92 research outputs found

    Elements of Ion Linear Accelerators, Calm in The Resonances, Other_Tales

    Full text link
    The main part of this book, Elements of Linear Accelerators, outlines in Part 1 a framework for non-relativistic linear accelerator focusing and accelerating channel design, simulation, optimization and analysis where space charge is an important factor. Part 1 is the most important part of the book; grasping the framework is essential to fully understand and appreciate the elements within it, and the myriad application details of the following Parts. The treatment concentrates on all linacs, large or small, intended for high-intensity, very low beam loss, factory-type application. The Radio-Frequency-Quadrupole (RFQ) is especially developed as a representative and the most complicated linac form (from dc to bunched and accelerated beam), extending to practical design of long, high energy linacs, including space charge resonances and beam halo formation, and some challenges for future work. Also a practical method is presented for designing Alternating-Phase- Focused (APF) linacs with long sequences and high energy gain. Full open-source software is available. The following part, Calm in the Resonances and Other Tales, contains eyewitness accounts of nearly 60 years of participation in accelerator technology. (September 2023) The LINACS codes are released at no cost and, as always,with fully open-source coding. (p.2 & Ch 19.10)Comment: 652 pages. Some hundreds of figures - all images, there is no data in the figures. (September 2023) The LINACS codes are released at no cost and, as always,with fully open-source coding. (p.2 & Ch 19.10

    Probabilistische Seismische Tomography mit gestreuten Wellen

    Get PDF
    This thesis presents a new probabilistic seismic tomography method with scattered waves to image the scattering and absorption that describe the small-scale structures of the Earth. I call it Adjoint Envelope Tomography (AET). The small-scale elastic structure of the subsurface at length scales below the resolution limits of waveform-based imaging methods can be described by random medium theory using statistical properties like correlation length of the fluctuations a or their amplitude Δ that characterizes the strength of the heterogeneity. Additionally, the quality factor Q or its inverse 1/Q is used to describe the intrinsic attenuation. High-frequency scattered seismic waves are frequently used to characterize small-scale heterogeneity, intrinsic attenuation or temporal changes of wave velocity with a variety of approaches. However, a rigorous framework for the iterative inversion of scatted waves to image the spatial variability of heterogeneity and high-frequency attenuation comparable to full-waveform inversion (FWI) is missing. A mathematical framework for an iterative inversion using forward and adjoint simulations of the radiative transfer equations is presented in this thesis, in full analogy to FWI which is based on the wave equation. The forward and adjoint problems are solved by modelling non-isotropic scattering in a random elastic medium with spatially variable heterogeneity and attenuation. The radiative transfer equations are solved with the Monte Carlo method. Recording of the specific energy density of the wavefield that contains the complete information about the energy density at a given position, time and propagation direction allows for calculating sensitivity kernels according to rigorous theoretical derivations. The practical calculation of the kernels involves the solution of the adjoint radiative transport equations. This thesis investigates sensitivity kernels that describe the relationships between changes of the model in P- and S-wave velocity, P- and S-wave attenuation and the strength of fluctuation on the one hand and seismogram envelope, traveltime changes and waveform decorrelation as observables on the other hand. These sensitivity kernels reflect the effect of the spatial variations of medium properties on the wavefield and constitute the first step in the development of the tomographic inversion approach for the distribution of small-scale heterogeneity based on scattered waves. To connect data misfits observed at multiple stations and extended lapse time windows to required model changes, misfit kernels are derived from the sensitivity kernels. The squared difference between observed and modelled envelopes is used as misfit function which is iteratively minimized with the Limited-memory Broyden–Fletcher–Goldfarb–Shanno algorithm (L-BFGS). In a first step AET is tested numerically in the acoustic approximation and shows that it is possible to image the spatial distribution of small-scale heterogeneity and attenuation in iterative inversions. The analysis shows that the relative importance of scattering and attenuation anomalies needs to be considered when the model resolution is assessed. The inversions confirm that the early coda is important for imaging the distribution of heterogeneity while later coda waves are more sensitive to intrinsic attenuation and this dependency can be used to cope with the trade-off that exists between both material properties. Building on the conclusions from the numerical test, AET is applied to a laboratory experiment in a second step. Using ultrasound data from embedded sensors in a meter-sized concrete specimen, the distribution of absorption and heterogeneity is respectively imaged using a later and earlier coda time window. The results successfully locate an area of salt concrete with increased scattering and concentric anomalies of intrinsic attenuation. The resolution test shows that the recovered anomalies constitute reasonable representations of the internal structure of the specimen. Both numerical and experimental results show the effectiveness of AET. Lastly, an initial attempt to investigate the 1D spherically symmetric scattering model of the Earth is conducted. The preliminary work about global stacking of Pdiff coda is finished. The global scattering of seismic waves from large deep earthquakes is simulated with the Monte Carlo method with a published heterogeneity model that matches the observations of Pdiff coda to some degree. This model is waiting for improvement with AET in the future. An analysis about the origin of Pdiff coda is discussed using the simulation with different scattering layers from lithosphere to Core-Mantle Boundary (CMB) and the available scattering area for Pdiff coda and its duration are demonstrated under the assumption of single-scattering showing that the Pdiff coda at high frequency originates from whole-mantle scattering.In dieser Arbeit wird hauptsĂ€chlich eine neue probabilistische seismische Tomographiemethode mit gestreuten Wellen vorgestellt, um die Streuung und Absorption abzubilden, die die kleinskaligen Strukturen der Erde beschreiben, genannt Adjoint Envelope Tomography (AET). Die kleinrĂ€umige elastische Struktur des Untergrunds auf LĂ€ngenskalen unterhalb der Auflösungsgrenzen der Wellenform kann in der Theorie der Zufallsmedien durch statistische Eigenschaften wie den intrinsischen QualitĂ€tsfaktor 1/Q oder die FluktuationsstĂ€rke Δ beschrieben werden, die die StĂ€rke der HeterogenitĂ€t charakterisiert. Hochfrequent gestreute seismische Wellen werden hĂ€ufig zur Charakterisierung kleinrĂ€umiger HeterogenitĂ€t, intrinsischer DĂ€mpfung oder zeitlicher VerĂ€nderungen der Wellengeschwindigkeit mit einer Vielzahl von AnsĂ€tzen verwendet. Es fehlt jedoch ein strenger Rahmen fĂŒr die iterative Inversion gestreuter Wellen zur Abbildung von HeterogenitĂ€t und hochfrequenter DĂ€mpfung, vergleichbar mit der Full-waveform inversion (FWI). In dieser Arbeit wird ein mathematischer Rahmen fĂŒr eine iterative Inversion unter Verwendung von VorwĂ€rts- und adjungierten Simulationen der Strahlungstransportgleichungen vorgestellt, in voller Analogie zur FWI, die auf der Wellengleichung basiert. Das VorwĂ€rts- und das adjungierte Problem werden durch die Modellierung von 2-D-Mehrfachstreuung in einem zufĂ€lligen elastischen Medium mit rĂ€umlich variabler HeterogenitĂ€t und DĂ€mpfung unter Verwendung der Strahlungstransportgleichungen gelöst, die mit der Monte-Carlo-Methode gelöst werden. Die Aufzeichnung der spezifischen Energiedichte des Wellenfelds, die die vollstĂ€ndige Information ĂŒber die Energiedichte an einem bestimmten Ort, zu einer bestimmten Zeit und in einer bestimmten Ausbreitungsrichtung enthĂ€lt, ermöglicht die Berechnung von SensitivitĂ€tskernen nach strengen theoretischen Ableitungen. Die praktische Berechnung der Kerne erfordert die Lösung der adjungierten Strahlungstransportgleichungen. In dieser Arbeit werden SensitivitĂ€tskerne untersucht, die die Beziehungen zwischen den Änderungen des Modells in der P- und S-Wellengeschwindigkeit, der P- und S-WellendĂ€mpfung und der StĂ€rke der Fluktuation einerseits und der SeismogrammhĂŒllkurve, den LaufzeitĂ€nderungen und der Wellenformdekorrelation als BeobachtungsgrĂ¶ĂŸen andererseits beschreiben. Diese SensitivitĂ€tskerne spiegeln die Auswirkungen der rĂ€umlichen Variationen der Eigenschaften des Mediums auf das Wellenfeld wider und stellen den ersten Schritt in der Entwicklung dieses tomographischen Inversionsansatzes fĂŒr die Verteilung kleinrĂ€umiger HeterogenitĂ€t auf der Grundlage gestreuter Wellen dar. Dann werden die Misfit-Kerne fĂŒr die quadratische Differenz zwischen den vollstĂ€ndigen beobachteten und modellierten HĂŒllkurven abgeleitet, die iterativ mit der L-BFGS-Methode minimiert werden. ZunĂ€chst wird AET numerisch in der akustischen NĂ€herung getestet und zeigt, dass es möglich ist, die rĂ€umliche Verteilung der kleinrĂ€umigen HeterogenitĂ€t und DĂ€mpfung in iterativen Inversionen abzubilden. Die Analyse zeigt, dass die relative Bedeutung von Streu- und DĂ€mpfungsanomalien bei der Bewertung der Modellauflösung berĂŒcksichtigt werden muss. Die Inversionen bestĂ€tigen, dass die frĂŒhe Coda fĂŒr die Abbildung der Verteilung der HeterogenitĂ€t wichtig ist, wĂ€hrend spĂ€tere Coda-Wellen empfindlicher auf die intrinsische DĂ€mpfung reagieren, und diese AbhĂ€ngigkeit kann genutzt werden, um den Kompromiss zwischen beiden Materialeigenschaften zu bewĂ€ltigen. Zweitens wird die Anwendung von AET in einem Laborexperiment umgesetzt. Unter Verwendung von Ultraschalldaten von eingebetteten Sensoren in einer meterlangen Betonprobe wird die Verteilung von Absorption und HeterogenitĂ€t in einem spĂ€teren bzw. frĂŒheren Coda-Zeitfenster abgebildet, was den Schlussfolgerungen aus den numerischen Tests entspricht. Die Ergebnisse lokalisieren erfolgreich einen Bereich des Salzbetons mit erhöhter Streuung und konzentrischen Anomalien der intrinsischen DĂ€mpfung. Die AuflösungsprĂŒfung zeigt, dass die gefundenen Anomalien die innere Struktur des Probekörpers angemessen wiedergeben. Sowohl die numerischen als auch die experimentellen Ergebnisse zeigen die Wirksamkeit von AET. Schließlich wird ein erster Versuch unternommen, AET fĂŒr die Untersuchung des sphĂ€risch symmetrischen 1D-Streumodells der Erde einzusetzen. Die Vorarbeiten zur globalen Stapelung der Pdiff-Koda sind abgeschlossen. Die globale Erdbebenstreuung wird durch die Monte-Carlo-Methode mit einem vorgeschlagenen Streumodell simuliert, das bis zu einem gewissen Grad mit der Beobachtung von Pdiff coda ĂŒbereinstimmt, die in Zukunft mit der AET-Methode verbessert werden soll. Eine Analyse ĂŒber den Ursprung von Pdiff coda wird durch die Simulation mit verschiedenen Streuschichten von der LithosphĂ€re bis zum CMB diskutiert und die verfĂŒgbare StreuflĂ€che fĂŒr Pdiff coda und ihre Dauer werden unter der Annahme von Einzelstreuung demonstriert

    ViCE: Visual Concept Embedding Discovery and Superpixelization

    Full text link
    Recent self-supervised computer vision methods have demonstrated equal or better performance to supervised methods, opening for AI systems to learn visual representations from practically unlimited data. However, these methods are classification-based and thus ineffective for learning dense feature maps required for unsupervised semantic segmentation. This work presents a method to effectively learn dense semantically rich visual concept embeddings applicable to high-resolution images. We introduce superpixelization as a means to decompose images into a small set of visually coherent regions, allowing efficient learning of dense semantics by swapped prediction. The expressiveness of our dense embeddings is demonstrated by significantly improving the SOTA representation quality benchmarks on COCO (+16.27 mIoU) and Cityscapes (+19.24 mIoU) for both low- and high-resolution images

    User grouping and power allocation in NOMA systems: a novel semi-supervised reinforcement learning-based solution

    Get PDF
    Author's accepted manuscriptIn this paper, we present a pioneering solution to the problem of user grouping and power allocation in non-orthogonal multiple access (NOMA) systems. The problem is highly pertinent because NOMA is a well-recognized technique for future mobile radio systems. The salient and difcult issues associated with NOMA systems involve the task of grouping users together into the prespecifed time slots, which are augmented with the question of determining how much power should be allocated to the respective users. This problem is, in and of itself, NP-hard. Our solution is the frst reported reinforcement learning (RL)-based solution, which attempts to resolve parts of this issue. In particular, we invoke the object migration automaton (OMA) and one of its variants to resolve the grouping in NOMA systems. Furthermore, unlike the solutions reported in the literature, we do not assume prior knowledge of the channels’ distributions, nor of their coefcients, to achieve the grouping/partitioning. Thereafter, we use the consequent groupings to heuristically infer the power allocation. The simulation results that we have obtained confrm that our learning scheme can follow the dynamics of the channel coefcients efciently, and that the solution is able to resolve the issue dynamicallyacceptedVersio

    Deep Learning in Medical Image Analysis

    Get PDF
    The accelerating power of deep learning in diagnosing diseases will empower physicians and speed up decision making in clinical environments. Applications of modern medical instruments and digitalization of medical care have generated enormous amounts of medical images in recent years. In this big data arena, new deep learning methods and computational models for efficient data processing, analysis, and modeling of the generated data are crucially important for clinical applications and understanding the underlying biological process. This book presents and highlights novel algorithms, architectures, techniques, and applications of deep learning for medical image analysis

    Seismic Event Coda-Correlation Imaging of the Earth's Interior

    Get PDF
    Seismic coda waves are the late part of the seismic energy generated by earthquakes. Global coda correlograms are constructed by cross-correlating and stacking seismic event late coda records that are noisy and seemingly useless, but they exhibit many prominent features sensitive to the Earth's internal structure. Thus, the coda correlation rises as a new paradigm in global observational seismology. As a new category of observations, the correlation features, if interpreted correctly, can provide new information about the Earth's interior. How to accurately utilise seismic event coda correlations, for instance, in "global coda correlation tomography," has been controversial and unresolved. Some attempts treat coda correlations as reconstructed seismic waves, which is on a par with methods developed in ambient-noise correlations, for they share similar data processing and computation routines. However, that introduces erroneous interpretation because theoretical analyses have demonstrated fundamental differences in the formation mechanisms of coda correlations and ambient-noise correlations. Therefore, we need a solution, a correct approach, to allow us to use a massive amount of coda correlation observables to increase constraints on the Earth's interior. This thesis consists of theoretical analysis, method developments, and applications for utilising seismic event coda correlations to image the Earth's interior. We first conduct comprehensive analyses to 'dissect' coda correlations for their formation mechanism quantitatively. The analyses reveal the mathematical relationship between coda correlations and the Earth's internal structure. Based on that, we build a novel framework toward global coda-correlation tomography. We verify the new framework in experiments and compare it with the method based on the assumption of seismic wave reconstructions. We illustrate significant inaccuracy in tomographic images can arise if coda correlations are treated as reconstructed seismic waves. Then, in an application, we provide a new class of observations for inner-core shear-wave anisotropy utilizing coda correlations in the new framework. We find that inner-core shear waves travel faster by at least ~5 s in directions oblique to the Earth's rotation axis than directions parallel to the equatorial plane (anisotropy of >0.8%). Our inner core shear-wave anisotropy observations place new constraints on the inner core mineral composition. Finally, we extend the principles to cross-correlations between source events and devise a new way to build global inter-source correlations. We demonstrate that a single seismic station is sufficient to construct a global correlogram. The correlogram exhibits prominent features sensitive to the internal planetary structures. We show implementations to constrain the Earth's and Martian cores' sizes and confirm a large Martian core. This provides a new paradigm for imaging planetary interiors on a global scale with currently realizable resources in planetary missions

    Phase-inherent linear visco-elasticity model for infinitesimal deformations in the multiphase-field context

    Get PDF
    A linear visco-elasticity ansatz for the multiphase-field method is introduced in the form of a Maxwell-Wiechert model. The implementation follows the idea of solving the mechanical jump conditions in the diffuse interface regions, hence the continuous traction condition and Hadamard’s compatibility condition, respectively. This makes strains and stresses available in their phase-inherent form (e.g. Δijα\varepsilon ^{\alpha }_{ij}, ΔijÎČ\varepsilon ^{\beta }_{ij}), which conveniently allows to model material behaviour for each phase separately on the basis of these quantities. In the case of the Maxwell-Wiechert model this means the introduction of phase-inherent viscous strains. After giving details about the implementation, the results of the model presented are compared to a conventional Voigt/Taylor approach for the linear visco-elasticity model and both are evaluated against analytical and sharp-interface solutions in different simulation setups

    Dissecting the Flow Physics of Aeroelastic Oscillations and Vortex-Dominated Flows: Computational and Data-Driven Methods

    Get PDF
    Fluid flows interact with flexible and moving bodies in a wide range of natural and engineering applications. These problems are often characterized by highly non-linear flow physics due to the generation and shedding of several vortices, their interactions, and the forces they induce on surfaces within the flow. However, our current understanding of the force producing mechanisms in these flows and our ability to accurately quantify their influence remains limited. As a result, the dynamics of many such vortex-dominated problems involving fluid-structure interactions and unsteady aerodynamics are notoriously hard to predict. In this work, we use computational modeling in conjunction with data-driven techniques to analyze the flow physics of these problems. We particularly focus on the flow-induced oscillation of wings and cylindrical bluff-bodies at low Reynolds numbers. Using simulations based on a sharp-interface immersed boundary method, we demonstrate the large range of oscillation responses and parametric dependencies that these systems exhibit. We propose an energy-based tool to analyze, predict and control the non-linear, and often non-intuitive, oscillation responses observed. We show that ``energy maps'' identify all possible flow-induced oscillation response branches and bifurcations in a system, thus allowing their use in the prediction and control of pitching oscillations for wings interacting with incoming gusts. Furthermore, we also formulate and demonstrate a novel extension to Dynamic Mode Decomposition for the analysis of such flows involving moving boundaries. A second focus of this work is on disentangling the various mechanisms at play in the generation of loads on surfaces within vortex-dominated flows. To this end, we develop a force and moment partitioning method which allows us to rigorously quantify the influence of distinct physical mechanisms as well as individual flow features in force/moment production. This method is combined with data-driven techniques to estimate the influence of each individual vortex in a flow-field, while also tracking and categorizing these vortices as they evolve with the flow. We utilize these tools to reveal new insights into fundamental phenomena in aerodynamics and flow-induced oscillations -- such as the mechanisms that drive the flow-induced oscillation of cylinders and the production of lift during dynamic stall and on three-dimensional wings

    Seismic wave propagation in Iran and eastern Indian shield

    Get PDF
    This dissertation addresses several important aspects of observational earthquake seismology: 1) methods for data management and processing large datasets, 2) analysis of seismic wave propagation at local to regional (up to about 700 km) source-receiver distances, 3) analysis of seismic coda, and 4) critical re-evaluation of the fundamental problem of seismic wave attenuation and measurement of the seismic “quality” factor (Q). These studies are carried out using new and previously analyzed earthquake data from Iran. In each of the four application areas above, innovative methods are used and significant new results are obtained. First, for efficient managing and processing of large earthquake datasets, I use a flexible, exploration-style open-source seismic processing system. Custom and problem-oriented scripts using Matlab or Octave software are included as tools in this processing system, allowing interactive and non-interactive analysis of earthquake records. In the second application, I note that the existing models for body-wave amplitudes are hampered by several difficulties, such as inaccurate accounts for the contributions of source and receiver effects and insufficient accuracy at the transition between the local and regional distances. Finding a reliable model for body-wave amplitudes is critical for many studies. To achieve such a reliable model, I use a joint inversion method based on a new parameterization of seismic attenuation and additional constraints on model quality. The joint inversion provides a correct model for geometrical spreading and attenuation. The geometrical-spreading model reveals the existence of an increase of body S wave amplitudes from 90 to about 115 km from the source which might be caused by waves reflecting from the crust‐mantle boundary. Outside of this distance range, amplitude decays are significantly faster than usually assumed in similar models. Third, in two chapters of this dissertation devoted to coda studies, I consider the concept of the frequency-dependent coda Q (Qc). Although this quantity is usually attributed to the subsurface, I argue that because of subjective selections of model assumptions and algorithms, Qc cannot be rigorously viewed as a function of surface or subsurface points. Also, frequency dependence of the measured Qc strongly trades off with the subjectively selected parameters of the measurement procedure. To mitigate these problems, instead of mapping a hypothetical in-situ Qc, I obtain maps of physically justified parameters of the subsurface: exponents of geometrical spreading (denoted ) and effective attenuation (denoted qe). For the areas of this study, parameter ranges from 0.005 s-1 to 0.05 s-1 (within Zagros area of Iran) and 0.010 s-1 to 0.013 s-1 (within the eastern Indian Shield). Finally, from both body- and coda-wave studies, I derive estimates of seismic attenuation within the study areas. In two areas of Iran and within the Indian Shield, weak attenuation with Q-factors of 2000–6000 or higher is found. In particular, coda envelopes can be explained by wave reverberations within elastic crustal structures, and the Q-type attenuation appears undetectable
    • 

    corecore