optical diagnosis of pulsed streamer discharge under ... · state a2Σ+(v’ = 3) using a tunable...

Optical Diagnosis of Pulsed Streamer Discharge under Atmospheric Pressure

Ryo Ono1 and Tetsuji Oda2 1High Temperature Plasma Center, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 227-8568, Japan 2Department of Electrical Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan

Abstract—This paper introduces recent optical measurements of radicals in atmospheric pressure non-thermal plasma. Laser-induced fluorescence (LIF), laser absorption, and optical emission spectroscopy (OES) techniques have been applied for density and temperature measurements of OH radicals, NO molecules, ozone, atomic oxygen, atomic nitrogen, and N2( A3Σ+ u ) metastables in pulsed corona and pulsed dielectric barrier discharges under atmospheric pressure. These radical measurements are indispensable to develop an efficient non-thermal plasma reactor for decomposing atmospheric pollutants because radicals play important roles in the decomposition processes of pollutants.

Keywords—Nonthermal plasma, Laser-induced fluorescence, Laser absorption, Radical

I. INTRODUCTION Optical measurement is a powerful tool for diagnosis

of discharge plasma. It is in situ and nonintrusive technique under proper conditions (e.g. using sufficiently low power laser) with high temporal and spatial resolutions. Recently, our group has measured some radicals in atmospheric pressure non-thermal plasma using laser-induced fluorescence (LIF), laser absorption, and optical emission spectroscopy (OES) [1–9]. The knowledge of radical behavior is indispensable to develop an efficient non-thermal plasma reactor for decomposing atmospheric pollutants because radicals play important roles in the decomposition processes of pollutants [10–12]. This paper introduces the measurement of radicals in pulsed corona discharge and pulsed dielectric barrier discharge (DBD) centered on our recent work.

II. STREAMER PROPAGATION

Non-thermal plasma is generated by pulsed discharge whose pulse duration is less than several 100 ns. In the pulsed discharge, the gas temperature is much lower than the electron temperature (1 to 10 eV [13]) because the discharge pulse duration is shorter than the time constant of the heat conduction from electrons to molecules. The pulsed discharge can produce highly reactive plasma with low energy consumption because the input energy is efficiently used for the production of radicals, ions, and excited particles without heating the gas.

The pulsed discharge often produces “streamer”, which is branched ionized filaments as shown in Fig. 1 [13–15]. The streamer propagates between the discharge gap very fast. Its time constant is on the order of only 1 to 10 ns. Therefore, a high-speed camera is needed to

observe the propagation of the streamer. Fig. 2 shows the streamer propagation observed using an image intensified CCD (ICCD) camera with an optical exposure time of 5 ns [16]. In this experiment, the discharge occurs between a point-to-plane gap with 13 mm gap length. Fig. 3 shows the electrical circuit for generating the discharge pulse. The charge stored in the 860 pF capacitor is discharged using the spark gap switch. The pulse duration of the discharge current is typically 50 to 200 ns, as shown in Fig. 4.

Fig. 1. Photograph of streamer between 13 mm point-to-plane gap.

(a) Dry air, V = 34kV

(b) Nitrogen, V = 18 kV

Fig. 2. Photographs of streamer propagation in pulsed positive corona discharge. A 13 mm point-to-plane gap is used [16].

Corresponding author: Ryo Ono e-mail address: [email protected] Presented at Tahiti Workshop in August 2007, Received in revised form: September 10, 2007 Accepted; September 25, 2007

123 International Journal of Plasma Environmental Science & Technology Vol.1, No.2, SEPTEMBER 2007

Fig. 2(a) shows the appearance of primary and secondary streamers, as is well known [13, 15]. In the photographs of 8 and 12 ns, a small luminous zone develops from the point anode to the plane cathode. This is the primary streamer. Then the secondary streamer develops from the anode toward the cathode as far as the halfway point of the gap. In Fig. 2(b), only the primary streamer is observed.

Fig. 5 shows the propagation of pulsed positive DBD. In addition to the primary and secondary streamers, the surface discharge on the glass plate is observed after the primary streamer bridges the discharge gap. This three phase propagation of DBD was also observed by Braun et al. [17] using a streak camera.

III. OPTICAL MEASUREMENT OF MOLECULES AND ATOMS

A. OH radical

OH radicals can be measured using laser-induced

predissociation fluorescence (LIPF) [18]. OH radicals in the ground state X2Π(v’’ = 0) are excited to the upper state A2Σ+(v’ = 3) using a tunable KrF excimer laser at around 248 nm, then subsequent fluorescence from A2Σ+(v’ = 3) to X2Π(v’’ = 2) is observed at around 297 nm. We measured the density of OH radicals in pulsed positive corona discharge using LIPF [1, 3, 9]. Fig. 6 shows comparison between the streamer photograph and the spatial distribution of OH radicals observed using two-dimensional LIPF technique. Discharge occurs between a 13 mm point-to-plane gap at sufficiently low repetition rate (< 8 pps). The result shows that OH radicals are produced in the streamer channels.

Fig. 7 shows the decay of OH density after discharge. It is observed around the tip of point electrode. The oxygen concentration is varied. It shows that the decay

Fig. 3. High-voltage pulse generating circuit.

Fig. 4. Typical voltage and current waveforms of pulsed positive corona discharge in air. V = 24 kV.

Fig. 5. Photographs of streamer propagation in pulsed positive DBD in dry air. A 1 mm thick glass plate is placed on the plate electrode. Gap distance

is 5 mm. V = 24 kV [7].

Fig. 6. (a) Streamer photograph and (b) OH density distribution [1].

Fig. 7. Decay of OH density after pulsed positive corona discharge in humid O2/N2 mixture for various O2 concentrations [3].

(a) (b)(a) (b)

Ono et al. 124

rate of OH density is faster at higher oxygen concentration. This result indicates that OH radicals react with some byproducts of oxygen, for example, atomic oxygen [9].

The density and decay rate of OH radicals depend on the position between the discharge gap. Fig. 8 shows the density of OH radicals at 1.0, 3.6, and 7.5 mm distance from the anode tip. The decay rate at 3.6 and 7.5 mm is much faster than that at 1.0 mm. It is probably caused by the difference in gas temperature in various positions [9].

LIPF can also measure the rotational temperature of OH radicals. We measured the gas temperature under an assumption that the OH rotational temperature is equal to the gas temperature. Fig. 9 shows the time evolution of gas temperature after discharge at 1.0 and 3.6 mm distance from the anode tip. The temperature increases after discharge although the discharge already finished. A possible explanation for that is the energy transfer from vibrationally excited molecules to the gas kinetic energy [19, 20].

B. NO molecule NO molecules can be observed by LIF using

excitation of X2Π(v’’ = 0) to A2Σ+(v’ = 0) at around 226 nm. Hazama et al. [21] and Kanazawa et al. [22] have measured NO molecules in corona streamer discharge using LIF. Fig. 10 shows an example of two-dimensional LIF of NO molecules using a dye laser [2]. The pulsed positive DBD occurs between 4 mm point-to-plane gap with NO(200 ppm)/N2 mixture flowing from the left side to the right side. Fig. 10 shows that NO molecules in the streamer channels are decomposed after discharge with a time constant of 10 μs.

C. Ozone

Ozone absorbs UV light with a large absorption cross

section. Ozone measurement by UV absorption is a popular technique and has been applied to the measurement in streamer discharge [23, 24].

We measured spatial distribution of ozone density using the two-dimensional laser absorption technique with a KrF excimer laser (248 nm) [4, 7]. In the experiment, we used multiple points-to-plane gaps, as shown in Fig. 11(a), to increase the absorption length. The laser beam is introduced into the reactor as shown in Fig. 11(b) (setup A) or (c) (setup B). After the beam passes through the reactor, the beam pattern is projected onto a fluorescent glass plate whose luminous intensity is proportional to the laser power. The projected beam pattern is observed with an ICCD camera and compared with a reference beam pattern to determine the beam absorption ratio for each pixel of the ICCD camera. Then ozone density is determined for each pixel of the ICCD camera from the beam absorption ratio. Fig. 12(a) and (b) show the discharge photograph and ozone density distribution measured using the setup A (Fig. 11(b)). It shows that ozone is produced in the streamer channels. Fig. 13 shows the time evolution of ozone density distribution after discharge. Ozone produced in the thin streamer channels diffuses with a diffusion coefficient of 0.1 to 0.2 cm2/s.

Fig. 14 shows ozone distribution measured using setup B (Fig. 11(c)). It shows that ozone is produced after discharge with a time constant of several tens μs via

Fig. 10. Two-dimensional distribution of NO density after pulsed positive DBD. V = 25 kV [2].

Fig. 8. Decay of OH density after pulsed positive corona discharge in humid O2(2%)/N2 mixture for various distance z from anode tip [9]..

Fig. 9. Time evolution of temperature after pulsed positive corona discharge in humid O2(2%)/N2 mixture for various distance z from anode

tip [9]..


the well known three-body reaction [25]:

(1)M.OMOO 32 +→++

Then ozone density decreases due to diffusion. Fig. 15(a) shows ozone density distribution at 500 μs after discharge for V = 18, 24, and 30 kV. It is shown that ozone is distributed only up to a certain distance from the anode. The length of the ozone production area, loz, indicated in each photograph, increases with discharge voltage, Fig. 15(b) shows streamer photographs for V = 18, 24, and 30 kV. The bright part of the streamer, indicated by a white arrow in the photograph, is the secondary streamer. Fig. 15(b) shows that the propagation length of the secondary streamer, ls, increases with discharge voltage, and that loz is almost equal to ls for all applied voltages. This result indicates

that ozone is mostly produced in the secondary streamer rather than in the primary streamer.

Fig. 16 shows ozone density distribution after pulsed positive DBD using similar multiple points-to-plane gaps shown in Fig. 11(a). A 1 mm thick glass plate is placed

Fig. 11. (a) Discharge reactor with multiple points-to-plane gaps. (b) and (c) Beam path for ozone measurement.

Fig. 12. (a) Photograph of pulsed positive corona discharge and (b) ozone density distribution. V = 36 kV. Ozone density is observed at 1 ms after

discharge [4].

Fig. 13. Temporal variation of ozone density distribution after pulsed positive corona discharge. V = 36 kV [4].

Fig. 14. Ozone density distribution after pulsed positive corona discharge [4].

Fig. 15. (a) Ozone density distribution at 500 μs after discharge and (b) streamer photographs and for V = 18, 24, and 30 kV [4].

Ono et al. 126

on the plane electrode. The gap distance is 5 mm. Ozone is produced in the surface discharge channels as well as the secondary streamer channels. In the surface discharge channels, ozone density increases after discharge with approximately equivalent time constant to that in the streamer channels, as shown in Fig. 17. It indicates that ozone in the surface discharge channels are also produced via reaction (1). In Fig. 16, ozone drifts towards the cathode at later post-discharge times (t > 1 ms). This ozone drift is not observed in pulsed positive corona discharge, shown in Fig. 14. The reason for the ozone drift is not yet known.

D. Atomic Oxygen

Atomic oxygen can be observed using two-photon

absorption laser-induced fluorescence (TALIF) [26,27]. Atomic oxygen in the ground state 2p3P (J’’ = 2) is excited to 3p3P level by two-photon absorption of 226 nm, then the fluorescence from 3p3P → 3s3S at 845 nm is observed. Figure 18 shows the decay of atomic

oxygen density after discharge in O2/N2 mixture. The decay rate increases with O2 concentration due to reaction (1). From this result, the rate coefficient of reaction (1) is estimated to be 2.1 × 10-34 cm6/s.

It is well known that ozone production is suppressed by increasing humidity. To investigate the reason for this phenomenon, we measured the influence of humidity on the behavior of atomic oxygen. Fig. 19 shows the decay of atomic oxygen for various H2O concentrations in humid air pulsed negative DBD. It shows that the atomic oxygen density just after discharge (t = 0 μs) is almost independent of humidity. However, the decay rate of atomic oxygen is considerably faster at a higher humidity. This tendency is observed also in the positive DBD. This result indicates that the admixture of humidity increases the decay rate of atomic oxygen, which leads to decreased ozone production in a humid environment.

Fig. 16. Ozone density distribution after pulsed positive DBD [7].

Fig. 17. Number of ozone molecules in “streamer area” and “surface discharge area” in pulsed positive DBD. V = 28 kV. [7].

Fig. 18. Decay of atomic oxygen density after pulsed positive corona discharge for various O2 concentrations [8].

Fig. 19. Decay of atomic oxygen density after pulsed negative DBD in humid air. V = −40 kV [6].


E. Atomic nitrogen

Atomic nitrogen can be observed using TALIF [26,

27]. Lukas et al. [28] measured atomic nitrogen in pulsed DBD using excitation from the ground state to 3p4S3/2 at 206.7 nm. The spatial distribution and decay of atomic nitrogen density after discharge were shown.

F. Metastables N2(A3Σ + u )

Metastable N2(A3Σ + u ) excites ground state NO(X2Π)

molecule via [29, 30]:

(2)).(XN)NO(A

)(AN)NO(X

g2

22

u3

22

Σ+Σ→

Σ+Π+

+

Then, the excited NO(A2Σ+) radiates the NO-γ emission via NO(A2Σ+) → NO(X2Π). If the background gas contains NO as a tracer, the behavior of N2(A3 Σ + u ) metastables can be indirectly observed using the NO-γ emission. The indirect measurement of NO(A2Σ+) in pulsed positive corona discharge suggested that N2(A3Σ+ u ) is mainly produced in the primary streamer channel, while NO(X2Π) is mainly produced in the secondary streamer channel [5].

IV. OTHER OPTICAL MEASUREMENTS Schlieren method can visualize the variation in gas

density in the post-discharge period. The density decrease in the streamer channel after discharge has been visualized using the schlieren method [31, 32]. Fig. 20 shows the example.

Laser spectroscopy can measure the density and temperature of electrons, as well as those of molecules and atoms. They can be measured by, for example, Thomson scattering and laser interference techniques. However, the measurement of electron parameters is difficult under atmospheric pressure discharge due to, for example, low single-to-noise ratio. The parameters of electrons are important for investigating the non-thermal plasma processes because they strongly influence the production of radicals. Therefore, they should be measured in the future work in the atmospheric pressure non-thermal plasma.

V. CONCLUSION

Optical measurement of radicals in pulsed streamer

discharge under atmospheric pressure was introduced centered on our recent work. In low pressure plasma and high temperature plasma, many other optical diagnostic techniques have been developed. We expect that new measurement techniques are developed also in the atmospheric pressure non-thermal plasma and they make clear the complicated reaction processes in the atmospheric pressure non-thermal plasma.

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Fig. 20. Temporal variation of schlieren images after positive pulsed corona discharge. V = 30 kV [32].

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optical diagnosis of pulsed streamer discharge under ... · state a2Σ+(v’ = 3) using a tunable...

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