[en] Hard x-ray (''Thin wall'') imaging will be employed on the National Ignition Facility (NIF) to spatially locate laser beam energy deposition regions on the hohlraum walls in indirect drive Inertial Confinement Fusion (ICF) experiments, relevant for ICF symmetry tuning. Based on time resolved imaging of the hard x-ray emission of the laser spots, this method will be used to infer hohlraum wall motion due to x-ray and laser ablation and any beam refraction caused by plasma density gradients. In optimizing this measurement, issues that have to be addressed are hard x-ray visibility during the entire ignition laser pulse with intensities ranging from 10¹³ to 10¹⁵ W/cm², as well as simultaneous visibility of the inner and the outer laser drive cones. In this work we will compare the hard x-ray emission calculated by LASNEX and analytical modeling with thin wall imaging data recorded previously on Omega and during the first hohlraum experiments on NIF. Based on these calculations and comparisons the thin wall imaging will be optimized for ICF/NIF experiments

[en] High convergence, hohlraum-driven implosions will require control of time-integrated drive asymmetries to 1% levels for ignition to succeed on the NIF. We review how core imaging provides such asymmetry measurement accuracy for the lowest order asymmetry modes, and describe recent improvements in imaging techniques that should allow detection of higher order asymmetry modes. We also present a simple analytic model explaining how the sensitivity of symmetry control to beam pointing scales as we progress from single ring per side Nova cylindrical hohlraum illumination geometries to NIF-like multiple rings per side Omega hohlraum illumination geometries and ultimately to NIF-scale hohlraums

[en] We have used a large format (4000x4000) high resolution (9 μm pixels) charge coupled device (CCD) to record images from the rear of a gated micro-channel plate (MCP) intensifier, and compared the results with conventional film recording. Measurements of linearity, dynamic range, dark noise, and distortion all show that the CCD is a superior replacement for film. Furthermore, its excellent registration allows for easy flat fielding, using data from a uniformly exposed MCP. As we increase the signal level to where the signal to noise is not dominated by photon counting statistics, we find that this flat fielding procedure produces a significant improvement in signal to noise. The small spatial scale of this noise has led to its identification as high spatial frequency variations in the MCP phosphor

[en] For ICF hohlraums driven by long pulses, such as will be needed for ignition on the NIF, the high-Z wall must be held back to avoid excessive laser spot motion and time-dependent symmetry swings. One means of accomplishing this is to fill the hohlraum with a low density, low-Z gas. We report on gas-wall interface characterization by axial x-ray backlighting and self-emission, on gas filled hohlraums fielded at the Omega facility. Up to 30 drive beams are fired, forming a single ring of illumination on the hohlraum wall to emulate the near 2D cylindrically symmetric NIF hohlraum drive conditions. We compare the observed motion with predictions. In addition, the gas-gold interface is Rayleigh-Taylor (R-T) unstable during deceleration. This R-T instability could be further exacerbated in NIF ignition hohlraums designed with intentionally roughened walls to provide smoothing of infrared heating used to prepare smooth DT ice layers in the capsule. We have therefore intentionally prepared initial perturbations on one half of the gold wall to quantify the amount of increased penetration, due to mix of the gold into the gas, at stagnation

[en] We have made the first detailed measurements of a diffusive supersonic radiation wave in the laboratory. A 10 mg/cm³ SiO₂ foam is radiatively heated by the x-ray flux from a laser-irradiated hohlraum. The resulting radiation wave propagates axially through the optically thick foam and is measured via time-resolved x-ray imaging as it breaks out the far end. The data show that the radiation wave breaks out at the center prior to breaking out at the edges, indicating a significant curvature in the radiation front. This curvature is primarily due to energy loss into the walls surrounding the foam. (c) 2000 The American Physical Society

[en] Coupling efficiency, the ratio of the capsule absorbed energy to the driver energy, is a key parameter in ignition targets. The hohlraum originally proposed for NIF coupled ∼11% of the absorbed laser energy to the capsule as x-rays. We describe here a second generation of hohlraum target which has higher coupling efficiency, ∼16%. Because the ignition capsule's ability to withstand 3D effects increases rapidly with absorbed energy, the additional energy can significantly increase the likelihood of ignition. The new target includes laser entrance hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive ICF. The LEH shields are high Z disks placed inside the hohlraum to block the capsule's view of the cold LEHs. The LEH shields can reduce the amount of laser energy required to drive a target to a given temperature via two mechanisms: (1) keeping the temperature high near the capsule pole by putting a barrier between the capsule and the pole, (2) because the capsule pole does not have a view of the cold LEHs, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple 140 kJ of x-rays to a capsule out of 865 kJ of absorbed laser energy and produce ∼10 MJ of yield. In the current designs, which are not completely optimized, the addition of the LEH shields saves ∼95 kJ of energy (about 10%) over hohlraums without LEH shields

[en] Hard x-ray ('thin wall') imaging will be employed on the National Ignition Facility (NIF) to spatially locate laser beam energy deposition regions on the hohlraum walls in indirect drive Inertial Confinement Fusion (ICF) experiments, relevant for ICF symmetry tuning. Based on time resolved imaging of the hard x ray emission of the laser spots, this method will be used to infer hohlraum wall motion due to x ray and laser ablation and any beam refraction caused by plasma density gradients. In optimizing this measurement, issues that have to be addressed are hard x-ray visibility during the entire ignition laser pulse with intensities ranging from 10¹³ to 10¹⁵ W/cm², as well as simultaneous visibility of the inner and the outer laser drive cones. In this work we will compare the hard x-ray emission calculated by LASNEX and analytical modeling with thin wall imaging data recorded previously on Omega and during the first hohlraum experiments on NIF. Based on these calculations and comparisons the thin wall imaging will be optimized for ICF/NIF experiments

[en] Carbon isotopic evidence revealed Deepwater Horizon (DWH) oil entering coastal planktonic and lower terrestrial food webs. The integration of spilled oil into higher terrestrial trophic levels, however, remains uncertain. We measured radiocarbon (¹⁴C) and stable carbon (¹³C) in seaside sparrow ( Ammodramus maritimus ) feathers and crop contents. Lower ¹⁴C and ¹³C values in feathers and crop contents of birds from contaminated areas indicated incorporation of carbon from oil. Our results, although based on a small sample of birds, thus reveal a food-web link between oil exposure and a terrestrial ecosystem. They also suggest that the reduction in reproductive success previously documented in the same population might be due to the (direct) toxic effect of oil exposure, rather than to (indirect) ecological effects. We recommend future studies test our results by using larger samples of birds from a wider area in order to assess the extent and implications of DWH oil incorporation into the terrestrial food web. (letter)

[en] A new method for performing compressible hydrodynamic instability experiments using high-power lasers is presented. A plasma piston is created by supersonically heating a low-density carbon based foam with x-rays from a gold hohlraum heated to ∼200 eV by a ∼1 ns Nova laser pulse [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)]. The piston causes an almost shockless acceleration of a thin, higher-density payload consisting of a layer of gold, initially 1/2 μm thick, supported on 10 μm of solid plastic, at ∼45 μm/ns². The payload is also heated by hohlraum x-rays to in excess of 150 eV so that the Au layer expands to ∼20 μm prior to the onset of instability growth. The Atwood number between foam and Au is ∼0.7. Rayleigh-Taylor instability, seeded by the random fibrous structure of the foam, causes a turbulent mixing region with a Reynolds number >10⁵ to develop between piston and Au. The macroscopic width of the mixing region was inferred from the change in Au layer width, which was recorded via time resolved x-radiography. The mix width thus inferred is demonstrated to depend on the magnitude of the initial foam seed. For a small initial seed, the bubble front in the turbulent mixing region is estimated indirectly to grow as ∼0.036±0.19 [∫√(Ag)dt]² which would imply for a constant acceleration 0.036±0.019 Agt². More direct measurement techniques must be developed in larger scale experiments to remove potential complicating factors and reduce the error bar to a level that would permit the measurements to discriminate between various theories and models of turbulent mixing. (c) 2000 American Institute of Physics

[en] Indirect-drive inertial confinement fusion makes use of cavities constructed of high atomic number materials to convert laser power into x-rays for ablatively driving an implosion capsule. Obtaining spatially uniform drive on the capsule requires a careful balancing between the laser absorption region (high drive) and the laser entrance holes (low drive). This balancing is made difficult because of plasma expansion, and the associated movement of the laser absorption region with time. This paper reports the first experimental demonstration of compensation for this motion by using different laser beams at different times, in agreement with modeling. (c) 2000 American Institute of Physics