L. O. Silva

University of Lisbon, Lisboa, Lisbon, Portugal

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Publications (288)673.31 Total impact

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    Full-text · Article · Jan 2016 · Nature Communications
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    ABSTRACT: An investigation of magnetic fields generated in an expanding bubble of plasma with misaligned temperature and density gradients (driving the Biermann battery mechanism) is performed. With gradient scales $L$, large-scale magnetic fields are generated by the Biermann battery mechanism with plasma $\beta \sim 1$, as long as $L$ is comparable to the ion inertial length $d_i$. For larger system sizes, $L/d_e > 100$ (where $d_e$ is the electron inertial length), the Weibel instability generates magnetic fields of similar magnitude but with wavenumber $k d_e \sim 0.2$. In both cases, the growth and saturation of these fields have a weak dependence on mass ratio $m_i/m_e$, indicating electron mediated physics. A scan in system size is performed at $m_i/m_e = 2000$, showing agreement with previous results with $m_i/m_e = 25$. In addition, the instability found at large system sizes is quantitatively demonstrated to be the Weibel instability. Furthermore, magnetic and electric energy spectra at scales below the electron Larmor radius are found to exhibit power law behavior with spectral indices $-16/3$ and $-7/3$, respectively.
    No preview · Article · Dec 2015
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    DESCRIPTION: The paper presents a theoretical work on the dynamics of Coulomb explosion for spher- ical nanoplasmas composed by two different ion species. Particular attention has been dedicated to study the energy spectra of the ions with the larger charge-to-mass ratio. The connection between the formation of shock shells and the energy spread of the ions has been the object of a detailed analysis, showing that under particular conditions the width of the asymptotic energy spectrum tends to become very narrow, which leads to a multi-valued ion phase-space. The conditions to generate a quasi mono-energetic ion spectrum have been rigorously demonstrated and verified by numerical simulations, using a technique that, exploiting the spherical symmetry of the problem, allows one to obtain very accurate and precise results.
    Full-text · Research · Nov 2015
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    Full-text · Dataset · Nov 2015
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    Full-text · Dataset · Sep 2015
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    A. Stockem Novo · A. Bret · R. A. Fonseca · L. O. Silva
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    ABSTRACT: Collisionless shocks occur in various fields of physics. In the context of space and astrophysics they have been investigated for many decades. However, a thorough understanding of shock formation and particle acceleration is still missing. Collisionless shocks can be distinguished into electromagnetic and electrostatic shocks. Electromagnetic shocks are of importance mainly in astrophysical environments and they are mediated by the Weibel or filamentation instability. In such shocks, charged particles gain energy by diffusive shock acceleration. Electrostatic shocks are characterized by a strong electrostatic field, which leads to electron trapping. Ions are accelerated by reflection from the electrostatic potential. Shock formation and particle acceleration will be discussed in theory and simulations.
    Full-text · Article · Sep 2015 · Plasma Physics and Controlled Fusion
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    ABSTRACT: An emissivity formula is derived using the generalised Fermi-Weizacker-Williams method of virtual photons which accounts for the recoil the charged particle experiences as it emits radiation. It is found that through this derivation the formula obtained by Sokolov et al using QED perturbation theory is recovered. The corrected emissivity formula is applied to nonlinear Thomson scattering scenarios in the transition from the classical to the quantum regime, for small values of the nonlinear quantum parameter \chi. Good agreement is found between this method and a QED probabilistic approach for scenarios where both are valid. In addition, signatures of the quantum corrections are identified and explored.
    Full-text · Article · Jul 2015 · Plasma Physics and Controlled Fusion
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    ABSTRACT: The microphysics of relativistic collisionless sheared flows is investigated in a configuration consisting of a globally neutral, relativistic $e^-e^+$ beam streaming through a hollow plasma/dielectric channel. We show through multidimensional PIC simulations that this scenario excites the Mushroom instability (MI), a transverse shear instability on the electron-scale, when there is no overlap (no contact) between the $e^-e^+$ beam and the walls of the hollow plasma channel. The onset of the MI leads to the conversion of the beam's kinetic energy into magnetic (and electric) field energy, effectively slowing down a globally neutral body in the absence of contact. The collisionless shear physics explored in this configuration may operate in astrophysical environments, particularly in highly relativistic and supersonic settings where macroscopic shear processes are stable.
    Full-text · Article · Jun 2015 · Plasma Physics and Controlled Fusion
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    ABSTRACT: In this paper we show a plausible mechanism that could lead to the formation of the Dark Lanes in Lunar Swirls, and the electromagnetic shielding of the lunar surface that results in the preservation of the white colour of the lunar regolith. We present the results of a fully self-consistent 2 and 3 dimensional particle-in-cell simulations of mini-magnetospheres that form above the lunar surface and show that they are consistent with the formation of `lunar swirls' such as the archetypal formation Reiner Gamma. The simulations show how the microphysics of the deflection/shielding of plasma operates from a kinetic-scale cavity, and show that this interaction leads to a footprint with sharp features that could be the mechanism behind the generation of `dark lanes'. The physics of mini-magnetospheres is described and shown to be controlled by space-charge fields arising due to the magnetized electrons and unmagnetized ions. A comparison between model and observation is shown for a number of key plasma parameters.
    Full-text · Article · May 2015
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    E. P. Alves · T. Grismayer · R. A. Fonseca · L. O. Silva
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    ABSTRACT: Electron-scale surface waves are shown to be unstable in the transverse plane of a shear flow in an initially unmagnetized plasma, unlike in the (magneto)hydrodynamics case. It is found that these unstable modes have a higher growth rate than the closely related electron-scale Kelvin-Helmholtz instability in relativistic shears. Multidimensional particle-in-cell simulations verify the analytic results and further reveal the emergence of mushroom-like electron density structures in the nonlinear phase of the instability, similar to those observed in the Rayleigh Taylor instability despite the great disparity in scales and different underlying physics. Macroscopic ($\gg c/\omega_{pe}$) fields are shown to be generated by these microscopic shear instabilities, which are relevant for particle acceleration, radiation emission and to seed MHD processes at long time-scales.
    Full-text · Article · May 2015 · Physical Review E
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    ABSTRACT: Electron-positron pair plasmas represent a unique state of matter, whereby there exists an intrinsic and complete symmetry between negatively charged (matter) and positively charged (antimatter) particles. These plasmas play a fundamental role in the dynamics of ultra-massive astrophysical objects and are believed to be associated with the emission of ultra-bright gamma-ray bursts. Despite extensive theoretical modelling, our knowledge of this state of matter is still speculative, owing to the extreme difficulty in recreating neutral matter-antimatter plasmas in the laboratory. Here we show that, by using a compact laser-driven setup, ion-free electron-positron plasmas with unique characteristics can be produced. Their charge neutrality (same amount of matter and antimatter), high-density and small divergence finally open up the possibility of studying electron-positron plasmas in controlled laboratory experiments.
    Full-text · Article · Apr 2015 · Nature Communications
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    ABSTRACT: The spatial-temporal evolution of the purely transverse current filamentation instability is analyzed by deriving a single partial differential equation for the instability and obtaining the analytical solutions for the spatially and temporally growing current filament mode. When the beam front always encounters fresh plasma, our analysis shows that the instability grows spatially from the beam front to the back up to a certain critical beam length; then the instability acquires a purely temporal growth. This critical beam length increases linearly with time and in the non-relativistic regime it is proportional to the beam velocity. In the relativistic regime the critical length is inversely proportional to the cube of the beam Lorentz factor $\gamma_{0b}$. Thus, in the ultra-relativistic regime the instability immediately acquires a purely temporal growth all over the beam. The analytical results are in good agreement with multidimensional particle-in-cell simulations performed with OSIRIS. Relevance of current study to recent and future experiments on fireball beams is also addressed.
    Full-text · Article · Mar 2015 · New Journal of Physics
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    ABSTRACT: The onset and evolution of magnetic fields in laboratory and astrophysical plasmas is determined by several mechanisms, including instabilities, dynamo effects and ultra-high energy particle flows through gas, plasma and interstellar-media. These processes are relevant over a wide range of conditions, from cosmic ray acceleration and gamma ray bursts to nuclear fusion in stars. The disparate temporal and spatial scales where each operates can be reconciled by scaling parameters that enable to recreate astrophysical conditions in the laboratory. Here we unveil a new mechanism by which the flow of ultra-energetic particles can strongly magnetize the boundary between the plasma and the non-ionized gas to magnetic fields up to 10-100 Tesla (micro Tesla in astrophysical conditions). The physics is observed from the first time-resolved large scale magnetic field measurements obtained in a laser wakefield accelerator. Particle-in-cell simulations capturing the global plasma and field dynamics over the full plasma length confirm the experimental measurements. These results open new paths for the exploration and modelling of ultra high energy particle driven magnetic field generation in the laboratory.
    Full-text · Article · Mar 2015 · Nature Physics
  • M Vranic · J L Martins · J Vieira · R A Fonseca · L O Silva
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    ABSTRACT: Using full-scale 3D particle-in-cell simulations we show that the radiation reaction dominated regime can be reached in an all-optical configuration through the collision of a ∼1 GeV laser wakefield accelerated electron bunch with a counterpropagating laser pulse. In this configuration the radiation reaction significantly reduces the energy of the particle bunch, thus providing clear experimental signatures for the process with currently available lasers. We also show that the transition between the classical and quantum radiation reaction could be investigated in the same configuration with laser intensities of 10^{23} W/cm^{2}.
    No preview · Article · Sep 2014 · Physical Review Letters
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    J. Vieira · R. A. Fonseca · W. B. Mori · L. O. Silva
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    ABSTRACT: We explore the role of the background plasma ion motion in self-modulated plasma wakefield accelerators. We employ J. 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.
    Full-text · Article · Sep 2014 · Physics of Plasmas
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    ABSTRACT: The energy transfer by stimulated Brillouin backscatter from a long pump pulse (15 ps) to a short seed pulse (1 ps) has been investigated in a proof-of-principle demonstration experiment. The two pulses were both amplified in different beamlines of a Nd:glass laser system, had a central wavelength of 1054 nm and a spectral bandwidth of 2 nm, and crossed each other in an underdense plasma in a counter-propagating geometry, off-set by \$\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}10^\circ \$. It is shown that the energy transfer and the wavelength of the generated Brillouin peak depend on the plasma density, the intensity of the laser pulses, and the competition between two-plasmon decay and stimulated Raman scatter instabilities. The highest obtained energy transfer from pump to probe pulse is 2.5%, at a plasma density of \$0.17 n_{cr}\$, and this energy transfer increases significantly with plasma density. Therefore, our results suggest that much higher efficiencies can be obtained when higher densities (above \$0.25 n_{cr}\$) are used.
    Full-text · Article · Sep 2014
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    A. Stockem · T. Grismayer · R. A. Fonseca · L. O. Silva
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    ABSTRACT: A new magnetic field generation mechanism in electrostatic shocks is found, which can produce fields with magnetic energy density as high as 0.01 of the kinetic energy density of the flows on time scales $ \tilde \, 10^4 \, {\omega}_{pe}^{-1}$. Electron trapping during the shock formation process creates a strong temperature anisotropy in the distribution function, giving rise to the pure Weibel instability. The generated magnetic field is well-confined to the downstream region of the electrostatic shock. The shock formation process is not modified and the features of the shock front responsible for ion acceleration, which are currently probed in laser-plasma laboratory experiments, are maintained. However, such a strong magnetic field determines the particle trajectories downstream and has the potential to modify the signatures of the collisionless shock.
    Full-text · Article · Aug 2014 · Physical Review Letters
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    ABSTRACT: It is shown that co-linear injection of electrons or positrons into the wakefield of the self-modulating particle beam is possible and ensures high energy gain. The witness beam must co-propagate with the tail part of the driver, since the plasma wave phase velocity there can exceed the light velocity, which is necessary for efficient acceleration. If the witness beam is many wakefield periods long, then the trapped charge is limited by beam loading effects. The initial trapping is better for positrons, but at the acceleration stage a considerable fraction of positrons is lost from the wave. For efficient trapping of electrons, the plasma boundary must be sharp, with the density transition region shorter than several centimeters. Positrons are not susceptible to the initial plasma density gradient.
    Full-text · Article · Aug 2014 · Physics of Plasmas
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    J Vieira · L D Amorim · Y Fang · W B Mori · P Muggli · L O Silva
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    ABSTRACT: We study the evolution of the self-modulation instability for bunches with finite rise times. Using particle-in-cell simulations we show that, unlike with long bunches with sharp rise times, there are large variations of the wake amplitude and phase velocity with finite rise time bunches. These results show that bunches with sharp rise times are important to seed the self-modulation instability and to ensure stable acceleration.
    Full-text · Article · Jul 2014 · Plasma Physics and Controlled Fusion
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    ABSTRACT: Coulomb explosion of pure ion nanoplasmas is an important problem in the field of ultra intense laser-cluster interaction with relevance for plasma physics, fusion research [1, 2] and imaging by "diffraction before destruction" [3]. In the present paper, a study of Coulomb explosion in composite clusters consisting of two atomic species is presented. The work focuses on heavy-light systems made of hydride molecules composed by carbon and hydrogen, in order to collect valuable information for coherent diffractive imaging [4]. Numerical simulations have been performed by using the shell method [5] that, despite of its simplicity, allows one to capture all the relevant physics involved with a reduced computational time. Moreover, a theoretical model, which is useful for a deep comprehension of the explosion dynamics, has been developed; results have been compared with numerical simulations showing perfect agreement. Numerical results The explosion of a a pure ion sphere of initial radius R 0 = 20 nm composed by N 0 4.5 · 10 5 ions for an initial density of n 0 = 10 22 cm −3 has been modeled. The cluster is constituted by a mix of 30% carbon ions ionized once and 70% hydrogen ions, so that m C /m H = 12, q C /q H = 1 and N H /N 0 = 0.7. At the initial time, ions are at rest and uniformly distributed in the sphere. The evolution of the electric field and the hydrogen density are shown in Fig. 1. At the initial time, hydrogen and carbon ions contribute to create a monotonic increasing electric field, which accelerates the H + ions more with respect to C + ions, because of their smaller mass to charge ratio. Consequently, the lighter particles can overtake the heavier ones and propagate ahead of them. The radial electric field, which has a linear behavior inside the bulk sphere of the C + ions, decreases approximately as 1/r 2 outside. Therefore, faster light ions coming from the bulk tend to reach the ones initially on the periphery of the cluster, which are slower due to the decaying field, forming a thin hydrogen shell. Analytical model The species with a larger q/m moves faster with respect to the other, creating two concentric spherical regions, S 1 and S 2 , with radius R 1 (t) and R 2 (t) ≤ R 1 (t). The sphere S 2 contains a mixture of light and heavy particles, R 2 representing the frontline of the heavy ions. Instead, the
    Full-text · Conference Paper · Jun 2014

Publication Stats

3k Citations
673.31 Total Impact Points

Institutions

  • 2008-2015
    • University of Lisbon
      Lisboa, Lisbon, Portugal
    • Lawrence Berkeley National Laboratory
      Berkeley, California, United States
  • 1997-2015
    • Instituto Técnico y Cultural
      Santa Clara de Portugal, Michoacán, Mexico
  • 2014
    • New University of Lisbon
      Lisboa, Lisbon, Portugal
  • 2013
    • University of Nevada, Reno
      • Department of Physics
      Reno, Nevada, United States
  • 2011-2013
    • ISCTE-Instituto Universitário de Lisboa
      Lisboa, Lisbon, Portugal
    • Imperial College London
      • Department of Physics
      Londinium, England, United Kingdom
  • 2001-2013
    • Instituto Superior de Contabilidade e Administração de Lisboa
      Lisboa, Lisbon, Portugal
  • 1998-2012
    • University of California, Los Angeles
      • • Department of Electrical Engineering
      • • Department of Physics and Astronomy
      Los Angeles, California, United States
    • Ruhr-Universität Bochum
      Bochum, North Rhine-Westphalia, Germany
  • 2010
    • Iowa State University
      • Department of Physics and Astronomy
      Ames, Iowa, United States
  • 2007
    • ISG | Business & Economics School
      Lisboa, Lisbon, Portugal
  • 2004
    • University of Strathclyde
      • Department of Physics
      Glasgow, Scotland, United Kingdom
  • 2003
    • Technical University of Lisbon
      • Centro de Fisica dos Plasmas
      Lisboa, Lisbon, Portugal