Progress in direct-drive inertial confinement fusion

Significant progress in direct-drive inertial confinement fusion (ICF) research has been made since the completion of the 60-beam, 30-kJUV OMEGA Laser System [Boehly, Opt. Commun. 133, 495 (1997)] in 1995. A theory of ignition requirements, applicable to any ICF concept, has been developed. Detailed understanding of laser-plasma coupling, electron thermal transport, and hot-electron preheating has lead to the measurement of neutron-averaged areal densities of ∼200mg∕cm2 in cryogenic target implosions. These correspond to an estimated peak fuel density in excess of 100g∕cm3 and are in good agreement with hydrodynamic simulations. The implosions were performed using an 18-kJ drive pulse designed to put the converging fuel on an adiabat of two. The polar-drive concept will allow direct-drive-ignition research on the National Ignition Facility while it is configured for indirect drive. Advanced ICF ignition concepts—fast ignition [Tabak et al., Phys. Plasmas 1, 1626 (1994)] and shock ignition [Betti et al., Phys. Rev. Lett. 98, 155001 (2007)]—have the potential to significantly reduce ignition driver energies and/or provide higher target gain.