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The nonlinear physics of cross-beam energy transfer (CBET) for multi-speckled laser beams is examined using large-scale particle-in-cell simulations for a range of laser and plasma conditions relevant to indirect-drive inertial confinement fusion (ICF) experiments. The time-dependent growth and saturation of CBET involve complex, nonlinear ion and electron dynamics, including ion trapping-induced enhancement and detuning, ion acoustic wave (IAW) nonlinearity, oblique forward stimulated Raman scattering (FSRS), and backward stimulated Brillouin scattering (BSBS) in a CBET-amplified seed beam. Ion-trapping-induced detuning of CBET is captured in the kinetic linear response by a new δf-Gaussian-mixture algorithm, enabling an accurate characterization of trapping-induced non-Maxwellian distributions. Ion trapping induces nonlinear processes, such as changes to the IAW dispersion and nonlinearities (e.g., bowing and self-focusing), which, together with pump depletion, FSRS, and BSBS, determine the time-dependent nature and level of CBET gain as the system approaches a steady state. Using VPIC simulations at intensities at and above the onset threshold for ion trapping and the insight from the time-dependent saturation analyses, we construct a nonlinear CBET model from local laser and plasma conditions that predicts the CBET gain and the energy deposition into the plasma. This model is intended to provide a more accurate, physics-based description of CBET saturation over a wide range of conditions encountered in ICF hohlraums compared with linear CBET gain models with ad hoc saturation clamps often used in laser ray-based methods in multi-physics codes.

Laser plasma instabilities (LPI) reduce driver-target coupling, alter implosion symmetry, and therefore can fundamentally limit fusion performance in inertial confinement fusion (ICF). Developing a predictive modeling capability for LPI effects can critically advance the success of the field. We perform vector particle-in-cell simulations of multi-speckled laser beams undergoing stimulated Raman scattering (SRS) at various densities and intensities relevant to mainly indirectly driven and a subset of parameter space for directly driven ICF systems, focusing on the regimes with intensities above the onset of electron trapping. Based on the wavenumber of the SRS daughter electron plasma wave, we identify several regions with underpinning SRS saturation physics: the electron-trapping dominated region with intermediate kλD values, the strong Landau damping region at larger kλD values, and the region where the Langmuir decay instability arises at lower kλD values. We develop a nonlinear SRS reflectivity model that features the base trapping-dominated scaling of (kλD)−4 and its modifications. Electron trapping deforms the initialized electron distribution functions, and we have developed a new δf-Gaussian-mixture algorithm for an accurate characterization of the trapped hot electron population. With this SRS hot electron description, we construct a nonlinear energy deposition model and a hot electron source model—based on a modified Manley–Rowe relation—suitable for including SRS effects as a sub-grid module in a high-fidelity ICF design code.

Implosions of Hohlraum-driven double shell targets as an alternative inertial confinement fusion concept are underway at the National Ignition Facility. The double shell system relies on a series of energy transfer processes starting from thermal x-ray absorption by the outer shell, followed by collisional transfer of kinetic energy to a heavy metal inner shell, and finally, conversion to the internal energy of the deuterium-tritium fuel. During each of these energy transfer stages, low-mode asymmetries can act to reduce the ideal transfer efficiency degrading double shell performance. Mechanisms, such as hard x-ray preheat from the Hohlraum, not only decrease the efficiency of kinetic energy transfer but may also be a source of low-mode asymmetry. In this article, we evaluate the shape transfer processes through the time of shell collision using two-dimensional integrated Hohlraum and capsule computations. We find that the dominant mode of the shape transfer is well described using a “radial impulse” model from the shape of the foam pressure reservoir. To evaluate the importance of preheat on inner shell shape, we also report on first measurements of Au L-shell preheat asymmetry in a double shell with a tungsten pusher. These measurements showed a 65% higher preheat velocity at the pole of the capsule relative to the equator. We also found that the experiments provided rigorous constraints by which to test the Hohlraum model settings that impact the amount and symmetry of Au L-shell preheat via the plasma conditions inside the outer cone Au bubble.

The performance of modern laser-driven inertial confinement fusion (ICF) experiments is degraded by contamination of the deuterium–tritium (DT) fuel with high-Z material during compression. Simulations suggest that this mix can be described by the ion temperature distribution of the implosion, given that such contaminants deviate in temperature from the surrounding DT plasma. However, existing neutron time-of-flight (nTOF) diagnostics only measure the spatially integrated ion temperature. This paper describes the techniques and forward modeling used to develop a novel diagnostic imaging system to measure the spatially resolved ion temperature of an ICF implosion for the first time. The technique combines methods in neutron imaging and nTOF diagnostics to measure the ion temperature along one spatial dimension at yields currently achievable on the OMEGA laser. A detailed forward model of the source and imaging system was developed to guide instrument design. The model leverages neutron imaging reconstruction algorithms, radiation hydrodynamics and Monte Carlo simulations, optical ray tracing, and more. The results of the forward model agree with the data collected on OMEGA using the completed diagnostic. The analysis of the experimental data is still ongoing and will be discussed in a separate publication.

An investigation of twenty two-shock campaign indirectly driven capsules on the National Ignition Facility was conducted using the xRAGE computer code. The two-shock platform was developed to look at the sensitivity of fuel–ablator mix with shock timing, asymmetry, surface roughness, and convergence on roughly ignition size scale capsules. This platform used CH/CD (plastic/deuterated plastic) shell capsules that were about 685- μm outer radius and filled with D2 or hydrogen-tritium (HT) gas. The experimental radius and velocity vs time, neutron yield, burn averaged ion temperature (Tion), burn width, and self-emission image size were compared to one-dimensional (1D) and two-dimensional (2D) simulations. Our 2D simulations suggest that the mixing of glass from the fill tube was the dominant source of impurity in the gas region of the capsule during burn, along with fuel–ablator mix. The mass of glass mixed in is about 5–10 ng. Our 2D simulations capture most of the yield trends from different degradation mechanisms, and they match the observed burn width and Tion measurements. Our 2D models match all the available data to within 2.5 times the normalized experimental error for 19 of 20 capsules.

Leather is prepared from vertebrate skin by the chemical stabilization of the fibrous protein collagen, the main solid constituent of skin. The natural three-dimensional fibrous weave of the collagen fibrils is retained intact in the leather, and this paper describes in detail the relation between the fibrous weave as seen under the light and scanning electron microscopes and the physical properties of the leather. In the main, skins of cattle, sheep and goats are used. These differ in total thickness, fibre bundle size and weave pattern, but offer a variety of raw material from which the tanner can select the skin type best suited for a particular end use. The tanning process can also modify the natural weave in order to achieve the required physical properties. For example the processing can allow fine spaces to remain between the fibrils, so that they are free to move over each other within the fibre bundle. Such a bundle will be highly flexible as will be the leather as a whole. Conversely, if fibrils adhere to each other and are not free to move the leather will be firm. Such fine spaces can be recognized under the microscope as longitudinal striations. Another important feature which is influenced by the tanning process is the angle at which the fibres interweave in relation to the grain surface. A low angle of weave is required for high tensile strength as this allows the pull to be transferred along the fibre axis. A low angle of weave also allows more frequent interweaving of the fibres within a given thickness of leather, than if the angle is high. As frequent interweaving of the fibres is a prerequisite for strength the tear strength of leather is highly dependent on the angle of weave. This is particularly so in certain types of leather prepared from cattle skin, which is cut into layers to obtain the required leather thickness. For leather to accommodate stretching, such as that occurring when lasting shoe uppers, or compression and creasing, the individual fibrils need to be free to move within the bundle, and the bundle within the weave as a whole. The changes in the fibre structure that occur under such conditions can be followed microscopically and are described in detail in this paper.

SYNOPSIS A resume is given of the literature on the morphology of collagen fibre bundles, and the occurrence in them of elastin, reticular tissue, and a cement substance. Two methods of demonstrating reticular tissue have been investigated. (1) Silver impregnation. A number of variations of the technique have been tried, and Foot's balanced solutions method has been adopted as the most satisfactory for skin. It gives clear, sharp preparations, in which (after toning with gold) collagen becomes rose-red to rather purple in colour, and reticular tissue usually shows black. The fine binding fibrils around collagen fibre bundles did not stain as elastin, nor did they become black when impregnated with silver. Nevertheless, they are considered to be reticular tissue, since morphologically they resemble reticulin more closely than they do collagen. (2) Swelling of fibres. Fibre bundles teased from the skin of cattle (calf and ox) and sheep (3–18 months) were soaked at a range of pH values in hydrochloric acid solutions. Calf fibre bundles from the leg swelled more readily and at higher pH values than those of the ox from the same position, in which the restrictive binding tissue was present as an intermittent sheath. The collagen fibres twisted into a rope-like form and became entangled in the associated elastin fibres, which, therefore, played an important part in restricting the fibre bundles as they swelled. It is also inferred, though the investigation was not specially directed towards this point, that a cement substance is present, holding the collagen fibres into bundles even in the absence of reticular tissue and elastin.