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We show that the Weibel or currente filamentation instability can lead to the emission of circularly polarized radiation. Using particle-in-cell (PIC) simulations and a radiation post-processing numerical algorithm, we demonstrate that the level of circular polarization increases with the initial plasma magnetization, saturating at ~13% when the magnetization, given by the ratio of magnetic energy density to the electron kinetic energy density, is larger than 0.05. Furthermore, we show that this effect requires an ion-electron mass ratio greater than unity. These findings, which could also be tested in currently available laboratory conditions, show that the recent observation of circular polarization in gamma ray burst afterglows could be attributed to the presence of magnetized current filaments driven by the Weibel or the current filamentation instability., 8 pages, 5 figures submitted to Astrophysical Journal Letters

In this paper, we determine the growth rate of the exponential radiation amplification in the ion-channel laser, where a relativistic electron beam wiggles in a focusing ion channel that can be created in a wakefield accelerator. For the first time the radiation diffraction, which can limit the amplification, is taken into account. The electron beam quality requirements to obtain this amplification are also presented. It is shown that both the beam energy and wiggler parameter spreads should be limited. Two-dimensional and three-dimensional particle-in-cell simulations of the self-consistent ion-channel laser confirm our theoretical predictions. info:eu-repo/semantics/acceptedVersion

Collisionless shocks are ubiquitous in astrophysics and in the laboratory. Recent numerical simulations and experiments have shown how these can arise from the encounter of two collisionless plasma shells. When the shells interpenetrate, the overlapping region turns unstable, triggering the shock formation. As a first step toward a microscopic understanding of the process, we here analyze in detail the initial instability phase. On the one hand, 2D relativistic PIC simulations are performed where two unmagnetized, symmetric, and initially cold pair plasmas collide. On the other hand, the instabilities at work are analyzed, as well as the field at saturation and the seed field which gets amplified. For mildly relativistic motions and onward, Weibel modes with ω=0+iδ govern the linear phase. We derive an expression for the duration of the linear phase in reasonable agreement with the simulations.

We investigate the Weibel instability in counter-propagating electron–ion plasmas with focus on the ion contribution, considering a realistic mass ratio. A generalized dispersion relation is derived from the relativistic theory by assuming an initially anisotropic temperature, which is represented by a waterbag distribution in momentum space, which shows an enhanced growth rate due to ion response. Two-dimensional particle-in-cell simulations support the theoretical analysis, showing a further amplification of magnetic field on ion time scale. The effect of an initial anisotropic temperature is investigated showing that the growth rate is monotonously decreased if the transverse spread is increased. Nevertheless, the presence of ions generates that the instability can develop for significantly higher electron temperatures. Suppression of oblique mode is also explored by introducing a parallel velocity spread.

Relativistic collisional effects on the filamentation instability are analytically and numerically investigated by comparing collisionless and collisional scenarios for a fast ignition (FI) configuration. The theoretical kinetic model, including warm species and space charge effects, predicts the preferential formation of larger filaments and the inhibition/enhancement of the instability when collisions are accounted for. These collisional effects are qualitatively and quantitatively confirmed by 1D and 2D particle-in-cell (PIC) simulations, also providing a physical picture for the inhibition/enhancement regime due to collisions, based on the electron beam slowdown. By plugging typical FI parameters in the dispersion relation, the theoretical model predicts significant growth rates of the instability deep inside the FI target, thus showing the potential role of the filamentation instability as a mechanism for energy deposition into the pellet core.

The paper presents a theoretical work on the dynamics of Coulomb explosion for spherical 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-monoenergetic 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.

A new configuration for the laser accelerator is proposed, which is inspired by the relativistic photon mirror effect. The material mirror is replaced here by an intense laser pulse, acting as a photon mirror for the incoming charged particles. A sufficient condition for particle reflection at such a photon mirror is established and three types of particle trajectories are described. A snow-plow acceleration regime is identified and quantitatively defined. Production of intense radiation bursts by the charged particle beam during the reflection process is also demonstrated.

We discuss here some of the physical problems related to the recently proposed scheme for solar sailing, using an artificial magnetosphere. We will concentrate on the forces acting on the plasma bubble, and their transfer to the spacecraft. Upper and lower limits of the force acting on the bubble are established. The results of test particle dynamics are presented, concerning the interaction of the solar wind with the modified magnetic dipole resulting from the spacecraft coils and the plasma expansion. Propagation of the forces along the magnetic flux tubes, from the bubble magnetopause down to the spacecraft vicinity, is discussed by using a simple MHD theoretical model. Emphasis is made on the distribution of currents flowing in the immediate vicinity of the spacecraft. Finally, results of PIC code simulations of the magnetized plasma expansion are presented and an overall qualitative picture of the physical processes is given, with a discussion of the strategy for obtaining more quantitative estimates of the magnetospheric propulsion efficiency.

The collective kinetic interaction of a neutrino distribution with electrostatic waves propagating in a dense plasma is considered. A Klimontovich-type kinetic equation for the neutrino gas in a plasma is derived, and the resulting kinetic description is applied to study the processes of the neutrino Landau damping and the neutrino-beam–plasma instability.

It is a great honor and pleasure to dedicate this special issue of Journal of Plasma Physics to our dear friend and colleague Professor Tito Mendonça on the occasion of his 65th birthday on December 28, 2010. Professor Tito is one of the most inspiring scientists in plasma physics, with contributions touching many other fields, using cross-cutting methodologies and novel theoretical techniques, which have led to important contributions and insights. Moreover, he has inspired many students at the Instituto Superior Técnico in Lisbon and mentored a new generation of physicists in Portugal to pursue scientific careers in a wide spectrum of fields from plasma physics to nonlinear optics, from astrophysics to quantum optics.

Photon kinetic theory provides a framework for the study of intense electromagnetic fields interacting with underdense plasmas. it is particularly useful when studying forward scattering four-wave modulational type instabilities. the most powerful features are its simplicity and its strong connection with particle beams, and the natural way in which broadband and angular spread effects can be included in the formalism. In this paper we derive the dispersion equations describing four-wave stimulated Raman and Brillouin scattering and Langmuir wave modulational instability for induced spatially incoherent pump waves.