A Systematic Study of Laser Imprint for Direct Drive—From Seeds to Integrated Implosions

A study of laser imprint for laser direct drive (LDD) is presented through measurements of laser-imprint seeds, the associated hydrodynamic instability growth rates, the shell thickness, and a systematic integrated study of the performance of imploded cryogenic DT ice and gas-filled shell targets under varying imprint seed levels and for two adiabat conditions. An understanding of how those implosions are degraded with the seed level is of paramount importance for inertial confinement fusion research. The seeds for imprint come from perturbations on the target [debris, surface imperfections, and engineering features] and from the speckle pattern in the laser beams and are amplified by the Richtmyer–Meshkov and Rayleigh–Taylor instabilities. Target seeds are minimized by careful selection and the imprint seed is changed by varying the bandwidth on smoothing by spectral dispersion (SSD). The seeds were characterized using a 2-D VISAR diagnostic and compared to results from radiation-hydrodynamic simulations. Growth-rate measurements and effects of the instabilities on the in-flight shell thickness and shell trajectory are discussed. The integrated experiment uses the stagnation measurements (neutron yield, areal density, x-ray images of hot-spot formation, fusion burn history) as metrics to gauge the implosion performance versus SSD bandwidth. The SSD bandwidth is quantified using a model that relates it to the s_rms of the laser illumination. The emerging understanding of laser imprint from the OMEGA experiments will be discussed along with mitigation strategies and the implications for LDD ignition-scale targets for the National Ignition Facility.