Engineering topics

Nagayama, T.

Publications and source records attributed to Nagayama, T..

10 records

DOE OSTI2023

National Opacity Program - Tri-Lab Assessment of Measurements and Models (Milestone 7720)

Opacity-on-NIF has obtained opacity data under conditions similar to those achieved by the entirely different Opacity-on-Z platform. From low- and high-Z elements at different anchor points, rigorously compare the opacity data between the laboratories and to multiple opacity theory models. Compare and assess the data acquisition and processing methods for obtaining opacities and for measuring/inferring sample conditions. Explain, or develop hypotheses for, any discrepancies. Map progress to the National Opacity Strategy and define future directions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Measuring He i Stark Line Shapes in the Laboratory to Examine Differences in Photometric and Spectroscopic DB White Dwarf Masses

Abstract Accurate helium White Dwarf (DB) masses are critical for understanding the star’s evolution. DB masses derived from the spectroscopic and photometric methods are inconsistent. Photometric masses agree better with currently accepted DB evolutionary theories and are mostly consistent across a large range of surface temperatures. Spectroscopic masses rely on untested He i Stark line-shape and Van der Waals broadening predictions, show unexpected surface temperature trends, and are thus viewed as less reliable. To test this conclusion, we present in this paper detailed He i Stark line-shape measurements at conditions relevant to DB atmospheres ( T electron ≈12,000–17,000 K, n electron ≈ 10 17 cm −3 ). We use X-rays from Sandia National Laboratories’ Z -machine to create a uniform ≈120 mm long hydrogen–helium mixture plasma. Van der Waals broadening is negligible at our experimental conditions, allowing us to measure He i Stark profiles only. H β , which has been well-studied in our platform and elsewhere, serves as the n e diagnostic. We find that He i Stark broadening models used in DB analyses are accurate within errors at tested conditions. It therefore seems unlikely that line-shape models are solely responsible for the observed spectroscopic mass trends. Our results should motivate the WD community to further scrutinize the validity of other spectroscopic and photometric input parameters, like atmospheric structure assumptions and convection corrections. These parameters can significantly change the derived DB mass. Identifying potential weaknesses in any input parameters could further our understanding of DBs, help elucidate their evolutionary origins, and strengthen confidence in both spectroscopic and photometric masses.

Schaeuble, M. -A. (ORCID:0000000188695203)↗

DOE OSTI2022

Milestone 7714 National Opacity Program: High-Fidelity Iron Data on NIF

Establish the level of reproducibility of NIF iron opacity shots, quantify to-date improvements in data quality through, e.g. background reduction, improved signal, and improved analysis methods, and formalize the approach that will be taken in the future to compare NIF data, Z data, and theory.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2022

Introduction to spectral line shape theory

Spectral line-shape models are an important part of understanding high-energy-density (HED) plasmas. Models are needed for calculating opacity of materials and can serve as diagnostics for astrophysical and laboratory plasmas. However, much of the literature on line shapes is directed toward specialists. This perspective makes it difficult for non-specialists to enter the field. We have two broad goals with this topical review. First, we aim to give information so that others in HED physics may better understand the current field. Additionally, this first goal may help guide future experiments to test different aspects of the theory. Second, we provide an introduction for those who might be interested in line-shape theory, and enough materials to be able to navigate the field and the literature. We give a high-level overview of line broadening process, as well as dive into the formalism, available methods, and approximations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

All-Order Full-Coulomb Quantum Spectral Line-Shape Calculations

Understanding how atoms interact with hot dense matter is essential for astrophysical and laboratory plasmas. Interactions in high-density plasmas broaden spectral lines, providing a rare window into interactions that govern, for example, radiation transport in stars. However, up to now, spectral line-shape theories employed at least one of three common approximations: second-order Taylor treatment of broadening operator, dipole-only interactions between atom and plasma, and classical treatment of perturbing electrons. In this Letter, we remove all three approximations simultaneously for the first time and test the importance for two applications: neutral hydrogen and highly ionized magnesium and oxygen. Finally, we found 15%–50% change in the spectral line widths, which are sufficient to impact applications including white-dwarf mass determination, stellar-opacity research, and laboratory plasma diagnostics.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2021

Observation of ionization trends in a laboratory photoionized plasma experiment at Z

We report experimental and modeling results for the charge state distribution of laboratory photoionized neon plasmas in the first systematic study over nearly an order of magnitude range of ionization parameter ξ ∝ F/N e . The range of ξ is achieved by flexibility in the experimental platform to adjust either the x-ray drive flux F at the sample or the electron number density N e or both. Experimental measurements of photoionized plasma conditions over such a range of parameters enable a stringent test of atomic kinetics models used within codes that are applied to photoionized plasmas in the laboratory and astrophysics. From experimental transmission data, ion areal densities are extracted by spectroscopic analysis that is independent of atomic kinetics modeling. The measurements reveal the net result of the competition between photon-driven ionization and electron-driven recombination atomic processes as a function of ξ as it affects the charge state distribution. Results from radiation-hydrodynamics modeling calculations with detailed inline atomic kinetics modeling are compared with the experimental results. There is good agreement in the mean charge and overall qualitative similarities in the trends observed with ξ but significant quantitative differences in the fractional populations of individual ions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

Background measurement methods for opacity experiments conducted at the Z facility

Laboratory experiments typically test opacity models by measuring spectrally resolved transmission of a sample using bright backlight radiation. A potential problem is that any unaccounted background signal contaminating the spectrum will artificially reduce the inferred opacity. Methods developed to measure background signals in opacity experiments at the Sandia Z facility are discussed. Preliminary measurements indicate that backgrounds are 9%–11% of the backlight signal at wavelengths less than 10 Å. Background is thus a relatively modest correction for all Z opacity data published to date. Future work will determine how important background is at longer wavelengths.

47 OTHER INSTRUMENTATION↗

DOE OSTI2021

Experimental methods for laboratory measurements of helium spectral line broadening in white dwarf photospheres

White Dwarf (WD) stars are the most common stellar remnant in the universe. WDs usually have a hydrogen or helium atmosphere, and helium WD (called DB) spectra can be used to solve outstanding problems in stellar and galactic evolution. DB origins, which are still a mystery, must be known to solve these problems. DB masses are crucial for discriminating between different proposed DB evolutionary hypotheses. Current DB mass determination methods deliver conflicting results. The spectroscopic mass determination method relies on line broadening models that have not been validated at DB atmosphere conditions. We performed helium benchmark experiments using the White Dwarf Photosphere Experiment (WDPE) platform at Sandia National Laboratories' Z-machine that aims to study He line broadening at DB conditions. Using hydrogen/helium mixture plasmas allows investigating the importance of He Stark and van der Waals broadening simultaneously. Accurate experimental data reduction methods are essential to test these line-broadening theories. In this paper, we present data calibration methods for these benchmark He line shape experiments. We give a detailed account of data processing, spectral power calibrations, and instrument broadening measurements. Uncertainties for each data calibration step are also derived. We demonstrate that our experiments meet all benchmark experiment accuracy requirements: WDPE wavelength uncertainties are <1 Å, spectral powers can be determined to within 15%, densities are accurate at the 20% level, and instrumental broadening can be measured with 20% accuracy. Fulfilling these stringent requirements enables WDPE experimental data to provide physically meaningful conclusions about line broadening at DB conditions.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

The effect of laser entrance hole foil thickness on MagLIF-relevant laser preheat

The magnetized liner inertial fusion (MagLIF) scheme relies on coupling laser energy into an underdense fuel raising the fuel adiabat at the start of the implosion. To deposit energy into the fuel, the laser must first penetrate a laser entrance hole (LEH) foil which can be a significant energy sink and introduce mix. In this paper, we report on experiments investigating laser energy coupling into MagLIF-relevant gas cell targets with LEH foil thicknesses varying from 0.5 μm to 3 μm. Two-dimensional (2D) axisymmetric simulations match the experimental results well for 0.5 μm and 1 μm thick LEH foils but exhibit whole-beam self-focusing and excessive penetration of the laser into the gas for 2 μm and 3 μm thick LEH foils. Additionally, better agreement for the 2 μm-thick foil is found when using a different thermal conductivity model in 2D simulations, while only 3D Cartesian simulations come close to matching the 3 μm-thick foil experiments. The study suggests that simulations may over-predict the tendency for the laser to self-focus during MagLIF preheat when thicker LEH foils are used. This effect is pronounced with 2D simulations where the azimuthally symmetric density channel effectively self-focuses the rays that are forced to traverse the center of the plasma. The extra degree of freedom in 3D simulations significantly reduces this effect. The experiments and simulations also suggest that, in this study, the amount of energy coupled into the gas is highly correlated with the laser propagation length regardless of the LEH foil thickness.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2020

Effect of Electron Capture on Spectral Line Broadening in Hot Dense Plasmas

Accurate calculation of spectral line broadening is important for many hot, dense plasma applications. However, calculated line widths have significantly underestimated measured widths for Δn = 0 lines of Li-like ions, which is known as the isolated-line problem. In this Letter, scrutinization of the line-width derivation reveals that the commonly used expression neglects a potentially important contribution from electron-capture. Line-width calculations including this process are performed with two independent codes, both of which removed the discrepancies at temperatures below 10 eV. In conclusion, the revised calculations also suggest the remaining discrepancy scales more strongly with electron temperature than the atomic number as was previously suggested.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗