Showing papers by "Stephanie Hansen published in 2017"
TL;DR: A new model is introduced that brings the calculated bang time into better agreement with the measured bang time and predicts higher temperatures in the plasma than the old model and also predicts that at higher gas fill densities, a significant amount of inner beam laser energy escapes the hohlraum through the opposite laser entrance hole.
Abstract: For several years, we have been calculating the radiation drive in laser-heated gold hohlraums using flux-limited heat transport with a limiter of 0.15, tabulated values of local thermodynamic equilibrium gold opacity, and an approximate model for not in a local thermodynamic equilibrium (NLTE) gold emissivity (DCA_2010). This model has been successful in predicting the radiation drive in vacuum hohlraums, but for gas-filled hohlraums used to drive capsule implosions, the model consistently predicts too much drive and capsule bang times earlier than measured. In this work, we introduce a new model that brings the calculated bang time into better agreement with the measured bang time. The new model employs (1) a numerical grid that is fully converged in space, energy, and time, (2) a modified approximate NLTE model that includes more physics and is in better agreement with more detailed offline emissivity models, and (3) a reduced flux limiter value of 0.03. We applied this model to gas-filled hohlraum experiments using high density carbon and plastic ablator capsules that had hohlraum He fill gas densities ranging from 0.06 to 1.6 mg/cc and hohlraum diameters of 5.75 or 6.72 mm. The new model predicts bang times to within ±100 ps for most experiments with low to intermediate fill densities (up to 0.85 mg/cc). This model predicts higher temperatures in the plasma than the old model and also predicts that at higher gas fill densities, a significant amount of inner beam laser energy escapes the hohlraum through the opposite laser entrance hole.
59 citations
TL;DR: Through these comparisons, topics such as modeling of the dielectronic processes, density effects or the effect of an external radiation field were addressed, and the K-shell spectroscopy of iron plasmas was also addressed, notably through the interpretation of tokamak and laser experimental spectra.
32 citations
TL;DR: In this article, high-resolution spectroscopic data from iron impurities in beryllium liners driven by Sandia's Z machine to temperatures near 10eV and electron densities near 2'×'10 24 'cm −3, conditions independently diagnosed from the transmission depth and shape of the iron K-edge.
30 citations
TL;DR: A highly reproducible benchmark experiment is developed to study spectrum formation from a photoionized silicon plasma in a regime comparable to astrophysical plasmas, demonstrating that the resonant Auger destruction assumption used to interpret black hole accretion spectra is inaccurate.
Abstract: The interpretation of x-ray spectra emerging from x-ray binaries and active galactic nuclei accreted plasmas relies on complex physical models for radiation generation and transport in photoionized plasmas. These models have not been sufficiently experimentally validated. We have developed a highly reproducible benchmark experiment to study spectrum formation from a photoionized silicon plasma in a regime comparable to astrophysical plasmas. Ionization predictions are higher than inferred from measured absorption spectra. Self-emission measured at adjustable column densities tests radiation transport effects, demonstrating that the resonant Auger destruction assumption used to interpret black hole accretion spectra is inaccurate.
30 citations
TL;DR: In this article, a recent study of Fe xvii R-matrix calculations aimed at resolving outstanding opacity problems claimed that substantial photon absorption from atomic core ionization processes was not previously considered.
Abstract: A recent study of Fe xvii R-matrix calculations aimed at resolving outstanding opacity problems claimed that substantial photon absorption from atomic core ionization processes was not previously considered. It is shown, however, that major opacity models already include cross-sections that are equivalent to the enhancements reported by the R-matrix method. Furthermore, the R-matrix calculations neglected important cross-sections that help to explain why the resultant opacity is lower than other models in the spectral range measured in transmission experiments relevant to the solar interior.
18 citations
TL;DR: In this article, the authors describe calculations of two-photon absorption, which is a candidate for the observed extra opacity, but do not yet match the experiments but show that the two-phase absorption process is strong enough to require careful consideration.
15 citations
Journal Article•
14 citations
TL;DR: A new time and space resolved x-ray spectrometer is described for use in Z-pinch inertial confinement fusion and radiation source development experiments, to measure the time- and one-dimensional space-dependent electron temperature and density during stagnation.
Abstract: We describe the design and function of a new time and space resolved x-ray spectrometer for use in Z-pinch inertial confinement fusion and radiation source development experiments. The spectrometer is designed to measure x-rays in the range of 0.5-1.5 A (8-25 keV) with a spectral resolution λ/Δλ ∼ 400. The purpose of this spectrometer is to measure the time- and one-dimensional space-dependent electron temperature and density during stagnation. These relatively high photon energies are required to escape the dense plasma created at stagnation and to obtain sensitivity to electron temperatures ≳3 keV. The spectrometer is of the Cauchois type, employing a large 30 × 36 mm2, transmissive quartz optic for which a novel solid beryllium holder was designed. The performance of the crystal was verified using offline tests, and the integrated system was tested using experiments on the Z pulsed power accelerator.
5 citations