Bulletin of the American Physical Society
61st Annual Gaseous Electronics Conference
Volume 53, Number 10
Monday–Friday, October 13–17, 2008; Dallas, Texas
Session VF2: Electron and Ion Transport in Gases |
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Chair: Rainer Johnsen, University of Pittsburgh Room: Salon A-D |
Friday, October 17, 2008 8:00AM - 8:15AM |
VF2.00001: Variation of electron mobility with gas density Ardeshir Bagheri The density (N) dependence of electron mobility (U) in various dense gases (H2, N2, O2, CO2 and rare gases) has been calculated. The multiple scattering of electron shifts its kinetic energy and it also changes the distribution function of electrons. This unified approach predicts both the positive (increasing) and negative (decreasing) effects. We have assessed the data on momentum transfer cross-sections by comparing the mobility at very low densities (NU)o with those of experimental values. The calculated ratio (NU)/(NU)ois compared with the observed values and other theoretical work. The Legler model which assumes constant cross-section is inadequate for predicting the observed density dependence. We obtain good agreement with available experimental work for all the atomic and molecular species studied here. [Preview Abstract] |
Friday, October 17, 2008 8:15AM - 8:30AM |
VF2.00002: Direct Determination of He-O+ Interaction Potential from Gaseous Ion Mobility Data Amir-Hossein Jalili, Naser Seyed-Matin The analysis of charged particle transport in gases under the influence of electric fields has widespread applications in science and technology ranging from swarms experiments used to determine ion-neutral interaction cross-sections/potentials to plasma chemistry and atmospheric physics. In the last fifty years it has become possible to make accurate measurements of the mobility of trace amounts of ions through a neutral gas over a wide range of the ratio of electric field strength, E, to the neutral gas number density, N. In the 1970s the required theories has developed sufficiently to permit to obtain accurate information about ion-neutral potentials from analysis of such measurements. Here the direct inversion scheme of Viehland and co-workers is used to calculate the interaction potential of He-O+ from recently published gaseous ion mobility data for this system. The obtained potential (INVERT potential), which is in good agreement with the ab initio potential of Danailov and co-workers was employed to compute the ion mobility of O+ ion in He gas as a function of E/N ratio by the two temperature theory of Viehland and Mason. Results show that the INVERT potential reproduces the ion mobility within experimental accuracy. [Preview Abstract] |
Friday, October 17, 2008 8:30AM - 8:45AM |
VF2.00003: Electron swarm coefficients for H$_{2}$O and H$_{2}$O-N$_{2}$ A.M. Juarez, E. Basurto, J.L. Hernandez-Avila, J. de Urquijo We have used a pulsed Townsend technique to measure the electron drift velocity v$_{e}$, the density normalized longitudinal diffusion coefficient ND$_{L}$, and effective ionization coefficient ($\alpha -\eta )$/N, in water vapour and water vapour-nitrogen mixtures over the density-reduced electric field range E/N, 16-650 x 10$^{-17}$V cm$^{2}$. The v$_{e}$ values are in good agreement with previous ones, while those for ND$_{L}$ agree well with a previous calculation. The limiting value for E/N was found to be E/N$_{lim}$=137 x 10$^{-17}$ V cm$^{2}$. For E/N$<$70x10$^{-17}$ V cm$^{2}$, the v$_{e}$ curves lie below that for pure N$_{2}$; however, the 10{\%} H$_{2}$O-N$_{2}$ curve for v$_{e}$ shows the trend for negative differential conductivity. The ($\alpha -\eta )$/N curve for H$_{2}$O shows a shallow, negative minimum, in disagreement with a recent measurement using the steady-state Townsend technique. The H$_{2}$O-N$_{2}$ curves for ($\alpha -\eta )$/N show a progressively smaller minima, together with a trend to lower values of ($\alpha -\eta )$/N as the N$_{2}$ content in the mixture increases. This research aims to provide a complete set of self-consistent electron swarm parameters for the simulation of flue-gas discharges. [Preview Abstract] |
Friday, October 17, 2008 8:45AM - 9:00AM |
VF2.00004: Modeling of positron thermalization in collisional traps Ana Bankovic, Joan P. Marler, Gordana Malovic , Stephen J. Buckman, Milovan Suvakov, Zoran Lj. Petrovic A Monte Carlo code has been used to follow the orbits of positrons thermalizing from approximately 10 eV down to very low energies. This technique is appropriate as positrons suffer only a very few collisions so their transport is fully non-hydrodynamic. We have studied how the initial energy distribution of a moderated positron beam changes to a low energy, trapped, group of positrons, and which processes contribute to that. We followed how energy and momentum are dissipated in collisions, the thermalization time, the role of the magnetic field, the size of the Larmor radius and geometry of the trap, and how different abundances of gases in the mixture affect the result and other aspects of low energy positron traps. Most importantly in all cases we could sample the efficiency of thermalization by following the losses of positrons due to Ps formation. Complete sets of cross sections covering all major, number, momentum and energy balance processes were included for gases such as Ar, He, H$_{2}$, N$_{2}$ and N$_{2}$/CF$_{4}$ mixture. Fore example our results indicate thermalization times of Nt= 2.51*10$^{17}$ sm$^{-3}$ for pure nitrogen and 1.41*10$^{16}$ sm$^{-3}$ for N$_{2}$/CF$_{4}$ mixture. [Preview Abstract] |
Friday, October 17, 2008 9:00AM - 9:15AM |
VF2.00005: Complex interplay of collisional and RF electric/magnetic field effects for electron transport in gases S. Dujko, R.D. White, K.F. Ness, R.E. Robson Advancements in modern day technology associated with non-equilibrium low-temperature magnetized plasma discharges demand the most accurate modeling of the underlying transport processes involved. In this work, the non-equilibrium transport of electrons in gases under the influence of \textbf{E} and \textbf{B} fields is studied via a unified time-dependent multi term solution of the Boltzmann equation. ~We will focus on the time-dependent behavior of electron transport properties in ICP discharges where \textbf{E} and \textbf{B} are RF. We systematically investigate the explicit field effects including field to density ratios, field frequency to density ratio, field phases and field orientations. In addition we will highlight the explicit modification of transport coefficients brought about by attachment/ionization. A multitude of kinetic phenomena were observed that are generally inexplicable through the use of steady-state dc transport theory. Phenomena of significant note include the existence of transient negative diffusivity, time-resolved NDC and anomalous anisotropic diffusion. Most notably, a proposed new mechanism for collisional heating in ICPs has emerged. [Preview Abstract] |
Friday, October 17, 2008 9:15AM - 9:30AM |
VF2.00006: Experimental Investigation on the Boltzmann Relation for a Bi-Maxwellian Plasma Jin-Young Bang, Chin-Wook Chung The Boltzmann relation is applied to the electron density distribution in plasmas. However, Maxwellian electron distributions are assumed. New Boltzamnn relation with bi- Maxwellian electron distributions is derived from the fluid equation and compared with the experimental results in inductively coupled plasma. It is found that the spatial distribution of the electron density is governed by the effective electron temperature and that of the cold and hot electrons is governed by each electron temperature. The increase in the effective temperature around the plasma sheath interface is negligible, thus the effective temperature at the discharge center can be simply used as the temperature in the Boltzmann relation. [Preview Abstract] |
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