Demonstrating ignition and net energy gain in the near future on MJ-class laser facilities will be a major step towards determining the feasibility of Inertial Fusion Energy (IFE), in Europe as in the United States. The current status of the French Laser MégaJoule (LMJ) programme, from the laser facility construction to the indirectly driven central ignition target design, is presented, as well as validating experimental campaigns, conducted, as part of this programme, on various laser facilities. However, the viability of the IFE approach strongly depends on our ability to address the salient questions related to efficiency of the target design and laser driver performances. In the overall framework of the European HiPER project, two alternative schemes both relying on decoupling target compression and fuel heating—fast ignition (FI) and shock ignition (SI)—are currently considered. After a brief presentation of the HiPER project's objectives, FI and SI target designs are discussed. Theoretical analysis and 2D simulations will help to understand the unresolved key issues of the two schemes. Finally, the on-going European experimental effort to demonstrate their viability on currently operated laser facilities is described.

Recent changes in the manner of performing hohlraum drive experiments have significantly advanced the ability to diagnose, understand and control the x-radiation flux (or drive) inside a laser heated hohlraum. Comparison of modeling and data from a very broad range of hohlraum experiments indicates that radiation hydrodynamics simulation codes reproduce measurements of time dependent x-radiation flux to about ±10%. This, in turn, indicates that x-ray production and capsule coupling in ignition hohlraums will be very close to expectations. This article discusses the changes to experimental procedures and the broad variety of measurements and tests leading to these findings.

The objective of Target Physics Program at CEA is the achievement of ignition on the LMJ, a glass laser facility of 1.8 MJ which will be completed by 2008. They include theoretical work, experimental work and numerical simulations. An important part of experimental studies is made in collaboration with U.S. DOE Laboratories : Lawrence Livermore National Laboratory, Los Alamos National Laboratory and the Laboratory for Laser Energetics at the University of Rochester. Experiments were performed on Phebus, NOVA (LLNL) and OMEGA (LLE); they included diagnostics developments. Recent efforts have been focused on Laser Plasma Interaction, hohlraum energetics, symmetry, ablator physics and hydrodynamic instabilities. Ongoing work prepare the first experiments on the LIL which is a prototype facility of the LMJ (8 of its 240 beams). They will be performed by 2002. Recent progress in ICF target physics allows us to precise laser specifications to achieve ignition with reasonable margin.

The laser program developed at the Centre d'Etudes de Limeil-Valenton, Saint-Georges, France (CEL-V) is concentrated on a systematic investigation of indirect drive fusion; by comparison with direct drive, this process is expected to provide the required irradiation uniformity with relaxed constraints on laser beam quality. The main concerns are radiative transfer and preheat, hydrodynamic instabilities, and high-density X-ray driven implosions. Ablative implosion experiments have been conducted with the two beams at the Phebus facility (5 kJ, 1.3 ns, 0.35 μm). Symmetry was proved to be controlled by the casing structure, following scaling laws describing hohlraum physics. A compressed DT density ∼100 ρ0 (ρ0 liquid DT density) has been deduced from activation measurements. Different aspects of the soft X-ray transfer processes, and particularly of the ablation of a low-Z material, which drives the capsule implosion, are dealt with in detailed investigations. Reported here are results on X-ray reemission and penetration in several materials, and on induced hydrodynamics of accelerated foils. The laser energy required to reach fuel ignition conditions has been evaluated from numerical simulations as well as from analytical models, taking into account hohlraum physics, capsule implosion, hot spot formation, and burn propagation. Several crucial parameters have been drawn, the most important being the radiation temperature. A target gain in the order of 10 appears achievable with a 2-MJ laser.

Inertial-confinement fusion targets filled with argon are widely used to diagnose the compressed core in implosion experiments. Shape analysis of the compressed core by imaging technique can be very interesting especially to determine the effect of the irradiation symmetry on the implosion performance, For this purpose, we are developing a monochromatic imaging diagnostic ARGONIX which uses a flat multilayer mirror in order to select the required monochromatic energy and a coded aperture in order to increase the overall sensitivity. This diagnostic has been tested on Phebus in indirect drive mode by imploding glass microballoons filled with only a DT mixture and plastic microballoons filled with pure argon, Penumbral images of silicon background emission from the pusher and argon monochromatic annular coded image of argon He-beta line will be presented together with their unfolded images.

Implosion experiments performed at Centre d'Etudes de Limeil-Valenton in the indirect drive scheme using the two-beams Nd:glass laser facility Phebus at the energy level = 6 kJ (blue light) are presented. A final density of compressed DT close to 100 ρ0 has been obtained; it has been deduced from radiochemistry of the activated silicon atoms in the pusher. The best irradiance uniformity on the microballoon was evaluated to = 15% rms. Phebus has also been equipped with an optical fiber oscillator to study the effect of a smoothing technique on coupling processes: It appeared that at 0·53 μm absorption efficiency is increased by =15–20%. With the eight-beams Octal laser, hydrodynamic instabilities development in accelerated planar targets has been investigated both for direct and indirect drives; the mixing zone detected at the light-heavy interface does not present visible bubble-and-spike like structures and is less developed in the indirect configuration. Atomic physics in laser plasmas is also deeply studied; a particular effort has been made on absorption spectroscopy, a powerful diagnostic of ionization dynamics in cold and dense plasmas. Experiments have been realized either in multilayered targets or using rear-side X-ray emission of thin Au foils to heat the samples. To reach fuel ignition conditions, more powerful lasers, in the range of megajoule, will be needed. Their design needs further technological developments to reduce the capital cost in $/W. At Limeil, we work mainly on high-damage threshold optical coatings, using the sol-gel process, high-quality, low-cost mirror fabrication, using the replica technics, and incoherent laser pulse generation for beam smoothing.

We have developed a silicon radiochemistry diagnostic to measure the glass pusher areal density <ρΔR≳ of D-T filled targets and irradiated with the two beams of the Phebus laser. We describe the experimental setup (debris automated retrieval and counting systems) and its performance. This diagnostic has been used to characterize the implosions performed in direct and indirect drive schemes. DT final densities as high as 100×DT liquid have been achieved.

A scaling model for hot spherical ablative implosions in direct-drive mode is presented The model results have been compared with experiments from LLE, ILE, and LLNL. Reduction of the neutron yield due to illumination nonuniformities is taken into account by the assumption that the neutron emission is cut off when the gas shock wave reflected off the center meets the incoming pusher, i.e., at a time when the probability of shell breakup is greatly enhanced. The main advantage of this semiempirical scaling model is that it elucidates the principal features of these simple implosions and permits one to estimate very quickly the performance of a high-aspect-ratio direct-drive target illuminated by short-wavelength laser light.

We present results of X-ray emission by the rear side of gold thin targets irradiated by the Phebus laser at λ = 0.35 μm, τ = 0.7 and 1.3 ns, and E L = 1.5 kJ. A streak camera coupled with a transmission grating gave the time-resolved X-ray emission of the rear side. Also, a streak camera coupled with a slit allowed us to obtain information about the space and time evolution of the plasma. Some other diagnostics gave information about the energy balance and the X-ray conversion efficiency. The results are in good agreement with previous ones obtained with the Octal laser, particularly on optimum thicknesses for X-ray conversion efficiencies. Values of the thermal flux limiter are deduced. Simulations with FCI 1 code with multigroup radiative transfer and non-LTE ionization reproduce the experimental results only about some points. A number of reasons, such as 2-D effects and problems of opacity, are invoked.

The Phebus laser was used to irradiate gold targets at λ=0.35 μm; the laser pulse was 0.7 or 1.3 ns and the energy on target 1 to 1.8 kJ. The incident intensity was varied from 3×1013 to 3×1015 W/cm2. The time history of x-ray emission was monitored in the range 0.2–2.5 keV with a streak camera coupled with a transmission grating. Numerical simulations cannot replicate the temporal evolution of soft x-ray emission (hν<800 eV), although the total conversion efficiency is correctly reproduced. Some observations, including time and space-resolved images, show that unexpected soft x-ray emission is produced in the corona at large distances from the target surface. A possible explanation is that cold and dense jets are generated towards the underdense plasma by an instability.

The experiments reported in this paper demonstrate that the PHEBUS laser facility is now currently being operated with high performances (4 TW with 250 ps pulses at 0·527 μm wavelength).The output energy of the 2-beam PHEBUS laser system can be focused either in a small focal spot (80% of the incident energy is in a 220 μm diameter focal spot) for high intensity experiments (≥5 × 1015 W cm−2) or in very large spots (a few mm in diameter) at moderate intensities (1013 − 2·5 × 1014 W cm−2), for large scale experiments. It is shown that the spatial intensity distribution in the target plane is primarily due to intensity independent aberrations and to diffraction. Laser light absorption in plane aluminum and gold targets are interpreted in terms of inverse bremsstrahlung absorption that may account for 70 to 90% of absorbed energy. Finally, the plasma expansion is shown to be very planar and comparison with one-dimensional Lagrangian simulations gives flux limiter values of 0·03 and 0·02 respectively for Al and Au targets.

Velocities up to 108 cm s−1 have been obtained in laser implosion of gold-coated microballoons driven by irradiation at short wavelength. Despite a strongly nonuniform illumination, high neutron yield (i.e., high fuel temperature) has been obtained. Results analysis suggests that the glass shell is first decompressed by x-ray radiation from the gold coating and then accelerated by the ablation pressure.

The hydrodynamic behavior of double-foil targets irradiated with 0.35 μm laser wavelength at irradiance of 2×1014 W cm-2 has been investigated by means of time and space resolved X-ray backlighting. Comparison with a 1-D lagrangian code shows discrepancies attributed to energy losses in the transfer between two foils. First results obtained with a 2-D code are presented for a single thin target. We infer that the hydrodynamic lateral expansion of the accelerated layer is the dominant process of energy loss.