DEVELOPMENT OF THE INDIRECT-DRIVE APPROACH TO INERTIAL CONFINEMENT FUSION AND THE TARGET PHYSICS BASIS FOR IGNITION AND GAIN

Inertial confinement fusion (ICF) is an approach to fusion that relies on the inertia of the fuel mass to provide confinement. To achieve co nditions under which inertial confinement is sufficient for efficient thermonuclear burn, a capsule (generally a spherical shell) containing thermonuclear fuel is compressed in an implosion process to condition s of high density and temperature. ICF capsules rely on either electro n conduction (direct drive) or x rays (indirect drive) for energy tran sport to drive an implosion. In direct drive, the laser beams (or char ged particle beams) are aimed directly at a target. The laser energy i s transferred to electrons by means of inverse bremsstrahlung or a var iety of plasma collective processes. In indirect drive, the driver ene rgy (from laser beams or ion beams) is first absorbed in a high-Z encl osure (a hohlraum), which surrounds the capsule. The material heated b y the driver emits x rays, which drive the capsule implosion. For opti mally designed targets, 70%-80% of the driver energy can be converted to x rays. The optimal hohlraum geometry depends on the driver. Becaus e of relaxed requirements on laser beam uniformity, and reduced sensit ivity to hydrodynamic instabilities, the U.S. ICF Program has concentr ated most of its effort since 1976 on the x-ray or indirect-drive appr oach to ICF. As a result of years of experiments and modeling, we are building an increasingly strong case for achieving ignition by indirec t drive on the proposed National Ignition Facility (NIF). The ignition target requirements for hohlraum energetics, radiation symmetry hydro dynamic instabilities and mix, laser plasma interaction, pulse shaping , and ignition requirements are al consistent with experiments. The NI F laser design, at 1.8 MJ and 500 TW, has the margin to cover uncertai nties in the baseline ignition targets. In addition, data from the NIF will provide a solid database for ion-beam-driven hohlraums being con sidered for future energy applications. In this paper we analyze the r equirements for indirect drive ICF and review the theoretical and expe rimental basis for these requirements. Although significant parts of t he discussion apply to both direct and indirect drive, the principal f ocus is on indirect drive. (C) 1995 American Institute of Physics.