Electrical breakdown simulations are carried out for liquids in response to a sub-microsecond (∼100–200 ns) voltage pulse. This model builds on our previous analysis and focuses particularly on the polarity effect seen experimentally in point–plane geometries. The flux-corrected transport approach is used for the numerical implementation. Our model adequately explains experimental observations of pre-breakdown current fluctuations, streamer propagation and branching as well as disparities in hold-off voltage and breakdown initiation times between the anode and cathode polarities. It is demonstrated that polarity effects basically arise from the large mobility difference between electrons and ions. The higher electron mobility leads to greater charge smearing and diffusion that impacts the local electric field distributions. Non-linear couplings between the number density, electric field and charge generation rates then collectively affect the formation of ionized channels and their temporal dynamics.
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.
In this paper we present the results of spectroscopic studies on mass selected Sr+(H2O)n, n=1–4 and Sr+(D2O)n, n=1–6 clusters. Mass spectra of nascent clusters formed in our laser vaporization source show that hydrated metal ion species are predominant for n⩽4. Clusters larger than this size are more abundant in the hydrogen loss form SrOH+(H2O)n−1. The cluster size at which the product switching occurs is slightly larger (n=5) in the deuterated species. Photodissociation of all clusters results in both ligand loss and H/D atom loss occurring via an intracluster reaction. The monomer and dimer cluster species exhibit distinct absorption bands attributable to electronic excitation of the 5s valence electron of Sr+. Metal–ligand stretching frequencies are extracted from Franck–Condon progressions in the excited state. Spectroscopic parameters agree well with ab initio calculations [Bauschlicher et al., J. Chem. Phys. 96, 4453 (1992), and Sodupe et al., Chem. Phys. Lett. 212, 624 (1993)]. Analysis of the product branching ratios allows us to conclude that, when below the threshold for excited state dissociation, rapid internal conversion followed by an intracluster reaction preferentially occurs on the ground state surface, while evaporation occurs primarily in the excited state. We find evidence in the spectral profiles of clusters with n⩾3 for a consistent “substructure” characterized by a pπ state lacking σ-type interactions with the ligand molecules.
An electrical breakdown model for liquids in response to a submicrosecond (∼100ns) voltage pulse is presented, and quantitative evaluations carried out. It is proposed that breakdown is initiated by field emission at the interface of pre-existing microbubbles. Impact ionization within the microbubble gas then contributes to plasma development, with cathode injection having a delayed and secondary role. Continuous field emission at the streamer tip contributes to filament growth and propagation. This model can adequately explain almost all of the experimentally observed features, including dendritic structures and fluctuations in the prebreakdown current. Two-dimensional, time-dependent simulations have been carried out based on a continuum model for water, though the results are quite general. Monte Carlo simulations provide the relevant transport parameters for our model. Our quantitative predictions match the available data quite well, including the breakdown delay times and observed optical emission.
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