Multiphysics simulation of the initial stage of plasma discharge formation in liquids

Abstract

Plasma initiation simulations in homogenous liquids in response to ∼3.0–5.0 ns voltage pulse is conducted. An in-house numerical framework consisting of a compressible fluid solver together with charged species conservation and Poisson’s equation solver is employed for the simulations. The simulations are conducted for a needle-like powered electrode with two different voltage profiles—linear and exponential increase. The model predictions show that under the influence of nanosecond voltage rise the liquid experiences the formation of negative pressure region near the vicinity of the powered electrode and surpasses the cavitation threshold pressure. The cavitation locations initiate as sub-micron regions and then extends up to a few microns. The electrical forces which is a combination of electrostatic, polarization and electrostrictive ponderomotive forces contributes significantly in cavitating the medium and forming low-density region. The ponderomotive forces have the highest impact followed by the polarization forces. The effect of electrostatic forces only become significant when sufficient free charges are formed. Despite the formation of low-density region, the ionization process is still predominantly driven by field dependent ionization—Zener tunneling; as the electric field across the sub-micron to micron scale low-density regions are not sufficient for electron impact ionization to be significant. A parametric study on maximum driving voltage and voltage profile is conducted. The results indicate that at higher voltage both the exponential and linear voltage profile form a compression wave and an associated high-density region in the medium. The magnitude of the compressive waves is not representative of shock waves. The bulk liquid velocity can reach hundreds of meters per second but maintains subsonic conditions when the maximum driving voltage is increased by a factor of 2.5–15 kV suggesting shock like conditions will be formed under higher electric field conditions.