21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Spectroscopic and shadow-graphic study of underwater corona discharge
Kunihide Tachibana1 and Hideki Motomura2
1
Osaka Electro-Communication University, Neyagawa, Osaka 572-8530, Japan
2
Ehime University, Matsuyama, Ehime 790-8577, Japan
Abstract: Corona-like streamer propagation in underwater discharge was studied using an
imaging spectrometer and an ultrafast framing camera. Temporally varying electron density
was derived from the Stark-broadened line shapes of H, H and O lines, and the medium
density inside the streamers was estimated from the change in their thickness. .
Keywords: Streamer propagation, optical emission, electron density, medium density.
1. Introduction
Underwater discharge has been attracting much interest
because of its availability in various environmental and
biomedical applications. In recent years, there appeared
are many excellent papers concerning about initial mechanisms of the underwater discharge [1-5], but we think the
mechanisms are not understood comprehensively as yet.
Under these circumstances, present work aims at further
understandings of the discharge initiation mechanisms in
water by observing the streamer propagation image together with the emission spectra from the streamers.
2. Experimental procedure and results
Underwater discharge was performed using a pin-toplane electrode configuration. We used a truncated copper
(Cu) wire cable covered with silicone rubber insulator or
a tungsten (W) pin in a Teflon rod with exposing sharpened tip as the anode and a stainless plate as the cathode
placed at a distance of 20 mm. The electrode assembly
was immersed in tap water filled in a plastic vessel. The
electric conductivity and the pH value of the water were
measured beforehand as 142 Scm-1 and 7.55, respectively. We used two types of the high voltage (HV) power
supplies with a long pulse duration of about 2.5 s (LP)
and a short pulse duration of about 250 ns (SP), of which
the maximum voltage was 30 kV and the rise time (10 to
90%) was about 50 ns.
2.1. Shadow-graphic observations
For the observation of the streamer propagation, we
used an image-intensified ultrafast digital framing camera
(ULTRA Neo, nac Image Technology Inc.). The maximum rate was 2×108 frames per second (5 ns interval)
with 12 successive frames and additional 12 frames after
10 s intermittent interval at variable rates.
Figure 1 shows the observed one-shot image of the LP
discharge with Cu electrode at the early period up to 110
ns at an interval of 10 ns. Thin brush-like streamers are
expanding from a bright ball at the electrode tip, and then
several thicker branches become more noticeable. The
expanding speed of the streamers is estimated from the
0 ns
Insulator
5.4 mm
10 ns
20 ns
30 ns
40 ns
50 ns
60 ns
70 ns
80 ns
90 ns
100 ns
110 ns
Cu wire
Fig. 1 Images of long pulse discharge taken at 10 ns interval.
0.1 s
0.6 s
1.1 s
1.6s
2.1 s
2.6 s
3.1 s
3.6 s
4.1 s
4.6 s
5.1 s
5.6 s
Fig. 2 Shadow graphic images of long pulse discharge taken
at 500 ns interval.
Fig. 3 Shadow graphic images of long pulse discharge taken
at 10 s interval.
21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Fig. 4 Collapsing behavior of streamers in later times observed with 100 s interval.
change of the lengths as about 0.35 mm/10 ns (= 35 km/s).
Figure 2 shows a series of images of the LP discharge
taken at a 500 ns interval. Combining with other images
(not shown here), it is seen that most of the streamers are
established within the first 200 ns, and then some stronger
streamers are remaining until the fall of the applied voltage. As the shadow-graph shows, the diameter of the
streamers starts to grow from about 30 m at 1.1 s to
about 150 m at 5.6 s.
Figure 3 shows the images taken at an interval of 5 s.
In actual, no discharge occurs within this period and the
apparent emission seen in several images is the artifact of
the residual images in the preceding exposures. At 55 s
the estimated thickness reaches about 500 m with the
length shortened to about a half of the maximum length.
Figure 4 shows the images taken at an interval of 100
s to see the behavior of the streamers in later times. It is
interesting to see from the images in both Figs. 3 and 4
that the streamers tends to shrink toward the anode after
50 s with increasing thickness, and finally collapses into
a large bubble, colliding onto the surface of the anode
assembly to be broken into small bubbles after 300 s.
2.2. Spectroscopic observations
For the observation of discharge streamers, we used an
imaging prism spectrometer (Andor SL100M-UV, f = 10
cm) and an intensified-CCD (ICCD) camera (Andor
iSTAR 320T) coupled with a UV camera lens (Nikon).
The spectral range from 280 to 800 nm was covered by
connecting smoothly the three different measurements
with wavelengths centered at 400, 500 (or 550) and 700
nm. The spectral sensitivity of the whole system was calibrated with a standard tungsten ribbon lamp.
Figure 5 shows the calibrated spectra measured at several timings from 80 ns (minimum delay of the system) to
1530 ns after the HV application, whose intensity scales
are adjusted individually to exhibit the characteristic
changes of the spectra (see the relative scales). At earlier
times, a large continuum background due to the black
body radiation is seen in the shorter wavelength range.
Fig. 5 Calibrated spectra in LP discharge at several timings.
Fig. 6 Change in spectral shapes of H, H and O lines observed in LP discharge at timings of 0.48, 0.98, 1.48 and
1.98 s.
Another noticeable characteristic feature is the largely
broadened spectral lines of H (657 nm), H (488 nm) and
OI (777 nm) lines, which are narrowing in later times.
Figure 6 shows the spectral broadenings of those lines
separately with normalized intensities. Although slight
asymmetries are seen in the spectral shapes of H and H
at earlier times as reported in [5], we tried to fit those
shapes with Voigt profiles and attributed those Lorentzian
components to the Stark broadening for deriving the
temporally changing electron density ne in the streamer.
The results obtained from the H shapes are shown in
Fig. 7 for the LP and SP discharges. It is noted that in both
cases the initial values of ne become more than 2×1019
cm-3 in accordance with previous results [1,5]. The decay
Fig. 7 Electron temperature in propagating streamers estimated from Stark broadenings of H line.
21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia of ne follows the respective discharge durations. A small
shoulders appearing in the decay are due to the applied
voltage waveform distorted by the reflection of the initial
peak in the magnetic pulse compression.
From the data of imaging spectral measurements we
investigated to find spatial changes of the line broadenings along the streamer length, but we were unable to see
the differences in contrast to a previous report [2]. In our
measurements, typically 10 shots were superposed in order to increase the signal level, so that the delicate changes might possibly be smeared out due to the statistical
fluctuation of the shape and the length of streamers. We
will try again to take the data in one shot with an increased sensitivity.
The A2 – X2 (0–0) band of OH radicals peaking at
308 nm was observed as seen in Fig. 5. Its relative intensity tends to increase in comparison to other emission
lines of H and O radicals. Figure 8(a) shows the shapes of
the OH band at several time steps with adjusted peak intensities. Rotational structures are not separated due to the
finite resolution of our system, but the overall shape reflects the rotational temperature Tr. By using a
free-software, LIFBASE [6], we tried to derive Tr. The
results show a value of about 7000 K as shown in Fig.
8(b), and the value does not change noticeably while the
emission lasts.
When we used a truncated copper-cable anode with silicone insulator, intense Cu lines at 324.75 and 327.40 nm
were observed in the OH spectra as well as the green lines
at 510.55, 515.32 and 521.82 nm as shown in Fig. 9 due
to the sputtering with a certain delay.
3. Discussion and remarks
By using a ultra-fast framing camera we have succeeded in the observation of the streamer propagation behavior
in a corona-like underwater discharge. A series of successive images in a single discharge phenomenon was taken
with the faster frame rate up to 2×108 frames/sec in the
early period and with much slower rates in the later period
to see the whole behavior until the collapse of streamers.
From the shadow-graph observation, the thickness of a
typical streamer grows from about 30 m at 1.1 s, at
which the emission starts to decay, to 500 m at 55 s, at
which the streamer shrinks to about one half of the length.
That means the volume of a streamer increased by about
140 times during the period. If we could assume that the
pressure and the temperature inside the streamer have
relaxed down to the atmosphere at 55 s, the medium
density in the streamer would be of the order of 3×1019
cm-3. Therefore, the initial medium density N in a streamer at 1.1 s is estimated to be about 4×1021 cm-3. This
suggests that the medium density of inside the streamer is
rarefied from the density of water about an order of magnitude.
Fig. 10 Streamer model by Katsuki et al in Ref. [1]
Fig. 8 (a) OH spectra observed with tungsten electrode at several
timings and (b) a fitted result with simulation at an assumed rotational temperature Tr of 7000 K.
Fig. 9 OH spectra in UV and visible region observed with copper
electrode at several timings.
Let us borrow the streamer model appeared in Ref. [1]
and redraw it in Fig. 10. In the model the streamer head is
cleaving the propagating channel with the transport of
ions through the frontier by the extremely high electric
field due to the space-charge field superposed with the
external field. Here, we try to assume that in the streamer
column the discharge is sustained by the ionization of the
medium (mostly water molecules). In order to obtain a
reasonable ionization rate i /N of, e.g., 2×10-21 m2, the
reduced electric field E/N in the medium should be about
1.5×10-19 Vm2 based on the Boltzmann analysis by a
free-software, LXcat [7]. If we borrow a reported value of
1×108 Vm-1 as the electric field strength E measured by
the Kerr effect [8], The medium density N is estimated as
0.7×1021 cm-3. This vale is about 1/5 of the estimated val-
21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia ue mentioned above, but consistent with each other on the
order of magnitude.
Thinking of the initial value of ne of about 2×1019 cm-3
and the medium density N of 4×1021 cm-3 which is rarefied from the density of water at the standard state by an
order of magnitude, the estimated ionization degree on the
order of 10-2 also looks reasonable.
The temperature in the streamer channel estimated from
the rotational temperature of the OH emission was about
7000 K. This is also consistent with previously reported
values of about 5000 K [2]. This causes the continuum
background ranging from UV to visible region.
In conclusion, present work has clarified some characteristic features of the streamer propagation in a single
underwater discharge with a high-speed camera observation. Form the measured spectroscopic data with an imaging spectrometer the temporal behaviors of electron
density ne and gas temperature (through the rotational
temperature Tr) have been revealed, although the spatial
differences have not been noticed. However, we need
more comprehensive and quantitative arguments by referring previous results in more detail before we grasp the
heart of the discharge mechanisms.
4. References
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[7] http://www.lxcat.laplace.univ-tlse.fr/
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