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Targets experimented on the Laser Megajoule (LMJ) facility are composed of amorphous hydrogenated carbon capsules. Some of them present rippled surface features like sinusoidal functions. Other experimented targets are hohlraum-containing capsules. The main difficulty when analyzing the machined capsules is to characterize the feature’s orientation and the sinusoidal shape featured in the capsule thickness by laser machining. For the capsule enclosed by the hohlraum, the main challenge is to characterize the capsule centering inside the assembled hohlraum. X-ray tomography is used to realize measurement, and obtained results are presented in this paper.

Some experiments implemented on the Laser Megajoule facility (LMJ) require the use of the rare-earth (RE) elements, the lanthanides (57 < Z < 71). Rare-earth metals are known to be unstable under a...

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...

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.

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.

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...

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.

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.