spectroscopic and shadow-graphic study of underwater ... · streamers are remaining until the fall...

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

1Osaka Electro-Communication University, Neyagawa, Osaka 572-8530, Japan

2Ehime 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 mech-anisms 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 to-gether with the emission spectra from the streamers.

2. Experimental procedure and results

Underwater discharge was performed using a pin-to- plane 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 sharp-ened 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, respective-ly. 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 maxi-mum 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

Fig. 1 Images of long pulse discharge taken at 10 ns interval. 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.

0 ns 10 ns 20 ns 30 ns

40 ns 50 ns 60 ns 70 ns

80 ns 90 ns 100 ns 110 ns

Insulator5.4 mm

Cu wire

0.1 s 0.6 s 1.1 s 1.6s

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. 4 Collapsing behavior of streamers in later times ob-served 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 volt-age. 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 cal-ibrated with a standard tungsten ribbon lamp. Figure 5 shows the calibrated spectra measured at sev-eral 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 ob-served 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 esti-mated from Stark broadenings of H line.



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 broaden-ings 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 or-der to increase the signal level, so that the delicate chang-es 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 in-creased sensitivity. The A2 – X2 (0–0) band of OH radicals peaking at 308 nm was observed as seen in Fig. 5. Its relative inten-sity 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 in-tensities. Rotational structures are not separated due to the finite resolution of our system, but the overall shape re-flects 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 sil-icone insulator, intense Cu lines at 324.75 and 327.40 nm

Fig. 8 (a) OH spectra observed with tungsten electrode at several timings and (b) a fitted result with simulation at an assumed rota-tional temperature Tr of 7000 K.

Fig. 9 OH spectra in UV and visible region observed with copper electrode at several timings.

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 succeed-ed in the observation of the streamer propagation behavior in a corona-like underwater discharge. A series of succes-sive 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 stream-er 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 mag-nitude. Fig. 10 Streamer model by Katsuki et al in Ref. [1] 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-



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 rare-fied 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 charac-teristic features of the streamer propagation in a single underwater discharge with a high-speed camera observa-tion. Form the measured spectroscopic data with an im-aging 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 refer-ring previous results in more detail before we grasp the

heart of the discharge mechanisms. 4. References

[1] T. Namihira, S. Sakai, T. Yamaguchi, K. Yamamoto, C. Yamada, T. Kiyan, T. Sakugawa, S. Katsuki and H. Akiyama, IEEE Trans. Plasma Sci. 35, 614 (2007).

[2] W. An, K. Baumung and H. Bluhm, J. Appl. Phys. 101, 053302 (2007).

[3] J. F. Kolb, R. P. Joshi, S. Xiao and K. H. Schoenbach, J. Phys. D: Appl. Phys. 41, 234007 (2008).

[4] P. Bruggeman and C. Leys, J. Phys. D: Appl. Phys. 42, 053001 (2009).

[4] P. H. Ceccato, O. Guaitella, M. R. Le Gloahec and A. Rousseau, J. Phys. D: Appl. Phys. 43, 175202 (2010).

[5] M. Simek, M. Clupek, V. Babicky, P. Lukes and P. Sunka, Plasma Sources Sci. Technol. 21, 055031 (2012).

[6] http://www.sri.com/engage/products-solutions/ lifbase/

[7] http://www.lxcat.laplace.univ-tlse.fr/ [8] G. S. Sarkisov, N. D. Zameroski and J. R. Wood-

worth, J. Appl. Phys. 99, 083304 (2006).

spectroscopic and shadow-graphic study of underwater ... · streamers are remaining until the fall...

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