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Results for “non-LTE”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Non-LTE profiles of the Al I autoionization lines

A non-LTE formulation is given for the transfer of radiation in the autoionizing lines of neutral aluminum at 1932 and 1936 A through both the Bilderberg and Harvard-Smithsonian model atmospheres. Numerical solutions for the common source function of these lines and their theoretical line profiles are calculated and compared with the corresponding LTE profiles. The results show that the non-LTE profiles provide a better match with the observations. They also indicate that the continuous opacity of the standard solar models should be increased in this wavelength region if the center-limb variations of observed and theoretical profiles of these lines are to be in reasonable agreement.

Finn, G. D.↗
NASA NTRS1973

A non-LTE study of silicon line formation in early-type main-sequence atmospheres.

We have computed populations of 16 levels of Si III-V and radiation fields in all connecting transitions; in particular the first six Si III triplet levels, including the 4553 line, and the first six Si IV levels including 4089. The computations were done for four non-LTE H-He model atmospheres, provided by Auer and Mihalas. Estimates of corresponding MK types are B1.5 V, B0.5 V, O9 V, and O6. Solutions were obtained by iterating the linearized equations of radiative transfer and statistical equilibrium, except that for less important lines an approximate equivalent two-level atom treatment was used. Continuous opacities of C, N, O, and Ne were included. All abundances were solar values.

Kamp, L. W.↗
NASA NTRS1974

Non-LTE H2+ as the source of missing opacity in the solar atmosphere

The population of the various vibrational levels of the H2+ molecule has been calculated from the consideration of formation and destruction mechanisms. The resulting population is used in calculating the total absorption due to H2+ and is compared with the other known sources of opacity at several optical depths of the solar atmosphere. It is shown that the absorption due to H2+ can probably account for the missing ultraviolet opacity in the solar atmosphere.

Swamy, K. S. K.↗
NASA NTRS1974

Non-equilibrium ionized blast wave

The structure of a cylindrical blast wave with ionization at non-LTE conditions was calculated using equations previously developed by Wu and Fu (1970). The degree of ionization was predicted by a modified Saha equation. Temperature profiles show that the temperature at non-LTE conditions is lower than at LTE near the shock front. This corresponds to a higher degree of ionization for the non-LTE limit, which indicates that the neutral gas absorption is much more efficient at non-LTE than at the LTE limit. The decaying velocity under non-LTE is approximately 15% less than under LTE.

Wu, S. T.↗
DOE OSTI2025

The R-Process Alliance: Hunting for gold in the near-UV spectrum of 2MASS J05383296–5904280

Over the past few years, the R-Process Alliance (RPA) has successfully carried out a search for stars that are highly enhanced in elements produced via the rapid neutron-capture (r-) process. In particular, the RPA has identified a number of relatively bright, highly r-process-enhanced (r-II) stars, suitable for observations with the Hubble Space Telescope (HST), facilitating abundance derivation of elements such as gold (Au) and cadmium (Cd). This paper presents the detailed abundances derived for the metal-poor ([Fe/H] = −2.55) highly r-process-enhanced ([Eu/Fe] = +1.29) r-II star 2MASS J05383296–5904280. One-dimensional local thermodynamic equilibrium (LTE) elemental abundances were derived via equivalent width and spectral synthesis using high-resolution high signal-to-noise near-UV HST/STIS and optical Magellan/MIKE spectra. Abundances were determined for 43 elements, including 26 neutron-capture elements. In particular, abundances of the rarely studied elements Nb, Mo, Cd, Lu, Os, Pt, and Au are derived from the HST spectrum. These results, combined with RPA near-UV observations of two additional r-II stars, increase the number of Cd abundances derived for r-process-enriched stars from seven to ten and Au abundances from four to seven. A large star-to-star scatter is detected for both of these elements, highlighting the need for more detections enabling further investigations, specifically into possible non-LTE effects.

chemically peculiar stars↗
DOE OSTI2025

Hydrodynamic expansion and near-infrared absorption of x-ray heated aluminum plasmas

We use x-ray pulses from dense argon plasmas at the Z Machine (Sandia National Laboratories) to generate hypersonic aluminum plasmas akin to material ejecta during proposed planetary defense missions, fusion reactor wall excursions, and other high-energy density processes. Near-infrared absorption is used to diagnose the controlled expansion of the plasmas through cylindrical cavities following their generation from x-ray heating of solid aluminum 7075 alloy. The data are compared to multidimensional radiation hydrodynamics simulations utilizing the ALEGRA multiphysics code, accounting for the dynamics of radiation scattering, material phase change, plasma expansion, thermal re-irradiation, and interactions with the cavity and with the infrared beams. To allow for accurate simulation, density functional theory is used to apply the Hagen–Rubens relation for the far-infrared and is adjoined with a detailed configuration accounting model using the Propaceos code, producing opacities spanning 10 −1 –10 4 eV photon energy for aluminum 7075 alloy, and in comparison with pure aluminum. The model is found to agree with experimental data in the higher-fluence regime when the Hagen–Rubens relation is applied. The ejected material, which is observed to travel up to 55 km/s, is comprised of a strongly ionized, non-LTE plasma front at ∼10 eV temperature followed by a weakly ionized LTE gas at higher density. The present findings lend some confidence to the broad-range equation of state and infrared opacity models for weakly ionized aluminum plasmas while demonstrating an approach to their future refinement, with potential application to astrophysical plasmas and other extreme processes.

Adiabatic process↗
DOE OSTI2025

Resolving discrepancies in bang-time predictions for indirect-drive ICF experiments on the NIF: Insights from the Build-A-Hohlraum campaign

This study investigated discrepancies between measured and simulated x-ray drive in Indirect-Drive Inertial Confinement Fusion (ID-ICF) hohlraums at the National Ignition Facility. Despite advances in radiation-hydrodynamic simulations, a consistent “drive deficit” remains. Experimentally measured ID-ICF capsule bang-times are systematically 400–700 ps later than simulations predict. The Build-A-Hohlraum (BAH) campaign explored potential causes for this discrepancy by systematically varying hohlraum features, including laser entrance hole (LEH) windows, capsules, and gas fills. Overall, the agreement between simulated and experimental x-ray drive was found to be largely unaffected by these changes. The data allow us to exclude some hypotheses put forward to potentially explain the discrepancy. Errors in the local thermodynamic equilibrium (LTE) atomic modeling, errors in the modeling of LEH closure, and errors due to a lack of plasma species mix physics in simulations are shown to be inconsistent with our measurements. Instead, the data support the hypothesis that errors in NLTE emission modeling are a significant contributor to the discrepancy. X-ray emission in the 2–4 keV range is found to be approximately 30% lower than in simulations. This is accompanied by higher than predicted electron temperatures in the gold bubble region, pointing to errors in non-LTE modeling. Introducing an opacity multiplier of 0.87 on energy groups above 1.8 keV improves agreement with experimental data, reducing the bang-time discrepancy from 300 to 100 ps. These results underscore the need for refined NLTE opacity models to enhance the predictive power of hohlraum simulations.

Band emission↗
DOE OSTI2026

Velocity-space Origins of the Pressure–Strain Interaction in Multipopulation Distributions and Its Application to Magnetic Reconnection

A forefront research question is how energy evolves in weakly collisional plasmas for which departures from local thermodynamic equilibrium (LTE) are significant. The standard approach is studying the terms in the non-LTE energy evolution equation derived by taking the second moment of the Boltzmann equation, but the resultant fluid metrics do not retain information about which particles at which velocities drive energy evolution. A widely studied channel for internal energy density evolution is the pressure–strain interaction. Here, we employ the kinetic pressure–strain, a phase-space diagnostic whose velocity-space integral recovers the pressure–strain interaction to disambiguate the contributions to the pressure–strain interaction from disparate particle populations in composite phase-space densities. We develop phase-space analogs of the pressure–strain interaction decompositions to provide the phase-space origins of normal versus sheared flow. We introduce the “kinetic strain-rate” tensor, the phase-space analog of the strain-rate tensor, which we argue is needed to interpret the phase-space origins of the pressure–strain interaction. To demonstrate the utility of these quantities, we investigate them for composite electron distributions near the electron diffusion region in two-dimensional particle-in-cell simulations of antiparallel symmetric magnetic reconnection. We find that the phase-space-based diagnostics isolate the roles of distinct populations. These results contribute to a growing body of work providing new methods for quantifying phase-space energy evolution for a broad array of processes, from magnetic reconnection to collisionless shocks and turbulence, opening new pathways for answering longstanding problems of particle energization in weakly collisional plasmas.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1971

Analysis of solar ultraviolet lines

The formation of the strongest ultra-violet emission lines of Mg II, O I, C II, and C III in the solar atmosphere is studied in detail. The equations of statistical equilibrium and radiative transfer for each ion are solved using a general computer program that is capable of solving non-LTE line-formation problems for arbitrary atmospheric and atomic models. Interpreting the results in terms of the structure of the solar atmosphere, it is concluded that the HSRA atmosphere has a temperature too low by about 500 K near h = 1100 km and that a temperature plateau with T sub e approximately = 18,000 K and width close to 60 km exists in the upper chromosphere. The structure of the solar atmosphere in the range 20,000 to 100,000 K and the effects of microturbulence on the formation of lines are also investigated. Approximate analytic line-formation problems are solved, and more exact solutions are derived later. An attempt is made to make the best possible fit to the Ca II K line center-to-limb profiles with a one-component atmosphere, with an assumed source function and microturbulent velocity.

Chipman, E.↗
NASA NTRS1972

Line-blanketed model stellar atmospheres applied to Sirius

The primary goal of this analysis is to determine whether the effects of atomic bound-bound transitions on stellar atmospheric structure can be represented well in models. The investigation is based on an approach which is called the method of artificial absorption edges. The method is described, developed, tested, and applied to the problem of fitting a model stellar atmosphere to Sirius. It is shown that the main features of the entire observed spectrum of Sirius can be reproduced to within the observational uncertainty by a blanketed flux-constant model with T sub eff = 9700 K and Log g = 4.26. The profile of H sub gamma is reproduced completely within the standard deviations of the measurements except near line center, where non-LTE effects are expected to be significant. The equivalent width of H sub gamma, the Paschen slope, the Balmer jump, and the absolute flux at 5550 A all agree with the observed values.

Fowler, J. W.↗
NASA NTRS1972

Observations of zeta Puppis.

The visual line spectrum of zeta Puppis (O4f) cannot be matched by theoretical line profiles computed by Auer and Mihalas (1971) from their non-LTE models. The discrepancy between observation and prediction suggests that deviations from plane-parallel geometry and hydrostatic equilibrium must be accounted for in order for models to represent the atmospheric regions forming the observed lines. Relaxation of these classical assumptions may affect the value of effective temperature for this star derived from observations of its continuous spectrum.

Heap, S. R.↗
NASA NTRS1973

Line profiles and turbulence generated by acoustic waves in the solar chromosphere. II - Contours of the Ca II and Mg II K lines

Making use of the time-averaged absorption profiles derived by Oster and Ulmschneider, non-LTE line formation in the context of a two-level atom is investigated for an isothermal atmosphere and for the Ca II and Mg II K lines in the solar chromosphere as represented by the Harvard-Smithsonian Reference Atmosphere. Source functions and emergent line profiles are computed for a variety of assumptions concerning the acoustically broadened profiles and the solar velocity fields.

Shine, R. A.↗