ALBERT

Description: The effects of large-scale (with Legendre modes ≲ 10 ) laser-imposed nonuniformities in direct-drive cryogenic implosions on the OMEGA Laser System are investigated using three-dimensional hydrodynamic simulations performed using the newly developed code ASTER . Sources of these nonuniformities include an illumination pattern produced by 60 OMEGA laser beams, capsule offsets (∼10–20  μ m), and imperfect pointing, power balance, and timing of the beams (with typical σ rms ∼ 10 μ m, 10%, and 5 ps, respectively). Two implosion designs using 26-kJ triple-picket laser pulses were studied: a nominal design, in which an 874- μ m-diameter capsule is illuminated by about the same-diameter beams, and a more hydrodynamically efficient “ R 75” design using a 900- μ m-diameter capsule and beams of 75% of this diameter. Simulations show that nonuniformities caused by capsule offsets and beam imbalance have the largest effect on implosion performance. These nonuniformities lead to significant distortions of implosion cores, resulting in an increased residual kinetic energy and incomplete stagnation. The shape of distorted cores can be well characterized using neutron images but is less represented by 4–8 keV x-ray images. Simulated neutron spectra from perturbed implosions show large directional variations because of bulk motion effects and up to an ∼2 keV variation of the hot-spot temperature inferred from these spectra. The R 75 design suffers more from illumination nonuniformities. Simulations show an advantage of this design over the nominal design when the target offset and beam power imbalance σ rms are reduced to less than 5  μ m and 5%, respectively.