Session YO03: HED Laboratory Astrophysics

Progress in understanding stellar interior opacity with laboratory experiments at Z

Discrepancies between the Standard Solar Model and helioseismology identified about 2 decades ago have yet to be resolved. Revising models for stellar matter opacity could be a key part of the explanation and laboratory experiments are underway to evaluate this possibility. Published data indicates that models underpredict iron opacity at stellar interior conditions but understanding why this is so remains elusive. Here we provide a summary of the effort centered on experiments at Z, including motivation, experiment methodology, expanded temperature/density regimes, measurements with multiple elements, and future directions.   

Time-resolved spectroscopy to advance opacity effort on Z

Opacity at solar interior conditions has been measured on Z and was found to be higher than predictions. This finding helps resolve the long-standing solar problem although no opacity-model revisions have been found to date. Sandia developed an ultrafast x-ray imager (UXI) that allows time-resolved absorption spectroscopy for the first time. Prior opacity data recorded on x-ray film had duration given by the 3-ns backlighter. One hypothesis for the opacity model-data discrepancy is that the temporal integration influenced the results. Time-resolved conditions have been constrained now and synthetic tests of temporal integration effect on past measurements did not resolve the source of the opacity discrepancy. But only actual time-resolved measurements would provide a model-free evaluation of this effect. So, the focus of the effort is now on measuring absolute time-resolved opacity. Besides, we will also discuss the first results from experiments designed to take advantage of newly acquired time-resolved knowledge to tailor the opacity experimental conditions. The strategy and prospects for obtaining multiple opacity measurements from a single Z experiment will be discussed.   

Oxygen opacity experiments for stellar interiors at Z

Much of our knowledge of the universe stems from 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 resolve this discrepancy is if the opacities of matter at solar interior conditions are higher than models predict. Experiments on the Z Machine have been investigating this by measuring the opacity of iron and oxygen at conditions near the solar convection zone base (CZB). From these, iron has shown a discrepancy between experiment and models at these conditions that has yet to be resolved. 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 the methods for measuring the opacity and for characterizing the plasma and what has been found.   

Platform-dependent advantages and challenges of laboratory stellar-opacity measurements

The accuracy of iron and oxygen opacity calculations critically affects solar/stellar modeling and numerous high-energy-density (HED) plasma simulations. The absence of benchmark opacity experiments introduces unknown uncertainty into these plasma simulations. The first iron opacity measurement at solar interior temperatures, performed at the Z-machine at Sandia National Laboratories, uncovered a greater-than-expected discrepancy between measured and modeled iron opacities at Te = 170-195 eV, ne = 1e22-4e22 e/cc. This can help resolve discrepancies between solar models and observations, but it also incited controversy within the HED community. An alternative experimental method developed at the National Ignition Facility (NIF) serves to independently examine these findings. The Z-machine employs an area backlight approach, while NIF uses point-projection. These methods offer unique benefits and challenges because of platform specificity. This talk discusses how these approaches complement each other and how this national collaboration will help provide unbiased constraints on the accuracy of iron and oxygen opacity.

Sandia National Laboratories is a multimission laboratory managed and operated by NTESS LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. DOE’s NNSA under contract DE-NA0003525.   

Constraining Hypotheses for Apparently High X-Ray Opacities on the National Ignition Facility

There is little spectrally resolved experimental opacity data of plasma at temperatures and densities relevant to the physics of the sun, other stars, and laboratory high-energy-density experiments. Prior and ongoing opacity experiments on the Sandia National Laboratories Z machine have shown up to factor-of-2 discrepancies with theory - a challenging puzzle.  Since 2015, and additional set of experiments on the NIF have begun measuring opacities of iron and oxygen at temperatures ~150 eV, and since 2021 those measurements have been extended to densities above 10²² electrons/cm3. The higher-density NIF data show typically higher opacities than expected theoretically. This talk summarizes the current NIF experiments and efforts to constrain various hypotheses for potential systematic errors. This data may have implications for modeling the structure of the sun and for determining the age of white dwarf stars.   

Spectral Modeling of Opacity-on-NIF experiments

The Opacity-on-NIF project has the goal of verifying the higher-than-expected iron opacity measurements done on the Z-machine. We describe our procedure for post-processing xRage radiation-hydrodynamics simulation results to create synthetic spectra for comparison to data collected by the spectrometer. These simulations are post-processed with the backlighter shining through the sample (or the hohlraum without a sample) to create synthetic spectra that include contributions from the sample and CH tamper, the tamper by itself, and the background that includes sample self-emission. We show select synthetic spectra for several different electron temperatures and electron densities for iron, compare them to the equivalent data and comment on the quality of the comparison.   

Platform development for Xflows: Exposé of Radiation flows on NIF

The goal of the Xflows¹ campaign is to bring the successful COAX² radflow platform from OMEGA to NIF to enable studies of radiation flow in supersonic, transonic, and subsonic phases with at least two foam types. The COAX platform^2-4 on OMEGA used same-shot spectroscopy and radiography to characterize the evolution of the radiation flow and developed shock, providing constraining data to radiation hydrodynamic simulations^3,4. The first Xflows shot day fired a transonic halfraum and the spectroscopic (capsule) and radiography (foil) backlighters separately. In this presentation we will discuss plans to complete the development of the target in 2024 based on the data from the first shot day. The first complete target physics shot will feature transonic flow driven into a COAX-like Ti-doped foam¹, with spectra collected to study radflow soon after firing the halfraum and radiography of the shock collected at later time. Finally, we will briefly overview plans to adapt the LLNL MSPEC-N spectrometer to the needs of this experimental campaign, with its first test planned for the final shot day in 2024.    

Design of halfraums for x-ray flow experiments on the NIF

We present a computational study to design and characterize halfraums for the XFLOWS x-ray flow experiments on the NIF. The propagation of radiative heat fronts in High Energy Density experiments has long been a challenging topic to study. Experimental efforts in the past three decades have had success creating relevant data in this regime. However, it has been a challenge for multi-physics simulation tools to accurately and unconditionally predict the behavior of these experiments, a task which is essential to furthering our understanding of Inertial Confinement Fusion and astrophysical processes. The COAX campaign introduced a novel spectroscopic diagnostic to study radiation flows on Omega-60 by measuring temperature and charge state across the heat front [1,2,3]. XFLOWS is the successor to COAX, and is fielded at NIF to access higher temperature regimes with stronger shocks and supersonic flows [4]. There is a need to design and characterize the halfraums that are used to drive these radiative heat waves in order to access the desired hydrodynamic and radiative regimes. Only by accurately characterizing the driving radiation field from the halfraum can we make substantive, quantitative predictions of the resulting flow of energy through the foam.

[1] Johns et al., HEDP 39, 100939, (2021)

[2] Fryer et al., HEDP 35, 100738, (2020)

[3] Coffing et al., POP 29, 8, (2022)

[4] Johns et al., RSI 94, 023502, (2023)   

Spectroscopically Characterizing Radiation Flow in Stochastic Media

Precise characterization of radiation flow experiments is required to distinguish among transport models. Cassio radiation-hydrodynamics simulations are used to design experimental platforms at the OMEGA Laser Facility that can measure the radiation flow using the spatially-resolved COAX absorption spectroscopy diagnostic. The simulations model radiation flow through stochastic media foam configurations containing optically thick clumps that are heterogeneously dispersed within an optically thin background aerogel. The radiation flow can be characterized within inhomogeneous configurations (specific initial conditions of a stochastic media), within homogeneous foams containing the same amount of component particles as the inhomogeneous configurations, and within the background-only material. Spectroscopy measurements are obtained of both the optically thick clumps and optically thin background, simultaneously.   

Capsule backlighter optimization for the OMEGA opacity experiments

Our study utilizes Kr gas-filled capsules to examine radiation flows in the COAX, Radishock, and XFOL experiments. To determine the spatial temperature profile of the radiation flow, which is induced by a high-energy laser drive through low-density foam, we leverage X-ray absorption spectroscopy with these gas-filled backlighters. This presentation will highlight our simulation results, particularly those centered around the Kr gas-filled capsule used in the XFOL experiments. To carry out these simulations, we utilized xRAGE, a radiation hydrodynamics adaptive mesh refinement code, in conjunction with a multigroup diffusion package. A total of 24 laser beams with a combined power of 10kJ were used to drive the capsule implosion. The opacity tables were generated using the TOPS code, and for the equation of state the SESAME library was utilized. The results were subsequently post-processed with SPECT3D and the OPLIB opacity tables, generating time-resolved spectra and angular flux. These are then compared with experimental results, contributing to our aim of enhancing our understanding and the reliability of such backlighters in terms of the timing jitter and flash intensity.   

Measuring Initial X-Ray Flux from a Halfraum for Radiation Flow Studies Using Dante and FIDUCIA

Radiation flows in stochastic media are not well understood from a radiation-hydrodynamics–simulation perspective. A study known as the X-Ray Flow Over Lumps Campaign seeks to validate radiation-hydrodynamics models by studying the radiation flow from a halfraum through doped foams with various sizes of inclusions.1 The inclusions change the absorption spectra of the foam as the radiation propagates from the halfraum; as a result, the changes to the absorption depend heavily on the physics assumptions used in the model. The first critical step to the measurements and validation of these models is to characterize the initial flux from the halfraum with a high degree of certainty. FIDUCIA, a method for inferring x-ray flux from the halfraum drive using cubic splines, generates a broadband picture of the initial flux with high precision.2 The x-ray flux is determined using monotonicity-conserving interpolation and a χ2 global minimization solver to produce only physically valid solutions. This new interpolation method reduces uncertainty in the x-ray flux measurement as validated by a Monte Carlo simulation.   

Radiative shocks in strongly coupled plasmas on the Omega-60 laser

Compact objects exist in some of the most extreme conditions of temperature and density in the universe. Neutron stars, an example of a compact object, are quite difficult to observe due to their small size and birth within clouds of gas after supernova explosions. Neutron star envelopes exist as a strongly coupled plasma of predominantly iron. These extreme conditions stress models of radiation hydrodynamics and radiation transport because it breaks the assumptions of weakly coupled plasma theory. The work presented here shows simulation and preliminary results from experiments at the Omega-60 laser at the Laboratory for Laser Energetics to study radiative shocks in strongly coupled plasmas at temperature and density conditions relevant to neutron star envelopes. These experiments use a capsule implosion with a 7 μm plastic layer and a layer of mid-Z metal on the interior surface. The capsule mass is constant through varying the metal layer thicknesses for different materials and different metals have different nuclear charges, which vary the ion coupling parameter as Z². Filtered x-ray framing camera measurements observe the K-shell emission of the metal layer and track the velocity of the inflowing plasma and rebounding shock. X-ray spectroscopy measurements provide information on the ionization states present and the effects of radiation transport in the system.   

A design of a Laser-Driven Experiment Scaled to Supernova Remnants

When a supernova erupts, the ejecta resulting from the explosion of the progenitor star expands into the surrounding circumstellar medium (CSM), forming three key elements: 1) a forward shock in the CSM, 2) a reverse shock in the ejecta, and 3) a contact discontinuity between the ejecta and the CSM. This contact discontinuity is unstable to both the Richtmyer-Meshkov instability, caused by the generation of shocks, and the Rayleigh-Taylor instability due to the higher pressure on the CSM side and lower density compared to the ejecta side. Understanding the evolution of this hydrodynamic instability is crucial for comprehending the formation of supernova remnants (SNRs).

 

We present a scaled experimental design for studying SNRs. Our design utilizes a graded-density layer to simulate the CSM. The dense side of this layer interfaces with an ablator, while the light side interfaces with a plastic layer, simulating the ejecta. This configuration effectively captures the key elements. By comparing rad-hydro simulations of the proposed experiments with the common self-similar model for SNRs, we observed good scaling between them in terms of 1D dynamics. Furthermore, we conducted 2D simulations of the proposed experiment, demonstrating that the evolution of the instability can be accurately measured within the limitations of a laser facility (OMEGA-EP). This experiment can pave the way for a platform to study SNRs across various astrophysical observations and different physical regimes.   

Laboratory Generated Heat Fronts Relevant to the Epoch of Reionization

Radiation-hydrodynamics is an active area in research across many fields such as High Energy Density Physics, Inertial Confinement Fusion, and Astrophysics. One phenomenon in radiation-hydrodynamics are photoionization fronts (PI Fronts or Ionization Fronts), which are a not well understood type of radiation-driven ionization wave. In astrophysical systems, PI fronts were formed during the Epoch of Reionization (EoR) when black holes and quasars emitted ionizing radiation into the neutral intergalactic medium (IGM). As the PI fronts propagated, they turned the neutral IGM into a cosmic ionizing background. James Webb Space Telescope (JWST) has a primary goal of measuring the EoR by surveying high red shift galaxies (z = 6-15) from this time. Stronger understanding of PI fronts can help enhance the understanding of high red shift galactic observations from JWST. In this effort, a platform has been developed to generate and measure PI fronts in the laboratory as part of the Z Astrophysical Plasma Properties Collaboration on the Z-Machine at Sandia National Laboratory. For the laboratory PI Front experiments, a radiation-driven ionization wave is driven into a nitrogen gas cell using a tungsten wire array as an x-ray source. Measurements of the front velocity have been made using photon-doppler velocimetry for a variety of gas pressures. Atomic kinetic calculations done in PrismSPECT are used to further understand the type of radiation front generated. Future work will include the verification of the radiation-driven ionization front as a PI front.   

Characterization of Space and Time Resolved Electron Density in the Photoionized Plasma Gas Cell Experiment at Z

The photoionized plasma gas cell experiment is an established platform to make at-parameter (ξ>>1 ergs cm s^-1) measurements of plasma properties for benchmarking astrophysical modeling codes. The experiments are performed on the Z Machine at Sandia National Labs where a cell filled with neon gas is driven by the broadband x-ray flux produced by a wire array z-pinch implosion. The recent fielding of the photonic Doppler velocimetry (PDV) diagnostic for measuring space- and time-resolved electron density¹ has confirmed the existence of a hydrodynamically unperturbed photoionized plasma that is driven only by radiation². We model the experiment with the 1D radiation hydrodynamics code HELIOS-CR and present a comparison of experimental and simulation electron density time histories to assess the qualitative and quantitative accuracy of the simulation predictions. The results suggest that the experiments qualitatively agree with the simulations, although there are discrepancies in the time evolution and spatial distribution of the electron density. ¹K. J. Swanson et al RSI 93, 043502 (2022), ²G.S. Jaar et al HEDP (submitted, 2023).