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[en] At Colorado University-Boulder the primary task is to extend our gyrokinetic Particle-in-Cell simulation of tokamak micro-turbulence and transport to the area of energetic particle physics. We have implemented a gyrokinetic ion/massless fluid electron hybrid model in the global δf-PIC code GEM, and benchmarked the code with analytic results on the thermal ion radiative damping rate of Toroidal Alfven Eigenmodes (TAE) and with mode frequency and spatial structure from eigenmode analysis. We also performed nonlinear simulations of both a single-n mode (n is the toroidal mode number) and multiple-n modes, and in the case of single-n, benchmarked the code on the saturation amplitude vs. particle collision rate with analytical theory. Most simulations use the f method for both ions species, but we have explored the full-f method for energetic particles in cases where the burst amplitude of the excited instabilities is large as to cause significant re-distribution or loss of the energetic particles. We used the hybrid model to study the stability of high-n TAEs in ITER. Our simulations show that the most unstable modes in ITER lie in the rage of 10 < n < 20. Thermal ion pressure effect and alpha particles non-perturbative effect are important in determining the mode radial location and stability threshold. The thermal ion Landau damping rate and radiative damping rate from the simulations are compared with analytical estimates. The thermal ion Landau damping is the dominant damping mechanism. Plasma elongation has a strong stabilizing effect on the alpha driven TAEs. The central alpha particle pressure threshold for the most unstable n=15 mode is about βα(0) = 0.7% for the fully shaped ITER equilibrium. We also carried nonlinear simulations of the most unstable n = 15 mode and found that the saturation amplitude for the nominal ITER discharge is too low to cause large redistribution or loss of alpha particles. To include kinetic electron effects in the hybrid model we have studied a kinetic electron closure scheme for the fluid electron model. The most important element of the closure scheme is a complete Ohm's law for the parallel electric field E||, derived by combining the quasi-neutrality condition, the Ampere's equation and the v|| moment of the gyrokinetic equations. A discretization method for the closure scheme is studied in detail for a three-dimensional shear-less slab plasma. It is found that for long-wavelength shear Alfven waves the kinetic closure scheme is both more accurate and robust than the previous GEM algorithm using the split-scheme, whereas for the ion-gradient-driven instability the previous algorithm is more efficient. This kinetic electron closure scheme will be implemented in GEM in the future. We have studied the beam driven Reverse Shear Alfven Eigenmodes (RSAE) observed in DIII-D discharge 142111. For this purpose a new scheme for obtaining the electric potential is implemented, i.e., by solving the gyrokinetic moment (GKM) equation, which is essentially the equation for ∂φ/∂t used in GEM's split-weight scheme, and then integrating in time. Due to charge-neutrality the ExB motions of the equilibrium densities of all species cancel each other and do not cause charge separation if there is no finite Larmor radius effect. The advantage of solving the GKM equation is that this lowest-order cancellation can be made explicit. The GKM approach is found to be more accurate and robust. GEM simulations have reproduced many features of RSAE seen in the experiment, such as frequency chirping and the chirping range. It has been reported by other simulation codes that the shearing direction of the mode structure in the poloidal plane disagrees with observation. We found that the mode structure, including the shearing in the poloidal plane, is in general sensitive to the beam distribution. Using the same beam density profile as in other codes but with a slowing-down distribution in velocity, GEM simulations reproduce the mode shearing direction seen in the experiment. We have carried out extensive nonlinear simulations of RSAE
[en] Because of their outstanding physical and chemical properties at high temperature, in comparison with metals, silicon carbide (SiC) composite materials are studied as possible nuclear fuel cladding materials either for future advanced fission/fusion reactors, or more recently, for the currently existing light water reactors. 2D-braided SiC/SiC composite tubes, manufactured by chemical vapor infiltration (CVI), exhibit an anisotropic, hardly deformable (1%) mechanical behavior. Understanding the relations between the microstructure, the damage mechanisms and the macroscopic behavior is essential to optimize the structural design of this material for the considered applications. One important manufacturing parameter is the braiding angle, i.e. the angle between the fiber tows and the tube axis. The objective of this work is to provide a comprehensive understanding of the damage-microstructure relations, in particular of the effects of the braiding angle on the damage mechanisms. For this purpose, an investigation combining experimental observations at macro and micro-scale and numerical simulations is developed. The composite tubes are first studied through in situ tensile testing under X-ray computed tomography. Experiments were carried out on the PSICHE beamline at synchrotron SOLEIL using a pink polychromatic beam. The recorded 3D images are processed using the digital volume correlation (DVC) technique, extended by a series of advanced image processing algorithms specifically developed in order to analyze the 3D microstructures, to measure the deformations through the tube thickness, and to detect and quantitatively characterize the network of micro-cracks created by the mechanical loading. In addition, numerical simulations are performed on the real microstructures as observed in the high-resolution images recorded during the in situ tests. Stress fields are calculated at the microstructural scale in the elastic regime using a numerical tool based on the Fast Fourier Transform (FFT). They help to better understand crack initiation and interpret the experimental observations within one-to-one comparisons. Both the experimental and numerical approaches are applied to three tubes with different braiding angles (30, 45 and 60 degrees). The effect of the braiding angle on the initiation and evolution of damage in the bulk of the composite materials can thus be highlighted. (author)
[fr]Du fait de leurs proprietes physiques et chimiques exceptionnelles a haute temperature par rapport aux metaux, les composites de carbure de silicium (SiC) sont etudies comme eventuel materiau de gainage du combustible nucleaire dans les reacteurs de fusion ou fission avancee futurs, ainsi que, depuis plus recemment, dans les reacteurs a eau legere existants. Les tubes composites SiC/SiC tresses en 2D, fabriques par procede d'infiltration chimique en phase vapeur (CVI), presentent un comportement mecanique anisotrope, faiblement deformable (∼ 1%). La maitrise des relations entre la microstructure, l'endommagement et le comportement macroscopique est essentielle pour optimiser precisement le dimensionnement structurel de ce materiau pour les applications envisagees. Un parametre de fabrication important est l'angle de tressage, angle entre les torons de fibres et l'axe du tube. L'objectif de ce travail est de fournir une comprehension detaillee de la relation endommagement-microstructure, en particulier des effets de l'angle de tressage sur les mecanismes d'endommagement. Dans ce but, une etude combinant observations experimentales a macro et micro-echelle et simulations numeriques est menee. Les tubes composites sont d'abord etudies par des essais de traction in situ sous tomographie par rayons X. Les experiences ont ete realisees sur la ligne PSICHE du synchrotron SOLEIL sous faisceau rose polychromatique. Les images tridimensionnelles sont analysees par la technique de correlation d'image volumique (DVC), completee par une serie d'algorithmes de traitement d'image originaux, developpes specifiquement pour analyser les microstructures 3D, mesurer les deformations a travers l'epaisseur du tube, detecter et caracteriser quantitativement le reseau de microfissures creees par le chargement mecanique. De plus, les microstructures reelles, decrites par les images de haute resolution issues des tests in situ, sont utilisees dans les simulations numeriques multi-echelle. Les champs de contrainte a l'echelle microstructurale sont calcules en regime elastique par une technique utilisant la transformee de Fourier rapide (FFT). Ils permettent de mieux comprendre l'initiation des fissures et d'interpreter les observations experimentales par une comparaison directe. Ces approches experimentales et numeriques sont appliquees a trois tubes presentant differents angles de tressage (30, 45 et 60 degres). L'influence de l'angle de tressage sur l'initiation et l'evolution de l'endommagement a oeur des composites est ainsi mise en evidence
[en] The emerging technology, transactive energy network, can allow multiple interconnected micro-grids (a.k.a. micro-grid clusters) to exchange energy for greater energy efficiency. Existing research has demonstrated that the micro-grid clusters can achieve some collective interests (e.g., minimizing total energy cost). However, some micro-grids may have to make sacrifices of their individual interests (e.g., increasing cost) for collective interests of the clusters. To bridge these research gaps, we propose four different transactive energy management models for micro-grid clusters where each micro-grid is allowed to have energy transactions with others. The first model focuses on maximizing collective interests, both the collective and individual interests are considered in the second model, and the last two models aim to maximize both the collective and individual interests. The performances of the proposed models are evaluated using a cluster of sixteen micro-grids with different energy profiles. It is demonstrated that 1) all of the four models can maximize the collective interests, 2) the third model can maximize the relative individual interests where each micro-grid can achieve the same percentage of cost savings as the clusters, and 3) the fourth model can maximize the absolute individual interests where each micro-grid can achieve the same amount of cost savings. - Highlights: • A modeling framework is developed for transactive energy management of the micro-grid clusters. • Four operation decision models are developed to balance the collective and individual interests. • The prices of local energy transaction are modeled. • The micro-grid clusters can achieve 15.34% energy cost savings.
[en] A δf particle simulation method is developed for solving the gyrokinetic-Maxwell system of equations that describes turbulence and anomalous transport in toroidally confined plasmas. A generalized split-weight scheme is used to overcome the constraint on the time step due to fast parallel motion of the electrons. The inaccuracy problem at high plasma β is solved by using the same marker particle distribution as is used for δf to evaluate the βmi/meApar term in Ampere's equation, which is solved iteratively. The algorithm is implemented in three-dimensional toroidal geometry using field-line-following coordinates. Also discussed is the implementation of electron-ion collisional effects which are important when kinetic electron physics is included. Linear benchmarks in toroidal geometry are presented for moderate β, that is, β<<1, but βmi/me>>1. Nonlinear simulation results with moderate β are also presented
[en] Absolute reaction rate constants for the ground state dichlorocarbene CCl2 with C2H2 and H2O have been measured (T=293 K). The CCl2 radicals were produced by laser photolysis of CCl4 molecules at 213 nm and then electronically excited to the CCl2 (A-tilde) state at 541.0 nm with Nd:YAG laser pumped dye laser. The technique of laser-induced fluorescence was employed to measure the relative concentrations at different reaction time and, then, obtain their rate constants. They are (3.3 ± 0.5) x 10-13 cm3 molecule-1 s-1 for the reaction of CCl2 + C2H2 and (4.8 ± 2.0) x 10-14 cm3 molecule-1 s-1 for the reaction of CCl2 + H2O (total pressure=12 Torr, T=293 K), respectively. It is found that the rate constant for reaction CCl2 + C2H2 slightly increases with the total pressure (P=10-50 Torr). But for the reaction CCl2 + H2O, it is independent on the total pressure. The mechanisms for these two reactions have been theoretically studied using a high-level ab initio calculation. For the reaction of CCl2 + H2O, it is found that there are two reaction mechanisms: insertion and addition-elimination. Three primary product channels, HCl + HClCO, HCl + trans-ClCOH and HCl + cis-ClCOH, are studied in detail. For the reaction of CCl2 + C2H2, there are also two reaction mechanisms: insertion and three-cyclo-addition. The addition of C atom in CCl2 radical to C-C triple bond in C2H2 molecule may be the dominant reaction channel giving a three-member-ring structure, CCl2 CHCH radical, as the primary intermediate
[en] A novel and simple method for preparing Mg–50%Al_4C_3 (hereafter in wt.%) master alloy has been developed by powder in-situ synthesis process under argon atmosphere. X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) results show the existence of Al_4C_3 particles in this master alloy. After adding 1.8% Mg–50%Al_4C_3 master alloy, the average grain size of α-Mg decreased from 360 μm to 154 μm. Based on the DTA test results and calculation of the planar disregistry between Al_4C_3 and α-Mg, Al_4C_3 particles located in the central regions of magnesium grains can act as the heterogeneous nucleus of primary α-Mg phase
[en] Pristine tetraaniline and three carboxyl-substituted tetraaniline derivatives, that is, tetraaniline ((Ani)4), carboxyl-capped tetraaniline ((Ani)4-COOH), amino/carboxyl-capped tetraaniline (NH2-(Ani)4-COOH) and carboxyl-capped star-shaped tetraaniline (Tri-(Ani)4-COOH) were synthesized, and the influence of substituent and molecular architecture on their electrochromic properties was investigated. Pristine (Ani)4 shows lowest electrochromic properties while the carboxyl-capped star-shaped Tri-(Ani)4-COOH possesses the best electrochromic properties with fast bleaching and coloring times of 2.04 s and 1.90 s, respectively, promising optical contrast of 74.2%, and improved coloration efficiency of 133.31 cm2C−1 at 750 nm. This enhanced electrochromic performance can be attributed to the introduction of carboxyl substituent and star-shaped molecular architecture, which are able to improve the interchain charge movement, electron movement between films and substrate and enhanced ion diffusion resulting from loosely packed molecular structure of films, and confirmed by cyclic voltammetry and atomic force microscope results, implying the introduction of special substituent and molecular architecture is an effective strategy to improve the electrochromic properties of aniline oligomers.
[en] We investigate the molecular environment of the Galactic supernova remnant (SNR) Kesteven 78 and perform an XMM-Newton X-ray spectroscopic study for the northeastern edge of the remnant. SNR Kes 78 is found to interact with the molecular clouds (MCs) at a systemic local standard of rest velocity of 81 km s–1. At around this velocity, the SNR appears to contact a long molecular strip in the northeast and a large cloud in the east as revealed in the 13CO line, which may be responsible for the radio brightness peak and the OH maser, respectively. The 12CO-line bright region morphologically matches the eastern bright radio shell in general, and the SNR is consistent in extent with a CO cavity. Broadened 12CO-line profiles discerned in the eastern maser region and the western clumpy molecular arc and the elevated 12CO (J = 2-1)/(J = 1-0) ratios along the SNR boundary may be signatures of shock perturbation in the molecular gas. The SNR-MC association places the SNR at a kinematic distance of 4.8 kpc. The X-rays arising from the northeastern radio shell are emitted by underionized hot (∼1.5 keV), low-density (∼0.1 cm–3) plasma with solar abundance, and the plasma may be of intercloud origin. The age of the remnant is inferred to be about 6 kyr. The size of the molecular cavity in Kes 78 implies an initial mass around 22 M☉ for the progenitor.
[en] Graphical abstract: - Highlights: • Li-Zn alloy substrate is novelly formed by Li electrodeposition on the Zn substrate precursor. • The coulombic efficiency of Li deposition/stripping on the Li-Zn alloy substrate remains high at 96.7% after 400 cycles. • The SEI film formed during the formation of Li-Zn alloy is stable during Li deposition/stripping cycling on the Li-Zn substrate. • The exchange current density of Li deposition on the Li-Zn substrate is 9.21 × 10"−"4 A cm"−"2 which is nearly eight times larger than that on the Cu substrate. - Abstract: The cycling performance of a Li metal electrode in rechargeable Li batteries is studied using a novelly formed Li–Zn alloy as a substrate. A Zn layer electrodeposited on a Cu disk with ultrasonic assistance is used as a substrate precursor. Li electrodeposition followed to form the Li–Zn alloy. The morphologies of the substrate before and after Li deposition and stripping are investigated by scanning electron microscopy (SEM), and the electrochemical properties of the substrate are investigated by galvanostatic charge-discharge and cyclic voltammetry (CV). The growth states of solid electrolyte interface (SEI) films of Li deposits on the Li–Zn alloy and Cu surfaces are compared by electrochemical impedance spectroscopy (EIS); exchange current densities of Li electrodeposition on Cu, Zn, and Li–Zn alloy substrates are also compared based on tests of constant current pulse deposition. The efficiency of Li deposition/stripping on the Li–Zn alloy substrate remains high at 96.7% after 400 cycles at a current density of 0.1 mA cm"−"2 and 250 cycles at the current density of 0.2 mA cm"−"2. These results can be attributed to the formation of a stable SEI film on the Li–Zn substrate and the high exchange current density of Li deposition and stripping on this substrate. The Li–Zn alloy proposed in this work may be a perfect substrate for enhancing the cycling capability of Li metal electrode