105 research outputs found
Beam scanning by liquid-crystal biasing in a modified SIW structure
A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium
1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface
A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance
Modélisation et simulation d'une plaque en acier 4340 chauffée par induction utilisant un modèle de simulation par éléments finis : prédiction et optimisation numérique /
RÉSUMÉ : Le traitement thermique par induction est une technologie importante dans l'industrie manufacturière. Avec sa vaste gamme d'applications allant du chauffage à la fusion, du brasage au soudage et du scellement des bouchons au collage, c'est souvent la clé pour ajouter de la valeur à un processus particulier dans l'industrie. Ceci est réalisé par le fait que le chauffage par induction fournit un taux de chauffage plus élevé que tout autre procédé de chauffage commercial disponible. En raison du chauffage rapide et de la bonne reproductibilité, il est couramment utilisé pour chauffer des matériaux conducteurs à la température exacte. La capacité de génération de chaleur en profondeur en combinaison avec une intensité de chaleur élevée rapidement et dans des régions bien définies sur la pièce est une caractéristique très attrayante de cette technologie conduisant à un temps de cycle de processus réduit avec une qualité reproductible. Néanmoins, beaucoup reste à faire pour construire le meilleur profil de température possible, c’est pour cela qu’il est nécessaire de caractériser et de contrôler l’effet des paramètres de contrôle du procédé. L’objectif de cette étude consiste alors à l’étude des paramètres géométrique, électromagnétique et mécanique intervenant en chauffage par induction, destinée à une géométrie tridimensionnelle à l’acier 4340 avec inducteur asymétrique. Pour bien mener cette étude, le projet est structuré en trois grandes parties combinant la simulation, la planification d’expérience, l’analyse statistique et l’optimisation pour la création d’un modèle prédictif du profil de température. À partir des équations de Maxwell pour l’électromagnétisme et les équations de transfert de chaleur, les phénomènes physiques ont été modélisés en éclairant les lois du comportement mécanique des matériaux et en établissant la densité de courant externe. Le problème a été entamé par la méthode des éléments finis avec un modèle en 3D et réalisé dans le logiciel COMSOL. Plusieurs simulations, et des mesures de profil de température ont été effectués pour chaque partie du modèle et permettent de confirmer avec succès les hypothèses du modèle et de vérifier la qualité des résultats de la simulation. Les conclusions de ce travail ont pointé l’aspect rentable des techniques utilisées et ont donné accès à l’industrie pour des recettes simples et fiables consacrées à la conception des profils de température uniformes et plus optimisés et qui rendent possible la production des composantes mécaniques de haute performance. -- Mot(s) clé(s) en français : Traitement thermique par induction, Acier 4340, Plaque, Simulation, Profil de température, Taguchi, ANOVA, Prédiction. --
ABSTRACT : Induction heat treatment is an important technology in the manufacturing industry. With its wide range of applications ranging from heating to fusion, brazing to welding, and cap sealing to gluing, this is often the key to adding value to a particular process in industry. This is achieved by the fact that induction heating provides a higher heating rate than any other commercial heating method available. Due to the rapid heating and good reproducibility, it is commonly used to heat conductive materials to the exact temperature. The ability to generate heat at depth in combination with high heat intensity quickly and in well-defined regions on the part is a very attractive feature of this technology leading to reduced process cycle time with reproducible quality. However, much remains to be done to build the best possible temperature profile, which is why it is necessary to characterize and monitor the effect of the process control parameters. The objective of this study is then to study the geometric, electromagnetic and mechanical parameters involved in induction heating, intended for a three-dimensional geometry in 4340 steel with asymmetric inductor. To properly conduct this study, the project is structured in three main parts combining simulation, experiment planning, statistical analysis and optimization to create a predictive model of the temperature profile. From Maxwell's equations for electromagnetism and heat transfer equations, physical phenomena have been modeled by illuminating the laws of mechanical behavior of materials and by establishing the external current density. The problem was started by the finite element method with a model in 3D and carried out in the software COMSOL. Several simulations, and temperature profile measurements were performed for each part of the model and allow to successfully confirm the model assumptions and to check the quality of the simulation results. The conclusions of this work pointed out the cost-effective aspect of the techniques used and gave access to the industry for simple and reliable recipes devoted to the design of uniform and more optimized temperature profiles and which make possible the production of mechanical components of high performance. -- Mot(s) clé(s) en anglais : Induction heat treatment, 4340 steel, Plate, Simulation, Temperature profile, Taguchi, ANOVA, Prediction
Metamaterial
In-depth analysis of the theory, properties and description of the most potential technological applications of metamaterials for the realization of novel devices such as subwavelength lenses, invisibility cloaks, dipole and reflector antennas, high frequency telecommunications, new designs of bandpass filters, absorbers and concentrators of EM waves etc. In order to create a new devices it is necessary to know the main electrodynamical characteristics of metamaterial structures on the basis of which the device is supposed to be created. The electromagnetic wave scattering surfaces built with metamaterials are primarily based on the ability of metamaterials to control the surrounded electromagnetic fields by varying their permeability and permittivity characteristics. The book covers some solutions for microwave wavelength scales as well as exploitation of nanoscale EM wavelength such as visible specter using recent advances of nanotechnology, for instance in the field of nanowires, nanopolymers, carbon nanotubes and graphene. Metamaterial is suitable for scholars from extremely large scientific domain and therefore given to engineers, scientists, graduates and other interested professionals from photonics to nanoscience and from material science to antenna engineering as a comprehensive reference on this artificial materials of tomorrow
Safety of Simultaneous Scalp and Intracranial Electroencephalography Functional Magnetic Resonance Imaging
Understanding the brain and its activity is one of the great challenges of modern science. Normal brain activity (cognitive processes, etc.) has been extensively studied using electroencephalography (EEG) since the 1930’s, in the form of spontaneous fluctuations in rhythms, and patterns, and in a more experimentally-driven approach in the form of event-related potentials allowing us to relate scalp voltage waveforms to brain states and behaviour. The use of EEG recorded during functional magnetic resonance imaging (EEG-fMRI) is a more recent development that has become an important tool in clinical neuroscience, for example, for the study of epileptic activity. The primary aim of this thesis is to devise a protocol in order to minimise the health risks that are associated with simultaneous scalp and intracranial EEG during fMRI (S- icEEG-fMRI). The advances in this technique will be helpful in presenting a new imaging method that will allow the measurement of brain activity with unprecedented sensitivity and coverage. However, this cannot be achieved without assessing the safety implications of such a technique. Therefore, five experiments were performed to fulfil the primary aim. First, the safety of icEEG- fMRI using body transmit RF coil was investigated to improve the results of previous attempts using a head transmit coil at 1.5T. The results of heating increases during a high-SAR sequence were in the range of 0.2-2.4 °C at the contacts with leads positioned along the central axis inside the MRI bore. These findings suggest the need for careful lead placement. Second, also for the body transmit coil we compared the heating in the vicinity of icEEG electrodes placed inside a realistically-shaped head phantom following the addition of scalp EEG electrodes. The peak temperature change was +2.7 °C at the most superior icEEG electrode contact without scalp electrodes, and +2.1 °C at the same contact and the peak increase in the vicinity of a scalp electrode contact was +0.6 °C (location FP2). These findings show that the S-icEEG-fMRI technique is feasible if our protocol is followed carefully. Third, the heating of a realistic 3D model of icEEG electrode during MRI using EM computational simulation was investigated. The resulting peak 10 g averaged SAR was 20% higher than without icEEG. Moreover, the superior icEEG placed perpendicular to B0 showed significant local SAR increase. These results were in line with previous studies. Fourth, the possibility of simplifying a complete 8-contact with 8 wires depth icEEG electrode model into an electrode with 1-contact and 1 wire using EM simulations was addressed. The results showed similar patterns of averaged SAR values around the electrode tip during phantom and electrode position along Z for the Complete and Simplified models, except an average maximum at Z = ~2.5 W/kg for the former. The SAR values during insertion depth for the Simplified model were double those for the Complete model. The effect of extension cable length is in agreement with previous experiments. Fifth, further simulations were implemented using two more simplified models: 8-contact with 1 wire shared with all contact and 8-contact 1 wire connected to each contact at a time as well as the previously modelled simplified 1-contact 1 wire. Two sets of simulations were performed: with a single electrode and with multiple electrodes. For the single electrode, three scenarios were tested: the first simplified model used only, the second simplified models used only and the third model positioned in different 13 locations. The results of these simulations showed about 11.4-20.5-fold lower SAR for the first model than the second and 0.29-5.82-fold lower SAR for the first model than the complete model. The results also showed increased SAR for the electrode close to the head coil than the ones away from it. For the multiple electrodes, three scenarios were tested: two 1-contact and wire electrodes in different separations, multiple electrodes with their wires separated and multiple electrodes with their wires shorted. The results showed interaction between the two tested electrodes. The results of the multiple electrodes presented 2 to ~10 times higher SAR for the separated setup than the shorted. The comparison between the 1-contact with 1 wire model and the complete model is still unknown and more tests are required to show it. From the findings of this PhD research, we conclude that a body RF coil can be utilized for icEEG-fMRI at 1.5 T; however, the safety protocol has to be implemented. In addition, scalp EEG can be used in conjunction with icEEG electrodes inside the body RF coil at 1.5 T and the safety protocol has to be followed. Finally, it is feasible to perform EM computational simulations using realistic icEEG electrodes on a human model. However, simplifying the realistic icEEG electrode model might result in overestimations of the heating, although it is possible that the simplification of the model can help to simulate more complex implantations such as the implantation of multiple electrodes with their leads open circuited or short circuited, which can provide more information about the safety of implanted patients inside the MRI
Electrostatic Design and Characterization of a 200 keV Photogun and Wien Spin Rotator
High-energy nuclear physics experiments at the Jefferson Lab Continuous Electron Beam Accelerator Facility (CEBAF) require high spin-polarization electron beams produced from strained super-lattice GaAs photocathodes activated to negative electron affinity in a high voltage photogun operating at 130 kV dc. A pair of Wien filter spin rotators in the injector provides precise control of the electron beam polarization at the end station target. An upgrade of the CEBAF injector to better support the upcoming Moller experiment requires increasing the electron beam energy to 200 keV, resulting in better transmission through injector apertures and improved photocathode lifetime. In addition, the energy increase is expected to reduce unwanted helicity correlated intensity and position systematics. These requirements led to the design of a shielding electrode described in this work, which minimizes the electric field at the triple-point junction and linearizes the potential along the insulator, thus reducing the risk of field emission induced insulator arcing. The Wien spin rotator design was modified for increasing the electric field from 1.6 to 2.7 MV/m and the magnetic field from 9.1 to 13 mT. The upgrades required detailed modeling in Solidworks, electrostatic simulations using CST, beam dynamics using GPT, device implementation, and in situ high voltage characterization of the world’s first 200 keV polarized photoelectron gun and compatible Wien filter spin rotator
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