Synchronized Observation of Plasma Emissions and Bubbles Generated by
Radio-Frequency Plasma in Water
S. Mukasa1, S. Nomura1, H. Toyota1
1
Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, 790-8577, Japan.
Abstract: In order to diagnose 27.12 MHz radio-frequency plasma in pure water, we
observed split two images, bubbles and H emissions. The electrode surface was covered
with the H emission at 10 kPa, while H distribution stretched upwards from the part of
the electrode surface at the atmospheric pressure. The plasma repeated stretch and
shrinkage owing to the bubble behavior. At 200 kPa or higher, the emission with continuous
spectrum spatially spread around the top of the stretched plasma.
Keywords: Plasma in water, Radio frequency, High-speed camera, Spectrum, Bubble
1.
Introduction
Plasma is generated in water and other liquids by
microwave and radio-frequency (RF) irradiation. The
plasma in these liquids is expected to degrade harmful
liquids, solutes and dispersoids, while synthesizing useful
materials and gases.
As another application of the plasma in liquids, we
purpose the production of hydrogen by decomposition of
methane hydrate which exists in a stable form in deep sea
deposits and permafrost at a low temperature [1]. At the
case, to clarify the physics of the plasma in highly
pressurized water becomes more important.
The plasma is generated from the tip of an immersed
electrode which is usually made of a metallic rod of
several millimeters in diameter. Liquids around the
plasma are evaporated by the plasma and therefore it is
more accurate to say that the plasma is generated not “in
liquids” but “in bubbles”. The bubbles repeatedly grew
and departed from the electrode by buoyancy in
approximately 10 to 30 ms. In pure water at a temperature
lower than the saturation temperature, immediately after
departure from the electrode the bubble size was reduced
due to condensation by water-cooling.
In the previous study [2], simultaneous measurement of
the spatial distributions of the hydrogen Balmer series,
H and H , emitted from 27.12 MHz RF plasma in water
was conducted using a high-speed camera and optical
devices. The distribution of the excitation temperature can
be calculated from the ratio of the local emission
intensities of H and H . The measurements have time
and spatial resolutions sufficiently precise for
examination of the bubble behavior.
In this study, we conducted mainly three experiments.
The first experiment was the simultaneous observation of
the bubble and emission image of H , the second was the
simultaneous observation of the emission images of H
and H from 27.12 MHz RF plasma in pure water under
the atmospheric pressure or higher pressure, and the third
was the measurement of emission spectrum in order to
confirm the emission of continuous spectrum.
2.
Experimental Procedure
The experimental apparatus used in the first experiment
is the same as that used in the previous study [2]. Plasma
was generated at the top of the electrode in the reactor by
supplying power of 130-150 W from a RF power supply
of 27.12 MHz. Flow of pure water was induced in the
experimental apparatus by a circulating pump with the
flow rate controlled at 1.5L/min by a valve. The pressure
in the experimental apparatus was controlled by a vacuum
pump, and was changed from 10 kPa to atmospheric
pressure. The electrode was made of a tungsten rod with a
diameter of 3.0 mm, inserted into glass tube with an outer
diameter of 6.0 mm and a thickness of 1.5 mm.
The experimental apparatus used in the second and
third experiments is schematically shown in Fig. 1. The
reaction vessel made from stainless steel was filled with
the pure water of 520 mL. The RF power supply and the
electrode were the same as those used in the first
experiment. The copper counter electrode of 13 mm in
diameter of which tap water was poured at the inside
served as cooler in the vessel. The distance between the
electrodes was set to approximately 20 mm.
After decompressing the inside of the vessel by the
aspirator and generating plasma, maintaining plasma,
the pressure was returned to the atmospheric pressure and
increased further using the pressure device. Supplied
power was set to 200 W at the atmospheric pressure, and
Vacuum
Counter electrode (Copper)
Cooling water (Tap water)
Pressure gauge
Pump
P
Bubble
Plasma
Electrode (Tungsten, 3)
Glass tube (Quartz)
Quartz window
Teflon
Matching
box
Fig. 1 Experimental apparatus for RF plasma in pure
water under atmospheric pressure or higher.
I (arb. unit)
1
H
10 kPa
OH
0.5
O
H
O
0
400
600
wavelength (nm)
800
Fig. 2 Emission spectrum of RF plasma in pure water.
(a)
Vesse
l
Black plate
Mirror
H -band pass filter
ND filter
High-speed camera
Plasma
Dichroic mirror
(b)
LED
H -band pass filter
H -band pass filter
Experimental apparatus
High-speed camera
Mirror
Paper
(a) 10 kPa
Plasma
Polycarbonate
Color filter
Dichroic mirror
Fig. 3 Optical systems for capturing two images of H
and H (a), bubble and H (b) by high-speed camera.
increased 100 W with the pressure increase of 100 kPa in
order to maintain plasma. The reflective power was
approximately 20 to 100 W.
The inside of the vessel was observable through two
transparent quartz glass windows. The emission spectrum
of plasma in pure water at 10 kPa is shown in Fig.2. The
main emission species are OH, H , H , and O, and the
difference in the emission species by the difference in a
pressure was not found.
The optical system for separating and capturing only
the emission images of H and H is shown in Fig.3a,
which was used in the second experiment. A dichroic
mirror (DIM-RED, sigma KOKI) penetrates the visible
light below approximately 570 nm, and reflects the visible
light more than approximately 630 nm. The band pass
filters which penetrate only the wavelength domains of
H and H were set in front of the high-speed camera,
and ND filter was set when the captured image was
overexposure.
The optical system for capturing the bubble image and
emission image of H is shown in Fig.3b, which was
used in the first experiment. Although it is essentially the
same as that of a previous optical system, the bubble
image is obtained by illuminating with blue
light-emitting-diode (LED) light towards the bubbles
from another observation window.
The sequential photographs were taken by 2000 fps
using the high speed camera (MEMRECAM GX-1, Nac
image technology). The gamma correction of the camera
(b) 101 kPa
Fig. 4 Sequential composite images of bubble and
emission distribution of H .
was made linear. The photographs were preserved by
TIFF16 (RAW) file in 12-bit gradation monochrome. The
contour of the bubble in the photograph was extracted by
performing image processing, and the volume of the
bubble was calculated on the assumption of the rotational
symmetry of the bubble.
A digital-still camera (D90, Nikon) with a single-focus
lens (MACRO 105mm EX DG, SIGMA) was used to
photograph the luminescence. A spectroscope (PMA-11,
Hamamatsu) was used to measure the spectrum.
3. Results and Discussions
3-1. Bubble and emission distribution of H
Sequential composite images of the bubble and the
emission distribution of H at 10 and 101 kPa are shown
(a)
Fig. 5 Composite images of bubbles and distribution of
H by RF plasma in pure water at 30 kPa.
in Fig. 4a and Fig. 4b, respectively. The colored areas in
the images indicate the emission distribution of H , and
the color turns from blue to red when the emission
intensity increases from the minimum to the maximum.
The area of strong emission is mostly on the surface of
the electrode, and was broadened at 10 kPa while
localized at higher pressure. The emission sometimes
seemed to be reflected on the surface of the bubble. When
the bubble rose in conjunction with the following
generated bubble, the following bubble was stretched
between the previous rising bubble and the electrode
surface, and the strong emission appeared in the stretched
bubble. Just after the separation of the previous bubble
from the electrode, the bubble became spherical with the
attenuation of the emission, and then started expanding
and rising. The emission of the H seemed to gradually
intensify with the expansion of the bubble at 101 kPa.
The plasma shrank just after the length reached a
maximum. This behavior was found to be related to the
behavior of the bubble. Because the lower bubble was
stretched just after the division, the bubble had a high
surface tension and the topside of the bubble rebounded
downwards. The top dove into the interior of the bubble
(see the red arrow in Fig. 5) and sometimes seemed to
reach the surface of the electrode. At that moment, the
plasma shrank greatly.
3-2. Ratio of emission intensities of H and H
An example of the distributions of H intensity, H
intensity and the intensity ratio compounded with the
electrode image is shown in Fig. 6. At 400 kPa, although
the distribution of H was stretched similarly to that at
101 kPa shown in Fig. 4b, the distribution of H was
sometimes spread around the top of the distribution, and
therefore the intensity ratio around the top of the
distribution became high.
If the density distribution of each energy level in
plasma follows the Boltzmann distribution, excitation
temperature Tex can be calculated using the intensity ratio
of H to H , IH /IH as follows:
(b)
(c)
Fig. 6 Distributions of emission intensity, H
H (b), and ratio of H to H (c) at 400 kPa.
attenuation coefficient of light of the optical system and
the wavelength dependability of the sensitivity of the
image sensor in the high-speed camera were not clarified.
Therefore, although the results must be argued only
qualitatively, the ratio of H to H around the top of the
distribution became apparently higher than 0.46. In order
to resolve the contradiction, the spectroscopic
measurement of plasma emission was conducted.
3-3. Spectroscopic measurements
Figure 7 shows the pictures of plasma taken by the
digital-still camera at 100 kPa (a), 200 kPa (b), 300 kPa
(c), and 500 kPa (d). At the atmospheric pressure, purple
luminescence stretched linearly from the surface of the
electrode, and the color at the tip of the luminescence
remained purple. Mainly two bright-line spectra, H and
H , appeared in the purple luminescence. The color at the
tip of the linear purple luminescence turned yellow at 200
kPa, and blue at 300 kPa while broadening. Over 300kPa,
the color seemed to the naked eye to remain blue at the tip
of the purple luminescence. The intensity of the entire
luminescence increased with the pressure, because the
supplied power was increased with pressure. Although the
center of the luminescence in Fig. 7d seems white, this
area was blown out because of overexposure. However,
blue on the right side of the blown-out area and yellow on
(a) 101 kPa
(b) 200 kPa
(c) 300 kPa
(d) 500 kPa
1
Tex
0.46
C ln
I H I Hα
,
(1)
where C is the constant value obtained from physical
properties. When IH /IH is over 0.46, Tex becomes
negative, and is not adequate. In this measurement, the
(a) and
Fig. 7 Photographs of RF plasma in pure water.
4.
Conclusions
In order to diagnose RF plasma in pure water, we
observed split two images, bubbles and H emissions.
OH(A-X)
H
×104 intensity (arb. unit)
2
H
O
200 kPa
101 kPa
black-body radiation
4700 K
1
0
300
400
500
600
700
800
wavelength (nm)
(a)
OH(A-X)
H
2
×104 intensity (arb. unit)
the left side can be confirmed. It indicates that there is a
distribution of color in the luminescence.
Blue or yellow luminescence was not always captured
by the camera. A bubble can be confirmed in Fig. 7d. The
blue or yellow luminescence appeared only in the grown
bubbles. The bubbles repeatedly grew and then reduced
after departing from the electrode by buoyancy. Therefore,
the condition of the luminescence, for example, color, size
and intensity, frequently changes with the bubble behavior.
However, because the cycle of the bubble growth was
approximately 10 to 30 ms, the luminescence does not
seem to change markedly to the naked eye.
The spectra of the luminescence are shown in Fig. 8a
and 8b. The detection time was 100 ms and detection area
was wide enough to include the entire luminescence. The
bright-line spectra such as H , H , OH(A-X) and O are
so strong that these peaks are out of the figure range.
When the pressure was 200 kPa or higher, continuous
spectra became apparent. The wavelength at the
maximum intensity of the continuous spectra was
approximately 620 nm at 200 kPa, and 430 nm at 300 kPa
or higher. The maximum intensity increased with the
pressure when the pressure was 300 kPa or higher.
Usually a continuous spectrum emitted thermally
conforms to the black-body radiation. When the peak of
the black-body radiation is 620 nm, which is the
wavelength at the maximum intensity of the continuous
spectrum at 200 kPa, the temperature is calculated to be
4700 K from the Wien's displacement law. However the
spectrum calculated from the Planck's law was quite
different from the continuous spectrum measured at 200
kPa. However at 300 kPa or greater, spectra over 450 nm
was found to conform to that of the black-body radiation.
Temperature calculated from the Planck’s law became
approximately 6200 K at 300 kPa, and 8800 K at 400 and
500 kPa.
The rotational temperature of hydroxyl radicals was
measured by fitting the OH (A-X) band to the results of
LIFBASE, software for spectrum simulation. This
resulted in temperatures of approximately 4200, 4800,
and 5000 K at 200, 300, and 400kPa, respectively [3]. The
present results seem to be overestimated when the
spectrum is calculated by application of the black-body
radiation. If the temperature at the surface of the bubble is
assumed to be near the saturation temperature of water, a
temperature distribution with a large temperature gradient
must exist. According to the concept of chemical
equilibrium, the temperature is approximately between
2300 and 4800 K, when the mole fraction of hydroxyl
radical produced by water degradation becomes greater
than 1%. At higher temperatures, the mole fraction of
hydroxyl radical decreases exponentially. Therefore the
OH (A-X) band was the dominant emission in the range
from approximately 2300 to 4800 K.
H
H
O
500 kPa
400 kPa
300 kPa
black-body radiation
6200 K
8800 K
8800 K
1
0
300
400
500
600
700
800
wavelength (nm)
(b)
Fig. 8 Emission spectra of RF plasma in pure water at
101-200 kPa (a), and 300-500 kPa (b).
The electrode surface was covered with the H emission
at 10 kPa, while H distribution stretched upwards from
the part of the electrode surface at the atmospheric
pressure. The plasma repeated stretch and shrinkage
owing to the bubble behavior. At 200 kPa or higher, the
emission with continuous spectrum spatially spread
around the top of the stretched plasma.
Acknowledgements
This work was supported by a Grants-in Aid from the
Ministry of Education, Culture, Sports, Science and
Technology of Japan (No 24560236).
References
[1] A. E. E. Putra, S. Nomura, S. Mukasa, H. Toyota,
International Journal of Hydrogen Energy, 37, 16000
(2012).
[2] S. Mukasa, S. Nomura, H. Toyota, Proceedings on
ISPC20, 540 (2011).
[3] S. Nomura, S. Mukasa, H. Toyota, H. Miyake, H.
Yamashita, T. Maehara, A. Kawashima, A. Abe, Plasma
Sources Science and Technology, 20, 034012 (2011).