Session GO04: HED Warm Dense Matter; EOS - Experiment

The Colliding Planar Shocks platform to study warm dense matter at the National Ignition Facility

We have developed an experimental platform to study warm dense matter at the National Ignition Facility that using colliding planar shocks to produce uniform plasma conditions and enable high-precision equation of state measurements [1]. The Colliding Planar Shocks (CPS) platform uses simultaneous x-ray Thomson scattering and x-ray radiography to measure the density, electron temperature, and ionization state at pressures approaching 100 Mbar. The CPS platform is designed to create a large volume of uniform plasma in the x-ray scattering volume, significantly improving the precision of the measurements necessary to test models for the equation of state and ionization potential depression in the warm dense matter regime. Here, we present the design of the CPS platform and compare hydrodynamic simulations to x-ray radiography and x-ray scattering data from initial experiments studying hydrocarbons.

 

[1] M. J. MacDonald et al., Phys. Plasmas 30, 062701 (2023).   

Using the Colliding Planar Shocks Platform at the National Ignition Facility for measuring the opacity of dense carbon plasmas

Opacity measurements in dense carbon plasmas play a vital role for understanding the radiative transfer, energy transport, and equation of state (EOS) of imploding inertial confinement fusion (ICF) capsules as well as many astrophysical phenomena. Widely used ionization models appear to underpredict ionization and thus overpredict opacity at high density conditions, even for low-Z materials. Accurate understanding of both the ionization and opacity of highly compressed carbon is crucial for improving predictive capabilities in high energy density (HED) applications including ICF and stockpile stewardship. Here we present an innovative experimental approach utilizing the Colliding Planar Shocks (CPS) platform [1] at the National Ignition Facility (NIF) for measuring the opacity of carbon at conditions where the K-shell is expected to ionize. Recent experiments using the CPS platform have demonstrated large volumes of highly compressed matter with minimal spatial gradients to make high precision measurements of materials under extreme conditions. Key variables such as the electron temperatures, electron densities, and ionization states can be extracted using simultaneous X-ray Thomson scattering (XRTS) and X-ray radiography. Initial experiments using the CPS platform have recently demonstrated the ability to compress solid CH and CH foams 3–8 times solid densities, as well as reach electron temperatures between 10-50 eV, and material pressures of 30–100 Mbar.   

Observation of density-driven line shifts at 100 TPa

The deviation of an atomic transition from its isolated-atom energy is a direct consequence of a change in the local environment around the atom. At the extreme pressures characteristic of stellar interiors and inertial fusion plasmas, transition energies shift due to electrostatic interactions between the atomic states and the dense-plasma environment. Understanding the mechanism that drives these line shifts, and particularly the dependence on the thermodynamic state, will enable new spectroscopic diagnostics of dense plasmas. We present time-resolved measurements of a blue-shifting absorption spectrum of a mid-Z tracer layer compressed to 100 TPa in a stagnated, laser-driven implosion. We map the increase in energy of the 1s--2p-type inner-shell transitions and relate this to the evolving thermodynamic conditions. Plausible mechanisms for the line shifts are discussed.   

Observing the onset of pressure-driven K-shell delocalization

We have developed an experimental platform for x-ray Thomson scattering (XRTS) at NIF to characterize plasma conditions in ICF indirectly-driven capsule implosions near stagnation [1,2]. This enabled us to investigate up to 30 times compressed ablator materials reaching pressures above 3 Gigabars, at conditions where the distance between the nuclei becomes comparable to the extent of the core shell bound states, which will eventually lead to their pressure ionization. In this talk we will present results from experiments with beryllium shells. We observe reduced elastic scattering for the most extreme conditions [2]. We interpret this reduction as the precursor of pressure ionization of the remaining K-shell electrons, that is, a strongly modified bound state. The beryllium charge state inferred from the data is considerable higher than standard models predict but agrees well with results from DFT simulations [2,3]. Accurate modelling of the K-shell occupation of light elements is imperative for creating predictive capabilities for ICF implosions. Our experiments yield valuable benchmarks for this process and demonstrating a complex pathway of pressure ionization.   

Observation of anomalous heating of compressed materials by adjacent diamond layers using EXAFS measurements

Large laser facilities have recently enabled compression of materials above TPa pressures. However, measuring the temperature of the compressed materials remains a challenging problem. We are developing a platform on NIF using EXAFS (Extended X-ray Absorption Fine Structure) to probe the bulk temperature of highly compressed materials. EXAFS refers to the oscillatory modulations in x-ray absorption spectra above an absorption edge, generated by interference between photoelectron waves and scattering by neighbor atoms. EXAFS is sensitive to temperature of materials because ionic thermal motion reduces the coherence of the interference, leading to decay of the modulations. Excellent EXAFS data have been obtained for Cu K-edge up to 10 Mbar. It is observed unexpectedly that the copper temperature is much higher than predicted when adjacent to diamond layer(s), showing the importance of heat transport on the thermal state of materials (Sio et al, submitted). The follow-up experiments and the prospect of such a new capability at NIF for probing thermal states of compressed materials will be discussed.   

Experimentally Measuring Thermal Conductivity in Warm Dense Tungsten Using Fresnel Diffractive Radiography

Transport properties in warm dense matter (WDM), such as thermal conductivity, have extensive theoretical predictions but lack experimental benchmarking [1]. We have developed a Fresnel Diffractive Radiography (FDR) platform at the Omega Laser Facility, which enables high spatial resolution measurements of the evolution of an isochorically-heated WDM interface [2-4]. Novel 1 µm-wide slits provide a spatially coherent X-ray source that, in the presence of sharp density gradients, result in distinct diffraction fringes. Isochoric X-ray heating of CH-coated metal wires sets up a temperature differential at the material interface. After pressure equilibration, the interface is hydrodynamically-stable, and the evolution of the interface is driven primarily through thermal conduction, which modifies the temperature and density profiles. We present results for the thermal conductivity of warm dense tungsten, obtained through analysis of the evolving diffraction pattern.

1) T. G. White et al. PTRSA 381, 20220223 (2023).

2) C. H. Allen et al. AO 61, 1987 (2022).

3) M. Oliver et al. RSI 93, 093502 (2022).

4) M. O. Schoelmerich et al. RSI 94, 013104 (2023).   

Electron-Ion Equilibration Rates Across the Solid-Liquid Phase Boundary in Warm Dense Gold

When a high-intensity laser is incident on a solid target, a highly non-equilibrium state is created through the process of preferential and rapid heating of one subsystem over the other^1;2. These transient, high-energy-density plasmas act as a precursor to warm dense matter (WDM) and serve as a testbed where we can validate quantum mechanical theories for electron-ion interactions. We have implemented a high-resolution (∼50meV) X-ray scattering platform³, designed for use with free-electron lasers, with a resolution capable of measuring changes to the quasi-elastic Rayleigh peak. The peak’s width is a direct measurement of the ions’ velocity distribution, essentially governed by Doppler broadening, which corresponds to a model-independent ion temperature measurement of the plasma. For a metallic thin gold film, we have measured the temporal evolution of the ion temperature over the first ∼20 ps after irradiation; in this time the ions are rapidly heated to electronvolt temperatures. The ion’s temperature evolution is used to determine the electron-ion equilibration in this regime. We will discuss the phase dependent nature of the equilibration rate and the unique behavior around the solid-liquid phase boundary.   

Initial high repetition rate compression experiments using the DIPOLE Laser at the European XFEL

The generation of high-energy-density matter by compression via nanosecond laser-ablation, with subsequent x-ray probing with FEL radiation, is a well established technique. However, hitherto, data collection rates have been limited to many minutes by the repetition rates of the optical drivers.  We report here on the first user experiment using the 100-J class DIPOLE laser at the HED instrument of the European FEL, where x-ray diffraction data from shocked materials was obtained at rates up to 1 Hz, along with VISAR data at comparative rates.  Illustrative examples of data for a variety of materials will be presented, including shocking matter into the warm dense liquid state.   

Characterization of Non-Thermal Phase Transitions in Ionic Compounds with Two-color X-ray Pulses.

High resolution crystallography has benefited from the availability of x-ray Free Electron Lasers (FEL). It has been possible to resolve hydrogen atoms and water molecules. [2] Intense x-ray FEL pulses interact with samples changing their electronic and atomic structure. To date, the experiments studying the x-ray FEL-matter interaction have predominantly examined semiconductors, such as diamond and silicon [4-5]. There is little known about how x-ray induced bond breaking occurs in a solid with more than one element or in a solid with ionic bonding[HP1] . A recent calculation has predicted a crystalline to disordered phase transition in the case of the high-intensity x-ray interaction with sodium chloride, an ionic solid [6]. With FEL x-ray pump and x-ray probe pulses, non-thermal phase transitions are predicted and new material phases can be detected. We have investigated the time-dependent intensity of diffraction peaks in sodium chloride (NaCl) and magnesium oxide (MgO). We will discuss the observed ultrafast responses of the materials through analysis of the observed diffraction peak intensities.   

High-pressure phase transformations in ramp-compressed SiO2

SiO₂ is one of the most widely studied materials at high-energy-density conditions because of its use as a standard in shock experiments and because of its geophysical importance. SiO₂ is considered an archetype for the silicates that dominate terrestrial mantles so measuring its structure at the relevant pressures and temperatures is important to understanding rocky exoplanets. In this work, we quasi-isentropically (ramp) compressed SiO₂ to ~400 GPa and probed its phase using in situ x-ray diffraction at the Omega laser facility. Both fused silica and novaculite, a microcrystalline quartz, were studied.   

Comparison of absolute Hugoniots in structured and unstructured CH foams

Many advanced ICF target designs utilize low density CH to increase ablation velocity and reduce instability growth. As such, precise determination of the CH foam Hugoniots is important for target design and modeling. Shocks driven at foam densities (50 to 150 mg/cm3) and Mbar pressures require piston velocities above the limits of explosively driven experiments, necessitating laser drivers. The Nike laser has been utilized to perform absolute Hugoniot measurements using structured (2PP) and unstructured (DvB) foams with

characteristic sizes varying from 1 to 50 μm. Nike produces shock pressures up to 9 Mbar with a highly uniform, time averaged, 400 μm flat-top beam profile, providing a unique platform for low-density equation-of-state experiments. Measurements were made using a high resolution monochromatic x-ray streaked imager which allowed independent measurement of the shock and piston velocities. Our experiments covered the shocked particle velocity range from 25 to 90 km/s and included inflight control of steadiness of the measured shock and piston velocities over the shock travel distance 400 μm. Comparisons are made between the various experimental results as well as numerical simulations using CALEOS in the NRL FASTRAD3D hydrocode. Work supported by US DoE/NNSA.   

Temperature and conductivity in shocked bridgmanite (MgSiO3) to 1.6 TPa

With the recent observation of an Earth-like rocky exoplanet by the James Webb Space Telescope, the need to characterize the interior components of planets becomes more urgent. To accurately model rocky exoplanets, the equation of state for planetary constituents must be constrained at pressures and temperatures relevant to mantles and cores. This work reports temperature and reflectivity measurements of shock compressed bridgmanite from 4,000 to 60,000 K, and these data are used to infer the melting curve and conductivity. Above 400 GPa, iron has a higher melting temperature than MgSiO3, implying that the mantle of rocky exoplanets with mass <2 M_E may solidify before the core. The conductivity of shocked bridgmanite rises significantly after melt to 2,200 Ω cm^-1. This suggests that a molten magma ocean may be conductive and produce a magnetic field.   

Compressibility, Structure, and Melting of Platinum to 500 GPa

Platinum is used as a pressure standard in both static and dynamic compression experiments, due in part to the stability of the face-centered cubic (fcc) phase over wide pressure–temperature states. To address discrepancies in the reported melting curves as well as a predicted phase transformation between 35 and 300 GPa [1], x-ray diffraction and optical pyrometry characterized shock and shock-ramped platinum. The fcc phase was measured for initial shocks between 83 and 200 GPa and subsequent compression up to 500 GPa. On the Hugoniot, the fcc phase remained stable upon compression until liquid diffraction was observed. From these results, a melt curve is constructed incorporating previous melt measurements in static and dynamic compression experiments. Optical pyrometry provides a lower bound on the Hugoniot melting temperature. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.

 

[1] L. Burakovsky, S. P. Chen, D. L. Preston, and D. G. Sheppard, J. Phys.: Conf. Ser. 500, 162001 (2014).   

Shock compression of H-Ne mixtures at gas giant interior conditions

Ne has been observed to be strongly depleted in the atmospheres of gas giant planets relative to bulk solar compositions [1]. A proposed mechanism for this depletion is the phase separation of Ne from H, the main component of gas giant atmospheres [2]. While recent work shows that H and He phase-separate at conditions present in the outer layers of Jupiter and Saturn [3], the miscibility of Ne with other planetary components at similar conditions has yet to be experimentally constrained. We combined static and dynamic compression to investigate the equation of state and reflectivity (an indicator of miscibility) of H-Ne (20% mol Ne) at the conditions of gas giant interiors. The novel band gap behavior or Ne, whereby it is predicted to remain insulating into the warm dense regime [4], could have a bearing on these results. Contrary to expectation, the data indicate mixing of H and Ne at temperatures up to 20,000 K and pressures up to 150 GPa. Follow-up experiments will utilize different mixing ratios and probe a wider range of pressure-temperature conditions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.

[1] Niemann et al., Science 1996

[2] Wilson and Militzer, Phys. Rev. Lett., 2010

[3] Brygoo et al., Nature, 2021

[4] Tang, J., Chem. Phys., 2019