B.G. Wilson

Laboratory measurement of opacity for stellar envelopes

Abstract We have measured the frequency dependent opacity of a low density iron plasma in Local Thermodynamic Equilibrium (LTE). The measured iron plasma conditions of 20 eV temperature and 10 −4 g/cc density, match those of stellar envelopes where iron dominates the radiative transport. Properties of the M-shell Δn = 0 transition arrays in iron are measured in this experiment, providing the first direct test of opacity models used in stellar pulsation and evolution calculations. We describe new methods to obtain LTE opacity data for plasmas at 100 times lower density than previous measurements. Experimental requirements include: high spectral resolution, large homogenous plasma sources, and Planckian radiation fields lasting tens of nanoseconds. These conditions were achieved using the 500 kJ SATURN facility at Sandia National Laboratory.

Opacity measurements: Extending the range and filling in the gaps

A series of experiments to explore Ge opacity at temperatures where the M-shell is almost filled will be discussed. Data are obtained at lower temperatures than previously explored and allow us to investigate the role of atomic structure calculations and their impact on opacity scalings. The experiment uses the Nova laser to irradiate a gold hohlraum within which a CH-tamped Ge sample is radiatively heated. A Nd backlight probes the sample 2 ns later to produce Ge spectral absorption features in the 1.2-1.5 keV energy range. Temperature is monitored by the use of an Al dopant and density is monitored by measuring the edge-on expansion of the sample. Temporal resolution of about 200 ps is obtained by using a short pulse backlight. Calculations in this photon energy region show significant changes in the spectral features.

Quantitative measurement of mid-z opacities

Abstract Results of recent experiments measuring x-ray absorption by a hot, dense, germanium plasma are presented. A general discussion of the experimental technique is given showing the requirements that must be met in order to extract quantitative transmission data. The resulting spectrally resolved absorption measurements can then be used to test the capabilities of LTE opacity codes. Meaningful comparisons require that the sample be in LTE, and that the temperature and density of the sample be uniform and accurately measured. Comparisons between the experiment and calculations are shown.

A new detailed term accounting opacity code for mid-Z elements: TOPAZ

A new opacity code, TOPAZ, that explicitly includes the detailed configuration term structure for mid-Z elements is under development and preliminary results are presented. The main purpose is to extend the current capabilities of opacity codes such as OPAL, which are limited to elements of astrophysical interest, towards heavier elements. Results from the new code are compared to several past experiments.

Accurate determination of the charge state distribution in a well characterized highly ionized Au plasma

Abstract The density, temperature and charge state distribution are accurately determined in a highly ionized non-LTE Au sample. Laser heated Au microdots buried in a thin Be foil, reach temperatures of 2 keV and ionize into the M-shell. During expansion, the tamped Au samples remain uniform and in near steady-state ionization equilibrium. The electron temperature is measured with time and space resolved Thomson scattering while the density is determined from time-gated X-ray imaging the expanded Au sample. The charge state distribution is obtained from analysis of emission measurements of Au 5f–3d transition arrays in the wavelength range 3.3–3.9 A, allowing the average charge to be determined to within ∼1% accuracy.

Statistical simulation of atomic data in opacity calculations

Abstract The most significant improvement in radiative opacity codes in recent years has been advances in the atomic data incorporated in the models. In particular, the configuration term structure, which significantly increases the amount of atomic data in the calculations, is now explicitly included for elements with low atomic number. For large atomic number, where explicit calculations are impractical, clever methods have been developed that conserve a few properties of the transition arrays rather than attempt to reproduce the detailed spectrum. There are, however, conditions where neglecting the resolved character of the transition arrays leads to large errors, but for which explicit calculations are either costly or impractical. Here, we test recently developed methods for statistically simulating resolved transition arrays. When possible, the results are compared to explicit calculations and experiments.

A deficiency of local density functionals for the calculation of self-consistent field atomic data in plasmas

Abstract In generating the atomic data base used in computing opacities of hot dense plasmas the local density functional approximation (LDA) is often employed. One advantage of the LDA is that the configuration average energy of a large number of ionization states and excited configurations can be computed compactly using a multi-variate Taylor series expansion in occupation numbers. The pitfalls of employing a local density functional and its impact on the quality of the Taylor series approximation are discussed. The consequences of modeling opacities with a LDA are illustrated by comparisons with experiment. A method for obviating the problems inherent in the LDA is presented.

NLTE ionization and energy balance in high-Z laser-plasmas including two-electron transitions

Abstract We describe a new non-LTE statistical atomic kinetics model of plasmas in which the two-electron transitions of auto-ionization and its inverse, resonant-capture, play a dominant role in establishing ionization and energy balance. We show that, compared with a familiar collisional–radiative-equilibrium model which includes only the one-electron bound–bound and bound–free transitions: (1) the two-electron transitions force recombination of the plasma with decreasing density, (2) the two-electron transitions nevertheless further act to greatly increase the radiative emissivity of the plasma, and (3) the relaxation of the two-electron transition driven sysems proceeds much faster.

Spectroscopic measurements of Rosseland mean opacity

Abstract 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.

Efficient isoparametric integration over arbitrary space-filling Voronoi polyhedra for electronic structure calculations

A numerically efficient, accurate, and easily implemented integration scheme over convex Voronoi polyhedra (VP) is presented for use in ab initio electronic-structure calculations. We combine a weighted Voronoi tessellation with isoparametric integration via Gauss-Legendre quadratures to provide rapidly convergent VP integrals for a variety of integrands, including those with a Coulomb singularity. We showcase the capability of our approach by first applying it to an analytic charge-density model achieving machine-precision accuracy with expected convergence properties in milliseconds. For contrast, we compare our results to those using shape-functions and show our approach is greater than 105 times faster and 107 times more accurate. A weighted Voronoi tessellation also allows for a physics-based partitioning of space that guarantees convex, space-filling VP while reflecting accurate atomic size and site charges, as we show within KKR methods applied to Fe-Pd alloys.