The paraxial propagation of a relativistic electron beam in a solid target is examined, within a three-dimensional model of particles interacting with the target electron return current via a diffusive electromagnetic field. Simulations of a modulated beam show amplification of the modulation seed, with growth rates comparing reasonably well with the linear analysis of the model. Scenarios of beam fragmentation are observed and discussed in more realistic conditions, when beam collisions on both target ions and electrons and the resulting solid heating and ionization are taken into account.

The advent of ultrahigh-power femtosecond lasers creates a need for an entirely new class of optical components based on plasmas. The most promising of these are known as plasma mirrors, formed when an intense femtosecond laser ionizes a solid surface. These mirrors specularly reflect the main part of a laser pulse and can be used as active optical elements to manipulate its temporal and spatial properties. Unfortunately, the considerable pressures exerted by the laser can deform the mirror surface, unfavourably affecting the reflected beam and complicating, or even preventing, the use of plasma mirrors at ultrahigh intensities. Here we derive a simple analytical model of the basic physics involved in laser-induced deformation of a plasma mirror. We validate this model numerically and experimentally, and use it to show how such deformation might be mitigated by appropriate control of the laser phase.

The interaction of intense laser beams with plasmas created on solid targets involves a rich nonlinear physics. Because such dense plasmas are reflective for laser light, the coupling with the incident beam occurs within a thin layer at the interface between plasma and vacuum. One of the main paradigms used to understand this coupling, known as Brunel mechanism, is expected to be valid only for very steep plasma surfaces. Despite innumerable studies, its validity range remains uncertain, and the physics involved for smoother plasma-vacuum interfaces is unclear, especially for ultrahigh laser intensities. We report the first comprehensive experimental and numerical study of the laser-plasma coupling mechanisms as a function of the plasma interface steepness, in the relativistic interaction regime. Our results reveal a clear transition from the temporally-periodic Brunel mechanism to a chaotic dynamic associated to stochastic heating. By revealing the key signatures of these two distinct regimes on experimental observables, we provide an important landmark for the interpretation of future experiments.

The propagation of a high-irradiance laser beam in a plasma whose optical index depends nonlinearly on the light intensity is investigated through both theoretical and numerical analyses. The nonlinear effects examined herein are the relativistic decrease of the plasma frequency and the ponderomotive expelling of the electrons. This paper is devoted to focusing and defocusing effects of a beam assumed to remain cylindrical and for a plasma supposed homogeneous along the propagation direction but radially inhomogeneous with a parabolic density profile. A two-parameter perturbation expansion is used; these two parameters, assumed small with respect to unity, are the ratio of the laser wavelength to the radial electric-field gradient length and the ratio of the plasma frequency to the laser frequency. The laser field is described in the context of a time envelope and spatial paraxial approximations. An analytical expression is provided for the critical beam power beyond which self-focusing appears; it depends strongly on the plasma inhomogeneity and suggests the plasma density tailoring in order to lower this critical power. The beam energy radius evolution is obtained as a function of the propagation distance by numerically solving the paraxial equation given by the two-parameter expansion. The relativistic mass variation is shown to dominate the ponderomotive effect. For strong laser fields, self-focusing saturates due to corrections of fourth order in the electric field involved by both contributions.

We report on shadowgraphic measurements showing the first space-and time-resolved snapshots of ultraintense laser pulse-generated fast electrons propagating through a solid target. A remarkable result is the formation of highly collimated jets (,20-mm) traveling at the velocity of light and extending up to 1 mm. This feature clearly indicates a magnetically assisted regime of electron transport, of critical interest for the fast ignitor scheme. Along with these jets, we detect a slower (ഠc͞2) and broader (up to 1 mm) ionization front consistent with collisional hot electron energy transport. 52.60. + h The fast ignitor scheme, which claims to relax some of the constraints hampering the standard approaches to inertial confinement fusion, has triggered a worldwide interest since its inception [1]. It hinges on the rapid additional heating of the core of a precompressed thermonuclear pellet due to the slowing down of a bunch of relativistic electrons generated by an ultraintense laser pulse. Now, the highly overcritical plasma surrounding the core should prevent any laser pulse from reaching it, whatever highintensity penetration mechanisms are at work (relativistic self-induced transparency [2] or ponderomotive hole boring [3]). An encouraging point is that particle-in-cell simulations predict a rather peaked hot electron distribution in the vicinity of the laser-solid interaction zone [4]. However, an efficient heating of the core requires the electron beam to remain collimated up to its final absorption zone, i.e., on a distance of several hundreds of microns. This can be achieved only through the pinching effect of the beam-driven magnetic field competing with multiple scattering. Therefore, fast electron transport from moderately to extremely dense regions appears as a key issue for the success of fast ignition, which must be thoroughly tackled both experimentally and theoretically.Over the past year, there has been a growing body of experimental evidence pointing to the existence of very collimated high intensity laser-produced electron jets traveling through solid targets. Tatarakis et al. have recently observed a narrow expanding plasma at the rear surface of thick plastic slabs irradiated by a 1 ps, 10 19 W͞cm 2 laser pulse [5]. By using a 2D Fokker-Planck hybrid code, they interpreted this localized rear heating as a magnetic field-enhanced electron energy deposition at the target/vacuum interface [6]. This effect has also been detected in other experiments [7]. Though very encouraging, these studies still provide an incomplete experimental picture of the phenomena arising in the bulk of the target.In the present paper, we report on optical shadowgraphic results showing what is, to our knowledge, the first comprehensive set of space-and time-resolved snapshots of fast electrons propagating through a solid target. In order to bypass the classical limitation of optical probing into an overcritical solid target, we use transparent glass slides. Our measurements pinpoint the existence of two types of fast...

Neutron production from a deuterated solid target irradiated by an ultraintense laser pulse is studied by means of kinetic numerical simulations. A two-dimensional particle-in-cell code is used to compute the velocity distribution of the deuterium ions accelerated during the interaction of the laser pulse with a hot deuterium plasma. A postprocessor has been designed to evaluate the slowing down of these ions in the solid unionized target, and the amount of neutrons produced by nuclear reactions between the accelerated and target ions. The energy and angle distributions of these neutrons are computed and compared to recent experimental results.

As a high-intensity laser-pulse reflects on a plasma mirror, high-order harmonics of the incident frequency can be generated in the reflected beam. We present a numerical study of the phase properties of these individual harmonics, and demonstrate experimentally that they can be coherently controlled through the phase of the driving laser field. The harmonic intrinsic phase, resulting from the generation process, is directly related to the coherent sub-laser-cycle dynamics of plasma electrons, and thus constitutes a new experimental probe of these dynamics.

The interaction of an ultraintense laser pulse with an overcritical collisionless plasma at normal incidence is investigated with 1.5 dimensional particle-in-cell simulations. Laser absorption and hot electron energy are reported for a large range of intensities and plasma densities. We observe a strong dependence of the electron temperature on the plasma density profile, and a transition between two different heating mechanisms when the gradient length is varied. For sharp edged profiles, the electron temperature is well below the laser ponderomotive potential. ͓S1063-651X͑97͒50401-6͔PACS number͑s͒: 52.40. Nk, 52.35.Mw, 52.60.ϩh