Session JI2: HEDP, Hydro Instability, Fast Ignition

Laser driven shockless compression for extracting near-isentrope equation-of-state measurements

Laser produced x-ray drive was used to shocklessly compress solid Al to a peak longitudinal stress of 110 GPa within 10 ns. Interface velocities versus time for multiple Al thicknesses were measured and converted to stress-density (P$_{x}-\rho )$ for near isentrope conditions using an iterative Lagrangian analysis. These are the most rapid shockless compression P$_{x}{\rm o}(\rho ){\rm p}$ data ever reported. P$_{x}{\rm o}(\rho ){\rm p}$ are stiffer than expected from models that are benchmarked against both static and shock experiments, suggesting a larger than expected time dependent viscoelastic response. In addition, we present the first results of a laser driven dynamic loading technique used to map out phase boundaries in Bismuth over ultra fast compression rates. Multi-grain Bi foils 20$\mu $m thick were shocklessly loaded to peak stresses of $\sim $11 GPa over 30ns. This time dependent compression causes the Bi sample to traverse several phase boundaries (I$\to $IV, Liquid) during a single shot. A time resolved velocity interferometer is used to measure the effects of new phases on a transmitted wave velocity profile. Ramp compression over several nanoseconds with initial temperatures spanning 410K to 521K results in over-driving of the Bi I-Liquid equilibrium phase boundary. A Bi multi-phase equation-of-state (EOS) for Bi I$\to $IV and Liquid together with a kinetics model give insights into the transition rates for different phase transitions.   

Material Hydrodynamics under Heated and Shocked Conditions

A critical goal in the inertial confinement fusion community is an understanding of the effects on capsule mixing due to target defects and surface perturbations. Ignition experiments typically rely on pre-shot target characterization to predict how initial perturbations will affect the late-time hydrodynamic mixing. However, it is the condition of these perturbations at the time of shock arrival that dominates their eventual late-time evolution. In some cases these perturbations are heated prior to the arrival of the main shock forming temperature and density gradients that may differ significantly from initial, pre-shot, conditions. A laser-based experimental platform has been developed to study these pre-heated hydrodynamic phenomena in a controlled manner. This new experimental design has recently generated extremely high quality image data on the OMEGA laser facility that has been quantitatively compared to simulation. The design allows for controlled x-ray preheat of a target and subsequent shock drive through gaps and perturbations. X-ray images have shown detailed evolution of heated gap structures at initial shock interaction and as the shock passes. Gaps are seen to ``heal'' and then reopen determined by the initial gap size and preheat conditions. Sufficient image resolution and dynamic range allow determination of detailed material locations and densities. These results give quantitative physical insight into the behavior of material evolution under shocked and heated conditions.   

Very-high-growth-factor planar ablative Rayleigh-Taylor experiments

The Rayleigh-Taylor instability is an important factor in bounding the performance envelope of ignition targets. This paper describes an experiment for ablative RT instability that for the first time achieves growth factors close to those expected to occur in ignition targets at the National Ignition Facility. The large growth allows small-seed perturbations to be detected and can be used to place an upper bound on perturbation growth at the ablation front resulting from microstructure in the preferred Be ablator. The experiments were performed on the Omega laser using a halfraum 1.2 mm long by 2 mm diameter with a 75{\%} laser entrance hole. The halfraum was filled with $\sim $ 1 atm of pentane to delay gold plasma from closing the diagnostic line of sight down the axis of the halfraum. The ablator was mounted at the base of the halfraum, and was accelerated by a two stepped X-ray pulse consisting of an early time section $\sim $ 100 eV to emulate the NIF foot followed by an approximately constant $\sim $ 150 eV drive sustained over an additional 5-7ns. It is this long pulse duration and late time observation that is different from previous experiments, and is responsible for the large growth achieved. The growth of a 2D sinusoidal perturbation machined on the drive side of the ablator was measured using face on radiography. The diagnostic view remained open until $\sim $ 10 ns at which time the growth factor was measured to be $\sim $ 200. The trajectory of the ablator was measured using streaked backlit radiography. The design and analysis of the experiments is described, and implications for experiments on ignition target ablators are discussed.   

Laser Plasma and Hydrodynamics Experiments with KrF Lasers

The proposed Fusion Test Facility (FTF) will exploit the unique features of Krypton Fluoride (KrF) lasers to achieve ignition and substantial gain ($>$20) at $<$500 kJ laser energies using direct drive.[1] The strategy uses highly uniform, high bandwidth, 248 nm KrF laser illumination at intensities near 2 x 10$^{15}$ W/cm$^{2}$ to accelerate low-aspect ratio pellets to implosion velocities of 400 km/s. Higher than usual implosion velocity allows ignition at substantially reduced laser energy. Amplitudes of both hydrodynamic instability during the pellet implosion and deleterious laser plasma instability (LPI) in the corona must be kept sufficiently low if one is to achieve ignition and gain. Increased laser intensity reduces hydrodynamic instability because it allows acceleration of thicker, low aspect ratio pellets, but is also more likely to produce deleterious LPI. The deep UV wavelength of KrF should allow use of these higher intensities. Studies of hydrodynamic instabilities and laser plasma instabilities (LPI) are the subject of ongoing experiments at the 2-3 kJ Nike KrF laser. The Nike laser has demonstrated highly uniform UV irradiation of planar targets at moderate laser intensities (I$\sim $10$^{14}$ W/cm$^{2})$, including the recent addition of short duration ``spike'' prepulses for hydrodynamic stability studies. A new effort in LPI physics is underway at the Nike facility where the peak intensity is being extended above 10$^{15}$ W/cm$^{2}$ by a combination of smaller focal diameters and shorter pulse lengths. This talk will discuss progress in the ongoing experiments at Nike in support of the FTF design. [1] S. P. Obenschain, et al., Phys. Plasmas\textbf{ 13} 056329 (2006).   

Simulations of High-Intensity Laser Interactions with Solid Targets and Implications for Fast-Ignition Experiments on OMEGA EP

High-intensity, laser--solid target-interaction experiments on the existing Vulcan\footnote{ C. N. Danson \textit{et al}., Nucl. Fusion \textbf{44}, S239 (2004).} and the future OMEGA/OMEGA EP Laser Facilities\footnote{ C. Stoeckl \textit{et al}., Fusion Sci. Technol. \textbf{49}, 367 (2006).} are modeled by a combination of techniques, including hybrid-implicit, particle-in-cell simulations,\footnote{ D. R. Welch \textit{et al}., Nucl. Instrum. Methods Phys. Res. A \textbf{464}, 134 (2001).} with the goal of predicting the performance of cone-in-shell fast-ignition experiments. The OMEGA EP Laser Facility will have an order of magnitude more energy available than on the Vulcan Laser System,$^{1}$ with an expected concomitant increase in total hot-electron energy. For interaction energies of $\sim $300~J to $\sim $5~kJ, low-mass, mid-$Z$ foil targets display some remarkable features that result from near-perfect hot-electron refluxing. They are efficient $K$-alpha radiators, with a yield that is insensitive to the details of the hot-electron spectrum.\footnote{ W. Theobald \textit{et al}., Phys. Plasmas \textbf{13}, 043102 (2006).} The absolute $K$-alpha yield is sensitive to the hot-electron conversion efficiency. This is investigated for the parameters of recent experiments.$^{4}$ Calculations with an appropriate electrical conductivity and equation of state\footnote{ R. R. Freeman, Bull. Am. Phys. Soc. \textbf{50}, 217 (2005).}$^{ }$show that target heating can be considerable. Additionally, hot surface layers$^{4}$ are attributed to surface retention and transport of part of the hot-electron spectrum. The degree to which these properties of ``isolated'' targets are active in cone geometries when refluxing is reduced by the target mass 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. Contributors: J. A. Delettrez, W. Theobald, C. Stoeckl, M. Storm, A. V. Maximov, and R. W. Short.   

Simulation and design study of cryogenic cone shell target for fast ignition FIREX project

The fast ignition (FI) scheme is attractive approach for the IFE, and many researches were conducted to understand its physics. In ILE Osaka Univ., FIREX (Fast Ignition Realization Experiment) project is in progress. Implosion experiments of the cryogenic target are scheduled in near future. In comparison with the central-hot-spot (CHS) approach, there are more unknown physics and variable parameters, which must be studied and determined to achieve the project's final goal of neutron yield 10\^{}14. Sophisticated target designs are required, in which not only the target structure and laser pulse shape but also the detail specifications of the high density fuel core plasma, trigger timing of the heating core by the peta-watt laser, heating mechanism, and so on, must be optimized. Fast Ignition Integrated Interconnecting code (FI$^{3})$ where radiation-hydro code, PIC code, and Fokker-Planck code are linked each others has been applied to the integrated simulation of FI. In the results, concerning the cone-guided implosion, we have learned that the uniformity of the implosion is not so critical issue as that in the CHS approach. Also, we have realized that heating efficiency strongly depend on the scale length of the pre-plasma in the conical target, and the density gap at the rear surface of the laser plasma interaction which locates the edge of the main fuel compressed by the implosion. That is, heating the core plasma with the peta-watt laser is not independent problem of laser plasma interaction. A desired fuel mass density profile of the core plasma is necessary. Recent design work of the cryogenic target is in progress for FIREX-I. Here, those important physics behind the FI are explained and some typical designs will be presented. \newline \newline Supported by MEXT, Grant-in Aid for Creative Scientific Research (15GS0214)