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International audience; RLCYC 75 is a new electrically calibrated laser calorimeter specially manufactured by Laser Metrology to calibrate energy diagnostics within the Laser MegaJoule (LMJ) facility. It consists of an optical cavity cooled by a hydraulic system. The system is designed to provide 1 m wavelength power laser measurements with uncertainty less than 1% at 2 kW and traceability to the International System of Units (SI). In this paper, the accuracy of RLCYC 75 measurements is studied. More precisely, three points are detailed: instrumentation uncertainty estimation, equivalence between optical and electrical supply and light absorption. To this end, electrical calibration campaigns and power laser measurement campaigns are conducted. Moreover, thermal and optical models are developed. Results show that RLCYC 75 design and instrumentation are efficient enough to reach the goal of relative uncertainty of about 1% at 2 kW. RLCYC 75 will become the 2 kW laser power primary standard for LMJ applications.

We present a 2D analysis of direct-drive shock ignition for the laser Megajoule. First, a target design is chosen in the HiPER-like target family generated by scale up and down of the original HiPER target. A first analysis is done considering the 1D fuel assembly and 2D shock ignition by means of the ring at polar angle of 33°2. The intensity profile is top-hat and calculations are done for several different radii. It is shown that larger the radius, lower the minimum spike power is. In addition, the intensity in each quad can stay below 4 × 1014 W/cm2 and is considered non crucial for parametric instabilities such as two plasmons. A 2D analysis of the fuel assembly is done in a second step by considering the two rings located at 49° and 59°5 and their symmetric by the equatorial plane symmetry. It is shown that low mode asymmetries are important at the stagnation and can significantly affect the areal density obtained. Finally, full 2D calculations of shock ignition is done, using all the beams of the LMJ and show that the spike power needed for ignition and gain is increased by a factor greater than 3 regarding the power needed in perfectly isotropic fuel assembly. This increase is mainly due to high level low mode asymmetries generated during fuel assembly.

The Laser MegaJoule (LMJ), built at CEA/CESTA near Bordeaux, France, will conduct research in the field of plasma physics and inertial confinement fusion. Some of these experiments are designed to produce during less than one nanosecond up to 10 19 neutrons at 14 MeV in indirect drive mode. The 300 m long building that hosts both laser sources and target chamber was designed to confine neutron and gamma fluxes in the experiment hall during a shot. Monte Carlo simulations were performed using a TRIPOLI 4 modelling of the facility to ensure that prompt doses in occupied areas inside the facility that host employees during a shot will be sufficiently low. Although significant radiation levels inside the experiment hall, prompt doses in occupied rooms are limited to a few μSv per shot which is fully compatible with radiation level limits.

As part of the French Inertial Confinement Fusion (ICF) program, CEA has developped Cryogenic Target Assemblies (CTAs) for the Laser MegaJoule (LMJ). These targets are filled by permeation with hig...

In this paper we present and discuss recent experimental and theoretical advances concerning the redistribution process, the control of target temperature, and the effect of deuterium-tritium (D-T) aging on the optimum laser shot temperature at the Laser Megajoule (LMJ) facility.We introduce two analytical models to provide a better understanding of thermal target behavior. On one hand the first model describes the evolution of the D-T layer temperature, which cannot be recorded experimentally. On the other hand the second model highlights the necessity for the optimum laser shot temperature (i.e., 1.5 K below the triple point) to be adapted to the aging of the target.The analytical considerations are completed with experimental results obtained with D2 taken as a reference system to investigate the properties of D-T in LMJ targets.

We have determined the thermal criterion for the Laser Megajoule cryogenic target that leads to a uniform layer of deuterium-tritium as specified for ignition. Thermal models were created, and computational fluid dynamics software was used to calculate this criterion in a spherical geometry as a function of capsule thermal conductivity. According to the values obtained, we have studied the possibility of removing anticonvection baffles in the cryogenic target. Results are presented in this paper and indicate that anticonvection baffles can be removed only if the gas density inside the cryogenic target is reduced and/or if the hydrogen content is reduced.

The cryogenic target assemblies (CTAs) designed for Laser Megajoule (LMJ) experiments have many functions and have to meet severe specifications imposed by implosion physics, the CTA thermal environment, and the CTA interfaces with the Megajoule laser cryogenic target positioner. Therefore, CTA fabrication uses many challenging materials and requires several technological studies. During the last 2 years, many developments have enabled better collection of comprehensive data on target constitutive materials and improvements in the fabrication of the CTA base, hohlraum, and aluminum turret.Studies have been carried out (a) to better characterize thermal properties of materials allowing optimization of the thermal simulation of the hohlraum, (b) to improve the CTA base fabrication process in order to optimize thermal studies of the LMJ experimental filling station (EFS), and (c) to determine coatings on the polyimide membrane that may limit the 300 K thermal effect on the microshell and increase the...

Smooth and uniform solid deuterium-tritium (DT) layers inside a spherical shell are needed in order to achieve ignition on the Laser Megajoule (LMJ) facility. The thermal environment around the cap...

The Laser Megajoule cryotarget positioner, PCC, will be used to set cryogenic targets in appropriate conditions for the laser shot in order to reach ignition. These conditions can be summarized as a few parameters of the deuterium-tritium (DT) solid layer: sphericity, roughness, and density of DT gas. The DT mixture is confined and held in a target assembly that is handled and cooled by the PCC. Thus, the parameters of the DT solid layer are controlled by the PCC. In particular, roughness depends on the control of the target base temperature (±1 mK), on the temperature slope while crossing the DT triple point (0.5 mK/min), and on the thermodynamic way followed to reach gas density conditions expected at the laser shot [slow cooling (0.5 mK/min), quenching (several kelvins per second), or rapid cooling (several kelvins per minute)]. Moreover, the required gas density needs high cryogenic power performances of the PCC to be fulfilled. As the target is gripped at cryogenic temperature by the PCC, the...

The Laser Megajoule (LMJ) cryogenic target is protected from ambient thermal radiation by a thermal shroud. When the cryotarget, held by the cryotarget positioner, is at the LMJ chamber center, the thermal shroud has to be removed just before the shot to allow the laser beams to reach the laser entrance hole of the cavity.The shroud remover, PET, will have to disconnect the thermal shroud from the cryogenic target base without disturbing the target base temperature regulation (˜18 K ± 2 mK), which guarantees the needed cryogenic target conditions to reach the ignition.The shroud withdrawal is divided into two successive phases: a slow withdrawal for the thermal disconnection between shroud and target base and a fast withdrawal for a quick extraction of the shroud out of the laser beamways pointing onto the cavity. The slow shroud withdrawal must be handled within 30 min to respect laser pointing stability. After the final target alignment at the chamber center, the shroud must be ejected 0.5 m awa...

The High Power laser Energy Research facility (HiPER) is a European project dedicated to demonstrating the feasibility of producing energy by laser-driven inertial confinement fusion. A first design of the fast ignition cryogenic target has been established. It is composed of a thin-walled microshell with an inserted gold cone and filled with deuterium-tritium (DT) fuel by means of a capillary (conically guided capsule). After assembly, targets must be tight at cryogenic temperatures (16 to 19.6 K).In order to evaluate the manufacturing feasibility of a single-shot target prototype, a program has been adapted from the Laser Megajoule (LMJ) cryogenic target fabrication know-how. Target component study for HiPER concerns a hollow gold cone (25-deg half-angle and ˜25-μm thickness), a thin polymeric microshell (2-mm diameter and 3- to 10-μm thickness), and a silica capillary (30-μm outer diameter).First gas-tight targets at 77 K have been produced (helium gas leak rate ˜1.4 × 10-11 Pa·m3/s). Major eff...

We use a self-consistent model combining the Quotidian Equation Of State and a nonrelativistic screened-hydrogenic model with l -splitting to study the thermodynamic conditions encountered along shock Hugoniot curves. We focus on the maximum compression ratio reached on shock Hugoniot curves for simple elements that are solid at normal temperature and pressure. Electron shell effect and pressure ionization can have a strong impact on the results. Numerical calculations are presented and discussed. Some of the physical points addressed in this work could be tested experimentally when the National Ignition Facility laser in the USA attains full power or when the Laser Megajoule in France comes online.

Nearly ten years ago, a research program concerning materials and technologies was defined to develop the very complex cryogenic target for obtaining the combustion of a deuterium-tritium mixture by inertial confinement fusion in the laser megajoule facility. The CEA target fabrication project includes research and development on various organic polymers and materials for the cryogenic laser megajoule target assembly as well as for other targets that can be useful for the fusion program (pre and post-ignition). Recent advances have been accomplished concerning the development of specific organic materials for the fabrication of targets components, including the synthesis of polymers for the laser megajoule microshells and metal-doped organic foams for the elaboration of doped-foam microshells or for the micromachining of components.

This paper presents a detailed assessment of the intermediate step design Laser Megajoule cryogenic target thermal behavior. Several simulations were carried out with a computational fluid dynamics code named FLUENT in order to better understand the effect of different parameters on this new design.

As part of the French Inertial Confinement Fusion program, Commissariat a l'Energie Atomique has developed cryogenic target assemblies (CTAs) for the Laser Megajoule (LMJ) and a program in two stages for the permeation filling of these CTAs: (a) the permeation filling studies with the Study Filling Station cryostats and (b) the design and manufacturing of the whole operational chain of CTA filling facilities. This paper deals with the description of both the cryogenic studying and the filling facilities for the LMJ targets.

Laser megajoule (LMJ) is a laser facility being built by the French nuclear science directorate, Commissariat a l'Energie Atomique. This facility is a large flashlamp pumped laser (1.8-MJ optical output). The pulsed power conditioning system of LMJ is based on a modular design of 480 modules (400-MJ energy capacitor bank). A new capacitor bank module (CBM) for the LMJ power conditioning system has been developed by the Institute of High Current Electronics (IHCE, Tomsk) in collaboration with International Technologies for High Pulsed Power (France). Two such modules were produced and tested in IHCE. The main parts of the CBM are the following: high-voltage power supplies, command control system, air conditioning system, capacitor block, and cables delivering the energy to a load. The parameters of the module are the following: total rated capacitance of 2900 (+4%) muF, charging voltage up to 24 kV, peak current amplitude of 240 kA, peak stored energy up to 864 kJ, pulsewidth at 10% of peak power of 360 mus, dimensions of 2.3 times 1.5 times 2.5 m3, and weight of ~2500 kg. In nominal regime (24-kV charging voltage, 40-m length cables), the energy delivered to flashlamps in each channel is 74 kJ (86% efficiency). No failures were observed in more than 1000 test shots, and all parameters of the CBM are well within the requirements. Detailed description of the module design, performance, and test results are stated in this paper.

As part of the French Inertial Confinement Fusion (ICF) experiments, cryogenic target assemblies (CTAs) for the Laser Megajoule (LMJ) program are manufactured and filled at CEA Valduc (Dijon) in tritium facilities. They will be moved at about 20 K into a transport cryostat for cryogenic targets, and will be driven from CEA/Valduc to CEA/CESTA (Bordeaux). This paper deals with the description of the tritium facilities for the LMJ cryogenic target. Twelve gloveboxes are needed to furnish 6 CTAs at the same time. These twelve gloveboxes make a relative independent set in the Valduc tritium building and house equipment to prepare the CTAs and the different vacuum vessels, to store and purify gas, to fill and cool the targets and transport them at cryogenic temperature.

The LASER MEGAJOULE (LMJ) will be an experimental fusion laser facility located at CEA-CESTA research center, close to Bordeaux. Fusion experiments will involve targets filled with tritium, requiring the design of containment devices to ensure worker protection against radiation exposure according to the French safety regulations. This paper reports the work performed by the LMJ safety team to design these confinement devices. At first, the paper describes the tritium inventory evaluated inside the facility. Then, specifications and allocations for the vacuum and air purge systems of the target bay are defined and explained. A spread model is described to evaluate the spatial repartition of the residual contamination in the chamber, concluding to an anisotropic process. Consequently, security factors have been defined according to the different locations inside the target bay. Definition of containment protections for the equipment of the target bay like fume hoods and glove boxes does not depend only on the effects of contamination (internal exposure hazards), but also on activation of materials (external exposure). Consequently, safety requirements have been defined in order to design containment devices. Finally, a specification of containment barriers performances is given (static and dynamic modes). Leakage of the barriers, negatives pressures, cleaning and conditioning are addressed.

International audience; The laser-induced damage of optical components of high-power laser systems such as the National Ignition Facility (NIF) (Murray, Proc SPIE 3492:1–10, 1998) and Laser MegaJoule (LMJ) (André, Fusion Eng Des 44:43–49, 1999) can have a significant impact on the operating costs of these laser systems since optimal shaping to carry out ignition is required. Laser-induced damage (LID) of fused silica appears on the exit surface, mostly at 3ω, and tends to grow exponentially with each laser shot. This damage affects the performance of optical components and limits their lifetime. The damage is due to thermal explosions of microdefects or microabsorbers, and these explosions emit a plasma that generates shock waves introducing cracks that increase the damage. This paper, thus proposes the deposit of an elastic layer on the exit surface to mitigate the shock waves and thereby decrease the extent of the damage volume. To this end, we have developed a mendable sol–gel-processed hybrid inorganic–organic layer. The hybrid layer is a nanocomposite made of silica and polydimethylsiloxane (PDMS). Their resultant mechanical properties can be tuned by adjusting the elastomer/silica ratio and catalysis conditions. The synthesis used to create these layers was investigated with an infrared spectrometer. The optical quality of the hybrid layers was checked by UV/visible/NIR spectrophotometry and thickness uniformity were mapped via reflectometry. Young’s modulus, Poison’s ratio, and the hardness have been measured in order to characterize the mechanical properties of these layers. The surface acoustic waves technique was used to determine Young’s modulus and Poison’s ratio of an optical layer obtained by a sol–gel process. The hardness has been measured with a homemade indenter. The results of first measurements of laser damage made with an excimer laser at 351 and 353 nm demonstrated a reduction of the laser-induced damage threshold with the PDMS/silica ratio.

SolGel coated fused silica samples were intentionally contaminated in a manner representative of the Laser Megajoule optics used in ISO 7 cleanrooms. They were then tested to assess the impact of this contamination on their LIDT.

In the context of high power laser systems, the laser damage resistance of fused silica surfaces at 351 nm in the nanosecond regime is a major concern. Under successive nanosecond laser irradiations, an initiated damage can grow which can make the component unsuitable. The localized CO2 laser processing has demonstrated its ability to mitigate (stopping) laser damage growth. In order to mitigate large damage sites (millimetric), a method based on fast microablation of silica has been proposed by Bass et al. [Bass et al., Proc. SPIE 7842, 784220 (2010)]. This is accomplished by scanning of the CO2 laser spot with a fast galvanometer beam scanner to form a crater with a typical conical shape. The objective of the present work is to develop a similar fast micro-ablation process for application to the Laser MegaJoule optical components. We present in this paper the developed experimental system and process. We describe also the characterization tools used in this study for shape measurements which are critical for the application. Experimental and numerical studies of the downstream intensifications, resulting of cone formation on the fused silica surface, are presented. The experimental results are compared to numerical simulations for different crater shape in order to find optimal process conditions to minimize the intensifications in the LMJ configuration. We show the laser damage test experimental conditions and procedures to evaluate the laser damage resistance of the mitigated sites and discuss the efficiency of the process for our application.

Recent progress in high-gain direct-drive inertial confinement fusion with the laser Megajoule is reviewed. A new baseline direct-drive target design is presented which implodes with a two-cones irradiation pattern of indirect-drive beam configuration and zooming. Perturbation amplitudes and correlated growth rates of hydrodynamic instabilities in the compressed core of a directly driven inertial confinement fusion capsule are analyzed in planar and spherical geometries, with and without heat conduction, in the unsteady state regime of the deceleration. Shock propagation in heterogeneous media is addressed in the context of first shock. The neutron and photon emissions of high-gain direct-drive target are characterized. Numerical interpretations of directly driven homothetic cryogenic D2 target implosion experiments on the Omega facility are presented.

A new solution for high-gain direct-drive fusion with the indirect drive beam irradiation of the Laser Megajoule is presented here. This solution uses 2 rings per hemisphere at polar angles of 49° and 59.5°. The 2D calculations with the hydrodynamics code FCI2 lead to a thermonuclear energy of 32 MJ with less than 1 MJ incident laser energy.

Petawatt lasers can generate extreme states of matter, making them unique tools for high-energy-density physics. Pressures in the gigabar regime can potentially be generated with cone-wire targets when the coupling efficiency is high and temperatures reach 2–4 keV (ref. 1). The only other method of obtaining such gigantic pressures is to use the megajoule laser facilities being constructed (National Ignition Facility and Laser MegaJoule). The energy can be transported over surprisingly long distances but, until now, the guiding mechanism has remained unclear. Here, we present the first definitive experimental proof that the heating is maximized close to the wire surface, by comparison of interferometric measurements with hydrodynamic simulations. New hybrid particle-in-cell simulations show the complex field structures for the first time, including a reversal of the magnetic field on the inside of the wire. This increases the return current in a spatially separated thin layer below the wire surface, resulting in the enhanced level of ohmic heating. There are a significant number of applications in high-energy-density science, ranging from equation-of-state studies to bright, hard X-ray sources, that will benefit from this new understanding of energy transport.

The Laser MegaJoule (LMJ) program plans to obtain Deuterium-Tritium (DT) mixture ignition leading to a fusion gain of ten. Cryogenic targets are hollow spheres whose interior is covered with a solid cryogenic fuel layer. The success of DT ignition depends on quality of the fuel layer uniformity. These targets must be cooled and kept at temperatures near the triple point (19.8 K) with a very good stability (+/-1 mK) for many hours, in the center of the 5 m radius experimental vacuum chamber with a position accuracy of a few microns. In order to validate our current device concepts, we have manufactured scale one prototypes to confirm all thermal and mechanical challenges, such as sharp thermal regulation, cooling autonomy and cryogenic target transfer.

The Laser Megajoule (LMJ) Cryotarget Positioner (PCC) will be used to set cryogenic targets in the vacuum chamber centre of this experimental facility for fusion by inertial confinement. In the Fre...

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.

Smooth solid DT layer inside a plastic spherical shell are needed in order to achieve the ignition on the Laser MegaJoule (LMJ) facility. This paper follows our previous one1 and presents the up to date results concerning the quenching and breathing techniques. New analysis software has been made which allowed us to update the results already obtained for the breathing technique. New experiments done on quenching have leaded to obtain a smooth layer for the higher mode down to 2.3 K below the triple point. The time where the roughness NIF specifications are obtained while the temperature is 1,5 K below the triple point is around 6 seconds using the quenching technique. Roughness analyses are made on one axis view. We also discuss other experimental aspects, such as shadowgraphy and redistribution process.

In this paper, we report the results of thermal simulations made for the cryogenic target of Laser MegaJoule (LMJ). Thermal simulations are done with FLUENT for a new 3-D model which takes in accou...

The Laser MegaJoule (LMJ) is a French high power laser project that requires thousands of large optical components. The wavefront performances of all those optics are critical to achieve the desired focal spot shape and to limit the hot spots that could damage the components. Fizeau interferometers and interferometric microscopes are the most commonly used tools to cover the whole range of interesting spatial frequencies. Anyway, in some particular cases like diffractive and/or coated and/or aspheric optics, an interferometric set-up becomes very expensive with the need to build a costly reference component or a specific to-the-wavelength designed interferometer. Despite the increasing spatial resolution of Fizeau interferometers, it may even not be enough, if you are trying to access the highest spatial frequencies of a transmitted wavefront for instance. The method we developed is based upon laser beam diffraction intermediate field measurements and their interpretation with a Fourier analysis and the Talbot effect theory. We demonstrated in previous papers that it is a credible alternative to classical methods. In this paper we go further by analyzing main error sources and discussing main practical difficulties.

An axially symmetric laser beam configuration irradiating a spherical capsule has been considered in the context of inertial confinement fusion (ICF). The laser beams are located at co-latitudes 49° and 131° and mimic the quad positions in the second cone of the Laser Mégajoule Facility. The capsule is directly irradiated by the laser beams whose energy deposition generates a nearly spherical shock wave. Two-dimensional hydrodynamic numerical simulations have been performed to analyse the non-uniformity of the shock wavefront launched inward throughout the target. Different laser intensity profiles, calculated by the illumination model, have been tested. The performance, in terms of shock non-uniformity, has been compared, and it is found that with an appropriate choice of the laser intensity profile it is possible to control the shock non-uniformity at early times.

The ablator material for inertial confinement fusion (ICF) capsules on the Laser Megajoule is a glow-discharge polymer (GDP) plastic. Its equation of state (EOS) is of primary importance for the design of such capsules, since it has direct consequences on shock timing and is essential to mitigate hydrodynamic instabilities. Using ab initio molecular dynamics (AIMD), we have investigated the 300-K isotherm of amorphous CH(1.37)O(0.08) plastic, whose structure is close to GDP plastic. The 300-K isotherm, which is often used as a cold curve within tabular EOS, is an important contribution of the EOS in the multimegabar pressure range. AIMD results are compared to analytic models within tabular EOS, pointing out large discrepancies. In addition, we show that the effect of oxygen decreases 300-K isotherm pressure by 10%-15%. The implication of these observations is the ability to improve ICF target performance, which is essential to achieve fusion ignition.

The Laser MegaJoule (LMJ) timing system has to synchronize 176 laser beams better than 40 ps rms to compress symmetrically the millimeter-size target in order to ignite the deuterium and tritium filled capsule despite the fact that the quadruplet laser sources are separated within the building by several hundreds of meters. This kind of performance is also required for fiducial pulses used to temporally mark laser and plasma diagnostics.

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.

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 Laser MegaJoule (LMJ) under construction at CESTA, a CEA laboratory located near Bordeaux, is designed to produce 240 beams, 1.8 MJ, 500 TW of ultraviolet light for high energy density physics experiments and thermonuclear burn of DT targets. The first step of LMJ program consisted in the construction of a full scale prototype the "Ligne d'Integration Laser" (LIL) which was developed to demonstrate LMJ performances and qualify all its components and systems as well as maintenance procedures. Commissioned in 2002, LIL facility mainly includes a laser bay which houses a bundle of 8 beamlines similar to the LMJ ones and a target bay where are located the 1ω-3ω conversion system, the focusing gratings and the 4.4-meter diameter target chamber equipped with plasma diagnostics. This paper describes the LIL facility as a tool for laser-matter interaction pointing out the laser system.

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.

The nominal configuration of inertial confinement fusion with the Laser MegaJoule (LMJ) uses a cylindrical hohlraum with two polar holes for the entrance of 60 laser quads. But the LMJ facility makes it possible to use a spherical hohlraum with more than two holes. We have studied two alternate configurations, in which 56 and 44 laser quads enter a spherical hohlraum through four and six holes respectively, with the same wall surface as in the nominal cylindrical hohlraum. We have estimated the intrinsic and random radiation asymmetries on the nominal capsule A 1040 in these two configurations.

Cryogenic inertial confinement fusion targets at the National Ignition Facility and the Laser Megajoule will be protected from thermal infrared radiation by a cold shroud. As the shroud is removed just before the laser pulse, infrared radiation will heat and possibly degrade the symmetry of the solid hydrogen fuel layer. A lumped component mathematical model has been constructed to calculate how long an indirect drive target can be exposed to thermal radiation before the fuel layer degrades. The allowed exposure time sets the maximum shroud removal time and therefore has important implications for the design of the cryogenic shroud systems. The model predicts that the maximum exposure time is approximately 0.18 s for plastic capsules inside hohlraums with transparent laser entrance holes. By covering the laser entrance holes with a partially reflective coating, the exposure time can be increased to approximately 1 s. The exposure time can be increased to about 2 s by using beryllium capsules. Several other design concepts could increase the exposure time even further. Lengthening of the allowed exposure time to 1 s or longer could allow a significant cost savings for the shroud system.

Several low atomic number materials as beryllium, polyimide or hydrocarbon can be suitable as an ablator for high energy class lasers. On CEA Laser "Megajoule " (LMJ) facility, amorphous hydrogenated carbon (a-C:H or CH x ), is the nominal ablator used to realize inertial confinement fusion (ICF) experiments. These capsules contain the fusible deuterium-tritium mixture to achieve ignition. Coatings are prepared by glow discharge polymerization (GDP) with trans-2-butene and hydrogen and can be easily doped with germanium adding tetramethylgermanium. Laser fusion targets must have optimized characteristics: diameter of 2.4 mm for LMJ targets, thickness up to 167.5 μm, sphericity and thickness concentricity better than 99% and an outer and an inner roughness of a few nanometers at high modes. The surface finish of these laser fusion targets must be extremely smooth in order to minimize hydrodynamic instabilities. With GDP techniques, it is possible to obtain coatings without any growing structures and thus only with very small nodules at the surface by controlling the coating parameters and having low deposition rates (lower than 0.5 μm/h). This allows to obtain high mode roughness lower than 10 nm. Nevertheless, this is not sufficient to obtain LMJ specifications especially at intermediate modes which are strongly degraded by local defects ("bumps"). In order to eliminate mostly of these singular points that could be the source of the defects, etching during the film growth can be useful. In this work, we show how we reduced the roughness of germanium doped CH x microshells by adding helium in the plasma or pulsing hydrogen in order to have an exacerbated etching effect and also a higher surface temperature and thus a better mobility of the adsorbed species. By controlling these parameters, we obtained germanium doped CH x microshells compatible with the LMJ specifications. The RMS roughness from modes 10 to 1000 is lower than 20 nm and from modes 2 to 10 is around 160 nm. New designs of graded germanium doped microshells improve the stability of the target to hydrodynamic instabilities. This allows relaxing the specification of the roughness. The first microshells were synthesized and the results are presented in this paper.

An arbitrary beam shaping-zooming concept is proposed for direct drive fusion on the Laser Megajoule and the National Ignition Facility. It is based on the intrinsic conception of these facilities which use four beamlets per beam. A substantial improvement of the laser–target coupling efficiency is achieved with a lowering of ignition threshold. A design providing a thermonuclear energy of about 30 MJ with less than 1 MJ energy-laser has been investigated. The control of low-modes irradiation uniformity is achieved dynamically with an enhancement of the hydrodynamic stability of the acceleration and deceleration phases.

The progress of the construction of very large laser facilities LMJ and NIF enables the prediction of inertial fusion achievement. These facilities will open new fields for research: the high energy density physics. Pressures of several 100 Mbars and temperatures of several 100 eV will be reached. Measurements of material properties (EOS and opacities) which have been demonstrated on current or former facilities will be possible at these never reached conditions. Pure hydrodynamics (instabilities) and radiative hydrodynamics astrophysical issues will be addressed.However, ignition and gain as a first proof of Inertial Confinement Fusion is a primary goal. The indirect drive route to inertial fusion has been prepared for many years by CEA (Commissariat a l'Energie Atomique). The last ten years were imprinted by a close collaboration between CEA and US-DOE in both the areas of facilities R&D and ignition target physics.The scientific issues are well known: the propagation of laser light through the very long plasma created inside the hohlraum has to be understood and mastered to be sure that less than 10% of laser energy will be backscattered by parametric instabilities. On the other hand, the stability of the capsule implosion has to be matched with the fabrication surface finish so as to avoid shell destruction and extinction of the central hot spot. Recent advances at CEA have allowed a better confidence of reaching ignition using the facility previously specified. These works used the CEA computing capability combined with plasma experiments on existing lasers facilities. Ignition achievement also supposes the realization of suitable cryogenic targets.CEA began the construction of the Laser Megajoule (LMJ), a 240-beam laser facility, at the CEA Laboratory CESTA near Bordeaux. The LMJ is designed to deliver 2 MJ of 0.35 µm light to targets for high energy density physics experiments.Four beams were operated for plasma experiments on the Ligne d'Integration Laser (LIL) at CESTA, for the end of 2004, meeting the specifications for LMJ. The realization phase of the LMJ facility was initiated in March 2003 with the start of construction of the building and the target chamber.

The Laser Megajoule (LMJ) facility, dedicated to inertial confinement fusion experiments, is under construction at CEA center near Bordeaux since summer 2003. Operation with hundred micrograms of deuterium–tritium will generate high 14 MeV neutron yields that will induce high neutron activation and high doses rates levels in the target chamber area. Previous calculations [Ph. Joyer, M. Dupont, H.P. Jacquet, Safety issues on Laser Megajoule (LMJ) facility, Nucl. Eng. Des. (2003)] have shown relatively high doses rates and the need to reduce activation in the experimental bay. Paths of activation and doses rates reduction were identified. The present paper gives the last results obtained.

The French Commissariat a l’Energie Atomique (CEA) began the construction of the Laser Megajoule (LMJ), a 240 beams laser facility, at the CEA Laboratory CESTA near Bordeaux. The LMJ will be a cornerstone of the CEA “Programme Simulation”, similar to LLNL “NIF” facility. The LMJ is designed to deliver 2 MJ of 0.35 μm light to targets for high energy density physics experiments and to ultimately obtain ignition and propagating burn with DT targets in the laboratory. The scientific conception and system design was completed in 1999 and was followed by the Demonstration of an Engineering Prototype which was achieved in early 2003 with operation of one beam of the Ligne d’Integration Laser (LIL) at CESTA, with 9.5 kJ of UV light (0.35 μm) in less than 9 ns from a single laser beam. The construction phase of the LMJ facility was initiated in March 2003 with the start of work on the building and the target chamber. This paper will present a summary of 2004 LIL results and a description of LMJ, which is expected to demonstrate first light performance through 240 beams by 2009.

The Laser Megajoule project is a major component of the French simulation program to study nuclear fusion by inertial confinement. The future Laser Megajoule facility requires control-command systems that will operate in a harsh radiative environment. Commercial off-the-shelf optical fiber data links are envisaged as a radiation tolerant solution for this application. In this paper, we present our preliminary study of their vulnerability. For this, we firstly have used an original method consisting of ultraviolet (/spl sim/5 eV) exposures of the fibers to identify the different germanosilicate optical fibers containing phosphorus, which leads them unacceptable for both steady state /spl gamma/-rays and successive pulsed X-ray irradiations. We have demonstrated the validity of the /spl gamma/-UV comparison by spectroscopic measurements. After this first selection, we have tested under pulsed X-rays (dose rate >10 MGy/s dose

Based on the results presented at the 20th IAEA Fusion Energy Conference 2004, this paper highlights the most important recent advances in inertial confinement fusion (ICF). With the construction of the National Ignition Facility (NIF) and the Laser Megajoule facility and many improvements in the target design, the conventional indirect-drive approach is advancing steadily towards the demonstration of ignition and high gain. The development of the polar direct-drive concept also made the prospects for direct-drive ignition on the NIF very favourable. Substantial progress was reported on the exploration of the fast-ignition approach to ICF. Parallel to that, multi-wire Z-pinches have become a competitive driver option for achieving ignition at the lowest possible cost. In heavy-ion fusion, experiments have been devoted so far to studying the generation, transport, and final focusing of high-current ion beams. A new concept for a power plant with a heavy-ion driver, based on a cylindrical direct-drive target compressed and ignited (in the fast-ignition mode) by two separate beams of very energetic (Ei 0.5 GeV u−1) heavy ions, has been proposed.

The French Commissariat a l'Energie Atomique (CEA) began the construction of the Laser Megajoule (LMJ), a 240-beam laser facility, at the CEA Laboratory CESTA near Bordeaux. The LMJ will be a cornerstone of the CEA 'Programme Simulation', the French analog of the US Stockpile Stewardship Program.The LMJ is designed to deliver 2 MJ of 0.35 µm light to targets for high energy density physics experiments and to ultimately obtain ignition and propagating burn with DT targets in the laboratory.The Scientific conception and system design was completed in 1999 and was followed by the Demonstration of an Engineering Prototype which was achieved in early 2003 with the operation of one beam of the Ligne d'Integration Laser (LIL) at CESTA, with 9.5 kJ of UV light (0.35 µm) in less than 9 ns from a single laser beam.The realization phase of the LMJ facility was initiated in March of 2003 with the start of construction of the building and the target chamber.This paper will present results from the commissioning phase of the LIL program in 2003 and 2004. The activation and commissioning of all eight beamlines of LIL over the next 2 years will be part of determining the final specifications and integration and commissioning plans for the LMJ, which is expected to demonstrate first light performance through 240 beams by 2009.

In the context of the French Laser-Mfusion-research programme, a direct-drive target design is developed. It is based on the use of a CH-foam ablator filled with cryogenic deuterium-tritium. One-dimensional optimization leads to a potential gain of 60 with a 1.5 MJ laser. The hydrodynamic stability of the implosion is investigated at the ablation front and the hot-spot surface by means of modelling and two-dimensional simulations. The effect of irradiation non-uniformities on low-mode (time-dependent) implosion asymmetries is studied by two-dimensional hydrodynamics simulations with three-dimensional laser-light raytracing. The effect of beam focal shapes on hot spot low-mode asymmetries is also addressed. PACS numbers: 52.273Bc

In the context of the French Laser Mégajoule (LMJ) fusion research program, direct drive is an alternate to indirect drive to reach ignition and thermonuclear burn. We present recent progress in the direct-drive fusion studies for LMJ. Calculations have shown that the LMJ irradiation uniformity is characterized by long wavelength asymmetries compatible with direct drive requirements. Calculations of the irradiation uniformity in the context of indirect drive beam positioning have been done. We show that non-uniformity can be minimized by repointing the beams. Unfortunately, a time analysis shows that this nonuniformity increases strongly in time above levels usually considered inconsistent for direct drive. Finally, a recent baseline target design is presented and consists of a DT ice shell surrounded by a low-density CH foam wicked with cryogenic DT. This design can potentially reach a gain of 90 with a 1-MJ on-target laser driver. Hydrodynamic stability is increased at the ablation front and the laser–target coupling efficiency achieves 85%.

The Laser Megajoule cryotarget positioner will be used to set the cryogenic targets at the center of the experimental vacuum chamber. It will consist of a 6-meter carbon boom at the end of which a target will be held by a cryogenic grip at 20 K. In the French concept, the targets will be transferred at 20 K to the cryotarget positioner using another cryostat. Some of the specifications are very ambitious. Indeed, the targets must be positioned with a high accuracy (+/-5 {mu}m), the temperature must be controlled with a very good stability and the cryostat must have a 5 day autonomy. To reach these requirements, some technical solutions have been considered. To validate these choices, a scale one prototype has been studied and built at the Low Temperature Laboratory (SBT) of CEA/Grenoble (France). We present here the first results obtained with this prototype.