Seminars

Achieving a Burning (and Igniting, by most definitions) Plasma on the National Ignition Facility (NIF) Laser*

December 16, 2021

Debra Callahan
Lawrence Livermore National Laboratory

One of the scientific milestones in fusion research on the path to ignition is creating a burning plasma. A burning plasma occurs when the energy deposited by the fusion-produced alpha particles is the dominant source of heating of the plasma – this is a necessary step to reach ignition. Over the last year, experiments on the NIF laser have reached this state using two indirect-drive designs; these two designs use larger capsules than had been used previously while maintaining the other important parameters of implosion velocity, low-mode symmetry, late-time ablation pressure, and high Z mix. To drive larger capsules with the same amount of laser energy, the larger capsules had to be driven in a similar size hohlraum, which makes maintaining a symmetric drive more difficult, and required the use of additional techniques to mitigate low-mode asymmetry.

One of these designs, Hybrid E, has also achieved ignition by the Lawson criterion. Under that definition of ignition, the fusion-produced alpha heating dominates the power balance in the hotspot – overcoming radiative and conduction losses. The experiment on Aug 8, produced 1.35 MJ, which is a capsule gain (yield/capsule absorbed energy) of about 6x. The target gain, yield/laser energy, was 0.7 – which did not meet the NAS definition of ignition = gain 1. This design is likely on the ignition cliff, which means it is in a sensitive part of parameter space – repeat experiments to date have all achieved capsule gain > 1 but with lower yields than the August experiment.

Since the start of experiments on NIF, progress has been made in steps. At each step, a combination of experimental data (including improved diagnostics), theory, and modeling is used to identify and understand the limiters in performance. New designs are then developed using this understanding – this generally results in an increase in performance until the next limiter becomes dominant. This cycle has produced several physics milestones on the way to ignition. First was fuel gain, where the neutron yield exceeds the energy in the deuterium-tritium (DT) fuel [1]. Next was “alpha heating,” in which the neutron yield is doubled due to the additional energy deposited in the DT fuel by alpha particle stopping [2,3]. Now, we have achieved the burning plasma state [4]. In this talk, we will review these new designs and experiments and show how the August experiment compares with several published ignition metrics.

[1] O. A. Hurricane, et al., Nature 506, 343 (2014)

[2] S. Le Pape, et al., Phys. Rev. Lett. 120, 245003 (2018)

[3] D. T. Casey, et al., Phys. Plasmas 25, 056308 (2018)

[4] A. B. Zylstra, O. A. Hurricane, et al. accepted for publication

*Work performed under the auspices of the U. S. Department of Energy by LLNL under contract DE-AC52-07NA27344

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Direct measurements of temperature and phase transitions along the MgO shock Hugoniot

December 2, 2021

June Wicks
John Hopkins University

Laser-driven shock compression enables experimental study of material properties at unprecedented pressures and temperatures. Experiments that probe these extreme conditions are conducted at timescales approaching the limits of atomic mobility, convoluting kinetic effects into observations of phase transitions. In this talk, I will present a suite of studies of the MgO Hugoniot aimed at constraining the phase diagram, comparing effects of crystallographic orientation, timescale (decaying vs steady shocks), and x-ray vs pyrometry diagnostics. I will discuss the implications of this work for our understanding of phase transitions under dynamic compression and our ability to experimentally extricate thermodynamics from dynamics.

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Generating burning plasmas with improved implosions in the Hybrid-E platform

November 18, 2021

Alex Zylstra
Lawrence Livermore National Laboratory

Improving the performance of implosions on NIF by increasing the energy coupled to the capsule has been a strategy pursued in the last several years. The main challenge in improving the energy coupling is in increasing the hohlraum efficiency while maintaining both control over the radiation drive symmetry and key implosion parameters, such as the implosion velocity and coast time. Improved hohlraum efficiency with symmetry control was recently demonstrated in the ‘Hybrid E’ platform [1], which introduces a small amount of wavelength detuning (Dl) to alter the amount of cross-beam energy transfer and increase drive on the hohlraum waist. Recent work has increased the velocity and increased the late-time ablation pressure as the capsule implodes, both of which lead to large increases in stagnation pressure and implosion performance, with the yield tripling from earlier results. These implosions have now entered the burning plasma regime [2-4], with Qa ~ 1.5. A detailed comparison of the increased stagnation pressure with these key design parameters, and expected scaling relations, will be discussed to guide the path forward towards even higher implosion performance.

[1] A.B. Zylstra et al., Phys. Rev. Lett. 126, 025001 (2021) [2] A.B. Zylstra, O.A. Hurricane, et al., submitted to Nature (2021) [3] J.S. Ross, J.E. Ralph, A.B. Zylstra, et al., submitted to Nature Physics (2021) [4] A.L. Kritcher, C.V. Young, H.F. Robey, et al., submitted to Nature Physics (2021).

This work was performed under the auspices of the U. S. Department of Energy by LLNL under contract DEAC52-07NA27344. LLNL-ABS-82291. 

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How the concepts of coast-time and radius of peak velocity were key to achieving capsule gain > 5 in inertially confined fusion

November 18, 2021

Omar Hurricane
Lawrence Livermore National Laboratory

Conventional belief in inertial confinement fusion (ICF) is that high laser power and low DT fuel adiabats are required for obtaining ignition. The textbooks in ICF make no mention of the concept of “coast-time,” yet implosion experiments on the NIF have repeatedly shown better success with higher adiabat designs driven to very short coast-times. Understanding why took time. Reduced coast-time (the time between peak ablation pressure and implosion bang-time) is important for maintaining shell compression and high stagnation pressure [1]. Theory [2] has recently revealed that coast-time has a subtle connection to the radius at which peak velocity (Rpv) is achieved. Reduced Rpv can compensate for high fuel adiabat and allow access to exceedingly high stagnation pressures. The ignition relevant metric EP2 (where E is hot-spot energy and P is hot-spot pressure) is exceedingly sensitive to Rpv, indicating that even relatively small decreases in Rpv can rapidly push an implosion towards igniting. This understanding was key to moving the Hybrid-E [3,4] implosion into the burning plasma regime [5], and now, to the scientific ignition threshold with a fusion yield of 1.35 MJ and capsule gain of >5 on August 8, 2021. In this talk, we tell the story of how we pieced together puzzling observations and theory of this obscure, but critical bit of ICF physics over a period of 8 years.

[1] O.A. Hurricane, et al., Phys. Plasmas, 24, 092706 (2017) [2] O.A. Hurricane, et al., Phys. Plasmas submitted, (2021) [3] A.L. Kritcher, et al., Phys. Plasmas, 28, 072706 (2021) [4] A.B. Zylstra, et al., PRL, 126, 025001 (2020) [5] A.B. Zylstra, O. A. Hurricane, et al. in preparation (2021).

This work was performed under the auspices of the U. S. Department of Energy by LLNL under contract DE-AC52-07NA27344. Sorting Category: 07.06. 

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Multiphase EOS development at LLNL and application to a wide-ranged semi-empirical multiphase EOS for iron

November 4, 2021

Christine Wu
Lawrence Livermore National Laboratory

An equation of state (EOS) plays a key role in encapsulating state-of-the-art scientific advances into a set of physical models that can then be utilized by hydrodynamics simulations to improve their predictive capability. In this talk, we first give a brief update on the status of multiphase EOS development at LLNL.  We then illustrate this development methodology by describing the construction of a wide-ranged multiphase EOS for elemental iron (Fe), consisting of five phases: α (bcc), ε (hcp), γ (fcc), δ (high-T/low-P bcc), and liquid. The free-energy models for the phases are constrained by fitting to experimental data in both lower-pressure and high-pressure regimes, and to the predictions of ab initio electronic-structure calculations at higher pressures and temperatures. We show that our resulting multiphase Fe EOS is in excellent agreement with a diverse range of static and dynamic high-pressure experimental data, and in many cases provides substantially better agreement than other currently used multiphase Fe EOS models. It should thus enable more accurate predictions of complex high-P,T processes involving Fe and Fe alloys in a host of high-energy-density physics problems, including those of great interest to planetary science. 

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Enabling high repetition rate laser wakefield acceleration at high and low plasma densities

October 28, 2021

Howard M. Milchberg
Lawrence Livermore National Laboratory

Recently, we have achieved record all-optical laser wakefield acceleration results at opposite ends of the laser energy and plasma density ranges. Using < 3 mJ/ pulse at 1 kHz and near- critical density, 100-micron thick plasmas, we have produced low divergence, 15 MeV quasi-monoenergetic electron bunches. At three to four orders of magnitude lower plasma density, we have generated bunches up to 5 GeV by injecting and guiding 13 J pulses in meter-scale plasma waveguides. I will describe these experiments and what led up to them.

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Stellar-Relevant Emission-Based Opacity Experiments at the Orion Laser Facility

October 21, 2021

Madison E. Martin
Lawrence Livermore National Laboratory

Short-pulse laser facilities, such as the Orion Laser Facility, can be used to directly heat targets to conditions consistent with the radiative zone of the sun, enabling the measurement of x-ray emission which can be used to infer opacity of materials at extreme conditions. Opacity is a critical parameter in the transport of radiation in high energy density (HED) systems such as inertial confinement fusion capsules and stars. The resolution of current disagreements between solar models and helioseismologically observations would benefit from experimental validation of theoretical opacity models. As a complement to existing platforms using long pulse lasers (ex. the National Ignition Facility) and pulsed power machines (ex. the Z Machine), short pulse lasers can be used to heat targets to even higher temperatures and densities. Time-integrated and time-resolved spectra of iron were measured at the Orion short-pulse laser. The K-shell spectra of sulfur, potassium, and chlorine were measured and used to diagnose the plasma conditions. We will discuss two main modeling approaches used to simulate spectra for comparison with measured spectral data: 1.) ray trace of 1-dimensional (1D) radiation-hydrodynamic models and 2.) stand-alone spectral calculations employing more sophisticated atomic physics. We show that while our 1D radiation-hydrodynamic methodology has been useful for predicting and matching many aspects of the experimental spectra, this simplified method does not create a consistent picture for all materials in the experiment. We therefore introduce future improvements to the methodology that may aid in understanding sensitivities of our emission-based opacity platform. 

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The High Energy Density Science Center: 2021 year in review

October 7, 2021 

Frank Graziani
Lawrence Livermore National Laboratory
High Energy Density Science Center Director

The High Energy Density Science Center (HEDSC) was established in late 2015 through a joint agreement between the Physical and Life Sciences, Weapons and Complex Integration, and NIF and Photon Science directorates. The Center’s goal is to build a high energy density community through support of, and collaboration with, academic partners and to integrate those efforts with the programs at LLNL. The Center is built around four focus areas: (1) education, (2) outreach through workshops and seminars, (3) workforce pipeline to the programs, and (4) enabling research collaborations between LLNL and academic partners. In this talk, we will present the year FY21 in review and look at what lies ahead in FY22. FY21 saw continuation in education programs with a new version of a quarter long course on HED diagnostics offered in collaboration with UCSD and featuring new distance learning technology and guest lecturers. In addition, the HED center education program is working with WCI to help offer training for new designers. Another new HEDS Center Fellow was selected and our current HEDS Center Fellow, Andrew Longman, completed his first year. Besides educating the next generation of HED scientists, the HEDS Center continues training students through internships and thesis opportunities. Our outreach and collaboration activities continue through a vibrant seminar series, university partnerships, a sabbatical program, and visits to community colleges and undergraduate institutions. Inclusion and diversity are an important aspect of our mission and the Consortium for HEDS (FAMU, UC Merced, Morehouse and LLNL) and the Center help a number of students within HEDS. The Consortium’s goal is to provide research and education opportunities in HEDS for under-represented groups and thereby increase diversity in the field. Under the restrictions of the pandemic, the summer internship program continued, on-line. The sabbatical program has now started after a hiatus in FY21. In FY22 we hope to expand the sabbatical program and begin outreach visits to K-12 and community and undergraduate colleges. A call for a new HEDS Fellow postdoc position is out and more HED education programs are planned. In FY22 we will continue to foster international collaborations in HEDS, supporting another thesis student from FAMU, as one finishes, and continue the internship program. 

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Thermonuclear Fusion in an Equilibrium Z Pinch

September 23, 2021

Uri Shumlak
University of Washington

The equilibrium Z pinch is a unique approach to magnetic confinement fusion that requires no external magnetic field coils. Equilibrium conditions are reached through the use of sheared plasma flows, which enhance stability and provide a path to thermonuclear fusion. Simple geometry and strong scaling of fusion gain with pinch current form the cornerstones of this compact fusion device. The sheared-flow-stabilized Z pinch has been developed through integrated computational and experimental investigations at the University of Washington in collaboration with Lawrence Livermore National Laboratory. Experimental results demonstrate plasma stabilization, sustained thermonuclear fusion, and agreement with theoretical and computational predictions. Building on these advances, Zap Energy Inc. is developing a low-cost fusion reactor core based on the equilibrium Z pinch.

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Low-velocity proton stopping power measurement in Warm Dense Matter

August 26, 2021

Sophia Malko
Princeton Plasma Physics Laboratory

Ion stopping in warm dense matter is a process of fundamental importance for the understanding of the properties of dense plasmas, the realization and the interpretation of experiments involving ion-beam heated warm dense matter samples, and for inertial confinement fusion research. The theoretical description of the ion stopping power in warm dense matter is difficult, notably due to electron coupling and degeneracy, and measurements are still largely missing. In particular, the low-velocity stopping range around the Bragg peak, that features the largest modelling uncertainties, remains virtually unexplored. We present proton energy loss measurements in warm dense plasma at lower projectile velocities than previous experiments, coming significantly closer to the Bragg-peak region. Our energy loss data, combined with a precise target characterization based on plasma emission measurements using two independent spectroscopy diagnostics, demonstrate a significant deviation of the stopping power from classical models in this regime. In particular, we show that our results are consistent with recent first-principles simulations based on time-dependent density functional theory.

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Using quantum computers to simulate a toy problem of laser-plasma interactions

August 5, 2021

Yuan Shi
Lawrence Livermore National Laboratory

Quantum computing may lead to game-changing capabilities for science and technology. However, many practical problems are classical, and exactly how quantum systems can be used to solve classical problems remains an open question. Moreover, for plasma physics, many problems are nonlinear, whereas quantum computers are designed to carry out unitary evolution in Hilbert spaces, which are fundamentally linear. In this seminar, I will present the first results using real quantum hardware to simulate a toy problem that is relevant to laser-plasma interactions. A generally applicable algorithm is derived, which encode three-wave interactions on quantum hardware. The algorithm is implemented using two compilation approaches. First, each simulation step is compiled as a sequence of standard gates. Using this approach, ~10 simulation steps can be carried out before results are corrupted by decoherence.  Second, each simulation step is compiled as a single customized gate, which is realized using optimal control. Using this approach, the simulation depth is extended to ~100. Our results highlight the advantage of using customized gates on noisy intermediate-scale quantum computers. The generalized nonlinear gates are potentially useful building blocks for solving a large class of problems in plasma/fluid dynamics, nonlinear optics, and lattice gauge theories on quantum computers.

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Quantitative Studies of Supersonic Microparticle Impacts on Metals

July 29, 2021

Christopher Schuh
Department of Materials Science and Engineering, MIT

When a microparticle projectile strikes a metal, the resulting physical phenomena that occur are diverse and depend significantly upon the particle velocity. Plasticity sets on at very low velocities, and follows a scaling law whereby the plastically-damped energy rises with velocity. Analysis of this scaling law, through direct measurement of the damped energy itself, can provide quantitative measurements of the prevailing strength of the metal at rates up to and even beyond 108 s-1. At higher velocities, new phenomena set on which disrupt this scaling law. In many metals, hydrodynamic jetting flow occurs beyond a certain critical velocity, leading to a solid-state splash that dissipates extra energy. This jetting involves significant local deformation at the particle surface, and is associated with rupture and flaking of the native oxide layer and the formation of metallic bonds when both impactor and substrate are metals. In other cases, plastic dissipation can lead to enough adiabatic heat production to cause local melting, which is also associated with shifts in the observed energy-velocity scaling law. Finally, when combined with post-mortem analysis of impact sites, such studies can lead to new insights on microstructure evolution within the metal, including the development of nanotwins and nanostructures uniquely associated with high-rate, high-strain deformation.

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Relativistically Transparent Magnetic Filaments: a short-pulse platform for megaTesla fields, direct electron acceleration, and efficient gamma radiation

July 22, 2021

Hans Rinderknecht
University of Rochester
Laboratory for Laser Energetics

In relativistically transparent interactions, lasers with intensity above 10²⁰ W/cm² drive relativistic current filaments in classically overdense plasmas, producing an azimuthal magnetic field that is comparable in strength to the laser field.  This magnetic filament traps electrons, which are directly accelerated by the laser pulse to 100’s of MeV and efficiently radiate MeV-scale photons by synchrotron radiation. This seminar introduces this novel process, presents scaling laws that describe the radiated photon energy and radiation efficiency, and shows the results of initial experiments performed at the Texas Petawatt Laser (TPW).  The analytical scaling laws are validated by 3-D particle-in-cell simulations in two regimes of focal radius.  The efficiency of gamma radiation is predicted to exceed 10% for optimized designs with laser intensity above 6×10²¹ W/cm².  Experiments at TPW using moderate laser intensity (10²¹ W/cm²) demonstrate the predicted signatures of electron acceleration and x-ray radiation from a subset of microchannel targets filled with low-density CH foam.  This ultrafast photon source may be optimized for a wide range of applications in HED, nuclear, and high-field physics.

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Laser-driven magnetic filaments as a platform for high-field science

July 15, 2021

Alexey Arefiev
University of California, San Diego

High-power high-intensity multi-beam laser systems that are becoming operational around the world can now be used to create a platform for high-field science that is based on relativistically transparent magnetic filaments driven by irradiating lasers within a dense plasma. The strength of the quasistatic field can be comparable to that of the laser, reaching the megatesla level. This talk will review several phenomena that can be studied with experimentally achievable laser intensities at multi-PW laser facilities. These include emission of dense gamma-ray beams in the quantum regime and electron-positron pair creation from light alone. Astrophysical environments are known for exotic physics regimes that involve generation of extreme magnetic fields and creation of matter and antimatter from light alone. The discussed platform provides a potential path towards recreating relevant regimes in laboratory conditions.

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Frontier of Dynamic Materials Using Ultrafast X-ray Radiography

July 1, 2021

Arianna E. Gleason-Holbrook
SLAC – National Accelerator Laboratory
Standford University

The study of matter under extreme conditions is a highly interdisciplinary subject with broad applications to materials science, plasma physics, geophysics and astrophysics. Understanding the processes which dictate physical properties in warm dense plasmas and condensed matter, requires studies at the relevant length-scales (e.g., interatomic spacing) and time-scales (e.g., phonon period). Experiments performed at XFEL light sources across the world, combined with dynamic compression, provide ever-improving spatial- and temporal-fidelity to push the frontier. This talk will cover a very broad range of conditions, and give examples of case-studies closely related to geophysics, Astro(bio)physics, planetary-, and fusion energy-sciences, as enabled by microstructure visualization and control from in situ, ultrafast X-ray imaging.

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Experimental Observations of Laser-Driven Tin Ejecta Microjet Interactions

June 24, 2021

Alison Saunders
Lawrence Livermore National Laboratory

The study of high-velocity particle-laden flow interactions is of importance for the understanding of a wide range of natural phenomena, ranging from planetary formation to cloud interactions. However, given the difficulty of generating high-velocity flows of many particles, experimental observations of particle dynamics under such conditions are sparse. The interactions of ejecta microjets offer a novel experimental mechanism to study particle interactions, as ejecta microjets are micron-scale jets formed by strong shocks interacting with imprinted surfaces to generate particle plumes traveling at several kilometers per second. We recently performed experiments on the OMEGA and OMEGA EP lasers to take the first time-sequences of x-ray radiography images of two interacting tin ejecta microjets. We observe different jet morphologies and characteristics, such as densities and velocities, for different tin shock pressures of 11.7 and 116.0 GPa. But most strikingly, we observe different interaction behaviors as a function of pressure, ranging from jets passing through each other unattenuated to jets generating a corona of material around the point of interaction. We perform simulations of particle collisions to model the interactions and propose future experiments to understand the effects dominating the observed interaction dynamics.

LLNL-ABS-823558. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by Laboratory Directed Research and Development (LDRD) Grant No. 18-ERD-060.

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Relativistic plasma mirrors for high-power ultrashort pulses from UV to soft x-rays

June 3, 2021

Julia Mikhailova
Princeton University

Plasma optics have emerged as an attractive toolkit to manipulate high-intensity light. Relativistic plasma mirrors can generate high-order harmonics of petawatt lasers with theoretical energy-conversion efficiencies exceeding 60%. This seminar will provide an overview of our work revealing the fundamental processes behind the efficient frequency up-conversion by plasma mirrors. We will discuss our recent experiments in which we have generated high-order harmonics in the relativistic regime using the laser beams from our 20-terawatt, 25-femtosecond-pulsed system focused onto solid targets. We will further examine remaining challenges in experimental methods and opportunities relating to driving waveform optimization.

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Wide Ranging Ionic Transport Coefficients for High-Energy-Density Applications

May 27, 2021

Luke Stanek
Department of Computational Mathematics, Science and Engineering
Michigan State University

Hydrodynamic models rely on closures via equations of state and trans­port coefficients. Due to the lack of experimental data for transport coefficients, we heavily rely on molecular dynamics simulations. The key input to these simulations is the force law, the choice of which can vary the computational cost by orders of magnitude.

In this talk, we discuss two complementary approaches: determining a computationally efficient, yet acceptable force law, and optimally using sparse transport coefficient data. We delineated the accuracy boundary of force laws by extracting force-matched pair-interaction potentials from Kohn-Sham molecular dynamics data. These pair-interaction potentials reduce the amount of computational resource needed by or­ders of magnitude allowing for large-scale simulations. Our results rule out the need for three-body potentials above specific element dependent temperatures. These conclusions highlight that accurate results can be obtained in different regimes using disparate methods. We discuss how machine learning provides an avenue to accurately predict transport co­efficients with sparse data spanning these regimes. Moreover, we utilize transport coefficient data from computationally cheap models to predict computationally expensive transport coefficients. Portions of this work were supported by SNL. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

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Direct Imaging of Extrasolar Planets

May 13, 2021

Bruce Macintosh
Kavli Institute for Particle Astrophysics and Cosmology
Department of Physics, Stanford University

More 4,000 extrasolar planets are now known, but almost all have been detected through  indirect methods—measuring the parent star’s Doppler shift or brightness variations. Direct detection refers to spatially separating the planet’s light from that of the star. It is extremely challenging—Jupiter in our solar system is 10^-9 the luminosity of the sun—but allows observations of planets inaccessible to other methods, particularly the outer parts of target systems, and allows spectral a characterization of a planet’s atmospheric properties.

I will discuss the optical physics that makes direct detection challenging, and the techniques—adaptive optics, coronagraphy, and image processing –that can overcome these challenges. To date, direct detection has been successful for young Jupiter-like planets, and I will show highlights of those discoveries, including those from the LLNL-led Gemini Planet Imager.

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First-Principles Equation of State (FPEOS) Database And Dilute Cores in Giant Planets

May 6, 2021

Burkard Militzer
Department of Earth and Planetary Science, University of California, Berkeley 

This talk will discuss two equation of state applications. First, we will describe a first-principles equation of state (FPEOS) database for matter at extreme conditions that we put together by combining results from path integral Monte Carlo and density functional molecular dynamics simulations of eleven elements and ten compounds [1]. For all these materials, pressure and internal energy are provided over a wide density-temperature range from 0.5 to 50 g/cc and from 105 to 109 K. Results from 5000 first-principles simulations were combined. In this talk, we focus on isobars, adiabats and shock Hugoniot curves of different silicates in the regime of L and K shell ionization. Second, we will discuss how uncertainties in the equation of state of hydrogen-helium mixtures affect our understanding of giant planets. We will review the gravity measurements of the Juno spacecraft that has been in orbit about Jupiter since 2016. Interpreting these measurements has been a challenge because it is difficult to reconcile the unexpectedly small magnitudes of the moments J4 and J6 with conventional interior models that assume an ab initio equations of state as well as a compact core of ice and rock. We conclude by discussing models for Jupiter’s interior that include cores that have been substantially diluted with hydrogen and helium.

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Simultaneous measurements of concentration and velocity fields at a shock-accelerated gas interface

April 29, 2021

Riccardo Bonazza
Department of Engineering Physics, University of Wisconsin-Madison

The Richtmyer-Meshkov instability (RMI) develops upon impulsive acceleration of the interface between fluids of different acoustic impedance. The RMI consists in the unbounded growth of any perturbations initially present on the interface, and it is a consequence of the baroclinical deposition of vorticity on the interface. Eventually, the interface develops into a turbulent layer leading to the mixing of the two fluids. Flows of this type occur in nature and in man-made settings, over very large ranges of length, time, and energy scales: from experiments for the attainment of inertial confinement fusion (ICF), to supersonic combustion systems, to supernovae explosions. Shock-induced mixing has very negative effects in the case of ICF while it holds great potential to improve the combustion processes in the case of hypersonic engines. Two of the fundamental parameters that govern these phenomena are the Atwood number (measuring the original density contrast) and the Mach number. The experiments described here take place in a vertical shock tube with large, square internal cross section. The initial condition consists of a shear layer between helium and argon. A downward-propagating planar shock wave of Mach number 1.6 or 2.2 accelerates the interface. Measurements of 2-D concentration and velocity fields in a vertical cross section of the flow are obtained using planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV), respectively. Field statistics and integral quantities are extracted from these data and turbulence and mixing parameters are evaluated. From them, the outer scale Reynolds number is estimated to reach between 10,000 and 20,000 at the late times, indicating that the flow has crossed the threshold of fully developed turbulence.

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The MegaJoule Direct Drive Campaign: NIF Experiments on the Pathway to MJ Yield

April 22, 2021

Michael Rosenberg
Laboratory for Laser Energetics – University of Rochester

Laser direct drive is one of three main approaches to achieving inertial confinement fusion ignition and high yield in the laboratory. While cryogenic direct-drive implosions designed to optimize implosion performance are conducted on the 30-kJ OMEGA laser, MJ-scale experiments on the National Ignition Facility (NIF) are critical to understand scale-dependent aspects of direct-drive physics – principally related to laser-plasma interactions – at ignition-relevant conditions. Spherical-geometry experiments over a range of convergence ratios have shown control of laser energy coupling and implosion symmetry using contoured shells and beam pointing, and wavelength detuning has been used to partially mitigate cross-beam energy transfer. Stimulated Raman scattering has been identified as a primary source of hot electrons and mid-Z layers have been found to reduce hot electron preheat in implosions. Planar and spherical experiments have generated x-ray radiography data to diagnose laser imprint and imprint mitigation using high-Z coatings. Low-convergence implosions are being explored as neutron sources, and a staged set of enhancements to NIF are proposed to allow for increased convergence and yield.

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Non-Maxwellian Distribution Functions and their Impacts on Crossed-Beam Energy Transfer and Absorption

April 15, 2021

David Turnbull
Laboratory for Laser Energetics – Rochester

Plasma heating by inverse bremsstrahlung absorption of electromagnetic radiation was predicted to result in non-Maxwellian electron distribution functions (EDFs) more than 40 years ago, self-consistently modifying the absorption itself¹ . Fokker-Planck simulations suggested that the bulk electrons in such EDFs should be close to super-Gaussian, with an exponent 2<m<5 that is a function of the overlapped laser intensity². In such plasmas, ion-acoustic wave (IAW) frequencies were also shown to increase monotonically with the super-Gaussian exponent, with implications for IAW instabilities including crossed-beam energy transfer (CBET)³ . It was also theorized, however, that the tails of the EDF might deviate from the bulk distribution. Despite the many implications for laser-plasma experiments, these predictions remained largely untested for decades. I will discuss recent experiments at the University of Rochester Laboratory for Laser Energetics that have validated many of these predictions, including the shape of the laser-heated EDFs⁴ and their impacts on CBET⁵ and absorption.

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Stochastic sampling of dense matter

April 8, 2021

Chris Pickard
University of Cambridge

High pressure and materials research has been transformed by the ability to predict both the structures and properties of materials from first principles. In many cases these predictions have been later confirmed by experiment. In others they have provided fruitful new directions to explore.

This progress has been achieved through the combination of stochastic approaches with reliable and efficient first principles methods. Diverse ensembles of initial structures can be generated, and structurally optimized. The resulting low energy structures are candidates for stable, and metastable, phases and/or defects and interfaces that might be experimentally realized. Success, of course, depends on a sufficiently broad and thorough sampling of configuration space. 

 A purely random strategy, as employed by Ab Initio Random Structure Searching (AIRSS),[1,2] is entirely parallel, and a natural fit to the high throughput computation (HTC) paradigm. Challenging cases can be tackled by designing the initial random structures so that they focus the search in regions of configuration space that are anticipated to yield success. 

The design of these random “sensible” structures will be explored, along with some new directions [3] which promise to help our understanding of, and accelerate random search, along with applications to high pressure and materials research – from dense hydrides approaching room temperature superconductivity, to surprising astrophysical reactions and complex interfacial materials.

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Laser-driven coils, how well do they work?

April 1, 2021

Jonathan Peebles
Laboratory for Laser Energetics – University of Rochester

Over the last decade, magnetic fields generated from a laser-driven current passing through a coil have become increasingly popular. The laser driven coil (LDC) fills a unique role in HED and potentially ICF science by providing magnetic fields to laser facilities without the need for bulky, destructive pulsed power devices, which are often incompatible with lasers due to debris. B-dot probes, Faraday rotation, and transverse (perpendicular to the coil axis) proton radiography have been used in previous LDC experiments, where fields of 1 kilo-Tesla over a millimeter cubed volume are commonly cited. However, in the body of recent work studying these coils, kilo-Tesla level fields are not the only measurements that have been made. Fields from LDCs vary over several orders of magnitude with different designs and drivers; in some cases, the same experiment has magnetic field measurements that differ by close to an order of magnitude depending on which diagnostic is being used.

To clarify the results of previous work, and to gain an understanding of which diagnostics are more reliable in this regime, we performed LDC experiments on OMEGA-EP using all of the aforementioned diagnostics, and have introduced a new technique called axial proton probing. The diagnostic responses for the experiment were compared with those for a known magnetic field. Not only was a consistent result found between the four primary diagnostics, but the results provided information on the diagnostics themselves, and some additional insight that may help explain the findings of other previous experiments.

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SPARC and the high-field path to fusion energy

March 25, 2021

Martin Greenwald
MIT – Plasma Science & Fusion Center

The SPARC tokamak is currently under design as a mid-sized, deuterium-tritium (DT) burning magnetic confinement experiment. By employing novel high-temperature superconducting magnets, it will operate at 12.2 T in a device of a size and configuration similar to many experiments that have been deployed in the world’s fusion research program. Operation at high field will however, allow SPARC to be the world’s first experiment to create and confine a plasma that produces net fusion power. The performance to satisfy that mission has been defined as a fusion gain, Q > 2 and PFusion > 50 MW which would be comfortably more than the 25 MW of RF input power. Achieving this goal, we believe, would be a sufficient demonstration to place fusion firmly into the world’s energy plans. Significant margin against uncertainties in performance assumptions has been built into the design such that well-established physics predicts that SPARC could produce more than 140 MW of fusion power with Q > 10. Successful operation of SPARC would inform and enable the construction of a fusion pilot plant – a device with a major radius of about 3 m, producing over 500 MW of fusion power.

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Materials microstructures in high pressure experiments

March 11, 2021

Sébastien Merkel
Université de Lille – CNRS 

Microstructures define the arrangement of elements within a material, including phase distributions, grain sizes or grain orientations, and play a key role in defining materials properties. In the geosciences, microstructures affect the propagation of seismic wave and can be used to infer dynamic processes deep inside the Earth based on seismic observations. In materials science, microstructures affect the material’s mechanical properties, such, as strength. Phase transformations and plastic deformation can affect microstructures in a material and both effects can be studied under extreme conditions. In this seminar, I will show how new experiments can be used to study microstructures and the underlying physical mechanisms, both in static and dynamic experiments. 

In the first part of the presentation, I will look at phase transformations in minerals. I will present the technique of multigrain crystallography in the diamond anvil cell and how it can be used for follow the distribution of phases, number of grains, and grain orientations in materials undergoing phase transformation and deformation, with applications in the field of geophysics. In the second part of the presentation, I will show how new designs of laser-driven compression experiments coupled with X-ray free electron lasers that can be used to study stress and texture in polycrystals at extreme strain rates. I will show how the technique can be used along with results on the hcp phase of Fe. 

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Cognitive Simulation: combining simulation and experiment with artificial intelligence

March 4, 2021

Brian Spears
Lawrence Livermore National Laboratory

Large-scale scientific endeavors often focus on improving predictive capabilities by challenging theory-driven simulations with experimental data. Yet, both simulation and experiment have become overwhelmingly rich, with a complex of observables including scalars, vector-valued data, and various images. At Lawrence Livermore National Laboratory (LLNL), we are using modern artificial intelligence (AI) technologies to combine predictive simulation models with rich experimental data. We call this set of methods cognitive simulation. We will describe a strategic LLNL research effort aimed at using recent advances in deep learning, computational workflows, and computer architectures to develop improved predictive models.

We will present work from a wide range of applications, including using inertial confinement fusion research and experiments at the world’s largest laser, the National Ignition Facility (NIF), as a testbed. We will describe advances in machine learning architectures and methods necessary to handle strong nonlinearities and multimodal data in ICF science and other applications. We will also cover state-of-the-art tools that we developed to steer physics simulation, uncertainty quantification, and model training at enormous scale. We finally describe our ongoing efforts to combine these technologies to support a broad spectrum of Lab science applications. We will especially emphasize development of self-driving, high-repetition-rate lasers as a prototype for embedding AI technologies in next-gen experimental facilities.

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Overview and progress of materials experiments using the NIF ramp compression platform

February 25, 2021

Suzanne Ali
Lawrence Livermore National Laboratory

It is important to a number of fields to be able to accurately simulate processes and material responses under extreme conditions. Using sophisticated hydrodynamic codes, we can model giant impacts, planetary formation, and inertial confinement fusion implosions. The degree to which these simulations reflect reality, however, is dependent on how well we understand the materials and physics involved. Dynamic compression experiments are one of our vital tools for improving our understanding of material properties under extreme conditions, and I will be presenting on the development and status of the Ramp Compression Campaign on the National Ignition Facility. Obtaining high-precision, accurate ramp-compression data on materials under extreme conditions requires a well-validated platform. To date, we have addressed key issues in the development of ramp compression on the NIF: we have demonstrated the ability to ramp-compress metals to high pressure, including copper to ~23 TPa, we have developed analysis tools and experimental design predictive capability, and we have performed cross-platform equation of state (EOS) experiments validating the NIF platform.

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Plasma acceleration and high energy density science

February 18, 2021

Cameron Geddes
BELLA Center – Lawrence Berkeley National Laboratory

Ultrashort pulse lasers enable resonant excitation of plasma waves, efficiently driving structures that can accelerate particles at rates of in the range of a GeV per centimeter.  Guiding of such lasers in plasma structures formed by laser heating, capillary discharges and most recently by combining these two techniques has extended the depth of interaction from millimeter to tens of centimeter scale, enabling energies up to 7.8 GeV to be achieved.  Experiments to combine two such stages at multi-GeV energies are being prepared. At the GeV energy scale, compact electron beams are being used to develop novel compact photon sources including free electron lasers, and MeV photons from Thomson scattering. The same laser pulses are being used for acceleration of ions from solid targets using target sheath fields, and experiments are being planned to access more efficient ion acceleration regimes and new frontiers in high energy density science in the near future, supported by a new short focal length capability. While these experiments are proving the building blocks of a future plasma based high energy physics collider and photon sources, laser development holds the key to scaling to the repetition rate and precision required. 

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Exploring the most Extreme Conditions of Matter with ultra-bright X-rays

February 11, 2021

Siegfriend Glenzer
SLAC – National Accelerator Laboratory
Standford University

Normally, what surrounds us are gases, liquids, or solids. But elsewhere in the universe, 99% of the observable matter exists under extreme conditions that lead to exotic states of matter and the formation of plasmas and warm dense matter. Specifically, near the center of Jupiter, hydrogen becomes liquid or even solid - a process important to understand the evolution of our solar system. In the center of the sun, hydrogen is a plasma that burns itself up by nuclear fusion - a process humans want to harness for clean energy production on earth. In the mantle of Neptune, hydrogen and carbon cannot mix and are postulated to form giant diamonds – a process that can explain Neptune’s excess heat generation.  On the other hand, very hot plasmas are postulated to eject particles that we can observe as cosmic rays and that are a million times more energetic than mankind’s largest machines. At SLAC, we are now studying these extreme states of matter in the laboratory. We apply enormous pressures to earthbound samples and use our X-ray laser, the Linac Coherent Light Source, to take split-second photographs of the states that result. This lecture will describe these experiments. The information we are gathering provides fundamental insights into the physical properties of matter in extreme conditions whose understanding is important for modeling astrophysical processes and for pursuing controlled fusion.  To further advance this field, a new upgrade has been proposed to bring state-of-the art lasers to the LCLX X-ray beam and which has recently been reviewed and endorsed by the US user community.

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Solving the Many-Electron Schrödinger Equation with Deep Neural Networks

February 4, 2021

Matthew Foulkes
Imperial College London

Given access to accurate solutions of the many-electron Schrödinger equation, much of chemistry could be derived from first principles. Exact wavefunctions of interesting chemical systems are out of reach because they are NP-hard to compute in general, but approximations can be found using polynomially-scaling algorithms. The key challenge for many of these algorithms is the choice of approximate or "trial" wavefunction, which must balance efficiency versus accuracy. Neural networks have shown impressive power as accurate practical function approximators and promise as a compact trial wavefunctions for spin systems, but problems in electronic structure require wavefunctions obeying Fermi-Dirac statistics. Here we introduce a new deep learning architecture, the Fermionic Neural Network, as a powerful trial wavefunction for many-electron systems. FermiNet is able to achieve accuracy beyond other variational quantum Monte Carlo trial wavefunctions for a variety of atoms and molecules. Using no data other than atomic positions and charges, we predict the dissociation curves of the nitrogen molecule and hydrogen chain, two challenging strongly-correlated systems, to significantly higher accuracy than the coupled cluster method, widely considered the most precise scalable method for quantum chemistry at equilibrium geometry. This demonstrates that neural-network wavefunctions can improve the simple and appealing variational quantum Monte Carlo method to the point where it outperforms much more complex ab-initio quantum chemistry methods, opening the possibility of accurate direct optimization of wavefunctions for previously intractable molecules and solids.

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An alternate approach to the ignition regime for inertial confinement fusion

January 28, 2021

Steve MacLaren
Lawrence Livermore National Laboratory

In inertial confinement fusion, the threshold for ignition is a highly dynamic quantity as the sources and sinks of power in the hot spot can vary rapidly. In this talk, we consider the ignition condition as a race between heating and disassembly rates and combine this ansatz with a prior solution to the fusion hot-spot thermodynamics to develop a Lawson-like ignition criteria for pressure X confinement time (p-tau) versus temperature. Low-Z capsule designs reach the temperature for this threshold using as much of the shell as feasible as ablator but then are limited in confinement time due to low stagnated mass. An alternate approach, the pushered single shell (PSS) introduces a dense inner layer of Mo-Be alloy that is smoothly graded outward to pure Be, increasing the confinement time at stagnation and lowering the temperature requirement at the ignition threshold.  We describe both the design of and initial experimental results from this type of capsule.

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Investigating the Physics of Burning Thermonuclear Plasmas

January 14, 2021

Brian Appelbe
Imperial College London

A burning thermonuclear plasma is a unique plasma environment. It is characterized by extremely large temperatures, densities and fluxes of α particles and radiation that evolve over very short temporal and spatial scales. A key question in ICF is how the transport processes accompanying these conditions affect the achievable yield. Furthermore, the effects of a magnetic field in a burning plasma deserve attention due to current research into magnetically-assisted ICF and Magneto-Inertial Fusion schemes, and the possible presence of self-generated magnetic fields in ICF experiments. 

This talk summarizes results from a theoretical and computational investigation of burning plasmas, with and without magnetic fields, with a particular focus on two novel results. Firstly, it is shown that the interaction of a flux of α particles with a thermal plasma can induce an electronic current in the plasma. This current leads to new transport processes in burning plasmas. Secondly, the presence of a magnetic field in a propagating thermonuclear burn wave can lead to the formation of transport barrier at the burn front, reducing the rate of burn propagation.

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Future Perspective for High-Intensity Laser HED Science

January 7, 2021

Tammy Ma
Lawrence Livermore National Laboratory

The Advanced Photon Technologies (APT) high-intensity HED science program at LLNL seeks to provide new capability in, and drive forward frontier HED with short-pulse laser science and applications. We will discuss recent work to develop laser-driven sources for DOE applications, including multi-ps, very high energy short pulse laser experiments on the NIF-ARC laser; and describe work towards a closed-loop paradigm to accelerate scientific discovery, integrating high-repetition-rate laser experiments, machine learning, and physics-based cognitive simulations.  Using these novel tools, areas of active research include high-intensity laser interactions with matter, laser-driven secondary sources, plasma optics, future light sources, and the development of high-throughput diagnostics, targetry, modeling, and machine learning.  Finally, we will touch on efforts to develop a purposeful set of partnerships and engagements to better leverage both national and international HED efforts. 

Magnetic Signatures of Radiation-Driven Double Ablation Fronts

December 7, 2020

Louise Willingale
University of Michigan

Laser-plasma interactions produce strong temperature and density gradients that generate megagauss strength magnetic fields through the Biermann-battery effect. We performed experiments using the OMEGA EP laser system and used proton radiography to measure the strength, spatial profile, and temporal dynamics of the self-generated magnetic fields. The target material was varied and the plastic (CH), aluminum, copper, or gold targets exhibited different magnetic field structures. Mid-Z targets had two distinct regions of magnetic field, one produced by gradients from electron thermal transport and the second from radiation-driven gradients. Extended magnetohydrodynamic simulations including radiation transport reproduced the key experimental features, including the magnetic field generation and double ablation front formation.

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Exploring the universe through Discovery Science on the National Ignition Facility (NIF)

December 3, 2020

Bruce Remington
Lawrence Livermore National Laboratory

Highlights from the NIF Discovery Science program will be presented. Examples include nuclear reactions relevant to stellar nucleosynthesis [1]; equations of state at high pressures relevant to planetary interiors [2, 3, 4] and white dwarf envelopes [5]; Rayleigh-Taylor instabilities relevant to supernovae and supernova remnant evolution [6, 7, 8]; relativistically hot plasmas [9] and target normal sheath acceleration of protons on NIF ARC [10]; magnetic reconnection at high energy densities [11]; and high velocity interpenetrating plasmas that generate collisionless astrophysical shocks, magnetic fields, and accelerate particles relevant to cosmic ray generation [12, 13]. 

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Latest results from surveying the high-energy sky

November 19, 2020

Marcus Brüggen
Universität Hamburg

The new generation of radio interferometers, such as MeerKAT, ASKAP and LOFAR, as well as the recently launched X-ray observatory eROSITA provide a new view of the high-energy Universe.

They deliver new insights into a large range of astronomical phenomena, such as active galactic nuclei, cosmic rays and magnetic fields, galaxy clusters, the Warm-Hot Intergalactic Medium and Dark Energy.

In my talk I will present new results from on-going radio surveys and the first data from the Russian-German space observatory eROSITA. I will focus on my work on galaxy clusters and the filaments that connect them, cosmic magnetic fields and particle acceleration in cosmological structures.

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Calculating Melting Curves for Crystallizing Stars

November 5, 2020

Simon Blouin
Los Alamos National Laboratory

White dwarfs are burned-out stars condemned to a slow cooling that extends over billions of years. Thanks to this simple evolution, it is relatively easy to measure their ages, making them useful cosmic clocks to study the history of our Galaxy. Eventually, the dense C/O plasma that makes up their cores becomes so correlated that it freezes. This process releases latent heat as well as gravitational energy due to the sedimentation of the O-enriched solid. This new energy source temporarily slows the cooling of the white dwarf and it is important to precisely model it if those stars are to be used as accurate cosmic clocks. Both the melting temperature and the importance of O sedimentation depend on the exact shape of the C/O phase diagram. To obtain an accurate version of this phase diagram, we have developed a new technique to compute the melting curves of multi-component plasmas. It is based on the Gibbs-Duhem integration technique and semi-grand canonical Monte-Carlo simulations of the liquid and solid phases of the screened, partially relativistic, fully ionized mixture. This approach is 200 times less costly than the competing technique where two-phase molecular dynamics simulations are used, thus allowing a finer sampling of the phase diagram and a vastly superior mitigation of finite-size effects. Our results lead to an improved match between white dwarf cooling models and astronomical observations, and highlight the role played by minor species in white dwarf evolution. This now well-tested method also presents an interesting potential for non-astrophysical systems (e.g., alloys).

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Computing the thermodynamic and transport properties of water and hydrogen using machine learning potentials

October 29, 2020

Bingqing Cheng
University of Cambridge

A central goal of computational physics and chemistry is to predict material properties using first principles methods based on the fundamental laws of quantum mechanics. However, the high computational costs of these methods typically prevent rigorous predictions of quantities at finite temperatures, such as chemical potential, heat capacity and thermal conductivity.

In this talk, I will first discuss how to enable such predictions by combining advanced statistical mechanics with data-driven machine learning interatomic potentials. As an example [1], for the omnipresent and technologically essential system of water, a first-principles thermodynamic description not only leads to excellent agreement with experiments, but also reveals the crucial role of nuclear quantum fluctuations in modulating the thermodynamic stabilities of different phases of water. As another example [2], we simulated the high pressure hydrogen system with converged system size and simulation length, and found, contrary to established beliefs, supercritical behaviour of liquid hydrogen above the melting line.

Besides the computation of thermodynamic properties, I will talk about transport properties:

Ref [3] proposed a method to compute the heat conductivities of liquid just from equilibrium molecular dynamics trajectories.

During the second part of the talk, I will rationalize why machine learning potentials work at all, and in particular, the locality argument.

I'll show that a machine-learning potential trained on liquid water alone can predict the properties of diverse ice phases, because all the local environments characterising the ice phases are found in liquid water [4].

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Adapting x-ray microcalorimeter spectrometers to diagnose magnetically confined plasma

October 22, 2020

Megan Eckart
Lawrence Livermore National Laboratory

X-ray microcalorimeter spectrometers have been developed for astrophysics space missions, providing imaging arrays with few-eV spectral resolution, but they also offer advantages for use on magnetic fusion energy devices. We recently deployed one of our LLNL/NASA x-ray microcalorimeter spectrometers to the Madison Symmetric Torus (MST) facility at the Wisconsin Plasma Physics Laboratory (WiPPL) and recorded x-ray photons emitted by the helium-like Al impurity ions in a majority deuterium plasma. In this talk we’ll introduce microcalorimeter detectors and discuss the diagnostic capability for magnetic fusion energy experiments, highlighting our experimental work toward demonstrating this capability at MST.

Microcalorimeter spectrometers combine the best characteristics of the x-ray instrumentation currently available on fusion devices: high spectral resolution similar to an x-ray crystal spectrometer, and the broadband coverage (from 0.1 keV to above 12 keV) of an x-ray pulse height analysis system. Fusion experiments are increasingly employing high-Z plasma-facing components, and require measurement of the concentration of all impurity ion species in the plasma. This diagnostic has the capability to satisfy this need for multi-species impurity ion data, and will also contribute to measurements of impurity ion temperature and flow velocity, Zeff, and electron density.

This work is funded by the DOE Office of Science, FES Measurement Innovation program. It is a collaboration between LLNL, WiPPL, and NASA Goddard Space Flight Center.

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Coupling of Laser Angular Momentum to Plasma for the Generation of Kilo-Tesla Magnetic Fields

October 15, 2020

Andrew Longman
University of Alberta  
Post Doc Fellow, HEDS Center

The generation and application of strong (kilo-Tesla) magnetic fields for the study of magnetized ICF, fast ignition, laboratory astrophysics, and particle acceleration/guiding is an area of great and growing interest. In this presentation, we will explore the novel approach for generating kilo-Tesla scale magnetic fields via the inverse Faraday effect driven by lasers carrying angular momentum.

The inverse Faraday effect describes the coupling of laser angular momentum directly to plasma electrons and has the unique advantage of not requiring a solid or structured target, making it suitable for both high repetition rate experiments, and in configurations where solid coils are not permissible. 

We will numerically explore the inverse Faraday effect in under-dense plasmas with both circularly polarized (CP) beams, and with structured beams carrying orbital angular momentum (OAM). The simulations are carried out using full 3D PIC calculations with Titan-like laser parameters. We demonstrate kilo-Tesla fields extending more than 100 microns in length, and with decay times on the order of picoseconds when driven by 100 femtosecond pulses. We will discuss the advantages and disadvantages of this approach, as well as discussing possible experimental configurations to demonstrate and measure the fields. 

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The High Energy Density Science Center: 2020 year in review

October 8, 2020

Frank Graziani
Lawrence Livermore National Laboratory  

The High Energy Density Science Center (HEDSC) was established in late 2015 through a joint agreement between the Physical and Life Sciences, Weapons and Complex Integration, and NIF and Photon Science directorates. The Center’s goal is to build a high energy density community through support of, and collaboration with, academic partners and to integrate those efforts with the programs at LLNL. The Center is built around four focus areas: (1) education, (2) outreach through workshops and seminars, (3) workforce pipeline to the programs, and (4) enabling research collaborations between LLNL and academic partners. In this talk, we will present the year 2020 in review and look at what lies ahead in 2021.  The year 2020 was a year of both growth and challenges. 2020 saw growth in education programs with a quarter long course on HED diagnostics offered in collaboration with UCSD in addition to the selection of a new HEDS Center postdoctoral fellow. Besides educating the next generation of HED scientists, the HEDS Center continues training students through internships and thesis opportunities. Our outreach and collaboration activities continue through a vibrant seminar series, university partnerships, a planned sabbatical program and visits to community colleges and undergraduate institutions. Inclusion and diversity is an important aspect of our mission and the Consortium for HEDS (FAMU, UC Merced, Morehouse and LLNL) received new funding from NNSA. The Consortium’s goal is to provide research and education opportunities in HEDS for under-represented groups and thereby increase diversity in the field.  With the arrival of COVID and working off-site, the HEDS Center had to react to the change. The summer internship program continued, on-line. The sabbatical program and visits to community and undergraduate colleges was put on hold and instead, the Center decided to produce a series of HEDS educational videos for the public. In 2021 we hope to bring back the sabbatical program and outreach visits to K-12 and community and undergraduate colleges. A call for a new HEDS Fellow postdoc position is out and more HED education programs are planned. In 2021 we will continue to foster international collaboration in HEDS, bring on another thesis student from FAMU and continue the internship program.

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MeV X-ray Radiography on High Intensity Lasers using Compound Parabolic Concentrators

September 10, 2020

Dean Rusby
Lawrence Livermore National Laboratory

High Intensity laser interactions produce a high current beam of electrons that can be used for many applications. One such applications is the production of a high energy and bright x-ray source via bremsstrahlung. These x-rays possess several qualities, such as pulse durations and high spatial resolutions, that make them ideal for radiography applications. The key to making the brightest x-ray source is converting as much energy as possible from the laser into high energy electrons, which often requires the highest intensity lasers. We have developed an alternative method using a novel target design, the compound parabolic concentrator (CPC) that can be used to confine, focus and couple more laser energy into the electrons. Experimental measurements have been performed that demonstrate the capabilities of CPCs targets and show that they can produce electrons and x-rays that far outperform planar targets. We also demonstrate the enhancement in x-ray radiography in the MeV regime.

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Solar Models and Opacities

August 27, 2020

Joyce Guzik
Los Alamos National Laboratory

In 2004, improved analyses of solar spectra resulted in a downward revision of the abundances of elements heavier than hydrogen and helium, in particular, the elements C, N, and O. Solar models evolved using these lower abundances showed discrepancies with inferences from solar oscillation observations (helioseismology). This problem has not been solved satisfactorily in the intervening 16 years. How serious/important is this problem? 

This talk will give an overview of how standard solar models are calculated and of the constraints from helioseismology. The talk will also present results of some attempts to change input physics or assumptions in the standard solar model, including opacity modifications, to try to resolve the discrepancies. I would like to have a discussion with the participants after the talk about which directions appear most promising to resolve the problem. 

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The response of solids to irradiation by massive particles

August 20, 2020

Michael Demkowicz
Department of Materials Science and Engineering, Texas A&M University

Solids are subjected to bombardment by high-energy, massive particles (electrons, neutrons, ions) in a wide range of energy, weapons, and astrophysical scenarios. I will summarize the fundamental mechanisms by which solids—especially metals—respond to such irradiation, from the dissipation of the particle’s energy in the course of a collision cascade to the generation and subsequent evolution of damage in the material. I will then present a selection of recent research on radiation response—carried out primarily using molecular dynamics simulations—focusing on amorphous and nanocomposite metals. I will conclude with a discussion of prospects for developing novel, radiation-resistant materials based on insights gained from this research.

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Speed-Limited Particle In Cell and Getting to the Answer Faster

August 6, 2020

John Cary
University of Colorado
Tech X

Speed-limited particle-in-cell (SLPIC) is an algorithm for achieving orders of magnitude speedup of simulations in cases where kinetic electron effects are important, yet the motion is slow compared with the electron thermal velocity.  This is the case for nearly all low-frequency plasma dynamics, but it is especially so for low-temperature plasma discharges where (1) the ions are cold compared with the electrons and (2) the ions are comparatively even more massive, e.g., in Argon discharges.  This method has several quirks, including the need to have both a charge and a current weight. This method has now been generalized to include collisions and secondary emission, thereby allowing it to be applied to determine the Paschen curve in irregular geometries.  The theory of this new algorithm will be presented along with results.  In addition, there will be a few remarks on other subjects, such as device computing and user-friendly code, that contribute to reducing time to solution.

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The Incorporation of Machine Learning into Scientific Simulations at Lawrence Livermore National Laboratory

July 30, 2020 

Katie Lewis
Lawrence Livermore National Laboratory

Scientific simulations have driven computing at Lawrence Livermore National Laboratory (LLNL) for decades.  During that time, we have seen significant changes in hardware, tools, and algorithms.  Today, data science, including machine learning, is one of the fastest growing areas of computing, and LLNL is investing in hardware, applications, and algorithms in this space.  While the use of simulations to focus and understand experiments is well accepted in our community, machine learning brings new challenges that need to be addressed.  I will explore applications for machine learning in scientific simulations that are showing promising results and further investigation that is needed to better understand its usefulness.

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NIF, An Unexpected Journey

July 9, 2020

Mike Campbell
Laboratory for Laser Energetics, University of Rochester

The Richtmyer-Meshkov instability (RMI) develops upon impulsive acceleration of the interface between fluids of different acoustic impedance. The RMI consists in the unbounded growth of any perturbations initially present on the interface, and it is a consequence of the baroclinical deposition of vorticity on the interface. Eventually, the interface develops into a turbulent layer leading to the mixing of the two fluids. Flows of this type occur in nature and in man-made settings, over very large ranges of length, time, and energy scales: from experiments for the attainment of inertial confinement fusion (ICF), to supersonic combustion systems, to supernovae explosions. Shock-induced mixing has very negative effects in the case of ICF while it holds great potential to improve the combustion processes in the case of hypersonic engines. Two of the fundamental parameters that govern these phenomena are the Atwood number (measuring the original density contrast) and the Mach number. The experiments described here take place in a vertical shock tube with large, square internal cross section. The initial condition consists of a shear layer between helium and argon. A downward-propagating planar shock wave of Mach number 1.6 or 2.2 accelerates the interface. Measurements of 2-D concentration and velocity fields in a vertical cross section of the flow are obtained using planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV), respectively. Field statistics and integral quantities are extracted from these data and turbulence and mixing parameters are evaluated. From them, the outer scale Reynolds number is estimated to reach between 10,000 and 20,000 at the late times, indicating that the flow has crossed the threshold of fully developed turbulence.

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Nuclear Science Experiments for National Security and Fundamental Science on NIF

July 2, 2020

Dawn Shaughnessy
Lawrence Livermore National Laboratory

The National Ignition Facility (NIF) is the world’s largest laser and is a major contributor to the Stockpile Stewardship Program conducting state of the art science in high energy density physics including fusion research. Developing the mission, science, technology, and support for projects of the scale of NIF is a demanding and multifaceted enterprise. There are many lessons to be learned from the NIF experience that can be applied in the quest to secure any future large-scale facility. This presentation will include a historical perspective on inertial confinement fusion (ICF) at Lawrence Livermore National Laboratory and the Stockpile Stewardship Program that motivated the NIF and the scientific and political strategy that ultimately secured the Facility.

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Graph theory ideas reveal long range conduction pathways

June 11, 2020

Maria Gomez
Department of Chemistry, Mount Holyoke College

Finding long range conduction pathways is challenging when the system includes both fast frequency modes needing short time step force integration and slow frequency modes requiring long time samples.  Methods such as kinetic Monte Carlo (kMC) avoid the integration of steps and instead use probabilities to choose the next moves and advanced the clock based on the move chosen.  Nevertheless, extracting fast conduction mechanisms between traps can be challenging.  Centrality methods based on the number of steps to return to key sites or vertices in a graph have been used to identify the most central areas in a graph.  However, physical systems with traps and highways have steps with distinctly different barriers making steps non-equivalent.  This seminar reviews both traditional and new graph theory schemes for finding long range pathways with a special focus on a centrality measure that instead of using the number of steps to return to sites considers the time of first returns [1] and its applications to proton conduction in doped perovskites[2].   In these systems shown below a single image including all proton binding sites with the most central sites shown in darker shades of grey shows both trapped regions and fast conduction highways. 

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Jupiter Revealed

June 4, 2020

Marius Millot
Lawrence Livermore National Laboratory

We will stream the documentary movie Jupiter Revealed from the BBC's Horizon show. This film describes new discoveries about the planet Jupiter following the success of the NASA Juno probe mission and how these discoveries help us better understand how planet form and evolve. One of the segments was filmed at the National Ignition Facility in Livermore. This segment highlights a series of Discovery Science experiments conducted at NIF to explore the exotic properties of metallic Hydrogen.

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Laser-Plasma Accelerators: Riding the Wave to the Next Generation X-Ray Light Sources

May 14, 2020

Félicie Albert
Lawrence Livermore National Laboratory

Particle accelerators have been revolutionizing discoveries in science, medicine, industry and national security for more than a century. An estimated 30,000 particle accelerators are active around the world. In these machines, electromagnetic fields accelerate charged particles, such as electrons, protons, ions or positrons to velocities nearing the speed of light. Although their scientific appeal will remain evident for many decades, one limitation of the current generation of particle accelerators is their tremendous size (typically a mile long) and cost, which often limits access to the broader scientific community. A plasma is a neutral medium composed of negatively charged free electrons and positively charged ions. Plasma can sustain electrical fields three orders of magnitude higher than that of conventional particle accelerators. Acceleration of electrons in laser-driven plasmas has been drawing considerable attention over the past decade. These laser wakefield accelerators promise to dramatically reduce the size of accelerators and revolutionize applications in medicine, industry and basic sciences.

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Harnessing Fusion: Building a Star on Earth

May 7, 2020

Tammy Ma
Lawrence Livermore National Laboratory

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) is the world’s largest and most energetic laser system.  The NIF is built to create very extreme states of matter – similar to those found in the interiors of stars and planets.  Here, scientists and engineers are working hard to demonstrate sustainable fusion burn – the same reaction that occurs in the sun – to one day harness as a source of limitless, clean energy.

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Towards Enabling Predictive Scale-Bridging Simulations through Active Learning

April 16, 2020

Jeffrey Haack
Los Alamos National Laboratory

Designing effective methods for multiscale simulation is a longstanding challenge. Our goal is to advance the state of the art for Machine Learning (ML) beyond sequential training and inference and facilitate scale bridging through novel techniques. Active Learning (AL) is a special case of semi-supervised ML in which a learning algorithm is able to interactively use the fine-scale model to obtain the desired outputs at new data points, making it ideal for concurrent scale-bridging. Our AL procedure will dynamically assess uncertainties of the ML model, query new fine scale simulations as necessary, and use the new data to incrementally improve our ML models. This capability will be demonstrated on two applications: transport in nanoporous media (e.g., for hydraulic fracturing) and inertial confinement fusion (ICF), validating against experimental data. Although the physics is quite dissimilar, both applications represent problems that suffer from inaccurate macro-scale predictions due to subscale physics that are ignored.

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Laboratory equation of state measurements of the carbon envelopes of white dwarf stars

March 26, 2020

Andrea Kritcher
Lawrence Livermore National Laboratory

White dwarfs (WD) represent the final state of evolution for the vast majority of stars 1–3 . Certain classes of white dwarfs pulsate4, 5 , leading to observable brightness variations whose analysis with theoretical stellar models uniquely probes their internal structure. Modeling of these pulsating WD stars provides stringent tests of white dwarf models and a detailed picture of the outcome of the late stages of stellar evolution 6. However, these high energy den- sity states are extremely difficult to access and diagnose in the labo- ratory and as a result theory is largely untested at these conditions. Here, we present equation of state (EOS) measurements of matter at pressures ranging from 100-450 million atmospheres, where the understanding of WD stars is sensitive to the EOS and where mod- els show significant differences. We measure the pressure-density relationship along the principal shock Hugoniot of hydrocarbon to within five percent. The observed maximum compressibility is consistent with theoretical models that include detailed electronic structure, are used in calculations of WD stars and inertial con- finement fusion (ICF) experiments7, 8 , and predict an increase in compressibility due to ionization of the inner core orbitals of car- bon. We also find that detailed treatment of the electronic structure and the electron degeneracy pressure are required to capture the measured shape of the pressure-density evolution for hydrocarbon before peak compression.

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Modern tests of vacuum-polarization effects in strong laser fields

March 5, 2020

Antonino Di Piazza
Max Planck Institute for Nuclear Physics

Quantum electrodynamics (QED) is a well-established physical theory, and its predictions have been confirmed experimentally in various regimes and with extremely high accuracy. However, there are still areas of QED that deserve theoretical and experimental investigation-- especially when physical processes occur in the presence of intense background electromagnetic fields, i.e., of the order of the so-called “critical” field of QED [1]. In the presence of electromagnetic fields of such high strength, for example, even the vacuum becomes unstable and electron-positron pair production spontaneously occurs.

After a broad introduction on strong-field QED, the seminar will describe different regimes of this theory and introduce present and upcoming experimental efforts to test it under such extreme conditions. As a prominent theoretical example of open problems, the talk will focus on how the properties of the vacuum are altered by intense background electromagnetic fields [1,2]. Unlike classical electrodynamics, QED predicts that electromagnetic fields also interact in vacuum through virtual electron-positron pairs. This is the physical origin of a number of so-called vacuum-polarization effects, which will be reviewed. Finally,  the talk will discuss recent experimental suggestions to observe these effects by means of intense laser beams [3].

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Theoretical foundations of quantum hydrodynamics for dense plasmas

February 27, 2020

Zhandos Moldabekov
Al Farabi Kazakh National University
Institute of Experimental and Theoretical Physics

Methods for large-scale simulation of time-dependent features of quantum plasmas are crucial for fundamental and applied problems. The microscopic theory of quantum hydrodynamics (MQHD), which goes beyond the random phase approximation by employing ab initio data for dynamic local field corrections, will be presented [1,2]. Quantum hydrodynamic theory is presented as the orbital averaging of the MQHD equations. The range of applicability of QHD is analyzed. Going beyond the gradient expansion approximation, a fully non-local Bohm potential is derived [1,2].

The results for the dynamic structure factor of ions at dense plasma and warm dense matter (WDM) conditions will also be presented [3,4]. The applicability of the classical generalized hydrodynamics and of the QHD for the description of collective oscillations of ions will be discussed. Additionally, the results for the energy loss characteristics of electrons at WDM and dense plasma conditions, where the static local field correction from ab initio QMC simulations [5] was used, will be presented. Finally, the first results for the friction coefficient acting upon ions due to electrons will be reported, where friction represents a correction to the Born-Oppenheimer approximation within Langevin dynamics simulation of ions.

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The Efficacy of Simple Models for Non-Ideal Plasmas

February 6, 2020

Liam Stanton
Department of Mathematics and Statistics, San Jose State University

Computational models are formulated in hierarchies of variable fidelity, often with no quantitative rule for defining the fidelity boundaries.  This talk will explore the extent to which simple screening models for ionic interactions can capture both the equilibrium structure and transport processes of non-ideal plasmas, such as dense plasmas found in inertial confinement fusion (ICF) experiments and ultra-cold neutral plasmas (UNPs). We have developed a simplified effective potential approach within a Boltzmann-type framework that yields accurate and computationally efficient fits for all of the relevant cross-sections and collision integrals needed to construct transport coefficients.  Our results, which span the UNP-to-ICF regimes, have been validated with molecular dynamics (MD) simulations for self-diffusion, inter-diffusion, viscosity, thermal conductivity, and stopping power with promising comparisons to UNP experiments as well.  MD simulations have also been used to examine the underlying assumptions of this effective Boltzmann approach through a categorization of behaviors of the velocity autocorrelation function.  In addition, we have constructed a dataset from a wide range of atomistic computational models to reveal the accuracy boundary between these higher-fidelity models and simpler screening models based on finite-temperature orbital-free density functional theory. The symbolic decision boundary is discovered by optimizing a support vector machine on the data through iterative feature engineering. This machine-learning approach reveals a symbolic rule that is independent of the computational method and directly reveals the central role of atomic physics in determining accuracy.

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Generalized Lawson Criteria for Inertial Confinement Fusion

January 23, 2020

Robert Tipton
Computation Physicist, Lawrence Livermore National Laboratory

In 1955 John Lawson proposed a criterion of ignition in the context of magnetic fusion. Over the decades, Lawson’s original criterion has been generalized in several different ways for the case of Inertial Confinement Fusion (ICF). This talk will describe the different generalizations and demonstrate their effectiveness with numerical simulations. The simulations indicate our best shots on NIF have a generalized Lawson criterion which is about 80% of the expected ignition value.

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Spin crossover in iron in lower mantle minerals

January 16, 2020

Renata Wentzcovitch
Department of Applied Physics and Applied Mathematics and
Department of Earth and Environmental Science, Lamont Doherty Earth Observatory, Columbia University

Pressure and temperature induced spin-state change in iron in lower mantle minerals is an unusual phenomenon with previously unknown consequences.  High pressure and high temperature experiments have offered a wealth of new information about this class of materials problems, which includes the insulator to metal transition in Mott systems. I will discuss key experimental data, contrast them with our ab initio results and thermodynamic models, show the implications for fundamental phenomena taking place at the atomic scale and their macroscopic manifestations, and discuss potential geophysical consequences of this phenomenon.   

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Simulating electronic structure under ultrafast compression with DFT and TDDFT

January 9, 2020

David Strubbe
Department of Physics, University of California, Merced

Many electronic-structure studies of warm dense matter have focused on the end-point of compression, in total thermal equilibrium, or perhaps a partial equilibrium with different electronic and lattice temperatures. In compression experiments at laser facilities such as NIF, the shock speed can reach 10 km/s, which is equivalent to 10 Å/fs. This means that a direct calculation of the compression process itself is actually feasible with first-principles methods because the compression operates on the natural length and time scales of these methods, such as time-dependent density-functional theory (TDDFT). The seminar will discuss progress toward TDDFT simulations of ultrafast compression processes, assessing numerical and conceptual issues of electronic dynamics with a rapidly varying unit cell. It will also present results on changes in band structure and optical absorption spectra of diamond structure silicon under extreme pressure but low temperature, in which transitions take place from semiconductor to semimetal to metal on short timescales before the structure can change to the other phases of silicon favored at high pressure. Finally, the seminar will cover  our work on non-additive kinetic energy functionals, which can provide a route to improved DFT-based average-atom or neutral pseudo-atom models for high energy density science. This work is being carried out as part of the Consortium for High Energy Density Science.

Thermonuclear reactions probed at stellar core conditions with laser-based inertial confinement fusion

November 14, 2019

Daniel Casey
Lawrence Livermore National Laboratory

Laser-based inertial confinement fusion (ICF) implosions provide a path to study reactions in thermonuclear plasmas. However, ICF experiments have significant challenges not found in accelerator experiments. This talk explored how these issues can be overcome and ICF implosions can be used to make nuclear measurements in some specific circumstances. In particular, the method of yield ratios is used to infer astrophysical S-factors for two reactions using a third as a reference. The resulting data show excellent agreement with evaluations and prior accelerator data for these two reactions, bolstering confidence in this method. This technique is now being explored as a candidate for a future plasma-electron-screening experiment to attempt to observe enhancements to reaction rates in the presence of plasma electrons.

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Astro-glaciology and habitability of planetary abysses: Experimental exploration of the richness of aqueous systems thermodynamics at high pressures

November 7, 2019

Baptiste Journaux
University of Washington

The presenter discussed his work related to studying the habitability of planetary interiors. The research team operated diamond anvil cell X-Ray diffraction experiments coupled with new advanced thermodynamic representation that allow scientists to accurately reproduce experimental data. This modular framework, based on local basis functions parameterization of a thermodynamic potential, allows accurate prediction of phase stability and realistic equilibrium thermodynamics in stable and metastable regime, and can be used in the representation of UHP experimental results.

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Determination of the ion temperature in high-energy-density plasmas using the Stark Effect and Zeeman Effect induced by intense laser light

November 4, 2019

Yitzhak Maron
Weizmann Institute of Science, Israel

The presenter discussed an experimental method to determine the ion temperature based on the effect of ion coupling on the Stark line shapes. It was implemented on a Z-pinch stagnation, confirming that the ion thermal energy is small compared to the imploding-plasma kinetic energy, where most of the latter is converted to hydromotion. The seminar also included a discussion of the Zeeman Effect induced by intense laser light, with the presenter describing an analysis of spectral line shapes of hydrogen-like species subjected to fields of electromagnetic waves showing that the magnetic component of an electromagnetic wave may significantly influence the spectra. In particular, the Zeeman Effect induced by powerful visible or infrared lasers can be experimentally observed.

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Material property measurements and theory into stellar regimes

October 31, 2019

Damian Swift
Lawrence Livermore National Laboratory

In real-world situations such as in stars, the behavior of matter at extreme conditions can be very sensitive to variations between widely used equation of state (EOS) models The presenter discussed work done by his team to develop experimental platforms accessing the EOS and atomic physics of matter in stellar regimes. Shock states up to ~80 TPa can be deduced from spherically converging shocks at LLNL’s National Ignition Facility, and the team recently improved the analysis to reduce uncertainties and infer off-Hugoniot data.

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Perspectives on research in computational plasma physics with applications to experiments

October 17, 2019

Bruce Cohen
Lawrence Livermore National Laboratory

The presenter discussed his perspective on the development and application of computational plasma physics to plasmas in nature and in the laboratory.  The examples described illustrated both fundamental plasma phenomena and the behavior of laboratory plasmas. Some of the specific examples included simulations of micro-instabilities in magnetic mirror, tokamak, and spheromak plasmas; laser-plasma interactions; and Knudsen-layer phenomena as it affects fusion performance. The examples also illustrated the development of models and algorithms that are well suited to simulating the phenomena of interest.

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The many faces of matter inside neutron stars

October 10, 2019

Fridolin Weber
San Diego State University and
University of California, San Diego

In this talk, the presenter provided an overview of our current understanding of the composition of neutron stars. Models for the equation of state of dense neutron star matter were also presented, which are constrained by the latest nuclear and astrophysical data. Particular emphasis was put on quark de-confinement in the core regions of neutron stars. Finally, the phase diagram of hot and dense proto-neutron star matter was discussed, and possible instabilities in such matter were pointed out.

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The High Energy Density Science Center: A year in review

October 3, 2019

Frank Graziani
Lawrence Livermore National Laboratory

In this talk, the director of LLNL’s HED Science Center presented the 2019 year in review and looked at what lies ahead in 2020. In 2019, the Center saw growth in its education programs, selected a deputy director, established a new HEDS Center postdoctoral fellowship, participated in LLNL’s Ambassador program, established closer ties with Japan’s HED efforts, delivered a vibrant seminar series, hosted summer student and thesis programs, and engaged in collaborations with many academic partners, including several minority serving institutions. The presenter also discussed plans for 2020, including a consortium of universities engaged in HED science education, selection of the HEDS Center postdoctoral fellow, hosting sabbatical programs, and various outreach programs.

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Continuum dynamics and reactions in light nuclei

September 19, 2019

Sofia Quaglioni
Lawrence Livermore National Laboratory

An overarching goal of nuclear physics is to arrive at the comprehensive understanding—in terms of the laws of quantum mechanics and the underlying theory of the strong force (quantum chromodynamics)—of atomic nuclei and their interactions, and to use this understanding to accurately predict nuclear properties that play a fundamental role in explaining the inner workings of the Universe or are critical to national security. The presenter discussed first-principles calculations of nuclear structural and reaction properties to predict thermonuclear reaction rates of interest for fusion energy technology and stellar nucleosynthesis.

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Overview of laser-produced relativistic electron-positron pairs

September 12, 2019

Hui Chen
Lawrence Livermore National Laboratory

Intense lasers are now the brightest source of relativistic electron-positron pair jets in the laboratory—up to 10¹² of highly relativistic pairs are routinely produced in experiments at LLNL and other high power, short pulse lasers with energies from 100-2000 J. This talk reviewed experimental results to date. In addition, the presenter discussed the near-future in this research area, exploiting laser-produced pair jets for fundamental plasma physics studies, for diagnosing high-energy-density physics, and for laboratory experiments mimicking the physics processes in various astrophysical observations including Gamma Ray Bursts.

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The exotic properties and structure of superionic water ice at Uranus and Neptune interior conditions

August 29, 2019

Federica Coppari
Lawrence Livermore National Laboratory

Since Bridgman’s discovery of five ice phases in 1912, studies on the extraordinary polymorphism of H₂O have documented 17 crystalline and several amorphous structures, as well as metastability and kinetic effects. Particularly intriguing is the prediction that water becomes superionic—with liquid-like hydrogens diffusing through the solid lattice of oxygen—when subjected to extreme pressures exceeding 100 GPa and temperatures above 2,000 K. Optical measurements along the Hugoniot curve of ice VII showed evidence of superionic conduction but did not confirm the microscopic structure of superionic ice. The presenter discussed recent experiments at the OMEGA Laser that used shockwaves to simultaneously compress and heat liquid water samples to 100–400 GPa and 2,000–3,000 K. In-situ x-ray diffraction shows that under these conditions, water solidifies within a few nanoseconds into nanometer-sized ice grains that exhibit unambiguous evidence for the crystalline oxygen lattice of superionic water ice. These data allow us to document a temperature- and pressure-induced phase transformation from a body-centered-cubic ice phase to a novel face-centered-cubic, superionic ice phase.

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Recombination in line broadening

August 15, 2019

Thomas Gomez
Sandia National Laboratories

Broadening of atomic lines is caused by a time-dependent perturbation on the atom due to the plasma ions and electrons surrounding it. The relative importance of each broadening mechanism depends on transitions and conditions. The broadening due to slow-moving ions is well understood compared with the broadening from electrons due to more complex time-dependent quantum-mechanical interaction with the radiating atom. In fact, there is a long-standing discrepancy between modeled and measured line width for these Li-like ions; this is known as the isolated-line problem. The presenter discussed common approximations and limitations in electron-broadening calculations, as well as the impact of our recent investigations on the isolated-line problem.

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Nuclear astrophysics experiments at heavy-ion storage rings

August 1, 2019

Yuri Litvinov
GSI Helmholtzzentrum für Schwerionenforschung, Germany

Storage of freshly produced radioactive particles in a storage ring enables efficient use of the same rare ion multiple times. Employing heavy-ion storage rings for precision physics experiments with highly-charged ions (HCI) is a rapidly developing field of research. At the heavy-ion storage ring ESR at GSI, construction of dedicated heavy-ion storage rings have enhanced experimental capabilities by enabling stored and cooled secondary HCIs in the previously inaccessible low-energy range. The presenter highlighted research programs at heavy-ion storage rings, including physics experiments at the intersection of atomic, nuclear and possibly plasma physics.

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Deep learning for non-local thermodynamic equilibrium in hydrocodes

July 25, 2019

Gilles Kluth
French Alternative Energies and Atomic Energy Commission, France
Commissariat à l'énergie atomique et aux énergies alternatives (CEA)

The presenter discussed the team’s work developing new techniques to accelerate radiation hydrodynamics simulations. The ultimate goal is to develop a fast representation of detailed atomic physics packages to give absorption coefficients, emissivities, and ionization states. The fast representation—in this case a deep neural network—can be called in place of a traditional physics package. The neural network is dramatically faster and uses substantially less memory. The team examined the NLTE physics of mid-Z materials for ICF simulations as a test application. They used Cretin for in-line collisional-radiative computations in the rad-hydrocode HYDRA, and they trained a deep neural network on a set of Cretin data under a broad set of plasma conditions. Finally, they replaced the in-line atomic physics computation in HYDRA with the well-trained neural network accelerator.

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Fusion, beams, and qubits

July 18, 2019

Thomas Schenkel
Lawrence Berkeley National Laboratory

The presenter discussed recent experiments at the BELLA petawatt laser that expanded its focus on electron acceleration to also include ion acceleration and laser-matter interactions with solid targets. The 1-Hz repetition rate of BELLA allows parametric explorations with thousands of shots. One example is nuclear reactions in plasmas and moderately hot targets, where unreasonably high values of apparent electron screening potentials have been observed in studies of light ion fusion that now can be investigated with higher precision. Pulsed ion beams provide access to the time domain of ion–solid interactions, and they have been used to drive materials very far from equilibrium. The talk reported on studies of color center qubit formation and damage accumulation in semiconductor devices with intense, pulsed ion beams.

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Experimental observation of ion acoustic waves in warm dense methane

June 27, 2019

Thomas White
University of Nevada, Reno

The study of warm dense matter (WDM) has important applications, such as understanding controlled thermonuclear fusion and in material processing. Laboratory experiments are now able to create WDM states with a range of techniques, allowing critical tests of theory and modelling. Still, the experimental possibilities to diagnose dense matter are rather limited. Recent advances in free-electron laser technology have produced X-rays with a high enough flux in a small enough bandwidth to allow the properties of WDM to be probed on a shot-by-shot basis. The presenter provided an overview of X-ray scattering experiments, along with preliminary results of an experiment principally designed to measure the ion-ion dynamic structure factor and sound speed in warm dense methane. Some of these results were also compared to results from quantum and classical atomistic simulations.

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Ion friction in strongly coupled plasmas

June 13, 2019

Scott Bergeson
Brigham Young University

The presenter discussed his research aimed at understanding ion-ion interactions in a multi-species, strongly coupled environment. Ion mass provides spatial confinement in high-density, laser-driven implosions of solid ices. Careful modeling of the target, as well as the laser profile and precision cryogenic targetry, are required to create targets of the right size, shape, structure, and density to optimize fusion yield. These efforts attempt to balance the competing demands of temperature, density, and size requirements for achieving self-sustained nuclear burn. Inertial confinement occurs as a competition between the ion response to the field generated by the other charges in the plasma (expansion) and the friction forces generated as distributions of ions move past each other (confinement). The presenter’s research team designed experiments to examine aspects of this confinement using a high-energy-density-plasma simulator.

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Frontiers of high-pressure research at the European Synchrotron Radiation Facility

June 6, 2019

Sakura Pascarelli
European Synchrotron Radiation Facility (ESRF)

The presenter provided an overview of static and dynamic compression activities and recent scientific results obtained at the ESRF. She also discussed future projects and the unique science opportunities offered by ESRF’s Extremely Brilliant Source (EBS), which will be operational in 2020. The EBS will offer significantly higher flux density and higher coherence, leading to important perspectives for extreme matter studies. She described three EBS-related projects: (1) a high-flux, nano-X-ray, diffraction beamline for science at extreme conditions; (2) pushing the limits of nuclear resonant scattering in energy and spatial resolution; and (3) a high brilliance EXAFS beamline optimized for time resolved and extreme conditions applications.

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Implications of Bell’s Theorem: What physicists get wrong

May 23, 2019

Kenneth Wharton
San Jose State University

Despite the fact that Bell’s Theorem tells us something profound about our universe, too many physicists dismiss it as a reconfirmation of something we already knew.  Many others have a misapprehension of the main result, incorrectly believing that it rules out hidden variables or does not apply to probabilistic models.  And even experts in quantum foundations are sometimes unaware of subtleties concerning the role of an “arrow of time” in Bell’s analysis, and an associated time-symmetric option for modeling entanglement phenomena in spacetime.  This talk attempted to clarify these and other issues, surveying Bell’s Theorem and its fascinating implications.

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Ab initio thermodynamic and dynamics results for the warm dense electron gas

May 16, 2019

Michael Bonitz
Kiel University, Germany

The presenter discussed his team’s work regarding warm dense matter (WDM), including obtaining ab initio thermodynamic results for the electron component in WDM based on novel quantum Monte Carlo (QMC) simulations. Based on these results, the first ab initio parametrization of the exchange-correlation free energy F_xc has been presented, which is a key input for DFT simulations of WDM.

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X-ray sources from laser-plasma acceleration: Development and applications for high energy density sciences

May 9, 2019

Félicie Albert
Lawrence Livermore National Laboratory

The presenter discussed particle acceleration in laser-driven plasmas as an alternative to generating x-rays via high brightness x-ray sources, such as synchrotrons and x-ray free electron lasers (XFELs). When short, intense laser pulses are focused into a gas, it produces electron plasma waves in which electrons can be trapped and accelerated, a process known as laser-wakefield acceleration (LWFA). Betatron x-ray radiation, driven by electrons from LWFA, has unique properties that are analogous to synchrotron radiation, with a 1000-fold shorter pulse. This source is produced when relativistic electrons oscillate during the LWFA process. This approach to generating x-rays via laser-plasma acceleration offers the potential to overcome some of the drawbacks of using synchrotrons and XFELs, including their size and cost.

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Understanding strongly correlated matter with experiments and simulations

May 2, 2019

Gianluca Gregori
University of Oxford, United Kingdom

Combining experiments at Free Electron Laser Facilities with state-of-art numerical simulations provide ways to further our understanding of warm dense matter. Perhaps the most successful and ubiquitous of these approaches has been the validation of density functional theory (DFT) against experimental data. However, despite the progress made by DFT and related schemes, there remain many problems that are intractable for existing methods, particularly if the full particle dynamics is needed. The presenter discussed how alternative approaches, based on the Bohmian trajectories formalism, can be used to treat the full particle dynamics with a considerable increase in computational speed, and they compare favorably against current experimental data. He also discussed the relevance of Bohm quantum mechanics in the wider context of correlated quantum systems.

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DIII-D and ITER: Latest experimental plans and diagnostic development

April 25, 2019

Rejean Boivin
General Atomics

The presenter discussed the DIII-D program’s latest experimental campaign, which will include new tools in support of the development of Advanced Tokamak research and the study of boundary solutions for fusion reactors. Many of these new tools aim at developing efficient mechanisms for current drive. In parallel, new diagnostics have been added and upgraded, allowing further testing of models and theories. In addition, progress has been made on the design, fabrication and assembly of the ITER device, remaining on track for the first plasma operation scheduled for 2025-2026. Diagnostic developments have reached levels where techniques can be applied or transferred to ICF devices. The presentation included a description of imaging polarimetry, divertor viewing systems, and gamma ray imaging.

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Isentropes extracted from proton-heated warm dense matter using streaked X-ray radiography

April 18, 2019

Matthew Hill
Atomic Weapons Establishment, United Kingdom

Obtaining off-Hugoniot data to improve equation of state models in the warm dense matter regime remains a high priority for HED science, but generating and diagnosing matter in this state to sufficient resolution has proved challenging. A series of laser-driven proton heating experiments conducted at the AWE Orion laser facility have achieved uniform isochoric heating to 5-10 eV in solid density boron carbide, plastics and diamond, with streaked X-ray radiography providing few-picosecond-resolution time-resolved density profiles of the subsequent isentropic expansion. Combined with ion spectra and streaked optical pyrometry, it is possible to construct model-independent isentropes from these data using a method published by M. Foord et al. and compare them to commonly used LEOS, SESAME and NuQEOS predictions. We find that at solid density all three models agree with the data to within experimental uncertainty, but at lower densities the data suggest they under-predict the pressure by as much as an order of magnitude.

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New approaches to temperature-dependent density functional approximations

April 11, 2019

Aurora Pribram-Jones
University of California, Merced

Thermal density functional theory is common in simulations of high-temperature, high-density materials, despite the scarcity of explicitly temperature-dependent electron interaction free energy approximations and disagreement over the impact of these missing thermal effects on calculated properties. Insights from both ensemble density functional theory and the electronic strong-interaction limit can be applied to thermal ensembles, creating new approximation schemes and serving to connect these branches of formal theory with thermal density functional theory and its applications. Numerical demonstrations using the finite-temperature asymmetric Hubbard dimer and the uniform electron gas will be used to examine the advantages and disadvantages of the two approaches.

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Fluid silicates at extreme conditions and the magma ocean

March 21, 2019

Lars Stixrude
University of California, Los Angeles

The Earth may have begun in a completely molten state, a global magma ocean, with silicate liquid extending from a dense silicate atmosphere to the boundary with the iron-rich core at a pressure of 140 GPa. Deep melt may exist in the Earth today, and the magma ocean may have left signatures of its presence. However, these signals are still uninterpretable because of a lack of basic knowledge of the behavior of fluid silicates at extreme conditions. To help answer fundamental questions, we have performed first principles quantum mechanical simulations in the range of pressure, temperature, composition relevant to the early Earth that have not yet been explored by experiment or theory. Simulations of liquid, vapor, and supercritical forms lend new insight into the crystallization history of the magma ocean, evaporation at its surface, and the possibility of a silicate dynamo early in Earth’s history.

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Simulations of dense hydrogen with quantum Monte Carlo

March 14, 2019

David Ceperley
University of Illinois, Urbana-Champaign

Hydrogen accounts for much of the visible mass in the universe. Even though hydrogen is the first element in the periodic table, calculating its properties is not simple since both the electronic and protonic correlations are quantum and correlated. It has long been an open question how hydrogen makes a transition from a molecular insulating state to an atomic metallic state. We use a quantum Monte Carlo method (Coupled Electron Ion Monte Carlo) where we start with the true interaction between the electrons and protons and treat both fully quantum mechanically. With this method we have studied molecular dissociation in liquid hydrogen and have observed clear evidence of a liquid-liquid phase transition. During the past few years, several experiments have reported observations of the transition we predicted.

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Overview of the Scarlet Laser Facility and the OSU high energy density physics group

March 7, 2019

Douglass Schumacher
Ohio State University

The Scarlet Laser Facility, built with support from the U.S. Department of Energy, is a 300 TW system delivering pulses up to 10 J in energy with durations as short as 30 fs. It is used to study the intense laser matter interaction from the damage threshold up to the creation of highly relativistic plasmas. The presenter described his recent work on structured targets that enhance the laser-plasma interaction, the mechanisms of laser-based ion acceleration, and plasma mirror physics. In addition, he discussed a new technology based on liquid crystals that rapidly forms films as thin as 20 nm for targets and plasma mirrors for high repetition rate, petawatt class lasers. In addition to showing particle-in-cell simulation (PIC) results in support of this work, he described a new approach to PIC modeling that incorporates atomic pair-potentials into the PIC cycle. His research team is using this code to study laser damage and hope to extend it to a more general study of warm dense matter.

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How artificial intelligence can advance inertial confinement fusion

February 28, 2019

Luc Peterson
Lawrence Livermore National Laboratory

Advancements in artificial intelligence technology are impacting scientific research in fields as diverse as drug discovery and fluid turbulence. Research in inertial confinement fusion (ICF) could also benefit from these technological advances. However, the challenges of ICF place some burdens on the immediate applicability of artificial intelligence. During this talk, the presenter discussed some of those challenges, such as data scarcity and the interdependence on computational modeling. In addition, the discussion explored how we are working to overcome these challenges to accelerate progress towards ignition.

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Understanding properties of materials under extreme conditions with first-principles methods for ICF/HED applications

February 7, 2019

Suxing Hu
University of Rochester

First-principles methods for quantum many-body systems, such as path-integral Monte Carlo (PIMC) and density functional theory (DFT) can provide a self-consistent way to predict material properties with the possibility of systematic improvement of their calculation accuracy. In particular, thermal DFT has revolutionized simulation and understanding of high-energy-density (HED) physics and chemistry over the past two decades. DFT-based quantum molecular-dynamics (QMD) has helped generate accurate predictions of static, transport, and optical properties of materials under HED conditions. In recent years, time-dependent DFT started to play a crucial role in studying dynamic and transport physics in HED plasmas. In this talk, the presenter discussed (1) the HED physics we have learned from PIMC and DFT-based QMD calculations; (2) how these first-principles results impact on our understanding of inertial-confinement fusion implosions and HED experiments in general; and (3) what challenges we are facing to further improve physics predictions of DFT for HED plasmas.

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Opacity of shock-heated boron plasmas

January 31, 2019

Walter Johnson
Notre Dame University

Standard measures of opacity, the imaginary-part of the atomic scattering factor, f₂,  and the mass attenuation coefficient are evaluated in shock-heated boron, boron carbide, and boron nitride plasmas. The Hugoniot equation, relating the temperature  behind a shock wave to the compression ratio across the shock front, is used in connection with the plasma equation of state to determine the pressure, effective plasma charge Z, and the K-shell occupation in terms of the compression ratio. Solutions of the Hugoniot equation reveal that the K-shell occupation in low-Z ions decreases rapidly from 2 to 0 as the temperature increases from 20eV to 500eV; a temperature range in which the shock compression ratio is near 4. The average-atom model is used to determine K-shell and continuum wave functions and the photoionization cross section for x-rays in the energy range from 1 to 10 keV, where the opacity is dominated by the atomic photoionization process. For an uncompressed boron plasma at 10 eV, where the K-shell is filled, the average-atom cross section, the atomic scattering factor and the mass attenuation coefficient are all shown to agree closely with previous (cold matter) tabulations. For shock-compressed plasmas, the opacity is found to be well approximated by scaling the cold-matter values by the relative K-shell occupation; however, there is a small correction to this rule caused by the energy dependence of the photoionization cross section. Attenuation coefficients, for a 9 keV x-ray are illustrated as functions of temperature along the Hugoniot for B, B₄C, and BN plasmas.

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Kinetic physics of magnetized plasmas and its impact on pulsed-power HED experiments

January 24, 2019

Genia Vogman
Lawrence Livermore National Laboratory

Pulsed-power experiments run mega-amps of current through a load to produce and study high-energy-density matter.  Experimental results show that the formation of low-density plasmas in the power feeds gives rise to parasitic currents, which affect load dynamics and prevent scaling of load parameters.  To understand the inimical transport properties of these low-density, magnetized, collisionless plasmas and how they affect experimental outcomes, the environment within the power feeds is studied using high-order time-dependent continuum kinetic simulations, which offer enhanced solution accuracy and can robustly capture equilibria.  The effects of drifts, anisotropies, finite Larmor motion, and sheared flow instabilities are examined.  The computational study is facilitated in part through the development of machinery for constructing self-consistent kinetic equilibria and through the generalization of existing fluid theory analysis.

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Charged-particle stopping power in dense plasmas

January 10, 2019

Alex Zylstra
Lawrence Livermore National Laboratory

As an energetic charged particle moves through plasma, it loses energy to the background electrons and ions; the rate of energy loss dE/dx is known as the stopping power. Sufficiently accurate stopping power models are required to model charged-particle transport in systems of interest, such as in ICF hot-spot ignition and burn propagation into the dense fuel where the DT as provide self-heating, or for secondary/tertiary neutron production in the ICF dense fuel. Calculating the stopping power in these regimes is a theoretical challenge, especially near the maximum in dE/dx (the ‘Bragg peak’) or in degenerate/strongly-coupled plasmas. In the past several years a few benchmark experiments have begun to provide direct measurements of dE/dx in ICF-relevant plasmas for the first time. This talk provided an overview of the results compared to several models.

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Goetz Lehmann
Institute for Theoretical Physics, Heinrich Heine University
September 28, 2017

Plasma-based Amplification and Manipulation of High-power Laser Pulses

In the last decade, the increasing availability of terawatt- and petawatt-class lasers with pulse durations on the order of 10 to 100 femtoseconds has intensified interest in the relativistic interaction between laser radiation and matter. Today laser intensities up to 10²² W/cm² can be achieved. Most high-intensity lasers today rely on amplification schemes that can barely be scaled to higher power levels due to material damage thresholds. An alternative approach that allows circumventing these issues is the use of plasma as an amplification medium. In the presentation, Prof. Lehmann discusses Brillouin-type plasma-based laser amplifiers and show that the ion plasma waves, driven by the two laser pulses, eventually form photonic crystals.

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Burkhard Militzer
Departments of Earth and Planetary Science, University of California, Berkeley
April 20, 2017

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