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In order to produce laser targets for laser plasma experiments, the target department of Commissariat a l’Energie Atomique operates in different fields of production techniques. Additive manufactur...

High-Z metallic foils including rare-earth (RE) elements are required for some experiments implemented on the Laser Megajoule. A specific process based on physical vapor deposition and laser machining was developed to produce high-Z material foils meeting strict specifications. This process allows pure metallic ultrathin foil fabrication from a few hundred nanometers to several microns of thickness of any high-Z materials. In the case of RE metals sensitive to oxidation, thin foils are buried under aluminum protective layers of about a few hundred nanometers. These metallic thin foils are flat, show thickness uniformity over 95%/cm2, and have roughness of about 10 nm. The foils are opaque to light, have a density similar to bulk material, present an oxygen content of about 1 at. %, and are stable over months under atmospheric conditions.

Pure gold-copper alloys are known to be difficult to electrodeposit because of a strong variation in composition after a few microns have been deposited. Commissariat al’Energie Atomique (CEA) stud...

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...

Hydrodynamics growth experiments involve rippled ablator samples (CHx, Ge:CH, or Si:CH). The rippled surface features a microscale mathematical shape (sinusoidal functions are widely used). Neverth...

To produce the laser targets needed for laser plasma experiments, the CEA target department uses different mechanical machining techniques and develops methods that are consistent with the target r...

Within the framework of fundamental physics studies at CEA, laser target experiments are conducted to investigate laser-matter interactions. Some of these experiments require metallic foams samples in a milimetric scale with an apparent volumetric mass within few hundreds milligrams per cube centimeters. Metallic foams materials are developed and studied since few years for their unique properties offered by their aerated structure. For example, due to a high specific surface, metallic foams could find applications in batteries, supercapacitors, as well as chemical sensors. However, all the metallic foams obtained by the processes referenced in the literature do not meet the specifications needed for laser experiments. This is why, the CEA has developed a new and innovative technique to synthetize metallic foams by plasma electrolysis deposition [1]. This process consists in applying a voltage within the 20-100V range at a cathode immerged in a solution containing metallic ions. Under these specific conditions, a uniform gaseous envelop is created around the cathode which is then isolated from the liquid. Due to the high electrical field applied at the electrode surface, several electrical plasma discharges are formed between the cathode and the gas/liquid interface. When the electrical discharges meet the solution, metallic cations are reduced to form metallic strands about 100 nm in diameter. The formed strands are all interconnected together to form metallic foam with micrometric porosities [2]. A few millimeters wide foam with a volumetric mass up to 100mg/cc is obtained in about 10 seconds which is very fast compared to classical electrochemical deposition techniques. The foams synthetized can be made up of pure metals (Cu, Au, Pt ...) or alloys (AuCu ...) in accordance to the metallic ions diluted in the solution. In order to control the shape and homogeneity of the synthetized foams, a better understanding of the mechanisms involved in the foams’ growth is required. By direct camera observation and Scanning Electron Microscopy (SEM), we can show that the growth behavior and the structure of the foams vary with the applied voltage. We observe three main growth patterns which seem to correspond to the different propagation modes of electrical discharges reported in the literature on plasma discharges in dielectric liquids [3,4]. The first one observed at “low” voltage is composed of very thin strands (Ø < 100 nm) growing linearly with few ramifications. The second one is formed by highly branched strands about 100 to 600 nm in diameter. And the third one obtained at the highest voltages is made up of strands above 1 µm thick with a shape very similar to the aggregates obtained in diffusion limited conditions [5]. Furthermore, we observe that the potential predominance range of each growing mode can be shifted by modifying the concentration or the nature of the metallic ions. Our investigations show that the mass transportation plays a major role with the applied voltage in defining the growth mode of the metallic foams. Electron BackScatterred Diffraction (EBSD) and X-ray Diffraction (XRD) analyses have been conducted to study the crystalline structure of each mode and thus providing information on the conditions of electro-crystallization of the strands. This work concludes by proposing an electrochemical model of the foam growth. References: [1] R.Botrel, US patent n° US 10,400,345 B2 (2019) [2] S.Rocher et al., Eur. Phys. J. Appl. Phys. 81, 10803 (2018 [3] R.E.Hebner, Series B: Physics. 193 (1987) [4] O. Lesaint, J. Phys. D: Appl. Phys., 49, 144001 (2016) [5] V.Fleury et al., Phys. Rev. A, 43, 12 (1991) Figure 1

Implosion experiments of an inertial confinement fusion (ICF) target on the laser megajoule (LMJ) and the National Ignition Facility require, for certain designs, a precise timing coalescence of fo...

International audience; The goal of this work is to realize metallic foams synthesized by contact glow discharge electrolysis with specific characteristics. In this paper, we show the results of our studies, consisting in investigating parameters that influence the foams characteristics. Thus, the morphology of metallic foams is examined through scanning electron microscopy (SEM) observations with the acid nature. Moreover, the evolution of the mass and the volume of metallic foams with two experimental parameters (overvoltage and gold concentration) is also investigated. The acid nature affects the foams microscopic structure highlighted by the SEM observations, but for now no valid explanation to this behaviour was found. We prove that the mass deposited on the electrode is dependent on the ionic salt concentration, whereas the overvoltage only affects the foam overall density.

Glass thin films appear particularly interesting as semipermeable barriers for many noncryogenic target applications. This functional layer can be sputtered from quartz targets onto CHx microshells...

In the context of National Ignition Facility Basic Science program we propose to study on the NIF ablative Rayleigh–Taylor (RT) instability in transition from weakly nonlinear to highly nonlinear regimes. Based on the analogy between flame front and ablation front, highly nonlinear RT instability measurements at the ablation front can provide important insights into the initial deflagration stage of thermonuclear supernovae of type Ia. NIF provides a unique platform to study the rich physics of nonlinear and turbulent mixing flows in High Energy Density plasmas because it can accelerate targets over much larger distances and longer time periods than previously achieved on the NOVA and OMEGA lasers. In one shot, growth of RT modulations can be measured from the weakly nonlinear stage near nonlinear saturation levels to the highly nonlinear bubble-competition, bubble-merger regimes and perhaps into a turbulent-like regime. The role of ablation on highly-nonlinear RT instability evolution will be comprehensively studied by varying ablation velocity using indirect and direct-drive platforms. We present a detailed hydrocode design of the indirect-drive platform and discuss the implementation plan for these experiments which only use NIF diagnostics already qualified.

Targets dedicated to high power lasers experiments require accurate and reliable manufacturing technologies. In that frame, laser micro-welding represents a potential powerful tool for targets assembly. The feasibility of joining highly reflective and thin materials has been studied. Demanding requirements have to be fulfilled (gas tightness and target's geometry).

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.

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.

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...

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.

Photon unoccupied states tailored by nanostructures present at sample surfaces can be probed using a scanning near-field optical microscope including large detection angles (Chicanne C. et al., Phys. Rev. Lett., 88 (2002) 097402-4). We demonstrate, theoretically and experimentally, how such setups equipped with a circularly polarized SNOM tip can deliver images of surface photonic states with a nanometer scale resolution.

For the “Megajoule” 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 contai...

We show that the spectral tailoring of optical local density of states (LDOS) may be achieved by lithographically designed nanostructures and that the subwavelength mapping of the spectral variation of the optical LDOS is feasible by varying the driving frequency of the effective dipole used in an illumination mode scanning near-field optical microscope.

We study the conditions to perform micro-spectroscopy with a sub-wavelength lateral resolution in the wavelength spectral range from 3 to 20 μ m, taking advantage of the infrared spectral signature of different chemical species. We utilised CLIO, a free electron laser, as the photon source. The transmitted photons were collected by either fluoride or chalcogenide glass fibres. Fibre tips were obtained through chemical etching by organic solvents. Metallisation of the tips permits to achieve lateral resolution of the order of the tip size. However, parasitic propagation of the light in the film reduces the contrast between irradiated and non-irradiated zones. We exemplify our set up with near-field infrared spectra obtained for polymer thin films deposited onto silicon wafers.

We have developed a scanning near field optical microscope (SNOM) based on bent optical fiber probes with a normal force atomic force microscopies (AFM) regulation. The optical fibers are used both as optical probes and AFM cantilevers. This SNOM was especially developed for transmission mode SNOM and it offers the possibility to realize simultaneously all AFM modes imaging and optical analysis. The fiber is used to illuminate the sample and the detection is performed through an inverted optical microscope. This article presents results obtained on dielectric, metallic, and magnetic samples with submicronic features. We have resolved the object of 80 nm in size, separated by 40 nm, and we illustrate the capabilities of our SNOM in investigating, locally, the variation of the optical properties of periodic samples, even though contaminant, and for a large variety of samples.

We use the LIL (Ligne d'Integration Laser ) facility to study the coalescence of two planar shocks in an indirectly-driven planar sample of polystyrene. This experiment represents the preliminary stage of the future shock-timing campaign for the Laser Megajoule (LMJ). The main objectives are to validate the experimental concept and to test the numerical simulations. We used a gold spherical hohlraum to convert into X-ray the 351 nm wavelength laser pulse and to initiate the two shocks in the sample. To access time resolved shock velocities and temperature, we used two rear-side diagnostics: a VISAR (Velocity Interferometer System for Any Reflection) working at two different wavelengths and a streaked optical self-emission diagnostic. We observed the coalesced shock, in good agreement with the numerical simulations. We also observed a loss of signal during the first nanoseconds probably due to sample heating from the hohlraum X-ray flux.

International audience; This paper reports the experimental observation, at optical frequencies, of the electromagnetic local density of states established by nanostructures corresponding to the recently introduced concept of optical corral [G. Colas des Francs et al., Phys. Rev. Lett. 86, 4950 (2001)]. The images obtained by a scanning near-field optical microscope under specific operational conditions are found in agreement with the theoretical maps of the optical local density of states. A clear functionality of detection by the scanning near-field optical microscope is thereby identified since the theoretical maps are computed without including any specific tip model.

Understanding and mitigating hydrodynamic instabilities and the fuel mix are the key elements for achieving ignition in Inertial Confinement Fusion. Cryogenic indirect-drive implosions on the National Ignition Facility have evidenced that the ablative Rayleigh-Taylor Instability (RTI) is a driver of the hot spot mix. This motivates the switch to a more flexible higher adiabat implosion design [O. A. Hurricane et al., Phys. Plasmas 21, 056313 (2014)]. The shell instability is also the main candidate for performance degradation in low-adiabat direct drive cryogenic implosions [Goncharov et al., Phys. Plasmas 21, 056315 (2014)]. This paper reviews recent results acquired in planar experiments performed on the OMEGA laser facility and devoted to the modeling and mitigation of hydrodynamic instabilities at the ablation front. In application to the indirect-drive scheme, we describe results obtained with a specific ablator composition such as the laminated ablator or a graded-dopant emulator. In application to the direct drive scheme, we discuss experiments devoted to the study of laser imprinted perturbations with special phase plates. The simulations of the Richtmyer-Meshkov phase reversal during the shock transit phase are challenging, and of crucial interest because this phase sets the seed of the RTI growth. Recent works were dedicated to increasing the accuracy of measurements of the phase inversion. We conclude by presenting a novel imprint mitigation mechanism based on the use of underdense foams. The foams induce laser smoothing by parametric instabilities thus reducing the laser imprint on the CH foil.

We present two designs relevant to ablative Rayleigh-Taylor instability in transition from weakly nonlinear to highly nonlinear regimes at the National Ignition Facility [E. I. Moses, J. Phys.: Conf. Ser. 112, 012003 (2008)]. The sensitivity of nonlinear Rayleigh-Taylor instability physics to ablation velocity is addressed with targets driven by indirect drive, with stronger ablative stabilization, and by direct drive, with weaker ablative stabilization. The indirect drive design demonstrates the potential to reach a two-dimensional bubble-merger regime with a 20 ns duration drive at moderate radiation temperature. The direct drive design achieves a 3 to 5 times increased acceleration distance for the sample in comparison to previous experiments allowing at least 2 more bubble generations when starting from a three-dimensional broadband spectrum.

To carry out laser plasma experiments on CEA laser facilities, a R&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.

The combination of atomic-force and scanning-near-field optical microscopies is useful for characterizing the physical and optical parameters of optoelectronic devices. With a commercial atomic-force microscope adapted to perform scanning-near-field optical measurements, we succeed in determining core diameters, localizing the erbium doping zone, and analyzing propagation modes in erbium-doped and multimodal optical fibers.