The Smoothing Performance of Ultrafast Pickets on the NIF

LLE Review, Volume 86 79 Introduction In the direct-drive approach to inertial confinement fusion (ICF), the focal spots of a symmetrically arranged cluster of high-intensity, ultraviolet (UV) laser beams directly irradiate a capsule.1,2 The base-line design for a direct-drive ignition target on the National Ignition Facility (NIF) uses shock preheat to control the isentrope of the ablation surface and the fuel. Control of the isentrope α (where α is the ratio of the fuel pressure to the Fermi-degenerate pressure) is achieved by changing the laser pulse shape. The UV pulse shape corresponding to α = 3 is chosen for the base-line design. This shape can be logically divided into two regions: a low-intensity “foot” (of duration ~4.2 ns) followed by a high-intensity “drive” (duration ~5 ns). This pulse shape represents a compromise that provides a certain safety margin for the implosion by reducing the target’s sensitivity to laser nonuniformity by ensuring that the target will remain intact during the drive portion of the pulse; however, laser nonuniformity still reThe Smoothing Performance of Ultrafast Pickets on the NIF