1,721,007 research outputs found
Target holder as a specific sensor for laser-induced plasma ablation
No full textWe have used grounded and isolated targets to measure currents of electrons escaping the pulsed laser-produced plasma. Presented experiments show that the laser-produced plasma is dominated by three phases such as the ignition phase, active and afterglow phases. The last two phases occurring after laser-plasma interaction are influenced by whether the target is isolated from the vacuum chamber or grounded. The voltage arising on the isolated target, which acts as a capacitor, mainly affects the active phase, where collisions of particles still form the plasma. This can cause reduction in ion emissions, as observed for heavier elements. The target charging accompanying the laser ablation was driven by nanosecond laser radiation with fluence ranging between 1–4 J/cm2
Effect of grounding and isolation of the target on the emissive properties of laser-produced plasma
No full textDuring laser ablation, ions are accelerated by electrons, some of which definitely escape from the plasma by passing through a potential barrier that is created on the outer regions of the plasma produced by the laser. The escaping electrons generate a net positive charge at the target surface. Depending on whether the target is grounded or isolated from the vacuum chamber, either this positive charge is neutralized by the current flowing from the ground or generates a voltage on the isolated target corresponding to its capacitance. Under certain experimental conditions, it can be expected that this charge can retroactively affect not only the expansion velocity of the emitted ions and their number, but also the total number of escaped electrons. In this work, we deal with ion emission and measuring the voltage arisen on an isolated target or the current of electrons flowing between the ground and a grounded target. For the first time, this made it possible to estimate the ratio of the number of electrons released from plasma to the number of electrons stopped by a potential barrier. Irradiating various targets with a 60-110 mJ energy delivered by a 23-ns KrF excimer laser, this portion is 10−3 for grounded targets and 10−5 for targets isolated from the vacuum chamber. The observation of target currents also makes it possible to identify the level of influence of the contamination of the target surface with chemisorbed hydrocarbons on plasma production and ion separation caused by their different atomic weights
A new acceleration technique by laser interaction upon doped targets.
No full textWe report characteristics of ion emission from Cu, Sn, Cu/Sn4, Cu/Be2, Ag/Cu7.5 and Ag/Cu28 plasmas generated by a KrF laser which was operated at a wavelength of 248 nm and delivered energy up to 50 mJ. The time-of-flight characteristics of ion beams were determined by using a Faraday cup. The employment of an expansion chamber, a new part of the experimental apparatus, allows to accelerate the created ion beam by HV potential up to 40 kV and to investigate the evolution of ToF signals at a distance. The analysis of ion characteristics is the main scope of our contribution. An influence of the dopant admixture, laser pulse energy and accelerating potential on the characteristics of the ion emission is demonstrated
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
Target charging during laser ablation of polyethylene
No full textIt is known that exposure of a target to a focused laser beam results in the occurrence of a time-varying current between the target and the grounded vacuum chamber. This current is composed by three distinct phases, namely, the ignition phase, in which the laser pulse drives the electron emission, while electrons coming from the ground through the target holder balance the positive charge generated on the target. The active phase appears at post-pulse times and it is characterized by the presence of peaked structures in the time-resolved current, representing characteristics of the target composition. Finally, the afterglow phase is determined by a current of electrons flowing from the target to the ground. In the active phase of target current resulting from polymers ablation with an UV KrF laser, negative target current peaks have been observed, whose origin is still unknown. We investigate the dependence of these current structures on the dimensions of the target, using ultra-high molecular weight polyethylene disks of different thickness. We provide evidence to explain the origin of such negative peaks. We found, indeed, that target initially charges positively under the action of the laser pulse, leading to a first negative target current peak. Then, a net charge unbalance is produced that further attracts free electrons on target surface. This behavior is enhanced if an opportune static electric field is imposed between the target and an opposite electrode
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