Publications


Focal aberrations of large-aperture HOPG von-Hàmos x-ray spectrometers

Journal of Instrumentation 7 (2012)

U Zastrau, CRD Brown, T Döppner, SH Glenzer, G Gregori, HJ Lee, H Marschner, S Toleikis, O Wehrhan, E Förster

Focal aberrations of large-aperture highly oriented pyrolytic graphite (HOPG) crystals in von-Hàmos geometry are investigated by experimental and computational methods. A mosaic HOPG crystal film of 100 μm thickness diffracts 8 keV x-rays. This thickness is smaller than the absorption depth of the symmetric 004-reflection, which amounts to 257 μm. Cylindrically bent crystals with 110mm radius of curvature and up to 100 mm collection width produce a X-shaped halo around the focus. This feature vanishes when the collection aperture is reduced, but axial spectral profiles show that the resolution is not affected. X-ray topography reveals significant inhomogeneous crystallite domains of 2±1mm diameter along the entire crystal. Rocking curves shift by about ±20arcmin between domains, while their full width at half-maximum varies between 30 and 50 arcmin. These inhomogeneities are not imprinted at the focal spot, since the monochromatically reflecting area of the crystal is large compared to inhomogeneities. Ray-tracing calculations using a Monte-Carlo-based algorithm developed for mosaic crystals reproduce the X-shaped halo in the focal plane, stemming from the mosaic defocussing in the non-dispersive direction in combination with large apertures. The best achievable resolution is found by analyzing a diversity of rocking curve widths, source sizes and crystal thicknesses for 8 keV x-rays to be ΔE/E ∼ 10 -4 . Finally a general analytic expression for the shape of the aberration is derived. © 2012 IOP Publishing Ltd and Sissa Medialab srl.


FLASH magnetohydrodynamic simulations of shock-generated magnetic field experiments

High Energy Density Physics 8 (2012) 322-328

P Tzeferacos, M Fatenejad, N Flocke, G Gregori, DQ Lamb, D Lee, J Meinecke, A Scopatz, K Weide

We report the results of benchmark FLASH magnetohydrodynamic (MHD) simulations of experiments conducted by the University of Oxford High Energy Density Laboratory Astrophysics group and its collaborators at the Laboratoire pour l'Utilisation des Lasers Intenses (LULI). In these experiments, a long-pulse laser illuminates a target in a chamber filled with Argon gas, producing shock waves that generate magnetic fields via the Biermann battery mechanism. We first outline the implementation of 2D cylindrical geometry in the unsplit MHD solver in FLASH and present results of verification tests. We then describe the results of benchmark 2D cylindrical MHD simulations of the LULI experiments using FLASH that explore the impact of external fields along with the possibility of magnetic field amplification by turbulence that is associated with the shock waves and that is induced by a grid placed in the gas-filled chamber. © 2012 Elsevier B.V.


Self-organized electromagnetic field structures in laser-produced counter-streaming plasmas

Nature Physics (2012)

NL Kugland, DD Ryutov, P-Y Chang, RP Drake, G Fiksel, DH Froula, SH Glenzer, G Gregori, M Grosskopf, M Koenig, Y Kuramitsu, C Kuranz, MC Levy, E Liang, J Meinecke, F Miniati, T Morita, A Pelka, C Plechaty, R Presura, A Ravasio, BA Remington, B Reville, JS Ross, Y Sakawa, A Spitkovsky, H Takabe, H-S Park


Resonant Kα spectroscopy of solid-density aluminum plasmas

Physical Review Letters 109 (2012)

BI Cho, K Engelhorn, SM Vinko, HK Chung, O Ciricosta, DS Rackstraw, RW Falcone, CRD Brown, T Burian, J Chalupský, C Graves, V Hájková, A Higginbotham, L Juha, J Krzywinski, HJ Lee, M Messersmidt, C Murphy, Y Ping, N Rohringer, A Scherz, W Schlotter, S Toleikis, JJ Turner, L Vysin, T Wang, B Wu, U Zastrau, D Zhu, RW Lee, B Nagler, JS Wark, PA Heimann

The x-ray intensities made available by x-ray free electron lasers (FEL) open up new x-ray matter interaction channels not accessible with previous sources. We report here on the resonant generation of Kα emission, that is to say the production of copious Kα radiation by tuning the x-ray FEL pulse to photon energies below that of the K edge of a solid aluminum sample. The sequential absorption of multiple photons in the same atom during the 80 fs pulse, with photons creating L-shell holes and then one resonantly exciting a K-shell electron into one of these holes, opens up a channel for the Kα production, as well as the absorption of further photons. We demonstrate rich spectra of such channels, and investigate the emission produced by tuning the FEL energy to the K-L transitions of those highly charged ions that have transition energies below the K edge of the cold material. The spectra are sensitive to x-ray intensity dependent opacity effects, with ions containing L-shell holes readily reabsorbing the Kα radiation. © 2012 American Physical Society.


Self-organized electromagnetic field structures in laser-produced counter-streaming plasmas

Nature Physics 8 (2012) 809-812

NL Kugland, DD Ryutov, PY Chang, RP Drake, G Fiksel, DH Froula, SH Glenzer, G Gregori, M Grosskopf, M Koenig, Y Kuramitsu, C Kuranz, MC Levy, E Liang, J Meinecke, F Miniati, T Morita, A Pelka, C Plechaty, R Presura, A Ravasio, BA Remington, B Reville, JS Ross, Y Sakawa, A Spitkovsky, H Takabe, HS Park

Self-organization occurs in plasmas when energy progressively transfers from smaller to larger scales in an inverse cascade. Global structures that emerge from turbulent plasmas can be found in the laboratory and in astrophysical settings; for example, the cosmic magnetic field, collisionless shocks in supernova remnants and the internal structures of newly formed stars known as Herbig-Haro objects. Here we show that large, stable electromagnetic field structures can also arise within counter-streaming supersonic plasmas in the laboratory. These surprising structures, formed by a yet unexplained mechanism, are predominantly oriented transverse to the primary flow direction, extend for much larger distances than the intrinsic plasma spatial scales and persist for much longer than the plasma kinetic timescales. Our results challenge existing models of counter-streaming plasmas and can be used to better understand large-scale and long-time plasma self-organization. © 2012 Macmillan Publishers Limited. All rights reserved.


Measuring electron-positron annihilation radiation from laser plasma interactions

Review of Scientific Instruments 83 (2012)

H Chen, R Tommasini, J Seely, CI Szabo, U Feldman, N Pereira, G Gregori, K Falk, J Mithen, CD Murphy

We investigated various diagnostic techniques to measure the 511 keV annihilation radiations. These include step-wedge filters, transmission crystal spectroscopy, single-hit CCD detectors, and streaked scintillating detection. While none of the diagnostics recorded conclusive results, the step-wedge filter that is sensitive to the energy range between 100 keV and 700 keV shows a signal around 500 keV that is clearly departing from a pure Bremsstrahlung spectrum and that we ascribe to annihilation radiation. © 2012 American Institute of Physics.


Revealing multiphoton resonant ionization in solid density plasmas with an x-ray free electron laser

2012 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2012)

B-I Cho, K Engelhorn, SM Vinko, JS Wark, RW Falcone, PA Heimann, IEEE


Studying astrophysical collisionless shocks with counterstreaming plasmas from high power lasers

High Energy Density Physics 8 (2012) 38-45

HS Park, DD Ryutov, JS Ross, NL Kugland, SH Glenzer, C Plechaty, SM Pollaine, BA Remington, A Spitkovsky, L Gargate, G Gregori, A Bell, C Murphy, Y Sakawa, Y Kuramitsu, T Morita, H Takabe, DH Froula, G Fiksel, F Miniati, M Koenig, A Ravasio, A Pelka, E Liang, N Woolsey, CC Kuranz, RP Drake, MJ Grosskopf

Collisions of high Mach number flows occur frequently in astrophysics, and the resulting shock waves are responsible for the properties of many astrophysical phenomena, such as supernova remnants, Gamma Ray Bursts and jets from Active Galactic Nuclei. Because of the low density of astrophysical plasmas, the mean free path due to Coulomb collisions is typically very large. Therefore, most shock waves in astrophysics are "collisionless", since they form due to plasma instabilities and self-generated magnetic fields. Laboratory experiments at the laser facilities can achieve the conditions necessary for the formation of collisionless shocks, and will provide a unique avenue for studying the nonlinear physics of collisionless shock waves. We are performing a series of experiments at the Omega and Omega-EP lasers, in Rochester, NY, with the goal of generating collisionless shock conditions by the collision of two high-speed plasma flows resulting from laser ablation of solid targets using ∼10 16 W/cm 2 laser irradiation. The experiments will aim to answer several questions of relevance to collisionless shock physics: the importance of the electromagnetic filamentation (Weibel) instabilities in shock formation, the self-generation of magnetic fields in shocks, the influence of external magnetic fields on shock formation, and the signatures of particle acceleration in shocks. Our first experiments using Thomson scattering diagnostics studied the plasma state from a single foil and from double foils whose flows collide "head-on" Our data showed that the flow velocity and electron density were 10 8 cm/s and 10 19 cm -3 , respectively, where the Coulomb mean free path is much larger than the size of the interaction region. Simulations of our experimental conditions show that weak Weibel mediated current filamentation and magnetic field generation were likely starting to occur. This paper presents the results from these first Omega experiments. © 2011.


X-ray Thomson scattering on shocked graphite

High Energy Density Physics 8 (2012) 46-49

D Kraus, A Otten, A Frank, V Bagnoud, A Blažević, DO Gericke, G Gregori, A Ortner, G Schaumann, D Schumacher, J Vorberger, F Wagner, K Wünsch, M Roth

We present measurements of the changes in the microscopic structure of graphite in a laser-driven shock experiment with X-ray scattering. Laser radiation with intensities of ∼2 × 10 13 W/cm 2 compressed the carbon samples by a factor of two reaching pressures of ∼90 GPa. Due to the change of the crystalline structure the scattered signals of the probe radiation were modified significantly in intensity and spectral composition compared to the scattering on cold samples. It is shown that the elastic scattering on tightly bound electrons increases strongly due to the phase transition whereas the inelastic scattering on weakly bound electrons remains nearly unchanged for the chosen geometry. © 2011 Elsevier B.V.


Molecular dynamics simulations of ramp-compressed copper

PHYSICAL REVIEW B 85 (2012) ARTN 024112

A Higginbotham, J Hawreliak, EM Bringa, G Kimminau, N Park, E Reed, BA Remington, JS Wark


Generation of scaled protogalactic seed magnetic fields in laser-produced shock waves.

Nature 481 (2012) 480-483

G Gregori, A Ravasio, CD Murphy, K Schaar, A Baird, AR Bell, A Benuzzi-Mounaix, R Bingham, C Constantin, RP Drake, M Edwards, ET Everson, CD Gregory, Y Kuramitsu, W Lau, J Mithen, C Niemann, HS Park, BA Remington, B Reville, AP Robinson, DD Ryutov, Y Sakawa, S Yang, NC Woolsey, M Koenig, F Miniati

The standard model for the origin of galactic magnetic fields is through the amplification of seed fields via dynamo or turbulent processes to the level consistent with present observations. Although other mechanisms may also operate, currents from misaligned pressure and temperature gradients (the Biermann battery process) inevitably accompany the formation of galaxies in the absence of a primordial field. Driven by geometrical asymmetries in shocks associated with the collapse of protogalactic structures, the Biermann battery is believed to generate tiny seed fields to a level of about 10(-21) gauss (refs 7, 8). With the advent of high-power laser systems in the past two decades, a new area of research has opened in which, using simple scaling relations, astrophysical environments can effectively be reproduced in the laboratory. Here we report the results of an experiment that produced seed magnetic fields by the Biermann battery effect. We show that these results can be scaled to the intergalactic medium, where turbulence, acting on timescales of around 700 million years, can amplify the seed fields sufficiently to affect galaxy evolution.


Precision X-ray spectroscopy of intense laser-plasma interactions

High Energy Density Physics 7 (2011) 105-109

NC Woolsey, RJ Clarke, D Doria, LA Gizzi, G Gregori, P Hakel, SB Hansen, P Koester, L Labate, T Levato, B Li, M Makita, RC Mancini, J Pasley, PP Rajeev, APL Robinson, E Wagenaars, JN Waugh, N Booth

Polarisation sensitive emission spectroscopy measurements are reported for a petawatt laser-solid target interaction at intensities up to 5 × 10 20 W cm -2 . These measurements were single-shot and used pairs of highly-orientated graphite spectrometers to resolve the sulphur Ly-α doublet. The sulphur Ly-α 1 component shows a large positive polarisation indicative of a low energy electron beam in the plasma, the Ly-α 2 component acts as a cross-spectrometer calibration. The measurements show a significant anisotropic or beam-like component to a cold return current. © 2011 Elsevier B.V.


Extent of validity of the hydrodynamic description of ions in dense plasmas.

Phys Rev E Stat Nonlin Soft Matter Phys 83 (2011) 015401-

JP Mithen, J Daligault, G Gregori

We show that the hydrodynamic description can be applied to modeling the ionic response in dense plasmas for a wide range of length scales that are experimentally accessible. Using numerical simulations for the Yukawa model, we find that the maximum wave number k(max) at which the hydrodynamic description applies is independent of the coupling strength, given by k(max)λ(s)≃0.43, where λ(s) is the ionic screening length. Our results show that the hydrodynamic description can be used for interpreting x-ray scattering data from fourth generation light sources and high power lasers. In addition, our investigation sheds new light on how the domain of validity of the hydrodynamic description depends on both the microscopic properties and the thermodynamic state of fluids in general.


Onset of Negative Dispersion in the One-Component Plasma

INTERNATIONAL TOPICAL CONFERENCE ON PLASMA SCIENCE: STRONGLY COUPLED ULTRA-COLD AND QUANTUM PLASMAS 1421 (2011)

JP Mithen, J Daligault, G Gregori


Reply to "Comment on 'Free-free opacity in warm-dense aluminum'"

High Energy Density Physics 7 (2011) 40-42

SM Vinko, G Gregori, JS Wark

We reply to the comment by Iglesias [HEDP, XXX] regarding our implementation of a solid-state pseudopotential in a model for the calculation of the free--free opacity in warm-dense aluminum [HEDP 5(2009), 124-131] . Some further details are given describing the method used to determine the adjustable parameter in the pseudopotential and several important limitations are discussed. © 2010 Elsevier B.V.


K-shell spectroscopy of Au plasma generated with a short-pulse laser

Canadian Journal of Physics 89 (2011) 647-651

C Zulick, F Dollar, H Chen, K Falk, G Gregori, A Hazi, CD Murphy, J Park, J Seely, CI Szabo, R Tommasini, R Shepherd, K Krushelnick

The production of X-rays from electron transitions into K-shell vacancies (K α,β ) emission) is a well-known process in atomic physics and has been extensively studied as a plasma diagnostic in low-and mid-Z materials. However, X-ray spectra from near neutral high-Z ions are very complex, and their interpretation requires the use of state-of-the-art atomic calculations. In this experiment, the Titan laser system at Lawrence Livermore National Laboratory was used to deliver an approximately 350 J laser pulse, with a 10 ps duration and a wavelength of 1054 nm, to a gold (Au) target. A transparent bent quartz crystal spectrometer with a hard X-ray energy window, ranging from 17 to 102 keV, was used to measure the emission spectrum. K α1,α2 and K β1,γ1 transitions were observed over a range of target sizes. Additionally, a series of shots were conducted with a pre-ionizing long pulse (3 ns, 1-10 J, 527 nm) on the backside of the target. FLYCHK, an atomic non-LTE code, designed to provide ionization and population distributions, was used to model the experiment. K α /K β ratios were found to be in good agreement with the predicted value for room temperature Au targets. © 2011 Published by NRC Research Press.


Decay of cystalline order and equilibration during the solid-to-plasma transition induced by 20-fs microfocused 92-eV free-electron-laser pulses.

Phys Rev Lett 106 (2011) 164801-

E Galtier, FB Rosmej, T Dzelzainis, D Riley, FY Khattak, P Heimann, RW Lee, AJ Nelson, SM Vinko, T Whitcher, JS Wark, T Tschentscher, S Toleikis, RR Fäustlin, R Sobierajski, M Jurek, L Juha, J Chalupsky, V Hajkova, M Kozlova, J Krzywinski, B Nagler

We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16)  W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.


X-ray scattering as a probe for warm dense mixtures and high-pressure miscibility

EPL 94 (2011)

K Wünsch, J Vorberger, G Gregori, DO Gericke

We develop a new theoretical approach that demonstrates the abilities of elastic X-ray scattering to yield thermodynamic, structural, and mixing properties of dense matter with multiple ion species. The novel decomposition of the electron structure factor in multi-component systems provides the basis to study dense mixtures as found in giant gas planets or during inertial confinement fusion. We show that the scattering signal differs significantly between single species, microscopic mixtures, and phase-separated fluids. Thus, these different phases can be distinguished experimentally via elastic X-ray scattering. © 2011 Europhysics Letters Association.


Simulations of neon irradiated by intense X-ray laser radiation

High Energy Density Physics 7 (2011) 111-116

O Ciricosta, HK Chung, RW Lee, JS Wark

We present simulations of the charge states produced by the interaction of intense X-ray laser radiation with a neon gas. We model the results of a recent experiment (Young et al., Nature 466, 56 (2010)), where mJ pulses of X-rays, with photon energies ranging from 800 to 2000 eV and pulse lengths ranging from 70 to 340 fs were incident on neon atoms at intensities of up to 10 18 W cm -2 . Simulations using an adapted version of the SCFLY collisional-radiative code, which included the effect of electron collisions and a simple self-consistent temperature model, result in charge state distributions that are in good agreement with the experimental data. We calculate the electron temperature of the system during the evolution of the plasma, and comment upon the role that collisions may play in determining the charge state distributions as a function of the neon ion number density. © 2011 Elsevier B.V.


Decay of Cystalline Order and Equilibration during the Solid-to-Plasma Transition Induced by 20-fs Microfocused 92-eV Free-Electron-Laser Pulses

PHYSICAL REVIEW SPECIAL TOPICS-ACCELERATORS AND BEAMS 14 (2011) ARTN 164801

E Galtier, FB Rosmej, T Dzelzainis, D Riley, FY Khattak, P Heimann, RW Lee, AJ Nelson, SM Vinko, T Whitcher, JS Wark, T Tschentscher, S Toleikis, RR Faeustlin, R Sobierajski, M Jurek, L Juha, J Chalupsky, V Hajkova, M Kozlova, J Krzywinski, B Nagler

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