1,721,069 research outputs found
Geolocation of multiple sociolinguistic markers in Buenos Aires
No full textAnalysis of language geography is increasingly being used for studying spatial patterns of social dynamics. This trend is fueled by social media platforms such as Twitter which provide access to large amounts of natural language data combined with geolocation and user metadata enabling reconstruction of detailed spatial patterns of language use. Most studies are performed on large spatial scales associated with countries and regions, where language dynamics are often dominated by the effects of geographic and administrative borders. Extending to smaller, urban scales, however, allows visualization of spatial patterns of language use determined by social dynamics within the city, providing valuable information for a range of social topics from demographic studies to urban planning. So far, few studies have been made in this domain, due, in part, to the challenges in developing algorithms that accurately classify linguistic features. Here we extend urban-scale geographical analysis of language use beyond lexical meaning to include other sociolinguistic markers that identify language style, dialect and social groups. Some features, which have not been explored with social-media data on the urban scale, can be used to target a range of social phenomena. Our study focuses on Twitter use in Buenos Aires and our approach classifies tweets based on contrasting sets of tokens manually selected to target precise linguistic features. We perform statistical analyses of eleven categories of language use to quantify the presence of spatial patterns and the extent to which they are socially driven. We then perform the first comparative analysis assessing how the patterns and strength of social drivers vary with category. Finally, we derive plausible explanations for the patterns by comparing them with independently generated maps of geosocial context. Identifying these connections is a key aspect of the social-dynamics analysis which has so far received insufficient attention
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Towards Single-Shot Characterization Of Near-Resonant Lwfa In A Differentially-Pumped Gas Cell
Full text linkWe report on progress in the development of a single-shot diagnostic for measuring near-resonant laser-driven wakefields. The development entails the following critical steps: 1. Increase in laser pulse energy 2. development and characterization of a differentially pumped gas cell 3. development of a single-shot diagnostic with low phase noise 4. analysis of laser/plasma parameter space for maximizing the measured phase shift. As a result of this development we expect to be have detection sensitivity of 50 - 100 mrad and we expect to be able produce and measure wakefields with dne/nesimilar to1 and phase shifts of several hundred mrad.Physic
A compact THz-driven electron gun
No full textNovel all-optical terahertz (THz)-based accelerators promise to enable new science by providing ultrafast and bright electron bunches at a small footprint. While practical prototypes of THz-based devices have been demonstrated and have shown exceptional capabilities to accelerate and manipulate electron beams on sub-ps timescales, the development of practical THz-driven photoguns has lagged behind due to challenges associated with physical miniaturization and the high THz pulse energy required. This thesis significantly advances the development of such THz-driven guns by setting a dual focus: First, systematic parameter scans are performed on tilted pulse-front setups providing crucial insights into the non-collinear interactionlengths, parameter sensitivities and physics inherent to high-energy THz generation. In addition, spatio-temporal manipulation of the pump pulse is explored both experimentally and by simulations and found as viable tool to scale tilted pulse-front based THz sources to such energies required for the next generation of compact particle accelerators. Application of the findings resulted in robust extraction of THz pulses with energies up to 400 μJ while operating well below the optical damage threshold. Secondly, these setups are used in in three different experiments aimed at developing a practical compact THz-driven gun. The extraction of multi-keV electrons from a triggerable THz photogun is demonstrated for the first time, and the parameter space and resulting performance are explored. Subsequently, multi-layered structures are developed that extend the interaction between electrons and THz pulses. A novel single-sided pumped THz accelerator is tested and a segmented dual-sided pumped THz photogun is developed. Finally, the instrumentation for commissioning and scaling THz driven gun technology to energies beyond 100 keV, comparable to the performance of modern compact DC electron guns, is presented and discussed. This work represents a critical step in the development of practical all optical THzdriven electron guns and paves the way towards more compact accelerators with fs bunch durations, low emittance and orders of magnitude higher brightness to power future ultrafast electron diffraction experiments and compact X-ray sources
Effiziente hochenergetische, lasergetriebene Multicycle-Terahertz-Erzeugung für Beschleuniger
No full textOptisch erzeugte Multi-Cycle-Terahertz-Strahlung (MC-THz) hat das Potenzial,
konventionelle Elektronenbeschleuniger und damit verbundene Forschungsgebiete zu revolutionieren. Bisherige Umwandlungseffizienzen von
optischer zu THz-Strahlung sind derzeit zu niedrig, um ausreichend hohe
Feldgradienten zur Beschleunigung von Elektronen auf hohe Energien zu
erreichen.
In dieser Arbeit wurde die MC-THz-Erzeugung durch Differenzfrequenzerzeugung
(DFG) in Magnesiumoxid-dotiertem, periodisch gepolten Lithiumniobat
(MgO:PPLN) und Rubidium-dotiertem, periodisch gepolten Kaliumtitanylphosphat (Rb:PPKTP) mittels eines Yb-basierten, speziell entwickelten
Multilinienlasers untersucht. Dieser Ansatz erweist sich als vielversprechend,
denn es konnte eine Steigerung der Konversionseffizienz auf 0,49%
mit einer Pulsenergie von 30 μJ bei 0,29 THz und 0,89% mit einer Pulsenergie
von 45 μJ bei 0,53 THz in MgO:PPLN sowie 0,16% mit einer Pulsenergie von
3 μJ bei 0,5 THz in Rb:PPKTP nachgewiesen werden. Diese Werte stellen die
zum heutigen Stand höchsten je gemessenen Werte dar. Die Ergebnisse sind
in sehr guter Übereinstimmung mit zweidimensionalen numerischen Simulationen.
Desweiteren wurden Skalierungsexperimente mit einem kommerziellen
Yb-basierten Laser (200 mJ, 500 fs, 50 Hz) durchgeführt. Eine Effizienz
von 0,29% mit Pulsenergien von 260 μJ bei 0,35 THz in MgO:PPLN-Kristallen
mit großer Apertur (10x15mm2) konnte demonstriert werden, obwohl dabei
eine bekannterweise weniger effiziente THz-Erzeugungsmethode, die optische
Gleichrichtung, benutzt wurde. Dies ist die höchste nachgewiesene Effizienz
basierend auf optischer Gleichrichtung für MC-THz Strahlung. Darüber
hinaus wurde die periodische Polung des nichtlinearen Mediums auch
über ein Wafer-Konzept erfolgreich realisiert. Dadurch war es möglich KTPund
LN-Wafer mit Aperturgrößen von 1” und 10x15mm2 zu verwenden,
um so die Pumpenergie im Vergleich zu herkömmlichen Kristallen hoch zu
skalieren. Dabei wurden zwei Methoden für die effiziente Benutzung des
Pumplasers untersucht: Einerseits wurden Wafer-Stapel hintereinander gereiht
und vermessen, andererseits eine Methode zur geseedeten MC-THz-
Erzeugung über einen rückreflektierten THz-Strahl getestet. In beiden Fällen
konnte eine erhöhte Effizienz beobachtet werden. Besonders im zweiten
Fall konnte eine Effizienz von 0,29% mit einer Pulsenergie von 280 μJ bei
0,16 THz erreicht werden, dreimal mehr als im Vergleich zum ungeseedeten
Fall. Diese Ergebnisse sind entscheidend für die Realisierung kompakter, auf
THz-Strahlung basierender Elektronenbeschleuniger der nächsten Generation.Optically generated, narrowband multi-cycle terahertz (MC-THz) radiation
has the potential to revolutionize electron acceleration, X-ray free-electron
lasers, advanced electron beam diagnostics and related research areas. However, the currently demonstrated THz generation efficiencies are too low to
reach the requirements for many of these applications.
In this project, a MC-THz generation approach via difference frequency
generation (DFG) driven by a laser with a multi-line optical spectrum was
investigated with the aim of increasing the conversion efficiency. For this purpose, a home-built, Yb-based laser source with a multi-line optical spectrum
was developed. This laser source was amplified to tens-of-millijoule using a
regenerative and a four-pass amplifier; it was used to generate MC-THz in
magnesiumoxid-doped periodically poled lithium niobate (MgO:PPLN) and
rubidium-doped periodically poled potassium titanyl phosphate (Rb:PPKTP).
With this laser system, the highest optical-to-THz conversion efficiencies (CE)
of 0.49% with a pulse energy of 30 mJ at 0.29 THz, and 0.89% with a pulse
energy of 45 mJ at 0.53 THz in MgO:PPLN were achieved. These results compare well with 2-dimensional numerical simulations. In addition, Rb:PPKTP,
which has a promising figure-of-merit compared to MgO:PPLN, achieved
a CE of 0.16% with a pulse energy of 3 mJ at 0.5 THz. Next, to scale this
laser system to tens of millijoule MC-THz output, large aperture crystals
for both MgO:PPLN and Rb:PPKTP were investigated using a commercial
laser, producing 200 mJ with a pulse duration of 500 fs at 1030 nm; although
in this case an older method of optical rectification (OR) was used, achieving
less efficiency than the multi-line source. With MgO:PPLN crystals of
aperture size 10x15mm2, a CE of 0.29% at 0.35 THz was achieved with a
pulse energy of 260 mJ. This is the highest known CE value using OR. In addition, wafer-stacks with alternating crystal-axis orientation of aperture size
of 1” for LN and 10x10mm2 for KTP were successfully tested. Two novel
experiments were performed with LN wafers: multi-stage wafer-stacks in
a serial configuration with multi-output THz radiation and back-reflected
seeded MC-THz generation. Both methods improved the efficiency of the
MC-THz generation, compared to a single stack. In particular, for the backreflected seeded MC-THz generation, pulse energies of 280 mJ with a CE of
0.29% was achieved; thus demonstrating the potential of seeded MC-THz
generation. These achievements are an important step for the realization of
next-generation, THz-driven electron accelerators
Efficient high energy laser-driven multicycle terahertz generation for accelerators
No full textOptically generated, narrowband multi-cycle terahertz (MC-THz) radiation has the potential to revolutionize electron acceleration, X-ray free-electron lasers, advanced electron beam diagnostics and related research areas. However, the currently demonstrated THz generation efficiencies are too low to reach the requirements for many of these applications. In this project, a MC-THz generation approach via difference frequency generation (DFG) driven by a laser with a multi-line optical spectrum was investigated with the aim of increasing the conversion efficiency. For this purpose, a home-built, Yb-based laser source with a multi-line optical spectrum was developed. This laser source was amplified to tens-of-millijoule using a regenerative and a four-pass amplifier; it was used to generate MC-THz in magnesiumoxid-doped periodically poled lithium niobate (MgO:PPLN) and rubidium-doped periodically poled potassium titanyl phosphate (Rb:PPKTP). With this laser system, the highest optical-to-THz conversion efficiencies (CE) of 0.49% with a pulse energy of 30 mJ at 0.29 THz, and 0.89% with a pulse energy of 45 mJ at 0.53 THz in MgO:PPLN were achieved. These results compare well with 2-dimensional numerical simulations. In addition, Rb:PPKTP, which has a promising figure-of-merit compared to MgO:PPLN, achieved a CE of 0.16% with a pulse energy of 3 mJ at 0.5 THz. Next, to scale this laser system to tens of millijoule MC-THz output, large aperture crystals for both MgO:PPLN and Rb:PPKTP were investigated using a commercial laser, producing 200 mJ with a pulse duration of 500 fs at 1030 nm; although in this case an older method of optical rectification (OR) was used, achieving less efficiency than the multi-line source. With MgO:PPLN crystals of aperture size 10^15 mm^2, a CE of 0.29% at 0.35 THz was achieved with a pulse energy of 260 mJ. This is the highest known CE value using OR. In addition, wafer-stacks with alternating crystal-axis orientation of aperture size of 1” for LN and 10^10 mm^2 for KTP were successfully tested. Two novel experiments were performed with LN wafers: multi-stage wafer-stacks in a serial configuration with multi-output THz radiation and back-reflected seeded MC-THz generation. Both methods improved the efficiency of the MC-THz generation, compared to a single stack. In particular, for the backreflected seeded MC-THz generation, pulse energies of 280 mJ with a CE of 0.29% was achieved; thus demonstrating the potential of seeded MC-THz generation. These achievements are an important step for the realization of next-generation, THz-driven electron accelerators
THz-Pulskompression
No full textCompact THz-based particle accelerators require Terahertz (THz) pulses of tens of mJ of energy to achieve high acceleration fields of hundreds of MV/m. Despite the promising prospects of scaling THz generation to these energy ranges by non-linear optical processes, their conversion efficiency for high-energy (HE) THz applications is currently too low.
Aiming to provide a new way of producing HE THz pulses, this thesis presents the development of a THz pulse compression system by which the signal emitted by a continuous-wave (CW) THz source is enhanced in intensity and compressed in duration into short pulses. For this purpose, a 900 mm long bow-tie enhancement cavity resonant with the Gaussian mode of a 100 GHz frequency CW incident electro-magnetic wave was implemented in a quasi-optical way. Inside the cavity, the intensity of the incident 100 GHz radiation was enhanced a certain number of times expressed through the parameter E. Subsequently, the cavity circulating THz radiation was extracted out in the form of a short pulse by using a laser-driven semiconductor switch. In particular, for a 145-μW cavity input light and a 532-nm, 7-ns, 50-mJ excitation laser pulse, the compression system was performed under two specific scenarios. First, for a E≈17.5 and by using an intrinsic silicon wafer, a maximum power of 940 μW was extracted, corresponding to ~38% of the cavity circulating power. Second, for a E≈6.5 and by utilizing an intrinsic gallium arsenide wafer, a maximum power of 635 μW was extracted, corresponding to ~67% of the cavity circulating power. In both cases, the duration of the extracted pulse was about ~28 ns, distributed in four main oscillations, each of them with a period of around 6~7 ns and different amplitude. These results proved that the magnitude, shape and duration of the extracted pulse were dependent on four factors: the cavity length, the parameter E, the energy of the excitation laser pulse and the switching dynamics of the semiconductor wafer.
Based on observations made from the current compression system, two conclusions can be drawn:
(i) The length of the resonator needs to be matched to the semiconductor response and the laser driver pulse length used.
(ii) The cavity and semiconductor switch size as well as laser energy needed needs to be scaled to a high power THz source such as gyrotrons. Such gyrotron sources have been developed to the multi-MW scale for electron-cyclotron resonance heating of fusion plasmas
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
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