Francesco Taccogna

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...

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.

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.

Three-dimensional structure of the extraction region of a hybrid negative ion source

A self-consistent three-dimensional particle-based model of the source extraction–acceleration transition region of a surface-produced negative ion source is developed. Some considerations are advanced on the characteristic of negative ion transport: it is purely electrostatic while collision-induced (charge exchange with atoms) and magnetic-induced (ion gyration around the filter field) transport contributions play no relevant role in H− extraction. In fact, the calculations presented here indicate that the key point is the penetration of the extraction grid field inside the plasma grid collar and the source region, which helps in removing the negative ions produced on the surface. This study suggests that the best plasma grid shape is characterized so as to allow the extraction field to arrive directly on the surface-emitting H− ions and that the best aperture size is directly related to the particular shape used.

Kinetic simulations of a plasma thruster

The modelling of the Hall thruster SPT-100 is a very important issue in view of the increasing importance of such propulsion devices in space applications. Only kinetic models can investigate the rich variety of physical mechanisms involved in the Hall discharge and in the plume emitted from the thruster. This paper collects a number of different particle-in-cell/Monte Carlo collision models which have been able to reveal different phenomena related to the peculiar physics of Hall thruster, such as sheath instability, azimuthal fluctuations and plume backflow.

Particle-in-Cell with Monte Carlo Simulation of SPT-100 Exhaust Plumes

A two-dimensional axisymmetric numerical code is developed for the simulation of a Hall thruster plume operating in various ambient plasmas. The code is based on a combination of particle simulation for the ionic components (Xe + and Xe + + ) and fluid computational techniques for electrons. In particular, we have used the Boltzmann relation modified in order to allow for the effect of nonisothermal electron temperature based on the adiabatic approximation. In our model several solutions, which have been sparsely considered in previous works, are jointly adopted. In particular, the electric field is computed by solving the Poisson equation without assuming quasi-neutrality, which often is violated in the near-field plume region; and collision processes are included by using two new techniques, the ion-neutral test-particle Monte Carlo collision model and the Nanbu cumulative small-angle collision theory for ion-ion coulombic collisions. Comparisons with experimental data suggest that the present simulation is accurately modeling the physics of the very near-field region of the plume.

Modeling of a negative ion source. II. Plasma-gas coupling in the extraction region

The production, destruction, and transport of H− in the extraction region of a negative ion source are investigated with a 1D(z)-3V particle-in-cell electrostatic code. The motion of charged particles (e, H+, H2+, and H−) in their self-consistent electric field is coupled with the neutral particles [H(n=1) and H2(X1∑g+,v=0,…,14)] dynamics and vibrational kinetics of H2. Neutral influxes into the domain are determined by the simulation of the expansion region. Surface and volumetric processes involving plasma and neutrals have been included by using different Monte Carlo collision methods. Calculations show the influence of the plasma grid bias and of the magnetic filter on the plasma parameter profiles. In particular, a transition from classical to complete reverse sheath is observed using a positively biased plasma grid. The influence of the magnetic filter is small. The importance of the hot-atom mechanism on the surface negative ion production is shown.

Modeling of a negative ion source I. Gas kinetics and dynamics in the expansion region

The vibrational population distribution of the electronic ground state of H2 in the expansion region of a negative ion source is investigated using a kinetic Monte Carlo model. Operative conditions are referred to the inductively coupled plasma radio frequency negative ion source developed at IPP-Garching. The different excitation and relaxation processes are discussed, both bulk and surface contributions. In particular, due to the relatively high plasma density, the relevant role of direct low energy electron-impact excitation, surface Auger neutralization, and vibration-translation deactivation are recovered. Results of the present model will be used as input data for the neutral source model in the extraction region.

Plasma flow in a Hall thruster

This work represents a two-dimensional (r,z)-3V axisymmetric fully kinetic particle-in-cell/Monte Carlo collision model of the plasmadynamics in the acceleration channel of a stationary plasma thruster. The model includes the process of secondary electron emission from the dielectric walls. In order to allow for a realistic simulation, differently from the previous fully kinetic model using a dummy mass ratio and vacuum permittivity or neglecting radial effects, a geometrical scaling of the channel is applied keeping the main dimensionless physics parameters constant. By this, the problem of the computational limits due to the very fast electron dynamics can be overcome. This model is able to give a clear picture of the plasma flow inside the acceleration channel. The results confirm the existence of an anode sheath with reverse ion flow, an ionization and acceleration region separated by a sonic transition point, and the ion flux distribution hitting the walls. Furthermore, the code is able to reproduce ...