Performance of beryllium targets with full-scale capsules in low-fill 6.72-mm hohlraums on the National Ignition Facility

When used with 1.06-mm beryllium (Be) capsules on the National Ignition Facility, gold hohlraums with the inner diameter of 5.75 mm and helium gas fill density of 1.6 mg/cm3 exhibit significant drive degradation due to laser energy backscatter (of order 14%–17%) and “missing” X-ray drive energy (about 32% during the main pulse). Also, hard to simulate cross-beam energy transfer (CBET) must be used to control the implosion symmetry. Larger, 6.72-mm hohlraums with fill densities ≤0.6 mg/cm3 generally offer improved drive efficiency, reduced hot-electron preheat, and better control of the implosion symmetry without CBET. Recently, we carried out an exploratory campaign to evaluate performance of 1.06-mm Be capsules in such hohlraums and determine optimal hohlraum parameters. Specifically, we performed a hohlraum fill-density scan with a three-shock, 9.5-ns laser pulse and found that an appropriate axial laser repointing and azimuthal outer-quad splitting resulted in significantly improved hohlraum energetics...