1,721,015 research outputs found
Characterisation of self-guided laser wakefield accelerators to multi-GeV energies
Full text linkThis thesis details experimental and theoretical work in the field of self-guided laser wakefield accelerators, characterising various aspects of the machine related to the driver laser and electron beam generation.
The spectral changes to the laser pulse driving a laser wakefield accelerator were characterised. It was found that the spectral blueshift is directly correlated to the length of the plasma cavity. Spectral phase changes of the driver pulse were measured to dramatically alter the interaction. Positive second order spectral phase was measured to increase electron beam energy, its charge and the spectral blueshifting undergone by the driver pulse.
The suppression of self-injection in laser wakefield accelerators operating in the highly non-linear bubble regime was observed. The use of ionisation impurity to pre-inject electrons into the plasma cavity was measured to alter the fundamental properties of the electron beam. Through particle-in-cell simulations it was shown that this effect arises from the repulsive electrostatic force from the beam load, preventing sufficient transverse momentum gain of sheath electrons.
Record electron beam energies of nearly 3 GeV were measured in the self-injecting, self-guiding regime of laser wakefield accelerators. These results were obtained at higher than expected plasma densities and are thought to be a direct result of increased energy coupling due to the use of a much longer main focussing optic. Very stable injection in the self-injection regime was observed allowing for experimental measurements of peak accelerating field within the bubble. The field E = (590 ± 180) GV/m is the highest value of the electric field reported.
The efficacy and long-term stability of self-guided, self-injecting laser wakefield electron acceleration was evaluated. Highest sustained laser energy to electron beam energy conversion efficiency of nearly 3% was measured. It was also shown the self-injection yields higher overall efficiencies than ionisation induced injection. Stability of injection and acceleration over more than half a thousand consecutive shots was studied and found to be directly dependent on the stability of the driving laser.Open Acces
FLASHForward - A Future-Oriented Wakefield-Accelerator Research and Development Facility at FLASH
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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
Paving the way of high repetition rate laser wakefield acceleration by solving heat induced grating deformation issues
No full textThe goal of this thesis was to enable high repetition rate Laser Wakefield Acceleration(LWFA) by solving heat induced grating deformation issues in a Chirped PulseAmplification (CPA) based 25 TW Ti:sapphire laser system centred at 800 nm. Thesepreviously limited the intensity and wavefront quality of the pulse. An increase of maximumrepetition rate, limited by heat induced grating deformation, from 1 to 10 Hz wasachieved. Through simulations it was shown that the grating deforms due to heatingup from absorbed laser energy which induces spatial-distortions in the wavefront. Byreplacing the NZK-7 based grating with one made from ZERODURR , a glass ceramicproduced by the Schott AG with a vanishing thermal expansion coefficient a, it wasexpected, and later verified through measurements, that the introduction of such distortionsinto the beam wavefront could be inhibited. During operating at 10 Hz, thefocal spot size of the far field was measured to increase from 51 mm to 204 mm inthe vertical-, and 61 mm to 202 mm in the horizontal axis within 5 minutes usingthe NZK-7 grating and kept growing to 244 mm in the vertical-, and 434 mm inthe horizontal axis over 8 hours. With the Zerodur grating installed it stayed constantin both axis at 51 mm during the whole measurement. It was also possible to reducethe change in position of the focus by one order of magnitude. Lastly, a RadioFrequency (RF) plasma cleaning setup was successfully tested and implemented intothe compressor chamber to remove the Carbon layer that was coated onto the gratingsurface by the high intensity of the laser. Heat induced grating deformation is one ofthe things that inhibit high repetition rate LWFA experiments. In summary, this thesispresents a possibility to resolve these challenges
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
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