Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011, pp. 3640∼3643

Long Gap Discharge in Water

Yun-Sik Jin,∗ Chuhyun Cho, Hong-Je Ryoo, Jong-Su Kim and Geun-Hie Rim

Korea Electrotechnology Research Institute, Changwon 641-120, Korea

SooWon Lim

University of Science and Technology, Daejeon 305-333, Korea

(Received 15 December 2010, in final form 6 September 2011)

A discharge in water with a long gap was realized at low voltage. The main feature of thisdischarge was the assistance of a high pressure gas jet. For a gap length of 5 cm in tap water, thedischarge or breakdown voltage was as low as 32 kV, which is five times lower than that of air- gapbreakdown. The breakdown voltage increased linearly with the gap distance. Helium, nitrogen,argon and air were used as injection gases. The discharge characteristics were investigated as afunctions of the injection gas species, the gap distance and the back pressure of the injection gas.Comparison of the discharge voltage with water surface discharge suggested that the gas jet assistedwater discharge is a kind of surface discharge. The long gap discharge in water can be utilized formany environmental applications, such as wastewater treatment.

PACS numbers: 52.80.Wq, 77.22.JpKeywords: Discharge in water, Gas jet injection, Breakdown voltageDOI: 10.3938/jkps.59.3640

I. INTRODUCTION

Electrical discharge in liquid is expected to be usedfor various industrial applications. Techniques to elimi-nate toxic substances from wastewater, wastewater fromwaste disposal, liquid phase industrial waste are underdevelopment [1,2]. The electrical discharge in water isalso a very promising method for treating ship ballastwater [3].

Since the breakdown field in water is several tens oftimes higher than that in the gas phase, an electricaldischarge in water is difficult to obtain. For instance,the electric breakdown strength of water in a centimetregap in a uniform electric field for a pulse width of 3 µsis 3 × 105 V/cm [4]. In order to generate a discharge inthe water, we need an ultra-high voltage source, whichneeds high cost and large volume, and high voltage is ac-companied by corona noise, X-ray and danger of electricshock. For a reduction of breakdown voltage in water,injection of gas bubbles between the electrodes was pro-posed. This method was effective to some extent, butthe discharge voltage was still high because the bubblesoccupy a very small part of discharge space [5,6].

In the present study, we propose a new method forreducing the breakdown voltage drastically for the firsttime. In this scheme, a high-pressure gas jet is injectedbetween the electrodes installed in the water. We call

∗E-mail: ysjin@keri.re.kr; Fax: +82-55-280-1458

Fig. 1. (Color online) Experimental setup for a gas-jet-assisted underwater discharge.

this discharge method as the gas-jet-assisted (GAJA)water discharge method. The electric discharge plasma,e.g., a GAJA discharge in water, produces various kindsof free radicals, strong ultraviolet radiation, ozone, andshock waves directly in the water. The breakdown volt-age was decreased by more than ten times compared witha discharge without gas jet injection. The characteris-tics of an electrical discharge obtained in water by usingthe GAJA method was investigated for various dischargeconditions.

II. EXPERIMENTAL METHOD

Figure 1 shows the experimental setup for a gas-jet-assisted discharge in water. A pair of electrodes is in-

-3640-

Long Gap Discharge in Water – Yun-Sik Jin et al. -3641-

Fig. 2. (Color online) (a) Voltage-current waveform of an air gap discharge (gap length: 2 cm). (b) Voltage-current waveformof a GAJA discharge in water (gap length: 2 cm, injection gas: air). (c) Voltage-current waveform of a GAJA discharge inwater (gap length: 2 cm, injection gas: Ar). (d) Voltage-current waveform of a GAJA discharge in water (gap length: 2 cm,injection gas: He).

Fig. 3. (Color online) Variation of breakdown voltage inGAJA discharge with gap distance.

stalled in a container filled with water. The water con-tainer is made of transparent acryl plate. For the gasinjection, a hole with a diameter of 0.5 – 1 mm wasbored in one electrode. The ends of the brass electrodeshave a hemisphere shape with a radius of 5 mm. Asthe injection gases, dry air, nitrogen, argon, and heliumwere tested. A semiconductor-switch-based high-voltage

pulsed power source was used to bias a high voltage be-tween the electrodes [7]. The pulse power system wasable to supply square-shaped microsecond pulses up to60 kV at a repetition rate of 1 kHz.

III. RESULTS AND DISCUSSION

The breakdown voltage of the GAJA water dischargewas compared to that of an air gap discharge. In the airgap discharge, two electrodes were separated 2 cm with-out a water filling. Figure 2(a) shows the voltage-currentwaveform of the discharge. The breakdown voltage wasabout 50 kV, and the delay time to the main dischargewas 2.5 µs. In the GAJA discharge, dry air, Ar and Hegas were injected through a 0.6 mm hole with a back pres-sure of 2 atm and the gap length was set to 2 cm. FromFig. 2(b), we can see that the breakdown voltage was 21kV and the delay time to the main discharge was 1 µs forair injection. When Ar or He was used as an injectiongas in the GAJA discharge, the discharge voltage wasdecreased to less than 18 kV (Fig. 2(c) and Fig. 2(d)).

-3642- Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011

Fig. 4. (Color online) Comparison of the discharge characteristics between tap water and distilled water (gap length: 2 cm,air injection): (a) Voltage-current characteristics before the breakdown occurs-tap water. (b) Voltage-current characteristicsbefore the breakdown occurs-distilled water. (c) Discharge characteristics of tap-water (applied voltage; 30 kV). (d) Dischargecharacteristics of distilled water (applied voltage; 30 kV).

This is about 1/3 the air-gap breakdown voltage. Thedelay time to the main discharge was decreased to lessthan 0.5 µs.

Figure 3 shows the discharge voltage as a function ofthe discharge gap length when air was used as the injec-tion gas. As the discharge gap length in the water in-creases, the discharge or breakdown voltage increases lin-early, but the rate of increase with increasing gap lengthis rather small, and the breakdown voltage is only 32 kVat 5 cm gap length (Fig. 3). This voltage is five timeslower than that of air-gap breakdown. The above ob-servation on breakdown voltage confirms that the GAJAdischarge in water is a very effective in generating a dis-charge plasma under water.

The discharge characteristics in water are influencedby the resistance of the water. The behaviour of GAJAdischarges in tap water and distilled water were inves-tigated. Figures 4(a) and (b) show the voltage-currentcharacteristics before the breakdown occurs. The gapdistance was 2 cm and air was used as the injection gas.In the case of tap water, the current wave form is similarto the voltage waveform, and the resistance was mea-sured to be 380 Ω. For the distilled water, the resistancewas so large that little current flowed and the voltagewaveform decayed slowly because of the large RC con-stant. Figures 4(c) and (d) show the discharge charac-

teristics when the applied voltage was 30 kV. There aresmall differences in the delay times to main dischargeand the breakdown voltages between the two kinds ofwater, but the discharge behaviours are very similar toeach other.

When the voltage was not high enough to form a sparkdischarge in water, the current was limited by the totalresistance of the circuit and the water. On the otherhand, when the voltage was high enough, the currentwas limited by only the circuit resistance because of thesmall resistance of the spark channel. This differencein the current appears between (a), (b) and (c), (d) inFig. 4. This means that the GAJA discharge in wateris little affected by the resistance of the water when thespark discharge is formed, so various kinds of water orliquids with different resistivites can be treated by usinga GAJA discharge.

In a GAJA water discharge, the gas jet ejected fromone electrode forms a gas channel and reaches the otherelectrode. When the gas channel is formed, it seems thatthe two electrodes are connected via the water surface ofthe gas channel. By applying a high voltage pulse on theelectrodes, flashover may occur along the surface of thegas channel, thus, the GAJA discharge can be regardedas a kind of water surface discharge. In order to ver-ify assumption, we executed a comparative experiment

Long Gap Discharge in Water – Yun-Sik Jin et al. -3643-

Fig. 5. (Color online) Comparison of discharge characteristics between (a) gas-jet-assisted water discharge and (b) watersurface discharge (gap length: 3 cm, 2-atm air).

between the GAJA discharge and a water surface dis-charge. Figures 5(a) and (b) show the voltage-currentbehaviours of each discharge when a dry air jet of 2 atmwas injected into a 3-cm gap. In the water surface dis-charge, half of the electrodes were exposed above thewater surface. The voltage and the current waveformsare almost the same in both discharges, which confirmsthat the GAJA discharge is a kind of surface discharge.

IV. SUMMARY

Long gap breakdown in water was possible with a rel-atively low voltage by using a gas-jet-assisted (GAJA)discharge method. In this method, a high-pressure gasjet is injected into a discharge gap from one electrode,and the gas jet makes a gaseous column between theelectrodes. At a 5-cm gap length, the discharge voltagein the water was 32 kV, which is five times lower thanthat in the atmosphere. Comparison of the dischargevoltage with a water surface discharge showed that thegas-jet-assisted water discharge is a kind of surface dis-charge. The discharge voltage in the GAJA method waslittle affected by the water’s resistivity, and this propertyallows the GAJA discharge to be used to treat variouskinds of waste water.

ACKNOWLEDGMENTS

The authors would like to thank Prof. H. Akiyamaand Prof. S. Katsuki of Kumamoto University for theirkind discussion and advice on the properties of waterdischarges.

REFERENCES

[1] B. R. Locke, M. Sato, P. Sunka, M. R. Hoffman and J-S.Chang, Ind. Eng. Chem. Res. 45, 882 (2006).

[2] H. Akiyama, IEEE Trans. Dielectr. Electr. Insul. 7, 646(2000).

[3] E. Tsolaki, P. Pitta and E. Diamadopoulos, J. Chem.Technol. Biotechnol. 85, 19 (2010).

[4] V. Ya Ushakov, Pulsed Electrical Breakdown of Liquid(Tomsk State University Publishers, Tomsk).

[5] H. Kitayama, H. Honma, N. Nakagawara and K. Yasuoka,IEEE Trans. Plasma Sci. 37, 897 (2009).

[6] T. Honda and Y. Minamitani, IEEE Trans. Plasma Sci.37, 179 (2009).

[7] H. J. Ryoo, G. Gussev and S. R. Jang, Acta Phys. Pol.A 115, 967 (2009).