Theory of Laser-Induced Adiabat Shaping in Inertial Fusion Implosions: The Decaying Shock

LLE Review, Volume 95 147 Introduction In inertial confinement fusion (ICF),1 a cryogenic shell of deuterium and tritium (DT) filled with DT gas is accelerated inward by direct laser irradiation (direct drive) or by the x rays emitted by a laser-illuminated enclosure of high-Z material (indirect drive). In the shell frame of reference, the acceleration points from the heavy shell toward the hot ablated plasma, making the shell’s outer surface unstable to the well-known Rayleigh–Taylor (RT) instability.2 In indirect-drive ICF, the high uniformity of the blackbody x-ray radiation results in a negligible level of imprinted perturbations on the shell’s outer surface. Indeed, the seeds of the Rayleigh–Taylor instability are mostly provided by the capsule’s surface roughness. In direct-drive ICF, the laser-beam intensity is not spatially uniform, and the direct illumination of the shell leads to high levels of laser imprinting that seed the RT instability. The use of random phase plates3 (RPP’s) has successfully shifted the spectrum of laser nonuniformities toward short wavelengths, and the implementation of either smoothing by spectral dispersion4 (SSD) or induced spatial incoherence5 (ISI) has provided significant smoothing by modulating the intensity speckle pattern in both space and time. Despite these important advances in smoothing techniques, the current level of imprinting in direct-drive ICF is still sufficiently large to substantially reduce the performance of low-adiabat implosions on the OMEGA laser and high-gain implosions on the National Ignition Facility (NIF).6