Propagation of a surface discharge on water is evaluated using simulation results of the voltage/current waveforms and discharge contact area. Voltage/current waveforms are calculated using an exponential function which assumes the resistance of water decreases with increasing discharge contact area. A conductive disk having a given potential is used to model the discharge contact area and the temporal variation of the radius of the disk is discussed as the discharge propagation. The calculation in the current field is replaced by a calculation of the electrostatic field using the similarity between current and electrostatic fields. The calculation of the electrostatic field is conducted by a charge simulation method. The discharge phenomena are classified into two stages which are the breakdown of needle-to-water gap and the surface discharge propagation on water. The electrostatic field calculation at needle-to-water gap is performed to determine the initial discharge contact area. Expansion of the contact area in the creepage direction, is evaluated by the electric field calculation at the edge of the conductive disk. The expansion terminates when the field becomes lower than 26 kV/cm. The velocity of the expansion increases with the applied voltage. The maximum contact radius increases with decreasing conductivity of water under the same applied voltage. The tendency of simulation results of the expansion of the contact area are consistent with the previous observation results of the discharge propagation.
Pulsed surface discharge on water has been given attention as a water purification technology and material synthesis method. The contribution of this study is to clarify the plasma state of a positive pulsed surface discharge on water. Although the pulse width of the current dramatically decreased with an applied voltage ranging from 10 to 20 kV under a conductivity of water of 1 mS cm−1 and a water depth of 2 mm, the plasma state in the vicinity of the needle electrode was a local thermodynamic equilibrium (LTE) state. Validation of the LTE was performed by the McWhirter criterion. On the other hand, the propagating discharge along the water surface was supposed to be in non-LTE state. Consequently, the pulsed surface discharge on water was in mixed states of LTE and non-LTE.
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