Photographic Study on Spark Discharge Generated by a Nanosecond High-Voltage Pulse over a Water Surface

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1 Photographic Study on Spark Discharge Generated by a Nanosecond High-Voltage Pulse over a Water Surface LI Wenqin (o ), WEN Xiaoqiong ( ), ZHANG Jialiang (Ü[û) Center for Plasma Science and Engineering, School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian , China Abstract Spark discharge generated by a nanosecond positive high-voltage pulse over a water surface at atmospheric pressure in air was studied using a high speed camera system. Faint streamers form near the pin electrode and propagate towards the water surface. The time for the streamer propagating across the air gap was estimated to be about 50 ns to 60 ns with a propagation velocity of m/s. It was found that the water conductivity and the gap distance have no significant effect on the propagation velocity of the streamer. After the streamers touch the water surface a brilliant spark channel forms across the air gap. The maximum diameter at the middle of the spark channel is about 1 mm, and approximately contracts with a radical velocity of about m/s. No significant dependence of the maximum diameter and decay velocity of the spark channel on the water conductivity and the gap distance were recognized in the present work. The maximum conduction current for a gap distance of 5 mm is significantly larger than that for a gap distance of 10 mm at the same water conductivity, and shows an increasing tendency with increasing water conductivity for a fixed gap distance. Based on the maximum conduction current, the effect of water conductivity and gap distance on the electron density of the spark discharge plasma at the peak current was investigated. Within the range studied, the electron density in the spark channel is about cm 3 and increases with water conductivity at a fixed gap distance. Keywords: spark discharge, water surface, nanosecond high-voltage pulse, high speed photography PACS: Mg DOI: / /15/10/11 1 Introduction Recently electrical discharges over a water surface in air at atmospheric pressure have attracted intensive attention for their potential applications in biomedical and surface treatment, chemical and biological decontamination, etc. These kind of discharges generate directly or indirectly many chemical reactive species ( OH, H 2 O 2,O 2,O 3 ) [1 3], which can attack and then degrade the organic pollutants contained in the water without secondary pollution. Additionally, the formation of oxygen-containing functional groups as well as the increase of surface energy allows the immobilizing of other specific chemical compounds that contain useful functional groups on the surface of polymer materials [4,5]. Electrical discharges over a water surface generated by DC or AC high voltage have been extensively studied. GAISIN et al. [6] found that the electrolyte solution, metal electrode structure and gap distance have a significant influence on the morphology and properties of the discharge. BRUGGEMAN et al. [7 10] and LU et al. [11 13] investigated in detail the effect of the voltage polarity on the discharge. They found that the discharge appears like a typical glow discharge if the water serves as an anode, while the discharge appears to be filamentary, forming multi-contact-points on the water surface, if the water serves as a cathode. BRUGGE- MAN et al. concluded that for a metal-to-water electrode system with r/d 1, the discharge could be considered as a glow-to-spark transition [8] for negative polarity, and a streamer-to-spark transition [9,10] for positive polarity of the pin electrode. There are few reports on the generation of electrical discharges over a water surface by a nanosecond high voltage pulse, although it has significant advantage in applications such as water treatment and polymer modification. For example, the spark discharges generated by nanosecond high voltage pulses can dissociate a large fraction of oxygen molecules and heat the gas to about 1000 K within a few tens of nanoseconds following each pulse [14]. In the present paper, spark discharge generated by nanosecond positive high-voltage pulse over a water surface at atmospheric pressure in air was studied by using a high speed camera system. The temporal evolution of the discharge ignition and the decay of the spark chan- supported in part by National Natural Science Foundation of China (No ) and the Fundamental Research Funds for the Chinese Central Universities (DUT11ZD(G)06) and (DUT13ZD(G)05)

2 nel were investigated for different water conductivities and gap distances. 2 Experiment The schematic of experimental setup was shown in Fig. 1. A pin-to-water electrode system was employed in the present work. The point anode was made of copper wire of 1 mm in diameter, the tip of which was tapered to 50 μm. A copper sheet of mm was set as a cathode at the bottom of the container. Deionized water with an initial conductivity of 1 μs/cm was used and its depth in the container was 70 mm. The conductivity of the water was adjusted by adding sodium chloride (analytical reagent). The gap distance between the point anode and the water surface was adjustable by moving the pin electrode. The spark discharge was generated in atmospheric air by applying a nanosecond positive high-voltage pulse. The discharge voltage and current were detected by a highvoltage probe (P6015A) and a current probe (Pearson 6585), respectively, and measured by a digital oscilloscope (Tektronix DPO4054). The time-resolved images of the discharge generated in the air gap were taken by an ultrahigh-speed frame camera system (PCO-HSFC) with four intensified charge-coupled device (ICCD). The trigger signal of the camera system was taken from the discharge voltage through a high-voltage probe (P6015A). The delay time of the camera was adjusted by comparing the voltage-current waveform and the trigger pulse. Fig. 2 shows typical waveforms of discharge voltage and current. The current measured by the digital oscilloscope consists of two parts, displacement current and conduction current. The first peak in the current waveform can be attributed to the displacement current caused by the rise of the high voltage pulse and the second peak arises from the discharge between electrodes. The displacement current was measured when there was no discharge between the electrodes. Then the conduction current was deduced by subtracting the displacement current from the discharge current. The maximum conduction current under different conditions was estimated and the results were plotted in Fig. 3. The maximum conduction current for a gap distance of 5 mm is significant larger than that for a gap distance of 10 mm at the same water conductivity, and it shows an increasing tendency with increasing water conductivity for a fixed gap distance. Fig.2 Typical waveforms of the discharge voltage and current measured at a gap distance of 10 mm and water conductivity of 1 µs/cm (color online) Fig.1 Experimental setup 3 Results and discussion Fig.3 The maximum conduction current estimated for different water conductivities and gap distances (color online) The time-resolved images of the ignition stage of discharge in the air gap were acquired by the ultrahighspeed frame camera system. Four successive images were taken from the same discharge pulse with 20 ns gate time and 0 ns inter-frame time. The typical images acquired at a 10 μs/cm water conductivity and 10 mm gap distance are shown Fig. 4. The time of the first image in Fig. 4 was set as 0 ns, which corresponds to point A marked in Fig. 2. Fig. 4 shows that faint streamers form near the pin electrode and propagate toward the water surface. The radius and light emission of the streamer increase (Fig. 4(2)), and many sub-branches emerge (Fig. 4(3)). The time for the streamer propagating across the air gap was estimated to be about 50 ns to 60 ns. 1021

3 Fig.4 Time-resolved images of streamer discharge at a gap distance of 10 mm and water conductivity of 10 µs/cm (20nsgatetimeand0nsinter-frametime) Fig.5 Time integrated images of spark discharge for different water conductivities and gap distances Based on the images, the propagation velocity of the streamer was deduced as v = Δd Δt, (1) where Δd is the length of the streamers and Δt is the time interval between the two successive images. It was found that the propagation velocity of the streamer is about m/s, and the water conductivity and the gap distance have no significant effects on it. NIKI- FOROV et al. [9] obtained a propagation velocity of the streamer of about 10 5 m/s with a DC discharge in a brass metal-to-water electrode system. However, LU et al. [15] deduced a propagation velocity of about 10 3 m/s in a 60 Hz AC discharge, which is much lower than that estimated in our experiments. It has been found [9] that a brilliant spark channel will form between the electrodes after the streamer touches the water surface. The maximum radius of the spark channel is of interest. In order to obtain the size of the maximum radius of the spark channel, the timeintegrated images of the spark discharge were recorded. The gate time of each image was 1 μs, which was chosen to match the discharge voltage pulse (FWHM of 500 ns). The typical time-integrated images of the spark discharge at different conductivities and gap distances are shown in Fig. 5. The maximum diameters at the middle of the spark channel were measured for different water conductivities and gap distances. The results are plotted in Fig. 6, which depicts that the water conductivity and gap distance have little influence on the maximum diameters of the spark channel within the range studied. The spark channel can spontaneously decay through electron attachment or electron-ion recombination after bridging the gaseous gap [16]. In order to study the Fig.6 Maximum diameters at the middle of the spark channel for different water conductivities and gap distances (color online) decay of the spark channel, four successive one-shot images with 20 ns gate time and 60 ns inter-frame time were taken from the same discharge pulse. Fig. 7 shows typical images of the decay of the the spark channel acquired at 40 μs/cm water conductivity and 5 mm gap distance. The time of the first image in Fig. 7 was set as 0 ns, which corresponds to point B marked in Fig. 2. It can be recognized that the spark channel gets gradually thinner and grayer. The diameter at the middle of the spark channel was measured for different water conductivities and gap distances, and the results are illustrated in Fig. 8 as functions of time. This reveals that the spark channel approximately contracts in a linear way with a radical velocity of about m/s, and has no significant dependence on the water conductivity and gap distance. In some previous simulation studies of spark discharges in air at atmospheric pressure [16,17], it was usually assumed that the radius of the spark channel was fixed or expanded in a certain approximate way. Our results provide support to some extent to these assumptions used in numerical simulations. 1022

4 into Eq. (2), the electron density n e can be given as [18] n e = 4NQ e h 8kB T e m e πe 2 D 2 di c, (4) V π where e and m e are the electron charge and mass, respectively. The electron-heavy collision frequency ν e h is acquired by Fig.7 Decay of the spark channel at a gap distance of 5 mm and water conductivity of 40 µs/cm (20 ns gate time and 60 ns interframe time) 8kB T e ν e h NQ e h, (5) πm e where N is the gas number density in the ideal gas law, Q e h is the electron heavy cross-section for momentum transfer, k B is Boltzmann s constant and T e is the electron temperature. It has been proved that the spark discharge maintains T g > 2000 K at all times [18]. In the present work, N is taken as cm 3,asthe gas temperature was 3000 K for the applied voltage of 30 kv. Q e h and T e are cm 2 and K, respectively [18]. The effect of water conductivity and gap distance on the electron density of the spark discharge plasma at the peak current was investigated. The results obtained for different water conductivities and gap distances are shown in Fig. 9. The electron density is of cm 3 within the range studied, and it increases with water conductivity at a fixed gap distance. PAI et al. [18] obtained a maximum electron density in the order of magnitude of cm 3 in a pin-pin electrode system and PILLA et al. [19] derived a value of cm 3 by the Stark broadening of H β in a spark discharge in a propane/air mixture. Our results agree well with these works. Fig.8 Diameters at the middle of the spark channel for different water conductivities and gap distances (color online) Based on the results on the maximum current and the maximum radius of the spark channel, the electron density of the spark discharge plasma at the peak current can be estimated approximately. The plasma resistance R p in the spark channel was approximately calculated by dividing the measured applied voltage V by the conduction current I c, and was related to the plasma conductivity (σ p )by R p = l / σp S, (2) where l is the gap distance and S is cross-sectional area of the spark channel, S = πd 2 /4. The maximum diameter d of spark channel was taken as 1 mm in the present work according to the results shown in Fig. 6. By substituting the plasma conductivity σ p = n ee 2 m e ν e h, (3) Fig.9 Electron density of the spark discharge plasma corresponding to the peak conduction current for different water conductivities and gap distances (color online) 4 Conclusion Spark discharge generated by nanosecond positive high-voltage pulse over a water surface has been studied by using a high speed camera system. Faint streamers form near the pin electrode and propagate toward the water surface. The time for the streamer propagating across the air gap was estimated to be of about 50 ns to 60 ns with a propagation velocity of m/s. It was found that the water conductivity and the gap distance have no significant effect on the propagation 1023

5 velocity of streamers. After the streamers touch the water surface a brilliant spark channel forms across the air gap. The maximum diameter at the middle of the spark channel is about 1 mm, and approximately contracts with a radical velocity of about m/s. No significant dependence of the maximum diameter and decay velocity of the spark channel on the water conductivity and gap distance were recognized in the present work. The maximum conduction current for a gap distance of 5 mm is significantly larger than that for a gap distance of 10 mm at the same water conductivity, and shows an increasing tendency with increasing water conductivity for a fixed gap distance. Based on the maximum conduction current, the effect of water conductivity and gap distance on the electron density of the spark discharge plasma at the peak current was investigated. Within the range studied, the electron density in the spark channel is of cm 3 and increases with water conductivity at a fixed gap distance. References 1 Lukes P and Bruce R L. 2005, J. Phys. D: Appl. Phys., 38: Janca J, Kuzmin S, Maximov A, et al. 1999, Plasma Chemistry and Plasma Processing, 19: 53 3 Burlica R, Kirkpatrick M J and Locke B R. 2006, J. Electrostat., 64: 35 4 Titova V A, Rybkina V V, Shikovaa T G, et al. 2005, Surface & Coatings Technology, 199: Choi H S, Shikova T G, Titov V A, et al. 2006, Journal of Colloid and Interface Science, 300: Gaisin A F. 2005, High Temp., 43: Mezei P and Cserfalvi T. 2007, Appl. Spectrosc., 42: Bruggeman P, Guns P, Degroote J, et al. 2008, Plasma Sources Sci. Technol., 17: Nikiforov A Yu and Leys Ch. 2006, J. Phys., 56: Bruggeman P, Ribezl E, Degroote J, et al. 2008, J. Optoelectron. Adv. Mater., 10: Laroussi Mounir, Lu Xinpei and Malott Chad M. 2003, Plasma Sources Sci. Technol., 12: Robinson J A, Bergougnou M A, Castle G S P, et al. 2001, IEEE Trans. Indust. Appl., 37: Lu Xinpei and Laroussi M. 2005, IEEE Trans. Plasma Sci., 33: Pai D. 2008, Nanosecond Repetitively Pulsed Plasmas in Preheated Air at Atmospheric Pressure [Ph.D]. Ecole Centrale Paris, Châtenay-Malabry (in French) 15 Lu Xin Pei and Laroussi M. 2003, J. Phys. D: Appl. Phys., 36: Aleksandrov N L and Bazelyan E M. 1999, Plasma Sources Sci. Technol., 8: Aleksandrov N L and Bazelyan E M. 1996, J. Phys. D: Appl. Phys., 29: Pai David Z, Deanna A Lacoste and Christophe O L. 2010, Plasma Sources Sci. Technol., 19: Pilla G. 2008, Etude expérimentale de la stabilisation de flames propane-air de préange par décharges nanoseconds impulsionnelles répétitives [Ph.D]. Ecole Centrale Paris, Chétenay-Malabry, 7: 385 (in French) (Manuscript received 22 June 2012) (Manuscript accepted 24 December 2012) address of corresponding author WEN Xiaoqiong: wenxq@dlut.edu.cn 1024