on the water surface and a positive column between them. A Faraday dark space is evident between the negative glow and the positive column. The light emission under the anode spot in Fig. 2 comes from the reflection of the water surface. The discharges shown in Fig. 2 have the same characteristic regions as the glow discharge at low pressure, i.e., negative glow, Faraday dark space, positive column and anode glow. Therefore, the discharge in atmospheric pressure air is a kind of APGD. It is worth pointing out that the anode spot usually stabilizes at the location with a minimum distance from the needle to the water surface, though it maybe slide occasionally on the water surface. (a), (b) and (c) are taken by a digital camera with exposure time of 8 ms and (d) is taken by an ICCD camera with exposure time of 100 ns. Other parameters: (a) U =1.9 kv, I=5 ma; (b) U =1.63 kv, I=8 ma; (c) U =1.53 kv, I=11 ma; (d) U =1.53 kv, I=11 ma Fig.2 Images of APGD with 6 mm air gap width and a 500 kω ballast resistor (color online) The dimensions of negative glow, anode glow and positive column change with increasing discharge current. Fig. 3 shows the diameter of the positive column as a function of the discharge current for different gap widths and different ballast resistors. It can be found that the diameter of the positive column increased with increasing discharge current or increasing gap width between the needle and the water electrode. Furthermore, it increases with decreasing value of the ballast resistor. The mechanism will be discussed qualitatively at the end of this paper.

3 LI Xuechen et al.: Characteristics of a Normal Glow Discharge Excited by DC Voltage Fig.3 The diameter of the positive column as a function of the discharge current with different gap widths (a) and different ballast resistors (b) (color online) Fig.4 The current-voltage curves of APGD in air. (a) Different gap widths with a constant ballast resistor of 500 kω, (b) Different ballast resistors with a 6 mm gap (color online) Typical current-voltage characteristics are presented in Fig. 4. The voltage is measured between the needle cathode and the ground water anode. From Fig. 4 it can be found that an increment in the voltage is accompanied by a decrement in the discharge current. The negative slope rate of the current-voltage curve indicates that a normal glow discharge is ignited. From Fig. 4, it can also be found that the voltage increases with increasing gap width or decreasing ballast resistor. The ballast resistor is used to restrict the increment of discharge current. As a result, the voltage increases with decreasing ballast resistor. As is well known, thermal instability will destroy the uniform glow discharge at high pressure, especially in atmospheric pressure air. In atmospheric pressure discharge, more heat is released from the discharge because of the higher breakdown voltage compared with the low pressure discharge. Let us suppose that an increment of discharge current density appears somewhere in the discharge. The gas will expand in the very location where the current density increases because of high gas temperature, compared with that in other locations. Consequently, the averaged free path for electrons will increase so that the electrons at this location have a higher electron temperature. Then, the α coefficient will increase at the location, which leads to a further increment of discharge current density. Consequently, the uniform glow discharge will constrict into a discharge filament, i.e., an arc or a streamer. Obviously, restriction of gas temperature increment is a good way for realizing APGD. The water electrode has a high thermal capacity to restrict the increment of gas temperature. Therefore, a water electrode is important for the realization of APGD in air. A typical emission spectrum radiated from the discharge is given in Fig. 5. The spectrum scanning from 300 nm to 800 nm includes mainly the second positive system of nitrogen molecules (337.1 nm), and the first negative system of nitrogen molecular ions (391.4 nm) shown in the enlarged part. Besides the emissions from nitrogen, a spectral line from an oxygen atom can be found at nm. Fig.5 An optical emission spectrum scanning from 300 nm to 800 nm. Other parameters are d=6 mm, R=500 kω, U =2.23 kv and I=3 ma As stated in Ref. [17], nitrogen molecules are mainly excited to the second positive system by collision with electrons higher than 11 ev, and the first negative system of nitrogen molecular ions are mainly populated by collisions with electrons higher than 18.7 ev. The intensity ratio of nm to nm stands for the electron temperature. The intensity ratio of nm to nm is shown in Fig. 6(a) as a function of the discharge current for different gap widths. From this figure, it can be found that the electron temperature 1151

4 Plasma Science and Technology, Vol.15, No.11, Nov increases with increasing discharge current or increasing gas gap width. The intensity of the second positive system of N2 is used to calculate the vibrational temperature [18]. Apparently, a high vibrational temperature represents basically the chemical reactivity of a vibrational excited species, and provides insight into the relative rates of vibration-vibration and vibrationtranslation energy exchange processes [19]. The vibrational temperature is presented in Fig. 6(b) as a function of the discharge current for different gap widths. The vibrational temperature also increases with increasing discharge current or increasing gas gap width. With a higher electron temperature, the electrons exchange more energy to molecules through collisions. Therefore, both the electron temperature and the vibrational temperature increase with increasing discharge current or increasing gas gap width. Fig.6 The intensity ratio of nm to nm (a) and the vibrational temperature (b) as functions of the discharge current for APGD with a ballast resistor of 500 kω (color online) Fig. 7(a) and (b) indicate the spatially resolved electron temperature and vibrational temperature, respectively. It can be found that the maxima of electron temperature and vibrational temperature appear in the vicinity of the needle electrode. The electron temperature almost keeps constant in the positive column and the anode glow, while the vibrational temperature decreases monotonically with increasing distance from the needle cathode. The discharge power increases with increasing discharge current. Then, the working gas tends to expand because of more heat released from the discharge, which results in an increment of the averaged free path for electrons. Consequently, the electron temperature increases with increasing discharge current. For the same reason, the electron temperature increases with increasing gas gap width or decreasing value of the ballast resistor, as shown in Figs. 6 and 7. The diameter of 1152 positive column is mainly determined by the diffusion of electrons along the radial direction. Therefore, the diffusion efficient is a key factor for the diameter of the positive column. According to the Einstein relation, the diffusion coefficient of electrons (De ) is given by De = µ kte e. Here µ is the drift mobility for electrons, k is the Boltzmann constant, e is the electrical quantity for electrons, and Te is the electron temperature. It is obvious that the diffusion coefficient of electrons is proportional to the electron temperature. Therefore, the diameter of the positive column increases with increasing electron temperature. As pointed out in Figs. 6 and 7, the electron temperature increases with increasing discharge current, increasing gas gap width, or decreasing ballast resistor. Consequently, the diameter of the positive column increases with increasing discharge current, increasing gas gap width, or decreasing ballast resistor, as shown in Fig. 3. Fig.7 The spatially resolved intensity ratio of nm to nm (a) and the vibrational temperature (b) for APGD in air with a gap width of 6 mm and a ballast resistor of 500 kω (color online) 4 Conclusion The characteristics of an atmospheric pressure glow discharge between a needle cathode and a water anode are investigated experimentally by visualization of the discharge and analyzing of its current-voltage curve. The APGD in air has the same characteristic regions as the glow discharge at low pressure, i.e., the cathode glow, Faraday dark space, positive column and an anode glow. The diameter of the positive column increases with increasing discharge current or increasing gas gap width. It also increases with decreasing value of the ballast resistor. The current-voltage characteristic verifies again that the APGD in air is a normal glow discharge. Optical emission spectrum is analyzed to calculate the electron temperature and vibrational

5 LI Xuechen et al.: Characteristics of a Normal Glow Discharge Excited by DC Voltage temperature as a function of the discharge current for different gap widths. Furthermore, spatially resolved electron temperature and vibrational temperature show that their maxima appear in the vicinity of the needle cathode. The diameter of the positive column and the electron temperature as functions of the discharge parameters are discussed qualitatively. References 1 Sun W, Li G, Li H, et al. 2007, Appl. Phys. Lett., 101: Zhang Z, Qiu Y, Lou Y. 2003, J. Phys. D: Appl. Phys., 36: Deng X T, Shi J J. 2005, Appl. Phys. Lett., 87: Plaksin V Y, Penkov O V, and Lee H J. 2010, Plasma Sci. Technol., 12: Kanazawa S, Kogoma M, Moriwaki T. 1988, J. Phys. D: Appl. Phys., 21: Massines F, Gherardi N, Naude N, et al. 2009, Eur. Phys. J.: Appl. Phys., 47: Wang Q, Sun J Z, Wang D Z. 2011, Phys. Plasmas, 18: Wang C, Zhang G, Wang X. 2012, Vacuum, 86: Yang D Z, Wang W C, Li S Z, et al. 2010, J. Phys. D: Appl. Phys., 43: Li X C, Niu D Y, Yin Z Q, et al. 2012, Phys. Plasmas, 19: Pei X, Lu X, Liu J, et al. 2012, J. Phys. D: Appl. Phys., 45: Machala Z, Laux C, Kruger C. 2005, IEEE Trans. Plasma Sci., 33: Machala Z, Jedlovsky I, Martisovits V. 2008, IEEE Trans. Plasma Sci., 36: Staach D, Farouk B, Gutsol A, et al. 2008, Plasma Sources Sci. Technol., 17: Bruggeman P, Liu J, Degroote J, et al. 2008, J. Phys. D: Appl. Phys., 41: Bruggeman P, Leys C. 2009, J. Phys. D: Appl. Phys., 42: Choi J H, Lee T I, Han I, et al. 2006, Plasma Sources Sci. Technol., 15: Li X C, Zhao N, Fang T Z, et al. 2008, Plasma Sources Sci. Technol., 17: Biloiu C, Sun X, Harvey Z, et al. 2006, Rev. Sci. Instrum., 77: 10F117 (Manuscript received 16 August 2012) (Manuscript accepted 18 January 2013) address of LI Xuechen: xcli@hbu.cn; plasmalab@126.com 1153