Evidence for a bubble-competition regime in indirectly driven ablative Rayleigh-Taylor instability experiments on the NIF.

We investigate on the National Ignition Facility the ablative Rayleigh-Taylor instability in the transition from weakly nonlinear to highly nonlinear regimes. A planar plastic package with preimposed two-dimensional broadband modulations is accelerated for up to 12 ns by the x-ray drive of a gas-filled Au radiation cavity with a radiative temperature plateau at 175 eV. This extended tailored drive allows a distance traveled in excess of 1 mm for a 130 µm thick foil. Measurements of the modulation optical density performed by x-ray radiography show that a bubble-merger regime for the Rayleigh-Taylor instability at an ablation front is achieved for the first time in indirect drive. The mutimode modulation amplitudes are in the nonlinear regime, grow beyond the Haan multimode saturation level, evolve toward the longer wavelengths, and show insensitivity to the initial conditions.

Simple solution to the Fresnel-Kirchoff diffraction integral for application to refraction-enhanced radiography.

We present a simple solution to the Fresnel-Kirchoff diffraction integral that is appropriate for x-ray radiography of strongly absorbing and phase-shifting objects in the geometrical optics regime, where phase contrast enhancements can be considered to be caused by refraction by a semi-opaque object. We demonstrate its accuracy by comparison to brute-force numerical ray trace and diffraction calculations of a representative simulated object, and show excellent agreement for spatial scales corresponding to Fresnel numbers greater than unity. The result represents a significant improvement over approximate formulas typically used in analysis of refraction-enhanced radiographs, particularly for radiography of transient phenomena in objects that strongly refract and show significant absorption.