Annarita Laricchiuta

Effect of electronic excited states on transport in magnetized hydrogen plasma

Multicomponent diffusion coefficients for magnetized, equilibrium hydrogen plasma have been calculated. The equilibrium composition of the plasma is determined by taking consistently into account the number of allowed atomic electronic excited states (EES) as determined by the average interparticle distance. The coefficients are shown to depend on the inclusion of realistic cross sections for the interactions with EES. The effect of an applied magnetic field on the diffusion coefficients and on derived quantities like the electrical conductivity and the internal and reactive thermal conductivity is studied and explained.

Non-equilibrium plasma kinetics: a state-to-state approach

State-to-state approaches are used to shed light on (a) thermodynamic and transport properties of LTE plasmas, (b) atomic and molecular plasmas for aerospace applications and (c) RF sustained parallel plate reactors. The efforts made by the group of Bari in the kinetics and dynamics of electrons and molecular species are discussed from the point of view of either the master equation approach or the molecular dynamics of elementary processes. Recent experimental results are finally rationalized with a state-to-state kinetics based on the coupling of vibrational kinetics with the Boltzmann equation for the electron energy distribution function.

Vibrational kinetics, electron dynamics and elementary processes in H2 and D2 plasmas for negative ion production: modelling aspects

We report current and past efforts made by our group in the ab initio modelling of different negative ion sources. In particular, we discuss the cross sections of elementary processes relevant to negative ion kinetics, including electron?molecule, atom?molecule and atom/molecule gas surface interactions, particularly emphasizing the role of vibrational excitation in affecting the cross sections. Attention is also paid to the elementary processes involving caesium in both volume and surface sources.Self-consistent models, which couple the Boltzmann equation and the vibrational kinetics, are used for describing multipole and rf discharges, while a PIC-MC (particle in cell) with Monte Carlo collisions is used to study electron and ion dynamics in a parallel plate reactor in the post-discharge regime. The present theoretical results should encourage further dedicated experimental work in the field.

Fundamental aspects of plasma chemical physics

Electron-molecule cross sections and rates involving rotationally, vibrationally and electronically excited states.- Reactivity and relaxation of vibrationally/rotationally excited molecules with open shell atoms.- Formation of vibrationally and rotationally excited molecules during atom recombination on surfaces.- Collisional-radiative models for atomic plasmas.- Collisional-radiative models for molecular plasmas.- Kinetic and Monte Carlo approaches to solve Boltzmann equation for the electron energy distribution functions.- Non-equilibrium plasma kinetics under discharge and post-discharge conditions: coupling problems for low pressure and atmospheric cold plasmas.- Ion transport under strong fields.- PIC (Particle In Cell ) models for low-pressure plasmas.- Negative ion H- for fusion.- Non equilibrium plasma expansion through nozzles.

Convergence of Chapman-Enskog calculation of transport coefficients of magnetized argon plasma

Convergence properties of the Chapman-Enskog method in the presence of a magnetic field for the calculation of the transport properties of nonequilibrium partially ionized argon have been studied emphasizing the role of the different collision integrals. In particular, the Ramsauer minimum of electron-argon cross sections affects the convergence of the Chapman-Enskog method at low temperature, while Coulomb collisions affect the results at higher temperatures. The presence of an applied magnetic field mitigates the slow convergence for the components affected by the field.

The role of electron scattering with vibrationally excited nitrogen molecules on non-equilibrium plasma kinetics

Electron energy distribution functions have been calculated by a self-consistent model which couples the electron Boltzmann equation with vibrationally and electronically excited state kinetics and plasma chemistry. Moderate pressure nitrogen gas discharges in the E/N range from 30 to 60 Townsend are investigated comparing an electron-impact cross section set considering transitions starting from all the vibrational states, with reduced models, taking into account only collisions involving the ground vibrational level. The results, while confirming the important role of second kind collisions in affecting the eedf, show a large dependence of the eedf on the set of inelastic processes involving vibrationally and electronically excited molecules, pointing out the need of using a cross section database including processes linking excited states in non-equilibrium plasma discharge models.

Transport of internal electronic energy in atomic hydrogen thermal plasmas

Reactive and internal thermal conductivities for equilibrium hydrogen plasma have been calculated by the Chapman-Enskog method. The equilibrium composition of the plasma is determined by taking consistently into account the number of allowed atomic electronic excited states (EES) as determined by the average interparticle distance. The coefficients depend on the inclusion of realistic cross sections for the interactions with EES. In particular, the interplay between the two coefficients that describe the transport of electronic and ionization energy is analyzed.

Thermodynamics, Transport and Kinetics of Equilibrium and Non-Equilibrium Plasmas: A State-to-State Approach

Thermal non-equilibrium plasmas have been deeply investigated theoretically by means of the state-to-state approach, offering the unique opportunity of a detailed information about internal distributions affecting thermodynamics, transport coefficients and kinetics, properly accounting for the presence of excited states. The efforts made in the construction of knowledge on the dynamics of elementary processes occurring in the plasma with resolution on internal degrees of freedom, required by the method, are discussed. Boltzmann equation is solved for electrons self-consistently coupled to the chemical species collisional dynamics, reproducing very interesting features of strongly non-equilibrium internal distributions, characterizing plasmas.

Vibrational excitation and dissociation mechanisms of CO2 under non-equilibrium discharge and post-discharge conditions

Different mechanisms of CO2 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.

Thermodynamics and transport properties of thermal plasmas: the role of electronic excitation

The role of electronic excited states in affecting the thermodynamic and transport properties of thermal plasma is investigated in the temperature range [300‐100 000 K] and in the pressure range [1‐10 3 atm] for hydrogen and [10 −2 ‐10 3 atm] for nitrogen. Thermodynamic functions have been calculated modelling in different ways the electronic levels of atomic species (ground-state, Debye‐H¨ uckel and confined-atom approximations). Frozen and reactive specific heats as well as isentropic coefficients are strongly affected by the electronic excitation whereas compensation effects smooth its influence on the total specific heat, i.e. the sum of frozen and reactive contributions. Higher-order approximations of the Chapman‐Enskog method have been used to evaluate transport coefficients, including electronically excited states as separate species. The importance of a state-to-state approach to calculate transport coefficients is presented taking into account the strong dependence of transport cross sections on the principal quantum number. Results for hydrogen, nitrogen and air plasmas are widely discussed.