Página 13 dos resultados de 1967 itens digitais encontrados em 0.004 segundos

## Ion motion in the wake driven by long particle bunches in plasmas

Vieira, J.; Fonseca, R. A.; Mori, W. B.; Silva, L. O.
Fonte: American Institute of Physics Publicador: American Institute of Physics
Tipo: Artigo de Revista Científica
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WOS:000337107200134 (Nº de Acesso Web of Science); We explore the role of the background plasma ion motion in self-modulated plasma wakefield accelerators. We employ Dawson's plasma sheet model to derive expressions for the transverse plasma electric field and ponderomotive force in the narrow bunch limit. We use these results to determine the on-set of the ion dynamics and demonstrate that the ion motion could occur in self-modulated plasma wakefield accelerators. Simulations show the motion of the plasma ions can lead to the early suppression of the self-modulation instability and of the accelerating fields. The background plasma ion motion can nevertheless be fully mitigated by using plasmas with heavier plasmas.

## Demonstration of relativistic electron beam focusing by a laser-plasma lens

Thaury, C.; Guillaume, E.; Döpp, A.; Lehe, R.; Lifschitz, A.; Ta Phuoc, K.; Gautier, J.; Goddet, J-P; Tafzi, A.; Flacco, A.; Tissandier, F.; Sebban, S.; Rousse, A.; Malka, V.
Fonte: Nature Pub. Group Publicador: Nature Pub. Group
Tipo: Artigo de Revista Científica
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Laser-plasma technology promises a drastic reduction of the size of high-energy electron accelerators. It could make free-electron lasers available to a broad scientific community and push further the limits of electron accelerators for high-energy physics. Furthermore, the unique femtosecond nature of the source makes it a promising tool for the study of ultrafast phenomena. However, applications are hindered by the lack of suitable lens to transport this kind of high-current electron beams mainly due to their divergence. Here we show that this issue can be solved by using a laser-plasma lens in which the field gradients are five order of magnitude larger than in conventional optics. We demonstrate a reduction of the divergence by nearly a factor of three, which should allow for an efficient coupling of the beam with a conventional beam transport line.

## Positron acceleration in a hollow plasma channel up to TeV regime

Yi, Longqing; Shen, Baifei; Ji, Liangliang; Lotov, Konstantin; Sosedkin, Alexander; XiaomeiZhang; Wang, Wenpeng; Xu, Jiancai; Shi, Yin; Zhang, Lingang; Xu, Zhizhan
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
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Nowadays, human's understanding of the fundamental physics is somehow limited by the energy that our high energy accelerators can afford. Up to 4 TeV protons are realized in the Large Hadron Collider (LHC). Leptons, such as electrons and positrons, however gained energies of about 100 GeV or less. Multi-TeV lepton accelerators are still lacking due to the relatively low acceleration gradient of conventional methods, which may induce unbearable cost. On the other hand, plasmas have shown extraordinary potential in accelerating electrons and ions, providing orders of magnitude higher acceleration fields of 10–100 GV/m. In such context, we propose a plasma-based high-energy lepton accelerator, in which a weakly focusing plasma structure is formed near the beam axis. The structure preserves the emittance of the accelerated beam and produces low radiation losses. Moreover, the structure allows for a considerable decrease of the witness energy spread at the driver depletion stage.

## Use of IAEA's phase-space files for the implementation of a clinical accelerator virtual source model

Rucci, José Alexis; Carletti, Claudia Elizabet; Cravero, Walter Ruben; Strbac, Bojan
Tipo: info:eu-repo/semantics/article; info:ar-repo/semantics/artículo; info:eu-repo/semantics/publishedVersion Formato: application/pdf
Português
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In the present work, phase-space data files (phsp) provided by the International Atomic Energy Agency (IAEA) for different accelerators were used in order to develop a Virtual Source Model (VSM) for clinical photon beams. Spectral energy distributions extracted from supplied phsp files were used to define the radiation pattern of a virtual extended source in a hybrid model which is completed with a virtual diaphragm used to simulate both electron contamination and the shape of the penumbra region. This simple virtual model was used as the radiation source for dosimetry calculations in a water phantom. The proposed model proved easy to build and test, and good agreement with clinical accelerators dosimetry measurements were obtained for different field sizes. Our results suggest this simple method could be useful for treatment planning systems (TPS) verification purposes.; Fil: Rucci, José Alexis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - CONICET - Bahía Blanca. Instituto de Física del Sur; Argentina; Universidad Nacional del Sur. Departamento de Física; Argentina;; Fil: Carletti, Claudia Elizabet. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - Conicet - Bahia Blanca. Instituto de Fisica del Sur; Argentina; Universidad Nacional del Sur. Departamento de Física; Argentina;; Fil: Cravero...

## Efficient Execution of Microscopy Image Analysis on CPU, GPU, and MIC Equipped Cluster Systems

Andrade, G.; Ferreira, R.; Teodoro, George; Rocha, Leonardo; Saltz, Joel H.; Kurc, Tahsin
Tipo: Artigo de Revista Científica
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High performance computing is experiencing a major paradigm shift with the introduction of accelerators, such as graphics processing units (GPUs) and Intel Xeon Phi (MIC). These processors have made available a tremendous computing power at low cost, and are transforming machines into hybrid systems equipped with CPUs and accelerators. Although these systems can deliver a very high peak performance, making full use of its resources in real-world applications is a complex problem. Most current applications deployed to these machines are still being executed in a single processor, leaving other devices underutilized. In this paper we explore a scenario in which applications are composed of hierarchical data flow tasks which are allocated to nodes of a distributed memory machine in coarse-grain, but each of them may be composed of several finer-grain tasks which can be allocated to different devices within the node. We propose and implement novel performance aware scheduling techniques that can be used to allocate tasks to devices. We evaluate our techniques using a pathology image analysis application used to investigate brain cancer morphology, and our experimental evaluation shows that the proposed scheduling strategies significantly outperforms other efficient scheduling techniques...

## Collisional Penrose process near the horizon of extreme Kerr black holes

Bejger, Michał; Piran, Tsvi; Abramowicz, Marek; Håkanson, Frida
Tipo: Artigo de Revista Científica
Português
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Collisions of particles in black holes' ergospheres may result in an arbitrarily large center of mass energy. This led recently to the suggestion (Banados et al., 2009) that black holes can act as ultimate particle accelerators. If the energy of an outgoing particle is larger than the total energy of the infalling particles the energy excess must come from the rotational energy of the black hole and hence this must involve a Penrose process. However, while the center of mass energy diverges the position of the collision makes it impossible for energetic particles to escape to infinity. Following an earlier work on collisional Penrose processes (Piran & Shaham 1977) we show that even under the most favorable idealized conditions the maximal energy of an escaping particle is only a modest factor above the total initial energy of the colliding particles. This implies that one shouldn't expect collisions around a black hole to act as spectacular cosmic accelerators.; Comment: 4 pages, 5 figures (minor corrections, matches PRL accepted version)

## The Astrophysics of Ultrahigh Energy Cosmic Rays

Kotera, Kumiko; Olinto, Angela V.
Tipo: Artigo de Revista Científica
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The origin of the highest energy cosmic rays is still unknown. The discovery of their sources will reveal the workings of the most energetic astrophysical accelerators in the universe. Current observations show a spectrum consistent with an origin in extragalactic astrophysical sources. Candidate sources range from the birth of compact objects to explosions related to gamma-ray bursts or to events in active galaxies. We discuss the main effects of propagation from cosmologically distant sources including interactions with cosmic background radiation and magnetic fields. We examine possible acceleration mechanisms leading to a survey of candidate sources and their signatures. New questions arise from an observed hint of sky anisotropies and an unexpected evolution of composition indicators. Future observations may reach the necessary sensitivity to achieve charged particle astronomy and to observe ultrahigh energy photons and neutrinos, which will further illuminate the workings of the universe at these extreme energies. In addition to fostering a new understanding of high-energy astrophysical phenomena, the study of ultrahigh energy cosmic rays can constrain the structure of the Galactic and extragalactic magnetic fields as well as probe particle interactions at energies orders of magnitude higher than achieved in terrestrial accelerators.; Comment: Draft of solicited review article; 44 pages and 12 figures; Final version to appear in Annual Review of Astronomy and Astrophysics vol. 49 (2011)

## Accelerating motif finding in DNA sequences with multicore CPUs

Perera, P.; Ragel, R. G.
Tipo: Artigo de Revista Científica
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Motif discovery in DNA sequences is a challenging task in molecular biology. In computational motif discovery, Planted (l, d) motif finding is a widely studied problem and numerous algorithms are available to solve it. Both hardware and software accelerators have been introduced to accelerate the motif finding algorithms. However, the use of hardware accelerators such as FPGAs needs hardware specialists to design such systems. Software based acceleration methods on the other hand are easier to implement than hardware acceleration techniques. Grid computing is one such software based acceleration technique which has been used in acceleration of motif finding. However, drawbacks such as network communication delays and the need of fast interconnection between nodes in the grid can limit its usage and scalability. As using multicore CPUs to accelerate CPU intensive tasks are becoming increasingly popular and common nowadays, we can employ it to accelerate motif finding and it can be a faster method than grid based acceleration. In this paper, we have explored the use of multicore CPUs to accelerate motif finding. We have accelerated the Skip-Brute Force algorithm on multicore CPUs parallelizing it using the POSIX thread library. Our method yielded an average speed up of 34x on a 32-core processor compared to a speed up of 21x on a grid based implementation of 32 nodes.

## Supersymmetric Candidates for Dark Matter (in German)

Steffen, Frank Daniel
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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The identity of dark matter is one of the greatest puzzles of our Universe. Its solution may be associated with supersymmetry which is a fundamental space-time symmetry that has not been verified experimentally so far. In many supersymmetric extensions of the Standard Model of particle physics, the lightest supersymmetric particle cannot decay and is hence a promising dark matter candidate. The lightest neutralino, which appears already in the minimal supersymmetric model, can be identified as such a candidate in indirect and direct dark matter searches and at future colliders. As the superpartner of the graviton, the gravitino is another candidate for the lightest superparticle that provides a compelling explanation of dark matter. While it will neither be detected in indirect or direct searches nor be produced directly at accelerators, the analysis of late-decaying charged particles can allow for an experimental identification of the gravitino at future accelerators. In this way, the upcoming experiments at the CERN Large Hadron Collider may become a key to the understanding of our Universe.; Comment: 14 pages, 4 figures, to appear in the Jahrbuch 2007 (German only) of the Max Planck Society

## Novel types of anti-ecloud surfaces

Montero, I.; Aguilera, L.; Dávila, M. E.; Nistor, V.; Galán, L.; González, L. A.; Raboso, D.; Pinto, P. Costa; Taborelli, M.; Caspers, F.; Ulrich, U.; Wolk, D.
Tipo: Artigo de Revista Científica
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In high power RF devices for space, secondary electron emission appears as the main parameter governing the multipactor effect and as well as the e-cloud in large accelerators. Critical experimental activities included development of coatings with low secondary electron emission yield (SEY) for steel (large accelerators) and aluminium (space applications). Coatings with surface roughness of high aspect ratio producing the so-call secondary emission suppression effect appear as the selected strategy. In this work a detailed study of the SEY of these technological coatings and also the experimental deposition methods (PVD and electrochemical) are presented. The coating-design approach selected for new low SEY coatings include rough metals (Ag, Au, Al), rough alloys (NEG), particulated and magnetized surfaces, and also graphene like coatings. It was found that surface roughness also mitigate the SEY deterioration due to aging processes.; Comment: 4 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba, Italy; CERN Yellow Report CERN-2013-002, pp.153-156

## A physical interpretation of the cubic map coefficients describing the electron cloud evolution

Demma, T.; Petracca, S.; Stabile, A.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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The Electron Cloud, an undesirable physical phenomena in the accelerators, develops quickly as photons striking the vacuum chamberwall knock out electrons that are then accelerated by the beam, gain energy, and strike the chamber again, producing more electrons. The interaction between the electron cloud and a beam leads to the electron cloud effects such as single- and multi-bunch instability, tune shift, increase of pressure and particularly can limit the ability of recently build or planned accelerators to reach their design parameters. We report a principal results about the analytical study to understanding a such dynamics of electrons.; Comment: 7 pages 9 figures, Proceedings of XVIII RINEm

## AC/RF Superconductivity

Ciovati, G.
Tipo: Artigo de Revista Científica
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This contribution provides a brief introduction to AC/RF superconductivity, with an emphasis on application to accelerators. The topics covered include the surface impedance of normal conductors and superconductors, the residual resistance, the field dependence of the surface resistance, and the superheating field.; Comment: 19 pages, contribution to the CAS-CERN Accelerator School: Superconductivity for Accelerators, Erice, Italy, 24 April - 4 May 2013, edited by R. Bailey

## Possible evidence for the pulsed X-ray emission from outer gap in PSR B1937+21?

Wang, H. G.; Xu, R. X.; Qiao, G. J.
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
16.63%
The fastest millisecond pulsar PSR B1937+21 presents an interpulse separated from the main pulse by nealy 180^o at radio frequencies. Recently, the ASCA observations (Takahashi et al. 2001) detected pulsed X-ray emission from this pulsar. Only a single narrow X-ray pulse is observed, which is coincident with the radio interpulse in phase. We investigate the possible origin of the pulsed X-rays from the polar cap (PC) accelerators or the outer gap (OG) accelerators in the frame of PC model and OG model, respectively, by assuming a dipolar magnetic field structure and the same radio emission pattern from its poles for the pulsar. The OG model can naturally explain the main observational facts. For the PC model, the coincidence between the X-ray pulse and the radio interpulse can not be reproduced in the assumed case. However when considering possible deviation from our assumption, PC model may still be valid for this pulsar in some cases.; Comment: 7 pages, to appear in Astrophysical Journal, or at http://vega.bac.pku.edu.cn/~rxxu/publications/P/wxq02.ps.gz

## Muon Acceleration in Cosmic-ray Sources

Klein, Spencer R.; Mikkelsen, Rune; Tjus, Julia K. Becker
Tipo: Artigo de Revista Científica
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Many models of ultra-high energy cosmic-ray production involve acceleration in linear accelerators located in Gamma-Ray Bursts magnetars, or other sources. These source models require very high accelerating gradients, $10^{13}$ keV/cm, with the minimum gradient set by the length of the source. At gradients above 1.6 keV/cm, muons produced by hadronic interactions undergo significant acceleration before they decay. This acceleration hardens the neutrino energy spectrum and greatly increases the high-energy neutrino flux. We rule out many models of linear acceleration, setting strong constraints on plasma wakefield accelerators and on models for sources like Gamma Ray Bursts and magnetars.; Comment: 5 pgs. submitted for publication

## Charged current weak electroproduction of Delta resonance

Alvarez-Ruso, L.; Singh, S. K.; Vacas, M. J. Vicente
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We study the weak production of $\Delta$ (i.e. $e^{-} + p \to \Delta^{0}+ \nu_{e}$ and $e^{+} + p \to \Delta^{++} + \bar{\nu}_{e}$) in the intermediate energy range corresponding to the Mainz and TJNAF electron accelerators. The differential cross sections $\sigma(\theta)$ are found to be of the order of $10^{-39}$ cm$^2$/sr, over a range of angles which increases with energy. The possibility of observing these reactions with the high luminosities available at these accelerators, and studying the weak N-$\Delta$ transition form factors through these reactions is discussed. The production cross section of N$^*(1440)$ in the kinematic region of $\Delta$ production is also estimated and found to be small.; Comment: 19 pages, REVTEX, 4 figures

## The $np \to \Lambda p$ reaction within a one-meson-exchange model

Parreño, A.; Ramos, A.; Kelkar, N. G.; Bennhold, C.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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With the advent of high precision proton accelerators, such as COSY in J\"ulich, or with medium energy accelerators like RCNP at Osaka, it may become possible to perform a direct study of the strangeness changing hadronic $np \to \Lambda p$ process. In this work we study this reaction using a one-meson-exchange weak potential and realistic $NN$ and $YN$ strong interactions to distort both the initial $np$ and the final $\Lambda p$ states. The calculated cross sections are very sensitive to the different mesons included in the weak transition, as well as to the strong $YN$ interaction model that generates the distorted final state.; Comment: 4 pages, 3 figures. Talk presented at the Conference on "Mesons and Light Nuclei", Prague-Pruhonice, Aug. 31 - Sep. 4 (1998)

## Beam Tests of Beampipe Coatings for Electron Cloud Mitigation in Fermilab Main Injector

Backfish, Michael; Eldred, Jeffrey; Tan, Cheng Yang; Zwaska, Robert
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Electron cloud beam instabilities are an important consideration in virtually all high-energy particle accelerators and could pose a formidable challenge to forthcoming high-intensity accelerator upgrades. Dedicated tests have shown beampipe coatings dramatically reduce the density of electron cloud in particle accelerators. In this work, we evaluate the performance of titanium nitride, amorphous carbon, and diamond-like carbon as beampipe coatings for the mitigation of electron cloud in the Fermilab Main Injector. Altogether our tests represent 2700 ampere-hours of proton operation spanning five years. Three electron cloud detectors, retarding field analyzers, are installed in a straight section and allow a direct comparison between the electron flux in the coated and uncoated stainless steel beampipe. We characterize the electron flux as a function of intensity up to a maximum of 50 trillion protons per cycle. Each beampipe material conditions in response to electron bombardment from the electron cloud and we track the changes in these materials as a function of time and the number of absorbed electrons. Contamination from an unexpected vacuum leak revealed a potential vulnerability in the amorphous carbon beampipe coating. We measure the energy spectrum of electrons incident on the stainless steel...

## Research on the secondary electron yield of tizrv-pd thin film coatings

Wang, Jie; Xu, Yan-Hui; Zhang, Bo; Wang, Yong; Wei, Wei
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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In particle accelerators, the build-up of electron cloud may have important influence on beam quality. Especially for the positron and proton accelerators, massive electrons lead to electron cloud, which affects the stability, energy, emittance and beam life adversely. A secondary electron emission (SEE) measurement system has been designed and used to study the SEE of palladium (Pd), TiZrV and TiZrV-Pd with an independently adjustable energy from 50 eV to 5 keV. Here, we obtained the characteristics of the SEE from Pd, TiZrV and TiZrV-Pd film coatings with different thickness under ultrahigh-vacuum (UHV) conditions. Moreover, the maximum secondary electron yield (SEY), {\delta}max, of the Pd, TiZrV and TiZrV-Pd film coatings under different primary electron doses were obtained, respectively. Finally, the variation of the secondary electron yield with the incident electron energy will be discussed for Pd, TiZrV and TiZrV-Pd thin film coatings. Low SEY is a new advantage of TiZrV-Pd films, besides high H2 absorption ability and prolonging the lifetime of TiZrV film, which will be of great value in the design of beam screen for Super Proton-Proton Collider (SPPC).

## Zgoubi: A startup guide for the complete beginner

Pressman, Annette; Hock, Kai
Tipo: Artigo de Revista Científica