S-R4-2
Investigation of streamer inception and branching conditions
using an ultra fast ICCD camera
ICCDカメラを用いた高時間分解計測によるストリーマ放電の開始・分枝条件の研究
Eiichi Takahashi
高橋栄一
National Institute of Advanced Science and Technology (AIST)
1-2-1 Namiki, Tsukuba Ibaraki 305-8564, Japan
産業技術総合研究所 〒305-8564 つくば市並木1-2-1
A recently developed ultrafast camera that can acquire 108 frames per second was used to investigate
streamer discharge. It enabled single-shot evaluation of streamer evolution without the need to consider
shot-to-shot reproducibility. This camera was used to investigate streamers in argon. Growing branches,
the transition when a streamer forms a return stroke, and related phenomena were clearly observed.

1. Introduction
Discharges are highly transient, probabilistic
phenomena and thus ultrafast instruments are
required to investigate them. In the past decade, thedevelopment of gated intensified CCD (ICCD)
cameras has enabled such phenomena to beobserved with a temporal resolution on the order ofnanoseconds [1]. Streamer tips propagate like tiny
spheres in air. Their morphology depends on factors
such as the impedance of the high-voltage discharge
circuit and the gas species.
In a previous study, we observed branchedstreamers in argon using an ICCD camera and weinvestigated the effect of preionization of thebackground gas on branching phenomena [2]. We
found that a background electron density of ne~105/cm3 suppresses branching.
In the present study, we used a recently developed
ICCD camera with a multi-frame capability (UltraNeo, NAC Inc.) to investigate such phenomena [3].
The optical system of this ICCD camera is based ona kaleidoscope. It can acquire more than 108 frames
per second and it has a high spatial resolution. It isa very promising camera for investigating ultrafast
probabilistic phenomena such as transientdischarges. We chose to investigate the detailed
evolution of positive streamers in argon because
discharges in argon exhibit very complex branchedstructures and because it is difficult to determine
the velocity of streamers in argon by single-shotmeasurements. Streamer tracks in argon remain
illuminated during the lifetime of the streamer. In
contrast, in nitrogen or air only the streamer tip
emits light and only for a very brief time.
Consequently, the tip velocity can be estimated bymeasuring the length of the light measured by an
ICCD camera with a certain gating time.

Fig. 1. (a) Growing positive streamer in argon taken by thefast-framing camera. Gate width of each image is 5 ns. Frame
number is shown at upper-left of images. Applied voltage andpressure were 21 kV and 1 atm, respectively. Makings show A:
stopped tip, B: growing tip, C: reconnecting streamer, D: finaljump. (b) Waveform of voltage and gating-duration.
12-pictures were taken in this duration. (c) Velocity distributionof the most elongated streamer tip.

2. Observation of growing streamer andsimilarity of streamer and other surface growthphenomena
Examples of growing streamer in argon observedby the fast-framing camera are shown in Fig. 1 and
2. Needle-to-plane electrode was used. The distance
between the electrodes was 3 cm. Positive
high-voltage pulses were applied to the needle
electrode for the case of positive streamer.
As noted on the figure 1, one of the tip wasstopped growing (region A) while another tip closer
to cathode kept on growing (region B). Immediately
after the first streamer reached to cathode, it turns
into arc mode. Remaining streamers closer to thearc channel were attracted to it and reconnected(region C). Final acceleration of the streamer
propagation was observed at region D [4]. The
velocity reaches almost 3 times faster than before(Fig.1 (c)). The formation of short circuit observed
at 7-th frame, this corresponds to the voltage dropin the Fig. 1(b).

extending branches while the positive streamer inargon shows very detailed branched structure. The
shape of the positive streamer is closer to DLA
phenomena [5]. The shape of the negative streamer
is rather similar to viscous fingers [6]. The
propagation velocity is almost half compared to thevelocity of positive streamer. The reconnection andfinal jumps are also observed for negative streamer.
The observed shielding effect in the positive
streamer is one of the features of the DLA.
However the observed reconnection behavior is notobserved in the DLA. The reconnection can be
explained by the model proposed by Dawson [7]:the model assumed that the streamer tips are
insulated from the anode and that tips can propagate
in zero-field strength by transferring electrostatic
energy to the next avalanche.
The origin of the self-similar structure in streamer
was discussed by some authors. The similarity ofdielectric barrier discharge on surface to DLA waspointed out by Niemeyer [8]. They showed that thedischarge structure has a Hausdorff dimension of
1.7 which is closer value for the DLA phenomena.
The possibility of Laplacian instability was also
shown by ref [8]. However, the theory to explain
the very diverse morphology of the discharge has
not been built yet.
6. Summary
The temporal evolution of streamers in argon wasinvestigatedbyperforming
single-shotmeasurements using a recently developed ultrafast
framing camera. The morphology is discussed bycomparing with the other surface growing
phenomena.
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2169[2] E. Takahashi, et. al., J. Phys. D, 44 (2011) 075204.
[3] E. Takahashi, et al. , J. Phys. D, 44 (2011) 302001.
[4] F. Grange, et. al., .J. Phys. D 28 (1995) 1619.
[5] P. Meakin, Phys. Rev. B 28 (1983) 5221
[6] D. Bensimon, Rev. Modern Phys., 58 (1986) 977[7] G. A. Dawson, et. al., Zeitschrift fur Physik A
Hadrons and Nuclei, 183 (1965) 159.
[8] L. Niemeyer, et. al., 52 (1984) 1033.
[9] M. Array’s et. al., Phys. Rev. E, 69 (2004) 036214.

Fig.2. Growing negative streamer in argon. Gate width was 10
ns. Other experimental conditions were same for Fig.1.

The negative streamer in Fig.2 shows differentbehavior. The negative streamer has more