Opacity Model Research Articles

Predicting Engine Parameters with Cost Efficient AI Models – An Experimental Method Validation

Artificial intelligence (AI) methods have rapidly become a best practice in various industrial applications due to their exceptional predictive abilities. Compared to traditional physical and chemical models, AI-driven approaches typically offer faster computation times without sacrificing accuracy. This makes them well-suited for enhancing or replacing conventional methods in engine technology. Building on this potential, our previous work focused on developing AI-based tools to accelerate the development of novel e-fuels for internal combustion engines. These AI tools require representative training datasets created through expensive engine dyno measurements. To address this challenge, we developed a general methodology to improve the cost-efficiency of dataset creation. This paper presents the experimental validation of this methodology by assessing its performance on seven different predictive tasks. Following the proposed framework, we designed and conducted an engine dyno experiment, then developed AI models to predict seven critical engine performance and emissions parameters: center of heat release, ignition delay, peak combustion temperature, peak pressure rise rate, brake thermal efficiency, exhaust opacity and NOx emissions. The results demonstrated the effectiveness of the methodology, with five out of seven models achieving excellent predictive performance on unseen test data (R2 > 0.97). Peak pressure rise rate and opacity models had slightly lower performance (R2 > 0.94), however, given the well-known challenges associated with predicting these parameters, the results are acceptable.

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.

Continuum Reverberation in Active Galactic Nuclei Disks Only with Sufficient X-Ray Luminosity and Low Albedo

Abstract Disk continuum reverberation mapping is one of the primary ways we learn about active galactic nuclei (AGN) accretion disks. Reverberation mapping assumes that time-varying X-rays incident on the accretion disk drive variability in UV–optical light curves emitted by AGN disks and uses lags between X-ray and UV–optical variability on the light-crossing timescale to measure the radial temperature profile and extent of AGN disks. However, recent reverberation mapping campaigns have revealed oddities in some sources, such as weakly correlated X-ray and UV light curves, longer than anticipated lags, and evidence of intrinsic variability from disk fluctuations. To understand how X-ray reverberation works with realistic accretion disk structures, we perform 3D multifrequency radiation magnetohydrodynamic simulations of X-ray reprocessing by the UV-emitting region of an AGN disk using sophisticated opacity models that include line opacities for both the X-ray and UV radiation. We find there are two important factors that determine whether X-ray irradiation and UV emission will be well-correlated: the ratio of X-ray to UV luminosity and significant absorption. When these factors are met, the reprocessing of X-rays into UV is nearly instantaneous, as is often assumed, although linear reprocessing models are insufficient to fully capture X-ray reprocessing in our simulations. Nevertheless, we can still easily recover mock lags in our light curves using software that assumes linear reprocessing. Finally, the X-rays in our simulation heat the disk, increasing temperatures by a factor of 2–5 in the optically thin region, which could help explain the discrepancy between measured and anticipated lags.

Searching for Internal Absorption Signatures in High-redshift Blazars

Abstract The γ-ray emission from flat spectrum radio quasars (FSRQs), a subclass of blazars, is believed to be generated through interactions of high-energy leptons and/or hadrons in the jet with the ambient photon fields, including those from the accretion disk, the broad line region (BLR), and the dusty torus. However, these same photon fields can also attenuate γ-rays through internal photon–photon (γ–γ) absorption, imprinting characteristic spectral features. Investigating the internal absorption is crucial for unraveling the complex structure of FSRQs and constraining the poorly known location of the γ-ray emission region. In this study, we select a sample of γ-ray detected FSRQs with high redshift (z ≳ 3) to search for absorption features appearing at lower photon energies due to a substantial redshift. We extract the Fermi Large Area Telescope γ-ray spectra of these sources and perform physical modeling using a detailed γ–γ opacity model, assuming that the BLR photon field dominates the absorption and focusing on the energy range ∼25 GeV/(1 + z), where the absorption feature due to Lyα photons is expected. Our analysis reveals a hint of internal absorption for one source (the lowest redshift object in our sample, z ≃ 3) and provides constraints on the location of its γ-ray emitting region along the jet. For the remaining, higher-redshift sources, the limited photon statistics prevent a reliable detection of internal opacity features.

Stochastic radiative transfer in random media. III. Effective opacity.

Homogenization of random media is a practical approach to efficiently simulate stochastic radiative transfer, which requires establishing a reliable effective opacity model. In this work, a new effective opacity model is developed by considering the two-point spatial correlations of the opacity fluctuation, which is further simplified by introducing the analytically empirical function of the optical depth. Our new model has been extensively verified by comparing with direct numerical simulations (DNSs) of stochastic radiative transfer in participating random media in two dimensions. A systematic comparison with existing effective opacity models in the literature is made. For more than 50 different sets of physical parameters of random media, including constant and temperature-dependent opacities, it shows that our new model is the most accurate, which can reproduce the DNS transport results within a relative error of 5% in most cases. In contrast, the performance of existing models is problem-dependent, and it yields considerable errors spanning over three orders of magnitude. The reasons why the newly developed model works well and why existing models fail are also discussed. For more realistic problems, radiative transfer in the aluminum-foam mixture is investigated, where our new model shows the best performance among analytical effective opacity models. In addition, the newly developed model's limitations are briefly analyzed.

Modelling Thomson Scattering in a Hydrogen Plasma at Stellar Interior Conditions Using the Hypernetted‐Chain Approach

ABSTRACTUnder the extreme conditions found in small stars, where electron degeneracy and Coulomb coupling are significant, accurate modeling of Thomson scattering is crucial for determining opacity, a primary quantity for stellar energy transport. We use hypernetted‐chain calculations, incorporating quantum pseudopotentials and electron‐exchange effects to obtain the electron–electron static structure factor to calculate the Thomson scattering transport cross‐section for conditions prevailing in the interior of small stars. These results are compared to those from average‐atom simulations and analytical calculations. Our findings support laboratory astrophysics experiments aimed at benchmarking opacity models for stellar interiors, particularly for red dwarf stars, and help to bridge theoretical models with observations.

Ab Initio Quantum Dynamics as a Scalable Solution to the Exoplanet Opacity Challenge: A Case Study of CO2 in a Hydrogen Atmosphere

Abstract Light–matter interactions lie at the heart of our exploration of exoplanetary atmospheres. Interpreting data obtained by remote sensing is enabled by meticulous, time- and resource-consuming work aiming at deepening our understanding of such interactions (i.e., opacity models). Recently, P. Niraula et al. pointed out that due primarily to limitations on our modeling of broadening and far-wing behaviors, opacity models needed a timely update for exoplanet exploration in the JWST era, and thus argued for a scalable approach. In this proof-of-concept study, we introduce an end-to-end solution from ab initio calculations to pressure broadening, and use a perturbation framework to address the need for precision to a level of ∼10%. We focus on the CO2–H2 system as CO2 is a key absorption feature for exoplanet research (primarily in many gas giants) at ∼4.3 μm as pressure-broadening parameters required for interpreting such observations remain sparse. We compute elastic and inelastic cross sections for the collisions of ortho-H2 with CO2, in the ground vibrational state, and at the coupled-channel fully converged level. For scattering energies above ∼20 cm−1, moderate precision intermolecular potentials are indistinguishable from high-precision ones in cross sections. Our calculations agree with the currently available measurements within 7%, i.e., well beyond the precision requirements.

First Measurement of Z Opacity Sample Evolution near Solar Interior Conditions Using Time-Resolved Spectroscopy.

Opacity model differences with Fe opacity measurements at high temperature (T>180 eV) and high electron density (n_{e}>3×10^{22} cm^{-3}) at nearly solar interior conditions have remained unresolved [Bailey etal., Nature 517, 56 (2015)10.1038/nature14048 and Nagayama etal., Phys. Rev. Lett. 122, 235001 (2019)PRLTAO0031-900710.1103/PhysRevLett.122.235001]. Systematic errors from temporal gradients are one hypothesis for the discrepancy. Past data recorded on x-ray film provided spectral measurements over a time determined by the 2.8-ns backlighter duration. Here, we present the first measurements of opacity sample temporal evolution using novel hCMOS ultrafast x-ray camera technology. The measured conditions, measured backlighter time history, and modeled opacities are used to show that temporal gradients do not resolve the model-data discrepancy. The methods demonstrated provide potential advantages, including opacities at more extreme conditions, spectral line shift measurements, and improved measurements at other facilities.

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 10−1–104 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.

Dust Scattering Albedo at Millimeter Wavelengths in the TW Hya Disk

Abstract Planetary bodies are formed by coagulation of solid dust grains in protoplanetary disks. Therefore, it is crucial to constrain the physical and chemical properties of the dust grains. In this study, we measure the dust albedo at millimeter wavelength, which depends on dust properties at the disk midplane. Since the albedo and dust temperature are generally degenerate in observed thermal dust emission, it is challenging to determine them simultaneously. We propose to break this degeneracy by using multiple optically thin molecular lines as a dust–albedo-independent thermometer. In practice, we employ pressure-broadened CO line wings that provide an exceptionally high signal-to-noise ratio as an optically thin line. We model the CO J = 2–1 and 3–2 spectra observed by the Atacama Large Millimeter/submillimeter Array at the inner region (r < 6 au) of the TW Hya disk and successfully derive the midplane temperature. Combining multiband continuum observations, we constrain the albedo spectrum at 0.9–3 mm for the first time without assuming a dust opacity model. The albedo at these wavelengths is high, ~0.5–0.8, and broadly consistent with the L. Ricci et al., DIANA, and DSHARP dust models. Even without assuming dust composition, we estimate the maximum grain size to be ~340 μm, the power-law index of the grain size distribution to be >−4.1, and the porosity to be <0.96. The derived dust size may suggest efficient fragmentation with a threshold velocity of ~0.08 m s−1. We also note that the absolute flux uncertainty of ~10% (1σ) is measured and used in the analysis, which is approximately twice the usually assumed value.

Impact of keratocyte differentiation on corneal opacity resolution and visual function recovery in male rats

Intrastromal cell therapy utilizing quiescent corneal stromal keratocytes (qCSKs) from human donor corneas emerges as a promising treatment for corneal opacities, aiming to overcome limitations of traditional surgeries by reducing procedural complexity and donor dependency. This investigation demonstrates the therapeutic efficacy of qCSKs in a male rat model of corneal stromal opacity, underscoring the significance of cell-delivery quality and keratocyte differentiation in mediating corneal opacity resolution and visual function recovery. Quiescent CSKs-treated rats display improvements in escape latency and efficiency compared to wounded, non-treated rats in a Morris water maze, demonstrating improved visual acuity, while stromal fibroblasts-treated rats do not. Advanced imaging, including multiphoton microscopy, small-angle X-ray scattering, and transmission electron microscopy, revealed that qCSK therapy replicates the native cornea’s collagen fibril morphometry, matrix order, and ultrastructural architecture. These findings, supported by the expression of keratan sulfate proteoglycans, validate qCSKs as a potential therapeutic solution for corneal opacities.

Measurement of 2p-3d absorption in a hot molybdenum plasma

We present measurements of the 2p-3d transition opacity of a hot molybdenum–scandium sample with nearly half-vacant molybdenum M-shell configurations. A plastic-tamped molybdenum–scandium foil sample is radiatively heated to high temperature in a compact D-shaped gold Hohlraum driven by ∼30 kJ laser energy at the SG-100 kJ laser facility. X rays transmitted through the molybdenum and scandium plasmas are diffracted by crystals and finally recorded by image plates. The electron temperatures in the sample in particular spatial and temporal zones are determined by the K-shell absorption of the scandium plasma. A combination of the IRAD3D view factor code and the MULTI hydrodynamic code is used to simulate the spatial distribution and temporal behavior of the sample temperature and density. The inferred temperature in the molybdenum plasma reaches a average of 138 ± 11 eV. A detailed configuration-accounting calculation of the n = 2–3 transition absorption of the molybdenum plasma is compared with experimental measurements and quite good agreement is found. The present measurements provide an opportunity to test opacity models for complicated M-shell configurations.

Investigation of healing strategies in a rat corneal opacity model with polychromatic light and stem cells injection

Investigation of healing strategies in a rat corneal opacity model with polychromatic light and stem cells injection

Little evolution of dust emissivity in bright infrared galaxies from 2 < z < 6

ABSTRACT Variations in the dust emissivity index, $\beta$, within and between galaxies, are evidence that the chemistry and physics of dust must vary on large scales, although the nature of the physical and/or chemical variations is still unknown. In this paper, we estimate values of $\beta$ and dust temperature for a sample of 109 dusty star-forming galaxies (DSFGs) over the range, $2 \ \lt\ z \ \lt\ 6$. We compare the results obtained with both an optically thin model and a general opacity model, finding that our estimates of $\beta$ are similar between the models but our estimates of dust temperature are not. We find no evidence of a change in $\beta$ with redshift, with a median value of $\beta = 1.96$ for the optically thin model with a confidence interval (16–84 per cent) of 1.67 to 2.35 for the population. Using simulations, we estimate the measurement errors from our procedure and show that the variation of $\beta$ in the population results from intrinsic variations in the properties of the dust in DSFGs. At a fixed far-infrared luminosity, we find no evidence for a change in dust temperature, $T_{\textrm {dust}}$, with redshift. After allowing for the effects of correlated measurement errors, we find an inverse correlation between $\beta$ and $T_{\textrm {dust}}$ in DSFGs, for which there is also evidence in low-redshift galaxies.

Statistical RTA simulations of atomic data for astrophysical opacity modeling in the context of kilonova emission

When considering some complex lanthanide ions characterized by a half-filled 4f subshell, the atomic structure Hamiltonian matrix sizes are so large that their diagonalization is challenging and therefore the atomic data of these ions are only used to compute the expansion opacity of a kilonova with difficulty. To avoid this problem, we propose a statistical simulation method to compute kilonova expansion opacities based on the resolved transition array (RTA) method of Bauche et al (1991 Phys. Rev. A 44 5707). The atomic structure relativistic Hartree–Fock (HFR) method has been employed to compute the radial integrals necessary for our statistical RTA simulations where the atomic data are randomly drawn using their corresponding statistical distributions and to determine the exact expansion opacities where the atomic data are obtained by the diagonalization of the Hamiltonian matrix. The statistical RTA simulations carried out for two specific ions, i.e. Sm VIII and Eu VI, for which it is still possible to diagonalize the Hamiltonian matrix, reproduce well the expansion opacities computed using HFR atomic data. Based on this good agreements, the statistical RTA method was used to compute the expansion opacity of Dy VIII, which is determined through diagonalization with difficulty. The proposed statistical RTA simulation method allows the computation of reliable astrophysical expansion opacities which are of paramount importance for kilonova light curve modeling and spectral analysis.

Measurement of mix at the fuel–ablator interface in indirectly driven capsule implosions on the National Ignition Facility

The interface between the capsule ablator and fuel ice layer is susceptible to hydrodynamic instabilities. The subsequent mixing of hot ablator material into the ice reduces fuel compression at stagnation and is a candidate for reduced capsule performance. The ability to diagnose ice–ablator mix is critical to understanding and improving stability at this interface. Combining the crystal backlighter imager with the single line of sight camera on the National Ignition Facility (NIF) allows direct measurement of ice–ablator mix by providing multiple quasi-monochromatic radiographs of layered capsule implosions per experiment with high spatial (∼12 μm) and temporal (∼35 ps) resolution. The narrow bandwidth of this diagnostic platform allows radiography of the inner edge of the capsule limb close to stagnation without capsule self-emission contaminating the data and removes opacity uncertainties typically associated with the spectral content of the radiograph. Analysis of radiographic data via a parameterized forward-fitting Abel inversion technique provides measurements of the distribution of mix mass inwards from the ice–ablator interface. The sensitivity of this mix measurement technique was demonstrated by applying it to layered experiments in which the stability of the ice–ablator interface was expected to vary significantly. Additional experiments suggest that high-density carbon capsules that employ a buried-layer dopant profile suffer from mixing at the innermost doped–undoped interface. Data from these experiments suggest that opacity models used in hydrodynamic simulations of NIF experiments can potentially over-predict the opacity of doped capsules. LLNL-JRNL-850535-DRAFT.

Origin and Extent of the Opacity Challenge for Atmospheric Retrievals of WASP-39 b

As the James Webb Space Telescope (JWST) came online last summer, we entered a new era of astronomy. This new era is supported by data products of unprecedented information content that require novel reduction and analysis techniques. Recently, Niraula et al. (N22) highlighted the need for upgraded opacity models to prevent facing a model-driven accuracy wall when interpreting exoplanet transmission spectra. Here, we follow the same approach as N22 to explore the sensitivity of inferences on the atmospheric properties of WASP-39 b to the opacity models used. We find that the retrieval of the main atmospheric properties from this first JWST exoplanet spectrum is mostly unaffected by the current state of the community’s opacity models. Abundances of strong opacity sources like water and carbon dioxide are reliably constrained within ∼0.30 dex, beyond the 0.50 dex accuracy wall reported in N22. Assuming the completeness and accuracy of line lists, N22's accuracy wall is primarily driven by model uncertainties on broadening coefficients and far-wing behaviors, which we find to have marginal consequences for interpreting the transmission spectra of large, hot, high-metallicity atmospheres such as WASP-39 b, in opposition to emission spectra and climate modeling, which depend on deeper parts of a planetary atmosphere. The origin of the opacity challenge in the retrieval of metal-rich hot Jupiters via transmission spectroscopy will thus mostly stem from the incompleteness and inaccuracy of line lists.

Radiation burnthrough measurements to infer opacity at conditions close to the solar radiative zone–convective zone boundary

Recent measurements at the Sandia National Laboratory of the x-ray transmission of iron plasma have inferred opacities much higher than predicted by theory, which casts doubt on modeling of iron x-ray radiative opacity at conditions close to the solar convective zone-radiative zone boundary. An increased radiative opacity of the solar mixture, in particular iron, is a possible explanation for the disagreement in the position of the solar convection zone-radiative zone boundary as measured by helioseismology and predicted by modeling using the most recent photosphere analysis of the elemental composition. Here, we present data from radiation burnthrough experiments, which do not support a large increase in the opacity of iron at conditions close to the base of the solar convection zone and provide a constraint on the possible values of both the mean opacity and the opacity in the x-ray range of the Sandia experiments. The data agree with opacity values from current state-of-the-art opacity modeling using the CASSANDRA opacity code.

Atomic structure considerations for the low-temperature opacity of Xenon

Atomic structure considerations for the low-temperature opacity of Xenon

Induction of Corneal Endothelial-like Cells from Mesenchymal Stem Cells of the Umbilical Cord

Because of the limited differentiation capacity of human corneal endothelial cells (CECs), stem cells have emerged as a potential remedy for corneal endothelial dysfunction (CED). This study aimed to demonstrate the differentiation of human umbilical cord-derived mesenchymal stem cells (UC-MSCs) into CECs and to investigate the efficacy of MSC-induced CEC injection into the anterior chamber in a rabbit model of CED. Human UC-MSCs were differentiated into CECs using medium containing glycogen synthase kinase 3β inhibitor and two types of Rho-associated protein kinase inhibitors. In the MSC-induced CECs, CEC-specific proteins were identified through immunohistochemistry and changes in CEC-specific gene expressions over time were confirmed through quantitative RT-PCR. When MSC-induced CECs were injected into a rabbit model of CED, corneal opacity and neovascularization were improved compared with the non-transplanted control or MSC injection group. We also confirmed that MSC-induced CECs were well engrafted as evidenced by human mitochondrial DNA in the central cornea of an animal model. Therefore, we demonstrated the differentiation of UC-MSCs into CECs in vitro and demonstrated the clinical efficacy of MSC-induced CEC injection, providing in vivo evidence that MSC-induced CECs have potential as a treatment option for CED.