1,721,098 research outputs found
Spatial properties of odd and even low order harmonics generated in gas
No full textHigh harmonic generation in gases is developing rapidly as a soft X-ray femtosecond light-source for applications. This requires control over all the harmonics characteristics and in particular, spatial properties have to be kept very good. In previous literature, measurements have always included several harmonics contrary to applications, especially spectroscopic applications, which usually require a single harmonic. To fill this gap, we present here for the first time a detailed study of completely isolated harmonics. The contribution of the surrounding harmonics has been totally suppressed using interferential filtering which is available for low harmonic orders. In addition, this allows to clearly identify behaviors of standard odd orders from even orders obtained by frequency-mixing of a fundamental laser and of its second harmonic. Comparisons of the spatial intensity profiles, of the spatial coherence and of the wavefront aberration level of 5I ‰ at 160â €...nm and 6I ‰ at 135â €...nm have then been performed. We have established that the fundamental laser beam aberrations can cause the appearance of a non-homogenous donut-shape in the 6I ‰ spatial intensity distribution. This undesirable effect can be easily controlled. We finally conclude that the spatial quality of an even harmonic can be as excellent as in standard generation
Free electron laser oscillator efficiency
No full textThe efficiency of Free Electron Laser (FEL) Oscillator devices is a fairly complicated function of the various parameters which characterize the device itself. We explore the relevant dependences by the use of the scaling formulas describing the FEL Oscillator dynamics and providing the relevant design elements. We obtain a quantitative dependence of the efficiency on the key FEL parameters (small signal gain coefficient, saturation intensity, electron beam qualities...) and discuss the use of the results obtained in the paper as a further element for the FEL Oscillator optimization. © 2018 Elsevier B.V
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
Large-amplitude Quasiperiodic Pulsations as Evidence of Impulsive Heating in Hot Transient Loop Systems Detected in the EUV with SDO/AIA
Get PDFShort heat pulses can trigger plasma pressure fronts inside closed magnetic tubes in the corona. The alternation of condensations and rarefactions from the pressure modes drive large-amplitude pulsations in the plasma emission. Here we show the detection of such pulsations along magnetic tubes that brighten transiently in the hot 94 EUV channel of the Solar Dynamics Observatory/AIA. The pulsations are consistent with those predicted by hydrodynamic loop modeling, and confirm pulsed heating in the loop system. The comparison of observations and model provides constraints on the heat deposition: a good agreement requires loop twisting and pulses deposited close to the footpoints with a duration of 0.5 minutes in one loop, and deposited in the corona with a duration of 2.5 minutes in another loop of the same loop system
Slice collective dynamics, projected emittance deterioration and free electron laser performances detrimental effects
No full textSelf-amplified spontaneous emission (SASE) free electron laser (FEL) devices have disclosed an unexpected interplay between the laser intensity growth and regions of the electron bunch of the order of the coherence length. They are usually identified with the bunch slice and contribute to the laser dynamics with their own characteristics. The dynamical effects inducing geometrical and phase space misalignment of bunch slice in X-ray operating FELs can be traced back to a plethora of phenomena, both in the Linac accelerating section or inside the beam transport optic magnet. They are responsible for spoiling of the beam projected qualities and, if not corrected properly, induce an increase of the saturation length and a decreasing of the output power. We discuss the inclusion of these effects in models employing scaling formulae
Two color FEL driven by a comb-like electron beam distribution
No full textWe discuss a new method for the production of trains of FEL radiation pulses based on the FEL emission driven by a comb-like electron beam. In addition, we present recent experimental results on the two color FEL emission as generated at the SPARC LAB facility: A train of two short (<200 fs) electron bunches, almost overlapped in time, with a comb-like energy distribution, has been injected in the undulator, giving rise to FEL pulses at two characteristic frequencies with multi-peaked time structure. This scheme shows also the versatility of the SPARC photo-injector to generate and manipulate such energy and time distributions. © 2014 Elsevier B.V
Elementi di meccanica quantistica
Get PDFNel libro vengono introdotti, seguendo un filo conduttore fenomenologico, i concetti della Meccanica Quantistica quali il modello atomico, il dualismo onda-particella, la fisica dei materiali, la spettroscopia. Discuteremo lo svilupparsi delle idee che portarono della nuova meccanica attraverso il dispiegarsi dei fatti sperimentali che decretarono la fine della concezione classica della natura. Mostreremo infatti come metodi matematici e sperimentali abbiano fornito una nuova chiave interpretativa del mondo atomico e non solo. Il mondo quantistico ed i concetti ad esso associati sono ancora difficilmente percepibili nella esperienza quotidiana, ciononostante esso è uno strumento su cui sempre di più si basano molti aspetti di tecnologie attualmente disponibili o in via di sviluppo. Lo scopo è quello di favorire una agevole e disincantata comprensione dei suoi aspetti essenziali.In this book the concepts of Quantum Mechanics, such as the atomic model, the wave-particle duality, the materials’
physics, spectroscopy... are introduced using a phenomenological approach. We study as experimental events led to the birth of Quantum Mechanics and how mathematical methods and experiments provided a new key to the interpretation of the atomic world and beyond. The purpose is to simplify the understanding of the essential points of a theory which is increasingly involved with many aspects of the present and in developing technologies, despite the quantum world is still hardly noticeable in everyday experience
Reconnection nanojets in the solar corona
No full textThe solar corona is shaped and mysteriously heated to millions of degrees by the Sun’s magnetic field. It has long been hypothesized that the heating results from a myriad of tiny magnetic energy outbursts called nanoflares, driven by the fundamental process of magnetic reconnection. Misaligned magnetic field lines can break and reconnect, producing nanoflares in avalanche-like processes. However, no direct and unique observations of such nanoflares exist to date, and the lack of a smoking gun has cast doubt on the possibility of solving the coronal heating problem. From coordinated multi-band high-resolution observations, we report on the discovery of very fast and bursty nanojets, the telltale signature of reconnection-based nanoflares resulting in coronal heating. Using state-of-the-art numerical simulations, we demonstrate that the nanojet is a consequence of the slingshot effect from the magnetically tensed, curved magnetic field lines reconnecting at small angles. Nanojets are therefore the key signature of reconnection-based coronal heating in action
Electron beam transverse longitudinal dynamics for SASE FEL operation
No full textWe present a procedure based on the combined use of analytical and numerical tools, useful for understanding the relative interplay between the various components of a free electron laser (FEL) self-amplified spontaneous emission (SASE) device. We take as reference case the Sorgente Pulsata Auto-amplificata di Radiazione Coerente (SPARC) experiment to analyze how electron beam transverse matching conditions affect the lasing conditions itself and discuss the possibility of implementing new configurations. In particular we show that the procedure can be extended to account for the effect of an on line inserted cavity, with the role of inducing a transverse longitudinal correlation allowing a transfer of the longitudinal and transverse emittance. We discuss the associated benefit on the FEL operation, with particular reference to the nonlinear harmonic generation. The possibility of completing the tool with regards to the beam degradation effects due to CSR is briefly accounted for. © 1965-2012 IEEE
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