arXiv subjects

Ricardo A. Fonseca

Publications and source records attributed to Ricardo A. Fonseca.

25 records · Page 2Linked to original sources

arXiv2014

Enhanced Stopping of Macro-Particles in Particle-in-Cell Simulations

We derive an equation for energy transfer from relativistic charged particles to a cold background plasma appropriate for finite-size particles that are used in particle-in-cell simulation codes. Expressions for one-, two-, and three-dimensional particles are presented, with special attention given to the two-dimensional case. This energy transfer is due to the electric field of the wake set up in the background plasma by the relativistic particle. The enhanced stopping is dependent on the $q^2/m$, where $q$ is the charge and $m$ is the mass of the relativistic particle, and therefore simulation macro-particles with large charge but identical $q/m$ will stop more rapidly. The stopping power also depends on the effective particle shape of the macro-particle. These conclusions are verified in particle-in-cell simulations. We present 2D simulations of test particles, relaxation of high-energy tails, and integrated fast ignition simulations showing that the enhanced drag on macro-particles may adversely affect the results of these simulations in a wide range of high-energy density plasma scenarios. We also describe a particle splitting algorithm which can potentially overcome this problem and show its effect in controlling the stopping of macro-particles.

physics.plasm-ph↗

arXiv2013

Modeling of Laser wakefield acceleration in Lorentz boosted frame using EM-PIC code with spectral solver

Simulating laser wakefield acceleration (LWFA) in a Lorentz boosted frame in which the plasma drifts towards the laser with $v_b$ can speedup the simulation by factors of $γ^2_b=(1-v^2_b/c^2)^{-1}$. In these simulations the relativistic drifting plasma inevitably induces a high frequency numerical instability that contaminates the interested physics. Various approaches have been proposed to mitigate this instability. One approach is to solve Maxwell equations in Fourier space (a spectral solver) as this has been shown to suppress the fastest growing modes of this instability in simple test problems using a simple low pass, ring (in two dimensions), or shell (in three dimensions) filter in Fourier space. We describe the development of a fully parallelized, multi-dimensional, particle-in-cell code that uses a spectral solver to solve Maxwell's equations and that includes the ability to launch a laser using a moving antenna. This new EM-PIC code is called UPIC-EMMA and it is based on the components of the UCLA PIC framework (UPIC). We show that by using UPIC-EMMA, LWFA simulations in the boosted frames with arbitrary $γ_b$ can be conducted without the presence of the numerical instability. We also compare the results of a few LWFA cases for several values of $γ_b$, including lab frame simulations using OSIRIS, a EM-PIC code with a finite difference time domain (FDTD) Maxwell solver. These comparisons include cases in both linear, and nonlinear regimes. We also investigate some issues associated with numerical dispersion in lab and boosted frame simulations and between FDTD and spectral solvers.

physics.comp-ph↗

arXiv2013

Exploiting multi-scale parallelism for large scale numerical modelling of laser wakefield accelerators

A new generation of laser wakefield accelerators, supported by the extreme accelerating fields generated in the interaction of PW-Class lasers and underdense targets, promises the production of high quality electron beams in short distances for multiple applications. Achieving this goal will rely heavily on numerical modeling for further understanding of the underlying physics and identification of optimal regimes, but large scale modeling of these scenarios is computationally heavy and requires efficient use of state-of-the-art Petascale supercomputing systems. We discuss the main difficulties involved in running these simulations and the new developments implemented in the OSIRIS framework to address these issues, ranging from multi-dimensional dynamic load balancing and hybrid distributed / shared memory parallelism to the vectorization of the PIC algorithm. We present the results of the OASCR Joule Metric program on the issue of large scale modeling of LWFA, demonstrating speedups of over 1 order of magnitude on the same hardware. Finally, scalability to over $\sim 10^6$ cores, and sustained performance over $\sim 2$ PFlops is demonstrated, opening the way for large scale modeling of laser wakefield accelerator scenarios.

physics.plasm-ph↗

arXiv2013

Full-scale ab initio 3D PIC simulations of an all-optical radiation reaction configuration at $10^{21}\mathrm{W/cm^2}$

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 $\sim$1 GeV laser wakefield accelerated (LWFA) electron bunch with a counter propagating laser pulse. In this configuration 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 classical and quantum radiation reaction could be investigated in the same configuration with laser intensities of $10^{24}\mathrm{W/cm^2}$.

physics.plasm-ph↗

arXiv2012

Numerical instability due to relativistic plasma drift in EM-PIC simulations

The numerical instability observed in the Electromagnetic-Particle-in-cell (EM-PIC) simulations with a plasma drifting with relativistic velocities is studied using both theory and computer simulations. We derive the numerical dispersion relation for a cold plasma drifting with a relativistic velocity and find an instability attributed to the coupling between the beam modes of the drifting plasma and the electromagnetic modes in the system. The characteristic pattern of the instability in Fourier space for various simulation setups and Maxwell Equation solvers are explored by solving the corresponding numerical dispersion relations. Furthermore, based upon these characteristic patterns we derive an asymptotic expression for the instability growth rate. The asymptotic expression greatly speeds up the calculation of instability growth rate and makes the parameter scan for minimal growth rate feasible even for full three dimensions. The results are compared against simulation results and good agreement is found. These results can be used as a guide to develop possible approaches to mitigate the instability. We examine the use of a spectral solver and show that such a solver when combined with a low pass filter with a cutoff value of $|\vec{k}|$ essentially eliminates the instability while not modifying modes of physical interest. The use of spectral solver also provides minimal errors to electromagnetic modes in the lowest Brillouin zones.

physics.plasm-ph↗

arXiv2004

Long-time evolution of magnetic fields in relativistic GRB shocks

We investigate the long-time evolution of magnetic fields generated by the two-stream instability at ultra- and sub-relativistic astrophysical collisionless shocks. Based on 3D PIC simulation results, we introduce a 2D toy model of interacting current filaments. Within the framework of this model, we demonstrate that the field correlation scale in the region far downstream the shock grows nearly as the light crossing time, lambda(t) ~ ct, thus making the diffusive field dissipation inefficient. The obtained theoretical scaling is tested using numerical PIC simulations. This result extends our understanding of the structure of collisionless shocks in gamma-ray bursts and other astrophysical objects.

astro-ph↗

arXiv2003

Kinetics of Particles in Relativistic Collisionless Shocks

Charged plasma particles form an anisotropic counter-streaming distribution at the front of a shock. In the near- and ultra-relativistic regimes, Weibel (or two-stream) instability produces near-equipartition, chaotic, small-scale magnetic fields. The fields introduce effective collisions and "thermalize" the plasma particles via pitch-angle scattering. The properties of jitter radiation emitted by accelerated electrons from these small-scale fields are markedly different from synchrotron spectra. Here we theoretical summary and the results of recent numerical 3D PIC plasma kinetic simulations. The relation of the obtained results to the theory of gamma-ray bursts is outlined.

astro-ph↗