Neutron yield study of direct-drive, low-adiabat cryogenic D2 implosions on OMEGA laser system

Neutron yields of direct-drive. low-adiabat (alpha approximate to 2 to 3) cryogenic D, target implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have been systematically investigated ming the two-dimensional (2D) radiation hydrodynamics code DRACO [P. B. Radha et al., Phys. Plasmas 12, 056307 (2005)]. Low-mode (l <= 12) perturbations, including initial target offset, ice-layer roughness, and laser-beam power imbalance, were found to be the primary source of yield reduction for thin-shell (5 mu m), low-alpha, cryogenic targets. The 2D simulations of thin-shell implosions track experimental measurements for different target conditions and peak laser intensities ranging from 2.5 x 10(14)-6 x 10(14) W/cm(2). Simulations indicate that the fusion yield is sensitive to the relative phases between the target offset and the ice-layer perturbations. The results provide a reasonable good guide to understanding the yield degradation in direct-drive, low-adiabut, cryogenic, thin-shell-target implosions. Thick-shell (10 mu m) implosions generally give lower yield over clean than low-l-mode DRACO simulation predictions. Simulations including the effect of laser-beam nonuniformities indicate that high-l-mode perturbations caused by laser imprinting further degrade the neutron yield of thick-shell implosions. To study ICF compression physics, these results suggest a target specification with a <= 30 mu m offset and ice-roughness of sigma(rms) <3 mu m are required. (C) 2009 American Institute of Physics. [doi:10.1063/1.3259355]