Time-Resolved Measurements of Electron Density in Nanosecond Pulsed Plasmas Using Microwave Scattering

In this work, Rayleigh microwave scattering was utilized to measure the electron number density produced by nanosecond high voltage breakdown in air between two electrodes in a pin-to-pin configuration (peak voltage 26 kV and pulse duration 55 ns). The peak electron density decreased from 1*10^17 cm^-3 down to 7*10^14 cm^-3 when increasing the gap distance from 2 to 8 mm (total electron number decreased from 2*10^13 down to 5*10^11 respectively). Electron number density decayed on the timescale of about several microseconds due to dissociative recombination.Comment: 12 pages, 5 figure

The physics of streamer discharge phenomena

In this review we describe a transient type of gas discharge which is commonly called a streamer discharge, as well as a few related phenomena in pulsed discharges. Streamers are propagating ionization fronts with self-organized field enhancement at their tips that can appear in gases at (or close to) atmospheric pressure. They are the precursors of other discharges like sparks and lightning, but they also occur in for example corona reactors or plasma jets which are used for a variety of plasma chemical purposes. When enough space is available, streamers can also form at much lower pressures, like in the case of sprite discharges high up in the atmosphere. We explain the structure and basic underlying physics of streamer discharges, and how they scale with gas density. We discuss the chemistry and applications of streamers, and describe their two main stages in detail: inception and propagation. We also look at some other topics, like interaction with flow and heat, related pulsed discharges, and electron runaway and high energy radiation. Finally, we discuss streamer simulations and diagnostics in quite some detail. This review is written with two purposes in mind: First, we describe recent results on the physics of streamer discharges, with a focus on the work performed in our groups. We also describe recent developments in diagnostics and simulations of streamers. Second, we provide background information on the above-mentioned aspects of streamers. This review can therefore be used as a tutorial by researchers starting to work in the field of streamer physics.Comment: 89 pages, 29 figure

Time-resolved nanosecond imaging of the propagation of a corona-like plasma discharge in water at positive applied voltage polarity

International audienceThe present paper is an experimental study of a pulsed filamentary plasma discharge inside liquid water in pin to plane electrode configuration. Time resolved electrical and imaging diagnostics have been performed. The initiation and the propagation of the discharge have been studied for several experimental parameters. The propagation is continuous and is followed by reilluminations at low water conductivity. The measured propagation velocity of the plasma discharge is 30km/s for the secondary positive mode. This velocity was found to be surprisingly constant whatever the experimental parameters and especially as a function of the water conductivity

Ultrashort filaments of light in weakly-ionized, optically-transparent media

Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration, high energies beyond the Joule level and peak powers exceeding several terawatt (TW). When such pulses propagate through optically-transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium, and the defocusing action of the electron plasma. Discovered one decade ago, this self-channeling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid to the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. Differences between femtosecond pulses propagating in gaseous or condensed materials are underlined. Attention is also paid to the multifilamentation instability of broad, powerful beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy, and the possibility to guide electric discharges in air are finally addressed on the basis of experimental results.Comment: 50 pages, 38 figure

Temporal dynamics of femtosecond-TALIF of atomic hydrogen and oxygen in a nanosecond repetitively pulsed discharge-assisted methane-air flame

The temporal dynamics of the spatial distribution of atomic hydrogen and oxygen in a lean methane-air flame, forced by a nanosecond repetitively pulsed discharge-induced plasma, are investigated via femtosecond two-photon absorption laser-induced fluorescence technique. Plasma luminescence that interferes with the fluorescence from H and O atoms was observed to decay completely within 15 ns, which is the minimum delay required for imaging measurements with respect to the discharge occurrence. During discharge, H atoms in the excited state rather than the ground state, produced by electron-impact dissociation processes, are detected at the flame front. It was found that the temporal evolution of H and O fluorescence intensity during a cycle of 100 µs between two discharge pulses remains constant. Finally, the decay time of O-atoms produced by the discharge in the fresh methane-air mixture was about 2 µs, which suggests a faster reaction between O-atoms and methane than in air

Computational modeling of high pressure plasmas for plasma assisted combustion, liquid reforming and thermal breakdown applications

The goal of the present work is to study high pressure non-equilibrium plasma discharges in chemically reactive systems. In this work, we present coupled computational studies of high pressure nanosecond pulsed plasmas for multiphysics applications ranging from plasma assisted combustion ignition, large gap thermal breakdown, to electric discharge in liquids for fuel reforming and biomedical applications. In the first part of the work, we report the results of a computational study which explores argon surface streamers as a low-voltage mechanism for thermal breakdown of large interelectrode gaps and investigate the effect of impurities (molecular oxygen) on the development of continuous surface streamer channels under atmospheric-pressure conditions. In pure argon, a continuous conductive streamer successfully bridges the gap between two electrodes indicating high probability of transition to arc. Presence of oxygen impurities in small concentrations (less than 5%) is found to be conducive to streamer induced thermal breakdown as it reduces the threshold voltage of streamer formation and minimizes unwanted streamer branching effects while maintaining a high probability of streamer to arc transition. Higher oxygen impurity levels > 5% are found to significantly deteriorate the continuous conductivity of streamer channel and lead to a much lower probability for transition to thermal arcs. In the second part of the work, we present a computational study of nanosecond streamer discharges in helium gas (He) bubbles suspended in distilled water (H₂O) for liquid reforming applications. The model takes into account the presence of water vapor in the gas bubble for an accurate description of the discharge kinetics. The objective is to study the kinetics and dynamics of streamer evolution and maximize active species production within the gas bubbles which is the quantity of interest for plasma processing of liquids. We investigate two parameters, namely a) trigger voltage polarity and b) the presence of multiple bubbles, which are found to significantly influence the characteristics of the discharge in gas bubbles. A substantial difference is observed in initiation, transition and evolution stages of streamer discharge for positive and negative trigger voltages. The volumetric distribution of species in the streamer channel is more uniform for negative trigger voltages on account of the formation of multiple streamers. In case of the presence of more than one gas bubble, we see the phenomenon of streamer hopping between bubbles where the high electric field in the sheath of the first bubble triggers the streamer discharge in the adjacent bubble. The presence of multiple immersed bubbles reduces the breakdown voltage of the plasma discharge and results in more uniform generation of active species. It is concluded that a negative pin trigger with multiple immersed gas bubbles maximizes the active species generation which is conducive to plasma assisted liquid reforming applications. In the final part of the work, a coupled two-dimensional computational model of nanosecond pulsed plasma induced flame ignition and combustion for a lean H₂ – air mixture in a high pressure environment is described. The model provides a full fidelity description of plasma formation, combustion ignition, and flame development. We study the effect of three important plasma properties that influence combustion ignition and flame propagation, namely a) plasma gas temperature, b) plasma-produced primary combustion radicals O, OH, and H densities, and c) plasma-generated charged and electronically excited radical densities. Preliminary zero-dimensional studies indicate that plasma generated trace quantities of O, OH and H radicals drastically reduces the ignition delay of the H₂ – air mixture and becomes especially important for high pressure lean conditions. Multi-dimensional simulations are performed for a lean H₂ – air mixture (φ=0.3) at 1 and 3.3 atm and a range of initial tem- perature from 1000 - 5000 K. The plasma is accompanied by fast gas heating due to N₂ metastable quenching that results in uniform volumetric heating in the interelectrode gap. The spatial extent of the high temperature region generated by the plasma is a key parameter in influencing ignition; a larger high temperature region being more effective at initiating combustion ignition. Plasma generation of even trace quantities (∼ 0.1%) of primary combustion radicals, along with plasma gas heating, results in a further fifteen-fold reduction in the ignition delay. The radical densities alone did not ignite the H₂ – air mixture. The generation of other plasma specific species results only in a slight ∼ 10 % improvement in the ignition delay characteristics over the effect of primary combustion radicals, with the slow decaying ions (H₂⁺, O₂⁻, O⁻ ) and oxygen metastable species (O₂ [superscript a1], O₂ [superscript b1], O₂ [superscript *]) primarily contributing to com- bustion enhancement. These species influence the ignition delay, directly by power deposition due to quenching, attachment and recombination reactions, and indirectly by enhancing production of primary combustion radicals.Aerospace Engineerin