Made available in DSpace on 2014-10-09T12:41:26Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:23Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:41:26Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:23Z (GMT). No. of bitstreams: 0 / Este trabalho apresenta o estudo do efeito do refletor sobre o tempo de vida neutrônico do Reator IPEN/MB-01. O método empregado requer uma abordagem que leve em conta tanto o núcleo quanto o refletor, de modo que as equações de cinética pontual, as quais constituem a base teórica de todo desenvolvimento matemático, contemplem ambas as regiões do reator. A partir dessas equações, conhecidas como equações de cinética pontual do modelo duas regiões, são obtidas as expressões teóricas para as APSDs (Auto Power Spectral Densities), as quais são utilizadas para o ajuste por mínimos quadrados aos dados das APSDs experimentais obtidas em vários estados subcríticos. O tempo de geração de nêutrons prontos, o tempo de vida dos nêutrons no refletor e a fração desses nêutrons que retornam ao núcleo, são obtidos como parâmetros do ajuste. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Ground penetrating radar (GPR) is widely used to identify locations of sinkholes in covered karst terrain in Florida. Some sinkholes serve as hydraulic conduits between the surficial and underlying aquifers. Their role is critical in determining the surficial aquifer response to pumping in deeper aquifers. Improved methods for discriminating between hydraulically active sinkholes and plugged sinkholes could help regional water management. In the covered karst of west-central Florida a clay-rich weathering horizon forms over the limestone. The clay-rich layer is in turn overlain by surficial sands. Ground penetrating radar profiles typically show a strong reflector from the top of clay-rich horizon as well as internal layering within sands. Active sinkholes are expected to have sandy conduits that broach the clay layer, and perhaps layering in the overlying sand indicative of ongoing subsidence. Three dimensional simulations of GPR profiles over sinkhole with and without conduits were run with the finite-difference time-domain (FDTD) program GPRMAX. Results from the synthetic surveys were then processed with standard techniques, including migration. The modeling confirms that conduits appear in GPR records primarily as gaps in the return from the clay layer. The modeling also shows that non-traditional survey geometries (varying antenna spacing and orientation) are unlikely to recover more information than traditional proximal transmitter-receiver separation. Also examined are GPR profiles and 3D grids over a set of active and inactive sinkholes in Tampa, Florida. Results from these surveys showed decent structural recovery of a small sinkhole similar in structure to that of the modeled ones. Indications of active subsidence and possible conduit structure were apparent from this data. Finally, the dense surveys served as a benchmark to compare interpretations taken with the same surveys at lower spatial resolutions and profiles with 2D-only processing methods in order to understand errors in analysis and interpretation that are possible from 2D surveys. Two-dimensional surveys, 2D processed and migrated, showed some similarity to the 3D results previously mentioned but contained more complexities and artifacts, which led to poorer interpretation ability.

No description available.

This project successfully designed, fabricated and characterized two highly reflective distributed Bragg reflectors for use with long wavelength vertical cavity surface emitting lasers. The first reflector consisted of 20 pairs of alternating lnP/Ino.64Gao.36Aso.777Po.223 layers grown on an InP substrate with a theoretically predicted normal incident reflectivity of 96.6% at a center wavelength of 1550nm. The second DBR had 20 pairs of alternating GaAs/Ino.484Gao.5i6P layers grown on a GaAs substrate with a theoretically predicted reflectivity of 94.9% at a center wavelength of 1550nm for normal incident light. Experimental results obtained using a specially designed reflectivity measurement setup confirmed reflectivity models and predictions at both normal and variable incident light angles. However, these measurements revealed a discrepancy between theoretical and experimental layer thickness values for both DBR structures. Applying perturbations to the theoretical models, the actual layer thicknesses of the DBRs were determined. X-ray analysis was employed to examine the periodicity of the super-lattices along with the accuracy of lattice matching to the substrate. Transmission electron microscopy revealed that no detectable drift in layer thickness was apparent during growth of the DBR structures. Photoluminescence was used to investigate any compositional variations of the quaternary layers in the first DBR stack. / Thesis / Master of Applied Science (MASc)

Future generations of space-based telescopes will require increasingly large primary reflectors, with very tight optical-quality tolerances. However, as their size grow, it becomes more and more difficult to meet the requirements, due to the manufacturing complexity and the associated costs. Chapters 2 and 3 propose two concepts of Adaptive Optics deformable mirrors, intended to be used as secondary corrector to compensate for manufacturing errors, gravity release and thermal distortion of large lightweight primary mirrors of space telescopes: (i) A scalable segmented bimorph mirror, based on independent PZT patches glued on Silicon wafers, providing a large number of degrees of freedom, a low mass while overcoming the problem of a low resonance mode; and (ii) A monolithic bimorph mirror, controlled by an array of independent electrodes, done by laser ablation on a single PZT patch. The modelling, the control strategy and the technological aspects are described. The performances of the manufactured prototypes are demonstrated experimentally. These prototypes have been developed in the framework of the ESA project, Bimorph Adaptive Large Optical Mirror Demonstrator (BIALOM). Chapter 4 introduces alternative designs, allowing to face the thermal distortion inherent to the bimorph architecture. They are compared in terms of stroke, voltage budget and first resonance frequency. These designs are required to be controlled in both directions using only positive voltages. Finally, the last chapter explores the feasibility of the shape control of a small size active thin shell reflector (with double curvature). The prototype is intended to be a technology demonstrator of a future large and very light active primary reflector. The behavior of the shell is studied through numerical simulations, and a preliminary design is proposed. This investigation is carried out in the framework of the ESA project: Multilayer Adaptive Thin Shell Reflectors (MATS). / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

This thesis investigates various active control aspects of large aperture telescopes; both Earth-based and space telescopes are considered.The first part proposes a concept of piezoelectric adaptive thin shell reflector for future space telescopes; it exhibits excellent areal density and stowability, and thus, paves the way to future large aperture space telescopes. Controlling the surface figure of spherical or parabolic shell with in-plane stresses induced by a piezoelectric layer raises two problems: (i) Doubly curved shells are significantly stiffer than flat plates (especially for the optical modes associated with hoop strains) and (ii) When using segmented electrodes with different voltages, the surface figure is subject to edge fluctuations with a characteristic length depending on the reflector curvature R_c and thickness t according to sqrt(R_ct). Accurate surface figure corrections require that the electrode size D_e satisfies D_e<sqrt(R_ct). This results in a very large number of electrodes, leading to ill-conditioning in the Jacobian matrix of the system; to solve this, a hierarchical approach is proposed to inverse the Jacobian, based on Saint-Venant's principle. This chapter also proposes a petal configuration which aims at reducing the hoop stiffness and improving the foldability of the reflector. A small scale technology demonstrator has been manufactured in the framework of the ESA-GSTP project Multilayer Adaptive Thin Shell Reflectors for Future Space Telescopes (MATS). The demonstrator includes a polymer substrate (PEEK) and a spin-coated PVDF-TrFE piezoelectric layer activated by independent electrodes; it is used to validate the manufacturing process and the independent actuation of the electrodes.The second part deals with control-structure interaction in flat deformable mirrors for Adaptive Optics. The problem arises because of the increasing size of AO mirrors, leading to lower resonance frequencies, and the control bandwidth requirements to achieve a good wavefront error compensation. This chapter studies the conditions for spillover instability and highlights the main parameters controlling the phenomenon: the ratio between the control bandwidth and the resonance frequency and the modal damping. Two methods for damping augmentation are discussed, one passive, using inductive shunting of piezoelectric elements, and the other active, using the wavefront sensor and the array of control actuators as a modal filter.The third part focuses on the field stabilization control of the tip/tilt mirror under wind disturbances of the E-ELT telescope (a distinctive feature of the E-ELT as compared to other smaller telescopes is that it will be a wind-limited instead of a seeing-limited telescope). A literature survey is conducted of the spectral content of the wind disturbances on large telescopes, with a special attention on the high frequency decay rate. The analysis confirms the adequacy of a decoupled design of the field stabilization (M5) control loop. However, the reaction torques necessary to control the tip/tilt mirror M5 have been found to depend critically on the asymptotic decay rate of the wind tilt disturbance. These torques act as a disturbance on the telescope structure and, if the wind disturbance does not decay fast enough with the frequency (a>-3), it may generate significant wavefront errors in the primary mirror M1, in a frequency range (30-100Hz) which may be difficult to eliminate by Adaptive Optics. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

The Internet traffic is constantly increasing following the emergence of new network applications like social networks, peer-to-peer, IP phone or IP television. In addition, these new applications request better path availability and path quality. Indeed the efficiency of these applications is strongly related to the quality of the underlying network. In that context network operators make use of traffic engineering techniques in order to improve the quality of the routes inside their network, but also to reduce the network cost of increased traffic handling with a better utilization of existing resources. This PhD thesis covers several topics of Traffic Engineering and Fast Restoration in IP/MPLS networks. Our first contribution is related to the definition of a well-engineered network. In the literature mathematical formulation of Traffic Engineering (TE) requirements are very diverse. We have thus performed a comparative study of many objective functions, in order to differentiate them and choose in a rational way the one that best reflects Traffic Engineering goals. We have also designed a method approaching optimal TE, whereby we divide the traffic matrix in N sub-matrices and route them independently, based on the derivatives of the objective function. The second topic addressed in this work concerns link weight optimizers (LWOs). Link weight optimization is the traffic engineering {it "standard"} technique in networks running link state routing protocols (which are widely used in transit networks). These link weight optimizers suffer from several limitations due to the BGP (Border Gateway Protocol) Hot-Potato rule, which is basically not considered by such optimizers. Therefore we have proposed a BGP-aware link weight optimization method that takes problematic Hot-Potato effects into account, and even turns them into an advantage. We have also studied how LWOs behave in big networks which have to use BGP route reflectors. Finally we have studied whether forwarding loops can appear or not when traffic is split among multiple equivalent egress routers, an optional BGP feature that we did use in our Hot-Potato aware LWO. Our last contribution concerns network resilience. We have proposed a solution for a rapid recovery from a link or node failure in an MPLS network. Our solution allows a decentralized deployment combined with a minimal bandwidth usage while requiring only reduced amount of information to flood in the network. This method is the first that makes possible a decentralized deployment combined with an optimal resource consumption. To easily simulate and test the methods proposed in this work, we have also contributed to the development of TOTEM - a TOolbox for Traffic Engineering Methods.

The optical design of a LED illuminator and camera imaging system were studied for potential use in a small medical "robotic type" probe to be used for minimally invasive abdominal surgery. Beam shaping optical reflectors were studied to increase the intensity distribution of the LED beam directed toward a close-by target surface. A CMOS/CCD camera and lens was used to image the targeted area. In addition, extensive optical ray tracing simulations were made to predict the intensity patterns. The experimental measurements and ray tracing simulations were in good agreement, and indicated that 20 degree cone reflectors for the LED sources and appropriate micro-lens/CCD chip imaging optics should provide a useful image at a working distance of about 5 cm.

This thesis is concerned with terahertz (THz) technology from both optical and electronic approaches. On the optical front, the investigation of optimised photoconductive switches included the characterisation, fabrication and testing of devices which can generate and detect THz radiation over the frequency range from DC to ~ 2.5 THz. These devices incorporated semiconductor photoconductors grown under low temperature (LT) Molecular Beam Epitaxy (MBE) conditions and using distributed Bragg reflectors (DBRs). The material properties were studied via numerous characterisation techniques which included Hall Effect and mid infrared reflections. Antenna structures were fabricated on the surface of the active layers and pulsed/continuous wave (CW) signal absorbed by these structures (under bias) generates photocurrent. With the help of the DBRs at certain wavelengths (800 nm and 1550 nm), the absorption coefficient at the corresponding illumination wavelength increased thus leading to significant increase of the THz output power while the materials kept the desirable photoconductive material properties such as high dark resistivity and high electron mobility. The inclusion of DBRs resulted in more than doubling of the THz peak signals across the entire operating frequency range and significant improvements in the relative THz power. For the THz electronic approach, a new type of InP-based Asymmetric Spacer Tunnel Diode (ASPAT), which can be used for high frequency detector, was studied. The asymmetric DC characteristics for this novel tunnel diode showed direct compatibility with high frequency zero-bias detector applications. The devices also showed an extreme thermal stability (less than 7.8% current change from 77 K to 400 K) as the main carrier transport mechanism of the ASPAT was tunnelling. Physical models for this ASPAT diode were developed for both DC (direct current) and AC (alternating current) simulations using the TCAD software tool SILVACO. The simulated DC results showed almost perfect matches with measurements across the entire temperature range from 77 K to 400 K. From RF (radio frequency) measurements, the intrinsic diode parameters were extracted and compared with measured data. The simulated zero biased detector circuits operating at 100 GHz and 240 GHz using the new InGaAs-AlAs ASPAT diode (4*4 micrometer square) showed comparable voltage sensitivities to state of the art Schottky barrier diodes (SBDs) detectors but with the added advantage of excellent thermal stability.