[en] The shock ignition of thermonuclear fuels allows an efficient direct drive laser ignition since the hot spot is put in ignition conditions by the overpressure given by the shock. The laser impulse that generates such a shock requires a final peak of high power. Preliminary studies at HIPER (High Power laser Energy Research) facility have shown the potentialities of shock ignition: a great tolerance for the Rayleigh-Taylor instability and a great acceptance of no perfectly symmetrical irradiation conditions. (A.C.)

[en] High convergence, hohlraum-driven implosions will require control of time-integrated drive asymmetries to 1% levels for ignition to succeed on the NIF. We review how core imaging provides such asymmetry measurement accuracy for the lowest order asymmetry modes, and describe recent improvements in imaging techniques that should allow detection of higher order asymmetry modes. We also present a simple analytic model explaining how the sensitivity of symmetry control to beam pointing scales as we progress from single ring per side Nova cylindrical hohlraum illumination geometries to NIF-like multiple rings per side Omega hohlraum illumination geometries and ultimately to NIF-scale hohlraums

[en] Polar direct drive (PDD), a promising ignition path for the NIF while the beams are in the indirect-drive configuration, is currently being investigated on the OMEGA laser system by using 40 beams in six rings repointed to more uniformly illuminate the target. The OMEGA experiments are being performed with standard, ''warm'' targets with and without the use of an equatorial ''Saturn-like'' toroidally shaped CH ring. Target implosion symmetry is diagnosed with framed x-ray backlighting using additional OMEGA beams and by time-integrated x-ray imaging of the stagnating core

[en] The elementary theory of phenomena occurring in indirect drive implosion of cryogenic capsule: ablation and shell acceleration by incoming X flux, shell deceleration and its inner ablation by electronic conduction which provides mass to hot spot, gives the final DT state in terms of initial target size and of laser power. The method to optimize pulse shape is given. The theory of ignition threshold is revisited with comparison to numerical simulations. Safety conditions, to prevent instabilities and implosion asymmetries, allow to define a target size and its laser energy which provide with a good safety a gain Thermonuclear energy/Laser energy of 20. (author)

[en] Layered and characterized cryogenic D₂ capsules have been imploded using both low- and high-adiabat (α, the ratio of the electron pressure to the Fermi-degenerate pressure) pulse shapes on the 60-beam OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the Laboratory for Laser Energetics (LLE). These experiments measure the sensitivity of the direct-drive implosion performance to parameters such as the inner-ice-surface roughness, the adiabat of the cryogenic fuel during the implosion, the laser power balance, and the single-beam nonuniformity. The goal of the direct-drive program at LLE is to demonstrate a high neutron-averaged fuel ρR at a significant fraction of the predicted one-dimensional (1-D) neutron yield using an energy-scaled, low-adiabat (α∼3) ignition pulse shape driving a hydrodynamically scaled deuterium-tritium ignition capsule. New results are reported from implosions of ∼920-μm-diam, thin (∼5 μm) polymer shells containing 100 μm D₂-ice layers with characterized inner-surface ice roughness of 3-12 μm rms. These capsules have been imploded using ∼17-23 kJ of 351 nm laser light with a beam-to-beam rms energy imbalance of less than 5% and full beam smoothing [1 THz bandwidth, two-dimensional (2-D) smoothing by spectral dispersion and polarization smoothing]. Near-1-D performance has been measured for a high-adiabat (α∼25) drive pulse, and the implosion performance with a low-adiabat (α∼4) pulse is in agreement with 2-D hydrocode predictions

[en] In the indirect driven implosion, the scattering laser from the hohlraum wall will partially irradiate on the capsule and influence its implosion. Based on the research of one-dimensional and two-dimensional numerical simulations, detailed analyses are pursued to give the influence of the scattering laser on the implosion and the explanation of the influence. (authors)

[en] Numerical studies show that a rugby-shaped hohlraum for indirect drive laser ignition has some advantages: it allows a better symmetry for the X-ray irradiation of the central target and it required less laser power. Rugby-shaped cavities have been tested successfully at the Omega facility. The energetic advantage is all the more important as the cavity is bigger. Simulations have shown that a rugby-shaped hohlraum plus adequate materials for the intern wall plus an optimization of the central target could open the way to an ignition with only 160 laser beams at the LMJ (Megajoule Laser) facility. (A.C.)

[en] The indirect drive method of inertial confinement fusion uses a high-Z radiation case to convert energy from high-powered laser beams to x rays which implode fusion capsules. Experiments have been performed on the Nova laser to characterize the x-ray production in high-Z cavities for studying the efficiency for x-ray production using two methods for characterization. One method measures the shock velocity produced in low-Z materials by the radiation. The shock velocity is measured by observing the optical signal from the rear of a stepped or continuously varying thickness of Al placed over a hole in the cavity wall. The other method measures the reradiated x-ray flux from the cavity wall viewing through a hole in the cavity. Both methods have been shown to provide a consistent characterization of the x-ray drive in the cavity target

[en] The LMJ target chamber is designed to withstand high fusion energy produced by 20 MJ yield shots and 600 no yield shots per year on a large variety of targets. We show how the target chamber and its equipment have to be protected from damaging by 14 MeV neutrons, high x-ray fluences, debris and shrapnel. A first wall made of louvers is required to prevent material ablated by x-rays from reaching the debris shields and reducing their life time. Constraints on the target design is also considered to limit the sources of vaporized or ionized mass and shrapnel, coming directly from the target. The use of pre-shield for large mass targets is suggested to meet the requirement on the frequency cleaning of the debris shields. Shock absorber structures have been successfully tested to prevent diagnostic and target positioner noses close to the target from being damaged. (authors)

[en] An experimental campaign dedicated to the study of the direct drive laser implosion has been performed at the Omega laser in Rochester. The Chronomix experiment has tested the stability of the implosion concerning the impact of all the defects generated either by the laser implosion or the target itself. It appears that the stability of the implosion depends only on the shape of the laser pulse. The shorter the pulse, the higher the stability. A series of simulations involving groups of homothetic targets has shed light on the ignition process. (A.C.)