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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
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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
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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]..
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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
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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].
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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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