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.

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The role of negative hydroxyl ions in liquid-phase plasma discharge formation is investigated using an inhouse modeling framework. Two tunneling sources for electrons are considered-tunneling ionization of water molecules and tunneling detachment of negative hydroxyl ions together with additional reaction steps. The simulations are conducted for a needle-like powered electrode with two different nanosecond rise time voltage profiles-a linear and an exponential rise. Both the profiles have a maximum voltage of 15 kV. The predictions show that the electron detachment, which has a much lower threshold energy requirement, provides a stream of electrons at low applied voltage during the initial rise time. The electrical forces from the electron detachment process generate stronger compression but a weaker expansion regime in the liquid resulting in ∼40% increase in the density and only ∼1% decrease. The electron detachment tunneling process is found to be not limited by the electric field, but rather by the availability of negative hydroxyl ions in the system and ceases when these ions are depleted. The tunnel ionization of water molecules forms the electron wave at a higher applied voltage, but the resulting peak electron number density is typically six orders of magnitude larger than the detachment tunneling. The higher electron number density allows the recycling of depleted negative hydroxyl ions in the system and can reestablish tunneling detachment. In addition, the system experiences a larger variation in density; specifically, a decrease in density due to tunnel ionization. The prediction also shows that irrespective of the initial electron sources (i.e. tunnel ionization or tunnel detachment) the reduced electric field is not sufficient enough to allow electron impact ionization to be active and make a significant contribution. Path flux analysis is conducted to determine the kinetics responsible for the recycling of the negative hydroxyl ions.

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