J. Tonge

Interpenetrating Plasma Shells: Near-Equipartition Magnetic Field Generation and Nonthermal Particle Acceleration

We present the first three-dimensional fully kinetic electromagnetic relativistic particle-in-cell simulations of the collision of two interpenetrating plasma shells. The highly accurate plasma-kinetic particle-in-cell (with the total of 108 particles) parallel code OSIRIS has been used. Our simulations show (1) the generation of long-lived near-equipartition (electro)magnetic fields, (2) nonthermal particle acceleration, and (3) short-scale to long-scale magnetic field evolution, in the collision region. Our results provide new insights into the magnetic field generation and particle acceleration in relativistic and subrelativistic colliding streams of particles, which are present in gamma-ray bursters, supernova remnants, relativistic jets, pulsar winds, etc.

On the role of the purely transverse Weibel instability in fast ignitor scenarios

The growth rate for the purely transverse Weibel instability is determined from relativistic kinetic theory using a waterbag distribution function in the momenta perpendicular to the main propagation direction of the beam. A parametric study is presented for conditions relevant to the fast ignitor. It is shown that for expected parameters the purely transverse Weibel instability will be significantly suppressed or even eliminated due to the transverse energy spread or emittance.

One-to-one direct modeling of experiments and astrophysical scenarios: pushing the envelope on kinetic plasma simulations

There are many astrophysical and laboratory scenarios where kinetic effects play an important role. These range from astrophysical shocks and plasma shell collisions to high intensity laser–plasma interactions, with applications to fast ignition and particle acceleration. Further understanding of these scenarios requires detailed numerical modeling, but fully relativistic kinetic codes are computationally intensive, and the goal of one-to-one direct modeling of such scenarios and direct comparison with experimental results is still difficult to achieve. In this paper we discuss the issues involved in performing kinetic plasma simulations of experiments and astrophysical scenarios, focusing on what needs to be achieved for one-to-one direct modeling and the computational requirements involved. We focus on code efficiency and new algorithms, specifically on parallel scalability issues, namely, on dynamic load balancing, and on high-order interpolation and boosted frame simulations to optimize simulation performance. We also discuss the new visualization and data mining tools required for these numerical experiments and recent simulation work illustrating these techniques is also presented.

Three-dimensional Weibel instability in astrophysical scenarios

Near equipartition magnetic fields are predicted by gamma ray bursters models and astronomical observations, in general associated with shocks or regions with colliding streams of particles. These scenarios require the conversion of kinetic energy in the outgoing plasma shells into B-fields. How the magnetic fields are generated and how particles are accelerated is still an open question, that can only be definitely addressed via fully kinetic three-dimensional (3D) numerical simulations. These shocks are collisionless because dissipation is dominated by wave–particle interactions, i.e., it is accomplished by particle scattering in turbulent electromagnetic fields generated at the shock front, or equivalently the mean free path is much longer than the shock front thickness (a few collisionless skin depths or a few Larmor radii, in magnetized plasmas). Plasma instabilities driven by streaming particles, such as the Weibel instability, are responsible for the excitation of these turbulent electromagnetic fields. Three-dimensional fully kinetic electromagnetic relativistic particle-in-cell simulations for the collision of two interpenetrating plasma shells were performed using the code OSIRIS [Fonseca et al., Lect. Notes Comput. Sci. 2331, 342 (2002)], showing (i) the generation of long-lived near-equipartition quasistatic (electro)magnetic fields, (ii) nonthermal particle acceleration, and (iii) short-scale to long-scale B-field evolution. These results may be important to understand magnetic field generation and particle acceleration in relativistic collisionless shock fronts, in gamma ray bursters, pulsar winds, and radio supernovae, and open the way to the full 3D kinetic modeling of relativistic shocks.Near equipartition magnetic fields are predicted by gamma ray bursters models and astronomical observations, in general associated with shocks or regions with colliding streams of particles. These scenarios require the conversion of kinetic energy in the outgoing plasma shells into B-fields. How the magnetic fields are generated and how particles are accelerated is still an open question, that can only be definitely addressed via fully kinetic three-dimensional (3D) numerical simulations. These shocks are collisionless because dissipation is dominated by wave–particle interactions, i.e., it is accomplished by particle scattering in turbulent electromagnetic fields generated at the shock front, or equivalently the mean free path is much longer than the shock front thickness (a few collisionless skin depths or a few Larmor radii, in magnetized plasmas). Plasma instabilities driven by streaming particles, such as the Weibel instability, are responsible for the excitation of these turbulent electromagnetic fi...

A global simulation for laser-driven MeV electrons in 50-μm-diameter fast ignition targetsa)

The results from 2.5-dimensional particle-in-cell simulations for the interaction of a picosecond-long ignition laser pulse with a plasma pellet of 50-μm diameter and 40 critical density are presented. The high-density pellet is surrounded by an underdense corona and is isolated by a vacuum region from the simulation box boundary. The laser pulse is shown to filament and create density channels on the laser-plasma interface. The density channels increase the laser absorption efficiency and help generate an energetic electron distribution with a large angular spread. The combined distribution of the forward-going energetic electrons and the induced return electrons is marginally unstable to the current filament instability. The ions play an important role in neutralizing the space charges induced by the temperature disparity between different electron groups. No global coalescing of the current filaments resulted from the instability is observed, consistent with the observed large angular spread of the ene...

Cyclotron maser radiation from astrophysical shocks

One of the most popular coherent radio emission mechanisms is electron cyclotron maser instability. In this article we demonstrate that electron cyclotron maser emission is directly associated with particular types of charged particle acceleration such as turbulence and shocks commonly inferred in astrophysical plasmas.

Evolution of a relativistic electron beam–plasma return current system

Evolution of a relativistic electron beam-plasma return current system has been studied using particle-in-cell simulations. The mode number-resolved linear growth rates of the oblique instabilities that the system suffers generally agree with the existing theory [A. Bret et al., Phys. Rev. E 72, 016403 (2005)]. The comparison of in- and out-of-plane simulations shows that two-stream type of instabilities dominates the early stage of energy transfer from the beam drift energy to the beam and plasma thermal energy. The end stage of the nonlinear evolution is dominated Weibel/filament type of instabilities, resulting a beam with a moderately increased angular spread, reduced drift energy, and no reduction in the initial cross section.

A simulation study of fast ignition with ultrahigh intensity lasersa)

The coupling efficiency between the ignition laser and the target core for the fast ignition concept is studied using two-dimensional particle-in-cell simulations. The details of the energy transport within the weakly collisional overdense plasma of a fast ignition target are examined by performing a series of particle-in-cell simulations, where the intensity incident on a 100 times critical plasma with 50 μm radius is varied between each simulation. The simulations show that the peak energy flux of the ignition electrons is significantly lowered as the electrons traverse the collisionless plasma from the critical density through a weakly collisional overdense plasma region. This allows higher intensity lasers to be used thereby improving the coupling efficiency. In addition, we find that a higher percentage of the ignition laser energy is delivered to the core of the simulation target at higher intensity. The coupling efficiency increases in time during the simulations which are run for 2.5 ps. For a las...

Three-dimensional particle-in-cell simulations of the Weibel instability in electron-positron plasmas

Three-dimensional (3-D) particle-in-cell simulations for the collision of an electron cloud with a positron cloud are presented, for conditions where the Weibel instability can develop. The 3-D features of the Weibel instability are identified in the magnetic field structure generated in the instability, and in the shape of the current filaments.

Efficient modeling of laser–plasma interactions in high energy density scenarios

We describe how a new framework for coupling a full-particle-in-cell (PIC) algorithm with a reduced PIC algorithm has been implemented into the OSIRIS code. We show that OSIRIS, with this new hybrid-PIC algorithm, can efficiently and accurately model high energy density scenarios such as ion acceleration in laser–solid interactions and fast ignition of fusion targets. We model, for the first time, the full-density range of a fast ignition target in a fully self-consistent hybrid-PIC simulation, illustrating the possibility of stopping the laser generated electron flux at the core region with relatively high efficiencies. Computational speedups greater than 1000 times are demonstrated, opening the way for full-scale multi-dimensional modeling of high energy density scenarios and the guiding of future experiments.