[en] In the Commissariat a l'Energie Atomique Laser Megajoule (LMJ) facility, amorphous hydrogenated carbon (a-C: H or CH_x) is the nominal ablator used to achieve inertial confinement fusion experiments. These targets are filled with of fusible mixture of deuterium-tritium in order to perform ignition. The a-C: H shell is deposited on a poly-alpha-methylstyrene (PAMS) mandrel by glow discharge polymerization with trans-2-butene, hydrogen, and helium. Graded germanium doped CH_x micro-shells are supposed to be more stable regarding hydrodynamic instabilities. The shells are composed of four layers for a total thickness of 180 μm. The germanium gradient is obtained by doping the different a-C: H layers with the addition of tetra-methylgermanium in the gas mixture. As the achievement of ignition greatly depends on the physical properties of the shell, the thicknesses, doping concentration, and roughness must be precisely controlled. Quartz microbalances were used to perform an in situ and real-time measurement of the thickness in order to reduce the variations and so our fabrication tolerances on each layer thickness. Ex situ control of the thickness of each layer was carried out, with both optical coherent tomography and interferometry, (wall-mapper). High-quality, PAMS and a rolling system have been used to lower the low-mode roughness [root-mean-square (rms) (mode 2) ≤ 70 nm]. High modes were clearly, reduced by, coating the pan containing the shells with polyvinyl alcohol + CH_x instead of polystyrene + CH_x resulting in an rms (≥mode 10) ≤ 20 nm, which can be ≤15 nm for the best micro-shells. The germanium concentration (0. 4 and 0. 75 at. %) in the a-CH layer is obtained by regulating the tetramethyl-germanium flow. Low range mass flow controllers have been used to improve the doping accuracy. (authors)

[en] At the CEA Laser 'Megajoule' facility, amorphous hydrogenated carbon (a-C:H or CH_x) is the nominal ablator used to achieve inertial confinement fusion experiments. These targets are filled with a fusible mixture of deuterium-tritium in order to perform ignition. Since the achievement of ignition greatly depends on the physical properties of the shell, there must be precise control of thicknesses, doping concentration, and roughness. Experimental devices associated with suitable characterizations are described in this paper. The tolerances and yields for each specification are also presented. Some specifications are largely reached; high-frequency surface roughness due to isolated surface defects appears to be the main yield-limiting factor. A microscopic approach of stress thin film measurement is described to examine oxygen uptake in CH_x film. (authors)

[en] To carry out laser plasma experiments on CEA laser facilities, a R and D program was set up and is still under way to deliver complex targets. For a decade, specific developments are also dedicated to 'Ligne d'Integration Laser' (LIL) in France and Omega facilities (USA). To prepare the targets intended for the first experiments on the Laser 'Megajoule' (LMJ) facility, new developments are required, such as cocktail hohlraum fabrication, gas barrier coating and foam shells developments. For fusion experiments on LMJ, an important program is also under way to elaborate the Cryogenic Target Assembly (CTA), to fill and transport the CTA and to study the conformation process of the DT layer.

[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.)

[en] 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 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 micro-shell and increase the deuterium-tritium fuel lifetime. CTAs have been produced to evaluate fabrication knowledge, to characterize CTAs, to study air tightness, and to study filling and D₂ ice layering on the EFS. An overview of the results that have been obtained during the past 2 years is presented in this paper. (authors)

[en] Highlights: • A dried droplet method is proposed to characterize matrix effects in a laser-produced plasma with a standardized approach • Iron lines from an iron solution residue on the sample surface are used for Boltzmann plots and Stark broadening measurements • The deposit does not affect the ablated mass, the electron excitation temperature T_e and number density n_e • Optimized with Al and Cu alloys, then applied to the characterization of matrix effects for 14 pure metals ablated at 266 nm • The largest differences are observed for the number of ablated atoms. They are much less pronounced for T_e and n_e The dried droplet method is proposed as a way of characterizing matrix effects in LIBS with a standardized approach. This method was introduced first in the field of LA-ICPMS for quantitative analysis of solids. It consists in depositing a droplet of an iron-containing solution on the sample surface and ablating the dry residue. Then, iron lines are used for spectroscopic diagnoses of the plasma. Along with white-light profilometry analysis for ablation craters measurements, this aims to accurately determine differences of ablated mass, electron excitation temperature and number density, including for pure metals. In this paper, we check that the presence of the dry residue does not influence those three factors. Then, the dried droplet method is applied to 14 pure metals. Results show that the number of ablated atoms varies by a factor of 25, while for the electron excitation temperature the maximum gap between the 14 metals is approximately 2000 K, i.e. a relative variation of ~30%. As for the electron number density, it could be estimated for only 6 metals and it varies within the measurement uncertainty between 6 and 8 10¹⁶ cm⁻³.

[en] We present in this article direct-drive experiments that were carried out on the Omega facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Two different pulse shapes were tested in order to vary the implosion stability of the same target whose parameters, dimensions and composition, remained the same. The direct-drive configuration on the Omega facility allows the accurate time-resolved measurement of the scattered light. We show that, provided the laser coupling is well controlled, the implosion time history, assessed by the “bang-time” and the shell trajectory measurements, can be predicted. This conclusion is independent on the pulse shape. In contrast, we show that the pulse shape affects the implosion stability, assessed by comparing the target performances between prediction and measurement. For the 1-ns square pulse, the measured neutron number is about 80% of the prediction. For the 2-step 2-ns pulse, we test here that this ratio falls to about 20%