Spatial Distribution of Atomic Radical Generated
by AC Excited Nonequilibrium Atmospheric Pressure Plasma
Keigo Takeda1,3, Masanori Kato1, Kenji Ishikawa2, Hiroki Kondo1, Makoto Sekine2,3, Masaru Hori1,2,3
1
2
3
Graduate School of Engineering, Nagoya University
Plasma Nanotechnology Research Center, Nagoya University
Japan and Core Research for Evolutional Science and Technology, Japan Science and Technology
Agency, Kawaguchi, Saitama 332-0012, Japan
Spatial distribution of absolute atomic radical density emitted from atmospheric pressure plasma has
been measured by using the vacuum ultraviolet absorption spectroscopy. Firstly, we carried out the 3D
measurement of O radical density emitted from Ar/O2 nonequilibrium atmospheric pressure plasma.
From the result, the absolute O radical density was 4.1×1014 cm-3 at the distance of 10 mm from the
electrode which made the plasma discharge. Although, the O radical density decreased with increasing
the distance from electrode, the decay of radical density at the long distance from electrode was lower in
the condition of the higher gas flow rate. From this result, it was found that the transport of radical due
to the gas flow was very important in the atmospheric pressure condition.
Keywords: Spatial distribution, Absolute density of atomic radical, Vacuum ultraviolet absorption
spectroscopy, Nonequilibrium atmospheric pressure plasma
1. Introduction
Nonequilibrium
atmospheric
pressure
plasmas are very attractive tool for many
industry applications required high speed dry
process due to the extremely high plasma
density compared with low pressure plasma.
Moreover, because the plasmas are able to be
generated under the atmospheric pressure
without some vacuum chamber, it is possible to
apply to the bio and medical treatments or liquid
material processes which are impossible with
low pressure plasmas. [1-6] In the plasma
processes, it is considered that the behaviors of
atomic radicals are important factor to
determine the process feature. However, the
discharge region is typically localized, and
reconbination rates of the atomic radicals in the
gas phase are very larger compared with lower
pressure conditions. Therefore, it is necessary to
understand three dimension (3D) of the radical
distribution for realizing the high precise
process control. Moreover, in the case of
atmospheric pressure plasma, it is considered
that the flow rate of discharge gases is a very
important factor to transport the radicals which
are generated by atmospheric pressure plasma to
the samples. Therefore, we carried out the 3D
measurement of atomic radical generated by
atmospheric pressure plasma. In order to
measure the atomic radicals in atmospheric
pressure plasma, spectroscopic methods ware
frequently adopted. However, in the cases of
optical emission and laser induced fluorescence
spectroscopy, the quenching effect of excited
radical is very large issue for obtain the absolute
density of radicals. Therefore, in this study,
vacuum ultraviolet absorption spectroscopy
(VUVAS) [7-10] has been used for measuring
the absolute density of atomic radical, because
the method is enable us to directly-measure the
absolute density of atomic radical in the ground
state. Using the VUVAS, we have carried out
O2+Ar
N2 purge gas
Electrode
Gas nozzle
Micro Hollow
Cathode Lamp
VUV Monochromator
Plasma
O2 or H2/He gas
Exhaust
MgF2 window
Exhaust
PMT
Exhaust
Figure 1 The schematic diagram of experimental setup for VUVAS measurement of O radical generated by AC power excited Ar/O2
nonequilibrium atmospheric pressure plasma.
the spatial density distribution of O radical
emitted from nonequilibrium atmospheric
pressure Ar/O2 plasma excited AC power.
2. Experimental setup
Figure 1 shows the schematic diagram of
experimental setup. In this study, the spatial
distribution of O radical emitted from Ar/O2
nonequilibrium atmospheric pressure plasma
excited by 60 Hz AC excited power were
measured. The plasma source has two metal
electrodes applied AC power for plasma
discharge. The distance between the two tips of
the electrodes is approximately 2 mm. The gas
flows into the discharge region through a gas
tube having an inner diameter of 1 mm, and the
distance between the top of the gas tube and the
electrodes is approximately 4 mm. The power
supply is a high voltage transmitter and is driven
by regulated 60 Hz AC voltage, which is
provided by an adjustable transformer. The high
voltage transformer provides a maximum
current of up to 20 mA, which can prevent
discharge from translating into the arc mode. At
an Ar and O2 mixture ratio (O2/(O2+Ar)) of 1%
and an ac power supply operating at 9.0 kV, a
stable plasma was generated along the gas flow.
The plasma was very small, triangular sheet
glow discharge in electrode plane with a higher
optical emission intensity along the three edges
of triangle, as shown in Fig. 2. In this study, the
Ar:3 slm
Radical generation region
Figure 2 Optical emission intensity distribution of the
nonequilibrium atmospheric pressure plasma.
Plasma source holder
Plasma source
Z axis
14
-3
O Radical Density (10 cm )
atomic O radical density was measured by using
vacuum ultraviolet absorption spectroscopy
(VUVAS) with a micro hollow cathode lamp
(MHCL) as a light source. The absorption
length was limited to several cm by two
stainless steel pipes with MgF2 windows. The
absorption intensity through the plasma remote
region was detected using a vacuum ultraviolet
monochromator, which was evacuated using a
turbo molecular pump. The N2 gas was used as a
purge gas to maintain the pressure of chamber at
atmospheric pressure. For achieving the three
dimensional measurement, the plasma source
was able to be moved around the measurement
point of VUVAS as a standard position. The
plasma source was moved along the direction of
the gas stream (Z) and the direction of the
diameter from the center of gas nozzle (R), as
shown in Fig.3.
8
Z=7
Z=10
Z=13
Z=16
6
4
2
0
0
1
2
3
4
Total Gas Flow (slm)
5
Figure 4 O radical densities as a function of total gas flow
rate at each point of Z axis.
measurements were carried out at each distance
(Z = 7, 10, 13, 16 mm) from electrodes to
VUVAS measurement point. Figure 4 shows the
measurement results of O radical density. We
found the O radical density at each Z axis point
increased with increase in total gas flow rate. It
is considered that O radicals were efficiently
generated by electron impact dissociation from
oxygen molecular in the condition of high flow
rate. On the other hand, the O radical density at
Z =10 mm was higher than that at Z=7 mm. The
plasma region was diffused along the gas flow
direction up to Z=10 mm. Moreover, the optical
emission intensity of O radical was higher
R axis
-3
O radical density (cm )
15
10
Pipe for VUVAS
Figure 3 Moving feature of plasma source for special
distribution measurement of O radical density.
14
10
1 slm
2 slm
3 slm
4 slm
5 slm
13
10
7
3. Results and discussion
Firstly, we have measured the O radical
density as a function of total gas flow rate. The
10
13
16
Z (mm)
Figure 5 the spatial distribution of O radical density as a
function of the distance along the Z axis from electrode to
VUVAS measurement point.
around the edge of plasma. Therefore, it is
supposed that the generation rate of O radical at
the point of Z=7 mm was lower than that at the
point of Z=10 mm.
Figure 5 shows the spatial distribution of O
radical density as a function of the distance
along the Z axis from electrode to VUVAS
measurement point. The O radical density was
4.1×1014 cm-3 at the gas flow rate of 1.0 slm
and the distance of 10 mm from the electrode.
The O radical density decreased with increasing
the distance from electrode. Moreover, the
decay of radical density at the long distance
from electrode was lower in the condition of the
higher gas flow rate. From this result, it was
found that the transport of radical due to the gas
flow was very important in the atmospheric
pressure condition.
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4. Conclusion
The spatial distribution of absolute O radical
density generated by AC power excited
nonequilibrium atmospheric pressure plasma
was measured by using VUVAS. From the
results, the O radical density was 4.1×1014 cm-3
at the gas flow rate of 1.0 slm and the distance
of 10 mm from the electrode. The O radical
density decreased with increasing the distance
from electrodes for generating the plasma.
However, the decrease in radical density at the
long distance from electrodes was lower in the
condition of the higher gas flow rate. It was
found that the transport of radical due to the gas
flow was very important in the atmospheric
pressure condition.
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