A comparison between characteristics of atmospheric ...

A comparison between characteristics of atmospheric-pressure plasma jets sustained by nanosecond- and microsecond-pulse generators in helium Cheng Zhang, Tao Shao, Ruixue Wang, Zhongsheng Zhou, Yixiao Zhou, and Ping Yan Citation: Physics of Plasmas (1994-present) 21, 103505 (2014); doi: 10.1063/1.4897322 View online: http://dx.doi.org/10.1063/1.4897322 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/21/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Comparison of the characteristics of atmospheric pressure plasma jets using different working gases and applications to plasma-cancer cell interactions AIP Advances 3, 092128 (2013); 10.1063/1.4823484 Atmospheric pressure He-air plasma jet: Breakdown process and propagation phenomenon AIP Advances 3, 062117 (2013); 10.1063/1.4811464 A pulse-modulated nonequilibrium atmospheric-pressure microwave argon plasma discharge preionized by a kilohertz excited plasma jet Appl. Phys. Lett. 100, 174101 (2012); 10.1063/1.4705433 Computational study of cold atmospheric nanosecond pulsed helium plasma jet in air Appl. Phys. Lett. 99, 111501 (2011); 10.1063/1.3636433 Atmospheric pressure nitrogen plasma jet: Observation of striated multilayer discharge patterns Appl. Phys. Lett. 93, 051504 (2008); 10.1063/1.2969287

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PHYSICS OF PLASMAS 21, 103505 (2014)

A comparison between characteristics of atmospheric-pressure plasma jets sustained by nanosecond- and microsecond-pulse generators in helium Cheng Zhang (章程),1,2 Tao Shao (邵涛),1,2,a) Ruixue Wang (王瑞雪),1,2 Zhongsheng Zhou (周中升),1 Yixiao Zhou (周亦骁),1 and Ping Yan (严萍)1,2 1

Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China Key Laboratory of Power Electronics and Electric Drive, Chinese Academy of Sciences, Beijing 100190, China 2

(Received 24 July 2014; accepted 22 September 2014; published online 6 October 2014) Power source is an important parameter that can affect the characteristics of atmospheric-pressure plasma jets (APPJs), because it can play a key role on the discharge characteristics and ionization process of APPJs. In this paper, the characteristics of helium APPJs sustained by both nanosecondpulse and microsecond-pulse generators are compared from the aspects of plume length, discharge current, consumption power, energy, and optical emission spectrum. Experimental results showed that the pulsed APPJ was initiated near the high-voltage electrode with a small curvature radius, and then the stable helium APPJ could be observed when the applied voltage increased. Moreover, the discharge current of the nanosecond-pulse APPJ was larger than that of the microsecond-pulse APPJ. Furthermore, although the nanosecond-pulse generator consumed less energy than the microsecond-pulse generator, longer plume length, larger instantaneous power per pulse and stronger spectral line intensity could be obtained in the nanosecond-pulse excitation case. In addition, some discussion indicated that the rise time of the applied voltage could play a prominent role on C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4897322] the generation of APPJs. V I. INTRODUCTION

Atmospheric-pressure plasma jet (APPJ) is considered as an efficient non-thermal plasma source for various applications, such as surface modification of polymers, treatment of heat-sensitive biomaterial, bacteria disinfection, and medical treatment of skin and teeth.1–5 In these applications, APPJs have many advantages compared with other plasma sources (corona, dielectric barrier discharge (DBD), and spark discharge), such as low temperatures, strong chemical activities, simple structure, and flexible handling.6,7 Therefore, more and more attentions have been paid on the generation of stable and efficient non-thermal plasma by APPJs.8,9 As to APPJs, the driving power source is one of the important parameters which could significantly affect the characteristics of plasma jets. In many cases, APPJs are excited by radio frequency (RF) and alternating current (AC) generators. Teschke et al. adopted a kHz high voltage power supply to sustain a DBD jet in helium and argon.10 A roomtemperature, APPJ could be achieved under such device by consuming approximately several watts. Walsh et al. studied the characteristics of APPJs sustained by an AC generator at a fixed frequency of 18 kHz. Three modes were found in helium APPJs, namely a plasma bullet, a chaotic mode, and a continuous mode.11 Waskoenig et al. investigated the atomic oxygen formation in a RF driven micro APPJ using laser spectroscopic measurement of absolute densities of ground state atomic oxygen, and the results revealed that there were steep gradients at the interface between the plasma core and a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]

1070-664X/2014/21(10)/103505/7/$30.00

the effluent region.12 In general, APPJs sustained by RF generators have low discharge voltage, high power utilization, and high particle density.13,14 For such kind of APPJs, the temperature of the plasma core in helium varies from 300 to 600 K, resulting in heat accumulation near the plasma core. Nevertheless, the temperature significantly falls outside the discharge region before the plasma interacts with any substrate. Thus, the RF APPJs can only be used in downstream treatments. APPJs excited by AC generator are also widely used in varies applications, and they can generate plasma plumes with a length of several centimeters.15–17 However, the accumulated heat due to high-frequency discharges still exists in AC APPJs, making the energy efficiency low. Thus, researchers make great efforts on the excitation of the power supply and optimal design for the APPJ structure to prevent over-heat damage. They found the pulsed power supply was a good choice for generating cold plasma plumes because it can generate large-volume density plasmas and limit the accumulated heat at atmospheric pressure.18–20 Moreover, pulsed generators are able to provide extremely high power density and reduced electric-field to accelerate electrons, thus, they can generate non-thermal plasmas of active particles with high chemical efficiency at atmospheric pressure.21,22 Recently, more and more attention focuses on generating APPJs by pulsed power supplies. Walsh et al. compared the electrical characteristics of cold APPJs sustained by pulsed and AC generators. It was concluded that pulsed excitation could reduce the electrical energy consumption in the process of producing the same density of oxygen atoms.23 Lu et al. investigated the characteristics of the propagation of the plasma plumes in a plasma jet array excited by pulsed RF and pulsed direct-current (DC) generators. Results showed that the pulsed RF generator could

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produce higher concentration of chemically active species than the pulsed DC generator in the plasma jet array.24 The effect of the rise time of the pulsed DC generator on the generation of APPJs was also presented. Results showed that both the plume length and the peak value of the discharge current increased with the decrease of the rise time.25 Li et al. reported a diffuse plasma plume produced by a DC driven plasma jet. The plasma plume behaved in the cupshaped diffuse mode, and the discharge was self-pulsed in spite of adopting a DC voltage.26 In all the aforementioned researches, the pulsed APPJ demonstrated its characteristics such as less accumulated heat, high electron energy, controllable active particles and high efficiency. Our previous work showed repetitive microsecond-pulse (ls-pulse) and nanosecond-pulse (ns-pulse) generators could excite not only APPJs but also helium plasma array, and a small amount of O2 additive could improve the spatial uniformity of the helium plasma array.27,28 In fact, the characteristics of APPJs are influenced by many different parameters of the pulsed generators, such as the rise time, full width at half maximum, power and energy. In this paper, characteristics of helium APPJs sustained by both ls-pulse and ns-pulse generators are compared. II. EXPERIMENTAL SETUP AND MEASUREMENT

Figure 1 shows the experimental setup and measuring system for the helium APPJs. In these experiments, two home-made pulsed generators are used. The generators have similar designs with those in Refs. 29 and 30. One is a compact ls-pulse generator, whose output pulse has a rise time of 300 ns and a pulse width of 5 ls. The other is a one-stage magnetic compression generator, whose output pulse has a rise time of 70 ns and a pulse width of 100 ns. In order to prevent the influence of the electromagnetic interference, the generators are trigged by a programmable pulse signal generator (PG-F1). In our experiments, both generators can produce repetitive voltage pulses ranged from 5 to 30 kV with the pulse repetition frequency (PRF) up to 3 kHz.

FIG. 1. Schematic diagram of experimental set-up and measurement system.

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A T-shaped cylindrical quartz tube is fixed in a polymethylmethacrylate column. The tube has an inner diameter of 3.2 mm, an outer diameter of 5.2 mm, and a length of 15 cm. The distance between the electrode end face and the tube nozzle is 3 cm. A tungsten needle with a length of 15 cm and a diameter of 2 mm is fixed in the center of the tube by a rubber plug. A part of the needle, whose length is 3 cm, is located outside the tube and connected to the output of the pulsed generator. A grounded copper sheet with a thickness of 180 lm and a width of 10 mm is wrapped at the outer wall of the tube. The distance between the sheet and the nozzle is 10 mm. The working gas, helium (99.999%), is controlled by the Sevenstar D08-4F mass flow meter. The flow rate of the working gas ranges from 0 to 15 standard liters per minute (SLM). The voltage applied to the electrodes is measured by a Tek P6015 high-voltage probe. The discharge current is measured by a Pearson Model 6595 current probe with a rise time of 2.5 ns and a current-to-voltage ratio of 0.2 A to 1 V. A Lecroy WR204Xi with a bandwidth of 2 GHz and a time resolution of 10 GS/s is used to record these signals. The emission spectra of the array are measured by a fiber optic spectrometer (Avantes, AvaSpec-2048-4, wavelength: 250 nm to 850 nm, resolution: 0.12 nm). The distance between the optic fiber and the tube nozzle is 1 cm. The images are taken by a Canon EOS500D digital camera with Tamron Lens (Model A001). III. EXPERIMENTAL RESULTS A. Discharge images of the pulsed APPJs

In our experiments, it could be observed that the pulsed APPJ extended its length with the increase of the applied voltage in both ls-pulse and ns-pulse excitation cases. Figure 2 illustrates the discharge images of helium APPJs at different applied voltages. The experimental conditions were as follows: a ls-pulse generator was used, the polarity of the applied voltage was positive, the helium flow rate was 3 SLM, the PRF was 1 kHz and the exposure time was 1 s. When the applied voltage was 2 kV, only weak luminance could be observed near the high-voltage electrode, which indicated that the APPJ initiated at the place where the small curvature radii was located. When the applied voltages were 3 kV and 4 kV, the APPJ extended its length and the plasma region was covered the whole area between the high-voltage and grounded electrodes, and the plasma channels appeared between the grounded electrode and the nozzle and outside

FIG. 2. Typical discharge images in the inceptive stage of the pulsed APPJ.


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of the nozzle. Note that when the applied voltage exceeded 7 kV, a stable APPJ could be obtained in our cases. To sum up, the pulsed APPJ was initiated near the high-voltage electrode with a small curvature radius, then the stable helium APPJ could be obtained with the increase of the applied voltage. Figure 3 shows the discharge images of the stable pulsed APPJs with different generator excitations. The images were captured with an exposure time of 1 s under the conditions that the applied voltage was 16 kV, the PRF was 1 kHz, and the helium flow rate was 3 SLM. It could be seen that the shapes of the plasma plumes were similar for both generators. The plasma plume length for the ns-pulse APPJ was approximately 6 cm, 1.5 cm longer than that for the lspulse APPJ. B. Voltage-current waveforms of the ns-pulse and ls-pulse APPJs

Figure 4 shows the corresponding voltage-current waveforms of pulsed APPJs excited by both the ns-pulse and lspulse generators. The experimental conditions were the same as those in Figure 3. It could be observed that the total current behaved bipolar though the applied voltages were unipolar, indicating there were two discharges appearing in each case. However, these two discharges occurred at different moments with different generators. In the case of the nspulse APPJ, the first discharge occurred at the rising edge of the applied voltage, behaving as a positive current pulse with a duration of 50 ns and an amplitude of 2.5 A. The second discharge occurred at the falling edge of the applied voltage right after the extinguishment of the first discharge for there was almost no time interval between the first and the second current pulses. Such phenomenon was similar to our previous experimental results on nanosecond-pulse DBD.31 The second discharge behaved as a negative pulse with a duration of 150 ns and an amplitude of 0.5 A, which was much smaller than that of the positive pulse. In the case of the lspulse APPJ, there were also the positive and negative current pulses appearing at the rising edge and falling edge of the applied voltage, respectively. However, there was a time interval (6 ls) between the positive and the negative current pulses, which was different from the former ns-pulse APPJ. In general, the measured current (the total current in Figure 3) is composed of the conduction current and the displacement current.6,32 The conduction current, which is directly produced by the APPJ, refers to the jet current in

FIG. 4. Typical voltage-current waveforms of nanosecond-pulse APPJ (a) and microsecond-pulse APPJ (b).

this paper. In the experiments, when there was no gas flow, no plasma jet was generated, thus, the measured current was mainly composed of the displacement current. To some extent, such measured current could be equal to the displacement current of the APPJ in helium flow. Therefore, the conduction current in the case of the helium APPJ could be calculated by subtracting the displacement current from the total current of the APPJ in helium flow.33 Figure 4 also shows the calculated jet current of the APPJ sustained by both pulsed generators. In both cases, it could be observed that the jet current was almost the same as the amplitude of the total current for the first discharge, but the jet current was smaller than the total current for the second discharge. Moreover, the amplitudes of the total and jet currents for the first discharge were prominently higher than those for the second discharge in both cases. In addition, the amplitude of the jet current in the case of the ns-pulse APPJ was higher than that of the ls-pulse APPJ case. C. Comparison between the discharge current characteristics of the ns-pulse and ls-pulse APPJs

FIG. 3. Discharge images of the pulsed APPJ with different generator excitations.

Figure 5 shows the dependence of the amplitude of the total current on the applied voltage at different PRFs. It could be observed that the amplitude of the total current increased with the applied voltage for both ns-pulse and ls-pulse helium APPJs. All other things being equal, an approximately three-fold increase in the amplitude of the


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experiment, it was concluded that a short rise time could lead to larger current and better performance of plasma plumes of APPJs sustained by the pulsed DC generator.25 D. Comparison between the instantaneous power and energy for the ns-pulse and ls-pulse APPJs

FIG. 5. Dependence of amplitude of total current on the applied voltage at different PRFs.

total current occurred when the generator exciting helium APPJs was changed from the ls-pulse to the ns-pulse generator. Furthermore, the amplitude of the total current decreased with the increase of the PRF for both the ns-pulse and lspulse APPJs, and such decrease was more significant in the ns-pulse APPJ case than in the ls-pulse APPJ case. As shown from Figure 4, the discharge current at the rising edge of the applied voltage was much larger than that at the falling edge. In order to conduct a further comparison between the current characteristics with both generators, the electrical waveforms at the rising edge of the applied voltage in the pulsed APPJs were enlarged, as shown in Figure 6. The experimental conditions were as follows: the applied voltage was 14 kV, the PRF was 1 kHz, and the helium flow rate was 3 SLM. It could be seen that the shorter the rise time was, the earlier the current pulse appeared. Note that the first current pulses in both the ns-pulse and ls-pulse APPJ cases had a time duration of 100 ns. However, the amplitude of the current for the ns-pulse APPJ was about 1.8 A, twice higher than that for the ls-pulse APPJ. Such phenomenon was consistent with the experimental results obtained by Lu et al. who used a pulsed DC power supply. In their

FIG. 6. Voltage-current waveforms at the rising edge of the applied voltage for nanosecond-pulse (a) and microsecond-pulse APPJs (b).

Power consumption and energy are two important parameters influencing the discharge characteristics. Figure 7 shows the calculated power and energy traces for both nspulse and ls-pulse APPJs. The experimental conditions were as the same as those in Figure 3. It could be seen that the peak of the instantaneous power was 30.8 kW and 6.9 kW for the ns-pulse and ls-pulse APPJs, respectively. It could be seen that the energy per pulse for the ls-pulse excitation was larger than that for the ns-pulse excitation. The ls-pulse APPJ consumed about 1 mJ more than the ns-pulse APPJ per pulse, because the pulse duration for the ls-pulse APPJ was longer than that for the ns-pulse APPJ, and such longer duration required more energy consumption for maintaining a stable APPJ. Figure 8 shows the energy consumption per second for the ns-pulse and ls-pulse APPJs at different PRFs. Note that the energy consumption increased with the PRF. Furthermore, the higher the PRF, the larger difference between the energy consumption per second for the ns-pulse and ls-pulse APPJs.

FIG. 7. Power and energy waveforms for nanosecond-pulse (a) and microsecond-pulse APPJs (b).


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there were some OH, N2, N2þ, and O in the emission spectra of the helium APPJ. It is because the working gas is impure and the surrounding air could be drifted into the working gas. An interesting results worthy to be pointed out here is that the generation of OH and O in the pulsed APPJ is beneficial for biomedical and material applications and environment protection.6,34 Furthermore, the optical emission intensities of all these spectral lines mentioned above for the ns-pulse APPJ were stronger than those for the ls-pulse APPJ. For example, the higher emission intensities at 308.8 nm (OH(A2R(v ¼ 0))!OH(X2R(v ¼ 0)) þ hk) and 777.8 nm (O (35S-35P)) indicated higher concentration of excited states of OH radicals and atomic oxygen in the active plume zone of the ns-pulse APPJ.24

FIG. 8. Energy consumption per second for nanosecond-pulse and microsecond-pulse APPJs at different PRFs.

E. Comparison between the optical emission spectra for the ns-pulse and ls-pulse APPJs

Figure 9 shows the emission spectra of the APPJs sustained by both the pulsed generators. The experimental conditions were as the same as those in Figure 3. Some typical spectral lines were shown in the emission spectra, such as OH(A2Rþ-X2P), N2(C3Pu-B3Pg), N2þ(B2Ru-X2Rg), He (33S-23P), He(31S-21P), He(31D-21P), and O(3p5S-3s5S), indicating the generation of stable helium APPJs.28 Note that

FIG. 9. Emission spectra for nanosecond-pulse and microsecond-pulse APPJs.

IV. DISCUSSION

Some characteristics of helium APPJs sustained by both the ns-pulse and ls-pulse generators are compared in this paper. Different from the AC or RF discharges, a pulsed discharge is a typical over-voltage discharge and it could provide a high instantaneous power density to accelerate high-energy electrons for gas ionization.19 Moreover, its short duration is able to limit the heat accumulation on the electrodes and produce stable non-thermal plasmas.20 Rise time is an important parameter of pulsed discharges, especially in molecular gases at the atmospheric pressure.35,36 Comparing with AC and RF excitations, the fast rise time of pulsed discharges is critical to achieve low gas temperature and obtain large-volume discharges. The voltage rises so fast that it is higher than the critical voltage required when the streamer has started developed and propagating.37 In our experiments, it could be seen in Figures 4 and 6 that the first discharge occurred at the rising edge of the applied voltage in both cases of the ns-pulse and ls-pulse APPJs. The rise time of the applied voltage for the ns-pulse APPJ was faster than that for the ls-pulse APPJ, and the current amplitude or the ns-pulse APPJ was higher than that for the ls-pulse APPJ. It is because the shorter rise time for the ns-pulse excitation could make higher elevated applied voltage and larger reduced electric field (E/N). Thus, it is more likely to obtain both high electron density and desirable active plasma chemistries in the case of ns-pulse APPJ.35 Furthermore, only certain excited states of some chemically active species in the active plume zone could be reflected by the spectral lines in optical emission spectroscopy because the plasmas are highly collisional at atmospheric pressure.14,38 It could be observed from Figure 9 that all the intensities of spectral lines in the optical emission spectra for the ns-pulse APPJ were stronger than those for the ls-pulse APPJ, indicating higher concentrations of the said species in certain exited states are obtained in the ns-pulse APPJ case with shorter rise time. Leiweke et al. investigated the effect of the rise time on argon metastable production efficiency in DBD, and they also found the metastable production efficiency for a short rise time was much higher than that for a long rise time.38 Note that the plume length for the ns-pulse APPJ was longer than that for the ls-pulse APPJ in Figure 3. However, such phenomenon is not consistent with the results presented


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by Lu et al., who used pulsed DC generator to excite APPJ.39 In their experiment, the length of the plasma plume with a pulse width of 10 ls was much longer than that with a pulse width of 300 ns, and the longest plasma plume was obtained when the pulse width was 1 ls. In our opinion, such phenomenon is also closely related to the rise time of the applied voltage. When the plasma plume propagates beyond the electrode, the plasma plume decelerates and tends to quench due to the high-rate collision at the atmospheric pressure. For pulsed discharge, their short rise time would permit the use of over-voltage, and the increase in E/N would lead to high density of initial electrons and high propagation velocity.40 In our experiments, the rising rate of the applied voltage for the ns-pulse APPJ is 228 V/ns, which is much higher than that for the ls-pulse APPJ (53 V/ns). The density of the initial electrons for the ns-pulse APPJ is also higher than that for the ls-pulse APPJ due to its faster ionization, resulting in higher velocity of the plasma bullets and longer distance they could go. It should be pointed out that because of the variation of parameters that could affect the pulsed generators, such as rise time, pulse width, energy consumption, and efficiency, the accurate effect of pulsed generators on plasma jets has been still not well understood. It can be seen from Figure 5 that the amplitudes of the total current for the ns-pulse APPJ are larger than those for the lspulse APPJ. This is also due to the effect of voltage rise time. Because the ns-pulse APPJ has shorter voltage rise time, its E/N is higher and could induce higher peak discharge currents.35 The current amplitude decreases with the increase of the PRF for both ns-pulse and ls-pulse APPJs, as shown in Figure 5. Two reasons could be used to explain such phenomenon. The first reason is about the dynamic of the plasma plumes at different PRFs.38 The densities of electrons and active particles increase with the PRF, making the direct electron impact excitation efficiency in the plasma plumes become higher. Thus, the equivalent gap capacitance decreases with the increase of the PRF, leading to the decrease of the circuit power deposition through the displacement current.41 Therefore, the total current decreases with the increase of the PRF. The second reason is the memory effect in pulsed discharges. Memory effect refers to the effect of the residual particles (ions and metastable species) on the characteristics of the discharge before the next pulse is applied.42,43 These particles are beneficial to the streamer propagation. They are generated in the previous discharge and could exist for several hundreds of microseconds or longer after the discharge extinguishes. The memory effect is prominent at a high PRF due to the short time interval between neighboring pulses, making the discharge voltage at a high PRF lower than that at a low PRF.31 Thus, the total current was smaller at a high PRF than at a low PRF. Detail reasons would be expected to be identified by some other measurements, such as electron density kinetics, recombination time under some certain conditions. V. CONCLUSION

A comparison between characteristics of helium APPJs sustained by the ls-pulse and ns-pulse generators is

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conducted by analyzing the plume length, discharge current, power and energy consumption and optical emission spectra in this paper. The pulsed APPJ initiated near the highvoltage electrode with a small curvature radius, and then a stable helium APPJ could be obtained with the increase of the applied voltage. Both the ns-pulse and ls-pulse generators could maintain a stable APPJ at a PRF of 1 kHz. The length of plasma plume for the ns-pulse APPJ is longer than that for the ls-pulse APPJ. Furthermore, all amplitudes of the total current, jet current and instantaneous power for the ns-pulse APPJ is twice or three times larger than those for the ls-pulse APPJ. Because of the memory effect at a high PRF, the total current decreases with the increase of the PRF. However, the ls-pulse APPJ consumes more energy than the ns-pulse APPJ. In addition, because of the larger density of initial electrons and fast ionization for the ns-pulse APPJ, all the intensities of the spectral lines for the ns-pulse APPJ are larger than those for the ls-pulse APPJ, which indicates the concentration of excited OH radicals and atomic oxygen in the active plume zone of the ns-pulse APPJ is higher than that of the ls-pulse APPJ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China under Contracts Nos. 51222701, 51207154, the National Basic Research Program of China under Contract No. 2014CB239505-3, and the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources under Contract No. LAPS14009. X. Lu, G. V. Naidis, M. Laroussi, and K. Ostrikov, Phys. Rep. 540, 123 (2014). 2 R. J. Leiweke, B. N. Ganguly, and J. D. Scofield, Phys. Plasmas 21, 083508 (2014). 3 X. Pei, Y. Lu, S. Wu, Q. Xiong, and X. Lu, Plasma Sources Sci. Technol. 22, 025023 (2013). 4 K. Kim, H. J. Ahn, J. H. Lee, J. H. Kim, S. S. Yang, and J. S. Lee, Appl. Phys. Lett. 104, 013701 (2014). 5 C. Chang, G. Liu, C. Tang, C. Chen, and J. Fang, Phys. Plasmas 18, 055702 (2011). 6 X. Lu, M. Laroussi, and V. Puech, Plasma Sources Sci. Technol. 21, 034005 (2012). 7 M. Qiang, C. Ren, D. Wang, Y. Feng, and J. Zhang, Plasma Sources Sci. Technol. 12, 561 (2010). 8 J. P. Boeuf, L. L. Yang, and L. C. Pitchford, J. Phys. D: Appl. Phys. 46, 015201 (2013). 9 S. Reuter, J. Winter, A. Schmidt-Bleker, H. Tresp, M. U. Hammer, and K. D. Weltmann, IEEE Trans. Plasma Sci. 40, 2788 (2012). 10 M. Teschke, J. Kedzierski, E. G. Finantu-Dinu, D. Korzec, and J. Engemann, IEEE Trans. Plasma Sci. 33, 310 (2005). 11 J. L. Walsh, F. Iza, N. B. Janson, V. J. Law, and M. G. Kong, J. Phys. D: Appl. Phys. 43, 075201 (2010). 12 J. Waskoenig, K. Niemi, N. Knake, L. M. Graham, S. Reuter, V. Schulzvon der Gathen, and T. Gans, Plasma Sources Sci. Technol. 19, 045018 (2010). 13 E. Wagenaars, T. Gans, D. O’Connell, and K. Niemi, Plasma Sources Sci. Technol. 21, 042002 (2012). 14 S. Hofmann, A. F. H. van Gessel, T. Verreycken, and P. Bruggeman, Plasma Sources Sci. Technol. 20, 065010 (2011). 15 J. S. Sousa, K. Niemi, L. J. Cox, Q. T. Algwari, T. Gans, and D. O’Connell, J. Appl. Phys. 109, 123302 (2011). 16 J. Furmanski, J. Y. Kim, and S. O. Kim, IEEE Trans. Plasma Sci. 39, 2338 (2011). 17 X. Li, P. Jia, N. Yuan, T. Fang, and L. Wang, Phys. Plasmas 18, 043505 (2011). 18 X. Cheng, J. Liu, B. Qian, Z. Chen, and J. Feng, IEEE Trans. Plasma Sci. 38, 516 (2010). 1


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