Improvement of Spatial Uniformity of Nanosecond-Pulse Diffuse Discharges in a Multi-Needle-to-Plane Gap

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1 Plasma Science and Technology, Vol.18, No.3, Mar Improvement of Spatial Uniformity of Nanosecond-Pulse Diffuse Discharges in a Multi-Needle-to-Plane Gap GU Jianwei ( ) 1,3, ZHANG Cheng ( ) 1,2, WANG Ruixue ( ) 1,2, YAN Ping ( ) 1,2, SHAO Tao ( ) 1,2 1 Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing , China 2 Key Laboratory of Power Electronics and Electric Drive, Chinese Academy of Sciences, Beijing , China 3 University of Chinese Academy of Sciences, Beijing , China Abstract Large-scale non-thermal plasmas generated by nanosecond-pulse discharges have been used in various applications, including surface treatment, biomedical treatment, flow control etc. In this paper, atmospheric-pressure diffuse discharge was produced by a homemade nanosecond-pulse generator with a full width at half maximum of 100 ns and a rise time of 70 ns. In order to increase the discharge area, multi-needle electrodes with a 3 3 array were designed. The electrical characteristics of the diffuse discharge array and optical images were investigated by the voltage-current waveforms and discharge images. The experimental results showed that the intensity of diffuse discharges in the center was significantly weaker than those at the margins, resulting in an inhomogeneous spatial uniformity in the diffuse discharge array. Simulation of the electric field showed that the inhomogeneous spatial uniformity was caused by the non-uniform distribution of the electric field in the diffuse discharge array. Moreover, the spatial uniformity of the diffuse discharge array could be improved by increasing the length of the needle in the centre of the array. Finally, the experimental results confirmed the simulation results, and the spatial uniformity of the nanosecond-pulse diffuse discharge array was significantly improved. Keywords: diffuse discharge array, nanosecond pulse, spatial uniformity, electric field PACS: s, v, Hc, Mg DOI: / /18/3/03 (Some figures may appear in colour only in the online journal) 1 Introduction Regular gas discharges (alter-current, direct-current, radiofrequency or microwave discharges) have various discharge forms, such as corona, glow, spark and arc. These discharges behave with different properties [1 3]. Pulsed discharge could generate non-thermal plasmas at atmospheric pressure, and its fast rise-time limits the formation of sparks. Thus, pulsed discharge has been widely used in various applications, covering the fields of surface modification, flow control, ignition and combustion, and environment protection [4 7]. Among the pulsed discharge sources, diffuse discharge could generate overlapped plasma channels in the gap without any spark, which is considered as a suitable sources for generating large-scale homogeneous plasma [8,9]. Different from the corona and spark discharges, a diffuse discharge can be identified as an intermediate state between corona and spark streamer discharges [10]. The diffuse mode was first proposed by Noggle et al. and it has drawn a great deal of attention from then on [11]. Pai et al. investigated the characteristics of repetitive nanosecond-pulse discharges in preheated air at atmospheric pressure [1,12 14]. Three different discharge modes, i.e. corona-like, diffuse-like and filamentary-like were observed in the discharges. Among which, the diffuse-like discharge had its own advantages, such as low light radiation, low electron temperature and low current [15]. By analysing the transform models from corona to diffuse and from diffuse to spark, the discharge modes were deeply understood at atmospheric pressure air. Comprehensive researches of diffuse discharges were also carried out by Tarasenko et al [16 18]. In their experiments, high voltage with a rise time of 1.5 ns and a pulse width of less than 5 ns was used to generate gas discharges in pin-to-pin or tube-to-plate gaps. Larger volume plasmas could be obtained under their experimental conditions. Moreover, it was found that the runaway electrons with high energy and the companied X- rays pre-ionized the gas and ignited diffuse discharges. Chollet et al. investigated the effect of the anode composition on the diffuse pattern in a nanosecond-pulse diffuse discharge [19]. The results showed that diffuse supported by National Natural Science Foundation of China (Nos , ) and the National Basic Research Program of China (No. 2014CB ) 230

2 GU Jianwei et al.: Improvement of Spatial Uniformity of Nanosecond-Pulse Diffuse Discharges discharges with tungsten anode were more homogeneous than those with copper anode. Luo et al. studied the influence of the applied voltage and gas pressure on the transition between patterned and diffuse discharge [20]. They found that lowering the reduced field was effective to generate the diffuse discharge. From the aforementioned research, it can be seen that diffuse discharges are able to produce at atmospheric pressure, and the uniformity is significantly affected by the experimental condition and the electrode arrangement. However, industrial applications generally require homogeneous and large-area plasmas. Li et al. obtained largescale plasmas in room-temperature atmospheric air and analysed the formation mechanism of the cold homogeneous plasmas [21,22]. They found that the conduction current density increased when the diffuse mode transformed to an arc mode, and the area of air cold plasmas was obtained when the gap reached hundreds of square centimetres. The diffuse-to-arc transition could be prevented when an array electrode structure with more wire electrodes was adopted [23]. Packan et al. obtained diffuse discharges in 2000 K air and the calculated electron density reached cm 3 [24]. Obvious filamentary discharges appeared in the gap when the applied voltage increased. Liu et al. studied on large area diffuse discharge plasmas by using bipolar nanosecond pulses [25]. The results showed that the calculated electron density and the electron temperature were cm 3 and 4.4 ev, respectively. In order to obtain large area diffuse plasmas, Yang et al. excited a needle-array-to-plate electrode configuration by a bipolar nanosecond-pulse generator [26]. It was found that the gas temperature increased when the dielectric thickness decreased. Furthermore, the plasma gas temperature remained almost a room temperature, which was much lower than that sustained by sinusoidal voltages [27]. Shao et al. investigated the characteristics of diffuse discharges sustained by nanosecond-pulse and microsecond-pulse generators with knife blade and saw blade electrode structures [28]. They found that the nanosecond-pulse generator was better for sustaining diffuse discharges than the microsecond-pulse generator [29 32]. Zhang et al. studied the characteristics of large-area diffuse discharges in a 5 cm gap with a coaxial configuration [33]. The results showed that the discharge channels overlapped with each other at a low repetition rate rather than at a high repetition rate. It was more likely to generate large-volume diffuse discharges at a low repetition rate. Therefore, different methods should be used to achieve large-volume diffuse discharges at atmospheric pressure. In this paper, a multi-needle-to-plane gap is used to generate volume diffuse discharges. The characteristics of the diffuse discharge array are investigated. 2 Experimental setup Fig. 1 showed the schematic picture of the experimental setup and measurement system. A homemade nanosecond-pulse generator based on the magnetic compression system was used to generate diffuse discharges. The output voltage pulses had a rise time of about 70 ns and a pulse width at half maximum of about 100 ns. Diffuse discharges were created in an anode array in atmospheric air. The array consisted of nine needles in a 3 3 array. These needles were made of copper and had a diameter of 0.9 mm and a length of 50 mm. The distance between each needle was 15 mm. A glass covered by indium tin oxide (ITO) was grounded and severed as the cathode. The ITO glass had a thickness of 2 mm and a transparency of 85%. The distance between the needle tip and the cathode was 3 cm. Fig.1 Schematic picture of the experimental setup and measurement system The voltage applied to the electrodes was measured via a Tektronics P6015 high-voltage probe, and the division ratio was 1000:1. The current was measured by a Pearson Model 6595 current probe, which had a rise time of 2.5 ns and a current-to-voltage ratio of 0.5 A to 1 V. A Lecroy WR204Xi oscilloscope with a bandwidth of 2 GHz and a time resolution of 10 GS/s was used to record the above signals. The images were taken using a Canon EOS500D digital camera with a Tamron Lens (Model A001), which was parallel to the discharge area and was approximately 1 m away from it. 3 Experimental results 3.1 Characteristics of diffuse discharge array Fig. 2 showed typical discharge images from both side and front views. The experimental conditions were as follows: the applied voltage was 28 kv, the PRF was 1 khz and the gap distance was 3 cm. It could be seen that the plasma channels started from the bright spot on the needle tip and spread from the anode to the cathode. No spark appeared in the gap. The plasma channels overlapped with each other and bridged the whole gap, indicating the generation of the diffuse discharges [30]. Furthermore, discharge images revealed that the diffuse discharge in the center was weaker than those on the margins and the diffuse discharge in the center did not reach the cathode. Thus, plasma chan- 231

3 Plasma Science and Technology, Vol.18, No.3, Mar nels in the center were suppressed by those on the margins, making an inhomogeneous distribution in the diffuse discharge array. Fig.2 Typical discharge images from the side view (a) and the front view (b) The voltage and current waveforms of typical discharges were presented in Fig. 3. It was observed that the duration of the current pulse was shorter than that of the voltage pulse, and the discharge current behaved in a unipolar way. The amplitude of the discharge current was about 4 A, which was smaller than that for the spark mode [10]. Further investigation showed that diffuse discharges could remain in a relatively wide range of the applied voltage. Fig.3 Typical voltage-current waveforms of diffuse discharges The pulse repetition frequency plays an important role in the nanosecond-pulse discharge. The experiment conditions were the same as in Fig. 2 except the PRFs were selected at 100 Hz, 250 Hz, 500 Hz and 1000 Hz. Fig. 4 shows the discharge images of diffuse discharges from the side view at different PRFs. Results showed that diffuse discharges were generated at all four frequencies. However, the discharge intensity was significantly affected by the PRF. When the PRF was 100 Hz, a corona discharge appeared around the needle tips where a small curvature radius is located. When the PRF increased to 250 Hz, diffuse discharges were generated in the array, but the discharge intensity was small. Further increasing the PRF gave a rise to the discharge intensity. Therefore, it was likely to obtain intense diffuse discharges at high PRF. Furthermore, the distribution of spatial uniformity was inhomogeneous at all PRFs. Although the electrode structure in this paper was able to increase the area of diffuse discharges, the spatial uniformity required improvement. Fig.4 Discharge images of diffuse discharges at different PRFs: (a) 1000 Hz, (b) 500 Hz, (c) 250 Hz and, (d) 100 Hz 3.2 Simulation results of the electric field distribution in the diffuse discharge array From the above experimental results, it was shown that the diffuse discharge in the center was weaker than those on the margins in the array. In this section, in order to analyze the effect of electric field distribution on the characteristics of diffuse discharges, the electric field distribution in the diffuse discharge array was investigated by the simulation. The software COMSOL Multiphysics 3.5 was adopted to analyze the electrostatic field distribution in the diffuse discharge array. The geometric parameters were as follows: the multineedle array was 3 3 and each needle had a length of 50 mm and a diameter of 0.9 mm, the distance between the neighboring needles was 15 mm, and the gap distance was 30 mm. Fig. 5 shows the geometric model of the simulation. The simulation settings were as follows: the voltage amplitude at the anode was 28 kv, the cathode was grounded, the electrode material was set as copper, and the discharge was created in atmospheric air. In addition, the boundary of the simulation region was considered as the position of zero potential. Fig.5 Geometric model of the simulation 232

4 GU Jianwei et al.: Improvement of Spatial Uniformity of Nanosecond-Pulse Diffuse Discharges Fig. 6 presents the simulation results. The direction and length of each arrow indicate the direction and intensity of the electric field, as shown in Fig. 6(a). Note that the intensities of the electric field in the discharge gap were higher than those outside the gap. The electric field distribution from the radial section was shown in Fig. 6(b), and it could be seen that the highest electric fields appeared in the tips of needles. Fig. 6(c) and Fig. 6(d) showed that the axial sections of the electric field distributions were near the roots of the needles and near the tips, respectively. It could be observed that electric fields were higher near the roots than those near the tips. Furthermore, the electric field in the center was weaker than those on the margins, indicating an inhomogeneous distribution of the electric field in the diffuse discharge array. Therefore, it could be concluded that the inhomogeneous spatial uniformity of the diffuse discharges was closely related to the inhomogeneous distribution of the electric field. needle in the center. The aim of the improvement was to make the electric field of the needle in the center higher than those of the needle on the margins. Table 1 lists the simulation data of the electric field in the array in the case of a longer center needle under different conditions. Both the position of the highest electric field and the highest electric field intensity were presented. It could be seen that the position of the highest electric field varied when the exceeding length of the needle in the center increased. Here the exceeding length referred to the longer length of the needle in the center than those on the margin in the diffuse discharge array. Note that the maximum value of the electric field appeared in the center when the exceeding length of the needle in the center ranged from 1.89 mm to mm. The highest electric field intensity was obtained when the exceeding length was 2 mm. In order to confirm the reliability of the improvement of the spatial uniformity in the diffuse discharge array, an advanced 3 3 multi-needle array was made based on the simulation results. The length of the needle in the center was set at 52 mm, and the lengths of other needles were 50 mm. Discharge images of the new electrode structure are shown in Fig. 7. It can be seen that the intensity of the diffuse discharge in the center significantly increased after the improvement, and the diffuse discharge in the center is almost the same as those on the margins. Particularly, it can be observed that the diffuse discharge in the center bridged the whole gap, and the discharge intensity increased. Therefore, the spatial uniformity of the array was improved when the length of the center needle increased. Fig.6 Simulation results under initial conditions: (a) Distribution of electric field lines, (b) Electric field distribution from the radial section, and the axial section of the electric field distribution near the roots of the needles (c) and near the tips (d) 3.3 Improvement of the spatial uniformity in the array Based on the above simulation results, a feasible method for improving the spatial uniformity of the diffuse discharge array was to increase the length of the Fig.7 Discharge images when the advanced electrode configuration is used (a) from the side view and (b) the front view Table 1. Simulation results with the longer center needle The exceeding Position of the Value of the highest The exceeding Position of the Value of the length highest electric electric field intensity length highest electric field highest electric field (mm) field (10 6 V m 1 ) (mm) (10 6 V m 1 ) 0 Margin Center Margin Center Margin Center Margin Margin Margin Margin Margin Margin Margin Margin Margin Margin Center Margin

5 Plasma Science and Technology, Vol.18, No.3, Mar The voltage and current waveforms of the advanced electrode configuration are shown in Fig. 8. It can be seen that there are two peaks occurring in the current waveform. The amplitude of the current was about 3 A. The two peaks of the current waveform are much more obvious in Fig. 8 than in Fig. 3; this was induced by the change of the electric field distribution. The distribution of electric fields was more homogeneous when the electrode structure was changed, and the discharge tended to be more uniform. Furthermore, compared to our previous work on needle-to-plane structure [10], the waveforms of current and voltage were similar, indicating the generation of stable diffuse discharges in the array. Actually, it can be seen that there is a very small peak in the current waveform at about 120 ns in Fig. 3, which is much smaller than the first peak; such a phenomenon is due to the non-uniform distribution of electric fields and instable discharges. Therefore, the improvement of the spatial uniformity of the diffuse discharge array was effective. Both the runaway electrons and X-rays pre-ionize the gap and ignite the diffuse discharges [9]. The discharge conditions, such as gap distance and PRF, can affect the characteristics of diffuse discharges. The gap distance affects the distribution of the electric field. A large gap leads to a weak electric field. In our previous work [10,28], the diffuse discharge would transit to a corona discharge or spark discharge when the gap increased or decreased, respectively. The electric field decreased when the gap distance decreases, which leads to the relatively lower energy of runaway electrons. Therefore, in order to obtain homogeneous diffuse discharges, the gap distance should be properly chosen. Furthermore, the effect of PRF could be explained by the accumulated effect in nanosecond-pulse discharge [34]. Residual particles could exist for a relatively long time and provide some effective initial electrons before the next pulse arrives. Electron density has the positive correlation with the time intervals between two pulses. Therefore, high PRF with short time intervals will lead to high electron density and as a result it can get an intense diffuse discharge. As a result, other conditions being equal, both increasing the PRF and decreasing the gap distance would result in the increase of electric field. A more intense diffuse discharge will be obtained at a high PRF and a small gap when no diffuse-to-spark transition occurs. 5 Conclusion Fig.8 Voltage-current waveforms of diffuse discharge array when the advanced electrode configuration is used 4 Discussion Because of the fast rise-time and short duration of the pulse, the nanosecond-pulse generator can produce large-area diffuse discharge plasmas, which start near the high voltage electrode. The mechanisms of the diffuse discharge in an inhomogeneous electric field could be explained by the field emission process and the generation of runaway electrons [10]. When the voltage pulses are applied, initial electrons generate from the surface of needle tips and move towards the plate electrode due to the field emission, but this process usually occurs in a small region near the multi-needle electrode with a small radius of curvature. In this paper, the electric field of the multi-needle array ( 10 6 V/cm) is high enough to produce initial electrons. Moreover, these initial electrons continuously accelerate and gain energy, as well as the high over-voltage guaranteeing the generation of runaway electrons in the gap. Because the electric field of the multi-needle array could exceed the critical electric field of 270 kv/cm, and initial electrons turned into the runaway mode [10]. At the same time, X-rays may be produced by collisions between the runaway electrons and molecules and the plate electrode. Characteristics of the diffuse discharge array were investigated in this paper. The experiments showed that a large-scale diffuse discharge array could be obtained at atmospheric pressure. However, the discharge in the center was weaker than those on the margins of the array. According to the simulation results of the distribution of the electric field in the discharge array, the electric field at the tip of the needle in the center was smaller than those on the margins of the array, indicating the spatial uniformity of the diffuse discharge array was significantly affected by the inhomogeneous distribution of the electric field in the array. Furthermore, the improvement of the spatial uniformity of the diffuse discharge array was achieved by increasing the length of the needle in the center of the array. It can enhance the electric field in the center of the array. The exceeding length was 2 mm. Finally, the experimental results confirmed that the improvement of the spatial uniformity of the diffuse discharge array was effective. The results could provide a useful reference for generating large-scale and homogeneous diffuse discharges. References 1 Zhang C, Shao T, Yan P. 2014, Chin. Sci. 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