LMJ Target Research Articles

Development in Glass Shells Production from Silicon-Doped GDP for Noncryogenic LMJ Target

Glass shells made from the pyrolysis of silicon-doped glow discharge polymers (Si-GDP) are particularly interesting for many noncryogenic target applications. We investigated the possibility of developing millimeter glass shells with >10-μm-thick walls to achieve a half-life of several months. Although previous studies have already demonstrated their feasibility, important developments are still needed to finely understand the role each step plays on the final glass shell’s properties. The adjustment of plasma deposition parameters and pyrolysis conditions allowed us to control shell shrinkage and defect formation. In the case of 7.4 at. % Si-GDP slowly pyrolyzed, we obtained spherical and smooth glass shells with near 100% yield. We also demonstrated that adjusting sintering temperature can produce fully dense glass shells from 2.2 to 2.4 g/cm3. Finally, deuterium pressurized capsules >3 MPa with a half-life of 8 months are obtained.

Germanium Doped CHx Microshells for LMJ Targets

At the CEA Laser “Megajoule” facility, amorphous hydrogenated carbon (a-C:H or CHx) 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 CHx film.

Progress on LMJ targets for ignition

Targets designed to produce ignition on the Laser MegaJoule are presented. The LMJ experimental plans include the attempt of ignition and burn of an ICF capsule with 160 laser beams, delivering up to 1.4MJ and 380TW. New targets needing reduced laser energy with only a small decrease in robustness have then been designed for this purpose. Working specifically on the coupling efficiency parameter, i.e. the ratio of the energy absorbed by the capsule to the laser energy, has led to the design of a rugby-shaped cocktail hohlraum. 1D and 2D robustness evaluations of these different targets shed light on critical points for ignition, that can be traded off by tightening some specifications or by preliminary experimental and numerical tuning experiments.

New designs of LMJ targets for early ignition experiments

The LMJ experimental plans include the attempt of ignition and burn of an ICF capsule with 40 laser quads, delivering up to 1.4MJ and 380TW. New targets needing reduced laser energy with only a small decrease in robustness are then designed for this purpose. A first strategy is to use scaled-down cylindrical hohlraums and capsules, taking advantage of our better understanding of the problem, set on theoretical modelling, simulations and experiments. Another strategy is to work specifically on the coupling efficiency parameter, i.e. the ratio of the energy absorbed by the capsule to the laser energy, which is with parametric instabilities a crucial drawback of indirect drive. An alternative design is proposed, made up of the nominal 60 quads capsule, named A1040, in a rugby-shaped hohlraum. Robustness evaluations of these different targets are in progress.

Update on ignition studies at CEA

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.

Planar Rayleigh-Taylor and Feedthrough experiments with CH(Ge) on OMEGA

Germanium-doped CH (CHGe) is one nominal ablator for LMJ target design. To investigate its properties we performed indirect drive planar Rayleigh-Taylor experiments on the OMEGA laser facility [1]. On each shot foil motion and modulations growth were simultaneously measured by side-on and face-on radiography, while drive was assessed by measuring radiation escaping through the hohlraum laser-entrance-hole (LEH). This complete set of data allows a more stringent comparison between the hydrocode simulations and the experimental results. We compare CHGe perturbations growth with those acquired on CHBr in the same experimental configuration. These preliminary results are the first step toward a test-bed validation of CH(Ge) as an ablator on OMEGA and further on LiL [2].

Gas Etching to Obtain Germanium Doped CHx Microshells Compatible with the Laser Megajoule Target Specifications

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 CHx), 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 CHx 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 CHx 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.

Roughness Optimization at High Modes for GDP CHx Microshells

For the “Mégajoule” Laser (LMJ) facility of the CEA, amorphous hydrogenated carbon (a-C:H) is the nominal ablator to be used for 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. The films properties have been investigated. Laser fusion targets must have optimized characteristics: a diameter of about 2.4 mm for LMJ targets, a thickness up to 175 μm, a sphericity and a thickness concentricity better than 99% and an outer and an inner roughness lower than 20 nm at high modes. The surface finish of these laser fusion targets must be extremely smooth to minimize hydrodynamic instabilities.Movchan and Demchishin, and later Thornton introduced a structure zone model (SZM) based on both evaporated and sputtered metals. They investigated the influence of base temperature and the sputtering gas pressure on structure and properties of thick polycrystalline coatings of nickel, titanium, tungsten, aluminum oxide. An original cross-sectional analysis by atomic force microscopy (AFM) allows amorphous materials characterization and permits to make an analogy between the amorphous GDP material and the existing model (SZM). The purpose of this work is to understand the relationship between the deposition parameters, the growing structures and the surface roughness.The coating structure as a function of deposition parameters was first studied on plane silicon substrates and then optimized on PAMS shells. By adjusting the coating parameters, the structures are modified, and in some case, the high modes roughness decreases dramatically.

Dependability and Safety for Megajoule Laser Operations and Maintenance

Risk assessment of a complex system that uses new technologies is described: the MegaJoule Laser (LMJ). The steps to obtaining the safety and security goals for LMJ are highlighted, i.e., to ensure safe operation, to put in place detection systems and means to prevent or limit serious accidents, and to limit the consequences to the environment and the workers. The objective is to show what has been done during LMJ design and construction to prevent serious accidents during routine operations and maintenance.The risks during operations and maintenance of the LMJ facility are identified by a preliminary risk analysis. The safety management to address adequately the protective features is described. As an example, a main accidental scenario in the LMJ target area is emphasized: workers’ exposure to hazardous levels of nuclear radiation. The LMJ operation and maintenance safety requirements that must be taken into account by the industrial firms to which LMJ contracts have been awarded are discussed.

Thermal and Hydrodynamic Study of the LMJ Cryogenic Target

The temperature distribution within the LMJ target has been extensively studied during the past three years regarding its constitutive materials, its geometry and its thermal environment within the LMJ experiment chamber. By the way, the target definition has evolved and a new architecture is now under consideration. A complete three-dimensional thermal simulation of this prototype has been lead. Its results are described in the paper. At the same time, our calculation efforts were focused on cavity hydrodynamics, especially concerned with overcoming free convection. Previous simulations results have indeed indicated that the thermal distribution is dependent on the filling pressure when the latter is over 10 kPa. Cavity filling CFD simulations are also presented and discussed.

Thick GDP Microshells for LIL and LMJ Targets

Amorphous hydrogenated carbon (a-C:H) is the nominal ablator to be used in French inertial confinement fusion (ICF) experiments. These capsules, containing the deuterium-tritium mixture, are developed for the LIL (Laser Integration Line) and the future Megajoule laser (LMJ) of the CEA. Coatings are prepared by glow discharge polymerization (GDP) with trans-2-butene and hydrogen. The films properties have been investigated. Laser fusion targets must have optimized characteristics: a diameter of about 1 mm for LIL targets and about 2.4 mm for LMJ targets, a thickness up to 175 μm, an outer and an inner roughness lower than 20 nm at high modes, a sphericity and a thickness concentricity better than 99%. This paper presents the first microshells obtained at the CEA with a GDP (Glow Discharge Polymerization) coater. Amorphous hydrogenated carbon shells of 175 μm with 1 mm or 2.4 mm diameter have been successfully prepared. The measured roughness at high modes is lower than 10 nm for a 30×30 μm characterization window.