Pierpaolo Minelli

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.

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.

High-Temperature Thermodynamic Properties of Mars- Atmosphere Components

Methods of calculation of high-temperature thermodynamic properties for some selected Mars-atmosphere components in the temperature range from 200 to 50,000 K and results are discussed and compared with previous works. Aspects such as quasi-bound rotational states, cutoff criteria, and autoionizing states are considered.

Physics of Hall-effect thruster by particle model

A realistic three-dimensional fully kinetic particle simulation of a Hall-effect thruster discharge has been developed. The model consists of a Particle-in-Cell methodology tracking electrons, Xe+ and Xe++ ions in their selfconsistent electric field. A detailed secondary electron emission from lateral walls is also implemented in addition with electron-atom and electron-ion volume collisions. The model is able to capture the most relevant features of axial, radial and azimuthal behaviors of the start-up transient and steady state phases detecting inverted sheaths and azimuthal modulation in the acceleration region. The model has the potentiality to investigate the driving mechanisms at the origin of the electron anomalous cross-field transport.

Direct simulation of non-linear interparticle collisional relaxation of ensembles of two-level systems

Abstract In this work, we study the particle kinetics of self-interacting two-level systems in gas phase. The translational relaxation is described by means of the direct simulation Monte Carlo method. The quantum state of the ensemble is described by a distribution function on the Riemann sphere as suggested in a recent paper [S. Longo, D. Bruno, M. Capitelli, P. Minelli, Chem. Phys. Lett. 316 (2000) 311]. Detailed results are presented for a uniform gas of spin 1/2 atoms relaxing under superimposed fixed and rotating magnetic fields.

Direct Simulation Monte Carlo Modeling of Non Equilibrium Reacting Flows. Issues for the Inclusion into a ab initio Molecular Processes Simulator

Direct Simulation Monte Carlo (DSMC) method for the modeling of non equilibrium reacting flows is presented. The is particularly suitable for the simulation of gas-phase systems with complex boundary conditions and with varying degrees of thermal and chemical non equilibrium. Since the description is done at the kinetic level, detailed information about the elementary processes, as derived from ab initio molecular dynamics calculations, can be used as input physical data for the simulation. These features make the DSMC method an ideal candidate for inclusion into a ab initio Molecular Processes Simulator for the gas phase.

About the Extraction of Surface Produced Ions in Negative Ion Sources

The enhancement of extracted negative ion current in cesiated sources is usually explained by the surface production of negative ions. In this contribution, the self‐consistent production and transport of H− in the extraction region of a radio‐frequency driven negative ion source is modelled by means of a parallel two‐dimensional Particle‐in‐Cell/Monte Carlo Collision simulation. It is shown that the number of surface‐produced negative ions extracted is regulated by a potential well developed in front of the plasma grid such that the extracted current does not proportionally increase with the flux of negative ions emitted at the surface.