[en] We present a computational study of the formation of jets at strongly driven hydrodynamically unstable interfaces, and the interaction of these jets with one another and with developing spikes and bubbles. This provides a nonlinear spike-spike and spike-bubble interaction mechanism that can have a significant impact on the large-scale characteristics of the mixing layer. These interactions result in sensitivity to the initial perturbation spectrum, including the relative phases of the various modes, that persists long into the nonlinear phase of instability evolution

[en] The information that can be obtained from current laser driven high Mach number (compressible) hydrodynamics experiments using solid targets and foams is limited by the need to use X-ray diagnostics. These do well at providing the shape of gross 2D structures which we model well, but are a long way from being able to reveal detailed information at the smaller spatial scales, or in 3D turbulent flows, where most of the modeling uncertainties exist. Remedying this is, and will continue to be, an ongoing research effort. An alternative approach that is not being considered is to use gaseous targets coupled with optical diagnostics. The lower density of gases compared to solids or foams means we can use much larger targets for a given laser energy. This should significantly improve spatial resolution, and the dynamic range of scales that are resolvable. In addition, it may be possible to adapt powerful techniques, such as LIF, used by the low Mach number (incompressible) fluid/gas community so that they work in the high Mach number plasma regime. This would provide much more detailed information on turbulent flows than could be achieved with current X-ray diagnostics. We propose a small research effort to use established techniques such as optical interferometry (absolute electron density), and Schlieren photography (electron density gradient), to study compressible hydrodynamic instabilities. We also propose to explore whether techniques such as LIF may be adapted to the plasma regime, thus providing detailed information, particularly about turbulent flows, that is not currently obtainable in plasmas using X-ray diagnostics. The setting will be radiating blast waves, which avoids costly target fabrication, while promising a high physics payoff to the astrophysics community just from using the established diagnostics alone. We propose to conduct the work in collaboration with Dr Todd Ditmire at the University of Texas at Austin, principally on the Janus laser, and Ditmire's short pulse laser, which is expected to be operational towards the beginning of FY02. Dr Stephen Rose at AWE has expressed interest in collaboration and would provide computational support. He would also look into using the Helen laser at AWE, and developing a UK university contact

[en] Explore the combination of optical diagnostics and gaseous targets to obtain important information about compressible turbulent flows that cannot be derived from traditional laser experiments for the purposes of V and V of hydrodynamics models and understanding scaling. First year objectives: Develop and characterize blast wave-gas jet test bed; Perform single pulse shadowgraphy of blast wave interaction with turbulent gas jet as a function of blast wave Mach number; Explore double pulse shadowgraphy and image correlation for extracting velocity spectra in the shock-turbulent flow interaction; and Explore the use/adaptation of advanced diagnostics

[en] Progress in simulation and experiment enhances prospects for NIF double-shell ignition in FY2011. Simulation studies have identified a stable double-shell ignition design based on graded density shells and metallic foams. Recent double-shell implosion experiments on Omega have shown unprecedented performance. Identification of possible failure mechanisms has motivated new and improved double shell designs: e.g., like-material outer surface of inner shell and foam; greatly reduced gap size; and density-matched glue. Progress in double-shell research has inspired many exciting materials science advances on the path to ignition

[en] A major challenge in achieving ignition with double-shells is controlling the mix of the dense, high-Z pusher into the DT gas. During implosion, interface perturbations become unstable as they are subjected to either impulsive (Richtmyer-Meshkov) or time-dependent (Rayleigh-Taylor) accelerations. These processes are especially critical for double-shells since density gradient stabilization mechanisms (that play a key role in the baseline cryogenic target) are not present. To study the nonlinear RT evolution for such a large range in modes we use the parallel 3-D rad-hydro code HYDRA. Simulations have revealed a new pathway for the RT instability of perturbations on the outer surface of the inner shell leading to shell disruption. We demonstrate that this instability can be controlled by tamping the inner shell with a low-Z material but it is not entirely suppressed. We find that the pusher/tamper interface transitions to turbulence at late times with large Reynolds number but still the integrity of the pusher is maintained. Furthermore, numerical studies suggest that for perturbations with mode numbers (l > 600), the mix-width at the pushed tamper interface approaches a constant value. Finally, to avoid turbulence onset altogether we investigate a new pusher with an imprinted density-gradient scale length in combination with a CuO/Cu₂O foam. Preliminary 2-D simulations with mode numbers up tp l = 612 show virtually no growth in this design

[en] This memo reports on the analysis of some recent measurements of solution impurity levels in the three KDP and one DKDP Pilot Production 1000 liter growth tanks (Tanks B, C, D, ampersand F). Solution samples were taken on a weekly basis during recent crystal growth runs in each tank and were analyzed by inductively coupled plasma emission spectroscopy (ICP-ES). The solution history for five specific elements, Si, B, Al, Fe and Ca will be analyzed in detail. The first four of these elements are input into solution via slow dissolution of the glass vessel at a rate which is strongly dependent on the solution temperature. Si and B continuously accumulate in solution, since they are not incorporated into the crystal. Al and Fe by comparison are incorporated into the crystal (primarily the prismatic sectors) and present problems to inclusion-free growth (Al) and 30 damage (Fe). The level of these impurities initially increases when the crystal size is small but later decreases when the rate of incorporation into the crystal exceeds the rate of dissolution of the glass tank. The last element, Ca is of interest since it has recently been observed to be one of the elements found at the location of 3cu damage. For Si and B, the dissolution or leach rate from the glass tank is easily obtained from the results of the chemical analysis. The temperature dependent leach rates are shown to be comparable (within a factor of two) for all four tanks, with Tank B (DKDP) having the lowest rate of Si accumulation. The glass leach rates of the two incorporating elements Al and Fe require substantially more analysis as the daily variation of the crystal dimensions, the solution concentration, and the mass of KDP remaining in solution must be taken into account in order to separate the rate of impurity incorporation from the rate of dissolution of the glass. The method for accomplishing this separation is described, and the result obtained is that the leach rates of all four tanks are within a factor of three of each other. Tank B again shows the lowest leach rate for both Al and Fe. The results for Ca are less clear. From the present data, the level of Ca does not change appreciably during a run, indicating that it .is neither leaching from the glass strongly nor being incorporated into the crystal at a significant rate. It does increase with the age of the solution, however, as Ca is a small but measurable component of both the starting salt as well as the D,O. Older solutions that have successfully grown several crystals will therefore have higher accumulated levels of Ca, which will increase the driving force for Ca incorporation into the crystal

[en] Double-shell (DS) targets (Amendt, P. A. et al., 2002) offer a complementary approach to the cryogenic baseline design (Lindl, J. et al., 2004) for achieving ignition on the National Ignition Facility (NIF). Among the expected benefits are the ease of room temperature preparation and fielding, the potential for lower laser backscatter and the reduced need for careful shock timing. These benefits are offset, however, by demanding fabrication tolerances, e.g., shell concentricity and shell surface smoothness. In particular, the latter is of paramount importance since DS targets are susceptible to the growth of interface perturbations from impulsive and time-dependent accelerations. Previous work (Milovich, J. L. et al., 2004) has indicated that the growth of perturbations on the outer surface of the inner shell is potentially disruptive. To control this instability new designs have been proposed requiring bimetallic inner shells and material-matching mid-Z nanoporous foam. The challenges in manufacturing such exotic foams have led to a further evaluation of the densities and pore sizes needed to reduce the seeding of perturbations on the outer surface of the inner shell, thereby guiding the ongoing material science research efforts. Highly-resolved 2D simulations of porous foams have been performed to establish an upper limit on the allowable pore sizes for instability growth. Simulations indicate that foams with higher densities than previously thought are now possible. Moreover, while at the present time we are only able to simulate foams with average pore sizes larger than 1 micron (due to computational limitations), we can conclude that these pore sizes are potentially problematic. Furthermore, the effect of low-order hohlraum radiation asymmetries on the growth of intrinsic surface perturbations is also addressed. Highly-resolved 2D simulations indicate that the transverse flows that are set up by these low-order mode features (which can excite Kelvin-Helmholtz instabilities) are not large enough to offset the overall robustness of our current design

[en] There is a great deal of interest in studying the evolution of hydrodynamic phenomena in high energy density plasmas that have transitioned beyond the initial phases of instability into an Ely developed turbulent state. Motivation for this study arises both in fusion plasmas as well as in numerous astrophysical applications where the understanding of turbulent mixing is essential. Double-shell ignition targets, for example, are subject to large growth of short wavelength perturbations on both surfaces of the high-Z inner shell. These perturbations, initiated by Richtmyer-Meshkov and Rayleigh-Taylor instabilities, can transition to a turbulent state and will lead to deleterious mixing of the cooler shell material with the hot burning fuel. In astrophysical plasmas, due to the extremely large scale, turbulent hydrodynamic mixing is also of wide-spread interest. The radial mixing that occurs in the explosion phase of core-collapse supernovae is an example that has received much attention in recent years and yet remains only poorly understood. In all of these cases, numerical simulation of the flow field is very difficult due to the large Reynolds number and corresponding wide range of spatial scales characterizing the plasma. Laboratory experiments on high energy density facilities that can access this regime are therefore of great interest. Experiments exploring the transition to turbulence that are currently being conducted on the Omega laser will be described. We will also discuss experiments being planned for the initial commissioning phases of the NIF as well as the enhanced experimental parameter space that will become available, as additional quads are made operational

[en] Point design ignition capsules designed for the National Ignition Facility (NIF) currently use an x-ray-driven Be(Cu) ablator to compress the DT fuel. Ignition specifications require that the mass of unablated Be(Cu), called residual mass, be known to within 1% of the initial ablator mass when the fuel reaches peak velocity. The specifications also require that the implosion bang time, a surrogate measurement for implosion velocity, be known to +/- 50 ps RMS. These specifications guard against several capsule failure modes associated with low implosion velocity or low residual mass. Experiments designed to measure and to tune experimentally the amount of residual mass are being developed as part of the National Ignition Campaign (NIC). Tuning adjustments of the residual mass and peak velocity can be achieved using capsule and laser parameters. We currently plan to measure the residual mass using streaked radiographic imaging of surrogate tuning capsules. Alternative techniques to measure residual mass using activated Cu debris collection and proton spectrometry have also been developed. These developing techniques, together with bang time measurements, will allow us to tune ignition capsules to meet NIC specs

[en] The conversion efficiency of ultra short-pulse laser radiation to K-α x-rays has been measured for various chlorine-containing targets to be used as x-ray scattering probes of dense plasmas. The spectral and temporal properties of these sources will allow spectrally-resolved x-ray scattering probing with picosecond temporal resolution required for measuring the plasma conditions in inertial confinement fusion experiments. Simulations of x-ray scattering spectra from these plasmas show that fuel capsule density, capsule ablator density, and shock timing information may be inferred