Possibilities are discussed of using Brillouin cells to produce high-peak-power radiation with the diffraction-limited divergence. The major emphasis is on the problems of coherent beam coupling at the output of several optical channels and of laser-pulse compression.

Recent studies on short-pulse interaction with steep-density gradient plasmas (L/λ0) uncovered some new absorption mechanisms. One such process was proposed by Brunei (1987), in which a laser beam is obliquely incident on a perfectly conducting surface. During one half-cycle, electrons dragged from the surface into the vacuum region are returned to the plasma with velocities of the same order as that of the quiver electron velocity. We have reexamined this mechanism using a 1–D (Lagrangian) plasma model, but without Brunel's assumption that the electric field vanishes inside the plasma. Analytical and numerical calculations of electron trajectories and the self-consistent electric fields are presented and comparisons are made with 1–D particle-in-cell simulations.

The nonlinear evolution of electrostatic plasma waves excited by mode conversion is studied in an inhomogeneous, collisionless, unmagnetized plasma. Experiments in the microwave range (f = 3.5 GHz), performed in a plasma (ne ≃ 1011 cm−3) created in a multipolar discharge, show that the electrostatic plasma-wave evolution exhibits a transition from a nonlinear steady-state regime toward chaos, occurring when the pump field or the gradient length is increased. The Zakharov equations, which model the plasmawave evolution coupled with low-density perturbation, are solved numerically with parameters close to those of the experiment; this simulation allows a characterization of the two regimes.

Two spherically symmetric versions of a self-similar collapse are investigated within the framework of the Zakharov equations, namely, one relative to a vectorial electric field and the other corresponding to a scalar modeling of the Langmuir field. Singular solutions of both of them depend on a linear time contraction rate Ξ(t) = V(t* – t), where t* and V = – Ξ denote, respectively, the collapse time and the constant collapse velocity. We show that under certain conditions, only the scalar model admits self-similar solutions, varying regularly as a function of the control parameter V from the subsonic (V ≪ 1) to the supersonic (V ≫ 1) regime.

The anisotropic wave collapses governed by the Zakharov equations or by the nonlinear Schrödinger equation are investigated by means of a transformation group exhibiting self-similar anisotropic solutions characterized by two contraction rates. We show the existence of two infinite sets of anisotropic self-similar collapses corresponding, respectively, to “pancake” (oblate) and “cigar” (oblong) shaped collapsing solutions. Both of these classes of anisotropic structures are furthermore shown to be bounded from below by two peculiar solutions whose validity extends from the subsonic to the supersonic regime (the so-called “trans-sonic” solutions) and from above by the standard isotropic solutions. Spatial profiles of trans-sonic anisotropic solutions are analytically derived.

In recent years experimental results about the pulsation of reflectivity or 3ω/2 harmonics from laser-irradiated plasma with a period of 10–20 ps initiated a reconsideration of a broad field of phenomena of laser-plasma interaction. We present here numerical results from a very general hydrodynamic computation showing that the pulsation is due to a standing-wave-produced density ripple in the widespread plasma corona causing these after 2 ps at Laue-Bragg reflection at very low density, followed by a hydrodynamic relaxation of the ripple. This cycle has a turnaround time of about 6–10 ps and immediately explains the observation of pulsation. The suppression of pulsation and the achievement of smooth direct drive is then understood by a washing-out process of the ripples when using the random-phase plate (RPP) or broadband irradiation, or by just preventing the buildup of the ripple at the induced spatial incoherence (ISI), where coherence of the laser field is 1 or 2 ps only.

Impedance-matching experiments are realized with a 0.26-μm-wavelength high-intensity laser irradiation of two-layered targets (Al-Au). From an ablation pressure of 50 Mbars produced in Al, a 100-Mbar shock pressure is obtained in Au. The X-UV emissivity of this shock at its breakthrough at the rear of the target is analyzed with a transmission grating coupled to an X-ray streak camera. Results obtained with various thicknesses of Al and constant Au thickness are shown, and compared with hydrodynamic codes where radiative transfer processes are taken into account. These preliminary experiments show that the estimated shock temperatures are consistent with the high pressures generated. This technique appears to be very promising for the task of obtaining equation-of-state data in these high-pressure ranges.

Low-fluence laser interaction with metallic target is investigated in a wide range of irradiance, from 5 × 106 to 1011 W/cm2, with 4- and 30-ns laser-pulse durations and 1.06-μm wavelength. Edge effects are avoided by means of large surfaces of irradiation (5 cm2). Specific diagnostics such as a ballistic pendulum and a piezoelectric quartz gauge are developed to measure the total momentum imparted to the target, as well as the temporal evolution of the stress at the rear side of the target. Specular reflection and sidescattering of the laser light from the target are also measured. The behavior of 2024 aluminum alloy is particularly studied. The results are interpreted in terms of liquid-vapor transition and change in laser absorption.

In light of experimental data, the theoretical analysis of surface absorption is reviewed, showing the dependence on metal temperature and laser wavelength. The thermal and mechanical coupling is also calculated.

Numerical simulations are performed with a 1–D Lagrangian hydrodynamic code including modifications for low-pressure regimes. The multiphase equation of state is extended to take into account negative pressures. An elastoplastic module is introduced into the code. A quite good agreement is obtained with the measurements of the stress and of the mechanical coupling coefficient.

Cumulative effects in gases accompanying hydrodynamic motion due to powerful laser energy deposition deal with the problems of high-speed jet generation and plasma heating. In the present paper, on the basis of experimental simulation and theoretical analysis we investigate the behavior of a cumulative ring shock wave near the axis of symmetry. Such a shock wave arises after the energy deposition into the volume of a gas having the form of a thin torus.

In spherical pinch (SP) experiments, the plasma heated at the center of a cell to reach ignition temperature is confined by imploding shock waves for a time long enough to satisfy the Lawson criterion for plasma fusion. In earlier theoretical studies, the expansion of the central plasma either is neglected or is assumed to be radially uniform. In the present paper, the energy is considered to be deposited instantaneously at the center of the cell and the nonlinear heat conduction equation is solved to study the temporal evolution of the central plasma. Incorporating the ignition condition for the average temperature of the expanding fireball, and its confinement by imploding convergent shock waves, which may be fired from the periphery of the cell with some time delay, the scaling laws for satisfying the Lawson criterion are investigated in detail. The relevant calculations indicate that the cumulative effects of the convergent shock waves in the vicinity of the center of the cell play an important role in these scaling laws.

A method for the automatic, discrete rezoning of two-dimensional Lagrangian codes for inertial confinement fusion (ICF) implosions has been developed. The method, which applies to matrix-ordered, quadrilateral zone meshes, allows the preservation of the interface tracking property of the Lagrangian approach. Total mass and momentum are conserved exactly, total energy is approximately conserved, and numerical diffusion is kept to tolerable levels. The mesh generator, the mapping scheme, and the actual implementation in a two-temperature laser fusion code are described. The performance of the method is illustrated by several sample applications.

The process of Mach wave generation in air is studied in both plane and spherical geometries. The experimental results reported here are theoretically interpreted using the predictions of a self-similar model of strong explosion along with the hydrodynamic equations of a perfect gas, and a good agreement is found.

Femtosecond laser pulses are capable of producing very high light intensities at moderate pulse energies. We study the laser light absorption at flux densities of 1017 W/cm2 and higher during the plasma formation and the plasma heating process. In the first stage multiphoton and collisional ionization dominate. Modification of the inverse bremsstrahlung absorption occur owing to ionization dephasing. In the second stage beam energy conversion is mainly by collisional absorption at normal incidence, and by resonance absorption at oblique illumination. Simultaneously strong electron heat conduction provides for plasma formation and heating in deeper layers of the solid not accessible to the laser light. The penetration depth of the laser beam is modified by the anomalous skin effect. The electron-ion collision frequency at the high laser intensities under consideration is determined by the oscillation energy of the electrons rather than by their thermal motion. Although it reaches high values (1014-1016 S-1), all kinds of solid targets become strongly reflecting (>60%) owing to the formation of electron densities largely exceeding those of solid-state plasmas. Effects modifying the degree of absorption are briefly discussed.