Laboratory measurements of iron opacity made under conditions similar to those inside the Sun reveal much higher opacity than predicted, helping to resolve inconsistencies within stellar models of the internal temperatures of stars. Internal temperature profiles of the Sun and other stars are controlled in large part by the rate at which radiation is absorbed by stellar matter. Until now it has not been possible to determine the opacity of matter in star-like conditions in the laboratory, but James Bailey et al. have now achieved that feat using the Sandia National Laboratories' Z facility, the world's most powerful X-ray generator. The experiments reveal a wavelength-resolved iron opacity that is 30 to 400 times greater than predicted in conditions very similar to those at the radiation/convection zone boundary in the Sun. Previous measurements of stellar interiors have been based on observations of surface waves, and there were serious discrepancies between theoretical predictions and observations. The new measurements account for about half of adjustment in opacity figures required to restore agreement between standard solar models and observations. Nearly a century ago it was recognized1 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 models2,3,4,5. A particular problem arose2,3,6,7,8 when refined photosphere spectral analysis9,10 led to reductions of 30–50 per cent in the inferred amounts of carbon, nitrogen and oxygen in the Sun. Standard solar models11 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 predicted2,3,6,7,8, because increased opacity compensates for the decreased element abundances. Iron accounts for a quarter of the total opacity2,12 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) × 1022 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.

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

Pulsed power accelerators compress electrical energy in space and time to provide versatile experimental platforms for high energy density and inertial confinement fusion science. The 80-TW “Z” pulsed power facility at Sandia National Laboratories is the largest pulsed power device in the world today. Z discharges up to 22 MJ of energy stored in its capacitor banks into a current pulse that rises in 100 ns and peaks at a current as high as 30 MA in low-inductance cylindrical targets. Considerable progress has been made over the past 15 years in the use of pulsed power as a precision scientific tool. This paper reviews developments at Sandia in inertial confinement fusion, dynamic materials science, x-ray radiation science, and pulsed power engineering, with an emphasis on progress since a previous review of research on Z in Physics of Plasmas in 2005.

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Review of pulsed power-driven high energy density physics research on Z at Sandia

Theoretical opacities are required for calculating energy transport in plasmas. In particular, understanding stellar interiors, inertial fusion, and Z pinches depends on the opacities of mid-atomic-number elements over a wide range of temperatures. The 150–300 eV temperature range is particularly interesting. The opacity models are complex and experimental validation is crucial. For example, solar models presently disagree with helioseismology and one possible explanation is inadequate theoretical opacities. Testing these opacities requires well-characterized plasmas at temperatures high enough to produce the ion charge states that exist in the sun. Typical opacity experiments heat a sample using x rays and measure the spectrally resolved transmission with a backlight. The difficulty grows as the temperature increases because the heating x-ray source must supply more energy and the backlight must be bright enough to overwhelm the plasma self-emission. These problems can be overcome with the new generation...

Experimental investigation of opacity models for stellar interior, inertial fusion, and high energy density plasmas

Results of a niobium absorption experiment are presented that represent a major step in the development of techniques necessary for the quantitative characterization of hot, dense matter. The general requirements for performing quantitative analyses of absorption spectra are discussed. Hydrodynamic simulations are used to illustrate the behavior of tamped x-ray-heated matter and to indicate potential two-dimensional problems inherent in the technique. The absorption spectrum of a low-Z material, in this case aluminum, mixed with niobium provides a temperature diagnostic, which together with radiography as a density diagnostic fully characterizes the sample. A discussion is presented of opacity calculations and a comparison to the measurements is given that illustrates the need for experiments to provide a critical test of theory. The experimental technique is placed in context with a review of previous measurements using absorption spectroscopy to probe hot, dense matter. It is shown that the overall experimental concepts, although understood, were not always achieved in previous experiments. \textcopyright{} 1996 The American Physical Society.

Absorption experiments on x-ray-heated mid-Z constrained samples.

http://absimage.aps.org/image/DPP08/MWS_DPP08-2008-000780.pdf

Experimental investigation of opacity models for stellar interiors, inertial fusion, and high energy density plasmas

The first quantitative measurement of photoabsorption in the region determining Rosseland and Planck mean opacity, is obtained for an x-ray heated iron plasma, using novel techniques and instrumentation. The plasma density of 0.0113 ± 0.0013 g/cm3 and temperature of 59 ± 3 eV are accurately constrained experimentally by imaging plasma expansion and observing and modeling absorption in sodium dopant ions. The measured iron absorption spectrum is compared with several newly developed opacity models. The data constrains Rosseland and Planck group means with of order 15% precision. This is the first quantitative experimental certification of opacity models germane to radiative transfer in LTE plasmas.

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Spectroscopic measurements of Rosseland mean opacity

In order to test and improve opacity models, we are performing experiments to accurately characterize and measure the opacity of high‐Z plasmas In the experiments the NOVA laser at Lawrence Livermore National Laboratory is used to indirectly heat tamped thin foil targets containing mid to high‐Z ions of interest which are co‐mixed with low‐Z ions for spectroscopic determinations of plasma conditions One of the NOVA beams is used to create a source of x‐rays for backlighting the plasma face‐on, while another beam is used to perform a radiograph of the expanding sample edge‐on X‐rays that pass through and around the plasma are simultaneously measured on high resolution crystal spectrometers, and the opacity is measured by comparing the direct and attenuated x‐ray spectra Plasma density is determined from the spatial extent of the expanded plasma, and plasma temperature is inferred from the ionization distribution of the low‐Z ions doped in the plasma Predictions of the high‐Z opacity are then compared 

D. F. Fields

Papers

Absorption experiments on x-ray-heated mid-Z constrained samples.

Spectroscopic measurements of Rosseland mean opacity

Measurements and models of the opacity of hot, dense plasma