Standard solar models11 using the revised element abundances disagree with helioseismic observations that determine the internal solar structure using acoustic oscillations.
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A particular problem arose2,3,6-8 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. Laboratory opacity measurements, however, have never been performed at stellar interior conditions, introducing uncertainties in stellar models2-5. Nearly a century ago it was recognized1 that radiation absorption by stellar matter controls the internal temperature profiles within stars. Once the uncertainty is reduced, future NIF experiments will probe higher temperatures (170-200 eV) to address the ongoing disagreement between theory and Z data. The paths to reduction of the largest uncertainties are discussed. The NIF transmission data show statistical uncertainties of 2-10%, but various systematic uncertainties must be addressed before pursuing quantitative comparisons. These conditions are similar to prior Z measurements which agree better with theory. From these data, X-ray transmission spectra are inferred, showing Mg K-shell and Fe L-shell X-ray transitions from plasma at a temperature of~150 eV and electron density of~8 × 10 21 /cm 3. An X-ray spectrometer records the transmitted X-rays, the unattenuated X-rays passing around the sample, and the sample's self-emission. Another 64 beams implode a spherical plastic shell to produce a continuum X-ray flash which backlights the hot sample. In the NIF experiments, 64 laser beams indirectly heat a plastic-tamped rectangular iron-magnesium sample inside a gold cavity. To address these issues, this paper details the first results from new experiments under development at the National Ignition Facility (NIF), using a different method to replicate the prior experimental conditions.
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Another discrepancy, between theory and helioseismic measurements of the boundary's location, would be ameliorated if the experimental opacity is correct. Discrepancies exist between theoretical and experimental opacity data for iron, at temperatures 180-195 eV and electron densities near 3 × 10 22 /cm 3, relevant to the solar radiative-convective boundary.