st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Visualization of Dielectric Barrier Discharge Sound Field in Atmospheric
Pressure by Novel Method
T. Nakamiya1, F. Mitsugi, Y. Iwasaki, T. Ikegami, R. Tsuda, Y. Sonoda
1
2
Graduate School of Industrial Engineering, Tokai University, Kumamoto, Japan
Graduate School of Science and Technology, Kumamoto University, Kumamoto, Japan
Abstract: The sound signal generated by the Dielectric Barrier Discharge was detected by a
Fraunhofer diffraction effect and the discharge sound was analyzed. The strong sound area is
appeared around the electrodes and different with the frequency components of the discharge
sound. The sound signal from discharge device may be useful to examine the discharge phenomena around the electrodes.
Keywords: Discharge sound, Fraunhofer diffraction, Computerized tomography
1. Introduction
DBD (Dielectric Barrier Discharge) is widely used for
ozone synthesis, pollution control including the removal
of the environmental pollutant, surface modification, biomedical application and so on. Future applications may
include their use in greenhouse gas control technologies.
The characteristics of DBD are mainly examined by
applied voltage, discharge current, and luminescence of
discharge. There is almost no research paper that uses the
discharge sound to examine the electrical discharge phenomenon such as DBD. We have examined the sound
signals which are emitted from the discharge area [1-4].
Because the sound signals contain about the information
of discharge and atmospheric condition around the discharge.
However, it’s not easy to detect the sound signal in
plasma reactor by the conventional condenser microphone
technique. Therefore, we have developed a new diagnostic method, in which sound wave is measured by an optical sensor based on a Fraunhofer diffraction effect between the sound wave and laser beam. The light diffraction technique, which we call the “Optical Wave Microphone (OWM)” technique, is an effective sensing method
to detect the sound and is flexible for practical uses as it
involves only a laser and simple lens system [5-8]. This
technique is also very useful to detect the sound wave
without disturbing the sound field. Moreover, OWM can
be applied for the visualization of sound field by computerized tomography (CT) because the ultra-small modulation by the sound field is integrated along the laser beam
path [9,10]. The two-dimensional discharge sound distribution (Reconstruction image) can be obtained by inversely projecting the data. The projection data can get by
moving the discharge device mounted on the stage.
2. Fundamental equation of OWM-CT
Figure 1 shows the schematic diagram of OWM. When
the probing laser beam crosses perpendicular to refractive
index change with sound wave which passes along x0 di-
Fig.1 Schematic diagram of OWM.
rection at the beam waist plane (x0,y0,0), diffracted waves
are generated. The penetrating beam with the diffracted
waves propagates through a Fourier optical lens (lens 2)
and reach the observing detector which is set at
Fraunhofer diffraction region or at the back focal plane
(xf,yf) of the Fourier lens (lens 2).
If the sound pressure distribution is integrated along the
s-axis for the interaction length (L) between the sound
wave and the laser beam, the equation (1) can be obtained
[4,11].


 2 P   k (   1)  
2
2
I   0  i 0
 exp  u   u   
ac
2

p

  wf  







 exp  u2   u   2  sin  t  p( s )ds
p L

P0: laser power;
wf: beam waist in the back focal plane of lens 2;
u: normalized x-coordinate (xf/wf);
: normalized wave number (kpw0/2);
ωp: angular frequency of refractive index change;
ki: wave number of laser;
μ0: refractive index of atmosphere;
(1)
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
CT method can reconstruct a cross-sectional image of
the sound field using projected data from many directions
of at least 180 degrees. The two-dimensional sound field
S(x0, y0) along the propagation direction of the z0 is the
subject area of our present research. The projected data
D(r,) which is the detected signal can be written as the
following equation.
D(r , )   S ( x0 , y0 ) ds
(2)
L
where r-s coordinates are rotated by  from the x0-y0
coordinates [4]. The reconstruction image can be obtained
by inversely projecting the data to x0-y0 coordinates, in
which the filtered back-projection method and Lamp filter
function were used as the reconstruction algorithm [12].
4. Results and discussion
Figure 3 (a) shows waveforms of OWM measured in
the Ar atmosphere. Amplitude and frequency of the sine
curved applied voltage were fixed at 6 kV and 25 kHz.
20
18
O ptical W ave M icrophone (m V )
γ: specific heat ratio;
p: atmospheric pressure;
16
14
12
10
8
6
4
2
0
100
200
300
Tim e (μs)
400
500
80
100
(a)
0.005
Fig.2 OWM setup coupled with computerized tomography for
detection of discharge sound.
0.004
Intensity (a.u.)
3. Experimental Procedures
Figure 2 shows a schematic view of the experimental
setup to detect the sound signal from the DBD. The device consists of two electrodes buried under Al2O3 ceramic substrate (15 mm in width, 37 mm in length, and
1.25 mm in thickness). The discharge device, which was
mounted on a stage, was set up inside the acrylic chamber.
The chamber was filled with atmospheric gas of Ar. The
proving laser beam (685 nm, 28 mW, 2 mm in diameter)
of OWM passed through above the discharge device. The
sound signals of transmitter crossed the laser beam between the diode laser and Lens 1. The diffracted laser
beam was performed the optical Fourier transform by the
Lens 1 and guided to the photodiode detector by the multimode optical fiber. The diameter of the laser to reach to
the optical fiber was adjusted by the beam expender (Lens
2 and Lens 3). The acoustic signals detected by OWM
were stored in the digital oscilloscope (Tektronix
TDS3034) and analyzed. To obtain projection data for CT
analysis, the discharge device was rotated in the θ direction and moved toward the x direction.
0.003
0.002
0.001
0.000
20
40
60
Frequency (kH z)
(b)
Fig.3 Waveform of (a) discharge sound detected by the
OWM in Ar atmosphere and (b) the analytical result of discharge sound by the FFT method.
The glow-like discharge appeared on the surface of the
discharge device. When the high voltage was applied to
the electrodes, the discharge current appeared and the
discharge sound was generated. The discharge sound was
detected by the OWM. Figure 3 (b) shows the FFT (Fast
Fourier Transform) spectra of voltage wave-form of
OWM. It contains many frequency components such as
25, 50, 74.5 kHz. These values are almost same with the
fundamental, 2nd and 3rd harmonics frequency of applied
voltage frequency.
Figure 4 shows the projection data of discharge device.
The surface of discharge device was away 1 cm from the
proving laser beam (z =1 cm). The sound field was rotated
and moved in the x-θ direction while the probing laser
was fixed. Although the sound field is rotated and moved,
the measurement result is the same as that by scanning the
laser beam and the detection device. From Fig. 3, it can be
expected that the sound field has an obvious peak on the
axis. The sound field at the x - y plane was visualized by
a CT method.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
The rotation step angle was set to 20 degrees from 0 to
180 degrees, and the driving range in the x direction was
from -25 mm to 25 mm with the step length of 5 mm.
20
y (m m )
10
0
-10
-20
-20
-10
0
10
20
10
20
x (m m )
180
150
120
90
0
θ(degree) 60
10
30
0
20
-10
(a)
-20
x (m m )
20
Figure 5 (a) shows reconstructed images of sound
field at two-dimensional x - y plane emitted from discharges in Ar. Top view of electrode for the coplanar dielectric barrier discharge device is illustrated in Fig.5 (b).
The high voltage electrode appears on the surface. In
Fig.5 (a), this electrode is located 0 in x-coordinate and 0
to 4.5 mm in y-coordinate. The spread and blurred emission was observed in the discharge of Ar atmosphere. The
sound field along the y direction was the strongest at just
above and both sides of the top electrode. The similar
results were confirmed in air and N2 which were reported
in previous work [3,5]
10
y (m m )
Fig.4 Projection data of the sound emitted from the discharge
device using OWM-CT.
0
-10
-20
-20
-10
0
x (m m )
(b)
20
y (m m )
10
0
-10
-20
-20
-10
0
10
20
x (m m )
(c)
(a)
(b)
Fig.5 (a) The reconstructed image of sound field at x – y plane
emitted from discharges in Ar by the OWM-CT. (b) View of the
electrodes for the coplanar dielectric barrier discharge device.
The figures of (a) and (b) are the same reduced scales.
Fig.6 Reconstructed images of sound field at x – y plane emitted from discharges in Ar. (a) 25kHz component (b) 50kHz
component (c) 74.5kHz component
Figure 6 shows the reconstructed images of sound
field of each frequency components such as 25, 50 and
74.5 kHz. The intensity of the each frequency components were obtained by the FFT analysis shown in Fig. 3
(b). Figure 6 (b) shows the sound distribution of 50 kHz
frequency components. The strong sound field appeared
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
on the right side of top electrode. This frequency component of 50 kHz (Twice of applied voltage frequency)
concerns plasma conditions such as the density of ions,
the electric field, the emission and so on. On the other
hand, the strong area of 25 and 74.5 kHz components
appear around the top electrode of discharge device.
5. Conclusions
The OWM-CT was applied to detect the sound field
from the coplanar dielectric discharge in Ar. Visualization
of discharge sound field of each frequency components
was performed. The distribution of each frequency components in discharge sound is important to understand the
plasma conditions.
6. Acknowledgements
This work was supported by Grant-in-Aid for Scientific
Research (23560510), the Japan Science and Technology
Agency (15-138) and the Research and Study Project of
Tokai University Educational System General Research
Organization.
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