arXiv subjects

A. Macchi

Publications and source records attributed to A. Macchi.

28 records · Page 2Linked to original sources

arXiv2007

Dynamics of charge-displacement channeling in intense laser-plasma interactions

The dynamics of transient electric fields generated by the interaction of high intensity laser pulses with underdense plasmas has been studied experimentally with the proton projection imaging technique. The formation of a charged channel, the propagation of its front edge and the late electric field evolution have been characterised with high temporal and spatial resolution. Particle-in-cell simulations and an electrostatic, ponderomotive model reproduce the experimental features and trace them back to the ponderomotive expulsion of electrons and the subsequent ion acceleration.

physics.plasm-ph↗

arXiv2007

Electromagnetic pulse reflection at self-generated plasma mirrors: laser pulse shaping and high order harmonic generation

A thin layer of overdense plasma is created when an electromagnetic pulse interacts with a rapidly ionizing thin foil. This layer will reflect the incoming pulse, forming a so-called plasma mirror. A simple realistic model based on paired kinetic and wave equations is used to describe analytically the process of mirror formation and the reflection and transmission of the incident pulse. The model incorporates the exact description of the ionization process in the foil and the polarization and conduction currents that follow. The analytical description of the reflected and transmitted pulses as well as their dependence on foil parameters, and initial pulse amplitude and form are presented. Possible application and effectiveness of this process to improve laser pulse contrast are discussed. In the case of the linearly polarized incident pulse, there harmonic generation occurs, that is absent in the case of the circular polarization. The spectra of the reflected pulses for different initial forms and amplitudes are studied.

physics.plasm-ph↗

arXiv2007

Features of ion acceleration by circularly polarized laser pulses

The characteristics of a MeV ion source driven by superintense, ultrashort laser pulses with circular polarization are studied by means of particle-in-cell simulations. Predicted features include high efficiency, large ion density, low divergence and the possibility of femtosecond duration. A comparison with the case of linearly polarized pulses is made.

physics.plasm-ph↗

arXiv2007

Generation and Observation of Coherent, Long--Lived Structures in a Laser--Plasma Channel

In recent experiments of laser pulse interaction at relativistic intensities with a low density plasma, the proton radiography technique showed evidence of long--lived field structures generated after the self-channeling of the laser pulse. We present 2D particle-in-cell simulations of this interaction regime, where the dynamics of similar structures has been resolved with high temporal and spatial resolution. An axially symmetrical field pattern, resembling both soliton-like and vortex structures, has been observed. A study of the physics of such structures and a comparison with experimental data is in progress.

physics.plasm-ph↗

arXiv2006

A Femtosecond Neutron Source

The possibility to use the ultrashort ion bunches produced by circularly polarized laser pulses to drive a source of fusion neutrons with sub-optical cycle duration is discussed. A two-side irradiation of a thin foil deuterated target produces two countermoving ion bunches, whose collision leads to an ultrashort neutron burst. Using particle-in-cell simulations and analytical modeling, it is evaluated that, for intensities of a few $10^{19} W cm^{-2}$, more than $10^3$ neutrons per Joule may be produced within a time shorter than one femtosecond. Another scheme based on a layered deuterium-tritium target is outlined.

physics.plasm-ph↗

arXiv2003

Two-surface wave decay: improved analytical theory and effects on electron acceleration

Two-surface wave decay (TSWD), i.e. the parametric excitation of electron surface waves, was recently proposed as an absorption mechanism in the interaction of ultrashort, intense laser pulses with solid targets. We present an extension of the fluid theory of TSWD to a warm plasma which treats boundary effects consistently. We also present test-particle simulations showing localized enhancement of electron acceleration by TSWD fields; this effect leads to a modulation of the current density entering into the target and may seed current filamentation instabilities.

physics.plasm-ph↗

arXiv2003

Dynamical ionization ignition of clusters in intense and short laser pulses

The electron dynamics of rare gas clusters in laser fields is investigated quantum mechanically by means of time-dependent density functional theory. The mechanism of early inner and outer ionization is revealed. The formation of an electron wave packet inside the cluster shortly after the first removal of a small amount of electron density is observed. By collisions with the cluster boundary the wave packet oscillation is driven into resonance with the laser field, hence leading to higher absorption of laser energy. Inner ionization is increased because the electric field of the bouncing electron wave packet adds up constructively to the laser field. The fastest electrons in the wave packet escape from the cluster as a whole so that outer ionization is increased as well.

physics.atom-ph↗

arXiv2001

Surface Oscillations in Overdense Plasmas Irradiated by Ultrashort Laser Pulses

The generation of electron surface oscillations in overdense plasmas irradiated at normal incidence by an intense laser pulse is investigated. Two-dimensional (2D) particle-in-cell simulations show a transition from a planar, electrostatic oscillation at $2ω$, with $ω$ the laser frequency, to a 2D electromagnetic oscillation at frequency $ω$ and wavevector $k>ω/c$. A new electron parametric instability, involving the decay of a 1D electrostatic oscillation into two surface waves, is introduced to explain the basic features of the 2D oscillations. This effect leads to the rippling of the plasma surface within a few laser cycles, and is likely to have a strong impact on laser interaction with solid targets.

physics.plasm-ph↗

arXiv2001

C$_{60}$ in intense femtosecond laser pulses: nonlinear dipole response and ionization

We study the interaction of strong femtosecond laser pulses with the C$_{60}$ molecule employing time-dependent density functional theory with the ionic background treated in a jellium approximation. The laser intensities considered are below the threshold of strong fragmentation but too high for perturbative treatments such as linear response. The nonlinear response of the model to excitations by short pulses of frequencies up to 45eV is presented and analyzed with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond laser pulses of 800nm wavelength ionization is found to occur multiphoton-like rather than via excitation of a ``giant'' resonance.

physics.atm-clus↗

arXiv2000

"Single-cycle" ionization effects in laser-matter interaction

We investigate numerically effects related to ``single-cycle'' ionization of dense matter by an ultra-short laser pulse. The strongly non-adiabatic response of electrons leads to generation of a megagauss steady magnetic field in laser-solid interaction. By using two-beam interference, it is possible to create periodic density structures able to trap light and to generate relativistic ionization fronts

physics.plasm-ph↗