D. A. Steinman

Status of inertial fusion target fabrication in the USA

This paper summarizes the current techniques used in the USA for fabrication of targets for inertial confinement fusion (ICF) experiments at the five ICF laboratories in the USA. It reviews the current target specifications that can be achieved and discusses directions for development of targets for ignition in the National Ignition Facility.

Automated Batch Characterization of ICF Shells with Vision-Enabled Optical Microscope System

Abstract Inertial Confinement Fusion (ICF) shells are mesoscale objects with nano-scale dimensional and nanosurface finish requirements. Currently, the shell dimensions are measured by white-light interferometry and an image analysis method. These two methods complement each other and give a rather complete data set on a single shell. The process is, however, labor intensive. We have developed an automation routine to fully characterize a shell in one shot and perform unattended batch measurement. The method is useful to the ICF program both for production screening and for full characterization. It also has potential for Inertial Fusion Energy (IFE) power plant where half a million shells need to be processed daily.

White Light Interferometry for the Optical Characterization of Transparent ICF Shells

Abstract White light interferometry has been adapted to the characterization of transparent ICF shells and their precursor mandrels. The combination of an interferometric microscope, a precision z-stage, and simulation-derived analysis algorithms allow determination of the diameters of the inner and outer surfaces, their non-concentricity, the location of interfacial layers, the average index of refraction of the walls, and the thickness of discrete layers within the shell wall. The hard- and soft-ware required for these measurements are described.

NEW DEVELOPMENTS IN GLASS SHELLS FROM SiGDP — RESIDUAL GASES, GAS FILLING AND TAILORED HALF-LIVES

Abstract Progress has been made in reducing and quantifying residual gases in shells manufactured by the silicon doped glow discharge polymer (SiGDP) to glass process. Previously, glass shells were made using a high temperature, open-air box oven. If the temperature profile used was sufficient, clear, colorless shells were obtained which had ~1/3 of an atmosphere of residual gas consisting of a mixture of N2, O2, CO and CO2 with generally N2 and CO2 being the major constituents. Improvements to the process were made by utilizing a controlled atmosphere, high temperature oven and developing an improved temperature profile for the SiGDP to glass conversion process. It is now possible to manufacture clear, colorless glass shells containing noble gas(es), which is a first for the ICF program. In addition, the improvements in our process has led to shells containing less residual gas (N2, CO, and CO2) than previously obtainable. Tailored deuterium halflifes are also possible by adjusting the final sintering temperature which results in glass that is very near but not full density which allows in some cases for fielding of glass shells with half-lives which can be more suitable to the experimentalist.

INVESTIGATIONS TO REMOVE DOMES FROM PLASTIC SHELLS BY POLISHING

Abstract A problem often observed for thick wall plastic targets is the presence of surface domes. We have been successful in applying mechanical polishing to remove isolated surface domes from thick wall 2 mm shells during a preliminary investigation. The background surface roughness for polished shells was dramatically improved with final values typically around 10 nm RMS as measured by WYKO patch surface profiles. The polishing sequence applied was also examined using AFM spheremapper data that was obtained for shells after each polishing step. A two-step polishing approach was able to produce shells that had significant improvement in all AFM power modes except for modes (3.10). Further polishing development is needed to reduce AFM low and mid power modes for shells. Polishing of otherwise target quality 2 mm shells that have domes could be a future treatment for NIF targets.

Fabrication of Gas-Filled Tungsten-Coated Glass Shells

Abstract Deuterium (D2) filled glass shells coated with a high Z element are needed for high energy density (HED) experiments by researchers at Los Alamos National Laboratory. We report here on our initial attempt to produce such shells. Glass shells made using the drop tower technique were coated with gold, palladium or tungsten, or a mixture of two of these elements. It was found that gold and palladium coatings did not stick well to the glass and resulted in poor or delaminated films. Tungsten coatings resulted in films suitable for these targets. Bouncing of shells during coating resulted in uniform tungsten coatings, but the surface of such coatings were filled with small nodules. Proper agitation of shells using a tapping technique resulted in smooth films with minimal particulate contamination. For coating rates of ~0.15 μm/hr coatings with ~2 nm RMS surface finish could be deposited. The surface roughness of coatings at higher rates, 0.7 μm/hr, was considerably worse (~100 nm RMS). The columnar structure of the coatings allowed permeation filling of the tungsten coated glass shells with deuterium at 300°C.

Atomic Layer Deposition Coating for Permeation Half-Life Control of GDP Ablator Capsules

Abstract The atomic layer deposition technique generates very thin Al2O3 films to control the hydrogen diffusion half-life of glow discharge polymer (GDP) inertial confinement fusion shells. The films generated by this process have an easily controlled thickness and are pinhole free. As a result, they can be used to set the hydrogen diffusion half-life of a GDP shell to the required value of hours, from an uncoated value of minutes. Such diffusivity control is much harder to achieve with the currently used sputtered Al coating, which also renders the shell opaque, causing difficulties with ice-layer characterization. The [approximately]10-nm oxide is also less intrusive to target performance than an [approximately]100-nm (and highly nonuniform) metal coating such that it can be safely ignored by the target designer.

Fabrication and Characterization of Fast Ignition Targets

Abstract Fast ignition is a novel scheme for achieving laser fusion. A class of these targets involves cone mounted CH shells. We have been fabricating such targets with shells with a wide variety of diameters and wall thicknesses for several years at General Atomics. In addition, recently such shells were needed for implosion experiments at Laboratory for Laser Energetics (LLE) that for the first time were required to be gas retentive. Fabrication of these targets requires producing appropriate cones and shells, assembling the targets, and characterization of the assembled targets. The cones are produced using micromachining and plating techniques. The shells are fabricated using the depolymerizable mandrel technique followed by micromachining a hole for the cone. The cone and the shell then need to be assembled properly for gas retention and precisely in order to position the cone tip at the desired position within the shell. Both are critical for the fast ignition experiments. The presence of the cone in the shell creates new challenges in characterization of the assembled targets. Finally, for targets requiring a gas fill, the cone-shell assembly needs to be tested for gas retention and proper strength at the glue joint. This paper presents an overview of the developmental efforts and technical issues addressed during the fabrication of fast ignition targets.

Target fabrication for inertial confinement fusion and fast ignition

Summary form only given, as follows. Direct-drive inertial confinement fusion (ICF) offers the potential for high gain and is a leading candidate for an inertial fusion-energy power plant. Laser and target nonuniformities can seed hydrodynamic instabilities during the implosion that, in turn, can compromise target performance. This is the primary target physics issue for direct-drive ICF. Several methods have been devised to control these seeds and their subsequent growth, including laser beam smoothing, advanced pulse shaping, target design, etc. LLEs baseline direct-drive ignition design for the National Ignition Facility (presently under construction at the Lawrence Livermore National Laboratory) is composed of a thin (3-/spl mu/m) plastic shell enclosing a thick (350-/spl mu/m) deuterium-tritium (DT)-ice layer. It provides a gain of 45 in spherically symmetric calculations (30 m two-dimensional simulations which include the effects of laser and target nonuniformities). Recent improvements to the ignition target design include the addition of a picket to the beginning of the laser pulse shape that reduces both the seeds and growth rate of the hydrodynamic instabilities Experiments performed on the 60-beam, 30-kJ UV OMEGA laser on warm and cryogenic targets are diagnosed using X-rays and nuclear particles. Significant improvements in warm capsule implosion target performance have been observed with improvements in beam smoothing on OMEGA.

Developments in capsule gas fill half-life determination

Abstract ICF experiments routinely make use of capsules filled with precise quantities of gaseous hydrogen and helium isotopes. These two gases in particular readily permeate out of capsules at rates dependent upon variables including shell wall thickness, composition and integrity. Therefore it is important that the fill half-life of these capsules be precisely known so that the exact fill pressure at shot time can be deduced, enabling valid experimental results. This presentation will describe some of our efforts to determine ICF capsule gas fill half-lives. We will compare fill half-life data obtained using weighing, interferometry and mass spectrometry techniques. In addition, we will describe our use of glass shell standards to compare the aforementioned techniques.