Rayleigh–Taylor instability evolution in ablatively driven cylindrical implosions

Abstract

The Rayleigh–Taylor instability is an important limitation in inertial confinement fusion capsule designs. Significant work both theoretically and experimentally has been done to demonstrate the stabilizing effects of material flow through the unstable region. The experimental verification has been done predominantly in planar geometry. Convergent geometry introduces effects not present in planar geometry such as shell thickening and accelerationless growth of modal amplitudes (e.g., Bell–Plesset growth). Amplitude thresholds for the nonlinear regime are reduced, since the wavelength λ of a mode $m$ decreases with convergence $\mathrm{\lambda \sim R/m,}$ where $R$ is the radius. Convergent effects have been investigated using an imploding cylinder driven by x-ray ablation on the NOVA laser [J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 (1983)]. By doping sections of the cylinder with opaque materials, in conjunction with x-ray backlighting, the growth and feedthrough of the perturbations from the ablation front to the inner surface of the cylinder for various initial modes and amplitudes from early time through stagnation was measured. Mode coupling of illumination asymmetries with material perturbations is observed, as well as phase reversal of the perturbations from near the ablation front to the inner surface of the cylinder. Perturbation growth is observed due to convergence and compressibility alone, without the effects of acceleration, and scales as $\text{∼1/ρR,}$ where ρ is the mass density. Imaging is performed with an x-ray pinhole camera coupled to a gated microchannel plate detector.