Session TR4: Reaction and Electron Kinetics

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Predicting the direction of chemical reactions using thermodynamics requires an entropy extremum principle. Under the governance of local equilibrium, the system evolves towards a stationary equilibrium state, at which entropy attains a constant maximum value. The equilibrium state is constrained by a set of independent state variables, which are often taken to be the temperature, pressure, and relative amounts of different chemical elements. Nonequilibrium systems are classified into two regimes: linear and nonlinear. In the linear regime, it has been argued that the entropy generation rate, which is positive semidefinite according to the 2$^{\mathrm{nd}}$ law of thermodynamics, attains a \textit{minimum} value at stationary states. For chemical reactions, a criterion must be fulfilled for operation in the linear regime. The chemical affinity of a reaction must be much smaller than the thermal energy: $A_{\mathrm{i}}$/\textit{RT}$_{\mathrm{M}}$ \textless \textless 1, where $A_{\mathrm{i}}$ is the chemical affinity, $R$ is the ideal gas constant, and $T_{\mathrm{M}}$ is the temperature of the gas. In chemically reactive nonequilibrium plasmas, we have recently demonstrated that stationary states are reached at which $A_{\mathrm{i}}$/\textit{RT}$_{\mathrm{M}}$ \textgreater \textgreater 1, therefore the system is governed by nonequilibrium, nonlinear thermodynamics. It is currently unknown if an entropy extremum principle governs the nonlinear regime. Discovery of that principle would have immense impact on a broad set of disciplines, for example evolution of life on planet earth. In this presentation, our published experimental data will be used to test a recently proposed hypothesis that systems in the nonlinear regime evolve towards constrained stationary states at which the entropy generation rate is \textit{maximized}.   

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The Dry Reformation of Methane (DRM), a process converting CO2 and CH4 into value-added products (CO2 (g) + CH4(g) $\rightarrow$ 2CO(g) + H2(g)), is a promising lead in the research of efficient energy storage. However, though CO2 plasmas are widely studied for CO2 conversion, the molecule synthesis mechanisms in CO2-CH4 plasmas remain poorly known. The goal of this work is to provide experimental data allowing a detailed description of CO2-CH4 plasma kinetic, both in DC glow and in RF discharges, to develop kinetic models coupling electron, vibrational and chemical kinetic processes. To this aim a dedicated reactor at low pressure, in both flowing and “static conditions” (without any gas flow), has been used in which relevant parameters (gas temperatures, conversion rates, electric field). The vibrational temperatures of CO and CO2 are measured in situ with FTIR spectroscopy by fitting infrared spectras . FTIR spectroscopy is also used to monitor the appearance of CO2-CH4 conversion products in static conditions. Densities of atomic short-lived species such as O are measured in the plasma with actinometry by addition of small amounts of Ar in the CO2-CH4 mixture. Finally, the reduced electric field is estimated with the ratio of atomic emission line.   

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Most practical atmospheric pressure discharges under ambient conditions transition from a Townsend to streamer-like discharge with time depending on the external voltage, gas composition and pressure, and the electrode geometry. Streamers are therefore critical in determining the reactive species population. We report results of numerical studies of steamers in different gas compositions at or near atmospheric pressures. The main effort of this work is to understand the production of reactive species and how it is influenced by the discharge physics. Three-dimensional Fluid models with azimuthal symmetry coupled with the Poisson's equation is used to simulate the streamer formation and propagation. Monte Carlo method is used to derive the transport properties from electron-impact cross sections for various gas mixture of technological importance. The results show that the tip of streamer is very efficient in the production of reactive species but the bulk of the fully formed streamer channels a significant amount of energy into gas heating. Overall a more uniform Townsend type discharge is more efficient for most species-type. However, for some high energy threshold species the streamer discharge is better. The G-factor (free radicals/100 eV) of several important species such as OH and O and its dependence on discharge parameters will be presented.   

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Electron transport across the magnetic field lines plays an important role in low-temperature plasmas. The plasma waves induced by kinetic instabilities, such as the electron cyclotron drift instability (ECDI), can de-trap electrons, leading to fluctuation-induced electron transport. We have recently investigated the effects of multidimensional plasma waves, initiated by both ECDI and the ion-ion two stream instability due to streams of singly- and doubly-charged ions, on the cross-field electron transport. These instabilities have been shown experimentally and analytically to coexist in $E \times B$ plasma devices. The numerical results capture not only the presence of both instabilities, but also show that the multidimensional plasma wave structure arising from their presence enhances the electron transport across the magnetic field lines.   

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We perform graph-theoretical analysis for extracting inherent information from complex reaction kinetics and devise a systematic way to rescale the network, where the scale-freeness and self-similarity in the network topology are maintained and the primary species are selected considering its topological centrality. Through a numerical simulation on atmospheric He$+$O2 plasma, it is demonstrated that the implementation of a simplified kinetic model allows the code to reproduce the plasma behavior obtained with an extensive model. The present chemical compression dramatically reduces the computational load. The proposed analytical approach enables us to exploit the full potential of expansive chemical reaction data and to achieve a more efficient and more robust kinetic system.   

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Plasmas at low pressures and atmospheric pressures are often produced with time-varying or pulsed electric fields. The temporal dependence of the electron energy distribution function in these time-varying fields is determined by different mechanisms: 1) electron energy gain that can be described by an energy diffusion time and 2) electron energy loss that is described by one (or more) electron energy relaxation times. This presentation compares simple analytical expressions for the relevant timescales to the solution of the time-dependent Boltzmann equation.   

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One of the most important aspects of any plasma discharge is how electrons gain energy from the electric field and channel it into the ionization reactions producing new particles needed for the discharge sustainment. In the present work we use numerical modeling to advance this knowledge for rf-magnetrons. Although the magnetic field geometry in planar magnetrons requires at least a 2D description in the configuration space for a realistic description, we use a simpler 1D geometry in order to illustrate some of important phenomena connected with the energy exchange between electrons and the electric field followed by the use of so acquired electron energy in the discharge life cycle. To reproduce the geometrical asymmetry leading to the establishment of a large negative self-bias, we use cylindrical geometry with powered inner electrode and a blocking capacitor. By conducting corresponding simulations for argon, we demonstrate that the electric field reversal, which arises due to the need to maintain the current continuity in the bulk plasma and charged particle flux balance at the electrodes in presence of the confining magnetic field, generates strong Hall currents, producing Ohmic heating and fast electrons with enough energy to participate in direct impact ionization.