73 research outputs found
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
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 ”m) 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 ”m) 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
Etude du plasma secondaire créé dans le neutraliseur d'ITER pour la formation de neutres rapides
Pour rĂ©aliser les conditions des rĂ©actions de fusion thermonuclĂ©aire dans le tokamak ITER, des moyens additionnels de chauffage sont requis. L'une des principales mĂ©thodes pour chauffer les ions du plasma de coeur sera l'injection de neutres D0 Ă©nergĂ©tiques. Le neutraliseur est l'Ă©tape de l'injecteur de neutres d'ITER oĂč le faisceau de deutĂ©rium prend ses propriĂ©tĂ©s en termes de taux de neutres D0 et de direction de propagation. L'interaction entre le faisceau Ă 1MeV et le gaz D2 neutralisant (~0.1Pa) crĂ©e un plasma secondaire. Les phĂ©nomĂšnes physiques en jeu sont prĂ©sentĂ©s Ă travers l'analyse des rĂ©sultats du code OBI-2. OBI-2 est un code PIC-MCC (Particle In Cell Monte Carlo Collision) en gĂ©omĂ©trie cylindrique (2D3V) dĂ©veloppĂ© 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 Ă©tĂ© Ă©tudiĂ©e et comparĂ©e au deutĂ©rium. Une Ă©tude paramĂ©trique sur le neutraliseur basĂ© sur le lithium a Ă©tĂ© rĂ©alisĂ©e dans la mesure oĂč la longueur et/ou la densitĂ© de Li injectĂ©e peuvent ĂȘtre modifiĂ©es. Le profil de densitĂ© de Li a Ă©tĂ© estimĂ© par le code Monte-Carlo 3D MC-OLIJET dĂ©veloppĂ© au LPGP. Le profil rĂ©sultatnt a Ă©tĂ© implĂ©mentĂ© en entrĂ©e du code PIC-MCC. Les rĂ©sultats montrent la faisabilitĂ© du neutraliseur basĂ© sur le lithium, gardant la convergence correcte du faisceau et avec de meilleures performances en termes de durĂ©e de vie des cryompompes avant rĂ©gĂ©nĂ©ration, de neutralisation du faisceau, d'effet de rĂ©trodiffusion 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. Ă©lectronique (914719901) / SudocSudocFranceF
Modélisation de l'extraction d'ions négatifs d'une source de plasma d'hydrogÚne (application à l'injecteur de neutres d'ITER.)
Le dĂ©veloppement de la source d'ions nĂ©gatifs pour l injecteur de particules d ITER constitue une des Ă©tapes essentielles pour gĂ©nĂ©rer des neutres de haute Ă©nergie . Pour remplir les caractĂ©ristiques requises pour ITER en termes de chauffage et de courant Ă l'intĂ©rieur du rĂ©acteur principal, la source d'ions nĂ©gatifs doit dĂ©livrer 40A de D-. La crĂ©ation d'une telle source reprĂ©sente un dĂ©fi tant technique que scientifique et demande une meilleure comprĂ©hension des phĂ©nomĂšnes physiques impliquĂ©es . Les connaissances actuelles sur le mĂ©chanisme d'extraction d'ion nĂ©gatifs d un plasma Ă©lectronĂ©gatif sont limitĂ©es, spĂ©cialement concernant la comprĂ©hension des caractĂ©ristiques d'une gaine de plasma magnĂ©tisĂ© dans la rĂ©gion d intĂ©rĂȘt oĂč on constante Ă©galement l extraction des Ă©lectrons simultanĂ©ment avec les ions nĂ©gatifs qui. De plus, l'asymĂ©trie due Ă la configuration croisĂ©e du champ magnĂ©tique pour piĂ©ger les Ă©lectrons nĂ©cessite une Ă©tude du problĂšme en trois dimensions. Un code 3D Particle-In-Cell Ă©lectrostatique a Ă©tĂ© spĂ©cialement dĂ©veloppĂ© pour Ă©tudier ce problĂšme. Le code utilise les coordonnĂ©es cartĂ©siennes et peut prendre en compte des gĂ©omĂ©tries complexes. Le code nommĂ© ONIX Ă©tudie les propriĂ©tĂ©s du plasma et le transport des Ă©lectrons et des ions nĂ©gatifs au niveau de la zone d'extraction. Les rĂ©sultats sur la formation d'un mĂ©nisque de plasma et l'Ă©crantage du champ d'extraction par ce plasma, ainsi que les trajectoires des ions nĂ©gatifs, sont prĂ©sentĂ©s. L'efficacitĂ© de l'extraction d'ions nĂ©gatifs du volume et de la surface est investiguĂ©e et on trouve que les processus de crĂ©ation en surface des ions nĂ©gatifs jouent un rĂŽle 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. Ă©lectronique (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 ChildâLangmuir 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 0.1Â T/Pa, as revealed by the comparison to self-consistent 2D numerical simulations
2D PIC-MCC simulations of magnetron plasma in HiPIMS regime with external circuit
International audienc
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
SynthÚse par PECVD et caractérisation de nanotubes de carbone orientés
Ce travail est consacré à l'étude de la synthÚse 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 paramÚtres explorés sont la nature des catalyseurs (Ni, Fe, Pd) déposés par pulvérisation plasma (PVD), la température du substrat, la composition du gaz plasmagÚne (C2H2/NH3 ou C2H2/H2). Ce procédé ECR PECVD permet la synthÚse de NTC dÚs 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) révÚle 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'acétylÚne 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 electronâs 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 sourceâs axis is well reproduced for 1 ”s 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 electronâs 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 methodâs 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(â) linear accelerator named Linac4. The source should deliver 80 mA H(â) 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(â)(2) and Cs-density of n(Cs+) = 3.8 Ă 10(16) m(â)(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(â)(2) with n(Cs+) = 3.3 Ă 10(15) m(â)(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
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