Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session RR4: Microdischarges III |
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Chair: Jichul Shin, University of Ulsan Room: Salon DE |
Thursday, October 25, 2012 1:30PM - 2:00PM |
RR4.00001: Modeling transient and DC microdischarges Invited Speaker: Leanne Pitchford The considerable recent interest in ``microdischarges'' ( discharges in small, spatially-confined geometries) is largely due to their remarkable stability. That is, stable, non-thermal, high-pressure plasmas can be generated and maintained in electric discharges in small geometries operating in controlled atmospheres or in open air. Various ways of increasing the plasma volume have been investigated, including arrays of micro discharges or 3-electrode systems (an additional anode). Further interest in microdischarges is due to the fact that plasma jets, initiated from microdischarges operating with pulsed or RF excitation and with an axial helium flow, can propagate some centimeters in open air. Modeling has been an important tool for developing an understanding of these microdischarges and in helping guide the experimental optimization of devices based on microdischarges. This talk will focus on results from fluid modeling. Issues to be discussed include the extent to which pd (pressure x distance) and pt (pressure x time) scaling laws are valid for transient and DC microdischarges. That is, how do these microdischarges differ from their larger, lower-pressure counterparts? Nonlinearities due to gas heating, step-wise ionization, surface and high-pressure gas phase chemistry, and the high electric field strength at the cathode are factors that can cause departures from pd and pt scaling. Results from fluid models have also provided a framework for understanding of pulsed or rf plasma jets, but models allowing the prediction of the plasma chemistry in plasma jets are still evolving. [Preview Abstract] |
Thursday, October 25, 2012 2:00PM - 2:15PM |
RR4.00002: Properties of a field emission-driven Townsend discharge Paul Rumbach, David Go For half a century, it has been known that the onset of field emission in direct current (DC) microplasmas with gap sizes less than 10 $\mu $m can lead to breakdown at applied voltages far less than predicted by Paschen's law. It is still unclear how field emission affects other fundamental plasma properties at this scale. In this work, a one-dimensional fluid model is used to predict basic scaling laws for fundamental properties such as ion density, electric field due to space charge, and current voltage relations in the pre-breakdown regime. Computational results are compared with approximate analytic solutions. It is shown that ionizing collisions by field-emitted electrons produce significant ion densities well before Paschen's criteria for breakdown is met. When positive space charge densities become sufficiently large, the effect of ion-enhanced field emission leads to breakdown. Defining breakdown mathematically using a solvability condition leads to a full modified Paschen's curve, while defining it physically in terms of a critical ion density leads analytically to an effective secondary emission coefficient, $\gamma '$, of the form initially suggested by Boyle and Kisliuk.\footnote{Boyle, W.S. and Kisliuk, P., Phys. Rev. \textbf{97}, 255 (1955).} [Preview Abstract] |
Thursday, October 25, 2012 2:15PM - 2:30PM |
RR4.00003: Stationary wave of microplasmas and its propagation in the array of microchannels Jin Hoon Cho, Peter Kim, Sung-Jin Park, J. Gary Eden Propagation of a stationary wave of microplasmas in an array of microchannels has been observed in rare gases at atmospheric pressure, and investigated spatiotemporally. The microchannels, fabricated in a spiral geometry, have widths of 200-300 $\mu $m, and a length-to-width aspect ratio of $\sim $10$^{3}$:1. The devices were powered with a 20 kHz sinusoid voltage and operated at 300 -- 700 Torr in case of several rare gases. A gated, intensified camera and a telescope, with a frame resolution of $\sim $50 ns reveal the formation of unique standing wave structures of microplasmas and the rapid propagation of the plasma structure radially outwards. The scale, spacing and propagation rate of the standing waves of microplasmas are dependent on the discharge gases, and the pressure. It is observed that the wave properties can be controlled by the device structure, and numerical analysis of the observed stationary waves along the azimuthal coordinate reveal a phase shift, oscillation and self-arrangement. The detailed characteristics of plasma wave propagation and these stationary waves will be discussed. [Preview Abstract] |
Thursday, October 25, 2012 2:30PM - 2:45PM |
RR4.00004: Plasma ignition dynamics in atmospheric-pressure pulsed-microwave plasma Hirotaka Toyoda, Takuya Murase, Kazuki Egashira Atmospheric-pressure pulsed plasmas have been given much attention because of its various possibilities for industrial applications. In this study, temporal variations of both plasma density and microwave electric field in pulsed-microwave atmospheric-pressure plasma of Ar/H$_{2}$ and N$_{2}$/H$_{2}$ are measured using H$_{\beta }$ line measurement. From time-resolved measurement of Ar/H$_{2}$ plasma from the plasma ignition till the steady state, both increase in plasma density and decrease in electric field are observed up to $\sim $0.7 $\mu $s from the plasma ignition, and their relation is well explained by the decrease in the plasma resistivity and the current flowing through the plasma. After $\sim $0.7 $\mu $s, plasma density starts to decrease rather slowly at a time scale of a few $\mu $s till it reaches the steady state. Time scale of the plasma density decrease is similar to that of the gas temperature increase, suggesting heat expansion of the neutral gas as well as the plasma in the vicinity of the plasma region. [Preview Abstract] |
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