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2013 ВЕСТНИК САНКТ-ПЕТЕРБУРГСКОГО УНИВЕРСИТЕТА Сер. 4. Вып. 4
КРАТКИЕ НАУЧНЫЕ СООБЩЕНИЯ
UDC 533.9.02+533.98
A. M. Astafiev, S. A. Gutsev, A. A. Kudryavtsev
STUDY OF THE DISCHARGE WITH AN ELECTROLYTIC ELECTRODE (GATCHINA'S DISCHARGE)*
Atmospheric pressure DC discharges with liquid electrodes were previously studied in [1-4]. The formation of regular spots has been reported on the surface of both liquid and metal anodes under similar conditions [3]. In our work we study glow discharges between a low-conducting liquid surface and a pin electrode of metal or carbon. Distilled water, tap water, or aqua solution of baking soda or nitric acid poured to a glass jar served as a high-voltage liquid electrode. A carbon or metal rod fixed on a dielectric handle served as a grounded electrode, which could removed relative to the water surface. A DC voltage up to 25 kV was connected through a limiting resistor 40-540 kOhm to the discharge electrodes with a possibility of polarity switch to provide a maximum current up to 200 mA. Electric parameters of the discharge, signals from the electric probes placed under the water surface and radiation intensity in the visible spectral range were measured using a video recording through optical light filters. Substantial differences have been observed between the discharges with different polarity of electrodes.
By moving the pin electrode towards the water surface, we first observed the occurrence of a corona discharge together with a deformation of the water surface caused by electrostatic forces and the ionic wind. Then, streamer breakdown and an establishment of a stationary discharge took place. Depending on the waterconductivity, various ordered structures were formed of circles, rings, wheel spokes and their combination were formed at the plasma-liquid interface (see Fig. 1).
We observed that a characteristic feature of our discharges was a uniform current distribution at low current densities 10_1 A/cm2) on the surface of the liquid anode. In the case of opposite polarity, the glow discharge was established with a normal current density
100 A/cm2) on the surface of the liquid cathode [6]. We clarified the nature of the
Aleksandr Michailovich Astafiev — PhD student, Saint Petersburg State University; e-mail: astafev-aleksandr@yandex.ru
Sergei Anatolievich Gutsev — PhD, Saint-Petersburg University ITMO; e-mail: gsa_ges@mail.ru
Anatoly Anatolievich Kudryavtsev — Associate Profesor, Saint Petersburg State University; e-mail: akud53@mail.ru
* По материалам международного семинара «Collisional processes in plasmas and gas laser media», 22—24 апреля 2013 г., физический факультет СПбГУ.
Семинар был проведён при софинансировании фондом «Династия».
© A.M. Astafiev, S. A. Gutsev, A. A. Kudryavtsev, 2013
nuUDI
Fig. 1. Photos of glow discharge with rod carbon cathode (from left to right): the anode — distilled water, coal cathode; the anode — tap water, coal cathode; the cathode — tap water, coal anode; the cathode — water with baking soda addition, aluminium anode
current distribution with the help of additional experiments. To compare the strength of the electric field in the positive column of the discharge with that in the thin near-surface layer, the dependence of the voltage on the air gap between the carbon cathode and the water surface was measured for two values of the current: 140 and 44 mA (see Fig. 2). The anode was placed at the bottom of the jar. The voltage drop inside the water was defined with the electrode at the bottom of the jar and proved to be 227.3 V for the large current and 57 V for the small one. Then, the anode drop extrapolated to a zero gap, was found to be 515 and 610 V, correspondingly. If we assume the thickness of the dark area at the large current to be equal to 2 mm, then the voltage drop in the near-surface layer will be 571 V. So, we estimated the electric field strength near the anode about 2.85 kV/cm, which on 10 times larger than in the bulk of the plasma column.
>
2200-, 20001800-
& 1600 -f
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IS 1200 -f
g 1000-
800 600 400 -200 0
s
1.5 Gap, cm
Fig. 2. Voltage drop between electrodes (air+water) for two currents of 44 (1 ) and 140 mA (2): a AU is the voltage drop in the anode region; for curve 1 V = 742.4 + 279.2 V/cm, AU a = 610 V, for 2 V = 666.7 + 506.5 V/cm, AUa = 515 V
Increasing the interelectrode distance (up to 5 cm) led to discharge instabilities of different nature depending on the current and voltage. In the optimum mode, the anode spot
2
started moving along the water surface under the influence of a steam jet and new spots could arise at smaller distances from the cathode — see Fig. 3 (and Fig. 4 for the cathode). During a secondary breakdown, the field strength was many times lower than that at the primary breakdown. Besides breakdowns from the water surface, breakdowns between different parts of the curved plasma column were also observed. The speed of the breakdown movement was about 100 m/s depending on the discharge current and the gap length.
Full understanding of the physical processes in the anode region of glow discharges is still lacking even for the classical discharges with metal electrodes. Both positive and negative potential fall in the anode region have been observed. Analysis of the experimental data shows that in our case, positive potential drop and monotonically increased electric field occur near the anode. Under these conditions, the simple theory proposed in Ref. [5] can be applied to explain the formation of the anode spots.
Fig. 3. The motion of the anode spot over the water surface and the appearance of a new current channel
Fig. 4. The motion of the cathode spot over the water surface and the appearance of a new current channel
In the anode region, a two-dimensional redistribution of the electric potential takes place between the equipotential anode and positive column plasma. The radial electrostatic potential in the positive column is formed due to ambipolar diffusion. Thus, the potential drop in the anode region increases with radius, which favoursthe additional off-axis ionization on the anode surface and the formation of luminous rings on the anode. These rings break into separate spots with increasing currents as observed in the classical glow discharges [5].
References
1. Bruggeman P., Leys C. Non-thermal plasmas in and in contact with liquids 2009. Vol. 42. 053001.
J. Phys. (D).
2. Verreycken T., Bruggeman P., Leys C. Anode pattern formation in atmospheric pressure air glow discharges with water anode //J. Appl. Phys. 2009. Vol. 105. 083312.
3. ShiraiN., IbukaS., IshiiS. Self-organization pattern in the anode spot of an atmospheric glow microdischarge using an electrolyte anode and axial miniature helium flow // J. Appl. Phys. 2011. Vol. 2, N 3. P. 2652-2653.
4. Arkhipenko V. I., Safronau Y. A., SimonchikL. V., Tsuprik I. M. Self-organization of the anode spots and fluctuations of dc glow discharge parameters in atmospheric pressure helium // ESCAMPIG XXI. Viana do Castelo, Portugal. 2012. P. 56.
5. Golubovskii Y. B., Kolobov V. I., al-Hawat S. K. Anode region of low-current glow discharges at low and high pressures // Sov. Phys. Tech. Phys. 1990. Vol. 35. P. 747-749.
6. EmelinS. E, Astafiev A. M., Pirozerski A. L. Investigation of space-time structure of the discharge with an electrolytic anode and face-type, air half-space directed cathode (Gatchina's discharge) // Proc. ISBL-08. Kaliningrad, 2008. P. 42.
Статья поступила в редакцию 22 апреля 2013 г.
