Design of a foam-lined hohlraum for ICF implosions on NIF

We describe a foam-lined hohlraum design for driving ignition implosions on the NIF, designed using the radiation hydrodynamics code HYDRA. In a typical hohlraum with a low density (ρ < 0.6 mg/cm³) helium fill, the plasma "bubble" formed where the outer laser beams strike the wall moves into the path of the inner beams and absorbs them. In this design, inner beam propagation is improved by lining the hohlraum wall with tantalum oxide (Ta₂O₅) foam of density 20 mg/cm³, initially below the critical density for 3ω light. The laser compresses the foam to a critical surface that moves radially outward until colliding with the incoming x-ray-ablated wall. The ablated plasma from the foam is lower density than plasma ablated from a solid gold wall, and inner beam absorption is reduced. The optimum foam thickness depends on the hohlraum radius, since the NIF beam angles and laser spot sizes are fixed: larger hohlraums with thicker foams improve laser propagation for a longer time. A 6.72 mm diameter hohlraum with a 400 μm thick liner provides increased waist-hot drive compared to a solid gold hohlraum, enabling symmetric implosions with a larger capsule.