The near-field plume region of Hall-effect thruster is investigated using a three-dimensional Particle-in-Cell/Monte Carlo collision (PIC-MCC) model. A detailed electron-surface interaction model has been implemented on the thruster exit plane. Results show the important role of magnetic field in the first 4 cm and of the azimuthal fluctuation together with asymmetry driven by the cathode and with the plasma-surface interaction on the exit plane.

A fully kinetic self-consistent model of an absorbing particle immersed in stationary isotropic weakly collisionalplasma has been developed. The combined effects of particle size and ion-neutral charge exchange collisionshave been investigated for intermediate regimes, where no analytic theories are available. It is shown thatcollisional effects related to the ion orbital destruction (presence of extrema in ion flux collected on the particlesurface and in particle potential and charge) are important for small particles, while they are totally absent forlarge particles. The potential distribution around the particle is quite well represented by a Yukawa form, butwith an effective screening length that shows different dependences from the gas pressure for small and largeparticle size. Analytical fitting formulas of particle charge and potential and screening length depending on theparticle radius parameter and on the Knudsen number have been obtained.

In this second paper, the effect of secondary electrons on the charge and potential of a dust particle immersed in plasma has been studied. The processes of electron-induced ionization and those of photo-electron and secondary electron emission from the particle surface as a function of primary electron temperature have been taken into account. Starting from temperatures as low as 6 eV in an Ar plasma, ionization produces an extra ion flux to the dust surface comparable to that of the ion charge exchange effect. For what concerns the surface emission, results show that a transition from negative to positive dust charge/potential takes place, and that the transition regime is characterized by a non-monotonic behavior of the electric potential around the particle. In the case of photoelectric emission, the dust charge and potential are monotonic decreasing functions of the electron temperature, while in the case of emission induced by primary electrons a minimum charge/potential is reached before they grow towards positive values. In no case multiple dust charge states have been observed due to the presence of the potential well attached to the particle surface. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The particle-in-cell (PIC) method is used to study two different ion thruster concepts-stationary plasma thrusters (SPTs) and high-efficiency multistage plasma thrusters (HEMP-Ts)-in particular, the plasma properties in the discharge chamber due to the different magnetic field configurations. Special attention is paid to the simulation of plasma particle fluxes on the thrusters' channel surfaces. In both cases, PIC proves itself as a powerful tool, delivering important insight into the basic physics of the different thruster concepts. The simulations demonstrated that the new HEMP-T concept allows for a high thermal efficiency due to both minimal energy dissipation and high acceleration efficiency. In the HEMP-T, the plasma contact to the wall is limited only to very small areas of the magnetic field cusps, which results in small ion energy flux to the thruster channel surface. The erosion yields for dielectric discharge channel walls of SPT and HEMP-Ts were calculated with the binary collision code SDTrimSP. For HEMP, thruster simulations have shown that there is no erosion inside the dielectric discharge channel.

In the present work, laser ablation of a graphite target submerged inpure water was tested as a methodology for the production of carbon-basednanoparticles. The effect of varying the external pressure imposed to the liquidwas investigated for the first time, in the range from 1 to 146 atm. Single or doublelaser pulses were used to ablate the target and the produced nanoparticles werecharacterized by atomic force microscopy (AFM) and by Raman spectroscopy. Aspectroscopic study of the laser induced plasma features was carried out with a Titarget and interpreted in terms of laser-induced cavitation phenomena. Tubularnanoparticles of 25 nm average diameter were obtained only by single pulse (SP)ablation of graphite, while particles formed with the double pulse (DP) techniquemainly consisted of graphite particulates. At 1 atm, these tubular nanoparticles were few and mixed with diamondlike carbon,while at 146 atm they were produced in a larger amount, suggesting that the high density effect induced by pressure plays a keyrole for their generation.

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 potentialdistribution.

This work shows the basic foundation of the particle-based representation of low temperature plasma description. In particular, the Monte Carlo Collision (MCC) recipe has been described for the case of electron-atom and ion-atom collisions. The model has been applied to the problem of plasma plume expansion from an electric Hall-effect type thruster. The presence of low energy secondary electrons from electron-atom ionization on the electron energy distribution function (EEDF) have been identified in the first 3 mm from the exit plane where, due to the azimuthal heating the ionization continues to play an important role. In addition, low energy charge-exchange ions from ion-atom electron transfer collisions are evident in the ion energy distribution functions (IEDF) 1 m from the exit plane. Copyright © Cambridge University Press 2014.

More self-consistent injection boundary conditions from the source region have been used in the extraction region model to examine the negative ion formation and transport. Bulk kinetic, plasma-surface, and gas-surface processes have been all included. This work represents a first example of coupling between different models, and it shows the important role of positive ion conversion on plasma grid for the extracted negative ion current.

A two-dimensional particle-in-cell/Monte Carlo collision model has been developed and used to study low electronegative magnetized hydrogen plasma. A configuration characterized by four electrodes is used: the left electrode is biased at Vl 1/4 ?100 V, the right electrode is grounded, while the upper and lower transversal electrodes are biased at an intermediate voltage Vud between 0 and ?100 V. A constant and homogeneous magnetic field is applied parallel to the lateral (left/right) electrodes. It is shown that in the magnetized case, the bulk plasma potential is close to the transversal electrodes bias inducing then a reversed sheath in front of the right electrode. The potential drop within the reversed sheath is controlled by the transversal electrodes bias allowing extraction of negative ions with a significant reduction of co-extracted electron current. Furthermore, introducing plasma electrodes, between the transversal electrodes and the right electrode, biased with a voltage just above the plasma bulk potential, increases the negative ion extracted current and decreases significantly the co-extracted electron current. The physical mechanism on basis of this phenomenon has been discussed.

The collisionless plasma sheath represents an important example of Vlasov theory application. In this study, Particle-in-Cell/Monte Carlo Collision methodology has been used to study different examples of plasma sheaths under strong negative charge emission from surface. Secondary electrons emitted by primary electrons (acceleration region of Hall-effect discharge) and by photons (dusty plasma) are responsible for a complete inverse sheath: the potential monotonically increases toward a positively charged wall that is shielded by a single layer of negative charge. No ion-accelerating presheath exists in the bulk plasma region and the ion flux at the wall is zero. In the case of production of hydrogen negative ions by neutral conversion on the plasma grid in the extraction region of a negative ion source, a space-charge-limited regime occurs with the formation of a non-monotonic double layer in front of the grid.

Laser photo-detachment is used as a method to measure or determine the negative ion density and temperature in electronegative plasmas. In essence, the method consists of producing an electropositive channel (negative ion free region) via pulsed laser photo-detachment within an electronegative plasma bulk. Electrostatic probes placed in this channel measure the change in the electron density. A second pulse might be used to track the negative ion recovery. From this, the negative ion density and temperature can be determined. We study the formation and relaxation of the electropositive channel via a two-dimensional Particle-In-Cell/Mote Carlo collision model. The simulation is mainly carried out in a Hydrogen plasma with an electronegativity of alpha = 1, with a parametric study for alpha up to 20. The temporal and spatial evolution of the plasma potential and the electron densities shows the formation of a double layer (DL) confining the photo-detached electrons within the electropositive channel. This DL evolves into two fronts that move in the opposite directions inside and outside of the laser spot region. As a consequence, within the laser spot region, the background and photo-detached electron energy distribution function relaxes/thermalizes via collisionless effects such as Fermi acceleration and Landau damping. Moreover, the simulations show that collisional effects and the DL electric field strength might play a non-negligible role in the negative ion recovery within the laser spot region, leading to a two-temperature negative ion distribution. The latter result might have important effects in the determination of the negative ion density and temperature from laser photo detachment diagnostic. (C) 2014 AIP Publishing LLC.

Kinetic modelling of nonequilibrium flows is described as it applies to hypersonic phenomenology. A Monte Carlo method is described for the study of species separation on shock wave fronts; a Particle in Cell with Monte Carlo Collisions (PIC/MCC) technique is described for the simulation of dust in plasma flows; modelling of nonequilibrium radiation in shock heated gases; implementation of slip models for the description of separation zones occurring in shock- boudary layer interactions.

In this paper we present results of different particle-based models of expansion and extraction regions for the ITER negative ion source. Results show the important role of electron-induced processes (eV excitation and dissociation) and gas-surface and plasma-surface interactions (atom recombinative desorption and ion neutralization) for the production of negative ion precursors. The spatial distribution of neutral and ionized caesium as well as the drag due to the deuterium flow and its re-evaporation from the plasma grid is calculated. The self-consistent two-dimensional electric field distribution in the extraction region allows a better understanding of the transport of surface-produced negative ions towards the orifice. The model delivers an extraction probability of less than 25%.

There are still many missing elements to complete the full understanding of physical mechanisms at the basis of the Hall thruster functioning. The origin of the anomalous electron cross-field transport remains unrevealed, while electron-wall interaction is often studied by local and/or reduced dimensional models. In this study, we attempt a fully kinetic self-consistent 3D particle-based simulation of the Hall-effect discharge. Results show the presence of multi-dimensional sheath structures in the acceleration region characterized by azimuthal modulation and axial transition from classical to reversed sheath.

By means of a self-consistent three-dimensional particle model of the source-extraction transition region of a surface-produced negative ion source, the characteristics of negative ion transport have been revealed. It is purely electrostatic and collision-induced (charge exchange with atoms) and magnetic-induced (gyration around filter field) transport contributions play no relevant role for the H- transport. In fact, the key point is the penetration of the EG field inside the source, which helps removing negative ions produced on the surface. This study suggests that the best PG shape is characterized such to allow the extraction EG field from attaching the surface from where H- are produced and that the best aperture size is directly related to this particular shape.

State-to-state non-equilibrium plasma kinetics is widely used to characterize cold molecular and reentry plasmas. The approach requires a high level of dynamical information, and demands a large effort in the creation of complete databases of state-resolved cross sections and rate coefficients. Recent results, emphasizing the dependence of elementary process probability on both the vibrational and rotational energy content of the H2 molecule, are presented for those channels governing the microscopic collisional dynamics in non-equilibrium plasmas, i.e. electron-impact induced resonant processes, vibrational deactivation and dissociation in atom-diatom collisions and atomic recombination at the surface. Results for H2 plasmas, i.e. negative ion sources for neutral beam injection in fusion reactors, RF parallel-plate reactors for microelectronics, atmospheric discharges and the shock wave formed in the hypersonic entry of vehicles in planetary atmosphere for aerothermodynamics, are discussed.

Modern problems of radiative aerothermodynamics of entering space vehicles are demonstrated and analyzed in the paper.New radiative gas dynamic problems concerned to coupling processes of non-equilibrium dissociation with radiation heat transfer in shock layers generated above large scale re-entry space vehicles returning from orbital and super orbital space mission are considered in the first part.Three-dimensional numerical simulation data on radiative aerothermodynamics of Martian entry probes Pathfinder, Exomars and Mars Science Laboratory (MSL) are presented and analyzed in the second part. It is shown that integral radiative heating of leeward surface of the entry probes exceeds corresponding convective heating.The third part is dedicated to consideration preliminary numerical simulation results on radiative gas dynamics of Galileo probes. At first, a review of the available results obtained during the mission preparation and post-flight analyses has been undertaken to select a computational matrix. This matrix has been selected by accounting for previous numerical efforts from the literature to crosscheck the results. Then, a model based on previous efforts has been set up for computing the flow-field around the probe at high altitude. Finally the test case matrix has been computed and crosschecked with existing numerical predictions performed.Some possibilities of innovative magneto-hydrodynamic (MHD) technologies being applied to solve problems of re-entry vehicles heat protection are discussed in the fourth part.All presented data demonstrate necessity of further development of the radiative aerothermodynamics based on state-tostate approaches.

A three-dimensional hybrid particle-in-cell model of the plume emitted by a four-channel Hall-effect thruster configuration for a preliminary design study has been developed. Results show the presence of a potential well in the central region close to the exit plane of the configuration. Together with the typical characteristics of the single channel ion energy spectrum, a double peak structure in the low energy range is present in the four-channel configuration. It is due to the geometrical effects (four-channel source) and to the anisotropic nature of ion-atom scattering.

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.