Bulletin of the American Physical Society
64th Annual Gaseous Electronics Conference
Volume 56, Number 15
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session PR2: Collision Processes in Plasmas |
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Chair: Allan Stauffer, York University Room: 255E |
Thursday, November 17, 2011 2:00PM - 2:30PM |
PR2.00001: Electron attachment to radicals and other reactive molecules Invited Speaker: Electron attachment to highly reactive fluorocarbon species C$_2$F$_5$ and CF$_2$ has been studied experimentally with a TEM-TOF apparatus (TEM- Trochoidal Electron Monochrator; TOF - Time-of-Flight mass spectrometer; Field et al., J. Phys. B 38 (2005) 255). These reactive molecules have been generated {\it in situ} by fast atom reactions and by passing fluorocarbon precursors through a microwave discharge. In the case of C$_2$F$_5$ low energy electron attachment is observed with the formation of F$^-$ at close to zero electron energy, however, no electron attachment to CF$_2$ was observed, despite theoretical prediction of a significant dissociative electron attachment cross section (Rozum et al., J. Phys. Chem. Ref. Data 35 (2006) 267). An upper limit for the dissociative electron attachment cross section of CF$_2$ has been estimated as 5$\times$10$^{-4}$ {\AA}$^2$ at 1.8 eV, which is the thermodynamic threshold for F$^-$ formation from CF$_2$. (Graupner et al., New J. Phys. 12 (2010) 083035).\\[4pt] Work done in collaboration with Karola Graupner and Sean Haughey, Centre for Plasma Physics, Queens University, Belfast; Christopher Mayhew, Dept. Physics, University of Birmingham, UK; and Judith Langer, Institut fur Optik und Atomare Physik, TU Berlin, Germany. [Preview Abstract] |
Thursday, November 17, 2011 2:30PM - 2:45PM |
PR2.00002: Collisional Radiative Recombination Ar$^{+}$ + e + e at low temperatures from 77K to 180 K R. Johnsen, T. Kotrik, P. Dohnal, R. Plasil, S. Roucka, P. Jusko, J. Glosik The extremely strong (T $^{-4.5})$ temperature variation of collisional radiative recombination (CRR) makes it difficult to extend experimental tests of the frequently used ``Stevefelt formula'' to low temperatures, since the electron temperature must be precisely known and CRR adds heat to the electron gas. Using a newly developed low-temperature helium-buffered flowing afterglow (Cryo-FALP), we have determined CRR rate coefficients for of Ar$^{+}$ ions with electrons at electron temperatures from 77 K to 180 K at electron densities n$_{e}$ from~10$^{8}$ to 10$^{10}$~cm$^{-3}$. The measured ternary rate coefficient $K_{CRR}$~at 77 K of (1.0 $\pm $~0.4) $\times $~10$^{-17}$ cm$^{6}$s$^{-1}$ and the observed variation with electron temperature $T_{e}$ agree well with the $T_{e}^{-4.5}$ dependence predicted by the Stevefelt formula [Preview Abstract] |
Thursday, November 17, 2011 2:45PM - 3:00PM |
PR2.00003: Low pressure hydrogen discharges diluted with argon explored using a global (volume averaged) model Jon Tomas Gudmundsson, Aron Th. Hjartarson, Eythor G. Thorsteinsson A steady state global (volume averaged) model is used to explore the plasma parameters and the plasma chemistry of a low pressure (1--100 mTorr) high density hydrogen discharge that is diluted with argon [1]. The electron density increases, the dissociation fraction of hydrogen increases and the electron temperature decreases with increased argon dilution. We find that H$_3^+$ is the dominant positive ion up to about 30 \% argon dilution at 10 mTorr, were ArH$^+$ becomes the dominant positive ion. The reaction rates for the creation and destruction of various species are explored versus the discharge pressure. In particular we explore the role of the vibrationally excited levels of the hydrogen molecule in the creation of the negative ion H$^-$ through dissociative attachment. The role of the ArH$^+$ ion in the discharge chemistry is discussed and we find that ArH$^+$ plays a significant role in the destruction of the H$^-$ ion. Furthermore, the creation and destruction of H$_3^+$ and ArH$^+$ ions are explored. The electronegativity increases with increasing H$_2$ content and reaches a value of approximately unity in a pure H$_2$ discharge at 100 mTorr.\\[4pt] [1] A. T. Hjartarson, E. G. Thorsteinsson and J. T. Gudmundsson, Plasma Sources Sci. Technol. {\bf 19} (2010) 065008 [Preview Abstract] |
Thursday, November 17, 2011 3:00PM - 3:15PM |
PR2.00004: Passage of energetic radiation through air: collisional kinetics of photons and electron production Y.S. Dimant, G.S. Nusinovich, P. Sprangle, V.L. Granastein A novel technique for remote detection of concealed radioactive materials using a gyrotron-induced localized breakdown in air has been recently proposed. This technique requires an excessive density of free electron in a focused spot of the gyrotron radiation. Such electrons are produced at a multi-step process starting from the initial radiation of primary MeV photons by the radioactive material. On their passage through air, the photons undergo inelastic scattering and absorption accompanied by ionization of air molecules. The released free energetic electrons also collide with neutral molecules, initiating an avalanche release of other electrons. All free electrons undergo a sequence of further elastic and inelastic collisions leading to their gradual cooling down to sub-eV energies. Low-energy electrons that withstand recombination and attachment to neutral molecules can trigger the avalanche breakdown in the focused spot, provided the free-electron density exceeds the breakdown threshold. To assess how the breakdown conditions relate to the characteristics and location of the concealed radioactive material, one needs to model the coupled process of photon and electron collisional kinetics. This paper analyses the photon and electron momentum distributions as functions of the distance to the primary radiation source. [Preview Abstract] |
Thursday, November 17, 2011 3:15PM - 3:30PM |
PR2.00005: Anisotropy of H$\alpha $ Lines in Low Current Discharges in H$_{2}$ Discharges at High E/N Vladimir Stojanovic, Zeljka Nikitovic, Zoran Petrovic We study anisotropy of anomalously broadened H$\alpha $ lines in a low current Townsend discharge in hydrogen. In order to achieve consistency with results of other authors we select conditions of simulation appropriate for moderate E/N (E-electric field, N-gas density) that are selected from experimental Townsend discharges in pure H2 [1]. Best available cross section data and information on collisions with surfaces are used to predict angular distributions of heavy particles and spatial dependence of line profiles due to heavy particle excitation. Very different profiles are obtained for end on and side on observation. If experiments could select emission from different regions a very strong variation of anomalously broadened wings is observed along the axis of the discharge. Acknowledgments: Work at the Institute of Physics is supported by the MNTRS under grants 171037. \\[4pt] [1] A. V. Phelps, Phys. Rev.E 79, 2009, 066401. [Preview Abstract] |
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