Hydrodynamic target response to an induced spatial incoherence-smoothed laser beam

Emery, Mark H.; Gardner, John; Lehmberg, R. H.; Obenschain, S. P.

doi:10.1063/1.859976

Cited by 90 publications

(44 citation statements)

References 13 publications

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“…This is called laser imprint, which also causes a RTI seed in ICF implosion (Emery et al 1991;Ishizaki & Nishihara 1997Taylor et al 1997;Nishihara et al 1998;Matsui et al 1999;Goncharov et al 2000;Velikovich et al 2000). A rippled shock wave and ablation surface deformation caused by non-uniform laser irradiation on a smooth target were also evaluated with the deflagration model (Ishizaki & Nishihara 1997).…”

Section: (E) Rm-like Instabilities and Ablative Rmimentioning

confidence: 99%

“…The interaction between the rippled shock front and the ablation surface through sound waves may induce RM-like instability. It is important to study such growth of RM-like instabilities because they determine the seed of the RT instability that develops during the acceleration phase in ICF (Emery et al 1991;Desselberger et al 1995;Endo et al 1995;Azechi et al 1997;Ishizaki & Nishihara 1997Taylor et al 1997;Nishihara et al 1998;Goncharov 1999;Matsui et al 1999;Goncharov et al 2000;Velikovich et al 2000). A simple analytical model was developed to study propagation of a rippled shock associated with the initial surface roughness of a target in Ishizaki & Nishihara (1997.…”

Section: (E) Rm-like Instabilities and Ablative Rmimentioning

confidence: 99%

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Richtmyer–Meshkov instability: theory of linear and nonlinear evolution

Nishihara

Wouchuk

Matsuoka

et al. 2010

Phil. Trans. R. Soc. A.

128

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A theoretical framework to study linear and nonlinear Richtmyer-Meshkov instability (RMI) is presented. This instability typically develops when an incident shock crosses a corrugated material interface separating two fluids with different thermodynamic properties. Because the contact surface is rippled, the transmitted and reflected wavefronts are also corrugated, and some circulation is generated at the material boundary. The velocity circulation is progressively modified by the sound wave field radiated by the wavefronts, and ripple growth at the contact surface reaches a constant asymptotic normal velocity when the shocks/rarefactions are distant enough. The instability growth is driven by two effects: an initial deposition of velocity circulation at the material interface by the corrugated shock fronts and its subsequent variation in time due to the sonic field of pressure perturbations radiated by the deformed shocks. First, an exact analytical model to determine the asymptotic linear growth rate is presented and its dependence on the governing parameters is briefly discussed. Instabilities referred to as RM-like, driven by localized non-uniform vorticity, also exist; they are either initially deposited or supplied by external sources. Ablative RMI and its stabilization mechanisms are discussed as an example. When the ripple amplitude increases and becomes comparable to the perturbation wavelength, the instability enters the nonlinear phase and the perturbation velocity starts to decrease. An analytical model to describe this second stage of instability evolution is presented within the limit of incompressible and irrotational fluids, based on the dynamics of the contact surface circulation. RMI in solids and liquids is also presented via molecular dynamics simulations for planar and cylindrical geometries, where we show the generation of vorticity even in viscid materials.

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Section: (E) Rm-like Instabilities and Ablative Rmimentioning

confidence: 99%

Section: (E) Rm-like Instabilities and Ablative Rmimentioning

confidence: 99%

Richtmyer–Meshkov instability: theory of linear and nonlinear evolution

Nishihara

Wouchuk

Matsuoka

et al. 2010

Phil. Trans. R. Soc. A.

128

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“…Simulations have shown that laser imprint occurs primarily during the low intensity foot pulse. 7,8 A long scale buffering plasma forms during the higher intensity main pulse that smoothes the effects of laser illumination nonuniformity. The relatively cold (50-100eV) plasma that forms during the foot pulse does not have sufficiently good electron thermal conductivity between the target and the laser absorption region to allow such long scale plasmas to form.…”

Section: Introductionmentioning

confidence: 99%

Effects of thin high-Z layers on the hydrodynamics of laser-accelerated plastic targets

et al. 2002

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We present experimental results and simulations that study the effects of thin metallic layers with high atomic number (high-Z) on the hydrodynamics of laser accelerated plastic targets. These experiments employ a laser pulse with a low-intensity foot that rises into a high-intensity main pulse. This pulse shape simulates the generic Report Documentation PageForm Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. We present experimental results and simulations that study the effects of thin metallic layers with high atomic number (high-Z) on the hydrodynamics of laser accelerated plastic targets. These experiments employ a laser pulse with a low-intensity foot that rises into a high-intensity main pulse. This pulse shape simulates the generic shape needed for high-gain fusion implosions. Imprint of laser nonuniformity during start up of the low intensity foot is a well-known seed for hydrodynamic instability. We observe large reductions in hydrodynamic instability seeded by laser imprint when certain minimum thickness gold or palladium layers are applied to the laser-illuminated surface of the targets. The experiment indicates that the reduction in imprint is at least as large as that obtained by a 6 times improvement in the laser uniformity. We present simulations supported by experiments showing that during the low intensity foot the laser light can be nearly completely absorbed by the high-Z layer. X-rays originating from the high-Z layer heat the underlying lower-Z plastic target material and cause large buffering plasma to form between the layer and the accelerated target. This long-scale plasma apparently isolates the target from laser nonuniformity and accounts for the observed large reduction in laser imprint. With onset of the higher intensity main pulse, the high-Z layer expands and the laser light is transmitted. This technique will be useful in reducing laser imprint in pellet implosions and thereby allow the design of more robust targets for high-gain laser fusion. Prescribed by ANSI Std Z39-18 2 shape needed for high-gain fusion implosions. Imprint of laser nonuniformity during start up of the low intensity foot is a well-known seed for hydrodynamic instability. We observe large reductions in hydrodynamic instability seeded by laser...

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“…Such perturbations develop during the early temporal phases of a laser pulse, when thermal diffusion is ineffective at reducing large variations in the intensity that can occur on picosecond time scales, even for smoothed laser beams. 80 These laser imperfections beget areal-mass non-uniformities in the target that grow at early times due to the ablative RM instability and later due to the ablative RT instability. A study of perturbation growth due to laser imprint in ICF targets using the FastRad3D code is underway and will likely be the subject of a forthcoming publication.…”

Section: -10mentioning

confidence: 99%

Numerical simulations of the ablative Rayleigh-Taylor instability in planar inertial-confinement-fusion targets using the FastRad3D code

et al. 2016

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The ablative Rayleigh-Taylor (RT) instability is a central issue in the performance of laser-accelerated inertial-confinement-fusion targets. Historically, the accurate numerical simulation of this instability has been a challenging task for many radiation hydrodynamics codes, particularly when it comes to capturing the ablatively stabilized region of the linear dispersion spectrum and modeling ab initio perturbations. Here, we present recent results from two-dimensional numerical simulations of the ablative RT instability in planar laser-ablated foils that were performed using the Eulerian code FastRad3D. Our study considers polystyrene, (cryogenic) deuterium-tritium, and beryllium target materials, quarter- and third-micron laser light, and low and high laser intensities. An initial single-mode surface perturbation is modeled in our simulations as a small modulation to the target mass density and the ablative RT growth-rate is calculated from the time history of areal-mass variations once the target reaches a steady-state acceleration. By performing a sequence of such simulations with different perturbation wavelengths, we generate a discrete dispersion spectrum for each of our examples and find that in all cases the linear RT growth-rate γ is well described by an expression of the form γ=α [kg/(1+ϵ kLm)]1/2−βkVa, where k is the perturbation wavenumber, g is the acceleration of the target, Lm is the minimum density scale-length, Va is the ablation velocity, and ϵ is either one or zero. The dimensionless coefficients α and β in the above formula depend on the particular target and laser parameters and are determined from two-dimensional simulation results through the use of a nonlinear curve-fitting procedure. While our findings are generally consistent with those of Betti et al. (Phys. Plasmas 5, 1446 (1998)), the ablative RT growth-rates predicted in this investigation are somewhat smaller than the values previously reported for the same target and laser parameters. It is speculated that differences in the equation-of-state and opacity models are largely responsible for the discrepancy. Resolution of this issue awaits the development of better experimental diagnostics capable of measuring small-wavelength (5–20 μm) perturbation growth due to the ablative RT instability in the linear regime.

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Hydrodynamic target response to an induced spatial incoherence-smoothed laser beam

Cited by 90 publications

References 13 publications

Richtmyer–Meshkov instability: theory of linear and nonlinear evolution

Richtmyer–Meshkov instability: theory of linear and nonlinear evolution

Effects of thin high-Z layers on the hydrodynamics of laser-accelerated plastic targets

Numerical simulations of the ablative Rayleigh-Taylor instability in planar inertial-confinement-fusion targets using the FastRad3D code

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