The effects of convergent geometry on the ablative Rayleigh-Taylor instability in cylindrical implosions

James Bradley Beck, Purdue University

Abstract

The implosion of an inertial-confinement-fusion (ICF) capsule achieves maximum efficiency if the imploding flow is exactly spherically symmetric. Real ICF flows, however, are subject to a variety of fluid instabilities that destroy this symmetry. Of specific concern is the ablation-surface instability, where the ablation of the outer shell of the capsule creates a situation analogous to the classical Rayleigh-Taylor instability. The study of perturbation growth at this position is experimentally difficult in spherical geometry, while planar geometry neglects the effects of convergence. For that reason, this study examined the growth of unstable perturbations at the ablation front of an imploding cylinder. This enables the effect of convergence to be seen while still allowing experimental diagnosis of perturbation growth. A theoretical growth rate for the Rayleigh-Taylor instability in cylindrical geometry was derived in this thesis, based upon Plesset's theory. This growth rate showed two new effects due to the convergent geometry. First, the growth rate was enhanced due to the decrease in the perturbation wavelength during an implosion. Second, the derivation predicted that perturbations can grow in a convergent geometry even in the absence of an acceleration. With the use of this theory and results from a one-dimensional computational code capable of modeling radiatively driven cylindrical implosions, perturbation growth rates could then be predicted. An experimental campaign was conducted to test this new theory, with two separate cylindrical designs being used: (1) a low-perturbation growth design, and (2) a high-perturbation growth design. The experimental results were then compared with the theoretically predicted values, with excellent agreement. Included in this comparison was the result that perturbations were seen experimentally growing in amplitude when no acceleration was present. A second, purely computational, study was also conducted where the effects of cylindrical geometry were studied upon the nonlinear Rayleigh-Taylor growth. This study identified the "nozzle" effect, caused when the more dense spikes converge upon the origin in cylindrical geometry, and the "nesting" effect which allowed for nonlinear effects to form at different radii, enhancing the amount of mix when compared to planar geometry calculations.

Degree

Ph.D.

Advisors

Choi, Purdue University.

Subject Area

Nuclear physics|Fluid dynamics|Gases

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