Laser uniformity and hydrodynamic stability experiments at the OMEGA laser facility T.R. BOEHLY, 1 A. BABUSHKIN, 1 D.K. BRADLEY, 2 R.S. CRAXTON, 1 J.A. DELETTREZ, 1 R. EPSTEIN, 1 T.J. KESSLER, 1 J.P. KNAUER, 1 R.L. McCRORY, 1 P.W. McKENTY, 1 D.D. MEYERHOFER, 1 S. REGAN, 1 W. SEKA, 1 S. SKUPSKY, 1 V.A. SMALYUK, 1 R.P.J. TOWN, 1 and B. YAAKOBI 1 1 University of Rochester, Laboratory for Laser Energetics, 250 East River Road, Rochester, NY 14623 2 Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA94550 ~Received 25 January 1999; Accepted 25 January 1999! Abstract Experiments to demonstrate the effects of various beam-smoothing techniques have been performed on the 60-beam, 30-kJ UV OMEGA laser system. These include direct measurements of the effect beam-smoothing techniques have on laser beam nonuniformity and on both planar and spherical targets. Demonstrated techniques include polarization smooth- ing and “dual-tripler” third-harmonic generation required for future broad bandwidth ~;1 THz! smoothing by spectral dispersion ~SSD!. The effects of improvements in single-beam uniformity are clearly seen in the target-physics exper- iments, which also show the effect of the laser pulse shape on the efficacy of SSD smoothing. Saturation of the Rayleigh- Taylor ~ RT! growth of the broad-bandwidth features, in agreement with the Haan model ~ Haan, 1989!, produced by laser imprinting has also been observed. 1. INTRODUCTION The Laboratory for Laser Energetics’ ~ LLE’s! research program ~ McCrory et al., 1996! supports the US inertial confinement fusion ~ ICF ! effort with direct-drive ICF ex- periments that investigate the laser and target requirements for successful ignition experiments on the National ignition facility ~ NIF !~ Lindl, 1995!. Implosion experiments on OMEGA are energy scaled to be “hydrodynamically equiv- alent” to the NIF capsules in that they experience similar RT growth factors, similar hot spot convergence ratios ~ C R  20–25!, and similar implosion velocities ~ McCrory et al., 1996!. These conditions can be achieved on OMEGA using cryogenic capsules. The hydrodynamic stability of the im- plosion is a key physics issue that affects both target design and the requirements for laser performance. Target nonuni- formities can grow exponentially due to the Rayleigh-Taylor instability ~ RTI ! during the acceleration and deceleration phases of the implosion. Ultimately, one is concerned about the amount of mixing of the target shell and the thermo- nuclear fuel that is caused by this unstable behavior. Ideally, the shell should remain sufficiently intact to compress the fuel without introducing material that can degrade the tar- get’s performance into the core shell. In the final stage of the implosion, target perturbations on the inner shell surface grow at the classical RT rate as the shell decelerates. This growth can be seeded by existing inner- surface perturbations plus those that “feed through” the shell from the ablation surface. The ablation surface instability, which is driven by target acceleration and is moderated by the ablation process, can be seeded by existing outer-surface per- turbations and by perturbations created ~or imprinted! by the laser. In addition, inner-surface perturbations can, by the propagation of shock and rarefaction waves, “feed out” through the shell from the inner surface. Since each of these processes is dispersive in growth rate and experiences vari- ous levels of nonlinear and saturation effects ~ Haan, 1989!, it is imperative that all aspects of the evolution of target non- uniformities be understood and, to the extent possible, con- trolled. Perturbation sources such as target defects and laser imprinting need to be minimized to acceptable levels. Important aspects of the current scientific program at LLE include efforts to understand the RT instability and its evolution, and the development of means to control laser imprinting. The main approach to the latter is to reduce non- uniformities in the laser. In this paper, we review experi- ments on the 30-kJ, 60-UV-beam OMEGA laser system Address correspondence and reprint requests to: T.R. Boehly, University of Rochester, Laboratory for Laser Energetics, 250 East River Road, Roch- ester, NY 14623-1299. E-mail: trb@lle.rochester.edu Laser and Particle Beams ~2000!, 18, 11–19. Printed in the USA. Copyright © 2000 Cambridge University Press 0263-0346000 $12.50 11