22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Study of operation gas flow rate and kinds dependency on atmospheric
serpentine plasma
S. Aoqui1, F. Mitsugi2, H. Kawasaki3 and T. Ohsima3
1
Dept. Computer & Information Sciences, Sojo University, Ikeda 4-22-1, Kumamoto, 860-0082, Japan,
2
Faculty of Engineering, Kumamoto University, Kurokami 2-39-1, Kumamoto, 860-8555, Japan
3
Dept. Electrical & Electronics Engineering, Sasebo National College of Technology, Okishin-machi 1-1, Sasebo,
857-1171, Japan
Abstract: A development of exhaust gas handling for environmental application and
sterilization technology in a medical, agriculture, and biotech field using atmospheric
pressure plasma is remarkable now. In an atmospheric pressure electric discharge, we paid
attention to gliding arc (GA) discharge. Our previous study on simultaneous observation of
the dynamic behaviour of the plasma path via a high-speed camera and the corresponding
electrical properties revealed that the plasma path was very complicated due to gas flow
rate and reconnections were repeated especially in high gas flow to maintain plasma. In this
study, gas flow quantity and a discharge state were observed using a high-speed camera in
detail.
Keywords: gliding arc discharge, serpentine plasma, atmospheric pressure plasma
1. Introduction
There has been interest in the use of so-called gliding
arc discharge systems for various applications in material
processing, surface treatment, gas conversion, water
treatment, and biological disinfection [1-13]. Although
the static or time-accumulated measurements for electrical
properties, temperature distribution, power supply,
electrode geometry, optical emission spectroscopy, and
acoustic propagation for the gliding arc discharge system
have been done, investigations to realize more efficient,
functional, and low power system supported by evidence
of time-resolved understanding for electrical and optical
properties of gliding plasmas are required for further
progress. In gliding arc discharge systems, a breakdown
of atmospheric gas is generated at the shortest gap
between two divergent electrodes. After the breakdown,
the plasma keeps gliding along the same direction as the
gas flow, which causes an increase of applied voltage,
with a transition from thermal to non-thermal equilibrium
until the applied voltage reaches a critical voltage that
needed for next breakdown. The previous plasma
disappears at the moment of the next breakdown. This
cycle from the ignition to the gliding is repeated
continuously. The non-thermal region of the plasma is
suitable for surface treatment, water treatment and
biological inactivation at lower gas temperature with high
electron temperature while the thermal plasma near the
shortest gap between electrodes is more effective for gas
treatment or material synthesis due to high plasma power
density and fast chemical reaction rate in high gas
temperature. Therefore, the control of power, impedance,
and equilibrium state of plasma should be optimized
according to its application. Particularly, techniques to
suppress the applied voltage and the power consumption
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to generate relatively cold plasma are important issue
because the power of plasma and gas temperature is
relatively high in conventional gliding arc discharge
systems. Our fundamental study on the simultaneous
observation of the dynamic behaviour of plasma path with
a high-speed camera and the corresponding electrical
properties for the above mentioned system revealed that
the plasma path was serpentine due to gas turbulence and
reconnections were repeated frequently especially in high
gas flow to suppress the increase of plasma power.
Therefore, we distinguished this kind of non-equilibrium
plasma from normal arc plasma and named it serpentine
plasma. The plasma impedance per unit plasma length
was not as low as that of normal gliding arc discharge
plasma due to the assistance of photoelectric effect. In this
work, we report electrical properties and the
corresponding high-speed camera images that are
characterized by the relationship between gas kinds and
gas flow rate in gliding arc configurations. [14-16].
2. Experiment
Fig. 1 shows the schematic illustration of experimental
setup for the gliding arc discharge system with UV
assistance and equipment for observation of electrical
properties and dynamic behaviour. Two electrodes, which
are made of iron and pure carbon, are 100 mm height
knife edge-shaped and their shortest gap was 5 mm. The
electrodes were set inside an acrylic chamber which has
an outlet on the top for gas exhaust. An inlet for gas
supply to the chamber was placed at the bottom and at the
centre between two electrodes. Ar gas and helium gas was
used to investigate the influence of gas flow rate. The gas
flow rate was controlled with a digital flow instrument
from 5 to 50 litter / min. We used a low pressure mercury
1
lamp (Hamamatsu, L937-01) as an UV source of which
the photon energy for the main spectrum is about 5 eV.
The lamp was set behind the shortest gap of the electrodes
as shown in Fig. 1. High voltage (sine wave, 60 Hz) was
applied between two electrodes with a high-voltage
transformer (VIC international, 120:1). The amplitude
was adjusted with a voltage slide autotransformer.
Waveforms of applied voltage and discharge current were
measured with a high-voltage probe (Tektronix, P6015A)
and a current clamp (Tektronix, TCP2020), respectively.
Both waveforms were captured with a digital oscilloscope
(Lecroy WaveRunner 204Xi-A). Time-resolved digital
photographs for plasmas were recorded by a high-speed
digital camera (Nobby Tech. Ltd., Phantom V.1210) with
the frame rate of 20,000 frames per second and the
effective pixels of 640 x 480. The observation times for
the digital oscilloscope and the high-speed camera were
synchronized by an external trigger signal from a pulsed
signal generator (Hamamatsu, C10149).
continued to 5 to 20 litter / min for almost all time, and a
paths of electric discharge went up and down, but were
approximately near for a continuation discharge.
However, discharge became intermittent from conditions
of flow rate of 30 litter / min, and without paths of
discharge time increased. Time of from an arc discharge
lower electrode part to the upper end shortened with flow
rate increase. At argon 50 litter / min, next electric
discharge shows a state that already began with a lower
part by the thirteenth photograph.
Fig. 1. Experimental setup of synchronized high-speed
camera & I-V measurement system.
2
Current (A)
1.0
0.5
0.0
-0.5
-1.0
Applied Voltage (kV)
3. Result and discussion
Fig. 2 show a high-speed camera images in
synchronization with typical IV characteristics on Ar flow
rate of 20 litter /min [16]. Each line is equivalent with an
image. But the indication of IV characteristics is
abbreviated in these articles afterward.in these papers
afterward. A State of gliding arc discharge that is to say
the state of serpentine plasma strongly depends on an
introduced gas. In this study we report to 2 kinds of gas
that is Ar and He. However also we already investigated
even other gas such as CO 2 , N 2 , O 2 , CH 4 and Air. A
State of gliding arc discharge that is to say the state of
serpentine plasma strongly depends on an introduced gas.
A high-speed camera image of Ar flow rate from 5 litter /
min to 50 litter /min is shown in Fig. 3a to Fig.3d. 20,000
fps was used for all the photography frame speed. An
image begins with an image which an arc occurred at the
electrode lower part, and images every time are arranged
transversely then next line. Because movement of paths of
electric discharge is slow at argon 5 litter /min, it is an
image every 30 frames that is 1.5 ms and other case, the
time for image interval is 0.5 ms. A paths of discharge
5
4
3
2
1
0
-1
-2
-3
-4
-5
0
10
20
30
Time (ms)
Fig. 2. A series of time-resolved photographs and the
corresponding electric properties in Ar 20 litter / min.
Experimental setup of synchronized high-speed camera &
I-V measurement system [16].
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Fig. 3a Ar flow rate: 5 litter / min with time interval of
1.5ms (20,000 fps)
Fig. 3d Ar flow rate: 40 litter / min with time interval of
0.5ms (20,000 fps)
Fig. 3e Ar flow rate: 50 litter / min with time interval of
0.5ms (20,000 fps)
Fig. 3b Ar flow rate: 10 litter / min with time interval of
0.5ms (20,000 fps)
Fig. 3c Ar flow rate: 20 litter / min with time interval of
0.5ms (20,000 fps)
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A means of the Time to Live of paths of discharges
from an electrode bottom to the upper end were as follows
each. From 5 litter / min to 50 litter /min correspond to 26
ms, 10 ms, 8.3 ms, 6.7 ms and 5.7 ms. A length and a
shape of paths of discharge became a complicated shape
with flow rate increase greatly. Because the image of the
electrode cross direction was got with the high-speed
camera, a three-dimensional property of paths of
discharge became strong at the same time. In 30 litter
/min or more, the intense twist of 3 dimensional paths of
discharge occurred therefore the state that paths of
discharge were reconnected, and length changed
happened. In this condition, a current flow continued by a
measurement of IV characteristics measured by
synchronization system. But a sample rate of the
oscilloscope 100 MHz (Max rate: 20 GHz) was used.
There was a possibility that it was caught the change of a
current form if the sample rate of the oscilloscope was
more high rate such as 2 GHz or more. Relationship
between gas flow rate and the shapes of path of discharge
are well unknown. But because a discharge becomes
intermittent when flow rate increases, it will be proper to
3
think that cooling of the electrode lower part with the gas
should be related.
Next Fig. 4 shows a discharge result with helium gas.
Discharge conditions were totally different from a
discharge with Ar gas.
Fig. 4 He flow rate: 40 liter / min with time interval of
0.25ms (20,000 fps)
In case of He, as for the rise speed of paths of
discharge, around 3 times to 30 times faster in comparison
with Ar. There were three kinds of patterns of discharge
repetition average time in He, and it was 3.2 ms, 1.8 ms
and 0.25 ms each. In addition, the three-dimensional
properties were few. A paths of discharge went up only
to half of electrode length, too. The tendency was the
same by an experiment that changed flow rate of He. It is
unidentified why it is different by a gas kind greatly, but
cooling with the gas will be a big factor.
4. Summary
In gliding arc discharge equipment with a single-phase
alternating current power supply, a measurement was
carried out in a high-speed camera with synchronous
measurement of IV characteristic of discharge. Ar and He
were used as operation gas, and a high-speed camera
image was acquired with many flow rate of gas.
The followings were results of this study.
5. References
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1. In Ar, it was confirmed that a discharge state turned
great when flow rate exceeded 30 litter / min.
2. A discharge interval shortened with increase of the
flow rate of Ar.
3. It was confirmed that discharge states were totally
different with Ar and He.
4. In He, discharge interval was 3 times to 30 times
faster than Ar.
5. Cooling of electrode(s) lower part progresses when
gas flow rate increases is considered a factor.
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