C. Gorse

Progress in the non-equilibrium vibrational kinetics of hydrogen in magnetic multicusp H− ion sources

Abstract The non-equilibrium vibrational kinetics of H 2 in multicusp magnetic discharges has been studied by improving a previous model developed by our groups. In particular, a complete set of V-T (vibrational translation) rates involving H-H 2 ( v ) collisions, calculated by using a three-dimensional dynamics approach, has been inserted into our self-consistent model for better representing the corresponding relaxation. Different experimental situations are simulated with special emphasis on the temporal scales necessary for the different distributions (electron energy and vibrational distributions) to reach stationary values. Finally, a comparison between theoretical and experimental quantities such as vibrational temperature, electron temperature, electron number density and concentration of negative ions (H − ) shows a satisfactory agreement, thus indicating the basic correctness of our model.

Non-equilibrium dissociation and ionization of nitrogen in electrical discharges: The role of electronic collisions from vibrationally excited molecules☆

Abstract Dissociation and ionization rates by electron impact from vibrationally excited nitrogen molecules have been calculated by using a set of cross sections obtained with the Gryzinski method and a self-consistent electron energy distribution function. The results show that in both processes, the presence of vibrationally excited molecules enhances the direct electronic mechanisms (DEM). In the case of dissociation, DEM is competitive with the pure vibrational mechanism (PVM), while in the ionization process DEM gives ionization rates several orders of magnitude smaller than PVM. A satisfactory agreement is finally found between theoretical and experimental dissociation and ionization rates.

Quasi-classical dynamics and vibrational kinetics of N+N2(v) system

Abstract Vibrational–translational (V–T) energy transfer and dissociation cross-sections of collisions of nitrogen atoms with rovibrationally excited nitrogen molecules are calculated by a quasi-classical dynamical approach. The results, relative to the whole set of vibrational states and interpolated on rotation, including quasi-bound levels, are used in a kinetic scheme for studying the global rate of dissociation. The kinetic model, which solves a system of 68 vibrational levels submitted to the action of V–T and dissociation processes, reveals some distortions in the resulting vibrational distribution function for v >40. The calculated global dissociation rate is in good agreement with the experimental results in a wide temperature range.

Vibrational excitation and negative-ion production in magnetic multicusp hydrogen discharges

Abstract A self-consistent approach based on the simultaneous solution of the vibrational master equation, the Boltzmann equation and the plasma chemistry describing the dissociation process has been used to obtain: (a) vibrational distributions, (b) electron energy distribution functions, (c) electron number densities ( n c ) and electron temperatures ( T c ), (d) degree of dissociation, and (e) the concentration of negative ions ( N H − ) in magnetic multicusp H 2 plasmas. The approach is an extension of a model presented earlier. The main differences come from the insertion of a source term (an electron beam) instead of the electric field in the Boltzmann equation and from the inclusion of new important processes in the vibrational master equation. The results which are in satisfactory agreement with the experimental vibrational distributions obtained by CARS spectroscopy, with the n c , T c values obtained by probe techniques and with the production of negative ions obtained by a photodetachment technique, show the importance of hydrogen atoms and of wall deactivation in affecting the properties of multicusp H 2 plasmas.

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.

Deactivation dynamics of vibrationally excited nitrogen molecules by nitrogen atoms. Effects on non-equilibrium vibrational distribution and dissociation rates of nitrogen under electrical discharges

Abstract The non-equilibrium vibrational distribution and the dissociation kinetics of N2 in electrical discharges has been calculated by solving an appropriate vibrational master equation. Attention has been devoted to the role of nitrogen atoms in deactivation the vibrational distribution. To this end, a complete set of V—T (vibration—translation) deactivation rates of vibrationally excited molecules by nitrogen atoms has been calculated using a quasiclassical trajectory technique. The results show that nitrogen atoms formed by electron impact are able to deactive the high vibrational levels (v > 25) of N2 strongly affecting the heavy particle dissociation kinetics of N2.

The effect of N+N2 collisions on the non-equilibrium vibrational distributions of nitrogen under reentry conditions

Abstract Global (reactive+non-reactive) quasiclassical state-specific rate coefficients for the vibrational excitation and deexcitation of nitrogen molecules by collision with nitrogen atoms have been used to model a reacting flow. The one-dimensional model used describes the relaxation of N 2 vibrational distributions in the boundary layer surrounding a shuttle reentering the atmosphere. The results single out the strong non-equilibrium character of the vibrational distributions along the coordinate normal to the shuttle surface caused by the dissociation-recombination processes. The results also show the dependence of the vibrational distributions upon inclusion in the model of state-specific N+N 2 ( v ) rates.

Electron energy distribution functions under N2 discharge and post-discharge conditions: A self-consistent approach

Abstract Electron energy distribution functions (EDF) under N 2 discharge and post-discharge conditions have been calculated by self-consistently solving the Boltzmann equation, the vibrational master equation (including dissociation) and the kinetics of the most important electronic states of N 2 . The results show that the relaxation of EDF in the post-discharge is strongly linked to the residence time of N 2 in the discharge, which determines the initial conditions. In particular at low residence times the role of metastable states in affecting EDF through superelastic electronic collisions (SEC) overcomes the corresponding one from superelastic vibrational collisions (SVC). On the contrary, SVC dominate at long residence times, when a well-developed vibrational distribution has been built up by the discharge. At intermediate residence times both SVC and SEC affect the relaxation of EDF, which in general presents a non-monotonic behaviour.

V—V pumping up in non-equilibrium nitrogen: Effects on the dissoviation rate

Abstract Non-equilibrium dissociation rates of N 2 in electrical discharges have been calculated by solving a system of vibrational master equations, including e—V (electron—vibration), V—V (vibration—vibration), V—T (vibration—translation) energy exchanges. V—V and V—T rates have been obtained by using a semiclassical model, while the e—V rates have been calculated by averaging the corresponding cross sections with a self-consistent electron energy distribution function. The results show that in the reduced electric field range 3 × 10 −16 V cm 2 ≤ E/N ≤ 6 × 10 −16 V cm 2 , n e = 10 11 cm 3 , p = 5 Torr, T = 500 K the contribution to the dissociation rate coming from the vibrational mechanism overcomes the corresponding one from electronic dissociative collisions. A satisfactory agreement between the present theoretical results and the experimental ones is found specially at low E/N values.

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.