The first ignition experiments on the Laser Megajoule facility will use an indirect drive scheme. Our A1040 point design target is a graded doped plastic capsule filled by permeation within a gold cylinder. The deuterium-tritium ice layer may be formed either by classical slow cooling at 1.5 K below triple point, or by rapid cooling at 2.3 K below triple point. To complete the specifications, we first studied the robustness to all technological defects with the current CEA capabilities for these two options of ice formation. The technological imperfections taken into account are regrouped into ID errors, which keep the implosion spherical, and 3D errors, which induce a deformation of the shell. The 3D robustness is expressed in terms of deformation at peak velocity and compared to the deformation threshold obtained with 2D simulations. The ID robustness is given by the probability of exceeding 50% of nominal yield. We have taken into account 22 ID parameters and the fusion energy is approximated by a neural network based on 2000 simulations. Although the studies are not finished yet, the first results show that the A1040 design with rapid cooling has sufficient margins with respect to technological defects.

A base-line capsule design for the Laser MegaJoule facility is optimized by radially grading the Ge dopant in plastic ablator. A one dimensional robustness study is performed to optimize the implosion and the entropy deposition. Then, the tolerance versus the ablator roughness is examined for modes over 10 through direct two-dimensional simulations. The stability is significantly higher than that of an uniformly doped design.

This article sums up the theoretical and experimental studies about ignition. Three experiments are salient this year on the Omega laser in collaboration with DOE laboratories (1) 3 cones of beams allow to mimic the LMJ configuration and to get symmetry measurements. (2) We measured perturbations due to hydro-instability in CHGe planar samples with face-on and side-on radiographs. (3) We improved our nuclear diagnostics, particularly the neutron image system tested on direct drive implosions. As far as LMJ target design is concerned, we defined a preliminary domain corresponding to the possible operation at 2ω. At 3ω we studied the low mode instability effects on the DT deformation (due to the laser or to the target) and on the yield. The stability is clearly improved with graded doped CH for our nominal capsule L1215.

The adoption of a non-uniform dopant profile has substantially increased the tolerance to high mode deformations of our baseline indirect-drive design. In addition, a low deuterium-tritium (DT) gas density, obtained by 'dynamic quenching' at 2.3 K below triple point, could partly compensate for the decrease in robustness due to DT ageing. Finally, the net margin regarding all laser and target technological defects is about 2. As soon as a sufficient amount of laser beams and diagnostics is available, we will shoot pre-ignition experiments to tune the point design. We are studying new targets which need less energy for these campaigns. We have estimated different direct-drive schemes using indirect-drive beams. The optimal LMJ polar direct-drive configuration is a 2-cone one and leads to marginally igniting targets. A new 2-cone direct-drive scheme, associated with focal spot zooming, allows us to reach ignition with enough margin.

In the context of the French Laser-Mégajoule fusion-research program, the hydrodynamic stability of the baseline direct-drive target is investigated at the hot spot surface during the deceleration phase by means of modeling and simulations. Using the convergence of the flow towards a self-similar solution, a closed system of ordinary differential equations is derived for the main hydrodynamic variables. An exact linear stability analysis is performed to compute the Rayleigh-Taylor growths. All theoretical predictions are compared to one-dimensional and two-dimensional single-mode detailed numerical results.

This paper is devoted to the study of the deceleration phase of inertial confinement capsules. First the self-similar flow exhibited by Betti et al. [Phys. Plasmas 8, 5257 (2001)] is proved to be an attractor in the sense that arbitrary initial conditions converge towards this solution. The convergence rate depends on the ablation process and heat conductivity and it is shown to be a power law of the increase rate of the hotspot mass. Second the thin layer that separates the hotspot from the cold shell is described and it is shown that it also converges to a locally self-similar profile. By using and generalizing a shell model introduced by Betti et al. [Phys. Plasmas 9, 2277 (2002)] a closed system of ordinary differential equations for the main hydrodynamic variables is derived. Finally the linear growth rates of the deceleration phase Rayleigh-Taylor instabilities are computed taking into account ablation and spherical convergence. Significant differences are exhibited between directly and indirectly driven capsules. (C) 2005 American Institute of Physics.

First we report two studies aimed at preparing laser integration line (LIL) experiments (LIL is the prototype of LMJ): deflection of a beam with and without 'longitudinal' smoothing (associated with focusing by gratings) and the radiation temperature, T-r, in a hohlraum with long pulses (10-20 ns). Experimentally, we did not see any Langmuir decay instability able to saturate the stimulated Raman scattering in a gas bag irradiated with the Omega laser. Next, in our hydro-code FC12, we implemented an improved version of the non-LTE atomic physics model: the change in Tr in the hohlraum is negligible, but now the simulations are in agreement with experiments on x-ray conversion and on Rayleigh-Taylor instabilities (RTIs) in a spherical geometry. The RTls in polyimide foil at 70 Am were understood, but not those at 30 Am. Finally, for the target design, we confirm the hydro-stability of the four targets of the operational domain of LMJ: the doped CH ablator of the nominal target can withstand a roughness in the range 50-100 nm. The robustness studies use 19 uncertainties coming from the laser power, the beam pointing and the target fabrication. Finally, the burning of DT has been studied in detail, identifying three regimes.

The simplified perturbation method of Vandenboomgaerde et al. (2002) is applied to both the Richtmyer–Meshkov and the Rayleigh–Taylor instabilities. This theory is devoted to the calculus of the growth rate of the perturbation of the interface in the weakly nonlinear stage. In the standard approach, expansions appear to be series in time. We build accurate approximations by retaining only the terms with the highest power in time. This simplifies and accelerates the solution. High order expressions are then easily reachable. For the Richtmyer–Meshkov instability, multimode configurations become tractable and the selection mode process can be studied. Inferences for the intermediate nonlinear regime are also proposed. In particular, a class of homothetic configurations is inferred; its validity is verified with numerical simulations even as vortex structures appear at the interface. This kind of method can also be used for the Rayleigh–Taylor instability. Some examples are presented.

One proposed capsule design for the Laser Megajoule facility is analyzed to determine surface finish specifications required to achieve ignition and propagated burn. We estimate the sensitivity of this capsule to hydrodynamic instabilities by means of direct two-dimensional simulations. Configuring multimode perturbations located at the DT ice and ablator surfaces, the fusion yield is predicted.

In the frame of a CEA/US DOE collaboration, radiation driven spherically convergent experiments were performed on the Nova laser in order to measure the Rayleigh-Taylor growth at the ablation front. Numerical simulations using het 2D Lagrangian code PCI2 have correctly reproduced experiments in moderate convergent geometry. Experiments have addressed convergence ratios up to 4 by considering larger capsules, larger hohlraum and longer laser pulses. Numerical analysis of these high convergence implosions is presented, and the effect of convergence on the Rayleigh-Taylor growth is investigated.

Experiments were performed on the Nova laser [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)], using indirectly driven capsules mounted in cylindrical gold hohlraums, to measure the Rayleigh–Taylor growth at the ablation front by time-resolved radiography. Modulations were preformed on the surface of Ge-doped plastic capsules. With initial modulation amplitude of 2–2.5 μm, growth factors of about six in optical depth were seen, in agreement with simulations using the radiation hydrocode FCI2 [G. Schurtz, La fusion thermonucleaire inertielle par laser, edited by R. Dautray et al. (Eyrolles, Paris, 1994), Vol. 2]. With initial modulation amplitude of 0.5 μm and a longer drive, growth factors of about 100–150 in optical depth were seen. Comparable planar experiments showed growth factors of about 40 in optical depth. Analytical models predict the observed growth factors.

Large lasers such as Nova allow the possibility of achieving regimes of high-energy densities in plasmas of millimeter spatial scales and nanosecond timescales. In those plasmas where thermal conductivity and viscosity do not play a significant role, the hydrodynamic evolution is suitable for benchmarking hydrodynamics modeling in astrophysical codes. Several experiments on Nova examine hydrodynamically unstable interfaces. A typical Nova experiment uses a gold millimeter-scale hohlraum to convert the laser energy to a 200 eV blackbody source lasting about a nanosecond. The X-rays ablate a planar target, generating a series of shocks and accelerating the target. The evolving areal density is diagnosed by time-resolved radiography, using a second X-ray source. Data from several experiments are presented and diagnostic techniques are discussed.

We recall the main features of the LMJ. By using a simple but global model we determined different shells able to give a thermonuclear yield larger than 15 MJ; this model delimited an operating domain for the laser with a 25% margin to take into account the poorly understood phenomena. The different issues are related to the physics of the shell and to the physics of the hohlraum: optimisation of the shell implosion; laser-plasma interaction in the hohlraum; irradiation uniformity given by the hohlraum (2D simulations); implosion with nonuniformities; robustness against the experimental uncertainties; hydrodynamic instabilities during implosion. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.

We consider the Rayleigh-Taylor instability in linear density profiles and we derive the exact analytic expressions of the growth rates and associated eigenfunctions. We study the behavior of the multiple eigenvalues in both the short- and the long-wavelength limit. As the largest eigenvalue gamma(max) reduces to the classical Rayleigh growth rate; the other eigenvalues vanish as the front thickness tends to zero. Furthermore, the simple expression of gamma(max) exact to first order in the long-wavelength limit differs from the widely used estimate sqrt[Akg/(1+AkL0)], where g is the acceleration, A the Atwood number, k the wave number of the perturbation, and L0 the minimum density gradient scale length.

Nous rappelons les caracteristiques principales du LMJ. En employant un modele simple et global nous avons determine les differentes coquilles capables de degager une energie thermonucleaire superieure a 15 MJ; ce modele delimite un domaine de fonctionnement du laser avec une marge de 25% pour tenir compte des phenomenes mal compris. Les differents phenomenes ont trait a la physique de la coquille et a la physique de la cavite : optimisation de l'implosion de coquille; interaction laser-plasma dans la cavite ; uniformite d'irradiation donnee par la cavite (simulations 2D); implosion avec non-uniformites ; robustesse vis-a-vis des incertitudes experimentales; instabilites hydrodynamiques pendant l'implosion.