Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session MW1: High Pressure Discharges I |
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Chair: Patrick Pedrow, Washington State University Room: Amphitheatre 204 |
Wednesday, October 24, 2012 3:30PM - 4:00PM |
MW1.00001: Extremely far from equilibrium: the multiscale dynamics of streamers Invited Speaker: Ute Ebert Streamers can emerge when high voltages are applied to gases. At their tips, the electric field is strongly enhanced, and electron energies locally reach distributions very far from equilibrium, with long tails at high energies. These exotic electron energies create radiation and chemical excitations at very low energy input, as the gas stays cold while the ionization front passes. Applications are multiple: highly efficient O* radical production in air for disinfection, combustion gas cleaning, plasma assisted combustion, plasma bullets in medicine etc. In that sense, streamers can be considered as very efficient converters of pulsed electric into chemical energy, in particular, if the electric circuits are optimized for the application. Streamers are also ubiquitous in nature, e.g., in the streamer corona of lightning leaders, in sprite discharges high above the clouds; and streamers also seem to contribute to generating gamma-ray flashes and even to electron-positron beams in active thunderstorms. Unravelling the intrinsic mechanisms of streamers is challenging: they can move with up to one tenth of the speed of light, and they have an intricate nonlinear structure with a hierarchy of scales. I will review how theory and experiment deal with these structures, and I will discuss the basic differences between positive and negative streamers, electron acceleration at streamer tips and the consecutive radiation and chemical reactions, the propagation mechanism of positive streamers in different gases, streamer velocities and diameters varying over at least two orders of magnitude, streamer branching and interaction, and their three-dimensional tree structure. Both theory and experiment work with a patchwork of methods, and geophysics can provide movies that cannot be taken in the lab. I will sketch the state and outline open questions. [Preview Abstract] |
Wednesday, October 24, 2012 4:00PM - 4:15PM |
MW1.00002: Simulation of the propagation and reignition of atmospheric pressure air discharges behind a dielectric plane obstacle Francois Pechereau, Jaroslav Jansky, Anne Bourdon In recent years, experimental studies on flue gas treatment have demonstrated the efficiency of plasma assisted catalysis for the treatment of a wide range of pollutants at a low energetic cost. In plasma reactors, usual catalyst supports are pellets, monoliths or porous media, and then atmospheric pressure discharges have to interact with many obstacles and to propagate in microcavities and pores. As a first step to better understand atmospheric pressure discharge dynamics in these complex geometries, in this work, we have carried out numerical simulations using a 2D-axisymmetric fluid model for a point-to-plane discharge with a dielectric plane obstacle placed in the path of the discharge. First, we have simulated the discharge ignition at the point electrode, its propagation in the gap and its impact and expansion on the dielectric plane. Depending on the applied voltage, the dielectric plane geometry and permittivity, we have identified conditions for the reignition of a second discharge behind the plane obstacle. These conditions will be discussed and compared with recent experimental results on the same configuration. [Preview Abstract] |
Wednesday, October 24, 2012 4:15PM - 4:30PM |
MW1.00003: Measurement of ozone production scaling in a helium plasma jet with oxygen admixture Brian Sands, Biswa Ganguly Capillary dielectric barrier plasma jet devices that generate confined streamer-like discharges along a rare gas flow can produce significant quantities of reactive oxygen species with average input powers ranging from 100 mW to $>$1 W. We have measured spatially-resolved ozone production in a He plasma jet with O$_2$ admixture concentrations up to 5\% using absorption spectroscopy of the O$_3$ Hartley band system. A 20-ns risetime, 10-13 kV positive unipolar voltage pulse train was used to power the discharge, with pulse repetition rates varied from 1-20 kHz. The discharge was operated in a transient glow mode to scale the input power by adjusting the gap width between the anode and downstream cathodic plane. Peak ozone number densities in the range of 10$^{16}$ - 10$^{17}$ cm$^{-3}$ were measured. At a given voltage, the density of ozone increased monotonically up to 3\% O$_2$ admixture (6 mm gap) as the peak discharge current decreased by an order of magnitude. Ozone production increased with distance from the capillary, consistent with observations by other groups. Atomic oxygen production inferred from O-atom 777 nm emission intensity did not scale with ozone as the input power was increased. The spatial distribution of ozone and scaling with input power will be presented. [Preview Abstract] |
Wednesday, October 24, 2012 4:30PM - 4:45PM |
MW1.00004: How Can The Arrival Of A Pulsed Streamer At One Side Of A Thin Dielectric Ignite A Secondary Streamer On The Other Side? Tom Field, Qais Al-Gwari, Colm O'Neill, Deborah O'Connell, Bill Graham The interaction of pulsed streamers with thin dielectric materials has been investigated. The streamers were generated in an atmospheric pressure pulsed dielectric barrier helium discharge. A thin dielectric material was introduced into the path of the streamers so that the surface of the dielectric was perpendicular to the path of the streamer. Under favourable conditions, shortly after a streamer reached the dielectric barrier a similar secondary streamer was ignited on the other side of the dielectric. The secondary streamer continued in the same direction as the initial streamer moving away from the dielectric with a velocity which was perpendicular to its surface. This secondary streamer generation has been investigated with various different dielectric materials. The mechanism by which the secondary streamer is ignited will be discussed. [Preview Abstract] |
Wednesday, October 24, 2012 4:45PM - 5:00PM |
MW1.00005: High order fluid model for streamer discharges Aram Markosyan, Sasa Dujko, Ronald White, Jannis Teunissen, Ute Ebert We present a high order fluid model for streamer discharges. Using momentum transfer theory, the fluid equations are obtained as velocity moments of the Boltzmann equation. We solve Poisson equation to obtain space charge electric field. The high order tensors from the energy flux equation are specified in terms of low order moments to close the system. The average collision frequencies for momentum and energy transfer in elastic and inelastic collisions required as an input in high order fluid model of streamers in molecular nitrogen are calculated using a multi term Boltzmann equation solution. The results of simulations are compared with those obtained by a PIC/MC method and by the classical first order fluid model based on the drift-diffusion and local field approximation. The comparison clearly validates the high order fluid model, while the first order fluid model underestimates many aspects of streamer dynamics. Two important issues are discussed on the basis of fundamental kinetic theory developed in the past decade: (1) the correct implementation of transport data in fluid models of streamer discharges; (2) the accuracy of two term approximation for solving Boltzmann's equation in the context of streamer studies. [Preview Abstract] |
Wednesday, October 24, 2012 5:00PM - 5:15PM |
MW1.00006: STUDENT AWARD FINALIST: Simulation of the ignition of a H2-air mixture at atmospheric pressure by a nanosecond repetitively pulsed discharge Fabien Tholin, Anne Bourdon Nanosecond repetitively Pulsed Discharges (NRPD) have a great potential for many applications at atmospheric pressure due to their ability to produce efficiently many reactive chemical species at a low energy cost. Recent measurements have shown that in the ``spark'' regime of NRP discharges, an ultra-fast local heating of the gas could be obtained. This effect is of great interest for applications as flow control and plasma assisted combustion (PAC). In this work, we have carried out 2D numerical simulations of the coupling of the NRP discharge in air at atmospheric pressure in a point-point geometry with the background air. In particular, we have simulated shock waves generated by the NRPD in the spark regime and we have compared our results with experiments. Then, we have studied the production of active species by the NRP discharge in the spark regime. Finally, for plasma assisted combustion applications, we have simulated the ignition of a flame kernel in a lean H2-air mixture by a spark NRPD. Based on this work, the relative importance for the combustion ignition of gas heating and production of active species by the spark NRP is discussed. [Preview Abstract] |
Wednesday, October 24, 2012 5:15PM - 5:30PM |
MW1.00007: Match Design for VHF Atmospheric Plasmas Brandon Byrns, Steven Shannon Atmospheric pressure discharges driven at very high frequencies have demonstrated the potential for unique heating modes and plasma formation in a wide range of gases at high pressure. One of the challenges in designing these systems is the matching network needed to couple power from the generator to the discharge. In this talk, we present matching network design considerations for VHF atmospheric plasmas including tune space, stability, and efficiency. We will present both modeled and experimental data from a source driven at 162MHz at 1atm pressure using a plurality of gasses including air, N$_{2}$, CO$_{2}$, He, and Ar. These cases will be used to demonstrate the interaction of the matching system with the plasma load, and how this can influence plasma conditions at specific process set points. [Preview Abstract] |
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