Fulvio Cornolti

Laser Acceleration of Ion Bunches at the Front Surface of Overdense Plasmas

The acceleration of ions in the interaction of high intensity laser pulses with overdense plasmas is investigated with particle-in-cell simulations. For circular polarization of the laser pulses, high-density ion bunches moving into the plasma are generated at the laser-plasma interaction surface. A simple analytical model accounts for the numerical observations and provides scaling laws for the ion bunch energy and generation time as a function of pulse intensity and plasma density.

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 2 omega, with omega the laser frequency, to a 2D electromagnetic oscillation at frequency omega and wave vector k > omega/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.

Fluid and kinetic simulation of inertial confinement fusion plasmas

The main features of codes for inertial confinement fusion studies are outlined, and a few recent simulation results are presented. The two-dimensional Lagrangian fluid code DUED is used to study target evolution, including beam-driven compression, hydrodynamic stability, hot spot formation, ignition and burn. An electro-magnetic particle-in-cell (PIC) code is applied to the study of ultraintense laser–plasma interaction and generation of fast electron jets. A relativistic 3D collisionless fluid model addresses relativistic electron beam propagation in a dense plasma.

Expansion of a finite-size plasma in vacuum

The expansion dynamics of a finite-size plasma is examined from an analytical perspective. Results regarding the charge distribution as well as the electrostatic potential are presented. The acceleration of the ions and the associated cooling of the electrons that takes place during the plasma expansion is described. An extensive analysis of the transition between the semi-infinite and the finite-size plasma behaviour is carried out. Finally, a test of the analytical results, performed through numerical simulations, is presented.

Fundamental issues in fast ignition physics: from relativistic electron generation to proton driven ignition

In recent years, several schemes for laser-driven fast ignition (FI) of inertial confinement fusion targets have been proposed. In all schemes, a key element is the conversion of the energy of a Petawatt laser pulse into a beam of strongly relativistic electrons and the transport of the latter into a dense plasma or a solid target. The electron beam may either drive ignition directly or be used to accelerate a proton beam which is in turn used to ignite. Both ignition scenarios involve a number of physical processes which are widely unexplored and challenging for theory and simulation. In this contribution, we present theoretical and numerical investigations of several fundamental issues of relevance to FI from the stage of electron generation and transport to that of proton energy deposition, including electron beam instabilities, electron transport in solid-density plasma, proton transport in the coronal plasma, and requirements for proton beam driven ignition.

Two-Surface Wave Decay

The parametric excitation of pairs of electron surface waves (ESW) in the interaction of an ultrashort, intense laser pulse with an overdense plasma is discussed using an analytical model. The plasma has a simple step-like density profile. The ESWs can be excited either by the electric or by the magnetic part of the Lorentz force exerted by the laser and, correspondingly, have frequencies around ω/2 or ω, where ω is the laser frequency.

Dynamical symmetries and harmonic generation

We discuss harmonic generation in the case of laser field-dressed Hamiltonians that are invariant under so-called dynamical symmetry operations. Examples for such systems are molecules which exhibit a discrete rotational symmetry of order N (e.g. benzene with N = 6) interacting with a circularly polarized laser field and single atoms in a bichromatic field, with the two lasers having circular polarizations. Within a general group theory approach we study the harmonics one obtains from the interaction of a laser pulse and a circular molecule. When the system is in a pure field-dressed state the known selection rule kN ± 1, k = 1, 2, 3 ,... results. However, other lines are observed when recombinations with states of a symmetry different from the initial one become important. This is the case for realistic laser pulses (i.e. with a finite duration), in particular when the fundamental laser frequency (or one of its multiples) is resonant with a transition between field-dressed states. Numerical ab initio simulations, confirming our analytical calculations and illustrating the power of the group theory approach, are presented.

Modeling field ionization in an energy conserving form and resulting nonstandard fluid dynamics

A fluid model that takes the field ionization energy correctly into account is presented for the first time by introducing an energy conserving ionization current as a source term in the wave equation. Nonstandard type fluid equations result from the finite ejection energy of the electrons in the field ionization process. The energy and momentum distributions of the ejected electrons are obtained from the time-dependent Schrodinger equation and classical Monte Carlo calculations. Characteristic results of how field ionization influences the pulse propagation, and some extremely nonlinear features caused by the ionization current are given.

Charged state of a spherical plasma in vacuum

The stationary state of a spherically symmetric plasma configuration is investigated in the limit of immobile ions and weak collisions. Configurations with small radii are positively charged as a significant fraction of the electron population evaporates during the equilibration process, leaving behind an electron distribution function with an energy cutoff. Such charged plasma configurations are of interest for the study of Coulomb explosions and ion acceleration from small clusters irradiated by ultraintense laser pulses and for the investigation of ion bunches propagation in a plasma.

Macroscopic self-focusing effects in a laser-produced low-density plasma

SummaryLarge-scale self-focusing effects are evidenced by focusing a 300 MW Nd laser beam with aF/8 spherical lens in a fairly low-density gas (such thatne≈nc/100). The include important beam recollimation and formation of an «empty» channel which was visualized by holographic interferometry. In these conditions plasma expansion and laser energy deposition were both larger ahead than behind the focal point.RiassuntoEffetti di autofocalizzazione su larga scala si manifestano quando il fascio di un laser a Nd da 300 MW è focalizzato tramite una lenteF/8 in un gas di relativamente bassa densità (tale chene≈nc/100). Essi includono una notevole ricollimazione del fascio laser e la formazione di un canale «vuoto» che è stato evidenziato grazie all’interferometria olografica. In queste condizioni sia l’espansione del plasma che la deposizione di energia da parte del laser erano maggiori oltre il punto focale piuttosto che dietro di esso.РезюмеСамофокусирующие эффекты в больших масштабах наблюдаются при фокусировке с помощью линзыF/8 пучка неодимого лазера 300 MB мощности в газ сравнительно небольшой плотности (такой чтоne∼nc/100). Они включают важную реколлимацию лазерного пучка и образование «пустого» канала, выявленного голографической интерферометрией. В этих условиях распространение плазмы и поглощение лазерной энергии является большим перед фокальной точкой чем за ней.