APPLIED PHYSICS LETTERS 89, 161502 共2006兲

Characteristics of radio-frequency, atmospheric-pressure glow discharges with air using bare metal electrodes Hua-Bo Wang, Wen-Ting Sun, He-Ping Li,a兲 Cheng-Yu Bao, and Xiao-Zhang Zhang Department of Engineering Physics, Tsinghua University, Beijing 100084, People’s Republic of China

共Received 21 June 2006; accepted 30 August 2006; published online 16 October 2006兲 In this letter, an induced gas discharge approach is proposed and described in detail for obtaining a uniform atmospheric-pressure glow discharge with air in a ␥ mode using water-cooled, bare metal electrodes driven by radio-frequency 共13.56 MHz兲 power supply. A preliminary study on the discharge characteristics of the air glow discharge is also presented in this study. With this induced gas discharge approach, radio-frequency, atmospheric-pressure glow discharges using bare metal electrodes with other gases which cannot be ignited directly as the plasma working gas, such as nitrogen, oxygen, etc., can also be obtained. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2363040兴 In recent years, atmospheric-pressure, glow discharge 共APGD兲 plasmas using bare metal electrodes and driven by radio-frequency 共rf兲 power supply have attracted more and more attention of the researchers in the world. This is because comparing with low-pressure glow discharge plasmas, APGD plasmas can provide advantages with reducing the capital costs of equipment and eliminating constraints on object sizes imposed by vacuum compatibility in actual applications, such as plasma etching, deposition, decontamination of chemical and biological warfare agents, etc., due to the removal of the vacuum system.1 Comparing with atmospheric-pressure dielectric barrier discharge 共DBD兲, rf APGD plasmas with bare metal electrodes do not need dielectric layers covered on the electrodes, which results in a much lower breakdown voltage and more homogeneous discharge.1 But up to now, most of the researchers use helium or argon as a plasma working gas, to which a small fraction 共0.5%–3%兲 of reactive gases 共e.g., O2, CF4, or water vapor, etc.兲 is added in order to generate a flux of chemically active species.2–7 Wang et al.6 and Laimer et al.7 obtained rf APGD plasmas with Ar– 1.0% O2 and pure argon as working gas, respectively. Yang et al.8 reported the discharge characteristics for a gas mixture of He– 0.4% N2. In actual applications, the large volume consumption of helium or argon would increase the capital costs of this technology. So, it is one of the challenges to reduce the costs of the plasma forming gas, i.e., to obtain APGD plasmas with cheaper plasma forming gas, e.g., nitrogen or air. Although rf discharges in air at atmospheric pressure were reported in previous papers,9–11 initiated by first touching the electrodes together, setting the current to the desired value, and then drawing the electrodes apart, the discharges operated as either rf glow or rf arc discharges, which resulted in the asymmetric voltage-current characteristics.11 In this letter, a uniform, APGD plasma operated steadily in a ␥ mode with air using two planar water-cooled bare copper electrodes is obtained with an induced gas discharge approach. And a preliminary study on the discharge characteristics of air APGD plasmas is also presented.

The sparking field of air is as high as 30 kV/ cm;12 therefore, on one hand, it is difficult to ignite an air discharge at atmospheric pressure, and on the other hand, the discharge tends to transfer into a filamentary arc after breakdown due to intense avalanche of electrons under so strong electric field.13 So, it is necessary to control the avalanche amplification to avoid its too fast growth in order to obtain a glow discharge.13 In atmospheric-pressure dielectric barrier discharge plasmas, the dielectric barrier layers are employed to inhibit the occurrence of a filamentary arc. And for an atmospheric-pressure DBD, if there are enough seed electrons before the occurrence of another discharge under a low electric field, it would be helpful to form a uniform glow discharge.13,14 Based on these facts, an induced gas discharge approach is proposed in the present letter to obtain a rf, APGD plasma with air as plasma working gas using watercooled, bare metal electrodes as follows: the uniform glow discharge with helium operated in an ␣ mode is obtained first; then, with the increase of the input power, the discharge transfers into a ␥ mode; and then, increasing the air flow rate and, at the same time, decreasing the flow rate of helium, finally, a stable, uniform glow discharge with air working in the ␥ mode is obtained when no helium is added into the plasma working gas anymore. A schematic diagram of the experimental setup is shown in Fig. 1. The planar-type plasma generator is composed of two 5 ⫻ 8 cm2 planar, bare, water-cooled copper electrodes,

a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

FIG. 1. Schematic diagram of the experimental setup.

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Appl. Phys. Lett. 89, 161502 共2006兲

FIG. 2. 共Color online兲 Photographs of the rf APGD plasmas with gap spacing d = 3.0 mm, where the white solid lines indicate the positions of the electrodes. 共a兲 Helium discharge in the ␥ mode, gas flow rate QHe = 1.0 slpm, input power Pin = 211 W; 共b兲 helium-air gas mixture discharge in the ␥ mode, Qair = QHe = 1.0 slpm, Pin = 320 W; 共c兲 air discharge in the ␥ mode, Qair = 1.0 slpm, Pin = 327 W.

i.e., the rf 共13.56 MHz兲 powered top electrode and the grounded bottom electrode. Teflon spacers are used to seal the plasma generator on both sides and adjust the distance between the electrodes. The plasma forming gas 共helium with purity of 99.99% or better, and/or air supplied by an oil-free compressor made in China兲 is admitted into the plasma generator from the left side, ionized between electrodes, and flows out of the generator from the right side. The rms values and wave forms of current and voltage, as well as the current-voltage phase difference, are measured using a current probe 共Tektronix TCP202兲 and a high voltage probe 共Tektronix P5100兲 and are recorded on a digital oscilloscope 共Tektronix TDS3054B兲. The discharge images are taken by a digital camera 共Fujifilm S5500兲. Similar to discharges at intermediate pressure, rf APGD plasmas can exist in two distinctively different, but stable modes, i.e., ␣ mode 共sustained by volumetric ionization process兲 and ␥ mode 共ionization by secondary electrons from the electrode surfaces is important兲.3,15,16 The discharge photographs with different gas mixing ratios ␹ 关=Qair / 共Qair + QHe兲兴 for a gap spacing d = 3.0 mm are shown in Fig. 2. For showing the discharge patterns between electrodes more clearly, a short exposure time is selected which makes the other part between electrodes darker. The white solid lines in Fig. 2 indicate the positions of the electrodes. It can be seen from Fig. 2 that 共1兲 for a pure helium 关QHe = 1.0 slpm 共standard liters per minute兲兴 glow discharge operated in a ␥ mode, as shown in Fig. 2共a兲, the negative bright glows on both electrodes are connected with a less luminous positive column at the gap center. 共2兲 After air is admitted into the discharge region mixed with helium 共QHe = Qair = 1.0 slpm兲, the discharge image changes obviously compared with that of pure helium discharge. As shown in Fig. 2共b兲, there is a much brighter positive column across the gap between two electrodes, which is surrounded by a less luminous discharge region. 共3兲 For a pure air glow discharge with no helium addition, the luminous structure of the discharge, as shown in Fig. 2共c兲, is almost the same as that for a helium-air mixture discharge, but with a more clear discharge region surrounding the brighter positive column. In this study, the ␥ mode glow discharge is very stable and can last for several hours. The voltage and current wave forms of air 共Qair = 2.0 slpm兲 rf APGD plasmas operated in a ␥ mode with

FIG. 3. Wave forms of the voltage 共solid line兲 and current 共dashed line兲 of the air rf APGD plasmas in the ␥ mode for d = 1.5 mm and Qair = 2.0 slpm. 共a兲 Pin = 117 W; 共b兲 Pin = 325 W.

different rf power inputs using water-cooled bare copper electrodes for gap spacing d = 1.5 mm are illustrated in Fig. 3. Figure 3 shows that there are obvious distortions for both the current and voltage wave forms, and with increasing rf power input, the distortions of the voltage and current wave forms both become larger. By analyzing the wave forms using fast Fourier transform, it is found that the amplitudes of the third and fifth harmonics of the voltage wave form are 6.9% and 4.2% of the fundamental wave, while in the case of the current, the amplitudes of the third, fifth, and seventh harmonics reach the values of 1.4%, 7.0%, and 6.1% of the fundamental wave, respectively, for the input power Pin = 117 W, as shown in Fig. 3共a兲; for the case with a higher rf power input, Pin = 325 W, as illustrated in Fig. 3共b兲, the corresponding values are 17.5% and 11.1% for the voltage wave form, while 2.9%, 10.1%, and 12.2% for the current wave form, respectively. Figure 3 also indicates that the distortions of the current wave forms are more significant than those of the voltage ones, especially for the case with higher rf power input. The relationship between the amplitudes of the rf current and the discharge voltage can be illustrated with a I-V curve. In Fig. 4, the I-V characteristics of the rf APGD plasmas working in a ␥ mode with air 共Qair = 2.0 slpm兲 for different gap spacings are presented. It can be seen from Fig. 4 that 共1兲 keeping the gap spacing constant, the discharge voltages vary little with the variations of the rf current. 共2兲 The averaged discharge voltages for sustaining a ␥ mode discharge with air increase with the increase of the gap spacing. The

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charge voltages increase with the increase of the gap spacing, and the plasma working gas flow rates have little influence on the gas discharge voltages. The experiments in this study also show that 共1兲 helium can be replaced with argon to obtain the air rf APGD plasmas in a ␥ mode and 共2兲 using the induced gas discharge approach, the rf APGDs using bare metal electrodes with other gases which cannot be ignited directly, such as nitrogen, oxygen, etc., can also be obtained. Further studies on the discharge characteristics, including the current and voltage wave forms, the I-V characteristics, the luminous structures of the discharge, etc., of air rf APGD plasmas using bare metal electrodes for different gap spacings, different electrode materials, different input power levels, etc., need to be conducted in future work.

FIG. 4. 共Color online兲 I-V characteristics of the air rf APGD plasmas for different gap spacings, Qair = 2.0 slpm.

averaged values of the discharge voltages are 297.3± 3.1, 317.2± 0.3, and 355.7± 5.8 V for the gap spacings d = 1.5, 3.0, and 6.4 mm, respectively. 共3兲 The corresponding averaged electric fields are approximately 1982, 1057, and 556 V / cm, respectively, which are much lower than that of the sparking electric field for air discharge 共⬃30 kV/ cm兲.12 In Ref. 11, in a normal glow, the current density at the electrode was constant before the discharge covered the whole electrode surface, i.e., the area of the electrode covered by the discharge increased with current. But in our experiment, with the increase of the discharge current, no obvious enlargement of the discharge region is observed for a ␥ mode discharge of air. The influences of the plasma working gas flow rates on the gas discharge voltages for different gap spacings are also studied in this letter, which shows that the flow rates of the plasma forming gas have little effect on the discharge voltages for a fixed gap spacing. In summary, the rf APGD plasmas operated in a ␥ mode with air using water-cooled, bare copper electrodes are obtained in this study using the induced gas discharge approach. The experimental observations show that the luminous structure of the discharge region with air is very different from that of helium discharge. The primary studies on the discharge characteristics show that there are obvious distortions of the current and voltage wave forms, the dis-

This work has been supported by the project sponsored by SRF for ROCS, SEM. 1

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Characteristics of radio-frequency, atmospheric ...

Oct 16, 2006 - Hua-Bo Wang, Wen-Ting Sun, He-Ping Li,a) Cheng-Yu Bao, and Xiao-Zhang Zhang. Department of Engineering Physics, Tsinghua University, ...

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