Raman microscopy as a defect microprobe for hydrogen bonding characterization in materials used in fusion applications

We present the Raman microscopy ability to detect and characterize the way hydrogen is bonded with elements that will be used for ITER's plasma facing components. For this purpose we first use hydrogenated amorphous carbon samples, formed subsequently to plasma-wall interactions (hydrogen implantation, erosion, deposition...) occurring inside tokamaks, to demonstrate how this technique can be used to retrieve useful information. We pay attention in identifying which spectroscopic parameters are sensitive to the local structure (sp 3 /sp 2) and which gives information on the hydrogen content using isothermal and linear temperature ramp studies on reference samples produced by plasma enhanced chemical vapor deposition. We then focus on the possibility to use this fast, non-destructive and non-contact technique to characterize the influence of hydrogen isotope implantation in few nanometers of graphite and beryllium as C is still used in the JT-60 tokamak and Be is used in JET and will be used as plasma-facing component in the future reactor ITER. We also pay attention on implantation in tungsten oxide which may be formed accidently in the machine.Comment: Physica Status Solidi C, 201

Structure of the carbon layers deposited on the toroidal pump limiter of Tore Supra

International audienceScanning and transmission electron microscopy analyses have been performed for tiles extracted from the toroidal pump limiter of Tore Supra for erosion- and deposition-dominated zones. Deposit thicknesses have been estimated for the plasma-facing top and the gap side lateral surfaces. Deposit thickness profiles have been measured inside gaps, showing that deposition mainly occurs in the first millimetre and that both poloidal and toroidal gap deposition is asymmetric. Quantitative information on the deposit volume and on D-retention are thus obtained from these measurements. Carbon probed at the tile top surfaces is mainly amorphous carbon, due either to the amorphization induced by ion bombardment in the erosion dominated zone, or to deposit formation processes in the deposition-dominated zones. Deposits are tip-shaped and are oriented, which should give information on transport processes

Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H− yield

International audienceHighly oriented polycrystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond, ultra-nanocrystalline diamond and diamond-like carbon surfaces are exposed to low-pressure hydrogen plasma in a 13.56MHz plasma reactor. Relative yields of surface-produced H− ions due to bombardment of positive ions from the plasma are measured by an energy analyser cum quadrupole mass spectrometer. Irrespective of plasma conditions (0.2 and 2 Pa), HOPG surfaces show the highest yield at room temperature (RT), while at high temperature (HT), the highest yield (∼3-5 times compared to HOPG surface at RT) is observed on BDD surfaces. The shapes of ion distribution functions are compared at RT and HT to demonstrate the mechanism of ion generation at the surface. Raman spectroscopy analyses of the plasma-exposed samples reveal surface modifications influencing H− production yields, while further analyses strongly suggest that the hydrogen content of the material and the sp3/sp2 ratio are the key parameters in driving the surface ionization efficiency of carbon materials under the chosen plasma conditions

Effect of composition and surface characteristics on fuel retention in beryllium- containing co-deposited layers

We have investigated retention of deuterium in beryllium- containing, laboratory-made films whose properties resemble co- deposits observed on JET-ILW or predicted for ITER. The samples were prepared using High Power Impulse Magnetron Sputtering and Thermo-Vacuum Arc Deposition. We have observed that retention depends on the flux of D atoms on the growing film, but even more prominently on its composition, structure, and morphology. Especially, inclusion of carbon by 10-15 at.% in the layers can increase retention by a factor of 2-10. This we attribute to increasing number of defects as well as aromatic and aliphatic C-D bonds in the samples. Other impurities do not significantly alter the D inventory while more D is retained in samples with rough or highly modified surfaces. Our results show that reproducing the reported D concentrations of ~5 at.% in JET-ILW- like deposits requires keeping the sample temperature at 100- 200°C during the production phase and optimizing the uniformity of deposition fluxes. Data from Be-D samples further indicate that fuel retention in more ITER-relevant co-deposits would be around 1-2 at.%

Enhancing surface production of negative ions using nitrogen doped diamond in a deuterium plasma

The production of negative ions is of significant interest for applications including mass spectrometry, particle acceleration, material surface processing, and neutral beam injection for magnetic confinement fusion. Methods to improve the efficiency of the surface production of negative ions, without the use of low work function metals, are of interest for mitigating the complex engineering challenges these materials introduce. In this study we investigate the production of negative ions by doping diamond with nitrogen. Negatively biased ($-20$ V or $-130$ V), nitrogen doped micro-crystalline diamond films are introduced to a low pressure deuterium plasma (helicon source operated in capacitive mode, 2 Pa, 26 W) and negative ion energy distribution functions (NIEDFs) are measured via mass spectrometry with respect to the surface temperature (30$^{\circ}$C to 750$^{\circ}$C) and dopant concentration. The results suggest that nitrogen doping has little influence on the yield when the sample is biased at $-130$ V, but when a relatively small bias voltage of $-20$ V is applied the yield is increased by a factor of 2 above that of un-doped diamond when its temperature reaches 550$^{\circ}$C. The doping of diamond with nitrogen is a new method for controlling the surface production of negative ions, which continues to be of significant interest for a wide variety of practical applications

Etude de la conversion de spin nucléaire de l'eau en matrices de gaz rares : mesures, modélisation de l'influence des interactions spin-spin intermoléculaires et contexte astrophysique.

In cometary atmospheres, nuclear spin isomers' abundance ratios of molecules like H2O, NH3, CH4, and more recently CH3OH, have been evaluated. They are not in thermodynamic Boltzmann's equilibrium and the question is : is this non equilibrium the signature of the temperature of the primordial medium in which molecules have been formed?As a matter of fact, different nuclear spin isomeric forms can be identified for molecules which have equivalent protons of non-zero nuclear spin in symmetrical positions. The conversion between these isomers, which is forbidden at the first order of approximation in the gas phase, is very slow and depends upon the neighbouring of the molecules.The aim of the present work, both experimental and theoretical, is to identify some relevant parameters involved in the nuclear spin conversion of water molecules trapped in rare gas matrices. In such media, an enrichment of ortho or para nuclear spin isomers is obtained by a very fast change of the temperature of the sample. Then, FTIR spectra recorded successively in the Ν2 and Ν3 vibrationnal modes of water, allow a measurement of the nuclear spin conversion time.Varying water dilution in argon matrix (from [Ar/H2O]=5000 to 50) at 4,2 K, shows that two types of conversions are present. For dilutions higher than 1000, the conversion times (~670 minutes) are independent of water dilution. We show that this is not due to unwanted effects (molecular impurities, IR source exposure time, copper-gold surface influence,...). It is then due to an intramolecular coupling process, accelerated by the matrix.On the other hand, below a dilution of 1000, a strong acceleration of the conversion is observed (conversion time of ~180 minutes, at a dilution of 50). This acceleration of conversion is clearly of intermolecular origin. We have developed a model based on intermolecular nuclear spin (of protons) magnetic interactions and energy exchange with the matrix. In this model, the conversion depends on the energy difference between the lowest rotational levels and on a collisional relaxation rate of the molecule with the matrix atoms. Intermolecular conversion at 4,2 K is allowed via a conversion channel between the two first rotational levels of the molecule. The presence of a second channel, that is consistent with the acceleration of the intermolecular conversion when increasing temperature, is found to be due to a population effect. Matrix change confirms that the intermolecular conversion mechanism is more efficient in the matrix with the smallest interatomic distances, in accordance with our model.Les molécules ayant des protons en position échangeable possèdent des isomères de spin nucléaires différents. Leur interconversion, interdite à l'ordre zéro par la mécanique quantique, dépend en partie de l'environnement.Dans les atmosphères cométaires, la mesure du rapport d'abondance de ces isomères pour les molécules de H2O, NH3, CH4, et plus récemment de CH3OH, montre un écart à l'équilibre thermodynamique. Sa signification physique est à ce jour au cœur d'un débat : est- ce un indicateur de la température qui régnait dans l'environnement primordial au sein duquel se sont formées ces molécules ? Ce travail expérimental alliant techniques de vide et de cryogénie, vise à identifier, en amont d'une étude dans les glaces cométaires, certains paramètres impliqués dans la conversion de spin nucléaire de H2O isolée dans des environnements plus simples : les matrices de gaz rare. Dans un tel environnement, un enrichissement en isomère de spin ortho ou para est produit par un changement brutal de température (entre 4,2 et 30 K). La détermination du temps de retour à l'équilibre thermodynamique se fait à partir de l'enregistrement dans le temps des spectres infrarouges ro-vibrationnels des modes Ν2 (déformation angulaire) et Ν3 (élongation antisymétrique), au moyen d'un spectromètre commercial IR par transformée de Fourier.L'étude systématique en fonction de la concentration que nous avons réalisée à 4,2 K en matrice d'argon, montre qu'il existe deux régimes de conversion qui sont en compétition. Pour des dilutions [Ar/H2O] supérieures à 1000, les temps de conversion (~670 minutes) sont indépendants de la concentration et atteignent un plateau. Cette conversion qui n'est pas imputable à des effets parasites (impuretés moléculaires, exposition au rayonnement IR du spectromètre, influence du support, ...) est sans doute due à un processus d'origine intramoléculaire, accéléré par la matrice. Par contre, en dessous du 1000ème, une accélération très nette de la conversion avec l'augmentation de la concentration est observée (~180 minutes au 50ème), indiquant clairement une origine intermoléculaire. Nous avons, dans ce cadre, développé un modèle faisant intervenir des interactions magnétiques entre les spins nucléaires des protons appartenant à des molécules d'eau différentes, et des échanges d'énergie avec les phonons de la matrice. Ce modèle, qui ne dépend que de l'écart en énergie entre les niveaux de rotation et du taux de relaxation collisionnelle de la molécule isolée en matrice, autorise la conversion d'origine intermoléculaire à 4,2 K en ouvrant un canal de conversion couplant le premier niveau de rotation de chacun des isomères de spin. L'existence d'un deuxième canal, plus efficace d'un ordre de grandeur à 25 K, dû à un effet de population, explique l'accélération observée à température croissante.La substitution de l'argon par du néon, du krypton ou du xénon, a permis de montrer, en accord complet avec le modèle, que le mécanisme d'origine intermoléculaire est plus efficace dans la matrice qui a le paramètre de maille le plus faible

Etude de la conversion de spin nucléaire de l'eau en matrices de gaz rares (mesures, modélisation de l'influence des interactions spin-spin intermoléculaires et contexte astrophysique)

The aim of the present work, both experimental and theoretical, is to identify some relevant parameters involved in the nuclear spin conversion of water molecules trapped in rare gas matrices. In such media, an enrichment of ortho or para nuclear spin isomers is obtained by a very fast change of the temperature of the sample. Then, FTIR spectra recorded successively in the 2 and 3 vibrationnal modes of water, allow a measurement of the nuclear spin conversion time. By varying water dilution in argon matrix (from [Ar/H2O]=5000 to 50) at 4,2 K, we show that two types of conversions are present. For dilutions higher than 1000, the conversion time (~670 minutes) is independent of water dilution. We show that this is not due to unwanted effects. It is then due to an intramolecular coupling process, accelerated by the matrix. On the other hand, below a dilution of 1000, a strong acceleration of the conversion is observed (conversion time of ~180 minutes, at a dilution of 50). This acceleration of conversion is clearly of intermolecular origin. We have developed a model based on intermolecular nuclear spin (of protons) magnetic interactions and energy exchange with the matrix. In this model, the conversion depends on the energy difference between the lowest rotational levels and on a collisional relaxation rate of the molecule with the matrix atoms. Intermolecular conversion at 4,2 K is allowed via a conversion channel between the two first rotational levels of the molecule. The presence of a second channel, that is consistent with the acceleration of the intermolecular conversion when increasing temperature, is found to be due to a population effectCe travail expérimental vise à identifier, en amont d une étude dans les glaces cométaires, certains paramètres impliqués dans la conversion de spin nucléaire de H2O isolée dans des environnements plus simples : les matrices de gaz rare. La mesure du temps de conversion se fait à partir de l enregistrement dans le temps des spectres infrarouges ro-vibrationnels des modes 2 et 3, au moyen d un spectromètre IR par transformée de Fourier. L étude systématique en fonction de la concentration réalisée à 4,2 K en matrice de néon, argon, krypton et xénon montre qu il existe deux régimes de conversion. Pour des dilutions [Ar/H2O] supérieures à 1000, les temps de conversion (~670 minutes) sont indépendants de la concentration et atteignent un plateau. Cette conversion, qui n est pas imputable à des effets parasites, est sans doute due à un processus d origine intramoléculaire, accéléré par la matrice. Par contre, en dessous du 1000ème, une accélération très nette de la conversion avec l augmentation de la concentration est observée (~180 minutes au 50ème), indiquant clairement une origine intermoléculaire. Nous avons développé un modèle faisant intervenir des interactions magnétiques entre les spins nucléaires des protons appartenant à des molécules d eau différentes, et des échanges d énergie avec les phonons de la matrice. Ce modèle autorise la conversion d origine intermoléculaire à 4,2 K en ouvrant un canal de conversion couplant le premier niveau de rotation de chacun des isomères de spin. L existence d un deuxième canal, plus efficace d'un ordre de grandeur à 25 K, dû à un effet de population, explique l'accélération observée à température croissantePARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

The temperature dependence of optical properties of tungsten in the visible and near-infrared domains: an experimental and theoretical study

International audienceThe knowledge of optical properties of tungsten at high temperatures is of crucial importance in fields such as nuclear fusion or aerospace applications. The optical properties of tungsten are well known at room temperature, but little has been done at temperatures comprised between 300 K and 1000 K in the visible and near-infrared domains. Here we investigate the temperature dependence of tungsten reflectivity from the ambient to high temperatures (<1000 K) in the 500-1050 nm spectral range, a region where interband transitions have a strong contribution. Experimental measurements, performed via a spectroscopic system coupled with a laser remote heating, show that the tungsten reflectivity increases with temperature and wavelength. We have described these dependences through a Fresnel and two Lorentz-Drude models. The Fresnel model reproduces accurately the experimental curve at a given temperature, but it is able to simulate the temperature dependency of reflectivity only thanks to an ad hoc choice of temperature formulae for the refractive indexes. Thus, a less empirical approach is preferred based on Lorentz-Drude models to describe the interaction of light and charge carriers in the solid. The first Lorentz-Drude model, which includes a temperature dependency on intraband transitions, fits experimental results only qualitatively. The second Lorentz-Drude model includes in addition a temperature dependency on interband transitions. It is able to reproduce quantitatively the experimental results, highlighting a non-trivial dependence of interband transitions as a function of temperature. Eventually, we use these temperature dependent Lorentz-Drude models to evaluate the total emissivity of tungsten from 300 K to 3500 K and we compare our experimental and theoretical findings with previous results

Microporosity and nanostructure of activated carbons: characterization by X-ray diffraction and scattering, Raman spectroscopy and transmission electron microscopy

International audienceMicroporosity and structure of a set of activated carbons was studied by combination of N 2 and CO 2 adsorption, Transmission Electron Microscopy (TEM), X-ray diffraction and scattering and multiwavelength Raman spectroscopy. It is shown that correlations between measured parameteres may be established for a given set of activated carbons, most often obtained from a same precursor. Comparison of results of TEM images processing and of Small-angle scattering with adsorption data suggests that super-micropores (0.7-2 nm) are highly variable in shape and strongly deviate from the ideal slit pore model. These pores are likely located in between disordered continuous graphene stacks. It is shown that Small-angle scattering is mostly caused by supermicropores; contribution of other types of porosity is of secondary importance. For a set of carbons with similar structure, a reasonable correlation between Guinier radii and pore width obtained from N 2 adsorption can be found; however, the reason for the observed offset between the data sets remain uncertain. Sensitivity of the Raman scattering to atomic scale processes leads to poor or unclear correlations between the spectroscopic and structural data, although some notable exceptions are noted

Appendix A. Supplementary data for Advanced spectroscopic analyses on a:C-H materials: Revisiting the EELS characterization and its coupling with multi-wavelength Raman spectroscopy

Modified method used for the determination of the R ratio taking into account the contribution of hererospecies. Low-loss EELS spectrum showing the Drude model fit used to extract the plasmon energy. Calibration curve between HD/HG Raman parameters used in this paper to derive the H content. Respective contributions of the Cdouble bondC groups and carbo-oxygenated groups (related to A and B peaks) in the π* character for the samples annealed 1000 and 2500 min.Peer reviewe