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We derive the kinetic theory of fluctuations in physically and numerically stable particle-in-cell (PIC) simulations of electrostatic plasmas. The starting point is the single-time correlations at the simulation start between the statistical fluctuations of weighted densities of macroparticle centers in the plasma particle phase-space. The single-time correlations at all time steps and in each spatial grid cell are then determined from the Laplace-Fourier transforms of the discretized Klimontovich-like equation for the macroparticles and Maxwell's equations for the fields as computed by modern PIC codes. We recover the expressions for the electrostatic field and the plasma particle density fluctuation autocorrelations spectra as well as the kinetic equations describing the average evolution of PIC-simulated plasma particles, first derived by Langdon in 1970, using a macroparticle test approach perturbing a discretized Vlasovian plasma and then averaging the obtained physical quantity over the initial macroparticle velocity distribution. We generalize and extend these results to the modern algorithms in PIC codes and using arbitrary macroparticle weights. Analytical estimates of statistical fluctuations single-time correlation amplitudes are derived as a function of the plasma simulation parameters, using the central limit theorem in the limit of a large number of macroparticles per cell. The theory is then used to analyze the ensemble averaging technique of PIC simulations where statistical averages are performed over ensembles of PIC simulations, modeling the same plasma physics problem but using different statistical realizations of the initial distribution functions of the macroparticles., 38 pages, 11 figures

There is considerable interest in the propagation dynamics of intense neutrino beams in a background dispersive medium such as dense plasmas, particularly in the search for a mechanism to explain the dynamics of type II supernovae. Neutrino interactions with matter are usually considered as single particle interactions. All the single particle mechanisms describing the dynamical properties of neutrinos in matter are analogous with the processes involving single electron interactions with a medium such as Compton scattering, Cerenkov radiation, etc. However, it is well known that beams of electrons moving through a plasma give rise to a new class of processes known as collective interactions, such as two stream instabilities, which result in either the absorption or generation of plasma waves. Employing the relativistic kinetic equations for neutrinos interacting with dense plasmas via the weak force, we explore collective plasma streaming instabilities driven by neutrino beams. We examine the anomalous transfer between neutrinos and dense plasma via excitation of electron plasma waves. The nonlinear coupling between an intense neutrino beam and a plasma reveals the presence of two regimes, a hydrodynamic regime and a kinetic regime. The latter is responsible for Landau damping or growth of electron plasma waves. In dense fusion stellar plasmas neutrino Landau damping can play a significant role as an additional stellar plasma cooling process. Another interesting result is an asymmetry in the momentum balance imported by the neutrinos to the core of the exploding star due to symmetry breaking by the collapsed star's magnetic fields. This results in a directed velocity of the resulting neutron star or pulsar, explaining the so called 'birth' velocity.

We present full set of coupled equations describing the weakly relativistic dynamics of a laser in a plasma with transverse inhomogeneity. We apply variational principle approach to obtain these coupled equations governing laser spot-size, transverse wavenumber, curvature, transverse centroid, etc. We observe that such plasma inhomogeneity can lead to stronger self-focusing. We further discuss the guiding conditions of laser in parabolic plasma channels. With the help of multi-dimensional particle in cell simulations the study is extended to the blowout regime of laser wakefield acceleration to show laser as well as self-injected electron bunch steering in plasma to generate unconventional particle trajectories. Our simulation results demonstrate that such transverse inhomogeneities due to asymmetric self focusing lead to asymmetric bubble excitation, thus inducing off-axis self-injection.

We study the evolution of the self-modulation instability using bunches with finite rise times. Using particle-in-cell simulations we show that unlike long bunches with sharp rise times, there are large variations of the wake amplitudes and wake phase velocity when bunches with finite rise times are used. These results show that use of bunches with sharp rise times is important to seed the self-modulation instability and to ensure stable acceleration regimes., 11 pages, 5 figures

Under violent astrophysical conditions, intense fluxes of neutrinos can drive a novel class of plasma instabilities: the electroweak versions of the standard electron- and photon-driven forward scattering instabilities. Employing the relativistic kinetic equations for the neutrinos interacting with a plasma via the weak interaction force, we explore the different collective plasma instabilities driven by neutrinos. We examine the anomalous energy transfer between the neutrinos and the background plasma via excitation of electron plasma waves (neutrino streaming instability), and the generation of quasi-static B fields (electroweak Weibel instability). The relevance of the electroweak plasma instabilities for the extreme conditions occurring in the lepton era of the early universe, supernovae II, and gamma ray bursters is pointed out. The impact on type II supernovae dynamics is examined, and estimates for the energy deposited via collective mechanisms behind the outgoing shock are presented. We show that electroweak plasma instabilities might play an important role in the re-energization of the stalled shock in SNe II.

A review of recent work on collective neutrino-plasma interactions is presented. The basic physical concepts of this new field as well as some possible astrophysical problems where the physics of collective neutrino-plasma interactions can have a radical impact, are discussed.

New acceleration technology is mandatory for the future elucidation of fundamental particles and their interactions. A promising approach is to exploit the properties of plasmas. Past research has focused on creating large-amplitude plasma waves by injecting an intense laser pulse or an electron bunch into the plasma. However, the maximum energy gain of electrons accelerated in a single plasma stage is limited by the energy of the driver. Proton bunches are the most promising drivers of wakefields to accelerate electrons to the TeV energy scale in a single stage. An experimental program at CERN -- the AWAKE experiment -- has been launched to study in detail the important physical processes and to demonstrate the power of proton-driven plasma wakefield acceleration. Here we review the physical principles and some experimental considerations for a future proton-driven plasma wakefield accelerator., 12 pages, 5 figures, submitted to Plasma Phys. Control. Fusion special edition for LPAW 2013. Small updates after comments from Journal referees

WOS:000327844400033 (Nº de Acesso Web of Science) Collisionless plasma instabilities operating on the electron time scale can be of importance to explain magnetic field generation; however, their role has been addressed in scenarios where velocity shear is absent. We show that, whenever present, velocity shears must be considered in the electron time scale since electromagnetic perturbations are unstable, both in the parallel (two fluids Kelvin-Helmholtz instability) and in the transverse plane of the flow. Using the two-fluid formalism in the limit of cold plasma, we derive the dispersion relations of the instability in the parallel plane. As the instabilities grow, we also show, through a kinetic model, the development of a dc magnetic field, extending all over the shear region, that reaches a typical value at a saturation of eB(dc)(sat)/mc omega(p) similar to beta(0)root gamma(0), where beta(0) and gamma(0) are respectively the normalized velocity and the Lorentz factor of the plasma flow.

It is shown through analytical modeling and numerical simulations that external magnetic fields can relax the self-trapping thresholds in plasma based accelerators. In addition, the transverse location where self-trapping occurs can be selected by adequate choice of the spatial profile of the external magnetic field. We also find that magnetic-field assisted self-injection can lead to the emission of betatron radiation at well defined frequencies. This controlled injection technique could be explored using state-of-the-art magnetic fields in current/next generation plasma/laser wakefield accelerator experiments., 7 pages, 4 figures, accepted for publication in Plasma Physics and Controlled Fusion

We describe how a new framework for coupling a full-PIC algorithm with a reduced PIC algorithm has been implemented into the code OSIRIS. 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 for the guiding of future experiments., 14 pages, 8 figures

The modulational instability that occurs during the interaction of a long laser pulse and its own wakefield in an underdense plasma has been studied experimentally and theoretically. Recent experiments using laser pulses that are several times longer than the wakefield period have yielded transmission spectra that exhibit a series of secondary peaks flanking the main laser peak. These peaks are too closely spaced to be the result of Raman instabilities; their origin was found to be photon acceleration of the laser's photons in the wakefield instead. In the experiments described in this paper, a laser pulse of 50-200 fs containing 300-600 mJ was focused on the edge of a helium gas jet on a 25 νm focal spot. The observed transmission spectra show evidence of both ionization blueshift and modulation by the pulse's wakefield. The transmission spectra have also been modelled using a dedicated photon-kinetic numerical code. The modelling has revealed a direct correlation between the spectral modulations and the amplitude of the excited wakefield. By comparing the measured and simulating spectra, the origin of various spectral characteristics has been explained in terms of photon acceleration. The feasibility of using this effect as a wakefield diagnostic will be discussed. © 2009 IOP Publishing Ltd.

Recently, we investigated the interaction between broadband drift mode turbulence and zonal flows near the edge of a region of magnetized plasma (Trines et al 2005 Phys. Rev. Lett. 94 165002). Our simulation results showed the development of a zonal flow through the modulational instability of the drift wave distribution, as well as the existence of solitary zonal flow structures about an ion gyroradius wide, drifting towards steeper relative density gradients. Both the growth rate of the turbulence and the particle/energy transport across the plasma boundary can be stabilized by adjusting the plasma density gradient. This spontaneous formation of solitary wave structures has also been found in Cluster satellite observations (Trines et al 2007 Phys. Rev. Lett. 99 205006), confirming our earlier theoretical predictions. We will discuss the consequences of our results for our understanding of the Earth's magnetopause, as well as for the study of zonal flows in tokamaks.

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 modelling, but fully relativistic kinetic codes are computationally intensive, and the goal of one-to-one direct modelling 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., 10 pages, 5 figures

Here we describe a new experiment to test the shielding concept of a dipole-like magnetic field and plasma, surrounding a spacecraft forming a 'mini magnetosphere'. Initial laboratory experiments have been conducted to determine the effectiveness of a magnetized plasma barrier to be able to expel an impacting, low beta, supersonic flowing energetic plasma representing the solar wind. Optical and Langmuir probe data of the plasma density, the plasma flow velocity and the intensity of the dipole field clearly show the creation of a narrow transport barrier region and diamagnetic cavity virtually devoid of energetic plasma particles. This demonstrates the potential viability of being able to create a small 'hole' in a solar wind plasma, of the order of the ion Larmor orbit width, in which an inhabited spacecraft could reside in relative safety. The experimental results have been quantitatively compared with a 3D particle-in-cell 'hybrid' code simulation that uses kinetic ions and fluid electrons, showing good qualitative agreement and excellent quantitative agreement. Together the results demonstrate the pivotal role of particle kinetics in determining generic plasma transport barriers.

A numerical study on ion acceleration in electrostatic shock waves is presented, with the aim of determining the best plasma configuration to achieve quasi-monoenergetic ion beams in laser-driven systems. It was recently shown that tailored near-critical density plasmas characterized by a long-scale decreasing rear density profile lead to beams with low energy spread [F. Fi\'uza et al., Physical Review Letters 109, 215001 (2012)]. In this work, a detailed parameter scan investigating different plasma scale lengths is carried out. As result, the optimal plasma spatial scale length that allows for minimizing the energy spread while ensuring a significant reflection of ions by the shock is identified. Furthermore, a new configuration where the required profile has been obtained by coupling micro layers of different densities is proposed. Results show that this new engineered approach is a valid alternative, guaranteeing a low energy spread with a higher level of controllability., Comment: 10 pages, 6 figures

Fully kinetic analytic calculations of an initially Maxwellian distribution with arbitrary density and temperature gradients exhibit the development of temperature anisotropies and magnetic field growth associated with the Biermann battery. The calculation, performed by taking a small order expansion of the ratio of the Debye length to the gradient scale, predicts anisotropies and magnetic fields as a function of space given an arbitrary temperature and density profile. These predictions are shown to qualitatively match the values measured from particle-in-cell simulations, where the development of the Weibel instability occurs at the same location and with a wavenumber aligned with the predicted temperature anisotropy., Comment: 9 pages, 4 figures