Irradiation uniformity at the Laser MegaJoule facility in the context of the shock ignition scheme

The use of the Laser MegaJoule facility within the shock ignition scheme has been considered. In the first part of the study, one-dimensional hydrodynamic calculations were performed for an inertial confinement fusion capsule in the context of the shock ignition scheme providing the energy gain and an estimation of the increase of the peak power due to the reduction of the photon penetration expected during the high-intensity spike pulse. In the second part, we considered a Laser MegaJoule configuration consisting of 176 laser beams that have been grouped providing two different irradiation schemes. In this configuration the maximum available energy and power are 1.3 MJ and 440 TW. Optimization of the laser?capsule parameters that minimize the irradiation non-uniformity during the first few ns of the foot pulse has been performed. The calculations take into account the specific elliptical laser intensity profile provided at the Laser MegaJoule and the expected beam uncertainties. A significant improvement of the illumination uniformity provided by the polar direct drive technique has been demonstrated. Three-dimensional hydrodynamic calculations have been performed in order to analyse the magnitude of the azimuthal component of the irradiation that is neglected in twodimensional hydrodynamic simulations.

A two-dimensional finite element model of front surface current flow in cells under non-uniform, concentrated illumination

A two-dimensional finite element model of current flow in the front surface of a PV cell is presented. In order to validate this model we perform an experimental test. Later, particular attention is paid to the effects of non-uniform illumination in the finger direction which is typical in a linear concentrator system. Fill factor, open circuit voltage and efficiency are shown to decrease with increasing degree of non-uniform illumination. It is shown that these detrimental effects can be mitigated significantly by reoptimization of the number of front surface metallization fingers to suit the degree of non-uniformity. The behavior of current flow in the front surface of a cell operating at open circuit voltage under non-uniform illumination is discussed in detail.

Non uniform target illumination in deflagration regime : refractive smoothing

Refractive smoothing of weak non-uniformities in the illumination of laser targets is analyzed, assuming absorption at the critical density and restricting conduction to a thin layer, and using results from thermal smoothing, which is uncoupled from the refraction. Magnetic effects are included. Non-uniformity wavelengths comparable to the thickness of the conduction layer are considered; efficient smoothing exists at both short and long wavelengths in this range. Thermal focusing could make the ablated plasma unstable.

New concepts and techniques for the development of high-efficiency concentrating photovoltaic modules

El interés por los sistemas fotovoltaicos de concentración (CPV) ha resurgido en los últimos años amparado por el desarrollo de células multiunión de muy alta eficiencia basadas en semiconductores de los grupos III-V. Estas células han permitido obtener módulos de concentración con eficiencias que prácticamente duplican las del panel plano y que llegan al 35% en los módulos récord. Esta tesis está dedicada al diseño y la implementación experimental de nuevos conceptos que permitan obtener módulos CPV que no sólo alcancen una eficiencia alta en condiciones estándar sino que, además, sean lo suficientemente tolerantes a errores de montaje, seguimiento, temperatura y variaciones espectrales para que la energía que producen a lo largo del año sea máxima. Una de las primeras cuestiones que se abordan es el diseño de elementos ópticos secundarios para sistemas cuyo primario es una lente de Fresnel y que permiten, para una concentración fija, aumentar el ángulo de aceptancia y la tolerancia del sistema. Varios secundarios reflexivos y refractivos han sido diseñados y analizados mediante trazado de rayos. En particular, utilizando óptica anidólica y basándose en el diseño de una sola etapa conocido como ‘concentrador dieléctrico que funciona por reflexión total interna‘, se ha diseñado, fabricado y caracterizado un secundario con salida cuadrada que, usado junto con una lente de Fresnel, permite alcanzar simultáneamente una elevada eficiencia, concentración y aceptancia. Además, se ha propuesto y prototipado un método alternativo de fabricación para otro de los secundarios, denominado domo, consistente en el sobremoldeo de silicona sobre células solares. Una de las características que impregna todo el trabajo realizado en esta tesis es la aproximación holística en el diseño de módulos CPV, es decir, se ha prestado especial atención al diseño conjunto de la célula y la óptica para garantizar que el sistema total alcance la mayor eficiencia posible. En este sentido muchos sistemas ópticos desarrollados en esta tesis han sido diseñados, caracterizados y optimizados teniendo en cuenta que el ajuste de corriente entre las distintas subcélulas que comprenden la célula multiunión bajo el concentrador sea muy próximo a uno. La capa antirreflectante sobre la célula funciona, en cierto modo, como interfaz entre la óptica y la célula, por lo que se ha diseñado un método de optimización de capas antirreflectantes que considera no sólo el amplio rango de longitudes de onda para el que las células multiunión son sensibles sino también la distribución angular de intensidad sobre la célula creada por la óptica de concentración. Además, la cuestión de la falta de uniformidad también se ha abordado mediante la comparación de las distribuciones espectrales y espaciales de irradiancia que crean diferentes ópticas (simuladas mediante trazado de rayos y fotografiadas) y las pérdidas de eficiencia que experimentan las células iluminadas por dichas ópticas de concentración medidas experimentalmente. El efecto de la temperatura en la óptica de concentración también ha sido objeto de estudio de esta tesis. En particular, mediante simulaciones de elementos finitos se han dado los primeros pasos para el análisis de las deformaciones que sufren los dientes de las lentes de Fresnel híbridas (vidrio-silicona), así como el cambio de índice de refracción con la temperatura y la influencia de ambos efectos sobre el funcionamiento de los sistemas. Se ha implementado un modelo que tiene por objeto considerar las variaciones ambientales, principalmente temperatura y contenido espectral de la radiación directa, así como las sensibilidades térmica y espectral de los sistemas CPV, con el fin de maximizar la energía producida por un módulo de concentración a lo largo de un año en un emplazamiento determinado. Los capítulos 5 y 6 de este libro están dedicados al diseño, fabricación y caracterización de un nuevo concepto de módulo fotovoltaico denominado FluidReflex y basado en una única etapa reflexiva con dieléctrico fluido. En este nuevo concepto la presencia del fluido aporta algunas ventajas significativas como son: un aumento del producto concentración por aceptancia (CAP, en sus siglas en inglés) alcanzable al rodear la célula con un medio cuyo índice de refracción es mayor que uno, una mejora de la eficiencia óptica al disminuir las pérdidas por reflexión de Fresnel en varias interfaces, una mejora de la disipación térmica ya que el calor que se concentra junto a la célula se trasmite por convección natural y conducción en el fluido y un aislamiento eléctrico mejorado. Mediante la construcción y medida de varios prototipos de unidad elemental se ha demostrado que no existe ninguna razón fundamental que impida la implementación práctica del concepto teórico alcanzando una elevada eficiencia. Se ha realizado un análisis de fluidos candidatos probando la existencia de al menos dos de ellos que cumplen todos los requisitos (en particular el de estabilidad bajo condiciones de luz concentrada) para formar parte del sistema de concentración FluidReflex. Por ´ultimo, se han diseñado, fabricado y caracterizado varios prototipos preindustriales de módulos FluidReflex para lo cual ha sido necesario optimizar el proceso de fabricación de la óptica multicavidad a fin de mantener el buen comportamiento óptico obtenido en la fabricación de la unidad elemental. Los distintos prototipos han sido medidos, tanto en el laboratorio como bajo el sol real, analizando el ajuste de corriente de la célula iluminada por el concentrador FluidReflex bajo diferentes distribuciones espectrales de la radiación incidente así como el excelente comportamiento térmico del módulo. ABSTRACT A renewed interest in concentrating photovoltaic (CPV) systems has emerged in recent years encouraged by the development of high-efficiency multijunction solar cells based in IIIV semiconductors that have led to CPV module efficiencies which practically double that of flat panel PV and which reach 35% for record modules. This thesis is devoted to the design and experimental implementation of new concepts for obtaining CPV modules that not only achieve high efficiency under standard conditions but also have such a wide tolerance to assembly errors, tracking, temperature and spectral variations, that the energy generated by them throughout the year is maximized. One of the first addressed issues is the design of secondary optical elements whose primary optics is a Fresnel lens and which, for a fixed concentration, allow an increased acceptance angle and tolerance of the system. Several reflective and refractive secondaries have been designed and analyzed using ray tracing. In particular, using nonimaging optics and based on the single-stage design known as ‘dielectric totally internally reflecting concentrator’, a secondary with square output has been designed, fabricated and characterized. Used together with a Fresnel lens, the secondary can simultaneously achieve high efficiency, concentration and acceptance. Furthermore, an alternative method has been proposed and prototyped for the fabrication of the secondary named dome. The optics is manufactured by direct overmolding of silicone over the solar cells. One characteristic that permeates all the work done in this thesis is the holistic approach in the design of CPV modules, meaning that special attention has been paid to the joint design of the solar cell and the optics to ensure that the total system achieves the highest attainable efficiency. In this regard, many optical systems developed in the thesis have been designed, characterized and optimized considering that the current matching among the subcells within the multijunction solar cell beneath the optics must be close to one. Antireflective coating over the cell acts, somehow, as an interface between the optics and the cell. Consequently, a method has been designed to optimize antireflective coatings that takes into account not only the broad wavelength range that multijunction solar cells are sensitive to but also the angular intensity distribution created by the concentrating optics. In addition, the issue of non-uniformity has also been addressed by comparing the spectral and spatial distributions of irradiance created by different optics (simulated by ray tracing and photographed) and the efficiency losses experienced by cells illuminated by those concentrating optics experimentally determined. The effect of temperature on the concentrating optics has also been studied in this thesis. In particular, finite element simulations have been use to analyze the deformations experienced by the facets of hybrid (silicon-glass) Fresnel lenses, the change of refractive index with temperature and the influence of both effects on the system performance. A model has been implemented which take into consideration atmospheric variations, mainly temperature and spectral content of the direct normal irradiance, as well as thermal and spectral sensitivities of systems, with the aim of maximizing the energy harvested by a CPV module throughout the year in a particular location. Chapters 5 and 6 of this book are devoted to the design, fabrication, and characterization of a new concentrator concept named FluidReflex and based on a single-stage reflective optics with fluid dielectric. In this new concept, the presence of the fluid provides some significant advantages such as: an increased concentration acceptance angle product (CAP) achievable by surrounding the cell with a medium whose refractive index is greater than one, an improvement of the optical efficiency by reducing losses due to Fresnel reflection at several interfaces, an improvement in heat dissipation as the heat concentrated near the cell is transmitted by natural convection and conduction in the fluid, and an improved electrical insulation. By fabricating and characterizing several elementary-unit prototypes it was shown that there is no fundamental reason that prevents the practical implementation of this theoretical concept reaching high efficiency. Several fluid candidates were investigated proving the existence of at least to fluids that meet all the requirements (including the stability under concentrated light) to become part of the FluidReflex concentrator. Finally, several pre-industrial FluidReflex module prototypes have been designed and fabricated. An optimization process for the manufacturing of the multicavity optics was necessary to attain such an optics quality as the one achieved by the single unit. The module prototypes have been measured, both indoors and outdoors, analyzing the current matching of the solar cells beneath the concentrator for different spectral distribution of the incident irradiance. Additionally, the module showed an excellent thermal performance.

Resonadores piezoeléctricos de cizalladura: estudio de la formación de biofilms y caracterización de fluidos magneto‐reológicos

La presente tesis revisa y analiza algunos aspectos fundamentales relativos al comportamiento de los sensores basados en resonadores piezoeléctricos TSM (Thickness Shear Mode), así como la aplicación de los mismos al estudio y caracterización de dos medios viscoelásticos de gran interés: los fluidos magnetoreológicos y los biofilms microbianos. El funcionamiento de estos sensores está basado en la medida de sus propiedades resonantes, las cuales varían al entrar en contacto con el material que se quiere analizar. Se ha realizado un análisis multifrecuencial, trabajando en varios modos de resonancia del transductor, en algunas aplicaciones incluso de forma simultánea (excitación pulsada). Se han revisado fenómenos como la presencia de microcontactos en la superficie del sensor y la resonancia de capas viscoelásticas de espesor finito, que pueden afectar a los sensores de cuarzo de manera contraria a lo que predice la teoría convencional (Sauerbrey y Kanazawa), pudiéndonos llevar a incrementos positivos de la frecuencia de resonancia. Además, se ha estudiado el efecto de una deposición no uniforme sobre el resonador piezoeléctrico. Para ello se han medido deposiciones de poliuretano, modelándose la respuesta del resonador con estas deposiciones mediante FEM. El modelo numérico permite estudiar el comportamiento del resonador al modificar distintas variables geométricas (espesor, superficie, no uniformidad y zona de deposición) de la capa depositada. Se ha demostrado que para espesores de entre un cuarto y media longitud de onda aproximadamente, una capa viscoelástica no uniforme sobre la superficie del sensor, amplifica el incremento positivo del desplazamiento de la frecuencia de resonancia en relación con una capa uniforme. Se ha analizado también el patrón geométrico de la sensibilidad del sensor, siendo también no uniforme sobre su superficie. Se han aplicado sensores TSM para estudiar los cambios viscoelásticos que se producen en varios fluidos magneto-reológicos (FMR) al aplicarles distintos esfuerzos de cizalla controlados por un reómetro. Se ha podido ver que existe una relación directa entre diversos parámetros reológicos obtenidos con el reómetro (fuerza normal, G’, G’’, velocidad de deformación, esfuerzo de cizalla…) y los parámetros acústicos, caracterizándose los FMR tanto en ausencia de campo magnético, como con campo magnético aplicado a distintas intensidades. Se han estudiado las ventajas que aporta esta técnica de medida sobre la técnica basada en un reómetro comercial, destacando que se consigue caracterizar con mayor detalle algunos aspectos relevantes del fluido como son la deposición de partículas (estabilidad del fluido), el proceso de ruptura de las estructuras formadas en los FMR tanto en presencia como en ausencia de campo magnético y la rigidez de los microcontactos que aparecen entre partículas y superficies. También se han utilizado sensores de cuarzo para monitorear en tiempo real la formación de biofilms de Staphylococcus epidermidis y Eschericia coli sobre los propios resonadores de cristal de cuarzo sin ningún tipo de recubrimiento, realizándose ensayos con cepas que presentan distinta capacidad de producir biofilm. Se mostró que, una vez que se ha producido una primera adhesión homogénea de las bacterias al sustrato, podemos considerar el biofilm como una capa semi-infinita, de la cual el sensor de cuarzo refleja las propiedades viscoelásticas de la región inmediatamente contigua al resonador, no siendo sensible a lo que sucede en estratos superiores del biofilm. Los experimentos han permitido caracterizar el módulo de rigidez complejo de los biofilms a varias frecuencias, mostrándose que el parámetro característico que indica la adhesión de un biofilm tanto en el caso de S. epidermidis como de E. coli, es el incremento de G’ (relacionado con la elasticidad o rigidez de la capa), el cual viene ligado a un incremento de la frecuencia de resonancia del sensor. ABSTRACT This thesis reviews and analyzes some key aspects of the behavior of sensors based on piezoelectric resonators TSM (Thickness Shear Mode) and their applications to the study and characterization in two viscoelastic media of great interest: magnetorheological fluids and microbial biofilms. The operation of these sensors is based on the analysis of their resonant properties that vary in contact with the material to be analyzed. We have made a multi-frequency analysis, working in several modes of resonance of the transducer, in some applications even simultaneously (by impulse excitation). We reviewed some phenomena as the presence of micro-contacts on the sensor surface and the resonance of viscoelastic layers of finite thickness, which can affect quartz sensors contrary to the conventional theory predictions (Sauerbrey and Kanazawa), leading to positive resonant frequency shifts. In addition, we studied the effect of non-uniform deposition on the piezoelectric resonator. Polyurethane stools have been measured, being the resonator response to these depositions modeled by FEM. The numerical model allows studying the behavior of the resonator when different geometric variables (thickness, surface non-uniformity and deposition zone) of the deposited layer are modified. It has been shown that for thicknesses between a quarter and a half of a wavelength approximately, non-uniform deposits on the sensor surface amplify the positive increase of the resonance frequency displacement compared to a uniform layer. The geometric pattern of the sensor sensitivity was also analyzed, being also non-uniform over its surface. TSM sensors have been applied to study the viscoelastic changes occurring in various magneto-rheological fluids (FMR) when subjected to different controlled shear stresses driven by a rheometer. It has been seen that there is a direct relationship between various rheological parameters obtained with the rheometer (normal force, G', G'', stress, shear rate ...) and the acoustic parameters, being the FMR characterized both in the absence of magnetic field, and when the magnetic field was applied at different intensities. We have studied the advantages of this technique over the characterization methods based on commercial rheometers, noting that TSM sensors are more sensitive to some relevant aspects of the fluid as the deposition of particles (fluid stability), the breaking process of the structures formed in the FMR both in the presence and absence of magnetic field, and the rigidity of the micro-contacts appearing between particles and surfaces. TSM sensors have also been used to monitor in real time the formation of biofilms of Staphylococcus epidermidis and Escherichia coli on the quartz crystal resonators themselves without any coating, performing tests with strains having different ability to produce biofilm. It was shown that, once a first homogeneous adhesion of bacteria was produced on the substrate, the biofilm can be considered as a semi-infinite layer and the quartz sensor reflects only the viscoelastic properties of the region immediately adjacent to the resonator, not being sensitive to what is happening in upper layers of the biofilm. The experiments allow the evaluation of the biofilm complex stiffness module at various frequencies, showing that the characteristic parameter that indicates the adhesion of a biofilm for the case of both S. epidermidis and E. coli, is an increased G' (related to the elasticity or stiffness of the layer), which is linked to an increase in the resonance frequency of the sensor.

On sonic anemometer measurement theory

In this paper a model for the measuring process of sonic anemometers (ultrasound pulse based) is presented. The differential equations that describe the travel of ultrasound pulses are solved in the general case of non-steady, non-uniform atmospheric flow field. The concepts of instantaneous line-average and travelling pulse-referenced average are established and employed to explain and calculate the differences between the measured turbulent speed (travelling pulse-referenced average) and the line-averaged one. The limit k1l=1 established by Kaimal in 1968, as the maximum value which permits the neglect of the influence of the sonic measuring process on the measurement of turbulent components is reviewed here. Three particular measurement cases are analysed: A non-steady, uniform flow speed field, a steady, non-uniform flow speed field and finally an atmospheric flow speed field. In the first case, for a harmonic time-dependent flow field, Mach number, M (flow speed to sound speed ratio) and time delay between pulses have revealed themselves to be important parameters in the behaviour of sonic anemometers, within the range of operation. The second case demonstrates how the spatial non-uniformity of the flow speed field leads to an influence of the finite transit time of the pulses (M≠0) even in the absence of non-steady behaviour of the wind speed. In the last case, a model of the influence of the sonic anemometer processes on the measurement of wind speed spectral characteristics is presented. The new solution is compared to the line-averaging models existing in the literature. Mach number and time delay significantly distort the measurement in the normal operational range. Classical line averaging solutions are recovered when Mach number and time delay between pulses go to zero in the new proposed model. The results obtained from the mathematical model have been applied to the calculation of errors in different configurations of practical interest, such as an anemometer located on a meteorological mast and the transfer function of a sensor in an atmospheric wind. The expressions obtained can be also applied to determine the quality requirements of the flow in a wind tunnel used for ultrasonic anemometer calibrations.

Stochastic homogenization of the laser intensity to improve the irradiation uniformity of capsules directly driven by thousands laser beams

Illumination uniformity of a spherical capsule directly driven by laser beams has been assessed numerically. Laser facilities characterized by ND = 12, 20, 24, 32, 48 and 60 directions of irradiation with associated a single laser beam or a bundle of NB laser beams have been considered. The laser beam intensity profile is assumed super-Gaussian and the calculations take into account beam imperfections as power imbalance and pointing errors. The optimum laser intensity profile, which minimizes the root-mean-square deviation of the capsule illumination, depends on the values of the beam imperfections. Assuming that the NB beams are statistically independents is found that they provide a stochastic homogenization of the laser intensity associated to the whole bundle, reducing the errors associated to the whole bundle by the factor  , which in turn improves the illumination uniformity of the capsule. Moreover, it is found that the uniformity of the irradiation is almost the same for all facilities and only depends on the total number of laser beams Ntot = ND × NB.

Direct-Drive Shock-Ignition for the Laser Megajoule.

We present a review of direct-drive shock ignition studies done as alternative for the Laser Mega-Joule to achieve high thermonuclear gain. One-dimensional analysis of HiPER-like Shock-ignited target designs is presented. It is shown that high gain can be achieved with shock ignition for designs which do not ignite only from the laser compression. Shock ignition is achieved for different targets of the fast ignition family which are driven by an absorbed energy between 100 kJ and 850kJ and deliver thermonuclear energies between 10-130 MJ. Shock-Ignition of Direct-Drive Double-Shell non-cryogenic target is also addressed. 2D results concerning the LMJ irradiation geometry are presented. Few systematic analyses are performed for the fuel assembly irradiation uniformity using the whole LMJ configuration or a part of the facility, and for the ignitor spike uniformity. Solutions for fuel assembly and shock ignition on LMJ using 2D calculations are presented. It is shown that high-gain shock-ignition is possible with intensity of each quad less than 1e15 W/cm2but low modes asymmetries displace the ignitor power in the spike towards higher powers.

Assessing the solar irradiation potential for solar photovoltaic applications in buildings at low latitudes ¿ Making the case for Brazil

In Brazil, a low-latitude country characterized by its high availability and uniformity of solar radiation, the use of PV solar energy integrated in buildings is still incipient. However, at the moment there are several initiatives which give some hints that lead to think that there will be a change shortly. In countries where this technology is already a daily reality, such as Germany, Japan or Spain, the recommendations and basic criteria to avoid losses due to orientation and tilt are widespread. Extrapolating those measures used in high latitudes to all regions, without a previous deeper analysis, is standard practice. They do not always correspond to reality, what frequently leads to false assumptions and may become an obstacle in a country which is taking the first step in this area. In this paper, the solar potential yield for different surfaces in Brazilian cities (located at latitudes between 0° and 30°S) are analyzed with the aim of providing the necessary tools to evaluate the suitability of the buildings’ envelopes for photovoltaic use

Multimodal non-rigid image registration methods for therapy and surgery planning

La planificación pre-operatoria se ha convertido en una tarea esencial en cirugías y terapias de marcada complejidad, especialmente aquellas relacionadas con órgano blando. Un ejemplo donde la planificación preoperatoria tiene gran interés es la cirugía hepática. Dicha planificación comprende la detección e identificación precisa de las lesiones individuales y vasos así como la correcta segmentación y estimación volumétrica del hígado funcional. Este proceso es muy importante porque determina tanto si el paciente es un candidato adecuado para terapia quirúrgica como la definición del abordaje a seguir en el procedimiento. La radioterapia de órgano blando es un segundo ejemplo donde la planificación se requiere tanto para la radioterapia externa convencional como para la radioterapia intraoperatoria. La planificación comprende la segmentación de tumor y órganos vulnerables y la estimación de la dosimetría. La segmentación de hígado funcional y la estimación volumétrica para planificación de la cirugía se estiman habitualmente a partir de imágenes de tomografía computarizada (TC). De igual modo, en la planificación de radioterapia, los objetivos de la radiación se delinean normalmente sobre TC. Sin embargo, los avances en las tecnologías de imagen de resonancia magnética (RM) están ofreciendo progresivamente ventajas adicionales. Por ejemplo, se ha visto que el ratio de detección de metástasis hepáticas es significativamente superior en RM con contraste Gd–EOB–DTPA que en TC. Por tanto, recientes estudios han destacado la importancia de combinar la información de TC y RM para conseguir el mayor nivel posible de precisión en radioterapia y para facilitar una descripción precisa de las lesiones del hígado. Con el objetivo de mejorar la planificación preoperatoria en ambos escenarios se precisa claramente de un algoritmo de registro no rígido de imagen. Sin embargo, la gran mayoría de sistemas comerciales solo proporcionan métodos de registro rígido. Las medidas de intensidad de voxel han demostrado ser criterios de similitud de imágenes robustos, y, entre ellas, la Información Mutua (IM) es siempre la primera elegida en registros multimodales. Sin embargo, uno de los principales problemas de la IM es la ausencia de información espacial y la asunción de que las relaciones estadísticas entre las imágenes son homogéneas a lo largo de su domino completo. La hipótesis de esta tesis es que la incorporación de información espacial de órganos al proceso de registro puede mejorar la robustez y calidad del mismo, beneficiándose de la disponibilidad de las segmentaciones clínicas. En este trabajo, se propone y valida un esquema de registro multimodal no rígido 3D usando una nueva métrica llamada Información Mutua Centrada en el Órgano (Organ-Focused Mutual Information metric (OF-MI)) y se compara con la formulación clásica de la Información Mutua. Esto permite mejorar los resultados del registro en áreas problemáticas incorporando información regional al criterio de similitud, beneficiándose de la disponibilidad real de segmentaciones en protocolos estándares clínicos, y permitiendo que la dependencia estadística entre las dos modalidades de imagen difiera entre órganos o regiones. El método propuesto se ha aplicado al registro de TC y RM con contraste Gd–EOB–DTPA así como al registro de imágenes de TC y MR para planificación de radioterapia intraoperatoria rectal. Adicionalmente, se ha desarrollado un algoritmo de apoyo de segmentación 3D basado en Level-Sets para la incorporación de la información de órgano en el registro. El algoritmo de segmentación se ha diseñado específicamente para la estimación volumétrica de hígado sano funcional y ha demostrado un buen funcionamiento en un conjunto de imágenes de TC abdominales. Los resultados muestran una mejora estadísticamente significativa de OF-MI comparada con la Información Mutua clásica en las medidas de calidad de los registros; tanto con datos simulados (p<0.001) como con datos reales en registro hepático de TC y RM con contraste Gd– EOB–DTPA y en registro para planificación de radioterapia rectal usando OF-MI multi-órgano (p<0.05). Adicionalmente, OF-MI presenta resultados más estables con menor dispersión que la Información Mutua y un comportamiento más robusto con respecto a cambios en la relación señal-ruido y a la variación de parámetros. La métrica OF-MI propuesta en esta tesis presenta siempre igual o mayor precisión que la clásica Información Mutua y consecuentemente puede ser una muy buena alternativa en aplicaciones donde la robustez del método y la facilidad en la elección de parámetros sean particularmente importantes. Abstract Pre-operative planning has become an essential task in complex surgeries and therapies, especially for those affecting soft tissue. One example where soft tissue preoperative planning is of high interest is liver surgery. It involves the accurate detection and identification of individual liver lesions and vessels as well as the proper functional liver segmentation and volume estimation. This process is very important because it determines whether the patient is a suitable candidate for surgical therapy and the type of procedure. Soft tissue radiation therapy is a second example where planning is required for both conventional external and intraoperative radiotherapy. It involves the segmentation of the tumor target and vulnerable organs and the estimation of the planned dose. Functional liver segmentations and volume estimations for surgery planning are commonly estimated from computed tomography (CT) images. Similarly, in radiation therapy planning, targets to be irradiated and healthy and vulnerable tissues to be protected from irradiation are commonly delineated on CT scans. However, developments in magnetic resonance imaging (MRI) technology are progressively offering advantages. For instance, the hepatic metastasis detection rate has been found to be significantly higher in Gd–EOB–DTPAenhanced MRI than in CT. Therefore, recent studies highlight the importance of combining the information from CT and MRI to achieve the highest level of accuracy in radiotherapy and to facilitate accurate liver lesion description. In order to improve those two soft tissue pre operative planning scenarios, an accurate nonrigid image registration algorithm is clearly required. However, the vast majority of commercial systems only provide rigid registration. Voxel intensity measures have been shown to be robust measures of image similarity, and among them, Mutual Information (MI) is always the first candidate in multimodal registrations. However, one of the main drawbacks of Mutual Information is the absence of spatial information and the assumption that statistical relationships between images are the same over the whole domain of the image. The hypothesis of the present thesis is that incorporating spatial organ information into the registration process may improve the registration robustness and quality, taking advantage of the clinical segmentations availability. In this work, a multimodal nonrigid 3D registration framework using a new Organ- Focused Mutual Information metric (OF-MI) is proposed, validated and compared to the classical formulation of the Mutual Information (MI). It allows improving registration results in problematic areas by adding regional information into the similitude criterion taking advantage of actual segmentations availability in standard clinical protocols and allowing the statistical dependence between the two modalities differ among organs or regions. The proposed method is applied to CT and T1 weighted delayed Gd–EOB–DTPA-enhanced MRI registration as well as to register CT and MRI images in rectal intraoperative radiotherapy planning. Additionally, a 3D support segmentation algorithm based on Level-Sets has been developed for the incorporation of the organ information into the registration. The segmentation algorithm has been specifically designed for the healthy and functional liver volume estimation demonstrating good performance in a set of abdominal CT studies. Results show a statistical significant improvement of registration quality measures with OF-MI compared to MI with both simulated data (p<0.001) and real data in liver applications registering CT and Gd–EOB–DTPA-enhanced MRI and in registration for rectal radiotherapy planning using multi-organ OF-MI (p<0.05). Additionally, OF-MI presents more stable results with smaller dispersion than MI and a more robust behavior with respect to SNR changes and parameters variation. The proposed OF-MI always presents equal or better accuracy than the classical MI and consequently can be a very convenient alternative within applications where the robustness of the method and the facility to choose the parameters are particularly important.

Stopping power dependence of nitrogen sputtering yields in copper nitride films under swift-ion irradiation: Exciton model approach

Nitrogen sputtering yields as high as 104 atoms/ion, are obtained by irradiating N-rich-Cu3N films (N concentration: 33 ± 2 at.%) with Cu ions at energies in the range 10?42 MeV. The kinetics of N sputtering as a function of ion fluence is determined at several energies (stopping powers) for films deposited on both, glass and silicon substrates. The kinetic curves show that the amount of nitrogen release strongly increases with rising irradiation fluence up to reaching a saturation level at a low remaining nitrogen fraction (5?10%), in which no further nitrogen reduction is observed. The sputtering rate for nitrogen depletion is found to be independent of the substrate and to linearly increase with electronic stopping power (Se). A stopping power (Sth) threshold of ?3.5 keV/nm for nitrogen depletion has been estimated from extrapolation of the data. Experimental kinetic data have been analyzed within a bulk molecular recombination model. The microscopic mechanisms of the nitrogen depletion process are discussed in terms of a non-radiative exciton decay model. In particular, the estimated threshold is related to a minimum exciton density which is required to achieve efficient sputtering rates.

A molecular dynamics study of swift heavy ion irradiation of amorphous silica: the role of thermal effects

Irradiation with swift heavy ions (SHI), roughly defined as those having atomic masses larger than 15 and energies exceeding 1 MeV/amu, may lead to significant modification of the irradiated material in a nanometric region around the (straight) ion trajectory (latent tracks). In the case of amorphous silica, SHI irradiation originates nano-tracks of higher density than the virgin material (densification). As a result, the refractive index is increased with respect to that of the surroundings. Moreover, track overlapping leads to continuous amorphous layers that present a significant contrast with respect to the pristine substrate. We have recently demonstrated that SHI irradiation produces a large number of point defects, easily detectable by a number of experimental techniques (work presented in the parallel conference ICDIM). The mechanisms of energy transfer from SHI to the target material have their origin in the high electronic excitation induced in the solid. A number of phenomenological approaches have been employed to describe these mechanisms: coulomb explosion, thermal spike, non-radiative exciton decay, bond weakening. However, a detailed microscopic description is missing due to the difficulty of modeling the time evolution of the electronic excitation. In this work we have employed molecular dynamics (MD) calculations to determine whether the irradiation effects are related to the thermal phenomena described by MD (in the ps domain) or to electronic phenomena (sub-ps domain), e.g., exciton localization. We have carried out simulations of up to 100 ps with large boxes (30x30x8 nm3) using a home-modified version of MDCASK that allows us to define a central hot cylinder (ion track) from which heat flows to the surrounding cold bath (unirradiated sample). We observed that once the cylinder has cooled down, the Si and O coordination numbers are 4 and 2, respectively, as in virgin silica. On the other hand, the density of the (cold) cylinder increases with respect to that of silica and, furthermore, the silica network ring size decreases. Both effects are in agreement with the observed densification. In conclusion, purely thermal effects do not explain the generation of point defects upon irradiation, but they do account for the silica densification.

Non-uniform target illumination in deflagration regime : thermal smoothing

Thermal smoothing in the plasma ablated from a laser target under weakly nonuniform irradiation is analyzed, assuming absorption at nc and a deflagration regime (conduction restricted to a thin quasisteady layer next to the target). Magnetic generation effects are included and found to be weak. Differences from results available in the literature are explained; the importance of the character of the underdense flow at uniform irradiation is emphasized.

Field experiment of 800× off-axis XR-Köhler concentrator module on a carousel tracker

This paper presents the design and preliminary experimental results of a concentrator-type photovoltaic module based on a free-form off-axis 800×XR-Köhler concentrator. The off-axis XR-Köhler concentrator is one of the advanced concentrators that perform high concentration with a large acceptance angle and excellent irradiance uniformity on a solar cell. As a result of on-sun characterization of the unglazed single-cell unit test rig, the temperature-corrected DC module efficiency was 32.2% at 25 °C without an anti-reflective (AR) coating on the secondary optics, and the acceptance angle was more than ±1.0°. In addition, the non-corrected DC efficiency of an individual cell in a glazed 8-cell unit module mounted on a carousel tracking system was measured. The individual efficiency deviated in the range of 24.3-27.4%, owing to the mirror shape and alignment errors. The resultant series-connected efficiency was approximately 25% at direct normal irradiation (DNI) of 770 W/m2.

Optimization of optical systems for LED spot lights concerning the color uniformity

Spotlighting is one illumination field where the application of light emitting diodes (LED) creates many advantages. Commonly, the system for spot lights consists of a LED light engine and collimating secondary optics. Through angular or spatial separated emitted light from the source and imaging optical elements, a non uniform far field appears with colored rings, dots or patterns. Many feasible combinations result in very different spatial color distributions. Several combinations of three multi-chip light sources and secondary optical elements like reflectors and TIR lenses with additional facets or scattering elements were analyzed mainly regarding the color uniformity. They are assessed by the merit function Usl which was derived from human factor experiments and describes the color uniformity based on the visual perception of humans. Furthermore, the optical systems are compared concerning efficiency, peak candela and aspect ratio. Both types of optics differ in the relation between the color uniformity level and other properties. A plain reflector with a slightly color mixing light source performs adequate. The results for the TIR lenses indicate that they need additional elements for good color mixing or blended light source. The most convenient system depends on the requirements of the application.