Increasing scaled performance in direct-drive inertial confinement fusion implosions through high-intensity subscale experiments on OMEGA

Recent experiments in the cryogenic direct-drive implosion campaign on OMEGA have assessed the use of high intensities to increase yield and areal density. This was enabled in part by the use of Si-doped ablators that partially mitigate hot-electron generation by two-plasmon decay (TPD), allowing intensities to exceed 10¹⁵ W/cm² without significantly preheating the target. To achieve these greater intensities, “subscale” implosions were performed on OMEGA, where phase plates reduced the waists of the beams from a nominal 358 μm to 287 μm. Similarly, target diameters were also reduced by about 20% from a nominal 930 μm to as low as 740 μm. Using this subscale design, thick-ice capsules (68 μm of frozen DT) were fielded to provide hydrodynamically stable implosions and driven with intensities from 0.9 × 10¹⁵ W/cm² to 1.4 × 10¹⁵ W/cm². Measured areal density increased with intensity due to the greater ablation pressure before dropping at the highest intensity, likely due to hot-electron preheat limiting the capsule’s convergence, suggesting that TPD degradations are suppressed up to at least 1.4 × 10¹⁵ W/cm². Using this knowledge, subscale implosions are now being optimized to improve both yield and areal density. Early efforts have already achieved the scaled performance of their highly optimized full-scale counterparts, with projected Lawson parameters of χ_MJ ~ 0.87 when projected to 2.15 MJ of driver energy.