1,720,994 research outputs found
Laser cleaning of diagnostic mirrors from tungsten-oxygen tokamak-like contaminants
Full text linkThis paper presents a laboratory-scale experimental investigation about the laser cleaning of diagnostic first mirrors from tokamak-like contaminants, made of oxidized tungsten compounds with different properties and morphology. The re-deposition of contaminants sputtered from a tokamak first wall onto first mirrors' surfaces could dramatically decrease their reflectivity in an unacceptable way for the proper functioning of plasma diagnostic systems. The laser cleaning technique has been proposed as a solution to tackle this issue. In this work, pulsed laser deposition was exploited to produce rhodium films functional as first mirrors and to deposit onto them contaminants designed to be realistic in reproducing materials expected to be re-deposited on first mirrors in a tokamak environment. The same laser system was also used to perform laser cleaning experiments, exploiting a sample handling procedure that allows one to clean some cm2 in a few minutes. Cleaning effectiveness was evaluated in terms of specular reflectance recovery and mirror surface integrity. The effect of different laser wavelengths (λ= 1064, 266 nm) on the cleaning process was also addressed, as well as the impact of multiple contamination/cleaning cycles on the process outcome. A satisfactory recovery of pristine mirror reflectance (≥90%) was obtained in the vis-NIR spectral range, avoiding at the same time mirror damaging. The results here presented show the potential of the laser cleaning technique as an attractive solution for the cleaning of diagnostic first mirrors
Laser cleaning of pulsed laser deposited rhodium films for fusion diagnostic mirrors
No full textIn this paper an experimental investigation on the laser cleaning process of rhodium films, potentially candidates to be used as tokamak first mirrors (FMs), from redeposited carbon contaminants is presented. A relevant issue that lowers mirror's performance during tokamak operations is the redeposition of sputtered material from the first wall on their surface. Among all the possible techniques, laser cleaning, in which a train of laser pulses is launched to the surface that has to be treated, is a method to potentially mitigate this problem. The same laser system (Q-switched Nd:YAG laser with a fundamental wavelength of 1064-nm and 7-ns pulses) has been employed with three aims: (i) production by pulsed laser deposition (PLD) of Rh film mirrors, (ii) production by PLD of C deposits with controlled morphology, and (iii) investigation of the laser cleaning method onto C contaminated Rh samples. The evaluation of Rh films laser damage threshold, as a function of fluence and number of pulses, is discussed. Then, the C/Rh films have been cleaned by the laser beam. The exposed zones have been characterized by visual inspection and scanning electron microscopy (SEM), showing promising results
Versatile Synthesis of Nanofoams through Femtosecond Pulsed Laser Deposition
Full text linkNanofoam materials are gaining increasing interest in the scientific community, thanks to their unique properties such as ultralow density, complex nano- and microstructure, and high surface area. Nanofoams are attractive for multiple applications, ranging from advanced catalysis and energy storage to nuclear fusion and particle acceleration. The main issues hindering the widespread use of nanofoams are related to the choice of synthesis technique, highly dependent on the desired elemental composition and leading to a limited control over the main material properties. Herein, femtosecond pulsed laser deposition is proposed as a universal tool for the synthesis of nanofoams with tailored characteristics. Nanofoams made by elements with significantly different properties—namely, boron, silicon, copper, tungsten, and gold—can be produced by suitably tuning the deposition parameters. The effect of the background pressure is studied in detail, in relation to the morphological features and density of the resulting nanofoams and nanostructured films. This, together with the analysis of the specific features shown by nanofoams made of different elements, offers fresh insights into the aggregation process and its relation to the corresponding nanofoam properties down to the nanoscale, opening new perspectives toward the application of nanofoam-based materials
Pulsed laser deposition of carbon nanofoam
No full textWe present a comprehensive study about the fabrication of porous, fractal-like nanostructured carbon materials, known as carbon nanofoam, through the Pulsed Laser Deposition (PLD) technique with nanosecond (ns-PLD) and femtosecond (fs-PLD) laser pulses ablating a pyrolitic graphite target. Carbon nanofoams have attracted considerable interest for applications of great societal and technological relevance. Among different fabrication methods, PLD emerged as a versatile tool that allows the depositon of nanostructured films on virtually any kind of substrate. Here we investigate the role of key PLD parameters (laser pulse duration and fluence, background gas pressure) on nanofoam characteristics, with a specific focus on how fractal aggregate properties at the nanometric scale (nanoparticle size, fractal dimension, gyration radius) determine nanofoam averaged properties such as density and uniformity. We performed systematic deposition campaigns and characterizations through experimental analysis and mathematical models. Moreover, a first-of-its-kind comparison between ns-PLD and fs-PLD regimes is drawn. We discuss the physics of foam growth in both regimes, and we propose an analytical model based on the fractal scaling law to predict the nanofoam density from aggregates' properties. The new insights gained about deposition and characterization of carbon nanofoam open new perspectives in fabrication of nanostructured films with precisely controlled properties
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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