Rayleigh–Taylor and Richtmyer-Meshkov instability induced flow, turbulence, and mixing. II

The highly nonlinear evolution of the single-mode Rayleigh-Taylor instability (RTI) at the ablation front of an accelerated target is investigated in the parameter range typical of inertial confinement fusion implosions. A new phase of the nonlinear bubble evolution is discovered. After the linear growth phase and a short constant-velocity phase, it is found that the bubble is accelerated to velocities well above the classical value. This acceleration is driven by the vorticity accumulation inside the bubble resulting from the mass ablation and vorticity convection off the ablation front. While the ablative growth rates are slower than their classical values in the linear regime, the ablative RTI grows faster than the classical RTI in the nonlinear regime for deuterium and tritium ablators.

/pdf/bubble-acceleration-in-the-ablative-rayleigh-taylor-isicnqcoa7.pdf

Bubble Acceleration in the Ablative Rayleigh-Taylor Instability

Inertial fusion energy (IFE) has been considered a promising, nearly inexhaustible source of sustainable carbon-free power for the world’s energy future. It has long been recognized that the control of hydrodynamic instabilities is of critical importance for ignition and high-gain in the inertial-confinement fusion (ICF) hot-spot ignition scheme. In this mini-review, we summarize the progress of theoretical and simulation research of hydrodynamic instabilities in the ICF central hot-spot implosion in our group over the past decade. In order to obtain sufficient understanding of the growth of hydrodynamic instabilities in ICF, we first decompose the problem into different stages according to the implosion physics processes. The decomposed essential physics pro- cesses that are associated with ICF implosions, such as Rayleigh-Taylor instability (RTI), Richtmyer-Meshkov instability (RMI), Kelvin-Helmholtz instability (KHI), convergent geometry effects, as well as perturbation feed-through are reviewed. Analyti- cal models in planar, cylindrical, and spherical geometries have been established to study different physical aspects, including density-gradient, interface-coupling, geometry, and convergent effects. The influence of ablation in the presence of preheating on the RTI has been extensively studied by numerical simulations. The KHI considering the ablation effect has been discussed in detail for the first time. A series of single-mode ablative RTI experiments has been performed on the Shenguang-II laser facility. The theoretical and simulation research provides us the physical insights of linear and weakly nonlinear growths, and nonlinear evolutions of the hydrodynamic instabilities in ICF implosions, which has directly supported the research of ICF ignition target design. The ICF hot-spot ignition implosion design that uses several controlling features, based on our current understanding of hydrodynamic instabilities, to address shell implosion stability, has been briefly described, several of which are novel.

Theoretical and simulation research of hydrodynamic instabilities in inertial-confinement fusion implosions

The quest for controlled fusion has been ongoing since the middle of the last century. Recently, however, there have been great strides in inertial confinement fusion, a method based on creating high energy densities through implosions. This review covers the three major approaches being pursued in the U.S., discussing key concepts, principles, and technical challenges. 

Physics principles of inertial confinement fusion and U.S. program overview

Fast electrons produced by a 10 ps, 160 J laser pulse through
 laser-compressed plastic cylinders are studied experimentally and
 numerically in the context of fast ignition. K(alpha)-emission images
 reveal a collimated or scattered electron beam depending on the initial
 density and the compression timing. A numerical transport model shows
 that implosion-driven electrical resistivity gradients induce strong
 magnetic fields able to guide the electrons. The good agreement with
 measured beam sizes provides the first experimental evidence for
 fast-electron magnetic collimation in laser-compressed matter.

Magnetically guided fast electrons in cylindrically compressed matter.

A fully nonlinear sharp-boundary model of the ablative Rayleigh-Taylor instability is derived and closed in a similar way to the self-consistent closure of the linear theory. It contains the stabilizing effect of ablation and accurately reproduces the results of 2D DRACO simulations. The single-mode saturation amplitude, bubble and spike evolutions in the nonlinear regimes, and the seeding of long-wavelength modes via mode coupling are determined and compared with the classical theory without ablation. Nonlinear stability above the linear cutoff is also predicted.

Nonlinear theory of the ablative Rayleigh-Taylor instability.

Target concepts developed in the context of the HIDIF project (Heavy Ion Driven Inertial Fusion) are presented. Target design has evolved, from the beginning of the study in 1993, in parallel with the evolution of accelerator parameters. Two different phases can be distinguished: (a) up to 1998 the goal was just demonstration of ignition with heavy ions, (b) from 1998 target and driver are being upgraded to high-gain designs able to be used for energy production. Analytical models and numerical tools developed in this context are summarized, and future research directions are discussed.

European fusion target work

One-dimensional model for a laser-ablated slab under acceleration

A preliminary design of a hohlraum driven by heavy ions has been analysed. The hohlraum is based on using foams of different densities to form a cavity surrounding the fuel capsule. The parameters of the beams have been provided by the HIDIF study group. The fuel capsule and pulse shaping have been taken from NIF. Doping of the ablator with intermediate-Z materials has also been studied. Results pertaining to the efficiency of the hohlraum, the ion pulse, the symmetry and the energy necessary to get ignition are presented.

Two-dimensional simulation of heavy ion hohlraum targets

Heavy ion fusion research in Spain

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