Developments in Direct Drive Laser Fusion

Recent designs for laser driven, direct drive inertial confinement fusion (ICF) indicate that substantial gains (G>100) might be achieved with lower total laser energy (E~500 kJ) than previously considered possible. A leading contender is the shock ignition approach which compresses low aspect ratio pellets with high intensity laser pulses (10¹⁵ W/cm²) before achieving ignition with a final higher intensity spike (10¹⁶ W/cm²). Excimer laser systems based on a krypton-fluoride (KrF) medium are particularly well suited to these new ideas as they operate in the ultraviolet (248 nm), provide highly uniform illumination, possess large bandwidth (1-3 THz), and can easily exploit beam zooming to improve laser-target coupling for the final spike pulse. This paper will examine target physics advantages of KrF lasers in relation to the new implosion designs and the balancing of hydrodynamic instability and laser-plasma instabilities. Supporting experimental and theoretical studies of are being conducted by the Nike laser group at the U. S. Naval Research Laboratory. Recent experimental work has also shown that the high ablation pressures and smooth profiles obtained with the Nike laser can be used to accelerate planar targets to velocities consistent with the requirements of impact ignition.