Bulletin of the American Physical Society
66th Annual Gaseous Electronics Conference
Volume 58, Number 8
Monday–Friday, September 30–October 4 2013; Princeton, New Jersey
Session PR1: High Pressure Discharges: Dielectric Barrier Discharges, Coronas, Breakdown, Sparks III |
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Chair: David Scott, George Washington University Room: Ballroom I |
Thursday, October 3, 2013 1:30PM - 2:00PM |
PR1.00001: Meter-Scale Atmospheric-Pressure Microwave Plasma Using Sub-Millimeter-Gap Slot Invited Speaker: Hirotaka Toyoda Atmospheric-pressure pulsed plasmas have been given much attention because of its various possibilities for industrial applications such as surface wettability control, sterilization and so on. Among various atmospheric-pressure plasma sources, microwave plasma that is produced inside waveguide-slots is attractive because high-density plasma up to 10$^{15}$ cm$^{-3}$ can be easily produced along very long waveguide with light-weight and rather simple antenna configuration. So far, we have investigated plasma production inside slot of the waveguide and in this talk, elongation of the plasma up to meter-scale with newly-designed plasma source will be presented. In this study, two types of antennas are proposed to elongate the atmospheric-pressure microwave plasma. Firstly, array-structured slot design with a closed-end waveguide is adopted using X-band microwave (10 GHz). In this structure, slot antennas with a total number of more than 40 are positioned with $\lambda_{g}$/2-pitch along $\sim$1m waveguide so as to utilize standing wave inside the waveguide and to increase the electric field inside the slot. By optimizing the antenna design, arrayed microwave plasmas are successfully produced along $\sim$1m-length waveguide. The arrayed-slot structure, however, the plasma is not completely uniform along the waveguide and plasma density drastically decreases between two adjacent slots. To solve this, an alternative type of antenna that is free from the standing wave effect is designed. In this new-type antenna, travelling wave inside the waveguide with no reflection wave is realized by a combination of a microwave circulator and a ring-structured waveguide. By this transmission line, microwave power flows only to one direction and the average microwave power becomes spatially uniform along the waveguide. By using a single but very long slot up to several tens cm, very uniform plasma is produced along the slot. The result strongly suggests easy scale-up of the plasma source more than one meter that can be applied to surface modification of large-scale devices. [Preview Abstract] |
Thursday, October 3, 2013 2:00PM - 2:15PM |
PR1.00002: Enhanced ozone production in a pulsed dielectric barrier discharge plasma jet with addition of argon to a He-O$_2$ flow gas Brian Sands, Biswa Ganguly, James Scofield Ozone production in a plasma jet DBD driven with a 20-ns risetime unipolar pulsed voltage can be significantly enhanced using helium as the primary flow gas with an O$_2$ coflow. The overvolted discharge can be sustained with up to a 5\% O$_2$ coflow at $<$20 kHz pulse repetition frequency at 13 kV applied voltage. Ozone production scales with the pulse repetition frequency up to a ``turnover frequency'' that depends on the O$_2$ concentration, total gas flow rate, and applied voltage. For example, peak ozone densities $>$10$^{16}$ cm$^{-3}$ were measured with 3\% O$_2$ admixture and $<$3 W input power at a 12 kHz turnover frequency. A further increase in the repetition frequency results in increased discharge current and 777 nm O($^5P$) emission, but decreased ozone production and is followed by a transition to a filamentary discharge mode. The addition of argon at concentrations $\geq$5\% reduces the channel conductivity and shifts the turnover frequency to higher frequencies. This results in increased ozone production for a given applied voltage and gas flow rate. Time-resolved Ar(1$s_5$) and He(2$^3S_1$) metastable densities were acquired along with discharge current and ozone density measurements to gain insight into the mechanisms of optimum ozone production. [Preview Abstract] |
Thursday, October 3, 2013 2:15PM - 2:30PM |
PR1.00003: Streamer dynamics scaling with positive polarity pulsed voltage and gas pressure in a helium plasma jet Robert Leiweke, Biswa Ganguly We have investigated the variation of streamer speed, current, diameter and emission intensity profiles as functions of positive polarity 20 ns rise time pulsed applied voltage ranging from 6 kV up to 11 kV, at 500 Torr gas pressure using a 2 mm diameter helium plasma jet entrained by N$_{2}$ co-flow in a 30 mm diameter pyrex cell. The streamer speed has been measured from the spatio-temporal intensity of He 3$^{3}$D$\to $2$^{3}$P transition at 589 nm using both PMT and 5 ns gated ICCD imaging; the streamer diameter is obtained from the ICCD imaging. The streamer current was measured using wideband width current sensor and it varied from 0.1 mA up to 0.8 mA. Streamer speed increased almost linearly and peak currents varied nonlinearly with the applied voltage. The streamer diameter, estimated from imaging, was nearly constant at 0.9 mm indicating that the observed streamer may be several overlapping streamers propagating through the helium gas channel. The peak electron density of $\approx$ 2 $\times$ 10$^{10}$ cm$^{-3}$ is estimated from current continuity and streamer diameter. We have also performed voltage dependent streamer dynamics measurements at gas pressures from 150 up to 740 Torr, and the data shows that streamer properties do not follow the same scaling behavior as in air plasma. [Preview Abstract] |
Thursday, October 3, 2013 2:30PM - 2:45PM |
PR1.00004: Further Investigations of Slow Lightning Karl Stephan, Gil Sheleg The phenomenon of ``slow lightning'' is a new type of tracking or sliding atmospheric-pressure resistive-barrier discharge on the surface of a weakly conducting electrolyte. It occurs during the production of water plasmoids (also called ``Gatchina discharges'') in which a high-voltage capacitor is discharged into an insulated cathode in limited surface contact with the electrolyte. Unlike conventional dielectric-barrier and most other resistive-barrier discharges, these novel discharges propagate on the surface relatively slowly, spreading at a speed of 1-10 meters per second. We have investigated this phenomenon in several ways, using high-speed videography, time- and space-resolved spectroscopy, and current-density profiling. The plasma produced at cathode spots forms the plasmoid, and this plasma is distinct from the plasma in the slow-lightning discharge above the electrolyte. The primary visible emission from the latter discharge is a continuum, probably due to free-bound transitions, although an N2$+$ band is also present as well as intense emission from OH radicals under certain conditions. Possible applications of this phenomenon include water purification and pollution control. [Preview Abstract] |
Thursday, October 3, 2013 2:45PM - 3:00PM |
PR1.00005: Student Award Finalist - Simulation of the reignition of atmospheric pressure air discharges behind dielectric obstacles: comparison with experiments Francois Pechereau, Anne Bourdon In recent years, experimental studies on plasma assisted catalysis for flue gas treatment have shown a significant reduction of pollutants at a low energetic cost. Catalyst supports are either random or organized two phase media such as pellets, monoliths or porous media. Then, in plasma reactors, atmospheric pressure discharges have to interact with many obstacles and to propagate in microcavities and pores. To better understand the discharge dynamics in these complex structures, experiments have been carried out at LPGP (Orsay, France) in a point-to-plane geometry with a dielectric plane obstacle placed in the discharge path. In this work, we have carried out discharge simulations in the experimental geometry. We have compared the dynamics of the discharge ignited at the point and its impact on the dielectric surface. Then, we have compared the conditions of a discharge reignition behind the dielectric obstacle. A good qualitative agreement with experiments has been obtained but to improve the quantitative comparison, we have carried out a detailed parametric numerical study. In this work, we will focus on the influence of the level of seed charges on the discharge reignition and discuss several physical processes that could have an impact on the level of seed charges. [Preview Abstract] |
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