st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Role of pulsed repetitive current for positive primary streamers in water
H. Fujita 1, S. Kanazawa2, K. Ohtani1, A. Komiya1, and T. Sato1
1
Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan
Department of Electrical and Electronic Engineering, Oita University, 700 Dannoharu, Oita 870-1192, Japan
2
Abstract: A series of primary streamer developments in water was visualized with an exposure time
of 5 ns at 200 mega frame per second (200 Mfps). The synchronization of pictures with the
discharge current showed that primary streamers propagated during the flow of pulsed repetitive
currents. Primary streamers initiated with a propagation velocity of 2.5 km/s, forming a
semi-spherical brush-like structure and propagated, changing the structure into a tree-like one.
Keywords: discharge in water, primary streamer, discharge current, visualization
1. Introduction
The terminology of a “streamer” was used for thin
luminous channels and currents caused by discharges in
atmospheric air and other gases. In the case of discharges
in liquids, lightning filaments or refractive-index changes
are also called “streamers” to express the pre-breakdown
phenomena. Streamers in liquids have been studied for a
long time [1]. Especially, the reports of streamers in water
are rapidly increasing in ten years for the expectation of
environmental and biomedical applications [2] since water
discharges generate ultraviolet rays, energetic electrons,
shock waves, microbubbles, and chemical active species
such as hydroxyl radicals [3]. However the initiation and
propagation mechanisms of streamers in water have been
unclear.
One of the controversial issues about streamers in water
is a propagation mode. Two propagation modes, classified
mainly by their structures and propagation velocities, are
reported for positive streamers [4]. Most of the literatures
refer to secondary streamer developments with a velocity
of 25-30 km/s, forming filamentary structure having a few
branches [4-7]. However there were few studies for the
initiation and propagation processes of primary streamers
[8].
In this work, we visualized the initiation and
propagation processes of primary streamers with an
exposure time of 5 ns at 200 Mfps and 10 Mfps
respectively, for the better understandings of primary
streamer characteristics. From the relationship of the
visualization results with the synchronized current
waveform, the development mechanisms of primary
streamers were discussed.
2. Experimental setup
Figure 1 shows a schematic of the experimental setup. A
single-shot positive pulsed high voltage was generated by
the use of a pulsed high voltage circuit comprised of a DC
power source, a MOS-FET switch, registers and capacitors
and applied to a needle-to-wire electrode system. A trigger
signal from a function generator was sent to a delay
generator, which allowed us to synchronize the discharge,
the camera gate, and the flash lamp timing. An applied
voltage and a discharge current were measured by using a
high voltage probe (LeCroy, PPE 20kV) and a current
probe (bergoz, FCT-016-1.25 WB), respectively. The
waveforms of the applied voltage, the discharge current
and the camera gate signals were monitored by an
oscilloscope. In this experiment, the single-shot pulsed
high voltages of 16.0 kV and 24.0 kV with 10 µs duration
were applied to a needle electrode, the tip radius of which
was sharpened to 40 µm. A grounded wire electrode was
set 6 mm below the needle electrode. Both electrodes were
made of platinum wires of 0.5 mm in diameter and covered
with insulation tubes except the discharge parts. The
Fig. 1 Schematic of experimental setup.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig. 2 A series of primary streamer initiation processes visualized with an exposure time of 5 ns at 200 Mfps when the
voltage of 16.0 kV was applied (a) and the synchronized waveforms of the applied voltage and the discharge current (b).
Magnification of the discharge current with the camera gate timings of (a) is shown in (c).
electrode system was placed in a quartz cell, which was 45
mm in height, 10 mm in width and 10 mm in depth. The
quartz cell was filled with ultrapure water, which was used
at 0.8 µS/cm in electrical conductivity after exposure to
atmospheric air.
A series of the propagation processes of primary
streamers was detected by an ultra high-speed camera
(NAC Image Technology, ULTRA Neo) with a
microscope lens (Keyence, VH-Z50L or VH-Z500R). A
flash lamp (Hamamatsu photonics, E6611) was used as a
light source. Flashlight was condensed by the combination
of two plano-convex lenses.
3. Results and Discussion
3.1 Initiation of primary streamers
Figure 2 (a) shows a series of primary streamer initiation
processes taken with an exposure time of 5 ns at 200 Mfps
when the voltage of 16.0 kV was applied. The
synchronized waveforms of the applied voltage and the
discharge current were shown in Fig. 2 (b). When the
voltage was applied, a displacement current was induced.
The influence of the displacement current continued as
pulsed noises until 1000 ns. About 1400 ns after the
voltage application, pulsed repetitive currents appeared
and a primary streamer comprised of filamentary channels
initiated. There was a initiation time delay observed,
defined as the time from the voltage application to the
streamer initiation, though the initiation time delay was not
constant every single-shot voltage application and
decreased when the voltage increased. During this time,
the shadow region was generated at the tip of the needle
Fig. 3 Time evolution of the radius of the semi -spherical
brush-like primary streamer shown on Fig. 2.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig. 4 A series of primary streamer propagation processes visualized with an exposure time of 5 ns at 10 Mfps when the
voltage of 24.0 kV was applied (a) and the synchronized waveforms of the applied voltage and the discharge current (b).
Magnification of the discharge current with the camera gate timings of (a) is shown in (c).
electrode as shown in Fig. 2 (a) t = 0 ns. It is considered
that it would be the time for the nucleation of microbubbles
in adjacent to the needle electrode [6]. Primary streamers
initiated from the shadow region as shown in Fig. 2 (a) t =
5 ns and propagated semi-spherically. The streamer
structure looks like “a brush”. So we call the first step of
the primary streamer propagation “a semi-spherical
brush-like primary streamer”. Figure 2 (c) shows the
magnification of the very beginning of pulsed repetitive
currents. In our experimental conditions, a continuous
component less than 20 mA with the duration of about 50
ns sometimes preceded pulsed repetitive currents.
However the structure and the propagation velocity of 2.5
km/s shown in Fig. 3 had no difference between the
continuous current and the pulsed repetitive currents.
3.2 Propagation of primary streamers
Figure 4 (a) shows a series of primary streamer
propagation processes taken with an exposure time of 5 ns
at 10 Mfps when the voltage of 24.0 kV was applied. The
initiation time delay shown on Fig. 4 (b) was shorter (about
700 ns) than that shown in Fig. 2 (b) (about 1500 ns) due to
the higher applied voltage. Figure 4 (c) is the magnified
current waveform with the camera gate timings. The
semi-spherical brush-like structure was observed until t =
100 ns. After that, most of the filamentary channels were
vanishing except a few channels. The survived channels
kept growing with a constant rate of 2.0 km/s until t = 400
ns as shown in Fig. 5, resulting in the formation of a
tree-like structure. We call this second step of primary
streamer propagations “a tree-like primary streamer”. The
discharge current from t = 400 ns to t = 600 ns seems a
noise induced by the preceded currents. In fact, the
Fig. 5 Time evolution of the length of the primary streamer
shown in Fig. 4.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Collaborative Research Project of the Institute of Fluid
Science, Tohoku University, and by a grant from Tohoku
University International Advanced Research and
Education Organization. The author wishes to thank
Tomoki Nakajima (Tohoku University) for technical
support.
Fig. 6 Illustration of a series of primary streamer
propagations during the flow of repetitive pulsed currents.
tree-like primary streamer did not develop during this
period. From t = 700 ns, the flow of pulsed repetitive
currents was newly detected and the primary streamer
propagation restarted. Because of the semi-spherical
structure and the propagation velocity of 1.8 km/s shown in
Fig. 5, it is considered that a semi-spherical brush-like
streamer regenerated from the tip of the channel. A
scenario of primary streamer developments is shown in
Fig. 6. The structure of primary streamers transfers a
semi-spherical brush-like structure into a tree-like
structure. In this process, the initiation point of a
semi-spherical streamer would become a branching node
of a tree-like streamer.
4. Conclusion
A series of primary streamer initiation and propagation
processes was visualized with an exposure time of 5 ns at
200 Mfps and 10 Mfps, synchronized with the discharge
currents. Primary streamers started to propagate with an
average velocity of 2.5 km/s, with a semi-spherical
brush-like structure when the pulsed repetitive currents
appeared on the current waveform. A continuous current
prior to pulsed currents was sometimes detected, but it did
not seem to affect the propagation velocity. The vanishing
of the channels of semi-spherical brush-like streamer
resulted in the change of the appearance structure into a
tree-like one. It was suggested that a semi-spherical
brush-like streamer could regenerate from the tip of the
tree-like primary streamer during the flow of pulsed
repetitive currents.
Acknowledgements
This study was partly supported by a Grant-in-Aid for
Scientific Research from JSPS, by a grant from the
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