COLLOQUIUM: In Pursuit of Ignition on the National Ignition Facility

The Inertial Confinement Fusion (ICF) Program is conducting experiments at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory with the goal of igniting a propagating thermonuclear burn wave in DT fuel leading to energy gain (defined as fusion yield/input laser energy >1). To do this the NIF laser delivers up to ~ 2 MJ of energy to a hohlraum (cylindrical cavity) which generates x-rays that implode a ~2 mm diameter spherical capsule filled with a solid layer of cryogenic deuterium-tritium (DT) fuel. For the fuel to ignite the implosion must create a central hot spot with a temperature ~ 5 keV and density ~ 100 g/cc. If the fuel has a total rhoR >~ 1g/cm2, it will be confined by its own inertia long enough for alpha deposition to self-heat the hot spot to the point that a self-sustaining burn wave is initiated – ignition. Achieving these conditions is extremely challenging and requires a nearly spherical, high velocity (~ 370 km/s), high convergence (CR~35) implosion. Initial attempts at ignition fell short of the mark for reasons that were not entirely clear, although there were strong indications that hydrodynamic instabilities were causing the capsule to break up during the implosion and that the x-ray drive on the capsule was not symmetric enough. Over the past 18 months experiments have focused on understanding why these initial attempts fell short of predictions. This has included developing an implosion with much reduced hydrodynamic instability – the so called “high foot” implosion – at the expense of compression and potential gain. These implosions perform close to 1D simulations and with a neutron yield of up to ~ 9x1016 (~26 kJ) have exceeded that of the previous design by more than 10X. More importantly, the alpha heating feedback on the hot spot which is critical for ignition is beginning to become significant resulting in ~ 2X enhancement in neutron yield. While there is much more to be done to reach ignition these results are encouraging. This talk will summarize the current status of our understanding of ignition experiments and future directions.