22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Pressure wave detection using fibered optical wave microphone for atmospheric
discharges
F. Mitsugi1, T. Ikegami1, T. Nakamiya2 and Y. Sonoda3
1
Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto, 860-8555, Japan
2
School of Industrial and Welfare Engineering, Tokai University, 9-1-1 Toroku, Kumamoto 862-8652 Japan
3
School of Industrial Engineering, Tokai University, 9-1-1 Toroku, Kumamoto 862-8652 Japan
Abstract: Multi-channel fibered optical wave microphone, which improves upon
conventional optical wave microphones those work based on Fraunhofer diffraction of
probe laser, was developed for detecting pressure waves emitted from atmospheric
discharges. In this work, applied frequency dependence on the generation of pressure
waves for surface barrier discharges was studied through four-channel fibered optical wave
microphone measurement.
Keywords: multi-channel fibered optical wave microphone, surface barrier discharge,
pressure wave
1. Introduction
Dielectric barrier discharge is one of the practical
forms of atmospheric pressure discharges, which has
been focused for plasma actuators because it induces
fluid flow in aerodynamic control. Total electrical
energy of dielectric barrier discharge is divided into the
excitation of molecules and atoms which causes optical
emission and chemical reaction, the generation of heat,
and the emission of pressure wave which is concern with
the mechanism of plasma actuators. In these years, we
have researched on pressure wave which is classified as
either shock wave or acoustic wave such as audible and
ultrasonic waves those are emitted at the generation of
plasma because our concern is the energy transfer from
plasma to atmosphere [1-9]. There is limitation for the
use of conventional condenser microphones or pressure
sensors because they disturb electric field close to
electrodes and sound field generated by discharges.
Although it is possible to observe intense shock waves
emitted from discharges with optical methods such as
Shadowgraph and Schlieren, the development of an
optical method which is sensitive to detect discharge
sound or tiny shock waves has been expected to examine
more details on pressure wave emitted from discharges.
The effective method that we developed was named
optical wave microphone, which works based on
Fraunhofer diffraction of a probe laser caused by
discharge sound.
Combined with a computer
tomography scanning, it can realize two dimensional
visualization for discharge sound field [8] although the
method required longer scanning time and assumption
for stability of plasma in time. Therefore, we have been
developing a multi-channel fibered optical wave
microphone system to realize real-time observation of
pressure waves around electrical discharges.
In this work, fundamental properties of four-channel
fibered optical wave microphone are evaluated using an
P-I-13-10
ultrasonic oscillator.
Detection of pressure wave
emitted from atmospheric pressure discharge was carried
out with the four-channel fibered optical wave
microphone. A surface barrier discharge device was
used as one of atmospheric pressure discharges.
Frequency characteristics for optical wave microphone
waveforms and discharge current waveforms were
compared each other by changing the frequency of
applied voltage. Applied frequency was changed from
5 kHz to 30 kHz and the frequency dependence on the
generation of pressure wave was studied.
2. Experimental setup
Fig. 1 shows the illustration of the four-channel fibered
optical wave microphone and the setup for measurement
of pressure waves which are emitted from surface barrier
discharges. The output of a fiber laser (637 nm,
190 mA) is divided into four through an optical divider
and single mode optical fibers (Diameter of core is
4.3 µm). The four probe laser beams are irradiated to
observation region after collimation with lenses. The
waist of each collimated laser beam is 0.75 mm. The
optical power of the each beam is about 7.6 mW. The
beams are Fourier transformed spatially by receiving
lenses and led into fibers those are connected with photo
detectors (Hamamatsu, S5935-01). The position of a
detector is adjusted precisely in the conventional optical
wave microphones while the adjustment of the incident
angle of the laser to the receiving lens with a kinetic
mount corresponds to that because the edge face of the
fiber core extracts a part of diffracted area where the
laser is focused by the receiving lens. The optimum
incident angle was set at 2/9 mrad in this setup.
According to numerical calculation, relationship between
the amplitude of optical wave microphone output and the
normalized wave number of pressure waves for the
fibered optical wave microphone, the most sensitive
1
Optical wave microphone (mV)
frequency of pressure waves is approximately 100 kHz
in this setup. There are four channels from CH1 to
CH4 and the distance between channels is 50 mm. The
use of the fiber makes it possible to put the detector
farther away from the high voltage source and the
surface dielectric barrier discharge device, which
increases the signal to noise ratio of the system. The
outputs of the detectors are captured by an oscilloscope.
The dielectric plate of a surface dielectric barrier
discharge device, which is set at 5 mm below from CH1
(x 0 = -5 mm), is Al 2 O 3 ceramic substrate (15 mm in
width, 37 mm in length, and 1.25 mm in thickness).
The distance between the top electrode (1.25 mm in
width, 29.5 mm in length, and 10 µm in thickness) and
the grounded electrode (6 mm in width, 24.5 mm in
length, and 10 µm in thickness) inside the substrate is
220 µm. The sinusoidal high voltage was applied to the
top electrode that was measured through a high voltage
probe. The frequency of the high voltage is ranged
from 5 kHz to 30 kHz. The discharge energy was
measured by Lissajous figure method using a series
capacitor of 100 nF.
CH1
CH2
CH3
CH4
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6
-7
-10
0
10
20
Time (ms)
30
40
Fig. 2. Waveforms of four-channel fibered optical wave
microphone for an ultrasonic oscillator that operates at
200 kHz.
4
0.04
0
0.00
Current (A)
Applied Voltage (kV)
2
-2
-0.04
-4
-400
-200
0
200
400
600
Time (ms)
Fig. 1. Illustration of experimental setup for a
four-channel optical wave microphone.
3. Results and discussion
Using an ultrasonic oscillator that operates at 200 kHz,
we measured the fundamental characteristics of the
four-channel fibered optical wave microphone. The
ultrasonic oscillator was placed 5 mm below the laser
(x 0 =-5 mm), alternatively, where the surface dielectric
barrier discharge device is set as seen in Fig. 1. As can
be observed from Fig. 2, ultrasonic waves that propagate
with 200 kHz and degenerate from CH1 to CH4 were
successfully observed with the four-channel fibered
optical wave microphone.
Fig. 3 shows waveforms of applied voltage and
discharge current during surface barrier discharge
operates by sinusoidal power supply with relatively low
frequency of 5 kHz. The number of pulsed current that
is caused by the generation of micro-plasma strongly
depends on the frequency of applied voltage. Because
only a few pulsed currents flowed in a half cycle of
applied voltage when the frequency was as low as 5 kHz
as can be confirmed in Fig. 3, waveform captured by
optical wave microphone can be analyzed. Fibered
optical wave microphone waveform, which was not
2
Fig. 3. Waveforms of applied voltage and current
between electrodes during surface barrier discharge that
operates at 5 kHz.
synchronized with Fig. 3, captured by CH1 of
four-channel optical wave microphone for surface
discharge operates at 5 kHz is enlarged in Fig. 4. It was
revealed from the waveform that the observed pressure
waves were not continuous pressure vibrations but were
composed from several individual pressure wave. The
each pressure wave has relation to what each pulsed
current concerns. Each micro-plasma induces local
heating which results in shock wave at the surface,
propagating as acoustic wave after degeneration to
supersonic level.
Applying frequency from 10 kHz to 30 kHz, we
investigated frequency dependence on the generation of
pressure waves. Figs. 5, 6, and 7 are four-channel
fibered optical wave microphone waveforms during
surface discharges operate at 10 kHz, 20 kHz, and
30 kHz, respectively. Fast Fourier Transform spectra
for CH1 are summarized as a function of applied
frequency in Fig. 8. It was obvious from these results
that frequency component of the pressure waves which
related to shock waves covers on the order of 100 kHz in
P-I-13-10
3
5 kHz (CH1)
1
Optical Wave Microphone (mV)
Optical wave microphone (mV)
2
0
-1
-2
-3
-4
50
60
70
80
90
100 110 120 130 140 150
Time (ms)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
CH4
CH3
CH2
CH1
0
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
100
150
Time (ms)
200
Fig. 7. Waveforms of four-channel fibered optical wave
microphone for surface barrier discharge that operates at
30 kHz.
CH4
30 kHz
0.00010
0.00008
CH3
0.00006
0.00004
CH2
0.00002
0.00000
0.00010
CH1
0
50
100
Time (ms)
150
200
Amplitude
Optical Wave Microphone (mV)
Fig. 4. Fibered optical wave microphone waveform
(CH1) for pressure waves emitted from surface barrier
discharge that operates at 5 kHz.
50
20 kHz
0.00008
0.00006
0.00004
0.00002
Fig. 5. Waveforms of four-channel fibered optical wave
microphone for surface barrier discharge that operates at
10 kHz.
0.00000
0.00010
0.00008
10 kHz
Optical Wave Microphone (mV)
0.00006
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
CH4
0.00004
0.00002
CH3
0.00000
0
100
200
300
400
500
Frequency (kHz)
CH2
CH1
0
50
100
Time (ms)
150
200
Fig. 6. Waveforms of four-channel fibered optical wave
microphone for surface barrier discharge that operates at
20 kHz.
P-I-13-10
Fig. 8. Fast Fourier Transform spectra of waveforms of
CH1 for Figs. 5, 6, and 7.
which the center frequency increased because incidence
of current pulses increase with the applied frequency.
Consequently, individual pressure wave could not be
recognized clearly when the applied frequency increased.
There was another pressure wave which was detectable
especially at 30 kHz as shown in Fig. 7. The frequency
components of the pressure waves those can be seen in
Fig. 8 were 60 kHz and 120 kHz which were
3
even-number multiples of the applied frequency.
Because a series of micro-plasmas generates in every
half period of the applied voltage, ultrasonic waves
which mainly composed of two times higher frequency
compared to that of applied voltage are able to be
induced by the acceleration of generated ions along with
electric field. It is suggested that the acceleration of
ions, which are generated by electron impact, in electric
field results in ultrasonic waves and their amplitude
depends on the quantity and lifetime of ions under
effective electric field.
Hence, the amplitude of
ultrasonic waves increased with applied frequency
because micro-discharges increase and acceleration of
ions becomes more effective when applied frequency
increases.
4. Summary
In this work, using the four-channel fibered optical
wave microphone, we measured pressure waves emitted
from surface barrier discharges. Changing frequency of
applied voltage from 5 kHz to 30 kHz, we obtained the
following results through comparison between the
optical wave microphone measurements and electrical
properties.
First, pressure waves, which are originated with shock
waves emitted from micro-plasmas and progress with
sound speed, could be detected with this method. Their
frequency component, which correspond with that of
pulsed discharge incidence, covers on the order of
100 kHz where conventional condenser microphones are
not sensitive. Second, generation of ultrasonic waves,
of which the main frequency component is even-number
multiples of applied frequency, was observed. The
amplitude of ultrasonic waves increased with the
increase of applied frequency. It is suggested that the
generation of ultrasonic waves is influenced by quantity
and lifetime of ions generated by micro-discharges.
4
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