Direct-drive implosion experiments on the GEKKO XII laser (9 kJ, 0.5 μm, 2 ns) with deuterium and tritium (DT) exchanged plastic hollow shell targets demonstrated fuel areal densities (ρR) of ˜0.1 g/cm2 and fuel densities of ˜600 times liquid density at fuel temperatures of ˜0.3 keV. (The density and ρR values refer only to DT and do not include carbons in the plastic targets.) These values are to be compared with thermonuclear ignition conditions, i.e., fuel densities of 500–1000 times liquid density, fuel areal densities greater than 0.3 g/cm2, and fuel temperatures greater than 5 keV. The irradiation nonuniformity in these experiments was significantly reduced to a level of <5% in root mean square by introducing random-phase plates. The target irregularity was controlled to a 1% level. The fuel ρR was directly measured with the neutron activation of Si, which was originally compounded in the plastic targets. The fuel densities were estimated from the ρR values using the mass conservation relation, where the ablated mass was separately measured using the time-dependent X-ray emission from multilayer targets. Although the observed densities were in agreement with one-dimensional calculation results with convergence ratios of 25–30, the observed neutron yields were significantly lower than those of the calculations. This suggests the implosion uniformity is not sufficient to create a hot spark in which most neutrons should be generated.

Recent improvements to the Nova laser and target experiments addressing laser–plasma interaction physics, hydrodynamic stability, implosion physics, and X-ray laser physics are summarized. Highlights include demonstrated laser performance exceeding Nova's original design goals, the observation of improved target compressions with temporally shaped laser pulses, and demonstrated lasing at wavelengths shorter than 45 Å.

The effect of long-wavelength irradiation nonuniformities on the performance of singleshell, reactor targets is investigated by means of 2-D numerical simulations. The stages of target collapse, hot-spot formation, ignition, and burn are illustrated. Evidence is shown for the occurrence of the Rayleigh–Taylor instability during target stagnation. It is then shown that the sensitivity of a given family of targets to the nonuniformities of the driving pressure critically depend on the 1–D ignition margin and on the spark convergence ratio of the target. The tolerable levels of nonuniformity as a function of the perturbation mode number are determined, for selected targets, by means of a parametric numerical study.

From analytic models of the ablation process in laser-plasma experiments, as well as simulations with Fokker–Planck and Spitzer codes, we have identified a new effect, thermal inertia smoothing. This effect results in a smoother Fokker–Planck temporal and spatial variation of the ablation pressure than that for Spitzer, for the same applied timevarying laser intensity. It is therefore advantageous for incoherent optical smoothing schemes. This is especially true at early times in the laser pulse (≤100 ps).

We studied the smoothing of nonuniformities in the quasisteady expansion of a plasm ablated from a spherical target under a weakly nonuniform laser irradiation with overall heat conduction and magnetic generation effects. We also considered light refraction effects.

The Rayleigh—Taylor instability is studied by means of a slab model and when slab thickness D is comparable to the ablation-to-critical surface distance. Under these conditions the perturbations originating at the ablation front reach the critical surface, and in order to determine the instability growth rate, we must impose boundary conditions at the corona. Stabilization occurs for perturbation wave numbers such that kD ˜ 10.

The design for an advanced, flash-lamp-pumped Nd: glass laser for inertial confinement fusion (ICF) research and applications is summarized. The laser consists of multiple beam lines, each of which delivers 145-kJ, O.35-μm, 8-ns laser pulses, with pulse shape and bandwidth capabilities consistent with requirements for driving ICF capsules to high gain.

This paper reviews the current status of high-power KrF lasers and considers their prospects for future development. The main laser requirements imposed by target physics considerations are identified and KrF lasers are shown to be uniquely capable of satisfying them. Some design considerations are given for (a) ultrashort-pulse high-power systems, (b) high-power coherent beam generation using Raman amplifiers, and (c) incoherent beam amplification.

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.