22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Effect of the argon bubble position on the plasma ignition modes in water
A. Hamdan, D. Lacoste and M.S. Cha
Clean Combustion Research Center, King Abdullah University of Science and Technology, SA-23955-6900 Thuxal,
Saudi Arabia
Abstract: A high-voltage nanosecond discharge with argon bubbles in distilled water is
presented. The ignition modes of the plasma inside the bubble were studied by using ICCD
imaging. Five ignition modes were characterized: from the cathode, from the anode, from
both electrodes, inside single isolated bubble and ignition induced by (plasma) bubble. The
main parameter controlling these ignition modes was the position of the bubbles in the gap.
Keywords: plasma in liquids, plasma in bubbles, nanosecond discharge, ICCD imaging
1. Introduction
During the last two decades, research activities in the
field of plasmas in and in contact with liquids have shown
a continuous increase. Such growing attention is mainly
due to its potential application to the diverse fields of
interest, in particular water treatment [1], nanomaterial
synthesis [2-4], fuel reforming [5], and biomedical
applications [6]. For the reason that the non-classical
properties of those plasmas, such as high electron density
(~1018 cmˉ3), high temperature (~10000 K), high pressure
(~100 bar), reduced lifetime (from ns to µs), etc., they
offer a new chemically reactive environment enabling to
increase the efficiency of targeted application.
Plasma in liquids is filamentary in nature, and the
propagation speed of a streamer is in the range of
1-100 km sˉ1 depending on the experimental conditions
[7]. The branching of discharge channels during the
propagation of a streamer can be related to the presence of
inhomogeneity such as low-density bubbles [8]. The
presence of bubbles in a streamer path changes the
dynamics of a streamer depending on the properties of
both bubbles and liquid.
Gaseous bubbles in liquids provide many phenomena of
significant importance, because they can affect to
physicochemical characteristics induced by plasma. As
an example, their presence in dielectric liquids has shown
a noticeable decrease in a breakdown voltage. In addition,
it has been observed that, depending on the experimental
details (liquid, bubble gas, bubble-electrode distance, HV
amplitude and frequency, etc.), it is possible to ignite
plasma inside a bubble without in-liquid streamers [9].
This means that the erosion of electrodes can be largely
reduced or even suppressed, which results in significant
improvement of the durability of devices.
To further understand the effects of bubbles on ignition
characteristics, we present in this paper an experimental
investigation of high-voltage nanosecond discharges with
argon bubbles in distilled water.
2. Experimental setup
The experimental setup is schematically presented in
Fig. 1. A container was filled with distilled water. Inside
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the water, pin-to-hollow needle electrodes were vertically
installed to generate discharges in a gap between two
electrodes. A high voltage (HV) was connected to a sharp
pin having a radius of ~100 µm at a tip, while a ground
electrode was a hollow needle with an inner and outer
radius of 0.6 and 0.3 mm, respectively. The gap distance
was fixed at 2.5 mm. Argon was supplied through the
needle using a mass flow controller to generate gaseous
bubbles. The flow rate of Ar was fixed at 2 cm3 min‾1,
which resulted in a bubble diameter of ~1 mm. A HV
pulsed power supply (FPG 25-15NM, FID), which is
capable of providing up to 25 kV (peak) with a full width
at half maximum (FWHM) = 10 ns, was used. We kept
applied voltage at 25 kV for all tested conditions in this
study.
Fig. 1. Schematic of experimental setup.
Nano-second imaging with an intensified charge
coupled device (ICCD) camera (PI-MAX2, Princeton
Instruments) was adopted to identify discharge
characteristics. We successfully captured light emissions
from discharges with a gate width down to two
nanoseconds. A synchronization scheme was enabled
with a delay generator (575, BNC). A light emitting
diode (LED) lamp (SST-90, Luminus) with such a short
gate time of ICCD camera.
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3. Results and discussion
Experimental parameters used in this study, such as a
bubble diameter, the gap distance, and applied voltage,
were chosen in a manner to obtain one event of
breakdown (in average) in every two high voltage pulses.
We also need to mention that ~5% of the discharges
transformed into an arc regime, which will not be covered
in this study.
As a result of our observation, we could identify there
are four distinctive ignition modes. Those modes were
closely related with the positions of bubbles in the gap
during the energized period of the upper pin electrode.
Ignition may initiate from the cathode, from the anode,
from the both electrodes independently, and in a floating
bubble in between the electrodes.
Other ignition mode was also found when two bubbles
are adjacent each other in the gap. A cathode-driven
ignited plasma bubble affected a neighboring bubble, thus
the floating bubble can be ignited consecutively. Detailed
characteristics of those ignition modes will be discussed
in the following.
3.1. Cathode driven ignition
For the reason that we applied positive HV pulse to the
upper pin electrode, the pin electrode was an anode and in
turn the hollow needle became a cathode. Since we
injected Ar through the cathode, during the formation
phase of the bubble, the cathode was in contact with Ar.
This situation is similar to a classical discharge in Ar gas
at atmospheric pressure. Due to the hollow structure of
the cathode, the edge of the hollow needle created an
intense electric field, which was sufficiently high to ignite
a discharge in Ar. I n most cases, ignition occurred just
after the bubble formation (Fig. 2). This was attributed to
the enhancement of the electric field on the bubble pole
near the cathode side. Once the ignition was finished, the
streamer seems to be localized on the gas-liquid interface
(Fig. 2a) similar to that observed for the positive
streamers in water with air bubble [8].
Fig. 2. ICCD images showing plasma ignition initiated
from the cathode. The exposure time was 2 ns.
In some cases, when the bubble detached from the
cathode (Fig. 2b), ignition can be obtained directly in the
bubble starting from the lower pole of the bubble near the
cathode. Then, the streamer propagated quickly towards
the upper pole of the bubble.
Another distinguishable observation near the cathode
was that the formation of rather energetic streamers at the
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lower pole of the bubble. The energetic streamer
penetrated into the bubble passing through its lower pole
towards the upper pole (Fig. 2c). It should be noted that
considering 2 ns exposure time and the lengths of
streamers in Fig. 2, the propagation velocity of the
streamers can be estimated as around 100-500 km sˉ1.
3.2. Anode driven ignition
We designed the electrode configuration to be vertical,
such that the generated bubble from the cathode was
supposed to migrate towards the anode along the axis of
the electrodes due to buoyancy. As a result, when the
bubble was near the anode, a breakdown ignited from the
anode, where the electric field is higher, could be
observed. In this case, the ignition is observed on the
upper pole of the bubble (contact point between the
bubble and the anode). Among the large number of
discharges, only a few of them (~5%) were ignited on the
anode. On the condition of misfire of the cathode driven
ignition or immature development of the bubble near the
cathode as shown in Fig. 3, we could observe the anode
driven ignition.
Fig. 3. ICCD images showing plasma ignition initiated
from the anode. The exposure time was 2 ns.
The images in Fig. 3 were integrated also during the
first 2 ns. Thus, the propagation speeds of the streamers
seem to be lower than those observed near the cathode.
Unusual mode of ignition was rarely observed near the
anode, and it seems to be rather homogeneous inside the
bubble (Fig. 3c). This mode possibly can be obtained due
to the enhancement of the electric field on the both poles
of the bubble, such that the plasma ignites more
homogenously and rapidly diffuse inside the bubble.
3.3. Co-ignition near the electrodes
Co-ignition of two bubbles near the each electrode
could be also found. This mode was observed when one
bubble was reaching the anode while another consecutive
bubble was forming near the cathode, simultaneously
(Fig. 4).
The two distinctive discharge channels could be found
close to the sharp edge of each electrode, where the
electric field intensity was higher (Fig. 4). The streamer
propagations seem to be localized at the gas-liquid
interface as similar to the cases with the cathode and
anode driven ignitions. This implies there might be little
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that one bubble presented near the cathode chasing the
former bubble. This former bubble either can be in
contact (Fig. 6a) with the latter bubble (which is near the
cathode) or be separated by a thin water layer from the
latter bubble.
Fig. 4. ICCD images showing plasma ignition initiated
simultaneously from the both electrodes. The exposure
time was 2 ns.
interactions between two igniting bubbles.
3.4. Ignition in an isolated bubble
Ignition in a single isolated bubble was observed with
around 5% probability in our experimental conditions. In
particular, this mode of discharge also can be
characterized as a modification of the bubble shape. The
initial spherical bubble was elongated in the direction of
electric field lines along the electrode axis demonstrating
rather cylindrical shape (Fig. 5).
Fig. 5. ICCD images showing plasma ignition in a single
isolated bubble. The exposure time was a) 2 ns, b) 50 ns,
and c) 200 ns.
Regardless of the exposure time between 2 and 200 ns,
the plasma looks like a single filament traversing the
bubble along its axis. The filamentary discharge was
confined inside bubble showing no connected channel
with either one of the electrodes. In addition, we may say
that that the plasma looks like more filamentary when the
bubble was close to one of the electrodes (Figs. 5a and
5c), while rather homogeneous feature when it locates at
the midway in the gap (Fig. 5b). This can be explained by
the enhancement of the electric field due to both
electrodes and bubble poles.
3.5. Igniting bubble driven ignition
One important result was also achieved in this study is
an interaction between two bubbles. The igniting bubble
with the cathode driven ignition could induce secondary
ignition to the adjacent bubble. This mode can be
obtained when the separation between two consecutive
bubbles was reduced (Fig. 6). More probable situation is
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Fig. 6. ICCD images showing ignition induced by a
plasma bubble. The exposure time was a) 2 ns, b) 50 ns,
and c) 200 ns.
As aforementioned, when the bubble was close to the
cathode, the ignition occurred in the lower pole of the
bubble, and the streamer propagated towards the upper
pole along the gas-liquid interface. The upper pole of
igniting latter bubble now can be considered as a virtual
electrode on which the electric field intensity is enhanced
due to the streamer head as well as the two facing poles of
two bubbles (i.e., contact point between the bubbles).
The streamer propagation inside the former bubble was
similar to that of the cathode driven ignition bubble.
However, due to significantly enhanced electric field, a
greatly higher propagation speed could be estimated. In
fact, Fig. 6a was taken with 2 ns exposure time, thus the
corresponding propagation speed can be around
1000 km sˉ1 showing two times higher than that in a
single bubble.
4. Conclusion
The investigation of high-voltage nanosecond discharge
in water with argon bubbles was presented in this paper.
The streamer ignition and propagation were characterized
with ICCD imaging with an exposure time down to 2 ns.
Different ignition modes were discussed: a cathode driven
ignition, an anode driven ignition, a co-ignition near
electrodes, an ignition in an isolated bubble, and an
igniting bubble driven ignition. The main parameter
controlling those modes was the relative position of the
bubbles with respect to the each other as well as the
electrodes.
In our conditions, the streamer was found to attach to a
gas-liquid interface and its propagation speed was around
several hundreds of kilometers per second. However, in
the case of the igniting bubble driven ignition, the
propagation speed was two times higher due to electric
field enhancement at a bubble-bubble interface.
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