1,721,125 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Study of novel electron injection mechanisms for laser-wakefield accelerators
Get PDFLaser-driven plasma wakefields are structures that can sustain large electric fields and offer a promising route to reducing the size and cost of electron (or positron) accelerators with applications in areas such as fundamental research, (biomedical) imaging or radiotherapy. In this thesis, I discuss novel mechanisms through which electrons are injected into the accelerating structure and methods used to achieve better control over experimental conditions. Experimental and numerical results are presented for injection of electrons into a wakefield driven by a short relativistic laser pulse propagating through a plasma of overcritical nanometre-sized clusters. The wakefield structure is found to be modified by the clusters and the creation of high-charge large energy spread electron beams is discussed. Furthermore, experimental and numerical results are presented for externally induced ionisation injection using two pulses of equal wavelength decoupling the processes of driving the accelerating structure from the injection into the structure finding quasi-monoenergetic electron beams. Finally, a reinforcement learning based algorithm for laser beam alignment is introduced and training results of alignment performance for an interferometer set-up as well as an off-axis parabolic mirror are presented
Relativistic channeling with applications to inertial confinement fusion
Get PDFThe goal of controlled fusion energy has been pursued by the international community for decades. The majority of research into inertial confinement fusion, one branch of fusion research as a whole, has been focused on central hot spot ignition. However, alternative approaches such as fast ignition present the opportunity to achieve higher gains with reduced symmetry requirements, making them ideal for fusion energy production.
One important component of the original fast ignition scheme is the formation of a persistent low-density channel through an inhomogeneous plasma. This thesis revolves around channeling with laser pulses of highly relativistic intensities through plasmas relevant to fast ignition research, an intensity regime which has not received thorough attention in the existing literature. In this thesis, several particle-in-cell simulations are run to analyze various schemes in an effort to gain greater control over the channeling process.
An expanded physical model of the hosing instability that includes relativistic laser intensities and near-critical densities is presented and derives the density dependence of the hosing equation. This is then tested through simulations, and an examination of its consequences on multiple-pulse channeling schemes is conducted. The results show that the hosing instability grows more rapidly in dense plasmas, reducing the benefits of the multiple-pulse scheme when using realistic pulses of relativistic intensity.
Phenomena such as a ponderomotive self-correction mechanism, self-focusing-induced filamentation, and a modulated wavepacket known as the "pathfinder" pulse all become more relevant at higher pulse intensities. These have interesting consequences for future channeling schemes.
Finally, an experiment performed at the OMEGA EP facility at the Laboratory for Laser Energetics in Rochester, NY, USA is presented and analyzed. This experiment explored the effects of pulse timing, intensity, and focal position on channel formation. The findings of this experiment have important consequences for experimental design and have already improved the results of integrated fast ignition experiments.</p
Quantitative optical probing of plasma accelerators
Get PDFFour novel methods to diagnose plasma wakefield accelerators using optical probes are presented in this thesis. The first method involves sending an optical probe pulse to cross the wakefield at an oblique angle of incidence. The wakefield then imprints a phase modulation onto the probe which is read using a spectral interferometry technique. At first, the method was developed analytically and verified against multi-dimensional Particle-in-Cell (PIC) simulations. These allowed the relation between the phase modulation of the probe and the electron density in the wakefield to be extracted. An experiment employing this technique was also performed and modulations with similar wavelength to those expected in the wakefield accelerator were detected. The second method, which I have named three-dimensional (3D) spectrometer for brevity, is based on the concept of compressed sensing. It involves the retrieval of multiple two-dimensional (2D) spectral profiles just from a single 2D image captured by a planar detector. Numerical tests show that it can retrieve up to ten sets of 2D spectral profile just from a single image. The retrieved signal is also robust for further post-processing. The third method is based on a numerical technique to retrieve information from shadowgraphy or proton radiography. The information is the self-generated magnetic and/or electric fields for proton radiography and the variation of refractive index for shadowgraphy. The technique is adapted from a computational graphics algorithm. Numerical simulations show that the retrieved information is accurate with an error of 10% using the method, even if caustics appear. The algorithm is also applied to retrieve the modulation of the refractive index from a real experimental result of plasma wakefield using shadowgraphy. The fourth and the final method is a novel optimisation algorithm and software based on neuromorphic computing to optimise systems via simulations. The software was employed to optimise the performance of a laser-driven plasma wakefield system by testing it using PIC simulations on a computer cluster. It is demonstrated that by running it and letting it learn laser-driven plasma wakefield parameters for a number of days, the software can find optimal parameters in a laser-plasma system without explicitly being taught
Modelling laser-plasma interactions for the next generation of high-power laser experiments
Get PDFInvestigating new regimes of laser-plasma interactions requires advances in at least two technologies: targets and laser systems. Exploring these regimes includes developing diagnostics to obtain more information about complex systems such as inertial confinement implosions and constructing models that consider entirely novel and unexplored phenomena. This thesis initially considers implementing state-of-the-art "mid-density" targets made of foam on current petawatt-class laser systems; particularly the application of foams to ion acceleration. It is highlighted that foam targets potentially pave the way for laser systems to realise collisionless shock acceleration as a high energy quasimonoenergetic ion source. A theoretical approach modelling the predicted energy distribution function for shock-accelerated ions is presented with both classical and relativistic formulations. The full, albeit narrow, parameter space for collisionless shock acceleration is mapped and it is shown that there are several pre-existing laser systems that could realise high-energy narrow-band proton beams. Impending multipetawatt laser systems are then considered but it is immediately highlighted that the absorption processes in this new regime may not be fully understood. Therefore, the thesis segues into developing a theory that models absorption in the newly dubbed ``post-ponderomotive'' regime. One absorption mechanism, the Zero Vector Potential process is given a complete description, the predictions of which are validated through numerical simulations. This discussion then considers the limit of the post-ponderomotive regime where quantum electrodynamic effects are predicted to be perturbations. The theory is modified to include the effects of electron-positron pair production and, again, a simulation campaign verifies the theory. Applications of the Zero Vector Potential mechanism to coherent x-ray generation are also explored, and in particular their potential use generating warm dense matter. Finally, given that no scientific hypothesis can be considered valid until it has been tested through experiment, three experimental designs are proposed that could test the theories and hypotheses presented herein
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Simulation of density measurements in plasma wakefields using photon acceleration
No full textThe data was used in a paper with the same title. They were created in May 2014, from a simulation run in SCARF machine at Rutherford Appleton Laboratory
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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