Francesco Califano

Hamiltonian magnetic reconnection

Magnetic reconnection in two dimensional (2D), collisionless, non-dissipative regimes is investigated analytically and numerically by means of a finite difference code in the nonlinear regime where the island size becomes macroscopic. The cross-shaped structure of the reconnection region, originally reported by Cafaro et al (1998 Phys. Rev. Lett. 80 20) is analysed as a function of the ratio between the ion sound Larmor radius and the inertial skin depth. This cross shape structure is found to survive in the presence of weak dissipation. Further insight on the quasi-explosive behaviour of the magnetic island width as a function of time and on the spatial structure of the perturbed current density is provided. We confirm that the amount of reconnected flux becomes of order unity on the time scale of the inverse linear growth rate. Results in the collisionless limit are interpreted on the basis of the Hamiltonian properties of the adopted collisionless, 2D, fluid model. Thus, collisionless reconnection is a fast, non-steady-state process, fundamentally different from 2D resistive magnetic reconnection, of which the Sweet-Parker model is the classic paradigm.

Generation of collimated beams of relativistic ions in laser-plasma interactions

A method is proposed for generating collimated beams of fast ions in laser-plasma interactions. Two-dimensional and three-dimensional particle-in-cell simulations show that the ponderomotive force expels electrons from the plasma region irradiated by a laser pulse. The ions with unneutralized electric charge that remain in this region are accelerated by Coulomb repulsive forces. The ions are focused by tailoring the target and also as a result of pinching in the magnetic field produced by the electric current of fast ions.

Nonlinear development of the Weibel instability and magnetic field generation in collisionless plasmas

The Weibel instability is expected to be one of the mechanisms at play in the generation of quasi-static magnetic fields by high-intensity laser pulses propagating with relativistic amplitudes in underdense plasmas. This instability can be excited in a plasma with two electron populations: the background cold electrons and the high energy electrons accelerated by the laser radiation. Here we investigate the nonlinear stage of the evolution of this instability both analytically and numerically. We consider both the non-relativistic and the relativistic regimes within the frame-work of a cold two-electron fluid plasma description, where ions are assumed to be at rest and to provide a uniform neutralizing background.

Electron parallel compressibility in the nonlinear development of two-dimensional collisionless magnetohydrodynamic reconnection

Some topological aspects of the magnetic reconnection phenomenon are summarized and recent numerical results, derived within a two-fluid model, of two-dimensional collisionless magnetic reconnection in presence of a strong guide field are reported. Both the Alfven and the whistler frequency range are investigated by including electron parallel compressibility effects that are related respectively to thermal effects and to density fluctuations. The Hamiltonian character of the system is emphasized as it drives the small scale dynamics through the presence of topological invariants. These determine the formation and the shape of small scale current and vorticity layers inside the magnetic island. Secondary fluid instabilities, mainly of the Kelvin–Helmholtz type, can destabilize these layers when a hydrodynamic type regime is achieved. The inclusion of parallel electron compressibility has stabilizing effects. In view of the limitations of the two-fluid modelling, possible developments are briefly discussed...

Generation of subcycle relativistic solitons by super intense laser pulses in plasmas

Abstract 2D and 3D particle in cell simulations of the interaction of ultra-short, high-intensity laser pulses with inhomogeneous plasmas show the formation of long-lived electromagnetic solitons. These solitons consist of electron density depressions and intense electromagnetic field concentrations with a bigger amplitude and a lower frequency than those of the laser pulse. A significant portion of the electromagnetic energy is trapped in the form of solitons. Due to the plasma inhomogeneity, the solitons propagate toward the plasma–vacuum interface where they radiate away their energy in the form of bursts of low frequency electromagnetic radiation.

Magnetic fields from high-intensity laser pulses in plasmas

In a plasma interacting with ultra-short, high-intensity laser pulses, the magnetic part of the Lorentz force on the electrons can become as important as the electric part. In this case, we can expect the magnetic field to change the pattern of the nonlinear laser-pulse - plasma interaction drastically. The relativistic nonlinearities introduced by the magnetic interaction are of general interest in relation to the field of ultra-strong electromagnetic waves propagating in media, high-energy space plasmas and laser - plasma interaction under laboratory conditions. We present a summary of analytical and numerical results concerning the generation of quasi-static magnetic fields by high-intensity laser pulses in underdense plasmas and in thin plasma foils, and discuss the dynamical effects of these fields on the plasma motion and on the pulse propagation. This analysis indicates that phenomena such as current pinching, reconnection of magnetic-field lines and vortex propagation etc, that have been previously discussed in the case of space and laboratory plasmas, are also important for laser - plasma interactions.

Finite Larmor radius effects in the nonlinear dynamics of collisionless magnetic reconnection

We provide numerical evidence of the role of finite Larmor radius effects in the nonlinear dynamics of magnetic field line reconnection in high-temperature, strong guide field plasmas in a slab configuration, in the large Δ regime. Both ion and electron temperature effects introduce internal energy variations related to mechanical compression terms in the energy balance, thus contributing to regularize the gradients of the ion density with respect to the cold regimes. For values of the Larmor radii that are not asymptotically small, the two temperature effects are no longer interchangeable, in contrast to what is expected from linear theory, and the differences are measurable in the numerical growth rates and in the nonlinear evolution of the density layers. We interpret such differences in terms of the change, due to ion temperature effects, of the Lagrangian advection of the plasma invariants that are encountered in the cold-ion, warm-electron regime. The different roles of the ion and ion-sound Larmor radii in the reconnection dynamics near the X- and O-points are evidenced by means of a local quadratic expansion of the fields.

Magnetic-field generation and wave-breaking in collisionless plasmas

We discuss the nonlinear evolution of the Weibe linstability of two counter-streaming electron beams in the relativistic and non-relativistic regimes in the framework of a a two-fluid deseription. We show the presence of two singularities per wavelength, responsible for the formation of very large density spikes In the case of non-symmetrie beams, we observe the compressive wave-break of the fast electron population, and the generation of a a dipolar magnetic field associated with a central fast thin current and two large slow return currents. This structure is very similar to those observed in PIC simulations of laser-plasma interactions.

Ultra intense magnetic fields in laser plasma interaction: their generation and influence on light propagation

Extremely large, quasi-stationary magnetic fields can be generated in plasmas by high intensity laser pulses. These fields can change the plasma dynamics and the pulse propagation. Several aspects of their generation and of their effect on the plasma and on the laser pulse are discussed in the relativistic pulse amplitude regime: (a) the formation of magnetic wakes, (b) the development of longitudinal and transverse Langmuir wake wavebreaks and (c) the magnetic field generation on a thin foil, viewed as a model for overdense plasmas with sharp boundaries.

Stress-driven mixed layer in a stably stratified fluid

Abstract We investigate numerically a two-dimensional flow where a shear layer is forced at the top of a linearly stratified fluid. As a consequence of the mechanical forcing, a statistically steady stress is exerted on the underlying fluid. The downwards transfer of momentum and buoyancy, characterized by the deepening of a mixed layer, is studied for three different values of the initial Brunt-Vaisala frequency N. We show that the flow reaches an asymptotic stage where the temporal evolution of the mixed layer becomes statistically self-similar. The spatial scaling factor is the mixed layer depth h(t) whose evolution is proportional to u&/N&&, where u& is a velocity associated to the stress τ = ϱ0u&2. These results are compared to previous theoretical and empirical models which have been proposed to describe the deepening of the oceanic mixed layer under the action of a wind stress. Astrophysical applications are also mentioned.