Intensity Effects in Dielectric Laser Accelerator Structures

Simulations of a Dielectric Laser Accelerator (DLA) are presented. Where in an electron beam is passed through a double grating structure of a dielectric material, whilst laser light polarized in the direction of travel of the electron beam is applied to both sides. This results in acceleration of the electron beam. The LSP (Large Scale Plasma) code is a particle in cell code which is applied in a novel way to model the DLA. Ultimately answers are needed to the questions: what is the acceleration gradient, how does this depend on the starting energy of the electrons and what is the current that can be accelerated through the DLA? DLA's have great potential in many fields, from providing compact linear accelerators, x-ray light sources and medical devices to high energy particle research.

Wake fields effects in addition to space charge forces may have an important impact during the emittance compensation process in a high brightness photo-injector. To study this effect we developed an upgraded version of the Homdyn code including off axis beam dynamics and wake fields. Homdyn describes a bunch as a uniformly charged cylinder, divided in cylindrical slices; in the upgraded version each slice's centroid can be transversally displaced from the nominal axis thus inducing wake fields. When the bunch is short as compared to the beam pipe radius, wake fields for a single cavity are calculated using methods of diffraction theory; instead we use, for a periodic collection of cavities, an asymptotic wake field obtained numerically at SLAC and then fitted to a simple function. As a first application we studied and verified a correction scheme for the SPARC photo-injector to control the bunch trajectory and angle at the entrance of the undulator. The correction scheme consists of a number of steering magnets and beam position monitors placed along the photo-injector. Two different steering approaches are analyzed and the emittance degradation is studied. The code demonstrates the steering positions and number do correct the bunch's orbit and angle and gives good results concerning the emittance degradation. The emittance and energy spread degradation due to wake fields in the emittance meter experiment is also discussed.

To study the beam instability caused by wake field effect, a dielectric wall accelerator module model consisting of symmetric pulse forming line, micro-stacked insulator and conductor layer was established. The finite integral method was used to simulate the wake field in the accelerating structure of dielectric wall accelerator (DWA) for high-current electron beam, wake potential and wake field impedance were calculated for the single and two accelerating units. Structural parameters of distance between two modules, thickness of metal and dielectric constant of insulator in micro-stacked insulator were changed to study the influence of different parameters on wake field. The results shows the wake potential and wake field impedance was small in x, y direction and different structural parameters can influence the wake field effect. Also it proved the advantage of accelerating cavity length and continuity in the DWA structure.

The transfer structure for generation of microwave power from the bunched drive beam presently consists of a smooth beam tube and a periodically loaded wave guide running in parallel and coupled to it by a slot. The bunches are in synchronous interaction with a forward 2/spl pi3-mode. Both the coupling from the beam to the output wave guide and the wave propagation down the tube have been measured on a scaled model at 8.6 GHz with the wire method, giving the longitudinal beam impedance and wake field. Transverse wake fields can be calculated from the longitudinal ones measured with the wire close to a shorting plate introduced in the centre of the structure. Quantitative estimates are then deduced for the real-scale transfer structure at 30 GHz and compared with results obtained from the three-dimensional MAFIA package, both being in good agreement. While the longitudinal wake fields from leading bunches are in phase with the following bunches, the transverse wake fields have a 90/spl deg/ phase offset. The wake field functions resulting from these studies have been inserted into the tracking program DTRACK and the transverse beam blow-up obtained seems to indicate that the effects of such wake fields remain tolerable. >

Capture, relaxation and recombination in quantum dots

With the continued down-scaling of the integrated circuits, the conventional dielectric oxide, SiO2, have been replaced by new dielectric materials, such as the high-κ oxides in CMOS transistors to avoid excess leakage current and the low-κ silicate insulating layers in-between copper wires to reduce the interconnect capacitances. To further control or improve their performance, a detailed understanding of the structural, electronic, dielectric, and the mechanical properties of these new dielectric materials are strongly in demand. In this thesis, we have employed first-principles density functional theory calculations to investigate the important properties of these new dielectric oxide materials in many fundamental aspects. For gate dielectrics in CMOS transistors, we have investigated the he structural, electronic, and dielectric properties of the Si- and La-doped HfO2, as well as the O vacancy formation and the relevant induced defect states in band gaps over a wide range of chemical compositions. For Si-doped HfO2, i.e. Hf-silicates (a-Hf1-xSixO2), our results show that the Si-rich Hf-silicates possess distinct structural characteristics from the Hf-rich ones and the electronic band gaps were found to vary nonlinearly with the Si content. Different from the previous theoretical studies, our results support a nonlinear dependence of the dielectric constants on the composition of silicates, which is mainly attributed to the rapid reduction of the low-frequency vibrational motions of Hf atoms with increasing the Si content. Furthermore, based on the generated structure models, we have identified the most probable O coordination structures in amorphous HfO2 and in Hf-silicates, respectively. Our calculations showed that the formation energies of O vacancy and the positions of the induced defect states in the band gap are largely dependent on the local structures of the vacancy site rather than on the compositions of Hf-silicates. Considering the measured valence band offset between Si and Hf-silicates, a considerable amount of O vacancies are likely to stay in the charge neutral state in Hf-silicates when the Fermi level lies in the band gap region of Si. Furthermore, the concentration of O vacancy in Hf-silicates was found to be much lower than that in HfO2 when the Fermi level lies in/below the mid-gap region of Si. Consequently, the flat band voltage shift and the transient threshold voltage instability can be significantly reduced in Hf-silicates in comparison to that in HfO2, indicating that the presence of a Hf-silicate layer in between the gate oxides and the Si substrate could be beneficial to the performance of the CMOS device. For the La-doped HfO2 (a-Hf1-xLaxO2-0.5x, x = 0 ~ 0.5), the mass density was found to decrease linearly with the La content, but the electronic band gap only reduces slightly by 0.29 eV as the La content increases from x = 0 to 0.5. Our Bader charge analysis shows that the average atomic charge on Hf (La) atoms can decrease from 2.31 (2.12) to 2.26 (2.06) as the La content increases from 0 to 0.5, which indicates that the ionicity of the Hf (La) atom tends to decrease as the La content increases. This results is consistent with our calculations for the Hf 4f core level energy, which reveal a binding energy shift of around 0.3 ~ 0.6 eV as the La content increases from 0 to 0.5, in good agreement with a recent experimental measurement. Our calculations also show that the dielectric constant of a-Hf1-xLaxO2-0.5x increases rapidly with the La content from x = 0 to 0.25 but it appears to be nearly unchanged as x ranges between 0.25 and 0.5, which is consistent with the recent experiments. Furthermore, based on the generated structure models, we have identified the most probable O coordination structures in a-Hf1-xLaxO2-0.5x. Similar to Hf-silicates, the formation energies are largely dependent on the local bonding configurations rather than on the compositions. However, O vacancy was found to be more likely to stay in positively charged states with lower formation energies than that in HfO2, implying that the flat band voltage shift and the threshold voltage instability could be enhanced due to La-doping. Furthermore, the trap energy level was found to decrease gradually with increasing the La contents, which indicates that the trap-assisted conduction can become more seriously in a-Hf1-xLaxO2-0.5x. For the interlayer insulating dielectrics, we have investigated the structure and mechanical properties of the methyl- and ethyl-bridged organosilicate hybrid glasses (OSG) using first-principles calculations. Our results show that the mass density of OSG can gradually decrease with increasing the carbon content, but the porosity may not necessarily increase with carbon-bridging units in the amorphous bond network. Our results also reveal that the elastic moduli and fractured surface energies of OSG models tend to decrease with increasing the alkylene-bridged units in the silicate bond network, which imply that the crack-resistance of the alkylene-bridged OSG can be much worse than fused silica based on the Griffith theory of brittle fracture. According to the calculations of decohesive energy, the mechanical strength of a-SiO2 still outperforms the methyl-bridged OSG models, indicating that the crack-resistance of these two materials is largely determined by the chemical bond strength. However, the decohesion energy of the ethyl-bridged OSG models turns out to be higher than a-SiO2 though their critical stress for crack propagation predicted by Griffith theory is simply half of that in a-SiO2. Our results further show that the enhanced fracture toughness in the ethyl-bridged OSG model can be mainly attributed to the plastic deformation and bonding reconfiguration induced by the ethyl-bridged units during fracture, which was not observed for the methyl-bridged OSG models and a-SiO2.

Advances in ultrafast laser technology and nanofabrication have enabled a new class of particle accelerator based upon miniaturized laser-driven photonic structures. However, developing a useful accelerator based on this approach requires control of the particle dynamics at field intensities approaching the damage limit. We measure acceleration in a fused silica dielectric laser accelerator driven by fields of up to 9 GV m−1 and observe a record 1.8 GV m−1 in the accelerating mode. At these intensities the dielectric is driven beyond its linear response and self-phase modulation changes the phase velocity of the accelerating mode, reducing the average gradient to 850 MeV m−1. We show that free-space optics can be used to compensate this dephasing and demonstrate that tailoring the laser phase and amplitude can facilitate optimization of the beam dynamics. This could enable MeV scale energy gain in a single stage and pave the way towards applications in scientific, industrial, and medical fields.

Collection of Solved Nonlinear Problems for Remote Shaping and Patterning of Liquid Structures on Flat and Curved Substrates by Electric and Thermal Fields

Effects of self energy of the ions on the double layer structure and properties at the dielectric interface

This thesis presents research on scheduling in an uncertain environment, which forms a part of the rolling stock life cycle logistics applied research and development program funded by Dutch railway industry companies. The focus therefore lies on scheduling of maintenance operations on rolling stock in the railway industry. The first chapter describes some of the history of the Dutch railways, focusing on the rolling stock used, and it introduces the context in which NedTrain, a major Dutch rolling stock maintenance company, operates. While maintenance of rolling stock is not a new problem, recent changes in the field are identified in this chapter which introduce new challenges: the first is the declining involvement of maintenance experts in the procurement of new rolling stock, the second is the on-going demand for increasing efficiency. Based on this discussion, the goal of this thesis is to devise a method with which schedules can be created based on a flexible process. The schedules resulting from this method are to be both flexible, meaning that they can be adapted easily, and robust, meaning that they are resilient to the effects of uncertain events. Lastly, our goal is to create the schedules in such a way that the effects of disturbances in one team have little or no effect on any other teams in the same workshop. Chapter 2 examines existing research in the context of the formulated research goals. It is shown that the well-known Resource-Constrained Project Scheduling Problem (RCPSP) can be mapped to the problem of scheduling maintenance tasks on trains in the NedTrain workshops. The most important methods to solve the RCPSP are discussed, among which are many exact methods. Both the scale of the problem and the requirement of having a flexible schedule instead of a fixed time assignment for the tasks prohibit using these methods. Among the heuristic methods discussed, the Precedence Constraint Posting method turns out to be an especially good fit: it is able to solve large-sized problems very quickly, and it uses a Simple Temporal Network (STN) as a way of representing many different solutions to the original problem, offering a lot of flexibility. Based on the research discussed, research questions are formulated to answer the research goals. - The first question asks how to define criteria encapsulating the notion of flexibility as posed by our research goals, and how to measure the flexibility of a schedule. - The second question asks how we can make the best use of available flexibility to attain robustness. - The third question asks if there is also a way to extend the concept of temporal flexibility to include a form of sequential flexibility. - The last question asks how we can use our work on robustness to decouple schedules, ensuring that disturbances in the schedule for one agent have no impact on any other agents. The third chapter investigates criteria corresponding to our intuitive notion of flexibility in scheduling. The concept of interval schedules serves as a basis for this investigation: the idea is that each task in the schedule is assigned an interval from which its start time can be picked, and the length of all these intervals serves as a representation for the flexibility of the schedule. We show that flexibility measures in existing research lead to an over-estimation of the total available flexibility, because dependencies between tasks are not taken into account properly: simply adding the interval sizes for two dependent tasks causes over-estimation, because the picked time for one task can reduce the interval size for another task. Our first major contribution is an independence requirement for the creation of these intervals: we state that intervals should be constructed in such a way that any choice in one interval has no impact on the available choices in any of the other intervals. Our second major contribution is a method to actually construct such intervals, which results in a valid interval schedule for an \stn, allowing efficient schedule execution under dynamic circumstances. Additionally, we note that an interval schedule can also serve as a mechanism to decouple a schedule, giving a partial answer to the last research question. In the fourth chapter, we investigate how to make use of the available flexibility to ensure that schedules are robust. We show that having a maximal total amount of flexibility does not imply that the schedule is as robust as possible: maximization might lead to a very skewed distribution of flexibility over the schedule. Different ways of distributing flexibility over a schedule are proposed and analyzed in an experimental setting. From the experiments it can be clearly concluded that sacrificing some of the total flexibility to improve the distribution of flexibility can have a positive influence on the robustness of the schedule. We also propose a different maximization strategy: one which maximizes the minimum of the interval sizes for the schedule. This strategy is shown to work especially well for small delays. So far we only discussed strategies concerning temporal flexibility, i.e., those in which start times remain flexible instead of fixed. Chapter 5 introduces a novel representation of solutions for planning problems in which not all ordering decisions needed to avoid resource contention are made in advance. This is achieved by grouping tasks which need to be executed sequentially together, in such a way that either ordering of the tasks remains feasible during execution time. An analysis using simulation experiments shows that this technique results in higher robustness, at the cost of somewhat lower resource utilization. The proposed algorithm to construct such schedules offers limited control over the size of the grouped tasks. The experiments show a slightly counter-intuitive results: schedules with larger groups offer lower robustness than those with smaller groups. A plausible explanation is the fact that delays can occur at any place in a schedule. Having multiple smaller groups instead of a few very large ones increases the chance that such a delay can be compensated for using a task group.

Fiber-reinforced polymer (FRP) composites offer several advantages in relation to traditional materials, such as high specific strength, good corrosion resistance, low thermal conductivity and rapid component installation. Despite the great potential of these materials, two major disadvantages limit their acceptance in civil engineering applications: their lack of inherent ductility and the fact that their fibrous and anisotropic character makes the joining of structural components difficult. The purposes of this research are to develop ductile structures using brittle all-FRP materials and to provide a corresponding design method assuring robustness and structural safety. The proposed concept for all-FRP structures incorporates system ductility through the use of ductile, adhesively-bonded joints and redundant (statically indeterminate) structural systems. The concept envisages adhesives with an initially elastic behavior sufficiently stiff to meet short- and long-term serviceability requirements. When serviceability and ultimate loads are exceeded, however, adhesive behavior should change and become plastic, or at least highly non-linear inelastic, with much lower stiffness. The ductile connections effect a favorable redistribution of internal and external forces in the redundant system in the same way as plastic hinges in statically indeterminate systems with ductile materials. In case of joint failure, the redundant system allows the creation of alternative load paths and redistribution of section forces preventing structural collapse. Numerical and experimental investigations of adhesively-bonded double-lap joints demonstrate that using an appropriate bilinear adhesive is more advantageous than using a traditional structural and stiff epoxy adhesive. These investigations show that ductile adhesives generate a linear axial strain distribution and consequently a constant shear stress distribution along the overlap. Numerical and analytical modelings are validated with experimental results. The numerical modeling allows evaluation of the effect of ductile adhesive behavior on joint behavior and establishment of recommendations for the selection of adhesive mechanical properties, according to serviceability and failure limit state requirements. Based on numerical modeling results, joints can also be classified according to stiffness (stiff or flexible). An analytical model, based on numerical and experimental results, is developed for predicting joint stiffness when using ductile adhesives, assuming that the adhesive deforms only and uniformly in shear and that adherends deform only in tension. The design method is developed in accordance with the new structural concept and Eurocode design philosophy. It is based on both numerical and analytical model results for adhesively-bonded double-lap joints. A case study illustrating the adhesively-bonded joint design and appropriate adhesive selection is carried out on continuous beams with adhesively-bonded joints at mid-support. Corresponding bending experiments on beams with ductile, adhesively-bonded connections demonstrate the new structural concept's feasibility and validate the proposed design method. An energy factor defined as the ultimate strength to serviceability limit state energy ratio is proposed to quantify and compare the robustness and safety of structures built from both ductile and brittle materials.

Rainfall intensity for a particular frequency and duration is one of the most important parameter for the hydrologic design of dam, reservoirs, storm sewers, culverts and many other hydraulic structures. This can be obtained from Intensity-Duration-Frequency (IDF) relationship, which is determined by frequency analysis of rainfall data. The goal of this research is to develop rainfall intensity-duration frequency relationship for the data scarce urbanized Guma catchment in Freetown the capital city of Sierra Leone using the Gumbel and Pearson Type III distributions. To achieve this goal, daily rainfall data for the period of 1991 to 2018 for the Guma catchment rainfall station was obtained from the Guma Valley Water Company and were converted to shorter duration (hourly) using the Indian Meteorological Department (IMD) method. Twenty-eight maximum daily rainfall events were converted to hourly rainfall events. Frequency analysis was conducted to develop the rainfall intensity-duration-frequency relationships using the Gumbel and Pearson Type III distributions. The Kolmogorov-Smirnov (K-S) goodness of fit test was utilized to determine which of the distributions have a better fit at 5 and 10% significant levels. The frequency analysis results show that the Gumbel distribution gives higher intensity for all return periods and durations than the Pearson Type III distribution. The result of the K-S goodness of fit test shows that all of the data fit the Gumbel distribution for different return periods (2, 5, 10, 25, 50 and 100 years) at the level of significance of which yield and which yield while the data do not give good fit using the Pearson Type III distribution at both levels of significance for the different return periods. Key words: Rainfall, Intensity-duration-frequency relationships, probability distributions, goodness of fit test.

In this paper we study the transverse wake field due to the bellows in the SSC and its effect on the transverse mode coupling instability. For long bunches this instability has a small growth rate which depends critically on the real part of the transverse impedance at ω≤1/σ_r where σ_r is the bunchlength in seconds. The instability is caused by the coupling of mode 0, the rigid dipole mode, and mode '--1', the mode shifted by <em>-ν₈</em> from the betatron tune. Thus, the instability needs a large frequency shift as well as a large 'off diagonal' element to couple the modes. The frequency shift is induced by the imaginary part of the transverse impedance at ω≤1/σ_r while the off diagonal element is related to the real part of the impedance at ω≃1/σ_r. In addition, if the beam current is large enough, there can be a coating-beam-like instability with a growth rate larger than the synchotron frequency. By comparing the transverse and longitudinal instability thresholds, it is straightforward to see that the single bunch transverse instability is more restrictive than the single bunch longitudinal instability in the SSC. there have been calculations which suggest that bellows can be a large source of transverse impedance, and thus can drive transverse instabilities. Therefore, in this paper we will emphasize the bellows impedance but also include the beam position monitor impedance. There are certainly other sources of transverse impedance which will be omitted here. In the following sections we first discuss the instability thresholds and then show calculations of the bellows wake fields. To reduce the wake field effects, another design with shielded bellows is then considered. Next we discuss briefly the impedance of the beam position monitors. Finally, we show the effect of the bellows and beam position monitors on transverse mode coupling.

Effective representation methods and proper signal priors are crucial in most signal processing applications. In this thesis we focus on different structured models and we design appropriate schemes that allow the discovery of low dimensional latent structures that characterise and identify the signals. Motivated by the highly non-linear structure of most datasets, we firstly investigate the geometry of manifolds. Manifolds are low dimensional, non-linear structures that are naturally employed to describe sets of strongly related signals such as the images of a 3-D object captured from different viewpoints. However, the use of manifolds in applications is not straightforward due to their usually non-analytic and non-linear form. We propose here a way to `disassemble' a manifold into simpler components by approximating it with affine subspaces. Our objective is to discover a set of low dimensional affine subspaces that can represent manifold data accurately while preserving the manifold's structure. To this end, we employ a greedy technique that iteratively merges manifold samples into groups based on the difference of local tangents. We use our algorithm to approximate synthetic and real manifolds and to demonstrate that it is competitive to state-of-the-art techniques. Then, we consider structured sparse representations of signals and we propose a new sparsity model, where signals are essentially composed of a small number of structured {\it molecules }. We define the molecules to be linear combinations of a small number of atoms in a redundant dictionary. Our multi-level model takes into account the energy distribution of the significant signal components in addition to their support. It permits to define typical visual patterns and recognise them in prototypical or deformed form. We define a new structural difference measure between molecules and their deformed versions, which is based on their sparse codes and we create an algorithm for decomposing signals into molecules that can account for different deviations in the internal molecule structure. Our experiments verify the benefits of the new image model in various restoration tasks and they confirm that the development of proper models that extend the mere notion of sparsity can be very useful for various inverse problems in imaging. Finally, we investigate the problem of learning molecule representations directly in the sparse code domain. We constrain sparse codes to be linear combinations of a few, possibly deformed, molecules and we design an algorithm that can learn the structure from the codes without transforming them back into the signal domain. To this end, we take advantage of our structural difference which is based on the sparse codes and we devise a scheme for representing the codes with molecules and learn the molecules at the same time. To illustrate the effectiveness of our proposed algorithm we apply it to various synthetic and real datasets and we compare the results with traditional sparse coding and dictionary learning techniques. From the experiments, we verify the superior performance of our scheme in interpreting and recognising correctly the underlying structure. In short, in this thesis we are interested in low-dimensional, structured models. Among the various choices, we focus on manifolds and sparse representations and we propose schemes that enhance their structural properties and highlight their effectiveness in signal representations.

In this study we present a general approach for the analysis of the wake field of a point charge moving in a vacuum tunnel bored in dielectric material that is uniform in the direction parallel to the motion of the bunch. In the transverse direction the structure surrounding the dielectric may have arbitrary geometry. A quasianalytic expression that relates the decelerating force with the first dielectric layer, the radius of the vacuum tunnel where the charge moves, and the reflection characteristics of the structure has been developed. Simulation results for a simple structure indicate that, if the effective location where the reflection occurs in the dielectric is sufficiently apart from the edge of the vacuum tunnel, it has no effect on the point charge. In fact, the decelerating field converges exponentially as this distance increases, to the asymptotic value determined by the first dielectric layer. An estimate of the trailing wake when the structure supports a specific mode is also provided.