Session ZO1: ICF V: Hydrodynamics and Transport

Studies of Adiabat Shaping in Direct-Drive, Cryogenic-Target Implosions on OMEGA

In recent years, techniques to shape the radial profile of the shell entropy in a direct-drive ICF implosion have been developed. The goal is to have a high entropy in the outer regions to reduce the Rayleigh--Taylor growth rate and a low entropy in the main fuel layer for high compressibility. This physics is being studied at the Laboratory for Laser Energetics using fully \textit{$\beta $}-layered cryogenic DT and IR-layered D$_{2}$ capsule implosions on the OMEGA laser. This paper will summarize the status of this research. This work was supported by the U.S. D.O.E Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Time-Dependent Absorption Measurements in Direct-Drive Spherical Implosions

In recent time-resolved absorption measurements of direct-drive spherical implosion experiments on OMEGA, we observed higher absorption than predicted on the basis of inverse bremsstrahlung absorption during the first 100 to 200 ps of irradiation. This enhanced absorption does not result in significant differences in the overall energetics. This effect can significantly influence the first shock launched into the plasma, however, and can thus affect the outcome of implosion experiments. We are currently investigating the source of this enhanced absorption. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Investigation of Direct-Drive Shock Heating Using X-Ray Absorption Spectroscopy

The shock-heated shell in a direct-drive imploding capsule has plasma conditions in the Fermi-degenerate, strongly and weakly coupled regimes ($T_{e}\sim $ 10 eV, $n_{e}\sim $ 10$^{23}$ cm$^{-3})$. Time-resolved Al 1$s$--2$p$ absorption spectroscopy was used to diagnose direct-drive, shock-heated plasmas. A surrogate target consisting of a CH planar foil (50 \textit{$\mu $}m) with a buried tracer layer of Al (1 to 2 \textit{$\mu $}m) was irradiated with 10$^{14}$ to 10$^{15 }$W/cm$^{2}$, and x rays (1.4 to 1.6 keV) from a point-source Sm backlighter were transmitted through the drive foil. The $T_{e}$ inferred with detailed atomic physics codes (PrismSPECT, Spect3D) from the measured spectral line shapes are close to the predictions of the 1-D hydrodynamics code \textit{LILAC}. The extension of this technique to infer density from the broadening of the spectral line shapes will be discussed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Analysis of the Compressibility Experiments Performed on the OMEGA Laser System

Successful compression of inertial confinement fusion (ICF) targets to the densities and pressures needed for ignition and burn requires an understanding of how compression and shock waves propagate in the shell. A series of compressibility experiments on planar plastic foils (CH) have been conducted on the OMEGA Laser Facility to test the predictive capabilities of the hydrodynamic codes. The foils were driven using both square (high-adiabat) and shaped (low-adiabat) pulses. Density profiles of the laser-driven CH targets are measured using x-ray radiography. Comparisons between 1-D \textit{LILAC} simulations and experimental observations suggested that 2-D effects may be important. Simulations and analyses of these experiments are further performed using the 2-D hydrocode \textit{DRACO}. Direct comparison of the simulation results to experimental measurements will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

K-Shell Absorption Spectroscopy at Low Temperatures in Preheat Experiments

K-shell absorption spectroscopy has been applied to thin aluminum layers in shock-heated planar foils. With quantitative analysis based on detailed atomic kinetics, line shapes, and K-edge shifts, this technique has been extended as a temperature ($T_{e})$ and density ($n_{e})$ diagnostic to the lowest applicable temperatures. Comparisons between the conditions inferred from both measured and simulated spectra reveal biases characteristic of absorption spectroscopy in sampling realistic ranges of conditions within the signature layer. The spectrum analysis is based on least-squares parameter estimation, which takes into account the degradation of $T_{e}$ and $n_{e}$ estimates by uncertainties in other unknown quantities such as background emission levels. Other absorption spectral effects, including 1$s$-3$p$ absorption lines and L-shell ionization edges, are evaluated as $T_{e}$ and $n_{e}$ diagnostics in other possible low-temperature applications. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Numerical simulations of thin-shell direct-drive Omega capsule implosions

An experimental effort is currently underway in which thin (4 $\mu $m) glass-shell capsules are fielded in direct-drive implosions at the Omega Laser Facility. The thin shells result in fast implosions and shock-heating of the gas to temperatures up to and in excess of 10 keV. One goal of these experiments is to obtain independent time-dependent measurements of ion, electron, and radiation temperatures in order to study matter- radiation coupling in a system that is nonequilibrium and can be made to be NLTE by adding high-Z dopants such as Xe. In this paper, we give an overview of the experimental setup and a summary of the results to date. We then discuss in some detail the simulations: how they are run, how well they compare with the data, and what they suggest about the physics of matter- radiation coupling and mix-induced yield degradation in ICF capsules.   

Hydrodynamics of foam cryogenic-deuterium target.

Hydrodynamic performance such as shock generation, compression, and target acceleration as well as energy transport including preheating of rear surface were investigated at Gekko-HIPER laser-irradiation system at ILE, Osaka. Cryogenic liquid deuterium target contained in a low-density plastic foam plane was irradiated with a 527-nm foot pulse and a 527-nm main-drive pulse. Hydrodynamics were well in good agreement with 1D simulations with Fokker-Planck electron transport. Rear surface temperature was found to be not significantly but slightly higher than the code prediction, indicating a possibility of target preheating due to energy transport that is not included in the code.   

Effect of Medium Inhomogeneity on Hydrodynamic Evolution

Experimental studies of hydrodynamic instabilities of interfaces related to inertial confinement fusion often utilize low-density foams. A detailed understanding of these instabilities necessitates an accurate knowledge of the materials in which they are evolving. Theoretical predictions and simulations of these instabilities often assume the foam to be a continuous solid medium. However, the foams are not a solid material; rather they are porous with cell sizes ranging from nanometers to microns. An experiment was performed on the OMEGA Laser to measure the temporal evolution of hydrodynamic jets in which the target material utilized foams with varying cell sizes. Snapshots of the jet's evolution were recorded with point-projection radiography at multiple times. Results and simulations of the experiment will be presented. This work is performed under the auspices of the U. S. DOE by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.   

Radiation Transport Through Inhomogeneous Materials

Calculations of radiation transport in heated materials are greatly complicated by the presence of regions in which two or more materials are inhomogeneously mixed This phenomenon is important in inertial confinement fusion (ICF), where mixing can occur from instability growth and in astrophysical systems where density clumps can be found in star-forming regions and molecular clouds. We describe laboratory experiments to test modeling of radiation transport through inhomogeneous plasmas. A laser-heated hohlraum is used as a thermal source to drive radiation through polymer foam containing randomly-distributed gold particles. We present experimental measurements of radiation transport in homogeneous foam and an inhomogeneous foam-gold particle mixture with micron and sub micron gold particles. We also compare simulation results to the experiment.   

Nonlocal Ion-Heat Transport in ICF Implosions

During shock propagation and coalescence in the vapor region of ICF targets, the ion mean free path can become large compared to relevant spatial scale lengths and to the size of computational cells in computer models. During this time, a local treatment of heat conduction is not valid. To investigate the effect of these long mean-free-path ions, we have developed a model for nonlocal heat transport and applied it to the modeling of experiments on the OMEGA laser. A description of this model and simulation results will be discussed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Effects of the Ion Viscosity on the Shock Yield and Hot-Spot Formation in ICF Targets

The formation of a central hot spot in direct-drive spherical implosions is investigated employing the hydrodynamic approach. The mean free path of ions in the hot spot could become comparable to the size of the hot spot, resulting in a violation of the hyrodynamic approximation. We treat such conditions by adding ion-viscous terms in hydrodynamic equations. These terms has been implemented in the 1-D hydrocode \textit{LILAC}, which was used to simulate the neutron yield from hot spots in ICF spherical implosions. The results of such simulations and comparison with the experimental data will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Thermal Transport Modeling in ICF Direct-Drive Experiments

Thermal transport plays an important role in inertial confinement fusion. A new nonlocal heat transport model\footnote{V. N. Goncharov \textit{et al}., Phys. Plasmas \textbf{13}, 012702 (2006). } has been implemented in the 1-D hydrocode \textit{LILAC}. The model is based on a solution of a simplified Boltzmann equation with a Krook-type collisional operator. The simulation results show enhanced laser absorption during the first 100 to 200 ps of the laser drive due to the resonance absorption. The hot electrons generated by the resonance field are treated using existing models.\footnote{ E. J. Valeo and W. L. Kruer, Phys. Rev. Lett. \textbf{33}, 750 (1974); V. B. Rozanov and S. A. Shumskii, Sov. J. Quantum Electronics \textbf{16}, 1010 (1986).} Simulations indicate that these electrons do not significantly modify the shell adiabat for directly driven, ignition-scaled targets. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Effects of Resonant Absorption in Direct-Drive Experiments on OMEGA

The resonant absorption mechanism enhances laser absorption and can result in the generation of fast electrons in direct-drive experiments on OMEGA. These effects influence the shock timing and can modify the adiabat in imploded targets. The effects of resonant absorption in planar OMEGA experiments using the 1-D code \textit{LILAC} are numerically studied. The code includes a direct solution of Maxwell's equations for the incident laser light. The simulation results indicate an important contribution of the resonance absorption during the first 100 to 200 ps regardless of the dissipation mechanisms of the absorbed laser energy near the critical surface. The results of the model will be presented and compared against the experimental data. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Strong reduction of nonlocal electron heat transport in magnetized laser produced plasma

We present a direct measurement of the reduction of nonlocal heat transport in laser-produced plasma by applying large external magnetic fields ($>$ 10 T). Imaging Thomson scattering measurements of the electron temperature profile show confinement of the heat wave transverse to a high-power laser beam resulting in strong local heating. The electron temperature increases from 200 eV to 800 eV when a 12 Tesla magnetic field is applied. We find agreement with hydrodynamic modeling (using LASNEX) when including a magnetic field model that self-consistently evolves the fields in the plasma.   

First experimental observations of the magnetic field effects on the nonlocal electron energy transport at the inertial fusion conditions

A correct modelling of the electron energy transport is essential for the simulation of laser-matter interaction and for the Inertial Confinement Fusion (ICF) target design. The classical Spitzer-H\"arm model does not reproduce experimental results. The nonlocality of the electron transport combined with the self-generated magnetic fields is often suggested as an appropriate model. In the recent experiment carried out on the LIL facility, the prototype of the Laser Mega Joule under construction in France, the effects of nonlocal transport combined with the self-generated magnetic fields were observed for the first time for the ICF conditions. The experimental results are interpreted by 2D numerical simulations including our new electron transport model [1]. We show that the model correctly reproduces the experimental results and affirms the role of the magnetic field on the nonlocal transport. \newline \newline [1] Ph. Nicola\"i, J.-L. Feugeas and G. Schurtz, Phys. Plasmas 13, 032701 (2006)