Session JO09: High Energy Density Science: Materials

Shock-Release Studies in Spherical Geometry

Direct-drive inertial confinement fusion implosions employ shocks to set up favorable compressibility and stability for optimal fusion yield and areal density. When these shocks traverse material interfaces, a process known as rarefaction causes the release of material upstream of the shock. This release happens multiple times in multiple locations in a typical high-performance cryogenic implosion—in particular, at the fuel/vapor interface where excess deuterium–tritium fuel is released into the center of the target, thereby setting the initial density and temperature conditions of the compression. Therefore, understanding rarefaction is critical in understanding fusion performance. Dedicated experiments to study the release of material in spherical geometry have been performed to directly measure the kinetic energy in the release. Comparisons of these experiments with simulation predictions and synthetic diagnostics will be presented. The physics implications of these comparisons for accurately modeling rarefaction will also be briefly discussed.   

Shock Release Physics at CH-DT Interface by Molecular Dynamics Simulations

Shock release commonly occurs in Inertial Confinement Fusion (ICF) and High Energy Density (HED) experiments, but it is challenging to model with hydrodynamic simulations. One important aspect of these processes is species separation, where lighter elements stream ahead of heavier ones. This phenomenon has been observed in CH polystyrene under strong shocks and during release, as revealed by large-scale molecular dynamics (MD) simulations (Zhang and Hu, Phys. Rev. Lett. 125, 105001 (2020)). These predictions were further shown to be consistent with experiments (Zhang et al., Phys. Rev. Research 4, 013126 (2022)). However, the significance of this physics in real multi-layer target systems and its impact on target performance in experiments remains unclear. In this talk, I will present extended MD simulations of CH-DT ice interfaces under single and double shocks and show results of shock release into a vacuum or DT gas. We will discuss the implications of these results when scaled to realistically sized targets, and how they compare with hydrodynamic simulations and newly conducted/planned experiments.   

EOS Measurements Using Refractive Enhanced Radiography of Cylindrically Converging Shock Waves*

We have developed an experimental platform for streaked refractive enhanced radiography (RER) [1] of cylindrically converging shock waves at the National Ignition Facility (NIF) for equation of state (EOS) measurements along the principal Hugoniot for low-Z elements with reduced bound-free absorption contrast. In addition to leveraging refraction enhancement to infer shock compression, RER enables high precision shock trajectory measurements including the rebound shock.

The shock waves are indirectly driven in 6.72 mm diameter gold hohlraums using a radiation drive adapted from the Gbar platform [2], generated with a 4 ns long square-shaped, 0.5 MJ laser drive. Due to convergence effects, the shock waves increase in strength and allow to probe a range of shock pressures (50 – 150 Mbar) in a given experiment. The first experiments used plastic and LiH cylinder samples with up to 1.4 mm diameter. We will report on first results from experiments at the Omega and NIF laser facilities.

 

[1] D.S. Montgomery, Rev. Scient. Instrum. 94, 021103 (2023).

[2] T. Doeppner et al., Phys. Rev. Lett. 121, 025001 (2018).   

Radiation Drive Designed to Extend the Pressure Ranges Measured inGbar Equation of State Experiments at the National Ignition Facility

We present the design and demonstration of a new radiation temperature drive to increase the

pressure range measured using the existing Gbar experimental platform at the National Ignition

Facility. Using the same peak radiation temperature as data previously published spanning 25–60

Mbar in polystyrene [1], post-shot hydrodynamic simulations suggest the new drive can measure 20–

200 Mbar with the same target design. The drive starts with a low radiation temperature that launches

a weak shock into the sample and is followed by a continuous increase in radiation temperature to

strengthen the leading shock. The additional shock strengthening elevates the pressures within the

sample beyond what is achievable by convergence alone. Design features of the new drive and

accompanying radiation hydrodynamics simulations are used to illustrate the method in which the

pressure range was increased. The design of this drive can be used to significantly increase the

amount of equation of state data collected on each experiment.

[1] T. Döppner et al, Phys. Rev. Lett. 121, 025001 (2018).   

X-ray-Generated Impulses in Cylindrical Samples at the National Ignition Facility

Experiments that study x-ray-generated impulses provide important insights into how materials respond in extreme environments. In such an experiment, high fluence x-rays deposit energy into a sample, causing rapid heating and subsequent expansion of vaporized material on the sample surface. The rapid material expansion generates a shock wave into the remaining cold material, which propagates through and can cause material damage upon transit or upon reflection from the sample rear surface. As the development of high-intensity x-ray line-emission sources advances[1], experiments on such x-ray-induced impulses in centimeter-scale coupon samples have been underway on the National Ignition Facility (NIF)[2]. In this talk, we present results from a novel extension to the x-ray effects platform that measures for the first time the effects of NIF x-rays on large-scale 10 cm cylindrical samples. We identify how the prompt impulse varies as a function of x-ray angle of incidence in the NIF target geometry. The development of this platform and results from initial experiments pave the way for future analysis on x-ray delivered impulses to more complex geometry samples. 

 

[1] M. J. May et al., Phys. Plasmas 26, 063105 (2019).

[2] D. Goto and S. J. Moon, LLNL-TR-817143 (2020).   

Effects of spectral variability on material response in x-ray-generated impulse experiments at the National Ignition Facility

Thermomechanical shock experiments aim to study the response of materials in extreme conditions. We use the National Ignition Facility (NIF) to generate high fluence x-ray atomic line emission sources, which irradiate material samples of interest. Under sufficiently high energy deposition, thermomechanical shocks are generated in the materials. While it is known that the characteristics of x-ray generated impulses vary as a function of incident x-ray spectra, unknown is how possible shot-to-shot spectral variations and uncertainties in NIF spectral reconstructions affect our understanding of the material response to impulsive loading. In this presentation, we show results of 1D radiation hydrodynamics simulations of x-ray induced thermomechanical shocks to investigate the effects of spectral variability in NIF line-emission sources on aluminum and titanium alloy samples. We compare the experimental results from NIF experiments to the simulated material response for several different cases of spectral reconstruction. We find that the material response is highly sensitive to assumptions made about the spectral contents and that knowledge of spectral uncertainties bounds our understanding of the resulting material response. The results of this effort help to extend our ability to extract quantitative material properties from x-ray experiments on NIF, which study material properties at the extremes of current laboratory capabilities.   

Investigating the Melt Curve of Sapphire Using Double Shock Compression

Sapphire is a commonly used window material for dynamic compression experiments [1] and plays a role in solid solution with the (Mg,Fe)SiO3 system in Earth’s lower mantle [2]. Therefore, it is important to understand its equation of state, optical properties, and melt curve.  By using a dynamic precompression platform, we can benchmark the equation of state of sapphire off its principal Hugoniot; namely along the melt curve at pressures nearly two time higher than using a single shock.  We present measurements on the sapphire melt curve above 1 TPa which show signatures of latent heat of melting, and present changes in the reflectivity across phase transitions in the doubly shocked sapphire.

[1] Z.-G. Li et. al., Physical Review B 98, 064101 (2018)

[2] S. Gréaux et. al., Geophysical Research Letters 43, 4239 (2016)   

Benchmarking of the D2 EOS with Double-shock Compression

We present data from recent D₂ experiments where off-Hugoniot (double-shock) states were measured directly and characterized. We deduce mechanical, thermal, and transport, properties of the double-shocked material to pressures of several Mbar with reflectivity, shock velocity, and temperature data obtained from velocity interferometry and optical pyrometry. We conclude with a comparison of our findings to recent experimental work by Fernandez-Panella et al. (2019) and theoretical work by Rygg et al. (2023).   

Development of a Nanosecond Raman Spectroscopy Platform to Diagnose Temperature and Chemistry in HED Matter

High-energy-density (HED) conditions are characterized by pressures exceeding ~100 GPa, where the pressure-volume work associated compression is comparable to the energies of chemical bonds. In this regime, exotic and unexplored chemistry occurs as materials behave unexpectedly compared to their nature at ambient conditions. Raman spectroscopy is a powerful tool for diagnosing pressure-induced changes in chemical bonding (e.g., bond strengthening, dissociation) in HED matter, but is a challenge for laser-driven experiments stemming from the short nanosecond time scales and small submillimeter sample sizes. We discuss plans to overcome these challenges and to develop a nanosecond UV (266 nm) Raman spectroscopy diagnostic for OMEGA to study chemistry and temperature of dynamically compressed matter.   

Extended X-ray Absorption Fine Structure Temperature Analysis at High-Energy-Density Conditions

A critical next step in understanding high-energy-density (HED) matter is to characterize the temperature of materials at HED conditions. Temperature measurements are historically difficult at low temperature HED conditions (above 100 GPa and below 5000 K) and extended x-ray absorption fine structure (EXAFS) spectroscopy is one of the few experimental techniques capable of constraining temperature at these conditions by measuring the variation in the distances between neighboring atoms. A variety of synthetic and experimental EXAFS spectra of iron, nickel and invar (Fe₆₄Ni₃₆) were analyzed with a Bayesian inference routine to infer the atom positions using both a cumulant expansion and a parametrized ion-distribution model. The parametrized ion-distribution model was found to improve the constraint on the true ion distribution for the synthetic asymmetric systems. These ion distributions were ultimately related to the temperature of the compressed sample using a variety of models that will be discussed herein.   

XAFS+SOP experiments provide a pathway towards thermal conductivity measurements at HED conditions

Directly measuring temperature as well as thermal conductivity at HED conditions is challenging. At the Laboratory for Laser Energetics (LLE), a Streaked Optical Pyrometer (SOP) measures the self-emission of heated samples above 3600 K and the X-ray Absorption Fine Structure (XAFS) spectroscopy platform measures simultaneous temperature, density and pressure. SOP and XAFS data of the Fe K-edge were collected at two times, 10 ns apart, within in a constant pressure hold and a clear temperature increase was observed. This data is used to estimate the thermal conductivity of Fe near 200 GPa.   

Probing thermal transport in dynamically-compressed materials using Cu Extended X-ray Absorption Fine Structure

While determining temperature in laser-driven material science platform remain challenging and is a major source of uncertainty in equation-of-state experiments, recent experiments using Extended X-ray Absorption Fine Structure (EXAFS) as a temperature diagnostic have shown great promise and also provided new insights into thermal transport in planar dynamic-compression targets. We discuss in this work the temperature of Cu dynamically compressed to 400 GPa determined using EXAFS at the National Ignition Facility, and how this temperature varies depending on the material adjacent to the Cu. The observed sensitivity to Cu thickness and the significantly higher temperature in Cu when adjacent to diamond suggest that thermal conduction plays an important role in the measured Cu temperature over the experiment timescale, and that the diamond temperature is higher than predicted by radiation-hydrodynamic simulations.   

EXAFS at L-edges for temperature measurements toward higher-Z materials at NIF

EXAFS (Extended X-ray Absorption Fine Structure) 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 in the range of 100-10000K because ionic thermal motion reduces the coherence of the interference, leading to decay of the modulations. The EXAFS measurements at L-edge are more challenging due to smaller EXAFS amplitude. The high x-ray flux at NIF has enabled L-edge measurements for higher-Z materials such as Ta at 10keV and Pb at 13keV. This talk will report recent progress in L-edge EXAFS measurements at NIF and platform development to extend the spectral range to 18keV. The good quality of EXAFS data at Ta L-edge made it possible to compare the resulting Ta temperatures from ramp and shock-ramp drive with various strength models. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Characterization of Non-Thermal Phase Transitions in MgO and NaCl with Two-color X-ray Pulses

With x-ray Free Electron Lasers (FEL), intense x-ray FEL pulses interact with samples, which can alter their electronic and atomic structures [1]. Previous experiments on x-ray FEL-matter interactions have mostly focused on semiconductors like diamond and silicon [2-4]. However, little is known about x-ray-induced bond breaking in multi-element solids or ionic solids. Recent calculations have predicted a crystalline-to-disordered phase transition in sodium chloride (NaCl) upon high-intensity x-ray interaction [5]. Using FEL x-ray pump and x-ray probe pulses, we can induce non-thermal phase transitions and detect new material phases. We have investigated the time-dependent intensity of diffraction peaks in NaCl and magnesium oxide (MgO). Our experimental observations are compared with density functional theory and particle-in-cell simulations. Our findings reveal the ultrafast responses of these materials, analyzed through the variations in diffraction peak intensities.