3D Broadband Bubble Dynamics for the Imprinted Ablative Rayleigh-Taylor Instability

An experiment was performed at the National Ignition Facility to investigate the three-dimensional ablative Rayleigh-Taylor Instability (RTI). To this end, a 300 µm thick polystyrene disk was irradiated with 450 kJ of 3ω (351 nm) laser light over 30 ns. The initial seeds of the instability were formed using a well-characterized imprinting laser beam both with and without continuous phase plate smoothing. Growth of the optical depth modulations and the acceleration of the foil were observed using time-resolved x-ray radiography. The results closely follow the classical self-similar distribution until late times when the bubble sizes approached the foil thickness. Due to the remarkably long laser drive and travel distance, multiple generations of merging were observed, and measurements of the nonlinear saturation velocities were extended to unprecedented long wavelengths. Smoothing of the imprint beam showed a large decrease in RTI modulations amplitude, though its late-time evolution approached the early-time distribution of the unsmoothed seed measurements, suggesting that the loss of memory of initial conditions has begun. These experiments are of crucial importance for turbulence studies and for benchmarking 3D radiation hydrodynamics codes used in Inertial Confinement Fusion.