Cryogenic-target performance and implosion physics studies on OMEGA

Recent progress in direct-drive cryogenic implosions on the OMEGA Laser Facility [T. R. Boehly, Opt. Commun. 133, 495 (1997)] is reviewed. Ignition-relevant areal densities of ∼200 mg/ cm ² in cryogenic D ₂ implosions with peak laser-drive intensities of ∼5× 10 ¹⁴ W/ cm ² were previously reported [T. C. Sangster, Phys. Rev. Lett. 100, 185006 (2008)]. The laser intensity is increased to ∼ 10 ¹⁵ W/ cm ² to demonstrate ignition-relevant implosion velocities of 3-4× 10 ⁷ cm/s, providing an understanding of the relevant target physics. Planar-target acceleration experiments show the importance of the nonlocal electron-thermal-transport effects for modeling the laser drive. Nonlocal and hot-electron preheat is observed to stabilize the Rayleigh-Taylor growth at a peak drive intensity of ∼ 10 ¹⁵ W/ cm ². The shell preheat caused by hot electrons generated by two-plasmon-decay instability was reduced by using Si-doped ablators. The measured compressibility of planar plastic targets driven with high-compression shaped pulses agrees well with one-dimensional simulations at these intensities. Shock mistiming has contributed to compression degradation of recent cryogenic implosions driven with continuous pulses. Multiple-picket (shock-wave) target designs make it possible for a more robust tuning of the shock-wave arrival times. Cryogenic implosions driven with double-picket pulses demonstrate somewhat improved compression performance at a peak drive intensity of ∼ 10 ¹⁵ W/ cm ².