We report on the most recent and successful effort at controlling the trajectory and symmetry of a high density carbon implosion at the National Ignition Facility. We use a low gasfill (0.3 mg/cc He) bare depleted uranium hohlraum with around 1 MJ of laser energy to drive a 3-shock-ignition relevant implosion. We assess drive performance and we demonstrate symmetry control at convergence 1, 3–5, 12, and 27 to better than ±5 μm using a succession of experimental platforms. The symmetry control was maintained at a peak fuel velocity of 380 km/s. Overall, implosion symmetry measurements are consistent with the pole-equator symmetry of the X-ray drive on the capsule being better than 5% in the foot of the drive (when shocks are launched) and better than 1% during peak drive (main acceleration phase). This level of residual asymmetry should have little impact on implosion performance.

Taking full advantage of the unique laboratory environment created by the National Ignition Facility (NIF) will require the availability of foam-lined indirect-drive inertial confinement fusion targets. Here, we report on a new approach that enables fabrication of target structures that consist of a thin-walled (<30 µm) ultra-low-density (<30 mg cm−3) hydrocarbon foam film inside a thick-walled, ∼2 mm diameter ablator shell. In contrast to previous work on direct-drive targets that started with the fabrication of foam shells, we use a prefabricated ablator as a mold to cast the foam liner within the shell. This work summarizes crucial components of this new approach, including the aerogel chemistry, filling of the ablator shell with the aerogel precursor solution with nanolitre precision, creating uniform polymer gel coatings inside the ablator capsule, supercritical drying and doping.

The National Ignition Facility (NIF) i,ii at Lawrence Livermore National Laboratory is a 192 beam, 1.8 MJ 0.35 µm laser designed to drive inertial confinement fusion (ICF) capsules to ignition iii. NIF was formally dedicated in May 2009. The National Ignition Campaign, a collaborative research undertaking by LLNL, LLE, LANL, GA, and SNL, has a goal of achieving a robust burning plasma by the end of 2012. In the indirect-drive approach iv , the laser energy is converted to thermal x-rays inside a high Z cavity (hohlraum). The x rays then ablate the outer layers of a DT-filled capsule placed at the center of the hohlraum, causing the capsule to implode, compress and heat the DT and ignite.

Application of a magnetic field to an indirect drive inertial confinement fusion target requires diffusion of the field through the high-Z and electrically conducting Hohlraum. The onset of the external field generates eddy currents in the Hohlraum wall that result in (1) a reduction of the peak field at the capsule, (2) heating of the Hohlraum wall through Ohmic dissipation, and (3) wall movement due to the inward force from the eddy current interacting with the field. Heating of the wall causes an increase in blackbody radiation which can preheat the capsule and frozen deuterium–tritium fuel, while wall motion leads to potential misalignment of the lasers at the Hohlraum wall. Limiting these detrimental effects sets requirements on the tolerable magnitude of each effect. We present a nonlinear model for B-field diffusion through an infinitely long thin-walled cylinder with a temperature dependent resistivity, to show that a 15 μm thick wall of pure gold fails to meet these requirements. A new Hohlraum material made from an alloy of Au and Ta has a measured resistivity of ≥60 times that of Au and is shown with the nonlinear model to meet the requirements for magnetization. We compare the nonlinear model to simulations of the actual Hohlraum target using a finite element code which includes temperature-dependent Hohlraum resistivity.

Phase-shifting interferometry reveals that a heat flux normal to the gas-solid interface reduces the surface roughness of thick ͑10-300 m͒ multicrystalline D 2 films. The initial roughness, caused by misaligned crystals and grain boundaries produced during the initial random nucleation and rapid crystal growth used in the experiment decreases with increasing heat flux. A simple energy minimization model quantitatively explains the functional relationship between surface roughness and heat flux.Very smooth and uniform 50-300 m-thick deuteriumtritium ͑D-T͒ layers on the interiors of 1-3 mm-diameter spherical capsules are required for ignitable inertial confinement fusion ͑ICF͒ targets for the National Ignition Facility. 1,2 Such D-T layers develop through a natural redistribution process driven by bulk-solid heating from tritium beta decay. 3-5 This process typically results in a multicrystalline D-T layer with the average solid-gas interface conforming to an isotherm of the spherical container. These thick multicrystalline films grown from the liquid or vapor are not perfectly smooth. The surface structure is a function of the distribution of crystallite sizes, orientations, etc., determined largely by the initial nucleation and growth. 6 Herring, 7,8 and Mullins 9 set the groundwork for understanding this surface structure. Typically, when a smooth or flat surface finish is required, slow, material-dependent techniques, such as epitaxial growth, are used. These techniques are not available for smoothing ICF fuel layers. A search for alternative methods motivated the present work. We show that a heat flux applied normal to the gas-solid interface smoothes 10-300 m-thick solid D 2 surfaces. This result may have more general application for controlling multicrystalline surface morphologies. An extensive literature exists on the theory of crystal shapes, but it does not apply to the present work. We therefore present a simple energy-minimization model of the effects of a thermal gradient on multicrystalline surface roughness, which quantitatively fits our data with reasonable choices of crystal parameters.The D 2 films were grown from the vapor phase by cooling through the triple-point temperature of 18.73 K in a cell schematically shown in Fig. 1. The top and bottom plates are MgF 2 -coated sapphire. The bottom plate is at temperature T 1 and serves as the substrate for growing D 2 films. The top plate is at temperature T 2 and allows optical access to the D 2 films. Raising T 2 several degrees above T 1 produces a heat flux F at the gas-solid surface. For films that are thin compared to xϭ3.84 mm, the distance between the top and bottom plates, Fϭ v (T 2 ϪT 1 )/x, where v is the vapor thermal conductivity for the average temperature between the plates. A fill tube ͑not shown͒ enables us to monitor the D 2 vapor pressure in the cell. Calibrated germanium resistance thermometers measure both T 1 and T 2 . Except when noted, the temperature stability was better than 0.005 K for a 2 h period.Surface roughness is determ...