静電気学会誌，44, 5（2020）216-217

J. Inst. Electrostat. Jpn.

ノ ー トStreamers Induced by Dielectric Barrier Electrohydraulic Discharge＊, 1＊＊

Masaaki OKUBOand Mitsuru TAHARA（Received March 10, 2020; Accepted March 25, 2020）

Streamers in water plasma induced by a dielectric barrier hydraulic discharge are observed with a high-sensitivity digital camera.
The barrier-type plasma reactor consists of a stainless-steel needle high-voltage (+35 – +65 kV) electrode partially immersed in asolution and a glass solution container wrapped with an aluminum film as the grounded electrode. Many filamentous discharges
or streamers are clearly observed in water under a high voltage of 50 kV. Efﬁcient dissociation of hydrogen peroxide generated
by the plasma under alkaline conditions is a key for water treatment or dye decolorization with radicals' generation.

1. Introduction
Recently, generating a plasma stably in water or anelectrohydraulic discharge plasma using pulsed power
technology with an extremely high voltage in a very short time(of the order of a nanosecond) has become possible. Therefore,studies on applications such as solution treatment, sterilization,and decolorization have received considerable interest1, 2).

However, when a pulsed high voltage is applied between theelectrodes, as the voltage is increased the streamer discharge(which has higher energy efficiency for the treatment) caneasily transit to a spark discharge. Such a spark discharge

Fig.1 Experimental setup.

requires high electrical energy and exhibits lower energy

wrapped around the outside and bottom surfaces of the beakerefficiency for the treatment. Therefore, maintaining a streamer

as a grounded electrode. A high-voltage pulse is applieddischarge is preferable. To do it, the dielectric barrier between

between the electrodes using a pulsed power source (with aelectrodes is effective as is the case with gas electricalgap of 4–6 mm) (Masuda Research Inc., Japan). The applied3)

discharge. In this study, following the previous one ,output voltage is 10–65 kV, the pulse width is 540 ns, and thecharacteristics of streamers induced by a dielectric barrier

pulse frequency is 120 pulses per second (Hz). The circuit

electrohydraulic discharge are investigated.

consists of a protection resistance (3 MΩ), a load resistance(50 kΩ), a capacitor (1 nF), and an inductor (4.7 mH).

2. Experimental apparatus and methods
A schematic diagram of the experimental setup is shown in

3. Experimental results and discussion

Fig. 1. A Pyrex glass beaker (with an inner diameter of 6.24

When the applied voltage is 50 kV, a series of electrohydrauliccm, a glass thickness of 1.8 mm, and a liquid volume of 200

discharge streamers that start from the tip of the needlemL) is filled with pure distilled water or a dye solution. A

electrode are observed, as shown in Fig. 2. A higher voltage

stainless-steel needle electrode (with a diameter of 1 mm) is

more than 50 kV can be maintained by the dielectric plasmacompletely covered in a polytetrafluoroethylene (PTFE) tube,reactor. As a result, high power density can be realized. The

except for the tip, which is immersed in water to a depth of 3

photographs in Fig. 2(a)–(c) are taken in distilled water withcm and used as a positive electrode. Aluminum foil tape isdifferent digital camera (Nikon Coolpix 5000) exposure times.
For example, streamers during (1/15)/(1/120) = 8 periods of

キ ー ワ ー ド：Electrohydraulic discharge, streamer, pulsed
high voltage, free radical, reactive dye＊Department of Mechanical Engineering, Osaka Prefecture
University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan＊＊
Formerly at Technology Research Institute of Osaka
Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan1
mokubo@me.osakafu-u.ac.jp

the high-voltage pulse are captured in Fig. 2(c). In the figures,a significant number of small bubbles can be seen to bereleased from the streamers. The composition of these bubbles
is investigated using a gas detection tube (Gastec Corp.). As aresult, ozone (~60 ppm) and hydrogen (~0.1％) were detected
near the water surface. Furthermore, free radicals such as

Streamers Induced by Dielectric Barrier Electrohydraulic Discharge（Masaaki OKUBO et al.）217（31）

Fig.2 Structures of streamers. Digital-camera exposure times are (a) 1/60 s, (b) 1/30 s, and (c) 1/15 s.
The locations of the needle electrode covered by the PTFE tube are indicated with dotted lines.

Fig.3 Optical absorbances in the dye decolorization: (a) untreated;
treatment times of (b) 10 min, (c) 20 min, and (d) 30 min.

Fig.4 Optical absorbances in the dye decolorization: (a) untreated,(b) immediately after 30-min plasma application, (c) 1 day
after it, and (d) 1 day after it with the addition of Na2CO3.

hydroxyl and hydrogen radicals were successfully detected inadvances under alkaline (Na 2CO 3) conditions , and thethe solution using electron spin resonance measurements .

superoxide anion (•O2) and perhydroxyl radical (•OOH), as3)

Fig. 3 shows time-dependent changes in the opticalabsorption of the dye (Reactive RED 106) solution with a4)

well as hydroxyl radicals (•OH), are induced. They are very
effective for decolorization.

concentration of 0.01 g/L after plasma application for 0–30
min. Under conditions of a dye solution concentration of 0.01

4. Conclusiong/L (electrical conductivity = 79 µS/cm) and an applied

The structures and treatment performance of streamers in a

voltage of 65 kV (with a pulse width of 540 ns and a pulse

water plasma induced by a dielectric barrier hydraulicfrequency of 120 Hz), a series of streamer discharges isdischarge are investigated. A higher voltage can be maintainedo b s e r v e d . A d i s c h a rg e e n e rg y o f a s i n g l e p u l s e i sby the dielectric plasma reactor. Efﬁcient dissociation of H2O2

approximately 0.46 J. The graph in Fig. 3 shows thatgenerated by the plasma under alkaline conditions is a key fordecolorization advances and optical absorption decreases withwater treatment with radicals’ generation.

the increase in the plasma treatment time. However, thedecolorization or decrease is not very large. Next, after treating

References

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