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Many inertial fusion designs use capsules made of beryllium, as its high mass ablation rate is advantageous. We present the first systematic experimental study of indirectly driven beryllium capsules with a cryogenic deuterium-tritium fuel layer. "Subscale" capsules, 80% of the nominal National Ignition Facility point design radius, show optimal performance with the remaining mass of ∼6–7%. A buoyancy-drag mix model explains the implosion performance, suggesting that fuel-ablator mix is the dominant degradation mechanism. Increasing the capsule scale is predicted to reduce the impact of fuel-ablator mix and achieve high performance. [ABSTRACT FROM AUTHOR]

Beryllium is a candidate ablator material for indirect-drive inertial confinement fusion experiments, motivated by its high mass ablation rate, which is advantageous for implosion coupling efficiency and stabilization of the ablation-front instability growth. We present new data on the shock propagation, in-flight shape, and hot spot self-emission shape from gas-filled capsules that demonstrate the feasibility of predictable, symmetric, controllable beryllium implosions at a case-to-capsule ratio of 3.7. The implosions are round (Legendre mode 2 amplitude ≲ 5 %) at an inner beam power and the energy fraction of 26%–28% of the total, indicating that larger beryllium capsules could be driven symmetrically using the National Ignition Facility. [ABSTRACT FROM AUTHOR]

Experiments at the National Ignition Facility (NIF) show that the implosion shape of inertial confinement fusion ablators is a key factor limiting performance. To achieve more predictable, shape tunable implosions, we have designed and fielded a large 4.2 case-to-capsule ratio target at the NIF using 6.72 mm diameter Au hohlraums and 1.6 mm diameter Cu-doped Be capsules. Simulations show that at these dimensions during a 10 ns 3-shock laser pulse reaching 275 eV hohlraum temperatures, the plasma flow from the hohlraum wall and ablator is not significant enough to impede beam propagation. Experiments measuring the shock symmetry and in-flight shell symmetry closely matched the simulations. Most notably, in two experiments, we demonstrated symmetry control from negative to positive Legendre P2 space by varying the inner to total laser power cone fraction by 5% below and above the predicted symmetric value. Some discrepancies found in 1st shock arrival times that could affect agreement in late time implosion symmetry suggest hohlraum and capsule modeling uncertainties do remain, but this target design reduces sensitivities to them. [ABSTRACT FROM AUTHOR]

Detailed radiation hydrodynamic simulations calibrated to experimental data have been used to compare the relative strengths and weaknesses of three candidate indirect drive ablator materials now tested at the NIF: plastic, high density carbon or diamond, and beryllium. We apply a common simulation methodology to several currently fielded ablator platforms to benchmark the model and extrapolate designs to the full NIF envelope to compare on a more equal footing. This paper focuses on modeling of the hohlraum energetics which accurately reproduced measured changes in symmetry when changes to the hohlraum environment were made within a given platform. Calculations suggest that all three ablator materials can achieve a symmetric implosion at a capsule outer radius of ∼1100 μm, a laser energy of 1.8 MJ, and a DT ice mass of 185 μg. However, there is more uncertainty in the symmetry predictions for the plastic and beryllium designs. Scaled diamond designs had the most calculated margin for achieving symmetry and the highest fuel absorbed energy at the same scale compared to plastic or beryllium. A comparison of the relative hydrodynamic stability was made using ultra-high resolution capsule simulations and the two dimensional radiation fluxes described in this work [Clark et al., Phys. Plasmas 25, 032703 (2018)]. These simulations, which include low and high mode perturbations, suggest that diamond is currently the most promising for achieving higher yields in the near future followed by plastic, and more data are required to understand beryllium. [ABSTRACT FROM AUTHOR]