No abstract

The National Ignition Facility has been used to compress deuterium-tritium to an average areal density of ~1.0±0.1 g cm(-2), which is 67% of the ignition requirement. These conditions were obtained using 192 laser beams with total energy of 1-1.6 MJ and peak power up to 420 TW to create a hohlraum drive with a shaped power profile, peaking at a soft x-ray radiation temperature of 275-300 eV. This pulse delivered a series of shocks that compressed a capsule containing cryogenic deuterium-tritium to a radius of 25-35 μm. Neutron images of the implosion were used to estimate a fuel density of 500-800 g cm(-3).

A detailed simulation-based model of the June 2011 National Ignition Campaign cryogenic DT experiments is presented. The model is based on integrated hohlraum-capsule simulations that utilize the best available models for the hohlraum wall, ablator, and DT equations of state and opacities. The calculated radiation drive was adjusted by changing the input laser power to match the experimentally measured shock speeds, shock merger times, peak implosion velocity, and bangtime. The crossbeam energy transfer model was tuned to match the measured time-dependent symmetry. Mid-mode mix was included by directly modeling the ablator and ice surface perturbations up to mode 60. Simulated experimental values were extracted from the simulation and compared against the experiment. Although by design the model is able to reproduce the 1D in-flight implosion parameters and low-mode asymmetries, it is not able to accurately predict the measured and inferred stagnation properties and levels of mix. In particular, the measured yields were 15%–40% of the calculated yields, and the inferred stagnation pressure is about 3 times lower than simulated.

Capsule implosions on the National Ignition Facility (NIF) [Lindl et al., Phys. Plasmas 11, 339 (2004)] are underway with the goal of compressing deuterium-tritium (DT) fuel to a sufficiently high areal density (ρR) to sustain a self-propagating burn wave required for fusion power gain greater than unity. These implosions are driven with a carefully tailored sequence of four shock waves that must be timed to very high precision in order to keep the DT fuel on a low adiabat. Initial experiments to measure the strength and relative timing of these shocks have been conducted on NIF in a specially designed surrogate target platform known as the keyhole target. This target geometry and the associated diagnostics are described in detail. The initial data are presented and compared with numerical simulations. As the primary goal of these experiments is to assess and minimize the adiabat in related DT implosions, a methodology is described for quantifying the adiabat from the shock velocity measurements. Results are contrasted between early experiments that exhibited very poor shock timing and subsequent experiments where a modified target geometry demonstrated significant improvement.

With the first four of its eventual 192 beams now executing shots and generating more than 100 kJ of laser energy at its primary wavelength of 1.06 m, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is already the world's largest and most energetic laser. The optical system performance requirements that are in place for NIF are derived from the goals of the missions it is designed to serve. These missions include inertial confinement fusion (ICF) research and the study of matter at extreme energy densities and pressures. These mission requirements have led to a design strategy for achieving high-quality focusable energy and power from the laser and to specifications on optics that are important for an ICF laser. The design of NIF utilizes a multipass architecture with a single large amplifier type that provides high gain, high extraction efficiency, and high packing density. We have taken a systems engineering approach to the practical implementation of this design that specifies the wavefront parameters of individual optics to achieve the desired cumulative performance of the laser beamline. This paper provides a detailed look at the causes and effects of performance degradation in large laser systems and how NIF has been designed to overcome these effects. We also present results of spot size performance measurements that have validated many of the early design decisions that have been incorporated in the NIF laser architecture.The optical quality or wavefront specifications for the laser slabs, crystals, windows, lenses, and mirrors that make up the National Ignition Facility ͑NIF͒ beamlines were established by flowdown analysis of those that were needed for these components to meet the primary criteria and functional requirements for the NIF laser system. In this work we summarize the NIF primary system criteria that are influenced by the wavefront quality of a beamline, and then describe how the beamline architecture and specifications for individual optical components have made it possible for NIF to meet its system criteria.The national motive for building the NIF laser centers around providing a laser driver for three missions of national purpose:• inertial confinement fusion ͑ICF͒ • the high-energy-density study of materials of interest to the stockpile stewardship program • the study of materials under very high temperatures and pressures of interest to the scientific community, particularly the astrophysical community.Of these user groups, the one that was most active in establishing the original specifications for NIF was the ICF community; their emphasis for the initial embodiment of NIF was for indirect drive ICF. For this mission, laser light enters a hohlraum through two laser entrance holes ͑LEH͒ to irradiate the walls of the hohlraum, subsequently generating x-rays that drive the implosion of a capsule centrally located within the hohlraum. 1 Thus, the primary wavefront requirement for NIF became its ability to provide focusable energy and power into the laser ...