Session BO09: High Energy Density Science: Opacity and Atomic Physics

Overview of current progress on Opacity-on-NIF and comparisons between Z and theory

The Opacity-on-NIF campaign^1,2,4,5 measures transmission to provide opacity data for Fe and other elements to help address the discrepancy between theory and Opacity-on-Z data³. The Opacity-on-NIF experiments have worked to improve reproducibility and have produced the first quantitative comparisons of experimental data from NIF with theory and Z data. Current status of the Opacity-on-NIF platform and uncertainties of these comparisons will be discussed.

 

Progress Extending Opacity Measurements on the National Ignition Facility (NIF) to Higher X-ray Energies (1.8-2.9 keV)

Measurements of X-ray opacity of hot dense plasmas are needed to help resolve discrepancies in solar and other stellar models.¹ Previous measurements at the Sandia Z Facility suggest opacities up to 2x higher than theory for the iron L-band, particularly for temperatures >160 eV and electron densities > 10²² e^-/cm³.¹ Initial NIF results tend to support the Z Facility data,² though uncertainties need to be reduced,³ raising the question whether the discrepancies might extend to higher X-ray energies. The NIF Opacity Spectrometer has recently been modified to measure X-ray energies from 1.8-2.9 keV, in addition to the usual 1.0-2.0 keV band.⁴ This talk presents the first NIF data using a "dual band" spectrometer configuration to record X-ray absorption spectra from 1.0-2.9 keV from samples in LTE at temperatures above 140 eV, including data from magnesium-oxide/silicon dioxide samples. Prospects for further experiments will be discussed. 

Results from the Opacity-on-the-National Ignition Facility Project: Comparison of Simulated and Experimental Opacities

Los Alamos and Livermore National Labs are engaged in a series of experiments on the National Ignition Facility to produce opacity data for iron and oxygen at conditions of 150 to 180 eV and 0.7 to 3.0 x 10²² electrons/cm³ to resolve a long-standing discrepancy between experimental data from the Z-machine and modern theoretical opacity calculations. The results could also impact models for the base of the Solar convection zone. Forward modeling of the experiment is important for characterizing backgrounds, interpreting the experimental results, and predicting the benefit of the time-gated spectroscopy over time-integrated.  Here we report on initial simulations of NIF opacity samples using the radiation-hydrodynamic code xRAGE  that have been post-processed with time dependence to determine how the desired spectral features of oxygen and iron compare with the background noise within the hohlraum.   

Progress Toward Oxygen Opacity Measurements at Solar Interior Conditions on the National Ignition Facility

The opacity campaign at the Lawrence Livermore National Laboratory (LLNL) National Ignition Facility (NIF) is in the process of measuring the soft x-ray opacity of oxygen at Solar interior conditions. These experiments are investigating a theorized solution to the Solar Problem, which, among other things, describes the discrepancy between the modeled and the helioseismically measured depth of the Solar convection zone boundary (CZB). The Solar Problem could be resolved if the theoretical Rosseland mean opacity near the CZB were increased by approximately 15 percent. Previous measurements on the Sandia National Laboratories (SNL) Z Facility (Z) show significantly higher opacity for iron than leading theoretical opacity codes near CZB conditions, accounting for approximately half of the discrepancy. The result prompted an opacity campaign at the NIF to verify the experimental results, as well as joint campaigns at the NIF and Z to measure oxygen opacity near CZB conditions. We present the current state of the data reduction and analysis for the NIF oxygen opacity campaign, and discuss future steps to ensure the resulting measurements are both accurate and precise.   

Progress towards absolute time-resolved opacity measurements at Z

Time-resolved spectroscopy using a novel hCMOS Ultra-fast X-ray Imager (UXI) is transforming stellar interior opacity measurements at the Sandia Z facility. Models for the Sun and stars are uncertain because opacity models are unable to reproduce previous iron opacity measurements at stellar interior conditions [Bailey et al. Nature (2015), Nagayama et al.  PRL (2019)]. The unprecedented novel time-resolved data help to resolve this dilemma in three important ways. First, one hypothesis for the opacity model-data discrepancy is that the temporal integration influenced the results. Time-resolved measurements of the backlighter history, sample evolution, together with calculated opacities at each time step allows film-based measurements to be synthesized. These tests show that the sample evolution cannot explain the reported discrepancy, unless opacity calculations are invalid. Second, measurements of the sample temperature and density evolution refine our understanding of the Z opacity platform and enable improved experimental design. Third, Sandia’s UXI technology enables measurements of iron opacities at multiple conditions from a single experiment. This increases the rate of learning, since more information is obtained to test the model predictions for trends in how opacity changes with plasma conditions. In this presentation, I will summarize the results on sample evolution in Fe experiments as well as progress towards the first extraction of absolute time-resolved opacity and remaining challenges to obtain that goal.   

Oxygen opacity experiments for stellar interiors at Z

Much of what we know about the universe is rooted in our understanding of the Sun. However, ongoing disagreement between solar models and helioseismic measurements of the interior structure of the Sun raises concerns about the accuracy of stellar models. One hypothesis that could help resolve this discrepancy is if the opacities of matter at solar interior conditions are higher than models predict. Experiments on the Z Machine investigate this by measuring iron and oxygen opacities at conditions near the solar convection zone base (CZB). The published iron measurements are higher than predictions, supporting this hypothesis. This talk will focus on the progress of the oxygen opacity experiments. Oxygen is the largest contributor to the opacity at the solar CZB and no experimental benchmark in this regime exists to date. We will discuss our methods for measuring the opacity and for characterizing the plasma and describe preliminary results.   

Characterizing the impact of material layers on Opacity-on-NIF Samples with Forward Modeling

Measurements taken during the Opacity-on-NIF experiment measure x-ray transmission, and thereby infer an opacity for a material of interest. Some samples for this experiment are built up in layers using a co-deposition method. Due to this fabrication method of the target samples, there can exist regions along the axis of the sample with significantly different material compositions, leading to a non-uniform opacity. Along with being the quantity of interest measured in the experiment, opacities also impact how energy is absorbed in the sample, causing temperature and density gradients in the sample during measurement time. This work details radiation-hydrodynamics simulations of target foils performed using the CASSIO code, and postprocessed using time-resolved methods to determine the impact that layered foils can have on both time-integrated and time-resolved opacity measurements.   

Experimental tests of astrophysical photoionized plasma models using the Z-machine at Sandia National Laboratories

The Z-machine at Sandia National Laboratories generates powerful X-ray radiation fluxes. This enables experiments to produce macroscopic plasmas at extreme conditions such as those found in accretion disk plasmas around black holes. Complex models for these non-Local-Thermodynamic-Equilibrium (non-LTE) plasmas remain mostly untested with laboratory data. We use a novel platform developed on the Z-machine to reach the same photon flux, density, and temperature conditions in black hole accretion disks. We will present model to data comparisons of the first ever high S/N iron L-shell x-ray emission spectra from a laboratory photoionized plasma. Such data have been a laboratory astrophysics goal for two decades but are even more critical now because of the “Super-Solar” iron abundance problem. Iron abundances inferred from x-ray spectra emitted by photoionized plasma around most black holes appear to contain 5-20 times more iron than the Sun. This contradicts the widely held expectation that most objects in the universe have the Sun’s composition. One prevailing theory is that effects of high electron density are not properly accounted for in the models. Reinterpreting the x-ray spectra with updated models resolved much of that discrepancy. However, a key question still remains: do photoionized plasma spectral models accurately account for x-ray emission? We will describe our progress in using this dataset to evaluate model accuracy and its potential to inform the super-solar iron abundance problem.   

Creating neutron star envelope matter on the Omega-60 laser

Neutron stars (NS) are some of the most extreme objects in the universe containing about 1-2 solar masses in a radius of about 10 km. The dimension and distance to these stars make it very difficult to observe the surface emission, of which there are only a handful of examples in the literature. The typical approach to modeling NSs, for simplicity, is to break them up into a solid crust and a liquid core with the neutron drip density setting the boundary between the two regions. The crust mediates the cooling of the NS, specifically the envelope, and theory predicts a relationship between the surface and boundary temperatures. However, the steepest region of the temperature gradient, where there is a boundary between radiation and degenerate electron dominated heat conduction, lies within the crust and is not observable. Laboratory experiments at these conditions can provide useful spectroscopic data to compare to models and inform the nature of emission from these sorts of plasmas to provide potential signatures for observatories.

 

The work presented here uses capsule implosions on the Omega-60 laser to create conditions relevant to boundary between radiation and degenerate electron heat conduction in NS envelopes. Simulations suggest a thin mid-Z metal layer on the interior surface of the capsule reaches extreme densities and temperatures of several 100s g/cc and over 1 keV. Filtered self-emission images show the evolution of the capsule metal and emission spectroscopy provide information on the conditions in the imploded metal and radiation transport.   

A Comparison of Plasma Conditions Inferred from Optical Thomson Scattering Measurements and X-ray Spectroscopy

The K-shell emission of low- and mid-Z elements are commonly used in HED plasma experiments to infer the plasma conditions. A study has been done utilizing simultaneous x-ray spectroscopy and Optical Thomson Scattering (OTS) measurements to compare the electron temperature inferred from the ion acoustic wave (IAW) feature and from fitting the measured x-ray spectra with atomic kinetics codes. A buried layer platform was used to create uniform non-local thermodynamic equilibrium plasmas (n_e ∼ few 10²¹ cm^-3, T_e ∼0.8 –- 1.2 keV) for this study. The target was a 250 μm diameter, 200 nm thick dot buried between two 1000 μm diameter, 5 μm thick beryllium foils. Lasers heat the target from both sides for 3ns. The density was inferred using both the electron plasma wave EPW feature as well as the size of the emitting volume measured with time resolved x-ray imaging. Two different target materials were used for the study: titanium and copper. The K-shell emission of the titanium and the K- and L-shell emission of the copper were measured time resolved. The comparison of the plasma conditions inferred from fitting the x-ray spectra with different atomic kinetic codes and the OTS measurements will be discussed.   

Development of a hybrid atomic model for application to the suite of numerical radiation magnetohydrodynamic and spectroscopic tools at NRL

We present on progress made to the development of a hybrid structure atomic model for application to the suite of NRL numerical radiation magnetohydrodynamic (RMHD) and spectroscopic codes. Current multi-physics simulations at NRL, such as MACH2-TCRE and DZAPP, employ atomic models with either limited detailed configuration structure or ground-to-ground approximations for two-electron processes which constrain the accuracy of equation-of-state calculations for RMHD modeling and predictive capabilities of high-fidelity spectroscopic modeling. The hybrid structure atomic model improves on the current implementation with a combination of detailed, fine-structure configurations for diagnostically-important resonance lines from the central charge states and compressed, configuration-averaged accounting for level structure and coupling completeness of satellite features from the neighboring, off-center charge states. Coupled collisional-radiative rate equations are solved for non-local thermodynamic equilibrium level populations and compared to alternative theoretical models by way of average ionization, desired equation-of-state parameters, and relative intensities of diagnostically-valuable Ar emission lines. The potential impact of a hybrid atomic model on RMHD simulations at NRL will be discussed in the context of future work.

 

This work is supported by the NRL Karle’s Fellowship.

 

Distribution Statement A: Approved for public release, distribution is unlimited   

Benchmarking of Hollow Fluorine Ion Modeling with Laser Produced Plasma Experiments

We continue our studies of exotic ions in dense plasmas with the major focus on KK hollow ions (with a completely empty K shell). Such exotic states manifest through the appearance of uncommon satellite lines (hypersatellites) that were originally observed in femtosecond laser produced plasmas. Since then, hollow ions have been found radiated from other dense plasma sources, but more benchmarking studies are needed to fully understand their relatively high intensities and applications. Here we present atomic data and modeling of radiation from KK hollow fluorine ions and benchmarking of line intensities of such hypersatellites with the experimental results. The experiments with Teflon foils were performed on the Leopard laser at UNR operating in femtosecond regime with different contrast. The major focus was on line radiation from KK hollow states of B- and C-like fluorine ions that produced the most intense hypersatellite spectral features in a spectral region 15.4-16.6 Å (between H- and He-like F fluorine resonance lines).  Future work is discussed.   

Probing ionization in warm dense copper using x-ray absorption spectroscopy

Warm dense matter exists at temperatures of order 10 eV and a few times solid density, where the complex balance of collective and quantum effects precludes standard approximations used in plasma physics or condensed matter physics. Understanding ionization in the warm dense matter regime, in particular, is an active area of research requiring additional experimental data to benchmark and improve current predictive capabilities. In this study, we present the experimental results and analysis of K-shell x-ray absorption spectra to infer temperature and ionization state distribution of copper uniformly heated to temperatures 10–30 eV and compressed to densities of 9–30 g/cm³. The experiments were conducted at the OMEGA laser facility using a buried layer of copper tamped by plastic on both sides, where the two sides were then irradiated by a symmetric laser drive and probed by a broadband x-ray source. Experimental results are compared to collisional-radiative models where we find large discrepancies in the predicted x-ray absorption spectra at these conditions. We also explore the role of including density effects in the underlying atomic data used in the collisional-radiative models and compare the results to our experimental data.   

Investigations of Multi-Ion and Kinetic Physics in Inertial Confinement Implosions with Mid-Z observer elements at OMEGA

The impacts of multi-ion and kinetic effects are not well-captured in models of inertial confinement implosions, leading to a degradation of observed yields as compared to fluid simulations. A series of experiments at OMEGA included mid-Z nitrogen gas fills in addition to D³He to accentuate the impact of multi-ion and kinetic effects for a range of capsule fill densities. The resulting nuclear and x-ray emission histories indicate the impact of the mid-Z elements on initial energy transfer to the ion species and subsequently to the electron population. Burn-averaged temperature shows a significant increase in the mid-Z gas fill case as compared to comparable capsule implosions with D³He fills only. The experimental data are compared to hydrodynamic simulations to understand the extent of the kinetic and muti-ion fluid effects coming into play during the shock-convergence and compression phases of the implosions.