A Monte Carlo method has been developed for the calculation of binary diffusion coefficients in gas mixtures. The method is based on the stochastic solution of the linear Boltzmann equation obtained for the transport of one component in a thermal bath of the second one. Anisotropic scattering is included by calculating the classical deflection angle in binary collisions under isotropic potential. Model results are compared to accurate solutions of the Chapman-Enskog equation in the first and higher orders. We have selected two different cases, H(2) in H(2) and O in O(2), assuming rigid spheres or using a model phenomenological potential. Diffusion coefficients, calculated in the proposed approach, are found in close agreement with Chapman-Enskog results in all the cases considered, the deviations being reduced using higher order approximations. (C) 2011 Elsevier Inc. All rights reserved.

The modeling of atmospheric gas, interacting with the space vehicles in re-entry conditions in planetary exploration missions, requires a large set of scattering data for all those elementary processes occurring in the system. A fundamental aspect of re-entry problems is represented by the strong non-equilibrium conditions met in the atmospheric plasma close to the surface of the thermal shield, where numerous interconnected relaxation processes determine the evolution of the gaseous system towards equilibrium conditions. A central role is played by the vibrational exchanges of energy, so that collisional processes involving vibrationally excited molecules assume a particular importance. In the present paper, theoretical calculations of complete sets of vibrationally state-resolved cross sections and rate coefficients are reviewed, focusing on the relevant classes of collisional processes: resonant and non-resonant electron-impact excitation of molecules, atom-diatom and molecule-molecule collisions as well as gas-surface interaction. In particular, collisional processes involving atomic and molecular species, relevant to Earth (N-2, O-2, NO), Mars (CO2, CO, N-2) and Jupiter (H-2, He) atmospheres are considered.

The process of dissociative attachment of electrons to molecular hydrogen and its isotopes in the energy range at approximately 14 eV is investigated. The dissociative electron attachment cross sections for all six hydrogen isotopes are calculated over an extended range of electron energies using the local complex potential model with the excited Rydberg 2?+g electronic state of H2- acting as the intermediate resonant state. A significant isotope effect in theoretical electron attachment cross sections is observed, in agreement with previous predictions and experimental observations. A two-parameter analytic expression for the cross section is derived from the theory that fits accurately the numerically calculated cross sections for all isotopes. Similarly, an analytic mass-scaling relation is derived from the theory that accurately reproduces the numerically calculated rate coefficients for all isotopes in the 0.1-1000 eV temperature range by using the rate coefficient for the H2 isotope only. The latter is represented by an analytic fit expression with two parameters only.

We report cross-sections and rate coefficients for excited states colliding with electrons, heavy particles and walls useful for the description of H-2/He plasma kinetics under different conditions. In particular, the role of the rotational states in resonant vibrational excitations of the H-2 molecule by electron impact and the calculation of the related cross-sections are illustrated. The theoretical determination of the cross-section for the rovibrational energy exchange and dissociation of H-2 molecule, induced by He atom impact, by using the quasi-classical trajectory method is discussed. Recombination probabilities of H atoms on tungsten and graphite, relevant for the determination of the nascent vibrational distribution, are also presented. An example of a state-to-state plasma kinetic model for the description of shock waves operating in H-2 and He-H-2 mixtures is presented, emphasizing also the role of electronically-excited states in affecting the electron energy distribution function of free electrons. Finally, the thermodynamic properties and the electrical conductivity of non-ideal, high-density hydrogen plasma are finally discussed, in particular focusing on the pressure ionization phenomenon in high-pressure high-temperature plasmas.

Non-equilibrium effects in hydrogen plasmas have been investigated in different systems, ranging from RF plasmas to corona discharges. The existing measurements of vibrational and rotational temperatures, obtained by different spectroscopical techniques, are reported, rationalized by results calculated by kinetic models. Input data of these models are discussed with particular attention on the dependence of relevant cross sections on the vibrational quantum number. Moreover, the influence of the vibrational excitation of H(2) molecules on the translational distribution of atoms in ground and excited levels is shown. Finally, a collisional radiative model for atomic hydrogen levels, based on the coupling of the Boltzmann equation for electron energy distribution function (EEDF) and the excited state kinetics, is presented, emphasizing the limits of quasi-stationary approximation. In the last case, large deviations of the EEDF and atomic level distributions from the equilibrium are observed.

A Graphics Processing Unit (GPU)-CUDA C and (Multi-core)-OpenMP versions of the Reaction Ensemble Monte Carlo method (REMC) are presented. The REMC algorithm is a powerful tool to investigate the equilibrium behavior of chemically reacting systems in highly non-ideal conditions. Both the GPU and the Multi-core versions of the code are particularly efficient when the total potential energy of the system must be calculated, as in the constant-pressure systems. Results, obtained in the case of Helium plasma at high pressure, show differences between real and ideal cases.

The space exploration is nowadays assisted by realisticmodeling of re-entry conditions of space vehicles inplanetary atmospheres, for design of thermal protection systems.The detailed kinetic modeling relies on the accuratedescription of elementary process dynamics, including therole of the excitation of the internal degrees of freedom ofchemical species. Efforts in the construction of a completeand consistent dynamical information are presented, focusingon hydrogen plasmas relevant to the Jupiter atmosphere.Examples of numerical experiments are given, emphasizingthe relevance of the adopted state-to-state approach inreproducing the onset of non-equilibrium internal distributionsgoverning the plasma-system evolution.

The role of vibrational excitation in affecting the dissociation under discharge conditions characterized by reduced electric field E/N ? 80 Td has been investigated in N2. The kinetic calculations have been performed using a self-consistent approach, solving at the same time the master equation for the composition and the distribution of internal states (vibrational and electronic) and the Boltzmann equation for the electron energy distribution function. The results show that vibrational mechanisms involving heavy particle excited states dominate electron impact dissociation mechanisms involving the whole vibrational ladder for E/N < 50 Td, the two mechanisms being competitive for E/N > 50 Td.

State-to-state non-equilibrium plasma kinetics is widely used to characterize cold molecular and reentry plasmas. The approach requires a high level of dynamical information, and demands a large effort in the creation of complete databases of state-resolved cross sections and rate coefficients. Recent results, emphasizing the dependence of elementary process probability on both the vibrational and rotational energy content of the H2 molecule, are presented for those channels governing the microscopic collisional dynamics in non-equilibrium plasmas, i.e. electron-impact induced resonant processes, vibrational deactivation and dissociation in atom-diatom collisions and atomic recombination at the surface. Results for H2 plasmas, i.e. negative ion sources for neutral beam injection in fusion reactors, RF parallel-plate reactors for microelectronics, atmospheric discharges and the shock wave formed in the hypersonic entry of vehicles in planetary atmosphere for aerothermodynamics, are discussed.

A simple equation describing the formation of plateaux induced by superelastic collisions in the electron energy distribution function (EEDF) of low temperature and afterglow plasmas is derived. The EEDFs predicted from this equation are in good agreement with those obtained from the numerical solution of the full Boltzmann equation in the presence of excited states.

A collisional-radiative model for the H2/He plasma, coupled to a Boltzmann solver for the free electron kinetics is used to investigate the non-equilibrium conditions created in the expansion of an high-temperature plasma flow through a converging-diverging nozzle, starting from the steady state composition at T 0 = 10 000 K and p 0 = 1 atm in the reservoir. It is shown that the plasma optical thickness plays a major role on the evolution of macroscopic quantities and internal distributions along the nozzle axis. Structured electron energy distribution functions, characterized by long plateaux and humps, are created due to superelastic collisions of cold electrons and electronically excited atomic hydrogen. The magnitudes of the plateaux are orders of magnitude higher in an optically thick plasma compared with a thin plasma, while the electron-electron collisions play a role in smoothing the peaks created by superelastic collisions between cold electrons and H (n > 2).

Transport properties of high-temperature helium and hydrogen plasmas as well as Jupiter atmosphere have been calculated for equilibrium and nonequilibrium conditions using higher approximations of the Chapman-Enskog method. A complete database of transport cross sections for relevant interactions has been derived, including minority species, by using both ab initio and phenomenological potentials Inelastic collision integrals terms, due to resonant charge-exchange channels, have been also considered.

Different mechanisms of CO<inf>2</inf> dissociation, in discharge and post-discharge conditions, have been computed by performing a parametric numerical solution of the electron Boltzmann equation as a function of the electric field, the ionization degree and the vibrational temperatures and by considering elastic, inelastic, superelastic and electron electron collisions. Emphasis is given to the role of superelastic electronic and vibrational collisions in affecting the electron energy distribution function and relevant rates. The results show that, at low E/N values, the dissociation rates from pure vibrational mechanism can overcome the corresponding rates of electron impact dissociation. In any case, the electron impact dissociation rates are largely dependent on the transitions from excited vibrational levels.