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Filamentary plasma discharge inside water : initiation and propagation of a plasma in a dense medium

Paul Ceccato
Abstract : This thesis presents an experimental study of a filamentary microplasma discharge inside liquid water. Such plasmas are used for liquid electrical insulations tests and for pollution control of water. Plasmas inside dense media are less understood than discharge inside gases. The purpose of the present thesis is to understand the physical mechanisms responsible for initiation and propagation of the discharge. A point to plane electrode configuration submerged in water has been constructed and was submitted to a high voltage pulse. Filaments inception and propagation and several discharges modes have been characterized with electrical measurements and time resolved nanosecond imaging. A Shadow diagnostic using 2 iCCDs was implemented to study the gas content and the shock wave emission from the discharge. The influence of the applied voltage polarity and the water conductivity was investigated. Spectroscopic measurements were performed on the OH emission band and the hydrogen emission lines. At positive high voltage the growth of the discharge begins by the nucleation of a microbubble at the needle electrode within a few microseconds at an applied voltage of 40kV, a hemispheric tree like filamentary structure grows at 3km/s during 100ns and is followed by the propagation of second filamentary structure ten time faster. This continuous propagation on a nanosecond time scale is followed by a stepwise propagation in case of distilled water. When the filaments reach the opposite electrode electrical breakdown occurs. At negative polarity the discharge is much slower 600m/s. The morphology of the gas cavity is driven by interface instability. Curiously, water conductivity has no influence at positive voltage polarity and even inhibits the propagation of the plasma filaments at negative voltage polarity. This thesis made possible to achieve a better understanding of the detailed phenomenology of electrical discharges in water.
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Submitted on : Monday, January 11, 2010 - 8:00:00 AM
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Paul Ceccato. Filamentary plasma discharge inside water : initiation and propagation of a plasma in a dense medium. Engineering Sciences [physics]. Ecole Polytechnique X, 2009. English. ⟨pastel-00005680⟩

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