Development of Discharge in a Saline Solution at Near-Threshold Voltages
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Development of Discharge in a Saline Solution at Near-Threshold Voltages Yu. D. Koroleva, b, c, *, I. A. Shemyakina, b, V. S. Kasyanova, b, V. G. Geymana, A. V. Bolotova, and V. O. Nekhorosheva a Institute
of High-Current Electronics, Siberian Branch, Russian Academy of Sciences, Tomsk, 634055 Russia b National Research Tomsk State University, Tomsk, 634050 Russia c Tomsk Polytechnic University, Tomsk, 634050 Russia *e-mail: [email protected] Received September 26, 2017; in final form, November 28, 2017
Abstract—The development of a discharge in a point−plane gap filled with a saline solution with a salt content of 3% was studied experimentally. The duration of the voltage pulse applied to the gap was about 2 ms. Data are presented on the formation dynamics of gas microcavities at near-threshold voltages at which gasdischarge plasma appears in some microcavities. The cavities are conglomerates of microbubbles with a typical size of ≈100 μm. At the threshold voltage (≈750 V), the active electrode is covered with a gas layer and the gap voltage is in fact applied to this layer, which leads to the development of discharges in individual microbubbles. In this case, the discharge operates in the form of short current pulses. The number of microcavities filled with plasma increases as the voltage grows above the threshold value. At the plasma boundary, new microbubbles are formed, in which discharges are ignited. As a result, the plasma front propagates from the active electrode into the gap with a characteristic velocity of 103 cm/s. DOI: 10.1134/S1063780X18060053
1. INTRODUCTION In recent years, pulsed discharges in electrolytes, including those in saline solutions, has attracted considerable interest. The physics of such discharges and their various applications were considered in review [1]. Studies in this field are stimulated by biomedical applications, among which discharges excited in the physiological solution in the so-called plasma scalpels [2–6], as well as in devices for sterilization of water and liquid aerosols [1, 7–9], occupy an important place. Considerable attention has been paid to applications associated with the formation of shock waves in high-current discharges [10, 11], in particular, as applied to problems of seawater hydroacoustics [6, 12, 13]. Although the concept of discharge (or breakdown) in liquid is commonly adopted, the discharge phenomena actually occur in gas-filled cavities that form in liquid for some or another reason. Electrode systems for exciting discharges in electrolytes usually consist of a small-area active electrode and a largearea return electrode [1, 14]. In the initial stage of discharge, microbubbles and other types of gas cavities appear near and on the surface of the active electrode. Since the conductivity of electrolytes is much higher than the conductivity of, e.g., distilled water, the cur-
rent density near the active electrode immersed in electrolyte can reach high values at relatively low voltages. Therefore, gas microcavities near a small
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