ABSTRACT. Magnetic reconnection is a ubiquitous fundamental process in space and astrophysical plasmas that rapidly converts magnetic energy into some combination of flow energy, thermal energy, and non-thermal energetic particles. Over the past decade, a new experimental platform has been developed [1] to study magnetic reconnection using strong coil currents powered by high power lasers at low plasma beta, typical conditions under which reconnection is energetically important in astrophysics. KJ-class lasers were used to drive parallel currents to reconnect MG-level magnetic fields in a quasi-axisymmetric geometry, similar to the Magnetic Reconnection Experiment or MRX [2], and thus this platform is named micro-MRX. This presentation summarizes two major findings from micro-MRX: direct measurement of accelerated electrons [3] and observation of ion acoustic waves [4] during anti-parallel reconnection. The angular dependence of the measured electron energy spectrum and the resulting accelerated energies, supported by particle-in-cell simulations, indicate that direct acceleration by the out-of-plane reconnection electric field is at work. Furthermore, a sudden onset of ion acoustic bursts has been measured by collective Thomson scattering in the exhaust of magnetic reconnection, followed by electron acoustic bursts with electron heating and bulk acceleration. These results demonstrate that the micro-MRX platform offers a novel and unique approach to study magnetic reconnection in the laboratory beyond the capabilities provided by traditional magnetized plasma experiments such as MRX and the upcoming FLARE or (Facility for Laboratory Reconnection experiments) [5]. Implications of these laboratory findings to astrophysical scenarios and future work on studying other particle acceleration mechanisms and ion acoustic waves during magnetic reconnection are discussed.

[1] L. Gao, H. Ji, G. Fiksel, W. Fox et al., “Ultrafast proton radiography of the magnetic fields generation by laser-driven coil currents,” Physis of Plasma 23, 043106 (2016) [2] M. Yamada, H. Ji, S. Hsu, T. Carter et al., “Study of driven magnetic reconnection in a laboratory plasma,” Physis of Plasmas 4, 1936-1944 (1997) [3] A. Chien, L. Gao, S. Zhang, H. Ji et al., “Non-thermal electron acceleration from magnetically driven reconnection in a laboratory plasma,” Nature Physis 19, 254-262 (2023) [4] S. Zhang, A. Chien, L. Gao, H. Ji et al., “Ion and electron acoustic bursts during anti-parallel magnetic reconnection driven by lasers,” Nature Physis 19, 909-916 (2023) [5] H. Ji, W. Daughton, J. Jara-Almonte, A. Le, A. Stanier, and J. Yoo, “Magnetic reconnection in the era of exascale computing and multiscale experiments,” Nature Reviews Physics 4, 263-282(2022)

ABSTRACT. Magnetic reconnection is a ubiquitous and fundamental process in plasmas by which magnetic fields change their topology and release magnetic energy. Despite decades of research, the physics governing the reconnection process in many parameter regimes remains controversial. Contemporary reconnection theories predict that long, narrow current sheets are susceptible to the tearing instability and split into isolated magnetic islands (or plasmoids), resulting in an enhanced reconnection rate. While several experimental observations of plasmoids in the regime of low-to-intermediate β (where β is the ratio of plasma thermal pressure to magnetic pressure) have been made, there is a relative lack of experimental evidence for plasmoids in the high-β reconnection environments which are typical in many space and astrophysical contexts. Here, we report strong experimental evidence for plasmoid formation in laser-driven high-β reconnection experiments.

ABSTRACT. We have been experimentally investigating magnetic reconnections driven by electron dynamics using high-power lasers. By controlling the external magnetic field strength, only electrons are magnetized in the experimental system. We have shown that magnetic reconnection can be driven by electron dynamics by imaging a cusp and plasmoid propagating at the Alfven velocity defined by electron mass [1]. Furthermore, by measuring the local electron and ion velocities, the pure electron outflows have been experimentally verified [2]. The electron dynamics can be essential in the Weibel instability in sub-relativistic collisionless shocks in the presence of a weak external magnetic field [3]. We report our recent efforts to extend reconnection experiments using relativistic intense lasers and also magnetic devices, together with magnetic field reconstruction with data science and informatics.

[1] Y. Kuramitsu et al., Nat. Commun., 9, 5109, 2018 [2] K. Sakai, Sci. Rep. 12, 10921, 2022 [3] Y. Kuramitsu, Y. Matsumoto, and T. Amano, Phys. Plasmas 30, 032109, 2023

ABSTRACT. Rotating plasma disks orbiting a central object, like a black hole, are ubiquitous in the universe. However, questions regarding their dynamical evolution, such as the mechanisms of angular momentum transport and the role of magnetic fields in seeding instabilities, turbulence and launching jets, remain outstanding.

In this talk I will give an overview of a new generation of laboratory experiments fielded at high-energy-density facilities (the MAGPIE pulsed-power generator at Imperial College London and the OMEGA laser at the University of Rochester), designed to probe plasma physics relevant to accretion disks and jet-launching regions [1-4].

In these experiments, a differentially rotating plasma column is driven and sustained by the collision of multiple inflowing plasma jets. The free-boundary design allows the plasma to expand axially, forming supersonic rotating jets which remain collimated as they propagate through the vacuum chamber. The rotating plasma flows are high magnetic Reynolds number (ranging from 10^2 to 10^3) and has a quasi-Keplerian stratification. The experiments are supported by 3-D MHD simulations performed on the code Gorgon, which are used to model the formation, evolution and structure of differentially rotating plasmas. I will discuss the potential of these experiments to study the magneto-rotational instability, the Omega-effect, and the overall effect of magnetic fields in high-Rm rotating plasmas on laboratory scales.

[1] Ryutov, Astrophys. Space Sci (2011) [2] Bocchi et al., The Astrophys. J. (2013) [3] Valenzuela-Villaseca et al., Phys. Rev. Lett. (2023) [4] Valenzuela-Villaseca et al., IEEE Trans. Plasma Sci. (under review, 2024)

ABSTRACT. It is considered that the Weibel instability amplifies the magnetic field in weakly magnetized collisionless shocks. Although the background magnetic field energy is much smaller than the ion kinetic energy, previous studies of astrophysical shocks imply that the background magnetic field could still have a large impact when the electrons are magnetized [1]. We show, by theory and particle-in-cell (PIC) simulations, that the magnetized electrons enhance the amplification of the Weibel-generated magnetic field and could trigger magnetic reconnection in the nonlinear evolution [2]. We apply this amplification mechanism to laboratory experiments utilizing high-intensity lasers [3]. We present the PIC simulation results of this setup and discuss possible applications, such as particle acceleration and fusion.

[1] Matsumoto Y., Amano T., Kato T. N., Hoshino M., 2015, Science, 347, 974 [2] Jikei T., Amano T., Matsumoto Y., 2024, ApJ, 961, 157 [3] Kuramitsu Y., Matsumoto Y., Amano, T., 2023, Physics of Plasmas, 30, 032109

ABSTRACT. Highly compressible magnetized turbulence is prevalent in most astrophysical systems in the interstellar and intergalactic mediums, exhibiting signs of high compressibility (i.e., large sonic Mach numbers, M > 1). Supersonic turbulence is known to play a critical role in determining the star formation rate, the star formation efficiency, and the stellar mass distribution. Stochastic fluctuations in turbulent, supersonic, magnetized plasmas also have a marked effect on the magnitude of the magnetic fields that permeate them, namely, dynamo action. Tapping into the plasma’s kinetic energy reservoir, the turbulent motions cause the magnetic fields to “stretch”, “twist”, and “fold”, a sequence of transformations that results in the amplification of the magnetic energy density. The latter quickly becomes a sizable fraction of the available kinetic energy density of the turbulent motions and the magnetic fields reach values consistent with observational data. While fluctuation dynamo is commonplace in astrophysical systems, it is hard to realize in terrestrial laboratories, especially in the supersonic limit where theory predicts that the mechanism is inefficient when compared to its subsonic counterpart. Here we demonstrate, using laser-driven experiments at the Omega Laser Facility, that supersonic turbulence is indeed capable of fluctuation dynamo action. The experiments exploit the mature TDYNO experimental platform we developed, which demonstrated turbulent dynamo in the laboratory for the first time [Tzeferacos et al. Nat. Comm. 9, 591, 2018], meticulously characterized it [Bott et al. Proc. Natl. Acad. Sci. U.S.A. 118, e2015729118, 2021] in the subsonic regime, and was extended to study supersonic magnetized turbulence [Bott et al. Phys. Rev. Lett. 127, 175002, 2021]. We detail the experiments we performed at Omega that led to this demonstration, as well as the FLASH simulation campaigns that we executed for the design and interpretation of the experiments.

The Flash Center for Computational Science acknowledges support by the U.S DOE NNSA under Awards DE-NA0002724, DE-NA0003605, DE-NA0003842, DE-NA0003934, DE-NA0004144, and DE-NA0004147, and Subcontracts 536203 and 630138 with LANL and B632670 with LLNL, and Awards. Support from the U.S. DOE ARPA-E under Award DE-AR0001272, U.S. DOE Office of Science, Fusion Energy Sciences under Award DE-SC0021990, and the National Science Foundation under Awards PHY-2033925 and PHY-2308844 is also acknowledged.

ABSTRACT. Over the last ten years a series of laser-plasma experiments have proven the feasibility of investigating dynamo processes in the laboratory. Key findings of these experiments include the demonstration of bona fide dynamo action in subsonic turbulent plasmas with both low- and order-unity magnetic Prandtl numbers, amplification of magnetic fields in supersonic plasmas, and significantly modified transport of fast particles and heat by dynamo-generated fields. In this talk, I will present new results that address a previously unsolved puzzle from these experiments: how dynamo action ceases. Based on data from Thomson-scattering, X-ray imaging, and proton-radiography diagnostics, we argue that, once the plasma cools below a critical temperature, magnetic-field amplification is not sustained, and the fields that were initially generated by the dynamo subsequently decay. The implications of these results for the critical magnetic Reynolds number of dynamo action in both subsonic and supersonic turbulent laser-plasmas will be discussed.

We acknowledge support by UKRI (grant number MR/W006723/1); EPSRC (grant numbers EP/M022331/1 and EP/N014472/1), the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreements nos. 256973 and 247039; the U.S. DOE NNSA under Awards DE-NA0002724, DE-NA0003605, DE-NA0003842, DE-NA0003934, DE-NA0003856, and Subcontracts 536203 and 630138 with LANL and B632670 with LLNL; the NSF under Award PHY-2033925; and the U.S. DOE Office of Science Fusion Energy Sciences under Award DE-SC0021990.

ABSTRACT. Magnetized collisionless shocks are ubiquitous in heliospheric and astrophysical environments, including planetary shocks, the heliopause, supernova remnants, and galaxy clusters. Magnetized shock dynamics are highly dependent on the angle θ between the upstream magnetic field and the shock propagation direction, with different physical processes active in quasi-perpendicular (θ > 45 deg) and quasi-parallel (θ < 45 deg) geometries. Of particular interest are particle heating and non-stationary dynamics in quasi-perpendicular shocks and particle acceleration in quasi-parallel shocks. However, we currently lack an understanding of key aspects of how these processes operate, with multiple competing theories and incomplete hints from satellite observations of shocks in space. Recent advances have enabled collisionless shocks to be created experimentally using high-powered lasers [1,2]. We present two new experimental platforms that combine unprecedentedly large magnetized volumes with strongly driven plasma flows to study magnetized collisionless shocks: a platform on the Z Machine at SNL to explore particle heating and non-stationary dynamics in quasi-perpendicular shocks; and a platform on the National Ignition Facility at LLNL to create and measure ion acceleration in quasi-parallel shocks. Details on these platforms and the collisionless shock physics they can access will be discussed.

[1] Schaeffer, et al., Phys. Rev. Lett. 119, 025001 (2017) [2] Schaeffer, et al., Phys. Rev. Lett. 122, 245001 (2019)

ABSTRACT. Our current understanding of shock-driven star formation and the mixing of stellar materials in the interstellar medium largely relies on limited observational data and sophisticated computational models. Despite their comprehensive scope, computational studies often face limitations in accurately capturing the full complexity of physical interactions. Turbulent mixing, resulting from the interaction between shocks and blast waves with denser clouds in the interstellar medium, simplifies under certain assumptions to become hydrodynamically self-similar. This self-similarity makes the problem suitable for detailed laboratory investigations over extended periods, providing a vital link between observational data and computational models.

The presented work focuses on the development of experimental platforms for conducting shock-cloud experiments. We have successfully utilized a modified 2-stage light-gas gun facility, enabling us to drive a planar Mach 3 shock through a 100 mbar nitrogen gas environment and across cylindrical foam targets with a 2 mm diameter and a density of 150 mg/cm3. The experimental outcomes revealed a cloud-crushing time of 1.8μs, aligning with the analytical solution derived by Klein et al. Compared to laser-driven shocks, our modified gas gun setup enables exploration in the low Mach number regime with larger targets for detailed observation. It sustains shock pressure for tens of microseconds, allowing for the observation of shock-target interactions over multiple cloud-crushing times.

We intend to expand our exploration of shock-driven mechanisms through the use of the inverse wire array Z-pinch pulsed power machine and high-power laser facilities. By employing a variety of target designs and cutting-edge diagnostics, we aim to study these interactions in different conditions, enhancing our understanding of the fundamental physics involved. This comprehensive insight is essential for unraveling the processes behind triggered star formation and for shedding light on similar turbulent mixing challenges seen in inertial confinement fusion experiments.

ABSTRACT. Magnetized collisionless shocks are ubiquitous in the universe and have been long presumed to be the source of some of the highest energy cosmic rays. Quasi-parallel collisionless shocks (in which the shock normal is approximately parallel to the background magnetic field) are believed to be more efficient accelerators of particles than quasi-perpendicular shocks. 2-D kinetic simulations confirm that quasi-parallel shocks are capable of energizing more particles and creating energy spectra that extend further than from quasi-perpendicular shocks. In the shock downstream, ions and electrons reach an energy partition Ti/Te ~ 1.3, implying a significant electron heating due to ion-electron energy exchange. A multi-fluid model shows a resonance between electron whistler waves and ion magnetohydrodynamic waves that may be responsible for the energy transfer from drifting ions to thermal electrons.

This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] University of Rochester “National Inertial Confinement Fusion Program” under Award No. DE-NA0004144, the Department of Energy under Award Nos. DE-SC0020431 and DE-SC0024566, and the resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility located at Lawrence Berkeley National Laboratory. The authors thank the UCLA-IST OSIRIS consortium for the use of OSIRIS.

ABSTRACT. The study of thermal transport in plasmas is necessary for understanding many extreme environments observed in the Universe, including the intracluster medium (ICM) of galaxy clusters and in inertial-confinement fusion (ICF) experiments. Theoretical modelling of transport in these systems is challenging due to the multi-scale nature of the physics that determines this transport. A recent experiment performed using the National Ignition Facility (NIF) showed that thermal conductivity induced by electron transport in a high-β, weakly collisional, turbulent plasma was suppressed by two orders of magnitude compared to the classical Spitzer value. However, the complicated field geometries and flows present in the plasma made it difficult to pinpoint the cause of the suppression. One possible mechanism considered was suppression induced by microinstabilities, notably the whistler heat flux instability which has been predicted to suppress conduction for similar plasmas. Making new experimental measurements of thermal transport in such plasmas, with simpler magnetic-field geometries, could help constrain theoretical models more tightly.

This talk will outline an experimental design to measure parallel thermal transport in weakly collisional, high-β plasma on the Orion laser facility. A front-side blow-off plasma will be used to produce a weakly collisional, planar plasma. This plasma will be magnetised due to fields generated by the Biermann-Battery during its ablation, with the magnetic field lines being aligned with the temperature gradient in the plasma. We will make use of a spatially resolved gated X-ray detector (GXD), and a spatially and temporally resolved X-ray spectrometer for the temperature diagnostics. These in tandem will be able to give measurements of temperatures in the regions of interest. Magnetic-field measurements will be taken using proton imaging with two perpendicular target-normal sheath accelerated (TNSA) proton beams. Preliminary analysis has been done using simulation data gathered from the 3D MHD fluid code FLASH, as well as the collisional-radiative spectral analysis code SPECT3D. Insights from these simulations will help us deliver a successful campaign.

ABSTRACT. Recent measurements at the Sandia National Laboratory of the x-ray transmission of iron plasma have inferred opacities much higher than predicted by theory which casts doubt on modelling of iron x-ray radiative opacity at conditions close to the solar convective zone-radiative zone boundary. An increased radiative opacity of the solar mixture, in particular iron, is a possible explanation for the disagreement in the position of the solar convection zone-radiative zone boundary as measured by helioseismology and predicted by modelling using the results of analysis of the elemental composition from the solar photosphere. Here we present data from radiation burn-through experiments which do not support a large increase in the opacity of iron at conditions close to the base of the solar convection zone and provide a constraint on the possible values of both the mean opacity and the opacity in the x-ray range of the Sandia experiments. The data agree with opacity values from current state-of-the-art opacity modelling using the CASSANDRA opacity code.

ABSTRACT. 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. Measurements of sample evolution of Fe at near solar interior conditions did not reveal an impact of temporal gradients on past film-based measurements. These time-resolved technique are now being applied to oxygen experiments, oxygen being the largest contributor to the Rosseland mean at the base of the solar convection zone. Besides, the focus of the effort is now on measuring absolute time-resolved opacity. We will also discuss the first results from experiments designed to take advantage of newly acquired time-resolved knowledge to better control and expand the opacity sample conditions. The strategy and prospects for obtaining multiple opacity measurements from a single Z experiment will be discussed.

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.

ABSTRACT. 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. 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.

ABSTRACT. New experimental platforms have been established at the OMEGA EP and NIF laser facilities with the goal of producing and characterizing laboratory photoionized plasma in steady state. The latter has been a standing challenge of photoionized plasma experiments and is key to the goal of benchmarking astrophysical theory and modeling codes employed in the analysis and interpretation of x-ray astronomy observations. In the experiments, a tamped-foil sample is heated and ionized by the broadband x-ray flux from an array of laser-driven halfraums that can have a duration of up to 30ns with radiation temperatures of 90eV and 160eV at OMEGA EP and NIF, respectively. Silicon and iron photoionized plasmas have been produced that maximize the population of ions with open n=2 atomic shell. The x-ray source performance is monitored with soft x-ray spectrometers, and density is determined from an imaging measurement of the photoionized plasma expansion. Diagnostics also include transmission and self-emission x-ray spectroscopy. A separate source of backlit photons is fired at different times to check that the charged state distribution determined from the transmission spectrum analysis is in steady state. Furthermore, a novel analysis method is employed to extract the plasma temperature1. We discuss the relevant timescales, the experimental design and observations, the analysis of the data, and the comparison of experimental results with simulations performed with several astrophysical and non-astrophysical models and codes. 1R. C. Mancini et al Phys. Rev. E 101, 051201 (2020). This work is supported by DOE grant DE-NA0004038, and the NLUF and NIF Discovery Science Programs.

ABSTRACT. Photoionization (PI) fronts are a type of radiation-driven ionization wave that occur in many astrophysical systems. PI fronts dictate important physics in the observations of flash supernova spectroscopy and Stromgren sphere structures around stars. Traditionally, many radiation transport models propagate energy by diffusion and assume an optically thick medium. Photoionization fronts are typically modeled using non-diffusive radiation transport in optically thin materials. The difference in diffusive vs nondiffusive radiation transport has important consequences in the evolution of radiatively driven systems, such as star formation rates in circumstellar medium. The Z-machine at Sandia National Laboratories can create high-energy-density conditions in the laboratory relevant to PI fronts. The Z-Pinch Dynamic Hohlraum generates a bright high flux x-ray source that are incident on a nitrogen gas cell. The x-rays penetrate the gas and locally ionize the electrons over a short mean free path (~100 um) as a front that then propagates down the length of the gas cell (20 mm). Initial results show the photoionization front propagates at supersonic velocities and displayed curvature consistent with non-diffusive radiation transport. Future experiments will be carried out to compare results to astrophysical observations and theory

ABSTRACT. Radiative shock waves can be found in a wide range of regimes, characterized mainly by three dimensionless parameters [1]. The Boltzmann number quantifies the effects of radiation flux, the Mihalas or radiative number quantifies the importance of radiative energy density, and the optical depth compares the photon mean free path and the characteristic length scale of the hydrodynamic system. However, a systematic classification has proven to be complex, as layers of optically thin and thick regions alternate to form precursors and relaxation regions, between which the hydrodynamic shock is embedded [2]. Here, we aim to study the effects of radiation on the formation of weak shocks when two radiative plasmas with different pressures are put in contact. The conditions upon which optically thin and thick solutions exist have been obtained and expressed as a function of the shock strength and Boltzmann number. The existence of an optically thin regime is related to the presence of an over-dense layer in the compressed material. Scaling laws for the characteristic time and length have been discovered for several regimes. The theoretical analysis is supported by FLASH simulations. Based on these findings, we investigate shock wave formation in gas-puff implosions with a high atomic number liner, which is supposed to enhance radiative shock effects during compression [3]. In doing that, we tried to identify the parameters for which the over-compression regime is relevant on the time and length scales characteristic of Z-pinch design-space [4]. Then, we show that the new capabilities of the FLASH code have made it possible to simulate such radiative setups in 2D, capturing the Rayleigh-Taylor instability (RTI). We take advantage of the code’s new capabilities to study the impact of radiation on RTI growth rate, attempting to infer the role of high energy fluxes on the possible mitigation of the instability. For this, we discuss a new theoretical study that could also be applied to the modification of RTI dynamics in astrophysical regimes such as supernovae remnants [5]. We acknowledge support by the Department of Energy (DOE) National Nuclear Security Administration (NNSA) under award numbers DE-NA0003856, DE-NA0003842, DE-NA0004144, and DE-NA0004147, under subcontracts no. 536203 and 630138 with Los Alamos National Laboratory, and under subcontract B632670 with Lawrence Livermore National Laboratory. We acknowledge support from the U.S. DOE Advanced Research Projects Agency-Energy (ARPA-E) under Award Number DE-AR0001272 and the U.S. DOE Office of Science under Award Number DE-SC0023246.

REFERENCES [1] Michaut, Claire, et al. "Classification of and recent research involving radiative shocks." Astrophysics and Space Science 322 (2009) [2] Drake, R. Paul. "Radiative shocks in astrophysics and the laboratory." High Energy Density Laboratory Astrophysics (2005) [3] E. Ruskov et al., “The staged Z-pinch as a potential fusion energy source”, Phys. Plasmas (2020) [4] F. Garcia Rubio et al., “Shock Wave Formation in Radiative Plasmas”, PRE, submitted (2024) [5] Kuranz, Carolyn C., et al. "How high energy fluxes may affect Rayleigh–Taylor instability growth in young supernova remnants." Nature communications 9.1 (2018).

ABSTRACT. Dynamic compression experiments using high-power lasers enable studies of ultradense matter to terapascal pressures. Using these techniques, the high-pressure states of geological materials can be characterized in order to better understand the deep interiors of planets. Here, we report on the high-pressure solid phases of SiO2, an archetype for the silicates that dominate terrestrial mantles. Fused silica was quasi-isentropically (ramp) compressed to ~500 GPa and its crystalline structure was probed in situ using simultaneous x-ray diffraction at the Omega Laser Facility. Preliminary results indicate the formation of the expected pyrite-type structure between ~250 and 500 GPa. We discuss future plans at the National Ignition Facility to extend this platform to higher pressure, where SiO2 is predicted to transform into more highly coordinated phases.

This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Grant No. DE-NA0004144. Funding for this research was provided by the Center for Matter at Atomic Pressures (CMAP), a National Science Foundation (NSF) Physics Frontiers Center, under Award PHY2020249.

ABSTRACT. An understanding of the pressure and temperature conditions of melt at extreme levels of compression is important for planetary interior and impact models, inertial confinement fusion designs, and the construction of predictive equation-of-state models. For many materials, shock compression studies provide the only method for experimentally constraining melt at extremes (100’s GPa, ~1 eV), but the material states accessed by such rapid compression remain unclear. Here we present data which constrain the phase diagrams of magnesium oxide (MgO) and silicon carbide (SiC) and draw direct comparisons with melt theory by conducting laser-compression experiments along the shock Hugoniot with in situ X-ray diffraction, velocimetry, and pyrometry measurements to simultaneously determine crystal structure, microstructural texture, temperature, density, and pressure.

An atomistic description of melting under shock loading conditions has been explored by theory for decades, with different processes proposed. In the equilibrium view (no kinetic effects), as the shock Hugoniot intersects with the melt line, liquid formation initiates. Increasing increments of shock pressure results in P-T states along the melt line, with the liquid volume fraction increasing at the expense of the solid, until eventually full melt is achieved. In contrast, non-equilibrium molecular dynamic (NEMD) simulations, which calculate the states accessed in uniaxial shock compression of single crystals, have predicted either the formation of superheated solid states or supercooled liquid states. Our combined shock front temperature and bulk structural data on SiC suggests the formation of a transient supercooled liquid state at the shock front followed by recrystallization into the high pressure B1 phase, consistent with predictions of shear induced melting in other systems. This, however, is a distinctly orientation-dependent effect, as evidenced by data collected from uniaxial compression from different crystal orientations. In this talk, I draw comparisons between experimental observations and state-of-the-art atomistic theory which reveal the complexities of melt at extremes of pressure and temperature.

ABSTRACT. The experiment described in this presentation explores the phase diagram of silicon near its isentrope from 40 to 400 GPa, by ramp compressing silicon by a laser drive. Thermodynamic states of silicon at these states are measured by velocimetry, and the crystal structure is determined by nanosecond in-situ x-ray diffraction. The experiment shows a significant increase of the stability range of the Si hcp phase compared to theoretical predictions. The hcp phase is observed at the pressure and temperature range where dhcp phase was predicted, and no evidence of the dhcp phase is observed. Furthermore, the hcp-fcc phase transition pressure is at least 93 GPa, much higher than the 55 GPa predicted by computation. This observation is consistent with previous shock compression experiments. The fcc phase is confirmed to remain stable to at least 400 GPa.

Currently, no temperature data exist from nanosecond pyrometric measurements on ramp compression experiments. Such measurements are difficult due to the low number of photons emitted from low temperature (lower than 4000 K) targets. In this work, we present the foundational framework for analyzing low signal-to-noise ratio data. This method yields identical results as traditional techniques at high temperatures but is more robust at low temperatures. This sets the stage for analyzing future low temperature pyrometry data.

ABSTRACT. We have used the high-power laser facility OMEGA-EP at the Laboratory for Laser Energetics to measure the liquid structure of the shock-compressed state of warm dense silicon. Using velocity interferometry and X-ray scattering techniques, concurrent characterization of the compressed sample provided direct measurement of the static structure of silicon in its liquid phase. By combining the predictions of an X-ray scattering model with the analytical technique of Markov-Chain Monte Carlo, convergence of the density and inferred pressure state was found for three unique ion-ion correlation models; effective Coulomb, Debye-Huckel and non-linear Hulthen. Mutual posterior distributions of the silicon state were found by comparing these convergences with the pressure-density state determined by impedance matching techniques. The subsequent parameter distributions on the silicon phase diagrams highlight a consistency between the non-linear Hulthen predictions and the principal Hugoniot. This is a powerful experimental development allowing for exploration of the equation-of-state of high-compression materials which are readily achieved at high-power laser facilities and reducing model selection biases.

This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856, the University of Rochester, and the New York State Energy Research and Development Authority. Part of this work was prepared by LLNL under Contract No. DE-AC52–07NA27344.