N2-H2 capacitively coupled radio-frequency discharges at low pressure. Part I. Experimental results: Effect of the H2 amount on electrons, positive ions and ammonia formation

The mixing of N2 with H2 leads to very different plasmas from pure N2 and H2 plasma discharges. Numerous issues are therefore raised involving the processes leading to ammonia (NH3) formation. The aim of this work is to better characterize capacitively-coupled radiofrequency plasma discharges in N2 with few percents of H2 (up to 5%), at low pressure (0.3-1 mbar) and low coupled power (3-13 W). Both experimental measurements and numerical simulations are performed. For clarity, we separated the results in two complementary parts. The actual one (first part), presents the details on the experimental measurements, while the second focuses on the simulation, a hybrid model combining a 2D fluid module and a 0D kinetic module. Electron density is measured by a resonant cavity method. It varies from 0.4 to 5 109 cm-3, corresponding to ionization degrees from 2 10-8 to 4 10-7. Ammonia density is quantified by combining IR absorption and mass spectrometry. It increases linearly with the amount of H2 (up to 3 1013 cm-3 at 5% H2). On the contrary, it is constant with pressure, which suggests the dominance of surface processes on the formation of ammonia. Positive ions are measured by mass spectrometry. Nitrogen-bearing ions are hydrogenated by the injection of H2, N2H+ being the major ion as soon as the amount of H2 is >1%. The increase of pressure leads to an increase of secondary ions formed by ion/radical-neutral collisions (ex: N2H+, NH4 +, H3 +), while an increase of the coupled power favours ions formed by direct ionization (ex: N2 +, NH3 +, H2 +).N. Carrasco acknowledges the financial support of the European Research Council (ERC Starting Grant PRIMCHEM, Grant agreement no. 636829). A. Chatain acknowledges ENS Paris-Saclay Doctoral Program. A. Chatain is grateful to Gilles Cartry and Thomas Gautier for fruitful discussions on the MS calibration. L.L. Alves acknowledges the financial support of the Portuguese Foundation for Science and Technology (FCT) through the project UID/FIS/50010/2019. L. Marques and M. J. Redondo acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2019

Real-Space Mesh Techniques in Density Functional Theory

This review discusses progress in efficient solvers which have as their foundation a representation in real space, either through finite-difference or finite-element formulations. The relationship of real-space approaches to linear-scaling electrostatics and electronic structure methods is first discussed. Then the basic aspects of real-space representations are presented. Multigrid techniques for solving the discretized problems are covered; these numerical schemes allow for highly efficient solution of the grid-based equations. Applications to problems in electrostatics are discussed, in particular numerical solutions of Poisson and Poisson-Boltzmann equations. Next, methods for solving self-consistent eigenvalue problems in real space are presented; these techniques have been extensively applied to solutions of the Hartree-Fock and Kohn-Sham equations of electronic structure, and to eigenvalue problems arising in semiconductor and polymer physics. Finally, real-space methods have found recent application in computations of optical response and excited states in time-dependent density functional theory, and these computational developments are summarized. Multiscale solvers are competitive with the most efficient available plane-wave techniques in terms of the number of self-consistency steps required to reach the ground state, and they require less work in each self-consistency update on a uniform grid. Besides excellent efficiencies, the decided advantages of the real-space multiscale approach are 1) the near-locality of each function update, 2) the ability to handle global eigenfunction constraints and potential updates on coarse levels, and 3) the ability to incorporate adaptive local mesh refinements without loss of optimal multigrid efficiencies.Comment: 70 pages, 11 figures. To be published in Reviews of Modern Physic

Irradiation de l'antimoine dans un microscope électronique à haute tension et observation de boucles de défauts ponctuels

We present the first results of the effects of electron irradiation in (111) thin foils of antimony single crystals (A7 structure) in a high voltage microscope. The voltage which separates the subthreshold events from the bulk radiation damage regime is estimated to be about 725 ± 20 kV. This threshold voltage corresponds to a maximum transferred energy T m of 22.6 eV, and thus to a threshold displacement energy E d of 15.7 or 8.2 eV for the assumed ejection directions or respectively. We have also observed the climbing of dislocations and the agglomeration of point defects into dislocation loops.Nous présentons les premiers résultats concernant l'irradiation électronique de lames monocristallines d'antimoine (structure A7) de plan moyen (111), dans un microscope à haute tension. Nous estimons à 725 ± 20 kV la tension qui sépare le régime d'irradiation sous le seuil du régime d'irradiation proprement dit. Cette tension seuil correspond à une énergie maximale Tm cédée lors d'un choc frontal égale à 22,6 eV, et donc à une énergie seuil de déplacement E d égale respectivement à 15,7 ou 8,2 eV selon que l'on admet que les atomes sont éjectés dans les directions ou . Nous avons également observé la montée des dislocations et la formation de boucles de défauts ponctuels

Irradiation de l'antimoine dans un microscope électronique à haute tension et observation de boucles de défauts ponctuels

Nous présentons les premiers résultats concernant l'irradiation électronique de lames monocristallines d'antimoine (structure A7) de plan moyen (111), dans un microscope à haute tension. Nous estimons à 725 ± 20 kV la tension qui sépare le régime d'irradiation sous le seuil du régime d'irradiation proprement dit. Cette tension seuil correspond à une énergie maximale Tm cédée lors d'un choc frontal égale à 22,6 eV, et donc à une énergie seuil de déplacement E d égale respectivement à 15,7 ou 8,2 eV selon que l'on admet que les atomes sont éjectés dans les directions ou . Nous avons également observé la montée des dislocations et la formation de boucles de défauts ponctuels

Tracing of polypropylene with rare earth oxides in view of automatic sorting of plastic wastes

International audienc

Recent developments in the TWODANT system of codes for criticality safety

The application of deterministic, discrete ordinates codes to criticality safety problems can be a very useful complement to analyses performed using Monte Carlo methods. This is especially so if there is a need for dose or flux maps of the system or if configuration perturbations are to be studied. In these latter two situations, it is difficult to obtain reliable Monte Carlo results due to statistical effects. Deterministic calculations are also useful as an independent check and verification of Monte Carlo results. The TWODANT system of discrete ordinates codes` has recently been enhanced for criticality safety analysis. In addition to ONEDANT, TWODANT, and TWOHEX, the TWODANT system now includes TWODANT/GQ and THREEDANT. These two new code modules expand the applicability of the TWODANT system. These new capabilities will be demonstrated on a representative sample of nuclear criticality safety problems. First, TWODANT/GQ and THREEDANT will be described. Salient features of each code module will be discussed. Calculational results obtained by each will be presented and compared with each other and with Monte Carlo results. Finally, some guidelines for the effective use of the TWODANT system on criticality applications will be given