The Use of Sacrificial Graphite-like Coating to Improve Fusion Efficiency of Copper in Selective Laser Melting

Thin and ultrathin carbon films reduce the laser energy required for copper powder fusion in selective laser melting (SLM). The low absorption of infrared (IR) radiation and its excellent thermal conductivity leads to an intricate combination of processing parameters to obtain high-quality printed parts in SLM. Two carbon-based sacrificial thin films were deposited onto copper to facilitate light absorption into the copper substrates. Graphite-like (3.5 µm) and ultra-thin (25 nm) amorphous carbon films were deposited by aerosol spraying and direct current magnetron sputtering, respectively. The melting was analyzed for several IR (1.06 µm) laser powers in order to observe the coating influence on the energy absorption. Scanning electron microscopy showed the topography and cross-section of the thermally affected area, electron backscatter diffraction provided the surface chemical composition of the films, and glow-discharge optical emission spectroscopy (GDOES) allowed the tracking of the in-deep chemical composition of the 3D printed parts using carbon film-covered copper. Ultra-thin films of a few tens of nanometers could reduce fusion energy by about 40%, enhanced by interferences phenomena. Despite the lower energy required, the melting maintained good quality and high wettability when using top carbon coatings. A copper part was SLM printed and associated with 25 nm of carbon deposition between two copper layers. The chemical composition analysis demonstrated that the carbon was intrinsically removed during the fusion process, preserving the high purity of the copper part

H- Beam formation simulation in negative ion source for CERN's Linac4 accelerator

The caesiated surface negative ion source is the first element of CERN's LINAC4 a linear injector designed to accelerate negative hydrogen ions to 160 MeV. The IS03 ion source is operated at 35 mA beam intensity and reliably feeds CERN's accelerator chain, H- ions are generated via plasma volume and caesiated molybdenum plasma electrode surface mechanisms. Studying the beam extraction region of this H- ion source is essential for optimizing the H- production. The 3D Particle-in-cell Monte Carlo code ONIX (Orsay Negative Ion eXtraction), written to study H- beam formation processes in neutral injectors for fusion, has been adapted to single aperture accelerator H- sources. The code was modified to match the conditions of the beam formation and extraction regions of the Linac4 H- source. A set of parameters was chosen to characterize the plasma and to match the specific volume and surface production modes. Simulated results of the extraction regions are presented and benchmarked with experimental results obtained at the Linac4 test stand

Etude du plasma secondaire créé dans le neutraliseur d'ITER pour la formation de neutres rapides

Pour réaliser les conditions des réactions de fusion thermonucléaire dans le tokamak ITER, des moyens additionnels de chauffage sont requis. L'une des principales méthodes pour chauffer les ions du plasma de coeur sera l'injection de neutres D0 énergétiques. Le neutraliseur est l'étape de l'injecteur de neutres d'ITER où le faisceau de deutérium prend ses propriétés en termes de taux de neutres D0 et de direction de propagation. L'interaction entre le faisceau à 1MeV et le gaz D2 neutralisant (~0.1Pa) crée un plasma secondaire. Les phénomènes physiques en jeu sont présentés à travers l'analyse des résultats du code OBI-2. OBI-2 est un code PIC-MCC (Particle In Cell Monte Carlo Collision) en géométrie cylindrique (2D3V) développé au LPGP qui permet de suivre la propagation du faisceau et les particules du plasma le long du neutraliseur.L'injection de lithium comme cible neutralisante a été étudiée et comparée au deutérium. Une étude paramétrique sur le neutraliseur basé sur le lithium a été réalisée dans la mesure où la longueur et/ou la densité de Li injectée peuvent être modifiées. Le profil de densité de Li a été estimé par le code Monte-Carlo 3D MC-OLIJET développé au LPGP. Le profil résultatnt a été implémenté en entrée du code PIC-MCC. Les résultats montrent la faisabilité du neutraliseur basé sur le lithium, gardant la convergence correcte du faisceau et avec de meilleures performances en termes de durée de vie des cryompompes avant régénération, de neutralisation du faisceau, d'effet de rétrodiffusion des ions positifs.To achieve thermonuclear fusion reactions in the ITER tokamak, additional heating is required. One of the main method to heat the core plasma ions will be the injection of energetic D0 neutrals. The neutralizer is the stage of the Neutral Beam Injector of ITER where the deuterium beam gets its properties in terms of neutral rate D0 and direction of propagation. The interaction between the 1MeV beam and the D2 neutralizing gas (~0.1Pa) creates a secondary plasma. These physical phenomena involved are presented through the analysis of the OBI-2 code results. OBI-2 is a PIC-MCC (Particle In Cell Monte-Carlo Collision) code in cylindrical geometry (2D3V) developed in the LPGP which allows to follow beam propagation and plasma particles along the neutralizer.The injection of lithium neutralizing target has been investigated and compared to deuterium one. Parametric study of the Li based neutralizer has been performed since the length and/or density of Li injected can be modified. The Li density profile has been estimated through the Monte-Carlo 3D code MC-OLIJET developed in the LPGP. The resulted profile has been implemented as an imput of the PIC-MCC code. Results show the feasibility of a lithium based neutralizer, keeping correct beam focusing and with better performance in terms of cryompump lifetime before regeneration, beam neutralization, positive ion backstreaming effect.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Modélisation de l'extraction d'ions négatifs d'une source de plasma d'hydrogène (application à l'injecteur de neutres d'ITER.)

Le développement de la source d'ions négatifs pour l injecteur de particules d ITER constitue une des étapes essentielles pour générer des neutres de haute énergie . Pour remplir les caractéristiques requises pour ITER en termes de chauffage et de courant à l'intérieur du réacteur principal, la source d'ions négatifs doit délivrer 40A de D-. La création d'une telle source représente un défi tant technique que scientifique et demande une meilleure compréhension des phénomènes physiques impliquées . Les connaissances actuelles sur le méchanisme d'extraction d'ion négatifs d un plasma électronégatif sont limitées, spécialement concernant la compréhension des caractéristiques d'une gaine de plasma magnétisé dans la région d intérêt où on constante également l extraction des électrons simultanément avec les ions négatifs qui. De plus, l'asymétrie due à la configuration croisée du champ magnétique pour piéger les électrons nécessite une étude du problème en trois dimensions. Un code 3D Particle-In-Cell électrostatique a été spécialement développé pour étudier ce problème. Le code utilise les coordonnées cartésiennes et peut prendre en compte des géométries complexes. Le code nommé ONIX étudie les propriétés du plasma et le transport des électrons et des ions négatifs au niveau de la zone d'extraction. Les résultats sur la formation d'un ménisque de plasma et l'écrantage du champ d'extraction par ce plasma, ainsi que les trajectoires des ions négatifs, sont présentés. L'efficacité de l'extraction d'ions négatifs du volume et de la surface est investiguée et on trouve que les processus de création en surface des ions négatifs jouent un rôle capital.The development of the negative ion source constitutes a crucial step in the construction of the neutral beam injector of ITER. To fulfil the ITER requirements in terms of heating and current drive, the negative ion source should deliver 40 A of D-. The achievement of such a source is challenging from technical and scientific points, and it requires a deeper understanding of the underlying physics. The present knowledge of the ion extraction mechanism from the negative ion source is limited due to the complexity of the problem that involves the comprehension of the behaviour of magnetized plasma sheaths when negative ions and electrons are pulled out from the plasma. Moreover, due to the asymmetry induced by the crossed magnetic configuration used to filter the electrons, any realistic study of this problem must consider the three spatial dimensions. To address this problem in a realistic way, a 3D Particles-in-Cell electrostatic code specifically designed for this system was developed. The code uses Cartesian coordinate system and it can deal with complex boundary geometry as it is the case of the extraction apertures. The complex magnetic field that is applied to deflect electrons is also taken into account. This code, called ONIX, was used to investigate the plasma properties and the transport of negative ions and electrons close to a source extraction aperture. Results on the formation of the plasma meniscus and the screening of the extraction field by the plasma are presented here, as well as negative ions trajectories. Negative ion extraction efficiency from volume and surfaces was investigated showing the capital importance of the surface negative ion production.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Two-dimensional analytical description of the plasma potential in a magnetron discharge

Abstract Simple analytical formulas are proposed to describe the plasma potential in a steady-state magnetron discharge, based on the results of various experiments and numerical simulations reported in the literature. The description is two-dimensional (2D), covering two main regions, the cathode sheath and the ionization region, both contributing to electron energization. A parabolic potential in the axial direction governs the cathode sheath. The thickness of the cathode sheath is obtained from the 1D collisionless Child–Langmuir law. A parabolic or linear potential in the axial direction characterizes the ionization region. The local ion current density to the cathode, estimated from the target erosion profile, sets the radial dependence of the potential. The proposed formulas use a set of input parameters that can be experimentally obtained. The analytical description captures all characteristics of the highly inhomogeneous plasma potential of a steady-state magnetron discharge operated in a reduced magnetic field B RT /p lower than 0.1 T/Pa, as revealed by the comparison to self-consistent 2D numerical simulations

Dynamic features of the electron drift and electron properties in a HiPIMS discharge

International audienceAbstract Information on the evolution of electron properties during High-Power Impulse Magnetron Sputtering (HiPIMS) operation of planar magnetrons enables the study of fundamental physical processes. In this work, incoherent Thomson scattering is implemented for the non-invasive, spatiotemporally-resolved characterization of electron properties and drifts in the HiPIMS regime of a planar magnetron. In the ionization region of argon and helium plasmas, the azimuthal electron drifts are directly measured perpendicular to the magnetic field and are found to evolve according to a changing balance of E x B and diamagnetic electron drifts, while radial electron drifts, measured parallel to the magnetic field, can be attributed to plasma expansion/contraction and centrifugal forces. The evolution of electron density and temperature in the afterglow plasma phase show the existence of two time scales for the variation of plasma properties. These characterizations provide detailed information on electron properties and dynamics in regions of the magnetic trap ordinarily inaccessible to invasive diagnostics

2D PIC-MCC simulations of magnetron plasma in HiPIMS regime with external circuit

International audienc

Synthèse par PECVD et caractérisation de nanotubes de carbone orientés

Ce travail est consacré à l'étude de la synthèse de nanotubes de carbone (NTC) orientés par plasma froid basse pression excité par résonance cyclotronique électronique (ECR PECVD). Ces travaux portent également sur la caractérisation structurale et physico-chimique des NTC. Les paramètres explorés sont la nature des catalyseurs (Ni, Fe, Pd) déposés par pulvérisation plasma (PVD), la température du substrat, la composition du gaz plasmagène (C2H2/NH3 ou C2H2/H2). Ce procédé ECR PECVD permet la synthèse de NTC dès 550C. La microscopie électronique à balayage et à transmission (MEB, MET) renseigne sur la vitesse de croissance et l'orientation des NTC. L'analyse de l'environnement chimique (XPS, XANES et EELS) révèle l'incorporation d'azote dans la structure des NTC synthétisés en C2H2/NH3 et indique les configurations les plus probables. Enfin, des nanofibres de carbone amorphe ont été obtenues à température ambiante en associant un plasma ECR d'acétylène et une source d'atomes d'azote.This work concerns the synthesis of oriented carbon nanotubes (CNT) using a low pressure microwave plasma excited by electron cyclotron resonance (ECR PECVD). The CNT structure and chemical environment are studied as a function of the nature of the metallic catalyst (Ni, Fe, Pd) deposited by plasma sputtering (PVD), of the substrate temperature and of the composition of the gas mixture (C2H2/NH3 or C2H2/H2). This ECR PECVD process allows the growth of oriented CNT from 550C. Information on the deposition rate and the structure of CNT are provided by scanning (SEM) and transmission (TEM) electron microscopy analyses. XPS, XANES and EELS analyses show that nitrogen is incorporated into the CNT walls when C2H2/NH3 plasma is used. As well, the dominant nitrogen environment was characterized. In addition, carbon nanofibers have been obtained at room temperature by associating to ECR acetylene plasma a nitrogen atom source.NANTES-BU Sciences (441092104) / SudocSudocFranceF

A Guiding Centre Approximation Approach for Simulation Electron Trajectories in ECR and Microwave Ion Sources

International audienceThis work presents a study on the feasibility of the implementation of the guiding centre (GC) approach in electron cyclotron resonance (ECR) ion sources, with the goal of speeding up the electron’s orbit integration in certain regimes. It is shown that the GC approximation reproduces accurately the trajectory drifts and periodic behaviour of electrons in the minimum-B field. A typical electron orbit far enough from the source’s axis is well reproduced for 1 µs of propagation time, with the GC time-step constrained below 100 ps, giving one order of magnitude gain in computation time with respect to Boris. For an electron orbit close to the axis a disphasement of the electron’s trajectory is observed, but the spatial envelope is conserved. A comparative study analyses electron trajectories in a flatter B-field, that in a microwave discharge ion source, where this method’s drawbacks may be avoided given a smaller magnetic field gradient and a shorter electron lifetime in the plasma chamber. In this regime electron trajectories were very well reproduced by the GC approximation. The time-step was constrained below 10 ns, providing up to 30 times faster integration compared to Boris

Beam formation in CERNs cesiated surfaces and volume H−^− ion sources

At CERN, a high performance negative ion (NI) source is required for the 160 MeV H(−) linear accelerator named Linac4. The source should deliver 80 mA H(−) ion beams within an emittance of 0.25 mm·mrad. For this purpose two ion sources were developed: IS01 is based on the NI volume production and IS02 provides additional NI by surface production via H interaction on a cesiated Molybdenum plasma electrode. The development of negative ion sources for Linac4 is accompanied by modelling activities. ONIX code has been modified and adapted to investigate the transport of NI and electrons in the extraction region of the CERN negative ion sources. The simulated results from modeling of IS01 and IS02 extraction regions, which were obtained in 2012 during source commissioning, are presented and benchmarked with experimental measurements obtained after 2013. The formation of the plasma meniscus and the screening of the extraction field by the source plasma are discussed. The NI production is compared between two types of sources, the first one based on volume production only and the second one encompassing NI cesiated surface production. For the IS02 source, different states of conditioning were simulated by changing the NI emission flux from the plasma electrode and Cs(+) density in the bulk plasma region. The numerical results show that in low work function regime, with high NI surface emission rate of 3000 A m(−)(2) and Cs-density of n(Cs+) = 3.8 × 10(16) m(−)(3), the total extracted NI current could reach ~80 mA. At the less favorable Cs-coverage, when the surface NI emission rate becomes significantly lower, namely 300 A m(−)(2) with n(Cs+) = 3.3 × 10(15) m(−)(3), the total extracted NI current only reaches ~20 mA. A good agreement between simulation and experimental results is observed in terms of extracted NI current for both extraction systems, including the case of reversed extraction potential that corresponds to positive (H(+)) ion extraction