OMEGA Laser Facility Research Articles

High-precision shock equation of state measurements for metallic fluid carbon between 15 and 20 Mbar

Diamond is an efficient ablator material to convert the energy of high-power giant lasers into ablation pressure with applications for High-Energy-Density (HED) science, planetary science, and Inertial Confinement Fusion (ICF) research at the National Ignition Facility (NIF). Unfortunately, current theoretical equation of state models cannot reproduce all the observed experimental data in the multi-megabar regime particularly relevant for HED and ICF research. New experimental data on the behavior of carbon at extreme pressures and temperatures are, therefore, essential to improve our predictive capability to design and analyze dynamic compression experiments for HED or ICF research and build improved equation of state models in the future. Here, we report high-precision laser-driven shock compression measurements on diamond single crystals at the Omega Laser Facility. Using ultrafast Doppler optical Velocimetry Interferometer System for Any Reflector (VISAR) to track the leading shock front and a quartz plate as an in situ reference, we obtain relative pressure-density shock equation-of-state measurements between 15 and 20 Mbar with an impedance-matching procedure. We also report shock-and-release measurements in a spherical geometry at the NIF. The new data provide tight constraints on the compressibility of warm dense carbon along the Hugoniot of full density diamond, allowing us to discriminate between existing theoretical equation-of-state models. We find that both LLNL LEOS 9061 and LANL Sesame 7835 models capture well the shock compressibility in the explored range. LANL Sesame 7835 also reproduces well the observed shock-and-release behavior of diamond near 10–20 Mbar.

Impact of spatiotemporal smoothing on the two-plasmon–decay instability

Higher levels of hot electrons from the two-plasmon–decay instability are observed when smoothing by spectral dispersion (SSD) is turned off in directly driven inertial confinement fusion experiments at the Omega Laser Facility. This finding is explained using a hot-spot model based on speckle statistics and simulation results from the laser–plasma simulation environment. The model accurately reproduces the relative increase in hot-electron activity at two different drive intensities although it slightly overestimates the absolute number of hot electrons in all cases. Extrapolating from the current ≈360-GHz system while adhering to the logic of the hot-spot model suggests that a larger SSD bandwidth should significantly mitigate hot-electron generation, and legacy 1-THz OMEGA experiments appear to support this conclusion. These results demonstrate that it is essential to account for laser speckles and spatiotemporal smoothing to obtain quantitative agreement with experiments. A compilation of hot-electron data from the past two decades reveals several other important points: (1) many prior experiments are more easily understood using recent results from multibeam absolute instability theory and (2) experiments with ignition-scale conditions produce less hot electrons compared to OMEGA spherical experiments for a given vacuum overlapped intensity, which is a promising result for validating performance predictions based on hydrodynamic scaling relations.

The rate of development of atomic mixing and temperature equilibration in inertial confinement fusion implosions

The MARBLE project is a novel inertial confinement fusion platform for studying the development of atomic mixing and temperature equilibration in inertial confinement fusion implosions and their impact on thermonuclear burn. Experiments involve the laser-driven implosion of capsules filled with deuterated engineered foams whose pores are filled with a gaseous mixture of hydrogen and tritium. By varying the size of the foam pores, we can study the timescale of the development of atomic mix relative to the development of thermal equilibrium between species. In contrast, previous separated reactant experiments have only provided information on the total amount of mix mass. We report on the series of MARBLE experiments [first reported in Haines et al., Nat. Commun. 11, 544 (2020)] performed on the University of Rochester's OMEGA laser facility and detailed and highly resolved three-dimensional radiation-hydrodynamic simulations of the implosions. In both the experimental and simulation results, we observe that the reactants do not achieve thermal equilibrium during the course of the implosion except in atomically mixed regions—i.e., that atomic mixing develops faster than thermal equilibration between species. The results suggest that ion temperature variations in the mixture are at least as important as reactant concentration variations for determining the fusion reaction rates.

Bayesian inference of deceleration-phase Rayleigh-Taylor growth rates in laser-driven cylindrical implosions

An iterative forward modeling approach has been developed and applied to the analysis of radiography data from cylindrical Rayleigh-Taylor instability studies performed at the Omega laser facility. Synthetic radiographs are generated and iterated using the Bayes’ Inference Engine [1] to produce maximum likelihood estimates for the time-dependent amplitude of deceleration-phase Rayleigh-Taylor modes seeded by a sinusoidal perturbation in the aluminum marker layer. This iterative forward modeling approach self-consistently fits the magnification and parallax in the image, both of which are sensitive at this scale (~13x magnification) to misalignments of the pinhole smaller than 200 μm. Systematic errors in the inference of growth factor as large as 10% have been identified and eliminated by this technique. Furthermore, the self-consistent modeling of image parallax reveals that these implosions are not elliptical as they may appear to the casual observer, but indeed highly symmetric. The regularizing priors adopted here, and further constraints that might be applied in future work, reduce experimental uncertainties and lend greater statistical significance to comparisons with hydrodynamic modeling.

Foil backlighter development at the OMEGA laser facility for extended x-ray absorption fine structure experiments.

Extended x-ray absorption fine structure (EXAFS) measurements require a bright and continuous x-ray source and a detection system with high spectral resolution to capture the modulations of the absorption coefficient above the material absorption edge. When performing EXAFS measurements under laser-driven dynamic compression, it is hence critical to optimize the backlighter x-ray emission. A series of experiments has been conducted at the OMEGA laser facility to characterize titanium (Z = 22), iron (Z = 26), germanium (Z = 32), molybdenum (Z = 42), silver (Z = 47), and gold (Z = 79) foil backlighters irradiated with 3 kJ-12 kJ of laser energy. The spectra have been recorded using a dual crystal spectrometer (DCS), a two-channel transmission spectrometer covering 11 keV-45 keV and 19 keV-90 keV energy bands. The DCS has been calibrated so that the spectral intensities can be compared between different campaigns.

Laser-plasma instabilities in long scale-length plasmas relevant to shock-ignition

We present 2D particle-in-cell simulations of laser-plasma instabilities (LPI) performed using conditions relevant to experiments on the OMEGA and NIF laser facilities. The laser intensity used is 2×1015 W cm−2, which allows comparison with previous experiments and simulations investigating shock ignition, though this is a relatively low intensity for the scheme. We find that the large convective gain of the instabilities leads to dynamics that are largely controlled by pump depletion and to scattering or absorption by LPIs predominantly occurring away from the quarter-critical density. The two-plasmon decay (TPD) instability is dominant for OMEGA-type conditions and remains important at the NIF-scale. In both cases, most absorption from TPD occurs near its Landau cutoff density, and we examine its nonlinear dynamics in this region, focusing on the ion-acoustic waves responsible for saturation. For NIF-type conditions, stimulated Raman scattering (SRS) and stimulated Brillouin scattering divert significant fractions of laser energy, and we find that SRS backscatter can occur with high gain at low densities due to kinetic effects. An in-depth analysis of hot-electron production is performed to identify the main sources and their characteristic electron temperature. For both simulations presented, the overall hot-electron temperature lies between 30 and 35 keV. This is relatively low and will likely increase shock pressure; however, the distributions also contain a significant number of high-energy (Ek>100 keV) electrons that would likely cause unacceptable preheat. We, therefore, suggest some strategies that may be utilized to minimize this high-energy component.

High-precision equation of state data for TiO2 : A structural analog of SiO2

The high-pressure response of titanium dioxide (${\mathrm{TiO}}_{2}$) is of interest because of its numerous industrial applications and its structural similarities to silica (${\mathrm{SiO}}_{2}$). We used three platforms---Sandia's Z machine, Omega Laser Facility, and density-functional theory-based quantum molecular dynamics (QMD) simulations---to study the equation of state (EOS) of ${\mathrm{TiO}}_{2}$ at extreme conditions. We used magnetically accelerated flyer plates at Sandia to measure Hugoniot of ${\mathrm{TiO}}_{2}$ up to pressures of 855 GPa. We used a laser-driven shock wave at Omega to measure the shock temperature in ${\mathrm{TiO}}_{2}$. Our Z data show that rutile ${\mathrm{TiO}}_{2}$ reaches 2.2-fold compression at a pressure of 855 GPa and Omega data show that ${\mathrm{TiO}}_{2}$ is a reflecting liquid above 230 GPa. The QMD simulations are in excellent agreement with the experimental Hugoniot in both pressure and temperature. A melt curve for ${\mathrm{TiO}}_{2}$ is also proposed based on the QMD simulations. The combined experimental results show ${\mathrm{TiO}}_{2}$ is in a liquid at these explored pressure ranges and is not highly incompressible as suggested by a previous study.

Solid Cherenkov detector for studying nucleosynthesis in inertial confinement fusion.

Measuring gamma rays emitted from nuclear reactions gives insight into their nuclear structure. Notably, there are several nuclear reactions that produce gamma rays at ∼1 MeV-3 MeV energies such as T(4He, γ)7Li, 4He(3He, γ)7Be, and 12C(p, γ)13N, which may solve questions lingering about big-bang nucleosynthesis and stellar nucleosynthesis. To observe 1 MeV-3 MeV gamma rays in an inertial confinement fusion system, a new style of the Cherenkov detector was developed using aerogel and fused silica as a Cherenkov medium. Utilizing the OMEGA laser facility, both aerogel and fused silica media were compared with the existing gas-medium Cherenkov detector to validate the concept. Gamma ray measurements from high yield inertial confinement fusion implosions (deuterium-tritium and deuterium-3He) demonstrated that aerogel and fused silica were viable Cherenkov media, paving the way for a potential optimized detector to make these cross section measurements on OMEGA or the National Ignition Facility.

Hydrodynamic conditions in laser irradiated buried layer experiments

The calculation of open shell ionization level and radiative properties of materials in Non-Local Thermal Equilibrium (NLTE) is currently still a major challenge for any atomic model. The predictions of various NLTE atomic codes at these conditions still differ significantly. In recent years, a new buried layer platform was developed at the Lawrence Livermore National Laboratory and the Laboratory for Laser Energetics. This platform is used to measure ionization distribution and emission of open L-shell, mid-Z ions and open M-shell, high-Z ions at NLTE conditions that are relevant in many laser plasma applications. These experiments offer a unique chance for benchmarking the atomic models. In order to perform these experiments, a uniform well characterized plasma source is required. In this work, we present one-dimensional (1D) and two-dimensional simulations of the experimental platform. These simulations were used for both the design and the analysis of the experiments. The simulations demonstrate the different phases of hydrodynamic evolution of the target and identify the time windows in which uniform conditions can be achieved. A 1D expansion of the target was found to be adequate to describe the target's evolution for most of the experiment duration. The fast 1D simulations were compared with recent experimental results from the Omega laser facility. The sensitivity of the results to several modeling parameters such as the electron flux limiter and laser resonant absorption is reported.

Experimental observation of elevated heating in dynamically compressed CH foam

We present an experimental result of significantly increased heating in a laser-driven blastwave experiment carried out at the OMEGA laser facility. Abnormally high temperatures were observed in warm dense CH compared to older experiments and theoretical predictions. The higher temperatures in compressed CH were linked to an improved smoothness of the laser intensity profile, which resulted in better efficiency of the drive and coupling of more energy into the system compared to previous similar experiments. Fifteen beams with combined intensity of ∼7× 1014 W cm−2 and a square intensity profile with 2 ns duration were used to drive a strong shock, which subsequently developed to a blastwave travelling through low density CH foam creating warm dense matter. Multiple diagnostics were used to examine the thermodynamic conditions in the warm dense CH foam. Velocity interferometry (VISAR) and streaked pyrometry (SOP) observed increased blastwave velocities, while x-ray Thomson scattering (XRTS) measured elevated temperatures of 17.5 − 35 eV in compressed CH foam. The experimental results were compared to hydrodynamic simulations and a potential contribution from x-rays to the elevated temperatures in the dense material was considered.

Anomalous Absorption by the Two-Plasmon Decay Instability.

Radiation-hydrodynamic simulations of directly driven fusion experiments at the Omega Laser Facility predict absorption accurately when targets are driven at low overlapped laser intensity. Discrepancies appear at increased intensity, however, with higher-than-expected laser absorption on target. Strong correlations with signatures of the two-plasmon decay (TPD) instability-including half-harmonic and hard-x-ray emission-indicate that TPD is responsible for this anomalous absorption. Scattered light data suggest that up to ≈30% of the laser power reaching quarter-critical density can be absorbed locally when the TPD threshold is exceeded. A scaling of absorption versus TPD threshold parameter was empirically determined and validated using the laser-plasma simulation environment code.

H2(p,γ)He3 cross section measurement using high-energy-density plasmas

An absolute cross section for the radiative capture reaction $^{2}\mathrm{H}(p,\ensuremath{\gamma})^{3}\mathrm{He}$ has been measured at the OMEGA laser facility using inertially confined plasmas. These high-temperature plasmas are created by imploding a fuel-containing capsule using laser ablation, and are advantageous in that they better mimic astrophysical systems. We measure an $S$ factor for this reaction of $0.429\ifmmode\pm\else\textpm\fi{}0.{026}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}0.{072}_{\mathrm{sys}}$ eV b at ${E}_{c.m.}=16.35\ifmmode\pm\else\textpm\fi{}0.40$ keV, which is higher than the adopted evaluations. This reaction is important as a source of nuclear energy in protostars and brown dwarfs. It is also a critical reaction during big-bang nucleosynthesis, and an accurate cross section can be used as a constraint on cosmology.

Plasma Collision in a Gas Atmosphere.

We present a study on the impact of a gas atmosphere on the collision of two counterpropagating plasmas (gold and carbon). Imaging optical Thomson scattering data of the plasma collision with and without helium in between have been obtained at the Omega laser facility. Without gas, we observed large scale mixing of colliding gold and carbon ions. Once ambient helium is added, the two plasmas remain separated. The difference in ionic temperature is consistent with a reduction of the maximum Mach number of the flow from M=7 to M=4. It results in a reduction of a factor ∼10 of the counterstreaming ion-ion mean free path. By adding a low-density ambient gas, it is possible to control the collision of two high-velocity counterstreaming plasma, transitioning from an interpenetrating regime to a regime in agreement with a hydrodynamic description.

Probing the Solid Phase of Noble Metal Copper at Terapascal Conditions.

Ramp compression along a low-temperature adiabat offers a unique avenue to explore the physical properties of materials at the highest densities of their solid form, a region inaccessible by single shock compression. Using the National Ignition Facility and OMEGA laser facilities, copper samples were ramp compressed to peak pressures of 2.30TPa and densities of nearly 30 g/cc, providing fundamental information regarding the compressibility and phase of copper at pressures more than 5 times greater than previously explored. Through x-ray diffraction measurements, we find that the ambient face-centered-cubic structure is preserved up to 1.15TPa. The ramp compression equation-of-state measurements shows that there are no discontinuities in sound velocities up to 2.30TPa, suggesting this phase is likely stable up to the peak pressures measured, as predicted by first-principal calculations. The high precision of these quasiabsolute measurements enables us to provide essential benchmarks for advanced computational studies on the behavior of dense monoatomic materials under extreme conditions that constitute a stringent test for solid-state quantum theory. We find that both density-functional theory and the stabilized jellium model, which assumes that the ionic structure can be replaced by an ionic charge distribution by constant positive-charge background, reproduces our data well. Further, our data could serve to establish new international secondary scales of pressure in the terapascal range that is becoming experimentally accessible with advanced static and dynamic compression techniques.

Transfer Learning to Model Inertial Confinement Fusion Experiments

Inertial confinement fusion (ICF) experiments are designed using computer simulations that are approximations of reality and therefore must be calibrated to accurately predict experimental observations. In this article, we propose a novel technique for calibrating from simulations to experiments, or from low fidelity simulations to high fidelity simulations, via “transfer learning” (TL). TL is a commonly used technique in the machine learning community, in which models trained on one task are partially retrained to solve a separate, but related task, for which there is a limited quantity of data. We introduce the idea of hierarchical TL, in which neural networks trained on low fidelity models are calibrated to high fidelity models, then to experimental data. This technique essentially bootstraps the calibration process, enabling the creation of models which predict high fidelity simulations or experiments with minimal computational cost. We apply this technique to a database of ICF simulations and experiments carried out at the Omega laser facility. TL with deep neural networks enables the creation of models that are more predictive of Omega experiments than simulations alone. The calibrated models accurately predict future Omega experiments, and are used to search for new, optimal implosion designs.

Experimental investigation of the collective stimulated Brillouin and Raman scattering of multiple laser beams in inertial confinement fusion experiments

The direct and indirect drive schemes for inertial confinement fusion (ICF) make use of a large number of laser beams arranged in a symmetric angular distribution. The preferential decay geometry of the three waves resonant couplings, mainly responsible for backscattered light in single beam experiments, may then be deeply modified in the region of crossing beams where collective laser plasma instabilities could develop. Such instabilities can occur for laser beams having a common symmetry axis along which they drive a common daughter wave. The collective coupling results in an increase of the growth gain with the increase of the number of interacting beams and produce energy losses in new backward directions. We have taken advantage of the multiple beams of the Omega laser facility and of its large battery of diagnostics to study the physics related to this multiple beams interaction in the regimes of high temperature plasmas relevant of the direct and indirect drive schemes to ICF. Experiments performed in a planar open geometry have evidenced the large amplification of stimulated Raman scattering (SRS) electromagnetic waves almost transverse to the density gradient as theoretically predicted 40 years ago. This was achieved in long scale-length high-temperature plasmas in which two beams couple to the same scattered electromagnetic wave further demonstrating this multiple-beams collective SRS interaction. The collective nature of the coupling and the amplification at large angles from the density gradient increase the global SRS losses and produce light scattered in novel directions out of the planes of incidence of the beams. Indirect drive experiments were performed in rugby ball shaped Hohlraum irradiated by 40 beams. Large instantaneous (peak reflectivity >30%) Brillouin sidescattering was evidenced to originate from the collective Brillouin amplification of a shared ion acoustic wave driven along the Hohlraum axis by a cone of 10 beams. In this paper, the scattering geometry is detailed for the two types of collective instabilities showing that they produce light scattered in novel very precise directions located far from the original aperture of the beams where the diagnostics are usually set-up. This scattered light could be measured on Omega thanks to the flexibility of the facility. Key features of the light scattered by collective instabilities are identified that would allow to recognize their signatures in more complex, less diagnosed experiments.

Optimized x-ray sources for x-ray diffraction measurements at the Omega Laser Facility.

The use of x-ray diffraction (XRD) measurements in laser-driven dynamic compression experiments at high-power laser facilities is becoming increasingly common. Diffraction allows one to probe in situ the transformations occurring at the atomic level at extreme conditions of pressure, temperature, and time scale. In these measurements, the x-ray source is generated by irradiation of a solid foil. Under certain laser drive conditions, quasimonochromatic He-α radiation is generated. Careful analysis of the x-ray source plasma spectra reveals that this radiation is not a single line emission and that monochromaticity is highly dependent on the laser irradiance. In this work, we analyze how the spectra emitted by laser-irradiated copper, germanium, and iron foils at the Omega Laser vary depending on different laser drive conditions and discuss the implications for XRD experiments.

A platform for thin-layer Richtmyer-Meshkov at OMEGA and the NIF

Imperfections at the interface between the ablator and fuel in an ICF capsule can give rise the Richtmyer-Meshkov instability (RMI). The effects of multiple shocks on this impulse-driven instability has been well studied in traditional, low energy density (LED) regimes, but work is limited in the high energy density (HED) regime. Instability and turbulent characteristics are difficult to diagnose in an ICF capsule, with its three dimensional and converging geometry. This paper highlights the platform development of a new planar HED experiment at the OMEGA and NIF laser facilities designed specifically to study the feedthrough of the RMI in a thin layer. Using a simple shock and then re-shock setup, Multi-shock (Mshock) has successfully redesigned the previous Reshock experiment to produce a well-defined mixing layer post-shock and reshock. Localized doping profiles have been developed to improve feature contrast, and reduce edge effects. A series of initial conditions have been precision machined and characterized for comparison, though the effects of preheat are distorting these profiles. First results quantifying the growth via the mixing layer width have been made with an 1D Eulerian code and an 1D RANS mix-model.

Progress on observations of interspecies ion separation in inertial-confinement-fusion implosions via imaging x-ray spectroscopy

We report on the analyses of x-ray-imaging spectroscopy data from experiments to study interspecies ion separation in direct-drive inertial-confinement-fusion experiments on the Omega laser facility. This is a continuation of recent, related research [S. C. Hsu et al., Euro Phys. Lett. 115, 65001 (2016); T. R. Joshi et al., Phys. Plasmas 24, 056305 (2017)]. The targets were argon (Ar)-doped, deuterium (D2)-filled spherical plastic shells of varying D2-Ar relative and total gas pressures. We used a time- and space-integrated spectrometer, streaked crystal spectrometer, and up to three time-gated multi-monochromatic x-ray imagers (MMIs) fielded along different lines of sight to record x-ray spectral features obtained from the implosions. The MMI data were recorded between first-shock convergence and slightly before the neutron bang time. We confirm the presence of interspecies ion separation as reported in our recent work. Extensions to the previous work include (a) the inclusion of shell mix in the data analysis, which slightly changes the amount of inferred species separation, (b) observation of species separation closer to the neutron bang time, and (c) fielding of the particle x-ray temporal diagnostic (PXTD) [H. Sio et al., Rev. Sci. Instrum. 87, 11D701 (2016)] to infer the relative timing between the neutron bang time and peak x-ray emission. Experimentally inferred species separation is compared with radiation-hydrodynamic simulations that include a multi-ion-species transport model.

Laboratory measurements of geometrical effects in the x-ray emission of optically thick lines for ICF diagnostics

Understanding the effects of radiative transfer in High Energy Density Physics experiments is critical for the characterization of the thermodynamic properties of highly ionized matter, in particular in Inertial Confinement Fusion (ICF). We report on non-Local Thermodynamic Equilibrium experiments on cylindrical targets carried out at the Omega Laser Facility at the Laboratory for Laser Energetics, Rochester NY, which aim to characterize these effects. In these experiments, a 50/50 mixture of iron and vanadium, with a thickness of 2000 Å and a diameter of 250 μm, is contained within a beryllium tamper, with a thickness of 10 μm and a diameter of 1000 μm. Each side of the beryllium tamper is then irradiated using 18 of the 60 Omega beams with an intensity of roughly 3 × 1014 W cm−2 per side, over a duration of 3 ns. Spectroscopic measurements show that a plasma temperature on the order of 2 keV was produced. Imaging data show that the plasma remains cylindrical, with geometrical aspect ratios (quotient between the height and the radius of the cylinder) from 0.4 to 2.0. The temperatures in this experiment were kept sufficiently low (∼1–2 keV) so that the optically thin Li-like satellite emission could be used for temperature diagnosis. This allowed for the characterization of optical-depth-dependent geometric effects in the vanadium line emission. Simulations present good agreement with the data, which allows this study to benchmark these effects in order to take them into account to deduce temperature and density in future ICF experiments, such as those performed at the National Ignition Facility.