Are microbubbles necessary for the breakdown of liquid water subjected to a submicrosecond pulse?

Abstract

Microbubble-based model analysis of liquid breakdown initiation by a submicrosecond pulse J. Appl. Phys. 97, 113304 (2005); 10.1063/1.1921338 Breakdown threshold and localized electron density in water induced by ultrashort laser pulses Electrical breakdown in homogeneous liquid water for an ϳ100 ns voltage pulse is analyzed. It is shown that electron-impact ionization is not likely to be important and could only be operative for low-density situations or possibly under optical excitation. Simulation results also indicate that field ionization of liquid water can lead to a liquid breakdown provided the ionization energies were very low in the order of 2.3 eV. Under such conditions, an electric-field collapse at the anode and plasma propagation toward the cathode, with minimal physical charge transport, is predicted. However, the low, unphysical ionization energies necessary for matching the observed current and experimental breakdown delays of ϳ70 ns precludes this mechanism. Also, an ionization within the liquid cannot explain the polarity dependence nor the stochastic-dendritic optical emission structures seen experimentally. It is argued here that electron-impact ionization within randomly located microbubbles is most likely to be responsible for the collective liquid breakdown behaviors.

Microbubble-based model analysis of liquid breakdown initiation by a submicrosecond pulse

“…We have also demonstrated through Monte Carlo calculations that freeelectron transport and possible electronic impact ionization in bulk water would be virtually absent. 21 The numerical analysis quantitatively demonstrated that strong elastic scattering coupled with the highly isotropic angular deflections work to prevent electrons from picking up the requisite ionization energy. As a result, neither electron-initiated charge creation within the homogeneous bulk nor the electronic injection process at the cathode can have much relevance for breakdown.…”

Initiation process and propagation mechanism of positive streamer discharge in water

“…35 In the case of sub-microsecond discharges, it has been proposed that streamer propagation is triggered by field emission at the interface of pre-existing microbubbles, in which electron impact ionization occurs. 36,37 In the case of microsecond or longer pulsed discharges, a bubble theory associated with thermal effects is most likely to be the main mechanism. A theoretical model has shown that Joule heating leads to the evaporation of liquid and the formation of gas bubbles, in which an electrical discharge propagates.…”

On the modeling of the production and drift of carriers in cyclohexane

“…The attachment loss term A is usually defined in the literature [9][10][11] through the attachment rate  a as:…”

“…In order to perform a more detailed analysis of the field emission at the cathode in that experiment, the model described in the previous section is used. As in the literature [9][10][11], the negative ion mobility  n is taken as the average electrohydrodynamic (EHD) mobility (10 -7 m 2 V -1 s -1 [5]) while the attachment rate constant  a is taken from the measured zero-field lifetime of electronic charges (500 ns for cyclohexane [5]). As shown in Fig.…”

“…This mechanism uses directly the Zener theory of tunneling in solids [8] to estimate the generation of electrons and positive ions in the liquid phase. Even though several parameters in Zener's theory are not known for liquids, recent publications have recently used this mechanism as the base for numerical models of breakdown [9][10][11]. However, there is little experimental evidence as to directly support the validity of this mechanism of charge generation.…”