A higher-than-predicted measurement of iron opacity at solar interior temperatures.

Nearly a century ago it was recognized that radiation absorption by stellar matter controls the internal temperature profiles within stars. Laboratory opacity measurements, however, have never been performed at stellar interior conditions, introducing uncertainties in stellar models. A particular problem arose when refined photosphere spectral analysis led to reductions of 30-50 per cent in the inferred amounts of carbon, nitrogen and oxygen in the Sun. Standard solar models using the revised element abundances disagree with helioseismic observations that determine the internal solar structure using acoustic oscillations. This could be resolved if the true mean opacity for the solar interior matter were roughly 15 per cent higher than predicted, because increased opacity compensates for the decreased element abundances. Iron accounts for a quarter of the total opacity at the solar radiation/convection zone boundary. Here we report measurements of wavelength-resolved iron opacity at electron temperatures of 1.9-2.3 million kelvin and electron densities of (0.7-4.0) × 10(22) per cubic centimetre, conditions very similar to those in the solar region that affects the discrepancy the most: the radiation/convection zone boundary. The measured wavelength-dependent opacity is 30-400 per cent higher than predicted. This represents roughly half the change in the mean opacity needed to resolve the solar discrepancy, even though iron is only one of many elements that contribute to opacity.

Systematic Study of L-Shell Opacity at Stellar Interior Temperatures.

The first systematic study of opacity dependence on atomic number at stellar interior temperatures is used to evaluate discrepancies between measured and modeled iron opacity [J. E. Bailey et al., Nature (London) 517, 56 (2015)NATUAS0028-083610.1038/nature14048]. High-temperature (>180 eV) chromium and nickel opacities are measured with ±6%-10% uncertainty, using the same methods employed in the previous iron experiments. The 10%-20% experiment reproducibility demonstrates experiment reliability. The overall model-data disagreements are smaller than for iron. However, the systematic study reveals shortcomings in models for density effects, excited states, and open L-shell configurations. The 30%-45% underestimate in the modeled quasicontinuum opacity at short wavelengths was observed only from iron and only at temperature above 180 eV. Thus, either opacity theories are missing physics that has nonmonotonic dependence on the number of bound electrons or there is an experimental flaw unique to the iron measurement at temperatures above 180 eV.

Hot-spot mix in ignition-scale inertial confinement fusion targets.

Mixing of plastic ablator material, doped with Cu and Ge dopants, deep into the hot spot of ignition-scale inertial confinement fusion implosions by hydrodynamic instabilities is diagnosed with x-ray spectroscopy on the National Ignition Facility. The amount of hot-spot mix mass is determined from the absolute brightness of the emergent Cu and Ge K-shell emission. The Cu and Ge dopants placed at different radial locations in the plastic ablator show the ablation-front hydrodynamic instability is primarily responsible for hot-spot mix. Low neutron yields and hot-spot mix mass between 34(-13,+50) ng and 4000(-2970,+17 160) ng are observed.

Pioneer saturn celestial mechanics experiment.

During the Pioneer Saturn encounter, a continuous round-trip radio link at S band ( approximately 2.2 gigahertz) was maintained between stations of the Deep Space Network and the spacecraft. From an analysis of the Doppler shift in the radio carrier frequency, it was possible to determine a number of gravitational effects on the trajectory. Gravitational moments ( J(2) and J(4)) for Saturn have been determined from preliminary analysis, and preliminary mass values have been determined for the Saturn satellites Rhea, Iapetus, and Titan. For all three satellites the densities are low, consistent with the compositions of ices. The rings have not been detected in the Doppler data, and hence the best preliminary estimate of their total mass is zero with a standard error of 3 x 10(-6) Saturn mass. New theoretical calculations for the Saturn interior are described which use the latest observational data, including Pioneer Saturn, and state-of-the-art physics for the internal composition. Probably liquid H(2)O and possibly NH(3) and CH(4) are primarily confined in Saturn to the vicinity of a core of approximately 15 to 20 Earth masses. There is a slight indication that helium may likewise be fractionated to the central regions.

Spectroscopic determination of temperature and density spatial profiles and mix in indirect-drive implosion cores.

In the field of inertial confinement fusion (ICF), work has been consistently progressing in the past decade toward a more fundamental understanding of the plasma conditions in ICF implosion cores. The research presented here represents a substantial evolution in the ability to diagnose plasma temperatures and densities, along with characteristics of mixing between fuel and shell materials. Mixing is a vital property to study and quantify, since it can significantly affect implosion quality. We employ a number of new spectroscopic techniques that allow us to probe these important quantities. The first technique developed is an emissivity analysis, which uses the emissivity ratio of the optically thin Lybeta and Hebeta lines to spectroscopically extract temperature profiles, followed by the solution of emissivity equations to infer density profiles. The second technique, an intensity analysis, models the radiation transport through the implosion core. The nature of the intensity analysis allows us to use an optically thick line, the Lyalpha, to extract information on mixing near the core edge. With this work, it is now possible to extract directly from experimental data not only detailed temperature and density maps of the core, but also spatial mixing profiles.

Numerical investigations of potential systematic uncertainties in iron opacity measurements at solar interior temperatures.

Iron opacity calculations presently disagree with measurements at an electron temperature of ∼180-195 eV and an electron density of (2-4)×10^{22}cm^{-3}, conditions similar to those at the base of the solar convection zone. The measurements use x rays to volumetrically heat a thin iron sample that is tamped with low-Z materials. The opacity is inferred from spectrally resolved x-ray transmission measurements. Plasma self-emission, tamper attenuation, and temporal and spatial gradients can all potentially cause systematic errors in the measured opacity spectra. In this article we quantitatively evaluate these potential errors with numerical investigations. The analysis exploits computer simulations that were previously found to reproduce the experimentally measured plasma conditions. The simulations, combined with a spectral synthesis model, enable evaluations of individual and combined potential errors in order to estimate their potential effects on the opacity measurement. The results show that the errors considered here do not account for the previously observed model-data discrepancies.

Calibrated simulations of Z opacity experiments that reproduce the experimentally measured plasma conditions.

Recently, frequency-resolved iron opacity measurements at electron temperatures of 170-200 eV and electron densities of (0.7-4.0)×10(22)cm(-3) revealed a 30-400% disagreement with the calculated opacities [J. E. Bailey et al., Nature (London) 517, 56 (2015)]. The discrepancies have a high impact on astrophysics, atomic physics, and high-energy density physics, and it is important to verify our understanding of the experimental platform with simulations. Reliable simulations are challenging because the temporal and spatial evolution of the source radiation and of the sample plasma are both complex and incompletely diagnosed. In this article, we describe simulations that reproduce the measured temperature and density in recent iron opacity experiments performed at the Sandia National Laboratories Z facility. The time-dependent spectral irradiance at the sample is estimated using the measured time- and space-dependent source radiation distribution, in situ source-to-sample distance measurements, and a three-dimensional (3D) view-factor code. The inferred spectral irradiance is used to drive 1D sample radiation hydrodynamics simulations. The images recorded by slit-imaged space-resolved spectrometers are modeled by solving radiation transport of the source radiation through the sample. We find that the same drive radiation time history successfully reproduces the measured plasma conditions for eight different opacity experiments. These results provide a quantitative physical explanation for the observed dependence of both temperature and density on the sample configuration. Simulated spectral images for the experiments without the FeMg sample show quantitative agreement with the measured spectral images. The agreement in spectral profile, spatial profile, and brightness provides further confidence in our understanding of the backlight-radiation time history and image formation. These simulations bridge the static-uniform picture of the data interpretation and the dynamic-gradient reality of the experiments, and they will allow us to quantitatively assess the impact of effects neglected in the data interpretation.

Measurement and analysis of x-ray absorption in Al and MgF2 plasmas heated by Z-pinch radiation.

High-power Z pinches on Sandia National Laboratories' Z facility can be used in a variety of experiments to radiatively heat samples placed some distance away from the Z-pinch plasma. In such experiments, the heating radiation spectrum is influenced by both the Z-pinch emission and the re-emission of radiation from the high-Z surfaces that make up the Z-pinch diode. To test the understanding of the amplitude and spectral distribution of the heating radiation, thin foils containing both Al and MgF2 were heated by a 100-130 TW Z pinch. The heating of these samples was studied through the ionization distribution in each material as measured by x-ray absorption spectra. The resulting plasma conditions are inferred from a least-squares comparison between the measured spectra and calculations of the Al and Mg 1s-->2p absorption over a large range of temperatures and densities. These plasma conditions are then compared to radiation-hydrodynamics simulations of the sample dynamics and are found to agree within 1sigma to the best-fit conditions. This agreement indicates that both the driving radiation spectrum and the heating of the Al and MgF2 samples is understood within the accuracy of the spectroscopic method.

Dopant radiative cooling effects in indirect-drive Ar-doped capsule implosion experiments.

We present results from simulations performed to investigate the effects of dopant radiative cooling in inertial confinement fusion indirect-drive capsule implosion experiments. Using a one-dimensional radiation-hydrodynamics code that includes inline collisional-radiative modeling, we compute in detail the non-local thermodynamic equilibrium atomic kinetics and spectral characteristics for Ar-doped DD fuel. Specifically, we present results from a series of calculations in which the concentration of the Ar is varied, and examine the sensitivity of the fuel conditions (e.g., electron temperature) and neutron yield to the Ar dopant concentration. Simulation results are compared with data obtained in OMEGA indirect-drive experiments in which monochromatic imaging and spectral measurements of Ar Hebeta and Lybeta line emission were recorded. The incident radiation drive on the capsule is computed with a three-dimensional view factor code using the laser beam pointings and powers from the OMEGA experiments. We also examine the sensitivity of the calculated compressed core electron temperatures and neutron yields to the radiation drive on the capsule and to the radiation and atomic modeling in the simulations.

X-ray absorption spectroscopy measurements of thin foil heating by Z-pinch radiation.

Absorption spectroscopy measurements of the time-dependent heating of thin foils exposed to intense z-pinch radiation sources are presented. These measurements and their analysis provide valuable benchmarks for, and insights into, the radiative heating of matter by x-ray sources. Z-pinch radiation sources with peak powers of up to 160 TW radiatively heated thin plastic-tamped aluminum foils to temperatures approximately 60 eV. The foils were located in open slots at the boundary of z-pinch hohlraums surrounding the pinch. Time-resolved Kalpha satellite absorption spectroscopy was used to measure the evolution of the Al ionization distribution, using a geometry in which the pinch served as the backlighter. The time-dependent pinch radius and x-ray power were monitored using framing camera, x-ray diode array, and bolometer measurements. A three-dimensional view factor code, within which one-dimensional (1D) radiation-hydrodynamics calculations were performed for each surface element in the view factor grid, was used to compute the incident and reemitted radiation flux distribution throughout the hohlraum and across the foil surface. Simulated absorption spectra were then generated by postprocessing radiation-hydrodynamics results for the foil heating using a 1D collisional-radiative code. Our simulated results were found to be in good general agreement with experimental x-ray spectra, indicating that the spectral measurements are consistent with independent measurements of the pinch power. We also discuss the sensitivity of our results to the spectrum of the radiation field incident on the foil, and the role of nonlocal thermodynamic equilibrium atomic kinetics in affecting the spectra.

Modification of the heat-setting characteristics of myosin by pressure treatment.

The effect on the heat-setting characteristics of myosin of pressure treatment up to 150 MPa at 0 to 40°C has been assessed by measuring the work done in inserting a plunger into samples after heating them at up to 70°C. The response depended upon the ionic strength and the pH of the myosin suspension, and the intensity and duration of pressure treatment. It was most pronounced at pressures of 75 MPa or greater applied for some minutes to myosin in 0·2-0·3m NaCl at about pH 6. It is suggested that the alteration in heat-setting properties is due to depolymerization, under pressure, of myosin filaments accompanied by a conformational change of the monomer so that it reaggregates in a different manner upon release of pressure.

Effect of restraint on the thermal transitions of tendon.

Sheep tendon samples that had been given a heat treatment in 0·9% sodium chloride solution at temperatures above their normal shrinkage temperature (ST) were examined by differential scanning calorimetry. If the tendon was not restrained from shrinkage during the heat treatment, no endothermic transition in the region of 60°C was subsequently detected by calorimetry. If it was restrained an endothermic transition was then detected, the magnitude of which was inversely related to the shrinkage allowed to occur during heat treatment. Values for transition heat (ΔH) and transition temperature (T(1,2)) of tendon samples restrained at their original taut length and heat treated 10°-15°C above the ST were similar to those found for unheated control samples. However, increase in the temperature of the heat treatment to 20°-30°C above the ST resulted in a decrease in both ΔH and T(1,2) and the transition became broader.

Effects of pressure treatment on the ultrastructure of striated muscle.

Pre- and post-rigor sheep semimembranosus muscles were subjected to a hydrostatic pressure of 100 meganewtons/m(2) at 25°C. The ultrastructure of the muscle fibres was compared with that of non pressure-treated samples. A conspicuous feature of pressure-treated post-rigor samples was the absence of the M-band in the central region of the A-band. Therefore it appeared that some, if not all, of the proteins in the M-line were very susceptible to disaggregation under high pressure. Another feature of the post-rigor pressure-treated sample was the loss of integrity and aggregation of I-band filaments which presumably involved an F-G transformation of actin. Pressure treatment of pre-rigor muscle resulted in extensive structural disruption with contraction band formation. It is suggested that a weakening of thin filaments and M-line bridges, when combined with a pressure-induced contraction, facilitates disruption of pre-rigor pressure-treated muscle.

Pressure-induced pH and length changes in muscle.

The response to increase in pressure of pre- and post-rigor muscle strips supporting a load has been investigated by measuring length changes with application of pressure up to 150 MPa in a windowed pressure vessel. The response of the pre-rigor strips depended on temperature; at 30°C the strips contracted, then, after some minutes, lengthened, presumably because of a breakdown of myofibrillar integrity. At 0°C the cold-shortened muscle lengthened but if pressure was released the strips shortened again. Loaded post-rigor muscle strips lengthened with application of pressure. It is suggested that the conditions that prevail in pre-rigor muscle are not as favourable for disaggregation of the myofilaments as those in post-rigor muscle. Measuresurement of pH changes in pre-rigor pressure-treated muscle showed these were accelerated at 30°C, but completely inhibited by treatment at 0°C for 3 or 24h.

Pressure-heat treatment of meat: Changes in myofibrillar proteins and ultrastructure.

The effects on muscle of a combined pressure-heat (P-H) treatment that overcomes myofibrillar toughness have been investigated using SDS gel electrophoresis and electron microscopy. Densitometer scans of polyacrylamide gels of muscle extracts revealed that P-H treatment caused greater degradation of connectin than did heat treatment alone. Breakdown of connection by P-H treatment was reduced in muscle that had been injected with the protease inhibitor pepstatin. However, pepstatin treatment did not reduce the effectiveness of P-H treatment for tenderizing meat, as would be expected if connectin was responsible for myofibrillar toughness. P-H treatment resulted in an increase in the intensity of a peak with M(r) ∼ 150 000, but this peak was also produced by non-tenderizing pressure treatments. The ultrastructural studies revealed that P-H treatment disrupted the thick and the thin filaments, leaving voids at the M-line region. It is suggested that P-H treatment achieves most of its effect by an irreversible disaggregation of the myosin of thick filaments.

Pressure-heat treatment of meat: A comparison of beef and buffalo meat.

A pressure-heat treatment, which disrupts the myofibrillar structure of meat but leaves the connective tissues essentially intact, was used to compare the connective tissue component of toughness in the Semimembranosus and Longissimus dorsi muscles from nine Brahman cross and nine buffalo steers, 24 to 29 months of age. For assessment of samples, peak force, initial yield force and peak force minus initial yield force values were determined from Warner-Bratzler shear force-deformation curves. In the control, non-pressure-heat treated samples, the only breed difference detected was in peak minus initial yield force value, which was significantly lower for the beef Semimembranosus muscles. However, for the pressure-heat treated samples of both muscles, peak force and peak minus initial yield force values were significantly lower for beef than for buffalo. The pressure-heat treatment could thus be used to detect differences in the contribution of connective tissue to toughness which would otherwise be obscured by the differences in the myofibrillar toughness.

Binding of comminuted meat: Effect of high pressure.

Patties prepared from comminuted meat were pressure-treated at up to 150 MPa at 0-3°C and the cohesion between meat particles in the cooked patty investigated from tensile strength measurements. Pressure treatment increased tensile strength, the magnitude of the increase depending upon the intensity and duration of pressure treatment, the concentration of salt in the patty and pH value. The effect was most pronounced in patties of pH 5 to 6 and with 1% salt in the aqueous phase. Under these conditions cooking losses were reduced. When compared with the effect of addition of 0·5% tetrasodium pyrophosphate in a patty with 1% salt, pressure treatment retained its effect at lower pH values.

Effect of pressure treatments on the mechanical properties of pre- and post-rigor meat.

Pressure-heat treatment of beef semitendinosus samples post-rigor gave shear and tensile results similar to those obtained with pressure treatment pre-rigor. Post-rigor pressure-heat treatment did not affect the contraction state, unlike pre-rigor pressure treatment which caused samples to contract by about 40%. Maximum tenderizing effect by pressure-heat treatment (150 M Nm(-2) at 60°C for 30 min) was achieved when samples were heated at 45°C for 45-180 min immediately before application of the treatment. As the pre-pressurization temperature was increased, the duration of heating became more critical until at temperatures ≥ 60°C the effects of subsequent pressure-heat treatment became very small. Pressure-heat treated samples did not show the increase in shear force values for cooking temperatures ≥ 60°C associated with myofibrillar hardening. It was concluded that pressure-heat treatment primarily affected the myofibrillar structure.

Pressure treatment of meat: Effects on thermal transitions and shear values.

The effects of pressure treatment (150 MN m(-2) for 3 h at 0°C) on the pH, thermal transitions, ultrastructure and Warner-Bratzler shear values of post-rigor beef semimembranosus and longissimus dorsi muscles have been investigated. Pressure treatment resulted in a slight but significant increase in pH. Differential scanning calorimetry revealed large changes in the thermograms of muscle samples as a result of pressure treatment, in particular a transition attributed to F-actin was absent in the pressure-treated sample. Examination of the ultrastructure also revealed extensive change as a result of pressure treatment, particularly in the I-band and M-line region. Pressure treatment either did not change shear values or increased them, according to whether the muscle was in the stretched or contracted state, respectively. The results are thought to support a theory for contraction state toughness proposed by Voyle (1969) in which increasing toughness is caused by an increasing incidence of sarcomeres in which thick filaments have been compressed onto the Z-line, thus removing the I-band as a zone of weakness.