S-R4-3
Dynamics of Underwater Discharge: A Scenario of Multiple Eventsfrom Pre-discharge to Post-discharge
水中放電のダイナミックス：放電前から放電後までのシナリオ
Seiji Kanazawa*, Ryuta Ichiki*, Toshikazu Ohkubo* and Takehiko Sato**
金澤誠司*, 市來龍太*，大久保利一*，佐藤岳彦*** Department of Electrical and Electronic Engineering, Oita University
700, Dannohara, Oita 870-1192, Japan
大分大学工学部電気電子工学科 〒870-1192 大分市旦野原700番地** Institute of Fluid Science, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
東北大学流体科学研究所 〒980-8577 仙台市青葉区片平2-1-1
A laser Schlieren technique has been applied for the visualization of the pressure field of a single-shotunderwater pulsed discharge. A needle-to-plane electrode submerged in distilled water was used. The
detailed time- and space-resolved observation of both streamers and pressure waves was performed. As aresult, several phenomena, such as the phase change prior to the initiation of the discharge, primary andsecondary streamers propagation, shock wave generation, and the bubble formation, were observed. Fromthese observations, a scenario of multiple events from pre-discharge to post-discharge was proposed.

1. Introduction
Recently, liquid-related electrical discharges,
such as discharges in water or discharges on thewater surface, have been widely studied forenvironmental and bio-medical applications as wellas nanomaterial processing.
However, theunderwater discharge mechanisms are more
complicated compared to the discharge in air. In
addition, several phenomena, such as the radicalformation, UV radiation, and shockwave generation,
are involved in the underwater discharge.
Although the discharge observation based on the
optical emission is widely performed, the obtained
information is limited on the streamer propagation
stage. To clarify the initiation and post-discharge
phenomena, another optical observation, such as
shadow/Schlieren imaging and Mach-Zehnderinterferometry has been investigated [1-2].
In this study, a laser Schlieren technique wasused for visualization of the pressure field ofunderwater discharge. To avoid the influence offormer discharge, a single-shot pulsed discharge
was used instead of repetitive pulsed discharges andliquid-gas dynamics was observed from theinitiation to after the extinction of the discharge.
2. Experimental Procedures
A laser Schlieren imaging technique was used tostudy the liquid-gas dynamics in the discharging
reactor. KrF excimer laser was used as a pulsed
light source. A needle-to-plane electrode with a 40mm gap submerged in distilled water ( 3 µS/cm)
was used. A pulsed high-voltage circuit with aMOS-FET switch was used to generate asingle-shot underwater pulsed discharge. The ICCD
camera was used to obtain Schlieren images atvarious time delays between the laser pulse and thepulsed high-voltage.
3. Results and Discussion
Fig.1 shows the typical temporal and spatial
evolution of the Schlieren images for various delay
times after the application of the high voltage pulse.
Each image presented in Fig.1 was selected from a
set of different observation at the same conditions(applied voltage 23 kV, pulse duration 10 µs, ICCD
gate time 300 ns).
At first, the appearance of opaque region with aspherical shape can be observed at the tip of theneedle electrode before the initiation of thedischarge as shown in Fig.1(a). It is considered thata phase change is occurred prior to the dischargeinitiation. One possible mechanism for theinstability initiation is modification of electric fieldwith a sharp variation in permittivity near thepowered electrode [3]. Since the change ofpermittivity or density variation leads to a change inrefractive index in water, therefore we can observe
this effect as the generation of opaque region by theSchlieren photograph. Pre-existing microscopic gas
bubbles within the opaque region may contributefor the initiation of electron avalanche, leading to
the generation of primary streamers with tree-likemorphology.
After primary streamers, secondary streamers
propagate beyond the opaque region and a few long

01020mm(a) - 30 ns(b) 600 ns(c) 4.7 µs(d) 9.0 µs(e) 17.6 µs(f) 50 µs(g) 87 µs(h) 140 µs

Fig. 1. Time evolution of Schlieren images.
(applied voltage: 23 kV, discharge current:4-6 A, needle-to-plane electrode gap: 40 mm, distilled water)

filamentary discharge channels create due to thefield induced dissociation and ionization at the tipof the streamer head [1]. The propagation velocityof the secondary streamer head is 3x104 m/s [4].
Moreover, the several shock waves appear with thestreamer propagation as shown in Fig. 1(b). The
reason for the multiple shock wave generation isdue to the step-like streamer propagation similar tothe stepped leader of a lightning.
After the propagation of secondary streamers,
the charge accumulation occurs at the interface
between gas and liquid, causing the re-illumination.
As a time elapses, the outgoing shock wave is often
generated from the tip of the needle electrode(Fig.1(c)). A spherical wave is also created at thenode of streamer branching, and by superposition of
all of the waves a complex wave pattern isgenerated (Fig.1(d)).
On the post discharge stage, tracks of streamer
propagation near the power electrode become
gaseous channels and those diameters become
thicker than that of secondary streamer (Fig.1(e)).
Although the applied pulsed voltage diminishes by~14 µs, the shock wave is still actively expanding
and then reflecting by the plane electrode as shownin Fig. 1(f). The gas channels are disrupted anddevelop into gas bubbles which grow and decay,
causing the generation of bubble wave (Fig. 1(g)).
Finally the trace of streamers fades away in timeand several disrupted bubbles are sustained in liquid,moving upward direction (Fig. 1(h)).
From these observations, Fig. 2 depicts aplausible scenario of multiple events frompre-discharge to post-discharge.
4. Conclusion
The commonly observed phenomenon ofunderwater discharge generated by a single shot
pulsedhigh-voltage
wasinvestigatedexperimentally. A definite sequence of events
starting with the generation of opaque region,
followed by the discharge initiation, theprimary/secondary streamers and re-illumination asa typical discharge mode, shock wave propagation,
localized gas channel at the trace of streamer
propagation, and eventual bubble formation at thetip of the needle electrode was observed by using alaser Schlieren technique.
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[4] S. Kanazawa, Y. Abe, Y. Kihara, M. Kocik and J.
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Fig. 2. Illustration of a scenario of multiple events from pre-discharge to post-discharge in underwater discharge.