Savino Longo

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.

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.

Plasma sheaths in Hall discharge

The sheath region of a Hall discharge is studied in a four-dimensional phase space which consists of one spatial (radial in cylindrical metrics) and three velocity dimensions by means of a particle-in-cell∕Monte Carlo model coupled with a probabilistic method for the secondary electron emission. Different axial regions (anode, ionization, and acceleration zones) of the channel have been investigated using the local field approximation and distinguishing between inner and outer walls. The presheath and sheath structures are different in the three regions simulated showing a charge saturated regime in the acceleration region. Small differences in behavior for the external and internal walls of the channel are detected. Further, trapped ions are found near the walls in the acceleration region which could have an important effect on the wall recombination enhancing the axial electron current. The results could be used to obtain boundary conditions and lateral wall losses which are suitable for incorporation i...

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.

Direct simulation of non-equilibrium kinetics under shock conditions in nitrogen

Abstract We study the interplay of vibrational kinetics, dissociation, translational and rotational relaxation in a strong shock wave in nitrogen by Direct Simulation Monte Carlo simulation (DSMC). The input data for vibrational and chemical processes are all in the form of cross-sections, mostly determined by molecular physics methods. In particular, we use for the first time very recent Quasi-Classical Trajectory (QCT) results for cross-sections of multi-quantum VT energy exchange and dissociation in N+N 2 collisions. Non-equilibrium distributions are observed and discussed.

Plasma-surface interaction model with secondary electron emission effects

This work represents the investigation of the region between a Maxwellian plasma source and a floating surface by a 1D–3V fully kinetic, electrostatic particle simulation. The electric field is self-consistently computed from the Poisson equation. The secondary electron emission is modelled rigorously by considering a realistic expression for the secondary emission coefficient dependent on the primary electron energy, the angle relative to the surface normal and surface materials and a realistic secondary electron distribution function is introduced at the collector surface. The minimum ion energy at the collector sheath edge is evaluated self-consistently by determining the plasma sheath, without the assumption of a monotonic potential. The model is able to simulate the space charge limited conditions as well as the positively charged wall cases. Results are compared with other secondary emission sheath theories and numerical models.

Modeling of a negative ion source. III. Two-dimensional structure of the extraction region

The self-consistent production and transport of H− in the extraction region of a hybrid negative ion source is modeled by means of a two-dimensional particle-in-cell/Monte Carlo simulation. The normal coordinate and one parallel coordinate with respect to the plasma grid are considered to analyze the transport of negative ions. Results show that, in order to establish space charge compensation, the extraction of surface-produced negative ions is limited by the flux of positive ions directed toward the plasma grid surface. An electrostatic barrier appears just in front of the wall, reflecting the majority of surface-produced H− and reducing by this their extraction probability to only 8.5%. Results reproduce the experimentally observed influence of the plasma grid bias voltage on the extraction identifying as a key element the presence of a saddle point in the electric potential distribution.

Monte Carlo models of electron and ion transport in non-equilibrium plasmas

The fundamentals of the Monte Carlo method for electron and ion transport in plasmas are explained with emphasis on the coupling of the particle transport with the space charge fields and chemical kinetics. In the first three sections the basics of the null-collision Monte Carlo methods and the particle in cell/Monte Carlo collision method (PIC/MCC) are explained with practical suggestions for numerical implementation. The fourth section shows that the Monte Carlo method can be derived from the linear Boltzmann equation. In the fifth section, we show how the charged particle transport can be self-consistently coupled to the chemical kinetics of the gas phase, in particular to the vibrational kinetics of molecules. In the penultimate section a one-dimensional, self-consistent model for a parallel-plate radio-frequency discharge in pure hydrogen is presented.

Effect of discharge voltage on capacitively coupled, parallel plate rf hydrogen plasmas

In this paper we present a parametric study of the effect of discharge voltage on capacitively coupled, parallel plate (CCPP) radio frequency discharges in pure hydrogen at low pressure, performed using a 1D(r)2D(v) particle in cell/Monte Carlo collision model with self-consistent neutral kinetics and also compare our results with experimental and theoretical ones reported in the literature. In the first part of the paper, we review the essential features of the numerical code, together with the database of plasma particles and neutral kinetics data. Results are discussed, in particular, for charged particle density and energy, the appearance of the double layer phenomenon, the plasma potential and the atom density. A possible role of photoelectric emission in the charged particle balance is also discussed.