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Experiments performed at the Laboratory for Laser Energetics with a continuous-wave (cw) x-ray source and on the OMEGA and OMEGA EP Laser Systems [Boehly et al., Opt. Commun. 133, 495 (1997) and Waxer et al., Opt. Photonics News 16, 30 (2005)] have utilized a Fresnel zone plate (FZP) to obtain x-ray images with a spatial resolution as small as ∼1.5 μm. Such FZP images were obtained with a charge-coupled device or a framing camera at energies ranging from 4.5 keV to 6.7 keV using x-ray line emission from both the cw source and high-intensity, laser-beam–illuminated metal foils. In all cases, the resolution test results are determined from patterns and grids backlit by these sources. The resolutions obtained are shown to be due to a combination of the spectral content of the x-ray sources and detector resolution limited by the magnification of the images (14× to 22×). High-speed framing cameras were used to obtain FZP images with frame times as short as ∼30 ps. Double-shell implosions on OMEGA were backlit by laser-irradiated Fe foils, thus obtaining a framing-camera–limited, FZP-image resolution of ∼3 μm–4 μm. [ABSTRACT FROM AUTHOR]

Pressure, by definition, characterizes the conditions within an isobaric implosion core at peak compression [Gus'kov et al., Nucl. Fusion 16, 957 (1976); Betti et al., Phys. Plasmas 8, 5257 (2001)] and is a key parameter in quantifying its near-ignition performance [Lawson, Proc. Phys. Soc. London, B 70, 6 (1957); Betti et al., Phys. Plasmas 17, 058102 (2010); Goncharov et al., Phys. Plasmas 21, 056315 (2014); and Glenzer et al., Phys. Plasmas 19, 056318 (2012)]. At high spectral energy, where the x-ray emission from an imploded hydrogen core is optically thin, the emissivity profile can be inferred from the spatially resolved core emission. This emissivity, which can be modeled accurately under hot-core conditions, is dependent almost entirely on the pressure when measured within a restricted spectral range matched to the temperature range anticipated for the emitting volume. In this way, the hot core pressure at the time of peak emission can be inferred from the measured free-free emissivity profile. The pressure and temperature dependences of the x-ray emissivity and the neutron-production rate explain a simple scaling of the total filtered x-ray emission as a constant power of the total neutron yield for implosions of targets of similar design over a broad range of shell implosion isentropes. This scaling behavior has been seen in implosion simulations and is confirmed by measurements of high-isentrope implosions [Sangster et al., Phys. Plasmas 20, 056317 (2013)] on the OMEGA laser system [Boehly et al., Opt. Commun. 133, 495 (1997)]. Attributing the excess emission from less-stable, low-isentrope implosions, above the level expected from this neutron-yield scaling, to the higher emissivity of shell carbon mixed into the implosion's central hot spot, the hot-spot "fuel-shell" mix mass can be inferred. [ABSTRACT FROM AUTHOR]

A high-performance cryogenic DT inertial confinement fusion implosion experiment is an especially challenging backlighting configuration because of the high self-emission of the core at stagnation and the low opacity of the DT shell. High-energy petawatt lasers such as OMEGA EP promise significantly improved backlighting capabilities by generating high x-ray intensities and short emission times. A narrowband x-ray imager with an astigmatism-corrected bent quartz crystal for the Si Heα line at ~1.86 keV was developed to record backlit images of cryogenic direct-drive implosions. A time-gated recording system minimized the self-emission of the imploding target. A fast target-insertion system capable of moving the backlighter target ~7c mi n ~100 ms was developed to avoid interference with the cryogenic shroud system. With backlighter laser energies of ~1.25 kJ at a 10-ps pulse duration, the radiographic images show a high signal-to-background ratio of >100:1 and a spatial resolution of the order of 10 µm. The backlit images can be used to assess the symmetry of the implosions close to stagnation and the mix of ablator material into the dense shell. [ABSTRACT FROM AUTHOR]

High-intensity, short-pulse laser-interaction experiments with small-mass, wedge-shaped-cavity Cu targets are presented. The diagnostics provided spatially and spectrally resolved measurements of the Cu Kα line emission at 8 keV. The conversion efficiency of short-pulse laser energy into fast electrons was inferred from the x-ray yield for wedge opening angles between 30° and 60° and for s- and p-polarized laser irradiation. Up to 36±7% conversion efficiency was measured for the narrowest wedge with p-polarization. The results are compared with predictions from two-dimensional particle-in-cell simulations. [ABSTRACT FROM AUTHOR]

X-ray-sensitive film is used for a variety of imaging and spectroscopic diagnostics for high-temperature plasmas. Replacement film must be found as older films are phased out of production. Biomax-MS is a “T-grain” class of film that is proposed as a replacement for Kodak DEF and a model of its response to x rays is presented. Data from dimensional measurements of the film, x-ray transmission measurements, scanning electron microscopy micrograph images, and x-ray calibration are used to develop this sensitivity model of Biomax-MS film as a function of x-ray energy and angle of incidence. Relative response data provide a check of the applicability of this model to determine the x-ray flux from spectrum data. This detailed film characterization starts with simple mathematical models and extends them to T-grain–type film. [ABSTRACT FROM AUTHOR]

Direct-drive, plastic shells imploded on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] with a 1 ns square pulse are simulated using the multidimensional hydrodynamic code DRACO in yield degradation in “thin” shells is primarily caused by shell breakup during the acceleration phase due to short-wavelength (ℓ>50, where ℓ is the Legendre mode number) perturbation growth, whereas “thick” shell performance is influenced primarily by long and intermediate modes (ℓ≲50). Simulation yields, temporal history of neutron production, areal densities, and x-ray images of the core compare well with experimental observations. In particular, the thin-shell neutron production history falls off less steeply than one-dimensional predictions due to shell-breakup-induced undercompression and delayed stagnation. Thicker, more-stable shells show burn truncation due to instability-induced mass flow into the colder bubbles. Estimates of small-scale mix indicate that turbulent mixing does not influence primary neutron yields. [ABSTRACT FROM AUTHOR]

A re-entrant, highly adaptable, x-ray streaked imager has been developed for OMEGA to increase the sensitivity and spatial resolution in hydrodynamic-stability experiments. It is based on a four-mirror Kirkpatrick–Baez (KB) microscope, coupled to a high-current streak tube. The unique mechanical assembly of the KB optic allows a choice between single- or multi-image modes and one- or two-dimensional imaging. Currently, the optic uses an Ir coating at a grazing angle of 2.1°. The incidence angle has been optimized to maximize throughput in the chosen energy band, centered at 1.5 keV with a full width at half maximum of about 0.4 keV. A calculated resolution of better than 5 μm over the central 200 μm of the field of view was verified in inertial confinement fusion experiments. New multilayer mirror elements for high-energy or multiband imaging, take advantage of the flexible mechanical design. Some important features of the PJX streak tube are described. [ABSTRACT FROM AUTHOR]

Mixing between the shell and fuel in directly driven single shell capsule implosions causes changes in yield, burn history, burn temperature, areal density, x-ray image shape, and the presence of atomic mix. Most observations are consistent with a mix model using the same values of its single free parameter as with indirectly driven single shell and double shell capsules. Greater mixing at lower gas pressure fills reduces capsule yield. Time dependent mixing growth causes truncation of the burn history. This emphasizes early yield from the center of the capsule, raising the observed burn temperature. Mixed fuel areal densities are lower because fuel moves through the shell and the observation weights earlier times when areal density is lower. Shell x-ray emission mixing into the fuel fills in the limb brightened image to produce a central peak. Implosions of ³He filled capsules with a layer of deuterated plastic show substantial atomic mix. [ABSTRACT FROM AUTHOR]

Three recent developments in direct-drive target design have enhanced the possibility of achieving high target gain on the National Ignition Facility (NIF): (1) Laser absorption was increased by almost 50% using wetted-foam targets. (2) Adiabat shaping significantly increased the hydrodynamic stability of the target during the acceleration phase of the implosion without sacrificing target gain. (3) Techniques to reduce laser imprint using pulse shaping and radiation preheat were developed. These design features can be employed for direct-drive-ignition experiments while the NIF is in the x-ray-drive configuration. This involves repointing some of the beams toward the equator of the target to improve uniformity of target drive. This approach, known as polar direct drive (PDD), will enhance the capability of the NIF to explore ignition conditions. PDD will couple more energy to the fuel than x-ray drive. The compressed fuel core can be more easily accessed for high-ρR diagnostic development and for fast-ignitor studies. Polar direct drive is examined in this manuscript using two-dimensional hydrodynamic simulations to determine the level of target performance that can be achieved. [ABSTRACT FROM AUTHOR]

Layered and characterized cryogenic D2 capsules have been imploded using high-contrast pulse shapes on the 60-beam OMEGA laser at the Laboratory for Laser Energetics [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. These experiments measure the sensitivity of the direct-drive implosion performance to parameters such as the inner-ice-surface roughness, the adiabat of the fuel during the implosion, and the laser power balance. The goal is to demonstrate a high neutron-averaged fuel ρR with low angular variance using a scaled, α∼3 ignition pulse shape driving a scaled all-DT ignition capsule. Results are reported with improvements in target layering and characterization and in laser pointing and target positioning on the OMEGA laser over previous experiments [T. C. Sangster et al., Phys. Plasmas 10, 1937 (2003)]. These capsules have been imploded using up to 23 kJ of 351-nm laser light with an on-target energy imbalance of less than 2% rms, full beam smoothing (1-THz bandwidth, two-dimensional smoothing by spectral dispersion, and polarization smoothing), and new, optimized, distributed phase plates. Pulse shapes include high-adiabat (∼25) square pulses and low-adiabat (<5) shaped pulses. The data from neutron and charged-particle diagnostics, as well as static and time-resolved x-ray images of the imploding core, are compared with one- and two-dimensional numerical simulations. Scaling of target performance to a weighted quadrature of inner-ice roughness at the end of the acceleration phase is investigated. [ABSTRACT FROM AUTHOR]

Investigates the target performance of stagnation and dependence on beam smoothing and pulse shaping of the OMEGA laser system. Diagnosis of compressed core conditions; Use of x-ray and neutron spectroscopy; Application of near-uniform illumination of spherical fuel-bearing targets with high-power laser beams.

The spectroscopic analysis of x-ray narrow-band images and space-integrated x-ray line spectra from argon-doped deuterium-filled inertial confinement fusion implosion experiments yields information about the spatial profiles in the compressed core. We discuss the analysis of direct-drive implosion experiments at OMEGA, in which images were obtained with a multimonochromatic imaging instrument. The analysis method considers data based on the argon Heβ and Lyβ spectral features and their associated Li- and He-like satellites. The temperature gradient structure is investigated by using the sensitivity of the Lyβ/Heβ emissivity ratio to the temperature. [ABSTRACT FROM AUTHOR]

We discuss a method for the simultaneous and self-consistent fitting of a set of intensity or emissivity spatial profiles from several narrow-band x-ray pinhole images from argon-doped inertial confinement fusion implosion cores, and the space-integrated line spectrum. A Pareto genetic algorithm (PGA) combines the search and optimization capabilities of a single-objective genetic algorithm with the Pareto domination technique of multiobjective optimization. Further, the PGA search is followed up by a fine-tuning step based on a nonlinear least-squares-minimization procedure. The result is a robust search and reconstruction method that finds the optimal core spatial structure subject to multiple constraints. This method is independent of geometry inversions and could take advantage of not only optically thin but also optically thick image data. Results are shown for two combinations of three-objectives based on gated argon Heβ and Lyβ image data and the line spectrum. [ABSTRACT FROM AUTHOR]

A framed, 16-image, Kirkpatrick–Baez (KB)-type x-ray microscope has been designed for use in imaging laser–plasma x-ray emission. The reflecting elements are 16 pairs of concave mirrors arranged to reflect and focus x rays emanating from a laser-produced plasma. The resolution of the elements is 3 μm at best focus and is better than 5 μm within a 400-μm-diam region. A framing camera will be used in combination with the KB optic to produce 16 gated x-ray images in the energy range from 1.5 to 7 keV over a typical interval of 1.5 ns. This system is designed for use on the University of Rochester’s OMEGA laser facility [T. R. Boehly et al., Opt Commun. 133, 495 (1997)]. [ABSTRACT FROM AUTHOR]

We present narrow-band self-emission x-ray images from a titanium tracer layer placed at the fuel-shell interface in 60-laser-beam implosion experiments at the OMEGA facility. The images are acquired during deceleration with inferred convergences of ~ 9-14. Novel here is that a systematically observed asymmetry of the emission is linked, using full sphere 3D implosion modeling, to performance-limiting low mode asymmetry of the drive. [ABSTRACT FROM AUTHOR]