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 FT1: Non-equilibrium Kinetics of Low-temperature Plasmas |
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Chair: Thomas Mussenbrock, Ruhr-University Bochum Room: Ballroom I |
Tuesday, October 1, 2013 3:30PM - 3:45PM |
FT1.00001: Active Plasma Resonance Spectroscopy: A Kinetic Functional Analytic Description Jens Oberrath, Thomas Mussenbrock, Ralf Peter Brinkmann The term ``active plasma resonance spectroscopy'' refers to a plasma diagnostic method which employs the natural ability of plasmas to resonate close to the plasma frequency. Essential for this method is an appropriate model to determine the relation between the resonance frequencies and demanded plasma parameters. Measurements with these probes in plasmas of a few Pa typically show a broadening of the spectrum that cannot be predicted by a fluid model. Thus, a kinetic model is necessary. A general kinetic model of electrostatic resonance probes has been presented by the authors [1]. This model is used to analyze the dynamic behavior of such probes, especially the spherical Impedance Probe and the Multipole Resonance Probe. It is shown, that damped resonances occur even in collisionless plasmas. Such a collisionless damping is caused by kinetic effects and is responsible for the broadening in a measured spectrum. Thus, the solution of the kinetic model can be used to determine the relation between the broadening of the resonance peak and the ``equivalent electron temperature.''\\[4pt] [1] J. Oberrath, T. Mussenbrock, R.P. Brinkmann, arXiv: 1305.7253 [physics.plasm-ph] [Preview Abstract] |
Tuesday, October 1, 2013 3:45PM - 4:00PM |
FT1.00002: Enhancement of the Coulomb collision rate by individual particle wakes Scott Baalrud, Brett Scheiner Charged particles moving in a plasma leave a trailing wake in their electric potential profile associated with the response function of the medium. For superthermal particles, these wakes can cause significant departures from the oft-assumed screened Coulomb potential profile. The wakes extend the interaction length scale beyond the Debye screening length for collisions between fast test particles and field particles in their wake. This can increase the Coulomb collision rate for velocities beyond the thermal speed. To demonstrate this effect, we consider the relaxation rate due to electron-electron collisions of an electron distribution function with initially depleted tails, as is common near boundary sheaths or double layers. This problem is related to Langmuir's paradox. We compare the standard Landau (Fokker-Planck) collision operator, which does not account for wakes, with the Lenard-Balescu collision operator, which includes wake effects through the linear dielectric response function. For this distribution, the linear dielectric is described by the incomplete plasma dispersion function.\footnote{S.\ D.\ Baalrud, Phys.\ Plasmas 20, 012118 (2013).} We compare the collision operators directly as well as the relaxation rate determined from a hybrid kinetic-fluid model. [Preview Abstract] |
Tuesday, October 1, 2013 4:00PM - 4:15PM |
FT1.00003: Particle Simulation of Expansion of Plasma Induced by Resonance Enhanced Multiphoton Ionization in Argon Gas Siva Sashank Tholeti, Venkattraman Ayyaswamy, Alina A. Alexeenko, Mikhail N. Shneider Rayleigh scattering of plasma generated by resonance enhanced multiphoton ionization (REMPI) provides a non-intrusive, time accurate measurement of electron formation and loss with many applications in plasma diagnostics. In this paper we study the non-equilibrium processes during the expansion of REMPI plasma. The particle in cell method with Monte Carlo collision (PIC/MCC) scheme is applied to account for the micron-sized characteristic length scale of the plasma and the non-continuum nature of the fast expansion process. Both 1D and 2D approximations are considered for the REMPI plasma expansion in Argon gas generated by a laser with a focal shape of a prolate ellipsoid at 100 torr and 5 torr pressures. The expansion of the plasma is found to be very sensitive to the initial velocity distribution of the electrons. REMPI plasma expansion is shown to be ambipolar in nature. Even though the radial expansion is predominant, there is a significant axial component requiring the 2D model. The deviation of the electron energy distribution functions (EEDFs) from that of the equilibrium Maxwell-Boltzmann energy distribution is presented both qualitatively and quantitatively. Based on this analysis the distinct plasma expansion phases have been delineated at various instances in time. [Preview Abstract] |
Tuesday, October 1, 2013 4:15PM - 4:30PM |
FT1.00004: Nitrogen atoms influence on associative ionization in nitrogen plasma Andrey Volynets, Alexey Zotovich, Dmitry Lopaev, Sergey Ziryanov, Nikolay Popov At the increased pressure the associative ionization is the main ionization mechanism in nitrogen plasma realizing due to reactions of metastable molecules N$_{2}$(A$^{3}\Sigma _{\mathrm{u}}^{+})$ and energy stored on high vibrational levels of the ground N$_{2}$ molecule state. While the role of these species is relatively well understood, the role of nitrogen atoms both in the ground and excited metastable states is not studied yet in detail. This work just deals with this problem and is focused on studying nitrogen atoms kinetics in DC glow discharge at increased pressures (5-50 Torr). Despite of the low dissociation degree the absolute concentration of nitrogen atoms is quite enough for the influence on the ionization mechanism through fast associative ionization reactions between metastable N($^{2}$P) atoms. The production of nitrogen atoms similarly to N$_{2}$(A$^{3}\Sigma_{\mathrm{u}}^{+})$ molecules is associated with the excitation degree of nitrogen vibrations. Thus energy stored in N$_{2}$ vibrations can be realized into the stepwise ionization through exciting N$_{2}$(A$^{3}\Sigma_{\mathrm{u}}^{+})$ and N($^{2}$P). [Preview Abstract] |
Tuesday, October 1, 2013 4:30PM - 4:45PM |
FT1.00005: Temporal evolution of OH density and gas temperature of nanosecond repetitively pulsed discharges in water vapor at atmospheric pressure Florent Sainct, Deanna Lacoste, Christophe Laux, Michael J. Kirkpatrick, Emmanuel Odic We present a study of plasma discharges produced by nanosecond repetitively pulses in water vapor at 450~K and 1~atm. The plasma was generated in water vapor with 20-ns duration high-voltage (0-20~kV) pulses, at a repetition frequency of 10~kHz, in the spark regime (2~mJ/pulse). To investigate plasma kinetics we focused on intermediate products of the discharge, in particular the hydroxyl radical. Between two discharges, the time-resolved density of OH was measured by Planar Laser Induced Fluorescence~(OH-PLIF). The temperature during the discharge was determined by optical emission spectroscopy, and between two pulses by two-color OH-PLIF. A 150~K preheating effect from the previous pulses is measured, with a maximum temperature elevation of 950 K during the first 10~$\mu$s following each pulse. The OH density measurements were compared with chemical kinetics simulations. The numerical results obtained with an initial OH density of 500~ppm show good agreement with the experimental data, thus providing a quantification of the OH density produced by the pulse. The electron number density is also measured via stark broadened H$_{\beta}$ lines. A kinetics model is proposed to interpret the measures. [Preview Abstract] |
Tuesday, October 1, 2013 4:45PM - 5:00PM |
FT1.00006: The electron energy distribution of microscale field emission-driven Townsend discharges Yingjie Li, David Go Attracted by the wide application potential of plasma-surface interactions, this work attempts to better understand the electron energy distribution (EED) of the free electrons in the discharge, which is critical in deciding the most favored type of electron-driven reaction. Particle-in-cell/Monte Carlo collision simulations are applied to study microscale Townsend discharges that can be formed in electrode gaps below 10 $\mu$m. Results show that the EED becomes non-continuous in this regime, generating several discrete peaks corresponding to specific inelastic collisions. The existence of these discrete peaks indicates that it is possible to enhance or eliminate certain types of reactions by manipulating the EED. The relative magnitude of these peaks and shape of the energy distribution can be controlled by both \textit{pd} and the applied potential. \textit{pd} dictates the number of inelastic collisions experienced by the emitted electrons, and the applied potential dictates the absolute maximum energy of the distribution. As shown in this work, at microscale dimensions, it is possible to control the energy distribution of free electrons to target specific, energy dependent gas-phase or surface reactions. [Preview Abstract] |
Tuesday, October 1, 2013 5:00PM - 5:15PM |
FT1.00007: Excited states and radicals formations in nanosecond pulse discharge and their evolution in afterglow Ivan Shkurenkov, David Burnette, Walter Lempert, Igor Adamovich The results of nanosecond pulse discharge and afterglow simulations carried out with developed one-dimensional self-consistent model in N$_{2}$/O$_{2}$ and H$_{2}$/N$_{2}$/O$_{2}$ gas mixtures and comparison with experimental data are presented. Excited states and radicals formations in the discharge as well as mechanisms of the NO formation and destruction are discussed. It was shown that NO is rapidly formed in the reaction between excited nitrogen N$_{2}$* (both triplet and singlet states) and atomic oxygen in afterglow and is destroyed by atomic nitrogen (reverse Zel'dovich mechanism). [Preview Abstract] |
Tuesday, October 1, 2013 5:15PM - 5:30PM |
FT1.00008: Vibrational kinetics in a Cl$_{2}$ inductively-coupled plasma Benjamin Pruvost, Jean-Paul Booth, Mickael Foucher, Pascal Chabert, Vasco Guerra, Ilya Fabrikant, Mark Kushner Inductively-coupled plasmas containing chlorine are widely used for conductor-etch applications, often using mixtures with HBr and O$_{2}$. We are carrying out an extensive comparison of experimental measurements with simulations using the Hybrid Plasma Equipment Model (HPEM). Vibrationally excited states of chlorine have historically been ignored in models, but recently we found that inclusion of a simple vibrational kinetic scheme in HPEM significantly improves the model agreement with experiment. Here we will present a more complete scheme, using calculated state-to-state cross-sections (up to v$=$5) for electron impact excitation and state-specific V-T (Cl$_{2}$-Cl$_{2}$ and Cl$_{2}$-Cl) and V-V (Cl$_{2}$-Cl$_{2})$ transfer rates. Initially the scheme has been implemented in a global model, which predicts vibrational temperatures up to 2500K at low pressure (3mTorr), dropping to $\sim$700K at 50 mTorr. We are attempting to measure the vibrational distribution using broadband ultraviolet absorption spectroscopy. Vibrationally excited states play a key role in gas heating, as well as significantly enhancing electron attachment, and should not be ignored. [Preview Abstract] |
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