Performance of Beryllium Targets with Full-scale Capsules in Low-fill 6.72-mm Hohlraums on the National Ignition Facility

When used with full-size beryllium (Be) capsules [1], high-fill 5.75-mm hohlraums exhibit significant drive degradation via laser backscatter and ``missing energy''. Also, hard to simulate cross-beam energy transfer (CBET) must be used to control the implosion symmetry. Low-fill ($\le $0.6 mg/cm$^{\mathrm{3}})$ 6.72-mm hohlraums offer improved drive efficiency and the symmetry tunability without the CBET. In FY16, we carried out an exploratory campaign to evaluate performance of full-size Be capsules in such hohlraums. Specifically, we have performed a fill-density scan with a three-shock, 9.5-ns pulse and found that an appropriate laser beam repointing and outer-quad splitting results in approximately 5{\%} laser backscatter at fill densities $\le $0.3 mg/cm$^{\mathrm{3}}$, with the implosion becoming less oblate as the fill-density decreases. We also plan to perform an implosion with a lower-foot, 12.6-ns pulse, to observe a more prolate symmetry. [1] J. L. Kline et al., Phys. Plasmas 23, 056310 (2016).