M. Theobald

Research Program for the Fabrication of the Cryogenic Target to Reach Ignition on the LMJ. Results and Prospects

Abstract The “CEA cryogenic target fabrication project” includes materials and technological researches on the Cryogenic Target Assembly (CTA), the DT filling and cryogenic transport to the LMJ site, and the conformation of the DT solid layer. The Research program to deliver CTAs to LMJ in 2010 is described. Some important experimental results have already been obtained and are presented.

Gas etching to obtain germanium doped CHX microshells compatible with the laser megajoule target specifications

Abstract 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

Abstract 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.

Thick GDP Microshells for LIL and LMJ Targets

Abstract 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.

THICKNESS, DOPING ACCURACY, AND ROUGHNESS CONTROL IN GRADED GERMANIUM DOPED CHX MICROSHELLS FOR LMJ

Abstract In the Commissariat à l’Energie Atomique Laser Megajoule (LMJ) 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. The a-C:H shell is deposited on a polyalphamethylstyrene (PAMS) mandrel by glow discharge polymerization with trans-2-butene, hydrogen, and helium. Graded germanium doped CHX microshells 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 tetramethylgermanium 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 (wallmapper). 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 + CHX instead of polystyrene + CHX resulting in an rms (>mode 10) < 20 nm, which can be <15 nm for the best microshells. The germanium concentration (0.4 and 0.75 at.%) in the a-CH layer is obtained by regulating the tetramethylgermanium flow. Low range mass flow controllers have been used to improve the doping accuracy.

Germanium Doped CHx Microshells for LMJ Targets

Abstract 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.

Graded Germanium Doped CHx Microshells Meeting the Specifications of the Megajoule Laser Cryogenic Target

Abstract For the CEA Laser “Mségajoule” (LMJ) facility, amorphous hydrogenated carbon (a-C:H or CHx), is the nominal ablator for inertial confinement fusion (ICF) experiments. These capsules contain the fusible deuterium-tritium mixture in order to achieve ignition. Coatings are prepared by glow discharge polymerization (GDP) with trans-2-butene and hydrogen. They can be easily doped with germanium by adding tetramethylgermanium. The GDP technique is well known today and largely used in American and French laboratories. But the microshells for laser fusion targets have many stringent characteristics. Although the feasibility of the shells has been demonstrated, the goal is now to obtain graded germanium doped shells meeting all the specifications with a yield compatible with a production step.

The LMJ cryogenic target assembly: Functions and fabrication

Abstract The Megajoule Laser cryogenic system fills, transports and inserts on the Cryogenic Target Positioner (CTP) individual Cryogenic Target Assemblies (CTAs), which are manipulated at about 20K by several cryogenic grippers. This CTA has to meet severe specifications imposed by implosion physics, its own thermal environment, and to respect a lot of interfaces with the permeation cell of the filling station, the several cryogenic grippers, the Megajoule laser interaction chamber,… Therefore, the CTA definition is very complex, and induces a lot of challenging tasks for its fabrication. During the last year, many improvements have been achieved allowing the realization of the first CTA prototype at scale one.

Tritium Ageing Studies for "LMJ Target" Applications: Poliymide and CHx Membranes Permeation Results

Abstract 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 high pressure deuterium-tritium gas mixture. The evolution of the materials physical properties, particularly organic ones (PI and CHx), which compose the target is unknown in these hard conditions. The polyimide and CHx membranes permeation was studied in this context. The hydrogen and helium permeation parameters are unaltered for polyimide membrane after tritium ageing in the 20 K-300 K temperature range: KHe = 3.10-14 exp(-16040/RT) mol.m-1.s-1.Pa-1 KH2 = 2.10-14 exp(-16950/RT) mol.m-1.s-1.Pa-1 First results for the CHx hydrogen permeation parameters without ageing tritium have been obtained: KH2 = 7.10-13 exp(-10550/RT) mol.m-1.s-1.Pa-1 New CHx permeation results after tritium ageing are shortly waited.