Laurent Jeannot

Spatial Reconstruction Algorithm of DT Layer in Cryogenic Targets Using Optical Techniques

Abstract The measurements of the solid DT layer, in terms of thickness and roughness, in the LMJ geometry (i.e. in a hohlraum) are not trivial. The DT layer measurements will be done using a Matsukov-Cassegrain telescope placed 39 cm away from the target. This telescope will be used to acquire shadowgraphy images on equators, and interferometric measurements on pole areas using optical coherence tomography (OCT). Optical coherence tomography allows determining the DT layer thickness on a few points, in the polar regions of the target. By scanning around the poles, several points can be acquired in order to calculate the roughness and the local shape of the DT layer at the pole. Both techniques were demonstrated on a 175 μm thick microshell with a 100 μm thick D2 layer. A reconstruction algorithm was designed to give the whole shape of the DT layer from the partial data given by shadowgraphy and OCT. A 3D spatial estimation of the DT layer can be obtained. The algorithm efficiency was improved, with the use of 360 points on shadowgraphic image and 11 points on each pole. An estimation of the spatial DT layer shape was given on the first 90 longitudinal modes and on the first 5 equatorial modes.

With Regard to the Low Mode Target Lifetime: An Analytical Model

Abstract Smooth and uniform solid deuterium-tritium (DT) layers inside a spherical shell are needed in order to achieve ignition on the Laser Mégajoule (LMJ) facility. The thermal environment around the capsule is the key to meeting the DT layer requirements. While keeping high mode roughness within the specifications at the shot temperature is now guaranteed by a rapid cooling technique, low mode roughness (“shape” of the layer) is still a complicated and demanding subject. A perfectly uniform temperature field around the capsule is needed. Final results of the constant thermal perturbation effects on the layer can be calculated, but the dynamic of reaction is not known. This paper presents a model that allows calculation of the low mode layer behavior depending on a change in the temperature field. This comes down to calculating a target lifetime for the low modes during a thermal transient state.

CHARACTERIZATION OF THE MICROSHELL SURFACE USING HOLOGRAPHIC MEASUREMENTS

Abstract To characterize the shape, the quality, and the roughness of microshells, typically used technologies are scanning electron microscopy, scanning interferometric microscopy, or atomic force microscopy. One of the drawbacks of these techniques is that they are generally slow because of their scanning process. Digital holographic microscopy technology is an innovation that can offer ability adapted to these studies. It captures holograms instead of intensity images, as done by conventional microscopes. The holograms are then digitally interpreted (10 per second) to reconstruct a double image, one for the intensity and another one for the phase. Using a rotation axis, the bump counting for the complete microshell surface is possible with a very high speed. Using an image stitching software, mapping can be done in a few minutes. Wavelets such as “Mexican hat” are used to model the bumps. Each bump can then be characterized on the map by its position, diameter, and height.

Characterization of the DT Layer of ICF Targets by Optical Techniques

Abstract The characterization of the solid DT layer, in terms of thickness and roughness, in the LMJ geometry (hohlraum) is not trivial. The DT layer measurements will be done using a Maksutov-Cassegrain telescope, 39 cm away from the target. This telescope will be used to acquire shadowgraphy images and spectral-interferometry measurements. Shadowgraphy imaging probes the DT layer geometry at the equator of the target. Spectral-interferometry gives the DT layer thickness on one spot on the shell, in the polar regions of the target. By scanning around the poles, several points can be acquired to probe the roughness and the local shape of the DT layer at the poles. This paper presents the spectra-interferometry technique and explains how the DT layer thickness could be deduced from channelled spectra. First experimental results on a 125 μm thick empty shell are also reported.

TPX Foams for Inertial Fusion Laser Experiments: Foam Preparation, Machining, Characterization, and Discussion of Density Issues

Abstract Low density foams (in this work, foam density refers to apparent density) are materials of interest for fusion experiments. Low density poly(4-methyl-1-pentene) (commercial name TPX) foams have been produced for ˜30 years. TPX foams have been shown to have densities as low as 3 mg·cm-3, which is very close to air density (1.2 mg·cm-3). Around this density foams are very light and highly fragile. Their fabrication is thus a real technological challenge. However, shrinking always appears in ranges ranking from 25% to almost 200%. As a result, the apparent density of the final foam never matches the expected value given by the precursor solution concentration. Besides, even if the mold dimensions are precisely known, shrinkage is never linear, and foams have to be machined for precise density measurement. In our work we present a fabrication process for TPX foams and discuss machining and density measuring issues. Particularly, we have found that there are volume and weight limits for a determination of density within the range of 3% uncertainty. This raises the question whether density should rather be determined directly on millimeter-sized targets or should be performed on a bigger scale sample prepared from the same batch.

THE CRYOGENIC STUDYING AND FILLING FACILITIES FOR THE LASER MEGAJOULE TARGETS

Abstract As part of the French Inertial Confinement Fusion program, Commissariat à l’Energie Atomique has developed cryogenic target assemblies (CTAs) for the Laser Mégajoule (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.

A Model to Characterize the D-T Layer of ICF Targets by Backlit Optical Shadowgraphy

Abstract A numerical model is presented in order to modelize the bright ring that appears in backlit optical shadowgraphy on a transparent hollow sphere with a solid deuterium-tritium layer inside. This novel model is based on computational calculations applied to the problem of the targets used in inertial confinement fusion. The model takes into account the influences of the optical imaging system (numerical aperture, source divergence, camera resolution, etc.) and the effect of the capsule itself, diameter, thickness, and refractive index, and allows one to analyze the inner surface of a capsule in terms of thickness and roughness.

Low-Mode Target Lifetime: Experimental Results

Abstract Smooth and uniform solid D-T layers inside a spherical shell are needed to achieve ignition on the Laser Megajoule (LMJ) facility. The thermal environment around the capsule is the key to reach the low-mode D-T layer requirements. During the nineteenth Target Fabrication Meeting in Orlando, Florida (2010), an analytical model was presented to predict the low-mode time evolution of a D-T layer in a capsule caused by a thermal perturbation. The model showed that the dynamical response is ruled by the redistribution time constant. To check the validity of the model, experiments have been done with deuterium layers inside an integrating sphere. The use of an infrared laser to generate a volumetric heating of the deuterium allowed us to tune the conformation time constant. The experimental setup has also been modified to allow or cancel 300-K infrared radiation entering the integrating sphere, producing a local warming on the capsule. Using shadowgraphy techniques, we have been able to follow the dynamical behavior of the deuterium layer. Analyses conclude that the analytical model is right and can be used with confidence.