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

Abstract

Neutron yields of direct-drive, low-adiabat (α≈2 to 3) cryogenic D2 target implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have been systematically investigated using 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 μm), low-α, cryogenic targets. The 2D simulations of thin-shell implosions track experimental measurements for different target conditions and peak laser intensities ranging from 2.5×1014–6×1014 W/cm2. 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-adiabat, cryogenic, thin-shell-target implosions. Thick-shell (10 μm) 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 μm offset and ice-roughness of σrms<3 μm are required.