Targets for the National Ignition Campaign

Abstract

The National Ignition Facility (NIF) is a 192 beam Nd-glass laser facility presently under construction at Lawrence Livermore National Laboratory (LLNL) for performing inertial confinement fusion (ICF) and experiments studying high energy density (HED) science. When completed in 2009, NIF will be able to produce 1.8 MJ, 500 TW of ultraviolet light for target experiments that will create conditions of extreme temperatures (>108 K), pressures (10 GBar) and matter densities (>100 g/cm3). A detailed program called the National Ignition Campaign (NIC) has been developed to enable ignition experiments in 2010, with the goal of producing fusion ignition and burn of a deuterium-tritium (DT) fuel mixture in millimeter-scale target capsules. The first of the target experiments leading up to these ignition shots will begin in 2008. The targets for the NIC are both complex and precise, and are extraordinarily demanding in materials fabrication, machining, assembly, cryogenics and characterization.The DT fuel is contained in a 2-millimeter-diameter graded copper/beryllium or CH shell. The 75-μm-thick cryogenic ice DT fuel layer is formed to sub-micron uniformity at a temperature of approximately 18 Kelvin. The capsule and its fuel layer sit at the center of a gold/depleted uranium ‘cocktail’ hohlraum. Researchers at LLNL have teamed with colleagues at General Atomics to lead the development of the technologies, engineering design and manufacturing infrastructure necessary to produce these demanding targets. We are also collaborating with colleagues at the Laboratory for Laser Energetics (LLE) at the University of Rochester in DT layering, and at Fraunhofer in Germany in nano-crystalline diamond as an alternate ablator to Beryllium and CH.The Beryllium capsules and cocktail hohlraums are made by physical vapor deposition onto sacrificial mandrels. These coatings must have high density (low porosity), uniform microstructure, low oxygen content and low permeability. The ablator capsule has a 5-μm-diameter hole laser drilled to permit removal of the mandrel and introduction of the DT fuel. A 10-μm-diameter fill tube is bonded to the capsule to enable filling with the DT gas. These components must then be assembled to tolerances of approximately 5—10 microns, with comprehensive characterization and metrology. The DT ice is formed through controlled seeding, aided by beta decay of the tritium to help smooth the layer, and differential heating of the hohlraum to counteract the effects of natural convection.We present an overview of the technologies for target fabrication, assembly and metrology and advances in growth and imaging of DT ice layers. The sum of these efforts represents a quantum leap in target precision, characterization, manufacturing rate and flexibility over current state-of-the-art.This work performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.