1,721,013 research outputs found
Energetic Radiation from Wakefield Acceleration and its Applications
Get PDFThe driving theme of this thesis is the experimental production and characterisation of high-energy gamma radiation using a laser wakefield accelerator (LWFA), and its application in the context of studies of fundamental QED phenomena.
An electron beam from shock injection of an energy up to 1.3 GeV was collided with a laser pulse at an intensity of a0 ~ 0.2 - 1 producing 10's of MeV photons from linear inverse Compton scattering (ICS). The emitted radiation was used to diagnose the properties of the electron beam and of the laser pulse at the interaction. It was demonstrated that this can also be used to systematically facilitate the spatio-temporal overlap of the electron beam and the laser pulse in future radiation reaction studies.
A relativistic electron beam of energy > 500 MeV was collided with a tightly focused laser pulse with a0 ~ 10. The interaction generated broadband synchrotron-like radiation from non-linear inverse Compton scattering (ICS) with critical energies > 30 MeV, which are the highest ICS photon energies reported from an all-optical setup at this time. The high photon energies in turn enabled a significant measurement of energy loss in the electron beam, rendering this the first published measurement of radiation reaction in an LWFA setup.
An electron beam from LWFA was used to commission a bremsstrahlung gamma-ray source reaching photon energies of several hundreds of MeV. Different materials and accelerator configurations were used to optimise the yield of photons and to mitigate the production of secondary particles.
The energetic gamma rays were then collided with the X-ray field emitted by a hot plasma in order to attempt the production of electron-positron pairs from the Breit-Wheeler process.Open Acces
Laser-plasma interactions as tools for studying processes in quantum electrodynamics
Get PDFConventional particle accelerators and astronomical observations have long been some of the only tools for studying processes in high energy physics. The development of laser-plasma sources and high gradient accelerators will therefore be a key asset to these studies. In particular, laser-plasma accelerators have favourable spatial and temporal properties for studies into intense processes, and can be readily coupled to a wide array of other laser-plasma sources creating unique environments. Here, coupling to an X-ray source and intense laser focus were used to study processes in quantum electrodynamics.
To study the linear Breit-Wheeler process, a 40 ps laser was used to drive a volumetric X-ray emitter. Line emission from a thin-foil Ge target, produced a highly efficient (3.4%), dense source of 1.3 − 1.9 keV X-rays, with 3 ± 1 (stat.) ±0.4 (sys.) ×10^{12} photons/eV/sphere. These X-rays were collided with bremsstrahlung gamma rays (with energies up to 800 MeV) to investigate electron-positron pair production. The X-ray source was well-optimised for studying this interaction, and would allow the detection of Breit-Wheeler pairs if used with a moderately improved electron beam for generating bremsstrahlung (3× the highest electron energy and 5× the total charge, as achieved previously). This would constitute the first laser-plasma photon- photon collider with low virtuality (energy off mass-shell ≈ 10^{−20} MeV^2).
In order to differentiate between competing models of electron radiation reaction in strong field quantum electrodynamics, a narrow energy-spread electron beam was studied. By utilising shock injection into a laser wakefield accelerator, a high energy (1260±40 MeV), narrow energy- spread (4.1±0.9 %) beam was generated. This is one of only a few studies that have successfully achieved these electron beam properties. While the shot-to-shot reproducibility of the electron beam was limited to 60%, the relative energy-spread was sufficiently small that differentiation of radiation reaction models could be readily achieved in future experiments.
With the upcoming commissioning of many multi-PW laser facilities, these studies demonstrate how active research into quantum electrodynamics can be achieved on the smaller, more accessible, laser-laboratory scale.Open Acces
Monte Carlo modelling of QED Interactions in laser-plasma experiments
Get PDFThis thesis is concerned with the development of a start-to-end Monte Carlo simulation capability for laser based QED experiments.
A physics package has been developed for Geant4 which models particle-photon interactions, including the Breit-Wheeler process and photon-photon scattering. The interactions are treated as a beam of particles travelling through a static photon field. Gaussian
process regression has been used to increase the event calculation rate by a factor of ∼ 1000.
A strong field QED event generator that models the nonlinear Breit-Wheeler process and nonlinear Compton scattering has been developed and integrated into Geant4. Deep learning has been used to emulate this package and increases the calculation rate by a factor of ∼ 1000, allowing the package to be used as a forward model for Bayesian inference to aid the design and analysis of strong field QED experiments.
The tools developed were used to design and analyse a photon-photon collider experiment at the Gemini laser facility. This collided a ∼ 50 fs beam of gamma rays (> 100 MeV) with a ∼ 40 ps beam of x-rays (∼ 1.5 keV). Using optimum beam conditions, simulations predict a signal-to-noise ratio of ∼ 0.1, meaning a statistically significant measurement of the Breit-Wheeler process could be made with ∼ 100 shots. However, during the experiment the electron beam obtained was sub-optimal, reducing the signal-to-noise ratio to ∼ 2 × 10 5 , making it unlikely that a measurement was possible.
No signatures of elastic photon-photon scattering were detected in the experiment, enabling a bound of 4.9 × 10^12 µb to be placed on the photon-photon scattering cross-section at a centre of mass energy ω_CM ≈ 0.66 me . While still far above the predicted QED cross-section, this is the most stringent bound placed to date, improving upon previous work by three orders of magnitude.Open Acces
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
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
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
Exploring novel regimes for ion acceleration driven by intense laser radiation
Get PDFThis thesis covers experimental and numerical studies on novel schemes of ion accelera-
tion with high intensity lasers. In particular, it discusses previously unexplored regimes
incorporating the radiation pressure of intense lasers. These schemes are of interest to potential
applications due to the emergence of improved ion beam properties that are detailed in this thesis.
The thesis discusses results from ion acceleration experiments using intense optical lasers on
ultra-thin targets at the Rutherford Appleton Laboratory, where the Vulcan Petawatt system was used
to irradiate nanometre thickness foils. In particular, the accelerated proton beam profiles from
these interactions showed a variety of features, such as Rayleigh-Taylor-like instability driven
spatial beam modulation, annular rings and a high-energy tail. A particularly interesting novel
observation is the emergence of a spectrally peaked on-axis component to the proton beam, which is
indicative of buffering of the proton layer ahead of a heating heavier ion species. These different
features will be analysed and discussed, and modelled using PIC simulation.
The thesis also includes the results from recent experiments studying the interaction of an intense
CO2 laser with an overdense plasma generated by a gas jet. A remarkably monoenergetic proton beam
was measured, in contrast to the majority of experiments performed previously on ion acceleration,
and was found by optical probing and numer- ical simulation to be a result of hole-boring generated
by the radiation pressure of the intense laser pulse acting on the plasma. The thesis will include
analysis of interferom- etry and shadowgraphy images of the plasma, and discussion of the plasma
dynamics and ion generation mechanisms involved, including the generation of radiation pressure
driven collisionless shock waves. The effects of the laser prepulse, electron transport effects and
non-linear post-soliton production will all be discussed. It will also present
detailed numerical particle-in-cell (PIC) simulation of the interaction.Open Acces
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