Voltage dip generator for testing wind turbines connected to electrical networks

This paper describes a new voltage dip generator that allows the shape of the time profile of the voltage generated to be configured. The use of this device as a tool to test the fault ride-through capability of wind turbines connected to the electricity grid can provide some remarkable benefits: First, this system offers the possibility of adapting the main features of the time–voltage profile generated (dip depth, dip duration, the ramp slope during the recovery process after clearing fault, etc.) to the specific requirements set forth by the grid operation codes, in accordance with different network electrical systems standards. Second, another remarkable ability of this system is to provide sinusoidal voltage and current wave forms during the overall testing process without the presence of harmonic components. This is made possible by the absence of electronic converters. Finally, the paper includes results and a discussion on the experimental data obtained with the use of a reduced size laboratory prototype that was constructed to validate the operating features of this new device

A simple example of "Quantum Darwinism": Redundant information storage in many-spin environments

As quantum information science approaches the goal of constructing quantum computers, understanding loss of information through decoherence becomes increasingly important. The information about a system that can be obtained from its environment can facilitate quantum control and error correction. Moreover, observers gain most of their information indirectly, by monitoring (primarily photon) environments of the "objects of interest." Exactly how this information is inscribed in the environment is essential for the emergence of "the classical" from the quantum substrate. In this paper, we examine how many-qubit (or many-spin) environments can store information about a single system. The information lost to the environment can be stored redundantly, or it can be encoded in entangled modes of the environment. We go on to show that randomly chosen states of the environment almost always encode the information so that an observer must capture a majority of the environment to deduce the system's state. Conversely, in the states produced by a typical decoherence process, information about a particular observable of the system is stored redundantly. This selective proliferation of "the fittest information" (known as Quantum Darwinism) plays a key role in choosing the preferred, effectively classical observables of macroscopic systems. The developing appreciation that the environment functions not just as a garbage dump, but as a communication channel, is extending our understanding of the environment's role in the quantum-classical transition beyond the traditional paradigm of decoherence.Comment: 21 pages, 6 figures, RevTex 4. Submitted to Foundations of Physics (Asher Peres Festschrift

Theory of Melting and the Optical Properties of Gold/DNA Nanocomposites

We describe a simple model for the melting and optical properties of a DNA/gold nanoparticle aggregate. The optical properties at fixed wavelength change dramatically at the melting transition, which is found to be higher and narrower in temperature for larger particles, and much sharper than that of an isolated DNA link. All these features are in agreement with available experiments. The aggregate is modeled as a cluster of gold nanoparticles on a periodic lattice connected by DNA bonds, and the extinction coefficient is computed using the discrete dipole approximation. Melting takes place as an increasing number of these bonds break with increasing temperature. The melting temperature corresponds approximately to the bond percolation threshold.Comment: 5 pages, 4 figure. To be published in Phys. Rev.

Ionic systems in materials research : new materials and processes based on ionic polymerizations and/or ionic liquids

Systems based on ionic interactions are usually related to reversible processes and/or transitory chemical states and, nowadays, they are believed to be key factors for the understanding and for the development of processes in several branches of chemistry and materials research. During the last decades, scientists have developed different approaches for the preparation of new materials and/or substances with outstanding properties based on ionic and other non-covalent interactions. In this thesis, different chemical systems, based on ionic interactions, have been employed for the preparation of different materials and for the development of more efficient synthetic methods in materials research. On the one hand "classical" and emerging applications of ionic interactions are utilized for the preparation of polymeric moieties and other materials, and on the other hand some of the results derived from these single approaches are combined to produce and investigate more complex systems. These systems may find applications in different fields of science and technology: From drug delivery and medical therapies to engineering devices and novel catalytic reactions systems. Thus, this dissertation is divided in three sections. In the first section, the synthetic approach for the well-established anionic polymerization procedure is enhanced by incorporating this technique into a high-throughput work-flow. The development of this experimental approach has allowed and accelerated the systematic synthesis of new block copolymer libraries. Some of the obtained block copolymers were utilized to prepare self-assembled micelles, which were investigated and characterized in detail. Furthermore, the proposed experimental approach was also applied for the development of a new synthetic route to prepare well-defined end-functionalized polymeric architectures bearing supramolecular moieties (e.g., terpyridine groups). In addition, this technique has shown to be a very useful tool for performing detailed kinetic investigations in a short time. Thus, the high-throughput approach was established for one of the most demanding experimental techniques in polymer synthesis. This new tool may help to speed-up research in this field, which will allow a better understanding of structure-property relationships in polymer science. In the second part of the thesis, ionic liquids are investigated as reaction media to carry out polymerizations by different reaction mechanisms. Due to their outstanding chemical and physical stabilities, ionic liquids are proposed as new ionic systems that can offer multiple advantages in polymer synthesis. Thus, it is demonstrated that ionic liquids can be efficiently utilized to perform homogeneous and heterogeneous polymerization reactions. In the homogeneous case, another important ionic polymerization mechanism, cationic ring opening polymerization, was selected as an example for the development of efficient and environmentally-friendly polymerization processes based on ionic liquids as reaction media and microwave irradiation as a heating source. Polymerizations performed in ionic liquids have shown faster reaction rates when compared to other solution polymerization methods, and also allow the synthesis of well-defined and chain extended polymers due to the fact that the investigated polymerization reactions reveal a "living" character. Furthermore, it is shown that the proposed synthetic method is not only limited to one reaction mechanism and can be readily extended to other types of polymerizations, such as free radical processes. Due to the fact that not all monomers and/or polymers are soluble in specific ionic liquids, it is also demonstrated that heterogeneous polymerization processes can be carried out in these substances. For these cases, ionic liquids do not only act as a reaction medium, but they also behave as surfactants to stabilize these heterogeneous systems. This has allowed the synthesis of polymer beads with controlled particle sizes and surface areas. For all the investigated polymerization reactions in ionic liquids, suitable and efficient approaches for the ionic liquid recycling and polymer isolation were developed by the use of water as secondary substance during the separation processes, which entirely avoids the use of volatile organic solvents. In addition, it is also demonstrated that, with the approaches proposed, cleaner and more efficient polymerization processes can be developed due to the known "green" characteristics of ionic liquids (e.g., negligible vapor pressure, negligible flammability, and liquids in a broad range of temperatures) and to the high efficiency of microwave irradiation in the presence of ionic liquids. The proposed environmentally-friendly polymerization processes certainly arise as alternative methods for reducing emissions of harmful volatile organic compounds still widely used throughout the polymer industry and for energy savings. In the last part of the thesis, the materials and/or concepts developed in the first two sections are combined in order to obtain more complex materials and systems. Specifically, amphiphilic block copolymers that were synthesized in the first part of this thesis, or obtained by other methods, are utilized for the preparation of self-assembled micelles in ionic liquids. This has revealed interesting properties due to the fact that these block copolymer micelles, with and without encapsulated guest molecules in their respective core, can be thermo-reversible transferred between two different phases (an aqueous phase and an ionic liquid phase). Furthermore, it is also demonstrated that the investigated block copolymer micelles provide confined environments that protect the encapsulated guest molecules from (sudden) external changes in the surroundings. Finally, the surfactant properties revealed by ionic liquids are utilized for the preparation of composite materials, which is illustrated by two examples: The utilization of ionic liquids has allowed for the efficient and homogeneous dispersion of inorganic materials (e.g. magnetite) into a polymeric matrix. Thus, polymer composites with both magnetic and conductive properties were prepared by an inexpensive method. In addition, this latter concept is also extended to the preparation of composite materials in a liquid state. As a result, novel magnetorheological fluids based on ionic liquids were prepared by dispersing magnetic particles in ionic liquids. The use of ionic liquids has allowed for the preparation of dispersions with low sedimentation rates and magnetorheological fluids with enhanced properties. A combination of the outstanding properties of ionic liquids with the magnetorheological technology led to the fabrication of new and "smart" fluids, which may find applications in several areas of research and technology, such as medical therapies (drug delivery and cancer therapeutic methods), engineering devices (dampers and breaks), as well as accurate transportation and delivery of substances in multiphase biological and chemical systems

Structure Formation, Melting, and the Optical Properties of Gold/DNA Nanocomposites: Effects of Relaxation Time

We present a model for structure formation, melting, and optical properties of gold/DNA nanocomposites. These composites consist of a collection of gold nanoparticles (of radius 50 nm or less) which are bound together by links made up of DNA strands. In our structural model, the nanocomposite forms from a series of Monte Carlo steps, each involving reaction-limited cluster-cluster aggregation (RLCA) followed by dehybridization of the DNA links. These links form with a probability $p_{eff}$ which depends on temperature and particle radius $a$. The final structure depends on the number of monomers (i. e. gold nanoparticles) $N_m$, $T$, and the relaxation time. At low temperature, the model results in an RLCA cluster. But after a long enough relaxation time, the nanocomposite reduces to a compact, non-fractal cluster. We calculate the optical properties of the resulting aggregates using the Discrete Dipole Approximation. Despite the restructuring, the melting transition (as seen in the extinction coefficient at wavelength 520 nm) remains sharp, and the melting temperature $T_M$ increases with increasing $a$ as found in our previous percolation model. However, restructuring increases the corresponding link fraction at melting to a value well above the percolation threshold. Our calculated extinction cross section agrees qualitatively with experiments on gold/DNA composites. It also shows a characteristic ``rebound effect,'' resulting from incomplete relaxation, which has also been seen in some experiments. We discuss briefly how our results relate to a possible sol-gel transition in these aggregates.Comment: 12 pages, 10 figure

State of Emergency Medicine in Switzerland: a national profile of emergency departments in 2006

BACKGROUND: Emergency departments (EDs) are an essential component of any developed health care system. There is, however, no national description of EDs in Switzerland. Our objective was to establish the number and location of EDs, patient visits and flow, medical staff and organization, and capabilities in 2006, as a benchmark before emergency medicine became a subspecialty in Switzerland. METHODS: In 2007, we started to create an inventory of all hospital-based EDs with a preliminary list from the Swiss Society of Emergency and Rescue Medicine that was improved with input from ED physicians nationwide. EDs were eligible if they offered acute care 24 h per day, 7 days per week. Our goal was to have 2006 data from at least 80% of all EDs. The survey was initiated in 2007 and the 80% threshold reached in 2012. RESULTS: In 2006, Switzerland had a total of 138 hospital-based EDs. The number of ED visits was 1.475 million visits or 20 visits per 100 inhabitants. The median number of visits was 8,806 per year; 25% of EDs admitted 5,000 patients or less, 31% 5,001-10,000 patients, 26% 10,001-20,000 patients, and 17% >20,000 patients per year. Crowding was reported by 84% of EDs with >20,000 visits/year. Residents with limited experience provided care for 77% of visits. Imaging was not immediately available for all patients: standard X-ray within 15 min (70%), non-contrast head CT scan within 15 min (38%), and focused sonography for trauma (70%); 67% of EDs had an intensive care unit within the hospital, and 87% had an operating room always available. CONCLUSIONS: Swiss EDs were significant providers of health care in 2006. Crowding, physicians with limited experience, and the heterogeneity of emergency care capabilities were likely threats to the ubiquitous and consistent delivery of quality emergency care, particularly for time-sensitive conditions. Our survey establishes a benchmark to better understand future improvements in Swiss emergency care

GRAVITY Spectro-interferometric Study of the Massive Multiple Stellar System HD 93206 A

Characterization of the dynamics of massive star systems and the astrophysical properties of the interacting components are a prerequisite for understanding their formation and evolution. Optical interferometry at milliarcsecond resolution is a key observing technique for resolving high-mass multiple compact systems. Here, we report on Very Large Telescope Interferometer/GRAVITY, Magellan/Folded-port InfraRed Echellette, and MPG2.2 m/FEROS observations of the late-O/early-B type system HD 93206 A, which is a member of the massive cluster Collinder 228 in the Carina nebula complex. With a total mass of about $90\,{M}_{\odot }$, it is one of the most compact massive quadruple systems known. In addition to measuring the separation and position angle of the outer binary Aa–Ac, we observe Brγ and He i variability in phase with the orbital motion of the two inner binaries. From the differential phase (${{\rm{\Delta }}}_{\phi }$) analysis, we conclude that the Brγ emission arises from the interaction regions within the components of the individual binaries, which is consistent with previous models for the X-ray emission of the system based on wind–wind interaction. With an average 3σ deviation of ${{\rm{\Delta }}}_{\phi }\sim 15^\circ$, we establish an upper limit of p ~ 0.157 mas (0.35 au) for the size of the Brγ line-emitting region. Future interferometric observations with GRAVITY using the 8 m Unit Telescopes will allow us to constrain the line-emitting regions down to angular sizes of 20 μas (0.05 au at the distance of the Carina nebula)

Near-field optical power transmission of dipole nano-antennas

Nano-antennas in functional plasmonic applications require high near-field optical power transmission. In this study, a model is developed to compute the near-field optical power transmission in the vicinity of a nano-antenna. To increase the near-field optical power transmission from a nano-antenna, a tightly focused beam of light is utilized to illuminate a metallic nano-antenna. The modeling and simulation of these structures is performed using 3-D finite element method based full-wave solutions of Maxwell’s equations. Using the optical power transmission model, the interaction of a focused beam of light with plasmonic nanoantennas is investigated. In addition, the tightly focused beam of light is passed through a band-pass filter to identify the effect of various regions of the angular spectrum to the near-field radiation of a dipole nano-antenna. An extensive parametric study is performed to quantify the effects of various parameters on the transmission efficiency of dipole nano-antennas, including length, thickness, width, and the composition of the antenna, as well as the wavelength and half-beam angle of incident light. An optimal dipole nanoantenna geometry is identified based on the parameter studies in this work. In addition, the results of this study show the interaction of the optimized dipole nano-antenna with a magnetic recording medium when it is illuminated with a focused beam of light

Searching for a Cosmological Preferred Axis: Union2 Data Analysis and Comparison with Other Probes

We review, compare and extend recent studies searching for evidence for a preferred cosmological axis. We start from the Union2 SnIa dataset and use the hemisphere comparison method to search for a preferred axis in the data. We find that the hemisphere of maximum accelerating expansion rate is in the direction $(l,b)=({309^\circ}^{+23^\circ}_{-3^\circ}, {18^\circ}^{+11^\circ}_{-10^\circ})$ (\omm=0.19) while the hemisphere of minimum acceleration is in the opposite direction $(l,b)=({129^\circ}^{+23^\circ}_{-3^\circ},{-18^\circ}^{+10^\circ}_{-11^\circ})$ (\omm=0.30). The level of anisotropy is described by the normalized difference of the best fit values of \omm between the two hemispheres in the context of \lcdm fits. We find a maximum anisotropy level in the Union2 data of \frac{\Delta \ommax}{\bomm}=0.43\pm 0.06. Such a level does not necessarily correspond to statistically significant anisotropy because it is reproduced by about $30$ of simulated isotropic data mimicking the best fit Union2 dataset. However, when combined with the axes directions of other cosmological observations (bulk velocity flow axis, three axes of CMB low multipole moments and quasar optical polarization alignment axis), the statistical evidence for a cosmological anisotropy increases dramatically. We estimate the probability that the above independent six axes directions would be so close in the sky to be less than $1$. Thus either the relative coincidence of these six axes is a very large statistical fluctuation or there is an underlying physical or systematic reason that leads to their correlation.Comment: 10 pages, 7 figures. Accepted in JCAP (to appear). Extended analysis with redshift tomography of SnIa, included errorbars and increased number of axes. The Mathematica 7 files with the data used for the production of the figures along with a Powerpoint file with additional figures may be downloaded from http://leandros.physics.uoi.gr/anisotrop