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Effect of pressure on the emission characteristics of surface dielectric barrier discharge plasma
Sensors and Actuators A 203 (2013) 1–5
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Effect of pressure on the emission characteristics of surface dielectric barrier discharge plasma Yun Wu a,b,∗ , Yinghong Li a , Min Jia a , Huimin Song a , Hua Liang a a b
Science and Technology on Plasma Dynamics Lab, Aeronautics and Astronautics Engineering College, Air Force Engineering University, Xi’an 710038, China Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 15 March 2013 Received in revised form 8 July 2013 Accepted 22 July 2013 Available online 12 August 2013 Keywords: Optical emission spectroscopy Plasma aerodynamic actuator Dielectric barrier discharge
a b s t r a c t This paper reports an experimental study of the emission characteristics of the surface dielectric barrier discharge plasma under different pressure. N2 (C3 IIu ) rotational temperature, electron temperature and density were used to quantify the plasma characteristics. At atmospheric pressure, when the inner gap width increases from 0 mm to 1 mm, emission characteristics change dramatically since the discharge is much more filamentary. N2 (C3 IIu ) rotational temperature, electron temperature and density are much higher with inner gap width of 1 mm. Along with the pressure decreasing, discharge transitions from filamentary to glow mode around 45 Torr. Variation laws of N2 (C3 IIu ) rotational temperature and electron density with inner gap width of 0 mm and 1 mm are different. While the variation law of electron temperature is similar. When the pressure is less than 8 Torr, plasma characteristics are very similar with inner gap width of 0 mm and 1 mm. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Plasma aerodynamic actuation is a novel flow control method to improve aircrafts’ aerodynamic characteristics and propulsion efficiency [1,2]. Plasma aerodynamic actuation generated by surface dielectric barrier discharge (SDBD) has been used to control flow separation, extend axial compressor stability and so on [3–5]. In order to get some new experimental results for further physical mechanism analysis, it is important to investigate the characteristics of SDBD plasma under different operating parameters. The physics of plasma actuator, such as discharge current, electric field, ion density, electron density, and electrohydrodynamic force, has been numerically investigated in [6,7]. High-speed photography, photomultiplier, optical emission spectroscopy, V-dot probe, mass balance, vertical pendulum, laser deflection technique, and phase-locked particle image velocimetry have been adopted to study the characteristics of plasma aerodynamic actuation experimentally [8–13]. Discharge during the negative-going portion of the discharge cycle is diffuse, while it is filamentary during the positive-going portion of the discharge cycle. 97% of the momentum coupling occurs during the negative-going portion of the discharge cycle [12]. Influence of applied voltage, actuator
∗ Corresponding author at: Science and Technology on Plasma Dynamics Lab, Aeronautics and Astronautics Engineering College, Air Force Engineering University, Xi’an 710038, China. Tel.: +86 02984787527. E-mail address: [email protected] (Y. Wu). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.07.023
configuration and dielectric material on the induced velocity and plasma characteristics of a SDBD plasma actuator has been investigated in [14–18]. With higher voltage and thinner electrode, the induced velocity is much higher. Plasma actuator with inner gap width of 1–2 mm was selected to maximize the induced velocity. Dielectric losses of quartz and Teflon are much smaller than other materials, such as printed circuit board and Bakelite. Characteristics of both the plasma and the induced flow field of plasma aerodynamic actuation generated by nanosecond pulsed discharge were different from the actuation generated by sine wave discharge [16–18]. Ambient pressure and temperature affect the characteristics of SDBD plasma actuator dramatically, as shown in [19–24]. As the pressure decreases, N2 (C3 IIu ) rotational temperature, vibrational temperature, electron temperature and density, and the induced velocity change dramatically. In this paper, we report an experimental study on emission characteristics of SDBD plasma with inner gap width of 0 mm and 1 mm under different pressures. 2. Experimental setup A schematic of the dielectric barrier discharge plasma aerodynamic actuator is shown in Fig. 1. The dielectric layer is RO4350B plate with relative permittivity constant of 3.48. The electrode is made of copper covered with lead-tin film and. The lower electrode, which is covered with silica gel, is grounded. The thickness of dielectric material is 0.5 mm. The electrode width is 1 mm. The electrode length and height are 80 mm and 0.035 mm, respectively. The inner gap width varies from 0 mm to 1 mm. The applied voltage
2
Y. Wu et al. / Sensors and Actuators A 203 (2013) 1–5
Fig. 1. A schematic of the asymmetric SDBD plasma aerodynamic actuator (not to scale).
waveform is sinusoidal wave. The output voltage and the frequency range of the power supply are 0–40 kV and 6–40 kHz, respectively. The applied voltage and the discharge current are measured by a high voltage probe (Tektronix P6015A) and a current probe (Tektronix TCP312+TCPA300). Signals are recorded on an oscilloscope (Tektronix DPO4104). Optical emission spectra are obtained using a charge coupled device spectrometer (Avantes 2048) through an optical fiber, located 1 mm above the surface of the dielectric layer and 1 mm downstream of the upper electrode. The exposure time is 1 s. The emission intensity is averaged temporally and spatially. The driving frequency is fixed at 23 kHz for both actuators in the experiment. The initial discharge voltage is higher for 1 mm gap actuator. 3. Experimental results and discussions N2 (C3 IIu ) rotational temperature, electron temperature and density were calculated to quantify the plasma characteristics [25]. Optical emission spectroscopy (OES), as a simple and non-intrusive diagnostic method, can be used to investigate atmospheric pressure air plasmas. OES has been used widely in estimating the rotational and vibrational temperatures, electron temperature and density in plasmas with gas pressure from several Pa to atmosphere, and temperature form hundreds to thousands of Kelvin [26–29]. The measurement uncertainty is almost unaffected by the pressure or gas temperature. For example, N2 (C) rotational temperature is obtained by fitting the measured profile of the emission band near 380.5 nm with that of a simulated one. By assuming a rotational temperature and considering the dipole radiation probability and the response function of the monochrometer, one can calculate the profile of a certain emission band. The actual rotational temperature can be determined through comparing the experimental measurement and theoretical calculation. OES has also been used in the investigation on surface barrier discharge plasma [30,31]. The rotational temperature comes to equilibrium with the translational temperature in less than 1 ns at atmospheric pressure [32]. The radiative lifetime of N2 (C) is around 40 ns, while the effective lifetime of N2 (C) is about 1 ns at one atmospheric pressure, respectively [33]. Rotational and vibrational temperatures have been used in plasmas with gas pressure from several Pa to atmosphere [26,27]. Rotational and vibrational temperatures with time interval of 2 ns have investigated in [34]. In this paper, the uncertainty of N2 (C) rotational temperatures is 3%. The uncertainty of electron temperature and density are both 3%. Since the accuracy of spectrometer is high and the calculation is not very complex, limiting factors of the resolution include the repetitiveness of discharge and theory used for diagnostics. The discharge is filamentary at atmospheric pressure and the streamer is distributed randomly at nanosecond scale, but the time-averaged result is much more stable. All experiments were conducted in air. The behavior of the DBD in nitrogen is very different from its behavior in air. The ground state N2 quenches N2 (C) and N2 (B) at rates much less than dissociative quenching by ground state O2 . Large numbers of N2 (A) is also
Fig. 2. Discharge images with different inner gap widths.
quenched by ground state O2 . Quenching of N2 (C) and N2 (B) by O2 is a main source of atomic oxygen production. Then oxygen negative ions are generated, which are the main source of the thrust for air during the negative half discharge cycle. In these low temperature discharges the molecular oxygen ion (O2 − ) is the most important. The role of photoionization is also very important in air. The role of oxygen quenching mechanism is considered in the estimation model of electron temperature and density, as shown in equation (12) and (13) in [25]. Spectral features and the values of the rate constants of collisional reactions of N2 (C), N2 (B) and N2 + (B) change significantly with pressure. The variation of rate constants of collisional reactions has been considered in the electron temperature and density estimation at varying pressure. When d increases from 0 mm to 1 mm, emission characteristics change dramatically at atmospheric pressure. Experiments with 0 mm and 1 mm gap actuators were conducted individually. Then the results were plotted in one figure. N2 (C3 IIu ) rotational temperature increases from 535 K to 880 K. The electron temperature increases from 1.63 eV to 1.73 eV. The electron density changes most, from 1.1 × 1011 cm−3 to 6.9 × 1012 cm−3 . The main reason for such dramatical change may be that the discharge with d of 1 mm is much more filamentary, as shown in Fig. 2. The plasma is uniform and diffused along the spanwise direction with d of 0 mm. While the discharge filaments are sparse, each filament is strong and bright with d of 1 mm. Discharge energy deposited on each filament is higher. Thus the N2 (C3 IIu ) rotational temperature, electron temperature and density increase a lot because it is determined by the optical emission from the discharge filaments. Turning up the voltage leads to higher heating of the dielectric barrier. Under low voltage, the discharge is limited in several filaments. As the voltage increases, the discharge is also filamentary, but more filaments are generated and discharge energy is distributed more averaged than the low voltage case, which is the reason for lower rotational temperature at higher voltage. When the voltage becomes even higher, the heat of dielectric layer is serious and the discharge is not stable. After just 1–2 s, dielectric breakdown happens and the discharge transitions to arc. At this time, the rotational temperature is much higher, which is not plotted since it is not stable. Due to lack of spatial resolution, the spatial-averaged rotational temperature is decreasing. In order to get the SDBD plasma characteristics at lower pressure, experiments with d of 0 mm and 1 mm were performed. Fig. 3 shows the applied voltage and discharge current traces at different operating pressures. At pressure higher than 50 Torr, the discharge is filamentary, indicated by many fine and narrow pulsed currents. As the pressure decreases, the number of narrow pulsed currents is less. At even lower pressure such as 2 Torr and 50 Torr,
Y. Wu et al. / Sensors and Actuators A 203 (2013) 1–5
3
emission intensity has been normalized by the emission intensity at 380.5 nm. As shown in Fig. 5, there is a significant difference between the pressure sensitivity of the 380 and 390 nm lines. For the spectral bands C (1–3), C (0–2), C (2–5), B (0–0), the emission of light of the corresponding wavelengths is caused by the elementary processes (1)–(8). The first negative system (FNS) overlaps the second positive system (SPS) in the 390 nm region. In the case of air discharge, if the relative intensity of the FNS and SPS of nitrogen molecules is changed, it means a change in average electron energy and electron energy distribution function (EEDF) because the ionization potential for the FNS of nitrogen molecules (18.7 eV) is higher than that of the SPS (11 eV) [35]. Therefore, the increase in the FNS of N2 + (B) with decreasing pressure has the same meaning as an increase in the number of high energy electrons. Increasing pressure changes the reduced field strength (E/N), thus reducing the ionization rate and changing the electron energy distribution. 3 e + N2 (X1 + g )=0 → N2 (C u ) =1 + e
N2 (C3 u ) =1 → N2 (B3 g ) =3 + h
(1) ( = 375.5 nm)
3 e + N2 (X1 + g )=0 → N2 (C u ) =0 + e
N2 (C3 u ) =0 → N2 (B3 g ) =2 + h
(3) ( = 380.5 nm)
3 e + N2 (X1 + g )=0 → N2 (C u ) =2 + e
N2 (C3 u ) =2 → N2 (B3 g ) =5 + h
the discharge is glow, shown by the current waveform. The discharge images with d of 1 mm correspond well with the current waveforms, as shown in Fig. 4. Intense and bright filaments diminished at low pressure. Therefore, the discharge transitions from filamentary discharge to glow discharge at a certain pressure. Influence of operating pressure on SDBD plasma characteristics with d of 1 mm has been investigated in [20]. In this paper, the difference plasma characteristics at different operating pressure with d of 0 mm and 1 mm is investigated in detail. Fig. 5 shows the optical emission spectra under different operating pressures. The
(4) (5)
( = 394.3 nm)
(6)
+ 2 + e + N2 (X1 + g )=0 → N2 (B ˙u ) =0 + 2e
(7)
+ 2 + (B2 + N+ u ) =0 → N2 (X g ) =0 + h 2
(8)
N2 (C3 IIu )
Fig. 3. Traces of the applied voltage and discharge current under different operating pressures.
(2)
( = 391.4 nm)
We calculate the rotational temperature, electron temperature and density and compare the values with d of 0 mm and 1 mm. The N2 (C3 IIu ) rotational temperature versus operating pressure is shown in Fig. 6. As the pressure decreases from atmospheric pressure, the electron collision frequency decreases, which leads to the decrease of the rotational temperature. When the operating pressure is higher than 45 Torr, N2 (C3 IIu ) rotational temperature with d of 1 mm is much higher than that with d of 0 mm. When the operating pressure is lower than 45 Torr, the rate of decrease of N2 (C3 IIu ) rotational temperature with d of 1 mm is much higher. When the operating pressure is lower than 8 Torr, N2 (C3 IIu ) rotational temperatures with d of 0 mm and 1 mm are very close, around 380 K. At high pressure, the discharge state with d of 0 mm and 1 mm is different, while it is similar at low pressure since the discharge is glow. The electron temperature versus operating pressure is shown in Fig. 7. The variation law of electron temperature in both discharges is very similar. With d of 1 mm, the electron temperature increases slowly from 1.73 eV at 760 Torr to 1.87 eV at 45 Torr, and then increases rapidly to 3.06 eV at 2 Torr. With d of 0 mm, the electron temperature increases slowly from 1.63 eV at 760 Torr to 1.91 eV at 40 Torr, and then increases rapidly to 3.01 eV at 2 Torr. Therefore, the electron temperature is mainly governed by operating pressure and discharge mode, not actuator configuration. Also, the transition pressure around 45 Torr can be inferred. The electron density versus operating pressure is shown in Fig. 8. The variation law of electron density in both discharges is very different. With d of 1 mm, the electron density decreases slowly from 6.9 × 1012 cm−3 at 760 Torr to 8.9 × 1010 cm−3 at 40 Torr, and then increases to 2.3 × 1011 cm−3 at 2 Torr. With d of 0 mm, the electron density increases from 1.1 × 1011 cm−3 at 760 Torr to 8.0 × 1011 cm−3 at 400 Torr, and then decreases to 2.0 × 1011 cm−3
4
Y. Wu et al. / Sensors and Actuators A 203 (2013) 1–5
Fig. 4. Surface dielectric barrier discharge plasma under different operating pressures (d = 1 mm).
Fig. 5. Optical emission spectra under different operating pressures.
at 2 Torr. When the operating pressure is lower than 8 Torr, electron density with d of 0 mm and 1 mm are very close. The electron density is governed by ionization rate and operating pressure, therefore the variation law is much more complex than other plasma parameters. When the pressure is higher than 300 Torr, the ionization rate with d of 1 mm is higher than that with d of 0 mm. Once
the pressure is lower than 300 Torr, the ionization rate with d of 1 mm is lower. From Figs. 6–8, it can be concluded that d and operating pressure play major roles in the plasma aerodynamic actuation behavior. When the operating pressure is high, the discharge is in filamentary mode. Plasma characteristics, especially N2 (C3 IIu )
Fig. 6. The N2 (C3 IIu ) rotational temperature versus the operating pressure.
Fig. 7. The electron temperature versus the operating pressure.
Y. Wu et al. / Sensors and Actuators A 203 (2013) 1–5
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discharges is very different. When the pressure is higher than 350 Torr, electron density with d of 1 mm is higher. When the pressure is lower than 350 Torr, electron density with d of 0 mm is higher. When the operating pressure is lower than 8 Torr, the electron density is very close. Acknowledgments This project is supported by the National Natural Science Foundation of China (51336011, 50906100) and the Science Foundation for the Author of National Excellent Doctoral Dissertation of China (201172). References
Fig. 8. The electron density versus the operating pressure.
rotational temperature and electron density, with d of 1 mm and 0 mm are very different. When the operating pressure is low, the discharge transitions to glow mode and plasma characteristics become similar. Although discharge with d of 1 mm and 0 mm are different, the electron temperature is similar and is mainly governed by the operating pressure. Since the nitrogen second positive emission has a very short lifetime (nanoseconds), the measurements of properties correlates with the time during which the discharge is active. The measurements are averaged over 1 s, while in fact the positive half cycle discharge is very different from the negative half cycle discharge. The positive cycle discharge is normally much more filamentary than the negative half cycle, so the parameters should be quite different. Therefore, the time-averaged value cannot indicate the differentiation between the positive and the negative half cycle discharges, which needs further investigation with fast optical emission spectroscopy. Also the result in this paper is spatialaveraged, while in fact the plasma is filamentary at high pressure. Therefore, the spatial-averaged value throughout the collection volume sampled is associated with the most luminous portion of the discharge, which needs further investigation with spatialresolved method. 4. Conclusions To summarize, d and pressure affects the plasma characteristics dramatically. When d increases from 0 mm to 1 mm, the discharge is much more filamentary at atmospheric pressure. N2 (C3 IIu ) rotational temperature, electron temperature and density are much higher with d of 1 mm. As the operating pressure decreases, both discharge transition from filamentary to glow mode around 45 Torr. N2 (C3 IIu ) rotational temperature with d of 1 mm is much higher when the operating pressure is higher than 45 Torr, while it is very close when the operating pressure is lower than 8 Torr. The difference of electron temperature with d of 1 mm and 0 mm is small and it is mainly governed by the operating pressure. When the operating pressure is higher than 45 Torr, the electron temperature increases slowly. When the operating pressure is lower than 45 Torr, the electron temperature increases sharply to around 3 eV. The variation law of electron density in both
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Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Effect of pressure on the emission characteristics of surface dielectric barrier discharge plasma Yun Wu a,b,∗ , Yinghong Li a , Min Jia a , Huimin Song a , Hua Liang a a b
Science and Technology on Plasma Dynamics Lab, Aeronautics and Astronautics Engineering College, Air Force Engineering University, Xi’an 710038, China Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 15 March 2013 Received in revised form 8 July 2013 Accepted 22 July 2013 Available online 12 August 2013 Keywords: Optical emission spectroscopy Plasma aerodynamic actuator Dielectric barrier discharge
a b s t r a c t This paper reports an experimental study of the emission characteristics of the surface dielectric barrier discharge plasma under different pressure. N2 (C3 IIu ) rotational temperature, electron temperature and density were used to quantify the plasma characteristics. At atmospheric pressure, when the inner gap width increases from 0 mm to 1 mm, emission characteristics change dramatically since the discharge is much more filamentary. N2 (C3 IIu ) rotational temperature, electron temperature and density are much higher with inner gap width of 1 mm. Along with the pressure decreasing, discharge transitions from filamentary to glow mode around 45 Torr. Variation laws of N2 (C3 IIu ) rotational temperature and electron density with inner gap width of 0 mm and 1 mm are different. While the variation law of electron temperature is similar. When the pressure is less than 8 Torr, plasma characteristics are very similar with inner gap width of 0 mm and 1 mm. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Plasma aerodynamic actuation is a novel flow control method to improve aircrafts’ aerodynamic characteristics and propulsion efficiency [1,2]. Plasma aerodynamic actuation generated by surface dielectric barrier discharge (SDBD) has been used to control flow separation, extend axial compressor stability and so on [3–5]. In order to get some new experimental results for further physical mechanism analysis, it is important to investigate the characteristics of SDBD plasma under different operating parameters. The physics of plasma actuator, such as discharge current, electric field, ion density, electron density, and electrohydrodynamic force, has been numerically investigated in [6,7]. High-speed photography, photomultiplier, optical emission spectroscopy, V-dot probe, mass balance, vertical pendulum, laser deflection technique, and phase-locked particle image velocimetry have been adopted to study the characteristics of plasma aerodynamic actuation experimentally [8–13]. Discharge during the negative-going portion of the discharge cycle is diffuse, while it is filamentary during the positive-going portion of the discharge cycle. 97% of the momentum coupling occurs during the negative-going portion of the discharge cycle [12]. Influence of applied voltage, actuator
∗ Corresponding author at: Science and Technology on Plasma Dynamics Lab, Aeronautics and Astronautics Engineering College, Air Force Engineering University, Xi’an 710038, China. Tel.: +86 02984787527. E-mail address: [email protected] (Y. Wu). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.07.023
configuration and dielectric material on the induced velocity and plasma characteristics of a SDBD plasma actuator has been investigated in [14–18]. With higher voltage and thinner electrode, the induced velocity is much higher. Plasma actuator with inner gap width of 1–2 mm was selected to maximize the induced velocity. Dielectric losses of quartz and Teflon are much smaller than other materials, such as printed circuit board and Bakelite. Characteristics of both the plasma and the induced flow field of plasma aerodynamic actuation generated by nanosecond pulsed discharge were different from the actuation generated by sine wave discharge [16–18]. Ambient pressure and temperature affect the characteristics of SDBD plasma actuator dramatically, as shown in [19–24]. As the pressure decreases, N2 (C3 IIu ) rotational temperature, vibrational temperature, electron temperature and density, and the induced velocity change dramatically. In this paper, we report an experimental study on emission characteristics of SDBD plasma with inner gap width of 0 mm and 1 mm under different pressures. 2. Experimental setup A schematic of the dielectric barrier discharge plasma aerodynamic actuator is shown in Fig. 1. The dielectric layer is RO4350B plate with relative permittivity constant of 3.48. The electrode is made of copper covered with lead-tin film and. The lower electrode, which is covered with silica gel, is grounded. The thickness of dielectric material is 0.5 mm. The electrode width is 1 mm. The electrode length and height are 80 mm and 0.035 mm, respectively. The inner gap width varies from 0 mm to 1 mm. The applied voltage
2
Y. Wu et al. / Sensors and Actuators A 203 (2013) 1–5
Fig. 1. A schematic of the asymmetric SDBD plasma aerodynamic actuator (not to scale).
waveform is sinusoidal wave. The output voltage and the frequency range of the power supply are 0–40 kV and 6–40 kHz, respectively. The applied voltage and the discharge current are measured by a high voltage probe (Tektronix P6015A) and a current probe (Tektronix TCP312+TCPA300). Signals are recorded on an oscilloscope (Tektronix DPO4104). Optical emission spectra are obtained using a charge coupled device spectrometer (Avantes 2048) through an optical fiber, located 1 mm above the surface of the dielectric layer and 1 mm downstream of the upper electrode. The exposure time is 1 s. The emission intensity is averaged temporally and spatially. The driving frequency is fixed at 23 kHz for both actuators in the experiment. The initial discharge voltage is higher for 1 mm gap actuator. 3. Experimental results and discussions N2 (C3 IIu ) rotational temperature, electron temperature and density were calculated to quantify the plasma characteristics [25]. Optical emission spectroscopy (OES), as a simple and non-intrusive diagnostic method, can be used to investigate atmospheric pressure air plasmas. OES has been used widely in estimating the rotational and vibrational temperatures, electron temperature and density in plasmas with gas pressure from several Pa to atmosphere, and temperature form hundreds to thousands of Kelvin [26–29]. The measurement uncertainty is almost unaffected by the pressure or gas temperature. For example, N2 (C) rotational temperature is obtained by fitting the measured profile of the emission band near 380.5 nm with that of a simulated one. By assuming a rotational temperature and considering the dipole radiation probability and the response function of the monochrometer, one can calculate the profile of a certain emission band. The actual rotational temperature can be determined through comparing the experimental measurement and theoretical calculation. OES has also been used in the investigation on surface barrier discharge plasma [30,31]. The rotational temperature comes to equilibrium with the translational temperature in less than 1 ns at atmospheric pressure [32]. The radiative lifetime of N2 (C) is around 40 ns, while the effective lifetime of N2 (C) is about 1 ns at one atmospheric pressure, respectively [33]. Rotational and vibrational temperatures have been used in plasmas with gas pressure from several Pa to atmosphere [26,27]. Rotational and vibrational temperatures with time interval of 2 ns have investigated in [34]. In this paper, the uncertainty of N2 (C) rotational temperatures is 3%. The uncertainty of electron temperature and density are both 3%. Since the accuracy of spectrometer is high and the calculation is not very complex, limiting factors of the resolution include the repetitiveness of discharge and theory used for diagnostics. The discharge is filamentary at atmospheric pressure and the streamer is distributed randomly at nanosecond scale, but the time-averaged result is much more stable. All experiments were conducted in air. The behavior of the DBD in nitrogen is very different from its behavior in air. The ground state N2 quenches N2 (C) and N2 (B) at rates much less than dissociative quenching by ground state O2 . Large numbers of N2 (A) is also
Fig. 2. Discharge images with different inner gap widths.
quenched by ground state O2 . Quenching of N2 (C) and N2 (B) by O2 is a main source of atomic oxygen production. Then oxygen negative ions are generated, which are the main source of the thrust for air during the negative half discharge cycle. In these low temperature discharges the molecular oxygen ion (O2 − ) is the most important. The role of photoionization is also very important in air. The role of oxygen quenching mechanism is considered in the estimation model of electron temperature and density, as shown in equation (12) and (13) in [25]. Spectral features and the values of the rate constants of collisional reactions of N2 (C), N2 (B) and N2 + (B) change significantly with pressure. The variation of rate constants of collisional reactions has been considered in the electron temperature and density estimation at varying pressure. When d increases from 0 mm to 1 mm, emission characteristics change dramatically at atmospheric pressure. Experiments with 0 mm and 1 mm gap actuators were conducted individually. Then the results were plotted in one figure. N2 (C3 IIu ) rotational temperature increases from 535 K to 880 K. The electron temperature increases from 1.63 eV to 1.73 eV. The electron density changes most, from 1.1 × 1011 cm−3 to 6.9 × 1012 cm−3 . The main reason for such dramatical change may be that the discharge with d of 1 mm is much more filamentary, as shown in Fig. 2. The plasma is uniform and diffused along the spanwise direction with d of 0 mm. While the discharge filaments are sparse, each filament is strong and bright with d of 1 mm. Discharge energy deposited on each filament is higher. Thus the N2 (C3 IIu ) rotational temperature, electron temperature and density increase a lot because it is determined by the optical emission from the discharge filaments. Turning up the voltage leads to higher heating of the dielectric barrier. Under low voltage, the discharge is limited in several filaments. As the voltage increases, the discharge is also filamentary, but more filaments are generated and discharge energy is distributed more averaged than the low voltage case, which is the reason for lower rotational temperature at higher voltage. When the voltage becomes even higher, the heat of dielectric layer is serious and the discharge is not stable. After just 1–2 s, dielectric breakdown happens and the discharge transitions to arc. At this time, the rotational temperature is much higher, which is not plotted since it is not stable. Due to lack of spatial resolution, the spatial-averaged rotational temperature is decreasing. In order to get the SDBD plasma characteristics at lower pressure, experiments with d of 0 mm and 1 mm were performed. Fig. 3 shows the applied voltage and discharge current traces at different operating pressures. At pressure higher than 50 Torr, the discharge is filamentary, indicated by many fine and narrow pulsed currents. As the pressure decreases, the number of narrow pulsed currents is less. At even lower pressure such as 2 Torr and 50 Torr,
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emission intensity has been normalized by the emission intensity at 380.5 nm. As shown in Fig. 5, there is a significant difference between the pressure sensitivity of the 380 and 390 nm lines. For the spectral bands C (1–3), C (0–2), C (2–5), B (0–0), the emission of light of the corresponding wavelengths is caused by the elementary processes (1)–(8). The first negative system (FNS) overlaps the second positive system (SPS) in the 390 nm region. In the case of air discharge, if the relative intensity of the FNS and SPS of nitrogen molecules is changed, it means a change in average electron energy and electron energy distribution function (EEDF) because the ionization potential for the FNS of nitrogen molecules (18.7 eV) is higher than that of the SPS (11 eV) [35]. Therefore, the increase in the FNS of N2 + (B) with decreasing pressure has the same meaning as an increase in the number of high energy electrons. Increasing pressure changes the reduced field strength (E/N), thus reducing the ionization rate and changing the electron energy distribution. 3 e + N2 (X1 + g )=0 → N2 (C u ) =1 + e
N2 (C3 u ) =1 → N2 (B3 g ) =3 + h
(1) ( = 375.5 nm)
3 e + N2 (X1 + g )=0 → N2 (C u ) =0 + e
N2 (C3 u ) =0 → N2 (B3 g ) =2 + h
(3) ( = 380.5 nm)
3 e + N2 (X1 + g )=0 → N2 (C u ) =2 + e
N2 (C3 u ) =2 → N2 (B3 g ) =5 + h
the discharge is glow, shown by the current waveform. The discharge images with d of 1 mm correspond well with the current waveforms, as shown in Fig. 4. Intense and bright filaments diminished at low pressure. Therefore, the discharge transitions from filamentary discharge to glow discharge at a certain pressure. Influence of operating pressure on SDBD plasma characteristics with d of 1 mm has been investigated in [20]. In this paper, the difference plasma characteristics at different operating pressure with d of 0 mm and 1 mm is investigated in detail. Fig. 5 shows the optical emission spectra under different operating pressures. The
(4) (5)
( = 394.3 nm)
(6)
+ 2 + e + N2 (X1 + g )=0 → N2 (B ˙u ) =0 + 2e
(7)
+ 2 + (B2 + N+ u ) =0 → N2 (X g ) =0 + h 2
(8)
N2 (C3 IIu )
Fig. 3. Traces of the applied voltage and discharge current under different operating pressures.
(2)
( = 391.4 nm)
We calculate the rotational temperature, electron temperature and density and compare the values with d of 0 mm and 1 mm. The N2 (C3 IIu ) rotational temperature versus operating pressure is shown in Fig. 6. As the pressure decreases from atmospheric pressure, the electron collision frequency decreases, which leads to the decrease of the rotational temperature. When the operating pressure is higher than 45 Torr, N2 (C3 IIu ) rotational temperature with d of 1 mm is much higher than that with d of 0 mm. When the operating pressure is lower than 45 Torr, the rate of decrease of N2 (C3 IIu ) rotational temperature with d of 1 mm is much higher. When the operating pressure is lower than 8 Torr, N2 (C3 IIu ) rotational temperatures with d of 0 mm and 1 mm are very close, around 380 K. At high pressure, the discharge state with d of 0 mm and 1 mm is different, while it is similar at low pressure since the discharge is glow. The electron temperature versus operating pressure is shown in Fig. 7. The variation law of electron temperature in both discharges is very similar. With d of 1 mm, the electron temperature increases slowly from 1.73 eV at 760 Torr to 1.87 eV at 45 Torr, and then increases rapidly to 3.06 eV at 2 Torr. With d of 0 mm, the electron temperature increases slowly from 1.63 eV at 760 Torr to 1.91 eV at 40 Torr, and then increases rapidly to 3.01 eV at 2 Torr. Therefore, the electron temperature is mainly governed by operating pressure and discharge mode, not actuator configuration. Also, the transition pressure around 45 Torr can be inferred. The electron density versus operating pressure is shown in Fig. 8. The variation law of electron density in both discharges is very different. With d of 1 mm, the electron density decreases slowly from 6.9 × 1012 cm−3 at 760 Torr to 8.9 × 1010 cm−3 at 40 Torr, and then increases to 2.3 × 1011 cm−3 at 2 Torr. With d of 0 mm, the electron density increases from 1.1 × 1011 cm−3 at 760 Torr to 8.0 × 1011 cm−3 at 400 Torr, and then decreases to 2.0 × 1011 cm−3
4
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Fig. 4. Surface dielectric barrier discharge plasma under different operating pressures (d = 1 mm).
Fig. 5. Optical emission spectra under different operating pressures.
at 2 Torr. When the operating pressure is lower than 8 Torr, electron density with d of 0 mm and 1 mm are very close. The electron density is governed by ionization rate and operating pressure, therefore the variation law is much more complex than other plasma parameters. When the pressure is higher than 300 Torr, the ionization rate with d of 1 mm is higher than that with d of 0 mm. Once
the pressure is lower than 300 Torr, the ionization rate with d of 1 mm is lower. From Figs. 6–8, it can be concluded that d and operating pressure play major roles in the plasma aerodynamic actuation behavior. When the operating pressure is high, the discharge is in filamentary mode. Plasma characteristics, especially N2 (C3 IIu )
Fig. 6. The N2 (C3 IIu ) rotational temperature versus the operating pressure.
Fig. 7. The electron temperature versus the operating pressure.
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discharges is very different. When the pressure is higher than 350 Torr, electron density with d of 1 mm is higher. When the pressure is lower than 350 Torr, electron density with d of 0 mm is higher. When the operating pressure is lower than 8 Torr, the electron density is very close. Acknowledgments This project is supported by the National Natural Science Foundation of China (51336011, 50906100) and the Science Foundation for the Author of National Excellent Doctoral Dissertation of China (201172). References
Fig. 8. The electron density versus the operating pressure.
rotational temperature and electron density, with d of 1 mm and 0 mm are very different. When the operating pressure is low, the discharge transitions to glow mode and plasma characteristics become similar. Although discharge with d of 1 mm and 0 mm are different, the electron temperature is similar and is mainly governed by the operating pressure. Since the nitrogen second positive emission has a very short lifetime (nanoseconds), the measurements of properties correlates with the time during which the discharge is active. The measurements are averaged over 1 s, while in fact the positive half cycle discharge is very different from the negative half cycle discharge. The positive cycle discharge is normally much more filamentary than the negative half cycle, so the parameters should be quite different. Therefore, the time-averaged value cannot indicate the differentiation between the positive and the negative half cycle discharges, which needs further investigation with fast optical emission spectroscopy. Also the result in this paper is spatialaveraged, while in fact the plasma is filamentary at high pressure. Therefore, the spatial-averaged value throughout the collection volume sampled is associated with the most luminous portion of the discharge, which needs further investigation with spatialresolved method. 4. Conclusions To summarize, d and pressure affects the plasma characteristics dramatically. When d increases from 0 mm to 1 mm, the discharge is much more filamentary at atmospheric pressure. N2 (C3 IIu ) rotational temperature, electron temperature and density are much higher with d of 1 mm. As the operating pressure decreases, both discharge transition from filamentary to glow mode around 45 Torr. N2 (C3 IIu ) rotational temperature with d of 1 mm is much higher when the operating pressure is higher than 45 Torr, while it is very close when the operating pressure is lower than 8 Torr. The difference of electron temperature with d of 1 mm and 0 mm is small and it is mainly governed by the operating pressure. When the operating pressure is higher than 45 Torr, the electron temperature increases slowly. When the operating pressure is lower than 45 Torr, the electron temperature increases sharply to around 3 eV. The variation law of electron density in both
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