22nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium

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Diagnostic of underwater discharge based on pin-hole configuration

V. Mazankova1, F. Krcma1 and B. Obradovic2

1 Faculty of Chemistry, Brno University of Technology, Purkynova 118, Brno 612 00, Czech Republic 2 University of Belgrade, Faculty of Physics, Studentski trg 12, 11000 Belgrade, Serbia

Abstract: A novel plasma jet based on generation of so-called pin-hole discharge is presented. The DC jet of both polarities in NaCl solutions of various conductivities was used for the presented study. The plasma rotational temperature, electron temperature and electron concentration were calculated from the discharge emission spectra in axial resolution.

Keywords: underwater discharge, plasma jet, optical emission spectroscopy

1. Introduction

Electrical discharges in liquids have been subject of many studies during the last years mainly focused on the formation of a conductive channel in the discharge gap filled with liquid, its diagnostic and various potential applications [1-4]. Nowadays, discharges in liquids are being studied extensively also for advanced chemical, biotechnology as well as medical applications. The new promising direction in application fields ranges chemical synthesis [5], surface activation and cleaning [6] and medical applications and sterilizations [7].

The creation of electric discharges in liquids is very complex and it is not fully understood, because they are operating under extreme conditions. Generally, there are two groups of theories describing the discharge ignition in liquids, electron and thermal (also called as bubble) theory [8]. The electron theory is based on the fact that water molecules are ionized and dissociated by the applied very high electric field, and plasma creation is more or less analogical to the Townsend’s theory of electron avalanches in gases. According to the thermal theory, liquid is heated by passing current which leads to its evaporation and bubble (or more exactly micro bubble) formation. Subsequently, the discharge is ignited in the gaseous phase inside the bubbles due to the potential gradient over the bubble size.

Another specificity of underwater discharges is their configuration. The most studied configuration is a point-to-plate electrode geometry [9] where DC high voltage up to tens of kV is applied on the small tip immersed into the grounded liquid. The coaxial configuration [10] is a modification of the previous one, and it is more suitable for water treatment applications in a flowing regime. The pin-hole systems where the discharge is created inside a small orifice connecting two chambers filled by any conductive solution (each chamber contains one of the electrodes) were intensively studied, too [11-13]. Besides the DC pulsed high voltage, also AC, high frequency, microwave or DC non-pulsed voltage regimes can be used for the generation of this kind of the under liquid discharge.

In the work presented here, we investigate the operation of an underwater discharge jet based on pin-hole configuration that was very recently developed by our research team. The electrical measurement, fast camera imaging and the optical emission spectroscopy were performed. 2. Experimental set up

The plasma jet based on the DC pin-hole configuration of underwater discharge was used for this experimental study. A simplified schematic drawing of the used experimental setup and its photo are given in Fig. 1.

Fig. 1. Schematic drawing of the plasma jet (left) and reactor with installed plasma jet (right). 1 – ceramic body, 2 - inner tungsten wire electrode (diameter of 0.6 mm), 3 – space with increased current density, 4 – outer copper electrode, 5 – silicon insulation.

A specially constructed discharge reactor (total volume

of 250 ml) was used in the study of the pin-hole discharge formation, see in Fig. 1 (right). There are two quartz glasses (diameter of 40 mm) in opposite walls of polycarbonate reactor chamber. The plasma jet consists of copper envelope as an outer grounded electrode (diameter of 12 mm), dielectric body from Macor ceramics and tungsten high voltage inner electrode (diameter of 0.6 mm). Distance between the inner electrode tip and the

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nozzle end in the ceramics is adjustable. The DC jet of both polarities (mean voltage of 650 V and mean current 0.12 A) in NaCl solutions of various conductivities (150-1200 μS/cm) was applied. The jet position was vertically adjustable using micro screws, with accuracy better than 0.1 mm.

The optical spectra were recorded by Jobin Yvon monochromator TRIAX 550 with CCD detector. A 300 g/mm grating was used for overview spectra acquisition in the range of 400–850 nm; the 3600 gr/mm grating was applied for Hβ line profile acquisition as well as for the well resolved OH radical A-X transition. The light emitted from the discharge was focused by quartz lens (f=100 mm) on the small orifice (diameter of 0.5 mm) mounted just at the front of the optical multimode quartz optical fibre leading light on the monochromator entrance slit. The emission spectra were recorded in axial resolution better than 0.5 mm. The atomic hydrogen (Hα, Hß, Hγ), oxygen and sodium spectral lines and OH bands were recorded in all spectra (Fig. 2). The plasma rotational temperature was calculated according Boltzmann plot technique using well resolved OH radical spectra. Electron temperature was calculated from Hβ and Hγ line intensities. The electron concentration was calculated using Stark broadening of Hβ line profile and simple deconvolution procedure. The mean energy dissipated in the system was calculated using integration of current and voltage over 0.1 s to eliminate discharge instabilities and its self-pulsing character (see Fig. 3).

Fig. 2. Overview spectrum of the plasma jet in NaCl solution. 3. Results

Fig. 3 shows typical waveforms of discharge voltage and the current measured under conditions used in this work in the both polarities. A positive polarity means that inner electrode is as anode and water solution was grounded, the positive high voltage was applied on liquid and inner electrode was grounded under negative polarity. In contrary to normal DC non pulsing voltage supplied pin-hole discharges [14] there are appeared high current peaks with very short duration. These pulses are also accompanied by very strong acoustic shock waves

generation. The energy supplied into plasma during these peaks also results in high drop of the discharge voltage. Thus, the mean electrical power was calculated as a quantity characterizing the discharge. Also due to this fact, all the spectroscopic measurements were carried out with integration times at least 0.1 s to minimize these strong discharge instabilities. Fig. 4 presents dependencies of calculated mean electrical power at the constant mean current of 150 mA (stabile discharge operation) on solution conductivity for both polarities. The electrical power is decreasing with the conductivity increasing at both polarities, as it was expected, and significantly higher power consumption is observed if discharge negative polarity is used.

Fig. 5 presents the mean (i.e. average over whole discharge volume) electron concentrations calculated from Hß line profile. The Doppler broadening was supposed nearly the same in all cases because rotational temperature obtained from OH radical spectra is slightly dependent of the discharge conditions, only, as it is demonstrated by Fig. 6. The electron concentration decreases with conductivity increasing, but the concentration in positive polarity is significantly lower mainly at the higher solution conductivities.

Fig. 3. Current and voltage wave forms of the plasma jet in NaCl solution (500 μS/cm) in the positive polarity (top) and negative polarity (bottom) of inner electrode.

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Fig. 4. Dissipated power of the plasma jet in NaCl solution.

Fig. 5. Mean electron density of the plasma jet in NaCl solution.

Fig. 6. Axial profiles of plasma rotational temperature at the selected discharge conditions.

Besides the global characteristics shown in Figs 3-5, the

axially resolved studies were carried out at the selected conditions. The discharge appearance strongly depends on the jet polarity as it is demonstrated by photos shown in Fig. 7. The discharge dimension is much higher in the negative polarity and also its overall brightness is higher.

The yellow colour in negative polarity is due to much stronger emission of sodium doublet at 589 nm.

Fig. 7. Photos of plasma jet in positive polarity (left) and negative polarity (right).

The OES spectra were measured in axial resolution. The intensity profiles for OH radical integral intensity and Hβ line are shown in Figs 8 and 9, respectively for the selected discharge conditions. These graph clearly demonstrate that maximum emission intensity is in higher distance from the nozzle tip in the negative polarity with nearly no conductivity influence. Also, the OH radical emission is visible from much larger volume that emission of atomic hydrogen.

Fig. 8. Axial profile of OH intensity for selected conductivities.

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Fig. 9. Axial profile of Hβ intensity for selected conductivities.

Probably the most interesting observation is demonstrated in the Fig. 8 and Fig: 9. While OH radical emission decreases with the solution conductivity increase in negative polarity, the opposite and even much stronger effect is visible in the case of Hβ

line intensity. All these effects will be studied in more detail very soon because the spatial distribution of discharge created active species is critical with respect to future applications. 4. Conclusion

The presented contribution gives the first results of electrical and OES measurement carried out on the newly developed plasma jet generated in liquids based on the pin-hole configuration. The results demonstrate mainly the strong dependence of the discharge characteristics on the jet polarity; the influence on the solution conductivity is pointed, too. The formation of active species like OH radicals and atomic hydrogen is studied in axial resolution and shows that maximal presence of these species is spatially non-uniform and depends on the species kind. The rotational temperature of plasma is nearly independent on the discharge conditions. The electron concentration increases with the solution conductivity however the dissipated power is decreasing. Much higher ionization is observed in the negative discharge.

Due to the strongly self-pulsing discharge operation, more experiments using time resolved imaging are planned for the near future to be able to understand fully the discharge creation and propagation as well as to determine production of the active species.

References [1] P. Sunka, V. Babicky, M Clupek, P. Lukes, M. Simek, J. Schmidt and M. Cernak Plasma Sourc. Sci. Technol., 8, 258 (1999) [2] P. Lukes, M. Clupek, P. Sunka, F. Peterka, T. Sano, N. Negishi, S. Matsuzawa and K. Takeuchi Res. Chem. Intermed., 31, 285 (2005) [3] K. R. Stalder, D. F. McMillen and J. Woloszko J. Phys. D: Appl. Phys. 38, 1728 (2005) [4] J. E. Foster, B. Weatherford, E. Gillman and B. Yee Plasma Sourc. Sci. Technol,. 19, 025001 (2010) [5] A. Hamdan, J. N. Audinot, S. Migot-Choux, C. Noel, F. Kosior, G. Henrion and T. Belmonte Adv. Eng. Mater., 15, 885 (2013) [6] G. Neagoe, O. Galmiz, A. Brablec, J. Rahel and A. Zahoranova Chem. Listy, 106, S1471 (2012) [7] A. I. Maksimova, I. K. Naumovab, and A. V. Khlyustovaa Energy Chemistry, 46, 212 (2012) [8] H. Fujita, S Kanazawa, K. Ohtani, A. Komiya, T. Kaneko and T. Sato J. Appl. Phys., 116, 213301 (2014) [9] M. J. Kirkpatrick and B. R. Locke, Industr. Engineer. Chem. Res., 44, 4243 (2005) [10] P. Sunka, V. Babicky, M. Clupek, M. Fuciman, P. Lukes, M. Simek, J. Benes, B. R. Locke and Z. Majcherova Acta Phys. Slovaca, 54, 135 (2004) [11] M. Sato, Y. Yamada and A. T. Sugiarto, Trans. Inst. Fluid-flow Machin., 107 95 (2007) [12] F. De Baerdemaeker, M. Simek, J. Schmidt and C. Leys Plasma Sourc. Sci. Technol., 16 341(2007) [13] Z. Stara, F. Krcma, M. Nejezchleb and J. D. Skalny, Desalination, 239, 283 (2009) [14] Z. Kozakova, F. Krcma, M. Vasicek, L. Hlavata, L. Hlochova, Eur. J. Phys. D (2015) – accepted Acknowledgments This work was done under COST Action TD1208 STSM and it was also supported by Czech Ministry of Education, Youth and Sports under project No. LD14014.