[en] Recent attention has focused on the effect of spherical convergence on the nonlinear phase of Rayleigh-Taylor growth. For instability growth on spherically converging interfaces, modifications to the predictions of the Layzer model for the secular growth of a single, nonlinear mode have been reported [D. S. Clark and M. Tabak, Phys. Rev. E 72, 0056308 (2005).]. However, this model is limited in assuming a self-similar background implosion history as well as only addressing growth from a perturbation of already nonlinearly large amplitude. Additionally, only the case of single-mode growth was considered and not the multimode growth of interest in applications. Here, these deficiencies are remedied. First, the connection of the recent nonlinear results including convergence to the well-known results for the linear regime of growth is demonstrated. Second, the applicability of the model to more general implosion histories (i.e., not self-similar) is shown. Finally, to address the case of multimode growth with convergence, the recent nonlinear single mode results are combined with the Haan model formulation for weakly nonlinear multimode growth. Remarkably, convergence in the nonlinear regime is found not to modify substantially the multimode predictions of Haan's original model

[en] The Layzer model for the nonlinear evolution of bubbles in the Rayleigh-Taylor instability has recently been generalized to the case of spherically imploding interfaces [D. S. Clark and M. Tabak, to appear, PRE (2005).]. The spherical case is more relevant to, e.g., inertial confinement fusion or various astrophysical phenomena when the convergence is strong or the perturbation wavelength is comparable to the interface curvature. Here, the model is further extended to the case of bubble growth during the deceleration (stagnation) phase of a spherical implosion and to the growth of spikes during both the acceleration and deceleration phases. Differences in the nonlinear growth rates for both bubbles and spikes are found when compared with planar results. The model predictions are verified by comparison with numerical hydrodynamics simulations

[en] Fast Ignition (FI) exploits the ignition of a dense, uniform fuel assembly by an external energy source to achieve high gain. In conventional ICF implosions, however, the fuel assembles as a dense shell surrounding a low density, high-pressure hotspot. Such configurations are far from optimal for FI. Here, it is shown that a self-similar spherical implosion of the type originally studied by Guderley [Luftfahrtforschung 19, 302 (1942).] may be employed to implode a dense, quasi-uniform fuel assembly with minimal energy wastage in forming a hotspot. A scheme for realizing these specialized implosions in a practical ICF target is also described

[en] Recently, attention has focused on the effects of spherical convergence on the nonlinear phase of Rayleigh-Taylor growth. In particular, for instability growth on spherically converging interfaces, modifications to the predictions of the Layzer model for the secular growth of a single, nonlinear mode have been reported. On the other hand, applications of interest involve surface perturbations which include the superposition of many unstable modes growing simultaneously. Such weakly nonlinear, multimode growth has previously been studied in the context of the well-known Haan model. Here, we combine the most recent results for enhanced nonlinear single mode growth on spherical interfaces with the Haan model formulation for multimode growth. Remarkably, the multimode results are found not to be substantially modified by including the effects of convergence

[en] The early nonlinear phase of Rayleigh-Taylor growth is typically described in terms of the classic Layzer model in which bubbles of light fluid rise into the heavy fluid at a constant rate determined by the bubble radius and the gravitational acceleration. However, this model is strictly valid only for planar interfaces and hence ignores any effects which might be introduced by the spherically converging interfaces of interest in inertial confinement fusion. Here a generalization of the Layzer nonlinear bubble rise rate is given for a self-similar spherically converging flow of the type studied by Kidder. A simple formula for the bubble amplitude is found showing that, while the bubble initially rises with a constant velocity similar to the Layzer result, during the late phase of the implosion, an acceleration of the bubble rise rate occurs. The bubble rise rate is verified by comparison with numerical hydrodynamics simulations

[en] Various gain models have shown the potentially great advantages of Fast Ignition (FI) Inertial Confinement Fusion (ICF) over its conventional hot spot ignition counterpart [e.g., S. Atzeni, Phys. Plasmas 6, 3316 (1999); M. Tabak et al., Fusion Sci. and Technology 49, 254 (2006)]. These gain models, however, all assume nearly uniform-density fuel assemblies. In contrast, conventional ICF implosions yield hollowed fuel assemblies with a high-density shell of fuel surrounding a low-density, high-pressure hot spot. Hence, to realize fully the advantages of FI, an alternative implosion design must be found which yields nearly isochoric fuel assemblies without substantial hot spots. Here, it is shown that a self-similar spherical implosion of the type originally studied by Guderley [Luftfahrtforschung 19, 302 (1942)] may be employed to yield precisely such quasi-isochoric imploded states. The difficulty remains, however, of accessing these self-similarly imploding configurations from initial conditions representing an actual ICF target, namely a uniform, solid-density shell at rest. Furthermore, these specialized implosions must be realized for practicable drive parameters and at the scales and energies of interest in ICF. A direct-drive implosion scheme is presented which meets all of these requirements and reaches a nearly isochoric assembled density of 300 g=cm³ and areal density of 2.4 g=cm² using 485 kJ of laser energy

[en] The recently proposed backward Raman laser amplification scheme utilizes the stimulated Raman backscattering in plasma of a long pumping laser pulse to amplify a short, frequency downshifted seed pulse. The output intensity for this scheme is limited by the development of forward Raman scattering (FRS) or modulational instabilities of the highly amplified seed. Theoretically, focused output intensities as high as 1025 W/cm² and pulse lengths of less than 100 fs could be accessible by this technique for 1 (micro)m lasers--an improvement of 10⁴-10⁵ in focused intensity over current techniques. Simulations with the particle-in-cell (PIC) code Zohar are presented which investigate the effects of FRS and modulational instabilities and of Langmuir wave breaking on the output intensity for Raman amplification. Using the intense seed pulse to photoionize the plasma simultaneous with its amplification (and hence avoid plasmas-based instabilities of the pump) is also investigated by PIC simulations. It is shown that both approaches can access focused intensities in the 1025 W/cm² range

[en] Encouraging progress is being made in demonstrating control of ablation front hydrodynamic instability growth in inertial confinement fusion implosion experiments on the National Ignition Facility [E. I. Moses, R. N. Boyd, B. A. Remington, C. J. Keane, and R. Al-Ayat, Phys. Plasmas 16, 041006 (2009)]. Even once ablation front stabilities are controlled, however, instability during the stagnation phase of the implosion can still quench ignition. A scheme is proposed to reduce the growth of stagnation phase instabilities through the reverse of the “adiabat shaping” mechanism proposed to control ablation front growth. Two-dimensional radiation hydrodynamics simulations confirm that improved stagnation phase stability should be possible without compromising fuel compression

[en] The work is summarized from several perspectives: 1D simulation perspective: Post-shot models agree with yield data to within a factor of ~2 at low implosion velocities, but the models diverge from the data as the velocity and convergence ratio increase. 2D simulation perspective: Integrated hohlraum-capsule post-shot models agree with primary data for most implosions, but overpredict yield and DSR for a few of the highest velocity implosions. High-resolution 3D post-shot capsule-only modeling captures much of the delivered performance of the one shot currently simulated.

[en] NIF capsules are supported in the hohlraum by two thin (∼15-110 nm) Formvar films (“tent”). Highly resolved HYDRA simulations indicate that a large (∼40% peak-average) areal density (ρR) perturbation develops on the capsule during acceleration as a consequence of this support geometry. This perturbation results in a jet of dense DT and, in some cases, CH that penetrates and cools the hot spot, significantly degrading the neutron yield (∼10-20% of 1D yield). We examine “low-foot” and “high-foot” pulse shapes, tent thicknesses, and geometries. Simulations indicate that thinner tents result in a smaller pR perturbation, however, the departure angle of the tent from the capsule surface is important, with steeper angles resulting in larger perturbations. (paper)