Initial processes of a pulsed micro-arc discharge between wire-to-plane electrodes in liquid

A lot of microdischarges in liquid are proposed and studied. It is a new frontier of plasma and is expected as a peculiar reaction field. Discharge process in electric discharge machinings (EDMs) is related to the physics of the pulsed micro-arc discharge in liquid. EDMs are well-known machining techniques developed since more than sixty years. However, the microdischarge between a workpiece and a wire electrode is not fully understood, because it includes the stochastic and transient processes appearing in a short period. In this paper, we characterize the initial stage of pulsed micro-arc discharges in liquid. A thin copper wire electrode was placed above a tungsten plate electrode with a separation of 200 mum in water To observe a single pulsed micro-arc discharge, the laser shadowgraphy was used. If a pulsed positive voltage was applied to the wire electrode, the electric breakdown was initiated from its surface with a short delay time of 1-2 mus. Then, the discharge developed toward the tungsten plate cathode. The discharge did not form a dendritic shape, which was observed often in the electrical breakdown in liquid, but an agglomerated structure in shadow images. When the shadow region reached to the tungsten plate electrode, the discharge channel was formed and expanded in the radial direction. This process produced the discharge time lag. When the applied voltage was not high enough for creating the discharge channel, the agglomerated structure disappeared in the middle region between the electrodes. When the polarity of the electrodes was reversed, the discharge developed from the wire cathode. The discharge time lag depends on the breakdown process and discharge position on the surface of the plate electrode. Consequently, if the time lag for the breakdown is examined in detail, machining mechanisms in the EDM will be clarified.