Publications by Justin Wark


High-resolution inelastic x-ray scattering at the high energy density scientific instrument at the European X-Ray Free-Electron Laser

Review of Scientific Instruments American Institute of Physics 92 (2021) 013101

V Cerantola, A Comley, J Eggert, L Fletcher, G Geloni, D Gericke, S Glenzer, S Göde, J Hastings, O Humphries, A Jenei, O Karnbach, Z Konopkova, R Loetzsch, B Marx-Glowna, E McBride, D McGonegle, B Ofori-Okai, C Palmer, C Plückthun, R Redmer, C Strohm, T Tschentscher, K Appel, G Gregori

We introduce a setup to measure high-resolution inelastic x-ray scattering at the High Energy Density scientific instrument at the European X-Ray Free-Electron Laser (XFEL). The setup uses the Si (533) reflection in a channel-cut monochromator and three spherical diced analyzer crystals in near-backscattering geometry to reach a high spectral resolution. An energy resolution of 44 meV is demonstrated for the experimental setup, close to the theoretically achievable minimum resolution. The analyzer crystals and detector are mounted on a curved-rail system, allowing quick and reliable changes in scattering angle without breaking vacuum. The entire setup is designed for operation at 10 Hz, the same repetition rate as the high-power lasers available at the instrument and the fundamental repetition rate of the European XFEL. Among other measurements, it is envisioned that this setup will allow studies of the dynamics of highly transient laser generated states of matter.


High energy density science with X-ray free-electron lasers

Proceedings of the International School of Physics "Enrico Fermi" 199 (2020) 147-170

JS Wark

© 2020 Società Italiana di Fisica. All rights reserved. Extreme states of matter with high temperatures and pressures can be created by irradiating matter with either intense X-rays emitted by X-ray free-electron lasers (FELs), and by heating and/or compression with optical lasers and then using the FEL X-rays as a probe. We provide here a very basic introduction to this burgeoning field, highlighting a few specific experiments, and signposting some directions for future exploration.


An approach for the measurement of the bulk temperature of single crystal diamond using an X-ray free electron laser

Scientific Reports Nature Research 10 (2020) 14564

J Wark, A Descamps, BK Ofori-Okai, K Appel

We present a method to determine the bulk temperature of a single crystal diamond sample at an X-Ray free electron laser using inelastic X-ray scattering. The experiment was performed at the high energy density instrument at the European XFEL GmbH, Germany. The technique, based on inelastic X-ray scattering and the principle of detailed balance, was demonstrated to give accurate temperature measurements, within 8% for both room temperature diamond and heated diamond to 500 K. Here, the temperature was increased in a controlled way using a resistive heater to test theoretical predictions of the scaling of the signal with temperature. The method was tested by validating the energy of the phonon modes with previous measurements made at room temperature using inelastic X-ray scattering and neutron scattering techniques. This technique could be used to determine the bulk temperature in transient systems with a temporal resolution of 50 fs and for which accurate measurements of thermodynamic properties are vital to build accurate equation of state and transport models.


Investigating off-Hugoniot states using multi-layer ring-up targets

Scientific Reports Springer Nature 10 (2020) 13172

D McGonegle, P Heighway, M Sliwa, C Bolme, A Comley, L Dresselhaus-Marais, A Higginbotham, A Poole, E McBride, B Nagler, I Nam, M Seaberg, B Remington, R Rudd, C Wehrenberg, J Wark

Laser compression has long been used as a method to study solids at high pressure. This is commonly achieved by sandwiching a sample between two diamond anvils and using a ramped laser pulse to slowly compress the sample, while keeping it cool enough to stay below the melt curve. We demonstrate a different approach, using a multilayer ‘ring-up’ target whereby laser-ablation pressure compresses Pb up to 150 GPa while keeping it solid, over two times as high in pressure than where it would shock melt on the Hugoniot. We find that the efficiency of this approach compares favourably with the commonly used diamond sandwich technique and could be important for new facilities located at XFELs and synchrotrons which often have higher repetition rate, lower energy lasers which limits the achievable pressures that can be reached.


A novel method to measure ion density in ICF experiments using X-ray spectroscopy of cylindrical tracers

Physics of Plasmas AIP Publishing 27 (2020) 112714

G Pérez callejo, MA Barrios, DA Liedahl, S Rose, J Wark

The indirect drive approach to inertial confinement fusion (ICF) has undergone important advances in the past years. The improvements in temperature and density diagnostic methods are leading to more accurate measurements of the plasma conditions inside the hohlraum and therefore to more efficient experimental designs. The implementation of dot spectroscopy has proven to be a versatile approach to extracting spaceand time-dependent electron temperatures. In this method a microdot of a mid-Z material is placed inside the hohlraum and its K-shell emission spectrum is used to determine the plasma temperature. However, radiation transport of optically thick lines acting within the cylindrical dot geometry influences the outgoing spectral distribution in a manner that depends on the viewing angle. This angular dependence has recently been studied in the high energy density (HED) regime at the OMEGA laser facility, which allowed us to design and benchmark appropriate radiative transfer models that can replicate these geometric effects. By combining these models with the measurements from the dot spectroscopy experiments at the National Ignition Facility (NIF), we demonstrate here a novel technique that exploits the transport effects to obtain time-resolved measurements of the ion density of the tracer dots, without the need for additional diagnostics. We find excellent agreement between experiment and simulation, opening the possibility of using these geometric effects as a density diagnostic in future experiments.


X-ray spectroscopic studies of a solid-density germanium plasma created by a free electron laser

Applied Sciences MDPI 10 (2020) 8153

J Wark

The generation of solid-density plasmas in a controlled manner using an X-ray free electron laser (XFEL) has opened up the possibility of diagnosing the atomic properties of hot, strongly coupled systems in novel ways. Previous work has concentrated on K-shell emission spectroscopy of low Z (<= 14) elements. Here, we extend these studies to the mid-Z(=32) element Germanium, where the XFEL creates copious L-shell holes, and the plasma conditions are interrogated by recording of the associated L-shell X-ray emission spectra. Given the desirability of generating as uniform a plasma as possible, we present here a study of the effects of the FEL photon energy on the temperatures and electron densities created, and their uniformity in the FEL beam propagation direction. We show that good uniformity can be achieved by tuning the photon energy of the XFEL such that it does not overlap significantly with L-shell to M-shell bound-bound transitions, and lies below the L-edges of the ions formed during the heating process. Reasonable agreement between experiment and simulations is found for the emitted X-ray spectra, demonstrating that for these higher Z elements, the selection of appropriate XFEL parameters is important for achieving uniformity in the plasma conditions.


Probing the Electronic Structure of Warm Dense Nickel via Resonant Inelastic X-Ray Scattering.

Physical review letters 125 (2020) 195001-

OS Humphries, RS Marjoribanks, QY van den Berg, EC Galtier, MF Kasim, HJ Lee, AJF Miscampbell, B Nagler, R Royle, JS Wark, SM Vinko

The development of bright free-electron lasers (FEL) has revolutionized our ability to create and study matter in the high-energy-density (HED) regime. Current diagnostic techniques have been successful in yielding information on fundamental thermodynamic plasma properties, but provide only limited or indirect information on the detailed quantum structure of these systems, and on how it is affected by ionization dynamics. Here we show how the valence electronic structure of solid-density nickel, heated to temperatures of around 10 of eV on femtosecond timescales, can be probed by single-shot resonant inelastic x-ray scattering (RIXS) at the Linac Coherent Light Source FEL. The RIXS spectrum provides a wealth of information on the HED system that goes well beyond what can be extracted from x-ray absorption or emission spectroscopy alone, and is particularly well suited to time-resolved studies of electronic-structure dynamics.


Probing the Electronic Structure of Warm Dense Nickel via Resonant Inelastic X-Ray Scattering

Physical Review Letters 125 (2020)

OS Humphries, RS Marjoribanks, QY Van Den Berg, EC Galtier, MF Kasim, HJ Lee, AJF Miscampbell, B Nagler, R Royle, JS Wark, SM Vinko

© 2020 American Physical Society. The development of bright free-electron lasers (FEL) has revolutionized our ability to create and study matter in the high-energy-density (HED) regime. Current diagnostic techniques have been successful in yielding information on fundamental thermodynamic plasma properties, but provide only limited or indirect information on the detailed quantum structure of these systems, and on how it is affected by ionization dynamics. Here we show how the valence electronic structure of solid-density nickel, heated to temperatures of around 10 of eV on femtosecond timescales, can be probed by single-shot resonant inelastic x-ray scattering (RIXS) at the Linac Coherent Light Source FEL. The RIXS spectrum provides a wealth of information on the HED system that goes well beyond what can be extracted from x-ray absorption or emission spectroscopy alone, and is particularly well suited to time-resolved studies of electronic-structure dynamics.


Mapping the electronic structure of warm dense nickel via resonant inelastic x-ray scattering

Physical Review Letters American Physical Society 125 (2020) 195001

O Humphries, RS Marjoribanks, Q Van Den Berg, EC Galtier, M Firmansyah, A Miscampbell, R Royle, J Wark, S Vinko

The development of bright free-electron lasers (FEL) has revolutionised our ability to create and study matter in the high-energy-density (HED) regime. Current diagnostic techniques have been successful in yielding information on fundamental thermodynamic plasma properties, but provide only limited or indirect information on the detailed quantum structure of these systems, and on how it is affected by ionization dynamics. Here we show how the valence electronic structure of soliddensity nickel, heated to temperatures of around 10 of eV on femtosecond timescales, can be probed by single-shot resonant inelastic x-ray scattering (RIXS) at the Linac Coherent Light Source FEL. The RIXS spectrum provides a wealth of information on the HED system that goes well beyond what can be extracted from x-ray absorption or emission spectroscopy alone, and is particularly well-suited to time-resolved studies of electronic-structure dynamics.


Measuring the oscillator strength of intercombination lines of helium-like V ions in a laser-produced-plasma

Journal of Quantitative Spectroscopy and Radiative Transfer Elsevier 256 (2020) 107326

G Pérez-Callejo, L Jarrott, D Liedahl, M Schneider, J Wark, S Rose

We present results of measurements of the oscillator strength of intercombination lines of He-like Vanadium ions in high energy density (HED) laser-produced-plasmas and compare them with the simulations from commonly used codes and data from the NIST database. Whilst not yet sufficiently accurate to constrain different trusted atomic-physics models for the particular system studied, our results are in agreement with the available data within experimental error bars, yet differ from cruder approximations of the oscillator strength used in certain atomic-kinetics packages, suggesting that this general method could be further extended to be used as a measurement of the oscillator strength of additional atomic transitions under the extreme conditions that are achieved in HED experiments.


Recovery of a high-pressure phase formed under laser-driven compression

Physical Review B American Physical Society 102 (2020) 24101

M Gorman, D McGonegle, S Tracy, S Clarke, C Bolme, A Gleason, S Ali, S Hok, C Greeff, P Heighway, K Hulpach, B Glam, E Galtier, H Lee, J Wark, J Eggert, J Wicks, R Smith

The recovery of metastable structures formed at high pressure has been a long-standing goal in the field of condensed matter physics. While laser-driven compression has been used as a method to generate novel structures at high pressure, to date no high-pressure phases have been quenched to ambient conditions. Here we demonstrate, using in situ x-ray diffraction and recovery methods, the successful quench of a high-pressure phase which was formed under laser-driven shock compression. We show that tailoring the pressure release path from a shock-compressed state to eliminate sample spall, and therefore excess heating, increases the recovery yield of the high-pressure ω phase of zirconium from 0% to 48%. Our results have important implications for the quenchability of novel phases of matter demonstrated to occur at extreme pressures using nanosecond laser-driven compression.


Time-Resolved XUV Opacity Measurements of Warm Dense Aluminum.

Physical review letters 124 (2020) ARTN 225002

S Vinko, V Vozda, J Andreasson, S Bajt, J Bielecki, T Burian, J Chalupsky, O Ciricosta, M Desjarlais, H Fleckenstein, J Hajdu, V Hajkova, P Hollebon, L Juha, M Kasim, E McBride, K Muehlig, T Preston, D Rackstraw, S Roling, S Toleikis, J Wark, H Zacharias

The free-free opacity in plasmas is fundamental to our understanding of energy transport in stellar interiors and for inertial confinement fusion research. However, theoretical predictions in the challenging dense plasma regime are conflicting and there is a dearth of accurate experimental data to allow for direct model validation. Here we present time-resolved transmission measurements in solid-density Al heated by an XUV free-electron laser. We use a novel functional optimization approach to extract the temperature-dependent absorption coefficient directly from an oversampled pool of single-shot measurements, and find a pronounced enhancement of the opacity as the plasma is heated to temperatures of order of the Fermi energy. Plasma heating and opacity enhancement are observed on ultrafast timescales, within the duration of the femtosecond XUV pulse. We attribute further rises in the opacity on ps timescales to melt and the formation of warm dense matter.


Time-resolved XUV opacity measurements of warm-dense aluminium

Physical Review Letters American Physical Society 124 (2020) 225002

S Vinko, V Vozda, J Andreasson, O Ciricosta, P Hollebon, M Kasim, DS Rackstraw, J Wark

The free-free opacity in plasmas is fundamental to our understanding of energy transport in stellar interiors and for inertial confinement fusion research. However, theoretical predictions in the challenging dense plasma regime are conflicting and there is a dearth of accurate experimental data to allow for direct model validation. Here we present time-resolved transmission measurements in solid-density Al heated by an XUV free-electron laser. We use a novel functional optimization approach to extract the temperature-dependent absorption coefficient directly from an oversampled pool of single-shot measurements, and find a pronounced enhancement of the opacity as the plasma is heated to temperatures of order of the Fermi energy. Plasma heating and opacity enhancement are observed on ultrafast timescales, within the duration of the femtosecond XUV pulse. We attribute further rises in the opacity on ps timescales to melt and the formation of warm dense matter.


X-ray diffraction at the National Ignition Facility

Review of Scientific Instruments AIP Publishing 91 (2020) 043902

J Rygg, A Lazicki, D Braun, D Fratanduono, J McNaney, D Swift, C Wehrenberg, F Coppari, M Ahmed, M Barrios, K Blobaum, G Collins, P Di Nicola, E Dzenitis, S Gonzales, B Heidl, M Hohenberger, N Izumi, D Kalantar, N Masters, R Vignes, M Wall, J Wark, A Arsenlis, J Eggert

We report details of an experimental platform implemented at the National Ignition Facility to obtain in situ powder diffraction data from solids dynamically compressed to extreme pressures. Thin samples are sandwiched between tamper layers and ramp compressed using a gradual increase in the drive-laser irradiance. Pressure history in the sample is determined using high-precision velocimetry measurements. Up to two independently timed pulses of x rays are produced at or near the time of peak pressure by laser illumination of thin metal foils. The quasi-monochromatic x-ray pulses have a mean wavelength selectable between 0.6 Å and 1.9 Å depending on the foil material. The diffracted signal is recorded on image plates with a typical 2θ x-ray scattering angle uncertainty of about 0.2° and resolution of about 1°. Analytic expressions are reported for systematic corrections to 2θ due to finite pinhole size and sample offset. A new variant of a nonlinear background subtraction algorithm is described, which has been used to observe diffraction lines at signal-to-background ratios as low as a few percent. Variations in system response over the detector area are compensated in order to obtain accurate line intensities; this system response calculation includes a new analytic approximation for image-plate sensitivity as a function of photon energy and incident angle. This experimental platform has been used up to 2 TPa (20 Mbar) to determine the crystal structure, measure the density, and evaluate the strain-induced texturing of a variety of compressed samples spanning periods 2–7 on the periodic table.


Non-isentropic release of a shocked solid

Physical Review Letters American Physical Society 123 (2019) 245501

PG Heighway, M Sliwa, D McGonegle, C Wehrenberg, CA Bolme, J Eggert, A Higginbotham, A Lazicki, HJ Lee, B Nagler, H-S Park, RE Rudd, RF Smith, MJ Suggit, D Swift, F Tavella, BA Remington, J Wark

We present molecular dynamics simulations of shock and release in micron-scale tantalum crystals that exhibit postbreakout temperatures far exceeding those expected under the standard assumption of isentropic release. We show via an energy-budget analysis that this is due to plastic-work heating from material strength that largely counters thermoelastic cooling. The simulations are corroborated by experiments where the release temperatures of laser-shocked tantalum foils are deduced from their thermal strains via in situ x-ray diffraction and are found to be close to those behind the shock.


Ab initio simulations and measurements of the free-free opacity in aluminum

Physical Review E American Physical Society 100 (2019) 043207

P Hollebon, O Ciricosta, MP Desjarlais, C Cacho, C Spindloe, E Springate, ICE Turcu, J Wark, SM Vinko

The free-free opacity in dense systems is a property that both tests our fundamental understanding of correlated many-body systems, and is needed to understand the radiative properties of high energy-density plasmas. Despite its importance, predictive calculations of the free-free opacity remain challenging even in the condensed matter phase for simple metals. Here we show how the free-free opacity can be modelled at finite-temperatures via time-dependent density functional theory, and illustrate the importance of including local field corrections, core polarization, and self-energy corrections. Our calculations for ground-state Al are shown to agree well with experimental opacity measurements performed on the Artemis laser facility across a wide range of extreme ultraviolet wavelengths. We extend our calculations across the melt to the warm-dense matter regime, finding good agreement with advanced plasma models based on inverse bremsstrahlung at temperatures above 10 eV.


Ultrafast laser-matter interaction with nanostructured targets

X-Ray Lasers and Coherent X-Ray Sources: Development and Applications XIII; 111110L (2019) Society of Photo-Optical Instrumentation Engineers (SPIE) 11111 (2019)

RS Marjoribanks, L Lecherbourg, JE Sipe, G Kulcsar, A Heron, J-C Adam, A Miscampbell, G Thomas, R Royle, O Humphries, RHH Ko, S Le Moal, A Tan, J Li, TR Preston, Q van den Berg, M Kasim, B Nagler, E Galtier, E Cunningham, JS Wark, S Vinko

Conventional solid-density laser-plasma targets quickly ionize to make a plasma mirror, which largely reflects ultra-intense laser pulses. This Fresnel reflection at the plane boundary largely wastes our e orts at ultra-intense laser/solid interaction, and limits target heating to nonlinear generation of high-energy electrons which penetrate inward. One way around this dual problem is to create a material with an anisotropic dielectric function, for instance by nanostructuring a material in such a way that it cannot support the material responses which generate a specularly reflected beam. We present linear theory for metallic and plasma nanowires, particle-incell simulations of the interaction of ultra-intense femtosecond pulses with nickel nanowires, showing penetration of laser light far deeper than a nickel skin-depth, helping to uniformly heat near-solid material to conditions of high energy-densities, and XFEL experiments giving insight into their ionization and excitation.


Molecular dynamics simulations of grain interactions in shock-compressed highly textured columnar nanocrystals

Physical Review Materials American Physical Society 3 (2019) 083602

P Heighway, F McGonegle, N Park, A Higginbotham, J Wark

While experimental and computational studies abound demonstrating the diverse range of phenomena caused by grain interactions under quasistatic loading conditions, far less attention has been given to these interactions under the comparatively dramatic conditions of shock compression. The consideration of grain interactions is essential within the context of contemporary shock-compression experiments that exploit the distinctive x-ray diffraction patterns of highly textured (and therefore strongly anisotropic) targets in order to interrogate local structural evolution. We present here a study of grain interaction effects in shock-compressed, body-centered cubic tantalum nanocrystals characterized by a columnar geometry and a strong fiber texture using large-scale molecular dynamics simulations. Our study reveals that contiguous grains deform cooperatively in directions perpendicular to the shock, driven by the gigapascal-scale stress gradients induced over their boundaries by the uniaxial compression, and in so doing are able to reach a state of reduced transverse shear stress. We compare the extent of this relaxation for two different columnar geometries (distinguished by their square or hexagonal cross-sections), and quantify the attendant change in the transverse elastic strains. We further show that cooperative deformation is able to replace ordinary plastic deformation mechanisms at lower shock pressures, and, under certain conditions, activate new mechanisms at higher pressures.


Identification of phase transitions and metastability in dynamically compressed antimony using ultrafast x-ray diffraction

Physical Review Letters American Physical Society 122 (2019) 255704

AL Coleman, R Briggs, RS McWilliams, D McGonegle, CA Bolme, AE Gleason, DE Fratanduono, RF Smith, E Galtier, HJ Lee, B Nagler, E Granados, GW Collins, JH Eggert, J Wark, MI McMahon

Ultrafast x-ray diffraction at the LCLS x-ray free electron laser has been used to resolve the structural behavior of antimony under shock compression to 59 GPa. Antimony is seen to transform to the incommensurate, host-guest phase Sb-II at ∼11 GPa, which forms on nanosecond timescales with ordered guest-atom chains. The high-pressure bcc phase Sb-III is observed above ∼15 GPa, some 8 GPa lower than in static compression studies, and mixed Sb-III/liquid diffraction are obtained between 38 and 59 GPa. An additional phase which does not exist under static compression, Sb-I', is also observed between 8 and 12 GPa, beyond the normal stability field of Sb-I, and resembles Sb-I with a resolved Peierls distortion. The incommensurate Sb-II high-pressure phase can be recovered metastably on release to ambient pressure, where it is stable for more than 10 ns.


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

Physics of Plasmas AIP Publishing 26 (2019) 063302

G Perez-Callejo, LC Jarrott, DA Liedahl, EV Marley, GE Kemp, RF Heeter, JA Emig, ME Foord, K Widmann, J Jaquez, H Huang, SJ Rose, J Wark, MB Schneider

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

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