Publications


Modeling HEDLA magnetic field generation experiments on laser facilities

High Energy Density Physics 9 (2013) 172-177

M Fatenejad, AR Bell, A Benuzzi-Mounaix, R Crowston, RP Drake, N Flocke, G Gregori, M Koenig, C Krauland, D Lamb, D Lee, JR Marques, J Meinecke, F Miniati, CD Murphy, HS Park, A Pelka, A Ravasio, B Remington, B Reville, A Scopatz, P Tzeferacos, K Weide, N Woolsey, R Young, R Yurchak

The Flash Center is engaged in a collaboration to simulate laser driven experiments aimed at understanding the generation and amplification of cosmological magnetic fields using the FLASH code. In these experiments a laser illuminates a sol id plastic or graphite target launching an asymmetric blast wave into a chamber which contains either Helium or Argon at millibar pressures. Induction coils placed several centimeters away from the target detect large scale magnetic fields on the order of tens to hundreds of Gauss. The time dependence of the magnetic field is consistent with generation via the Biermann battery mechanism near the blast wave. Attempts to perform simulations of these experiments using the FLASH code have uncovered previously unreported numerical difficulties in modeling the Biermann battery mechanism near shock waves which can lead to the production of large non-physical magnetic fields. We report on these difficulties and offer a potential solution. © 2012 Elsevier B.V.


Simulation of laser-driven, ablated plasma flows in collisionless shock experiments on OMEGA and the NIF

High Energy Density Physics 9 (2013) 192-197

MJ Grosskopf, RP Drake, CC Kuranz, EM Rutter, JS Ross, NL Kugland, C Plechaty, BA Remington, A Spitkovsky, L Gargate, G Gregori, A Bell, CD Murphy, J Meinecke, B Reville, Y Sakawa, Y Kuramitsu, H Takabe, DH Froula, G Fiksel, F Miniati, M Koenig, A Ravasio, E Liang, W Fu, N Woolsey, HS Park

Experiments investigating the physics of interpenetrating, collisionless, ablated plasma flows have become an important area of research in the high-energy-density field. In order to evaluate the feasibility of designing experiments that will generate a collisionless shock mediated by the Weibel instability on the National Ignition Facility (NIF) laser, computer simulations using the Center for Radiative Shock Hydrodynamics (CRASH) radiation-hydrodynamics model have been carried out. This paper reports assessment of whether the experiment can reach the required scale size while maintaining the low interflow collisionality necessary for the collisionless shock to form. Comparison of simulation results with data from Omega experiments shows the ability of the CRASH code to model these ablated systems. The combined results indicate that experiments on the NIF are capable of reaching the regimes necessary for the formation of a collisionless shock in a laboratory experiment. © 2013.


Magnetic field generation by Biermann battery and Weibel instability in laboratory shock waves

EAS Publications Series 58 (2012) 23-26

G Gregori, F Miniati, B Reville, RP Drake

Magnetic field generation in the Universe is still an open problem. Possible mechanisms involve the Weibel instability, due to anisotropic phase-space distributions, as well as the Biermann battery, due to misaligned density and temperature gradients. These mechanisms can be reproduced in scaled laboratory experiments. In this contribution we estimate the relative importance of these two processes and explore the laser-energy requirements for producing Weibel dominated shocks. © The Author(s) 2013.


Complete spatial characterization of an optical wavefront using a variable-separation pinhole Pair

2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013 (2013)

DT Lloyd, K O'Keeffe, SM Hooker

Characterizing the spatial and temporal properties of high harmonic generation (HHG) is crucial for understanding and controlling the dynamics of this extreme nonlinear process. Indeed, it has been shown that the macroscopic spatial properties of HHG encode details of the microscopic response of matter to an intense laser field: such as signatures of the interference of different quantum trajectories [1]. © 2013 IEEE.


Strength of Shock-Loaded Single-Crystal Tantalum [100] Determined using in Situ Broadband X-Ray Laue Diffraction

Physical Review Letters 110 (2013)

AJ Comley, BR Maddox, RE Rudd, ST Prisbrey, JA Hawreliak, DA Orlikowski, SC Peterson, JH Satcher, AJ Elsholz, HS Park, BA Remington, N Bazin, JM Foster, P Graham, N Park, PA Rosen, SR Rothman, A Higginbotham, M Suggit, JS Wark

The strength of shock-loaded single crystal tantalum [100] has been experimentally determined using in situ broadband x-ray Laue diffraction to measure the strain state of the compressed crystal, and elastic constants calculated from first principles. The inferred strength reaches 35 GPa at a shock pressure of 181 GPa and is in excellent agreement with a multiscale strength model, which employs a hierarchy of simulation methods over a range of length scales to calculate strength from first principles. © 2013 American Physical Society.


PROBING DYNAMIC MATERIAL STRENGTH USING IN SITU X-RAY DIFFRACTION

SHOCK COMPRESSION OF CONDENSED MATTER - 2011, PTS 1 AND 2 1426 (2012)

J Hawreliak, B El-Dasher, J Eggert, J Rygg, G Collins, H Lorenzana, G Kimminau, A Higginbotham, B Nagler, SM Vinko, WJ Murphy, T Whitcher, S Rothman, N Park, JS Wark


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 aper ture 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.


Direct measurements of the ionization potential depression in a dense plasma

Physical Review Letters 109 (2012)

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

We have used the Linac Coherent Light Source to generate solid-density aluminum plasmas at temperatures of up to 180 eV. By varying the photon energy of the x rays that both create and probe the plasma, and observing the K-α fluorescence, we can directly measure the position of the K edge of the highly charged ions within the system. The results are found to disagree with the predictions of the extensively used Stewart-Pyatt model, but are consistent with the earlier model of Ecker and Kröll, which predicts significantly greater depression of the ionization potential. © 2012 American Physical Society.


XUV spectroscopic characterization of warm dense aluminum plasmas generated by the free-electron-laser FLASH

Laser and Particle Beams 30 (2012) 45-56

U Zastrau, T Burian, J Chalupsky, T Döppner, TWJ Dzelzainis, RR Fäustlin, C Fortmann, E Galtier, SH Glenzer, G Gregori, L Juha, HJ Lee, RW Lee, CLS Lewis, N Medvedev, B Nagler, AJ Nelson, D Riley, FB Rosmej, S Toleikis, T Tschentscher, I Uschmann, SM Vinko, JS Wark, T Whitcher, E Förster

We report on experiments aimed at the generation and characterization of solid density plasmas at the free-electron laser FLASH in Hamburg. Aluminum samples were irradiated with XUV pulses at 13.5 nm wavelength (92 eV photon energy). The pulses with duration of a few tens of femtoseconds and pulse energy up to 100 μJ are focused to intensities ranging between 10 13 and 10 17 W/cm 2 . We investigate the absorption and temporal evolution of the sample under irradiation by use of XUV and optical spectroscopy. We discuss the origin of saturable absorption, radiative decay, bremsstrahlung and atomic and ionic line emission. Our experimental results are in good agreement with simulations. © 2012 Cambridge University Press.


Multiple pulse resonantly enhanced laser plasma wakefield acceleration

AIP Conference Proceedings 1507 (2012) 872-873

L Corner, R Walczak, LJ Nevay, S Dann, SM Hooker, N Bourgeois, J Cowley

We present an outline of experiments being conducted at Oxford University on multiple-pulse, resonantly-enhanced laser plasma wakefield acceleration. This method of laser plasma acceleration uses trains of optimally spaced low energy short pulses to drive plasma oscillations and may enable laser plasma accelerators to be driven by compact and efficient fibre laser sources operating at high repetition rates. © 2012 American Institute of Physics.


Electron acceleration driven in plasma channels at the Astra-Gemini laser facility

AIP Conference Proceedings 1507 (2012) 193-198

PA Walker, N Bourgeois, W Rittershofer, J Cowley, N Kajumba, AR Maier, J Wenz, CM Werle, DR Symes, PP Rajeev, SJ Hawkes, O Chekhlov, CJ Hooker, B Parry, Y Tang, VA Marshall, S Karsch, F Grüner, SM Hooker

The generation of GeV-scale electron beams in the plasma channel formed in a gas-filled capillary discharge waveguide is investigated. Electron beams with energies above 900 MeV and with root-mean-square divergence of 3.5 mrad are observed for plasma densities of 2.15 × 10 18 cm -3 and a peak input laser power of only 55 TW. The variation of the electron energy with the plasma density is measured and found to exhibit a maximum at plasma densities for which the dephasing length approximately matches the length of the plasma channel. Injection and acceleration of electrons at the relatively low plasma density of 3.2 × 10 17 cm -3 is observed. The energy spectra of the generated electron beams are shown to exhibit good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting for the effects on the measured energy spectrum of off-axis electron beam propagation are investigated. © 2012 American Institute of Physics.


Warm Dense Aluminum Plasma generated by the Free-Electron-Laser FLASH

17TH INTERNATIONAL CONFERENCE ON ATOMIC PROCESSES IN PLASMAS (ICAPIP) 1438 (2012) 61-64

U Zastrau, SM Vinko, JS Wark, S Toleikis, T Tschentscher, SH Glenzer, RW Lee, AJ Nelson, TWJ Dzelzainis, D Riley, B Nagler, E Galtier, FB Rosmej, E Foerster


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


Creation and diagnosis of a solid-density plasma with an X-ray free-electron laser.

Nature 482 (2012) 59-62

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

Matter with a high energy density (>10(5) joules per cm(3)) is prevalent throughout the Universe, being present in all types of stars and towards the centre of the giant planets; it is also relevant for inertial confinement fusion. Its thermodynamic and transport properties are challenging to measure, requiring the creation of sufficiently long-lived samples at homogeneous temperatures and densities. With the advent of the Linac Coherent Light Source (LCLS) X-ray laser, high-intensity radiation (>10(17) watts per cm(2), previously the domain of optical lasers) can be produced at X-ray wavelengths. The interaction of single atoms with such intense X-rays has recently been investigated. An understanding of the contrasting case of intense X-ray interaction with dense systems is important from a fundamental viewpoint and for applications. Here we report the experimental creation of a solid-density plasma at temperatures in excess of 10(6) kelvin on inertial-confinement timescales using an X-ray free-electron laser. We discuss the pertinent physics of the intense X-ray-matter interactions, and illustrate the importance of electron-ion collisions. Detailed simulations of the interaction process conducted with a radiative-collisional code show good qualitative agreement with the experimental results. We obtain insights into the evolution of the charge state distribution of the system, the electron density and temperature, and the timescales of collisional processes. Our results should inform future high-intensity X-ray experiments involving dense samples, such as X-ray diffractive imaging of biological systems, material science investigations, and the study of matter in extreme conditions.


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, APL 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. © 2012 Macmillan Publishers Limited. All rights reserved.


Plasma switch as a temporal overlap tool for pump-probe experiments at FEL facilities

Journal of Instrumentation 7 (2012)

M Harmand, D Murphy, D Brown, M Cammarata, T Döppner, S Düsterer, D Fritz, E Förster, E Galtier, J Gaudin, H Glenzer, S Göde, G Gregori, V Hilbert, D Hochhaus, T Laarmann, J Lee, H Lemke, KH Meiwes-Broer, A Moinard, P Neumayer, A Przystawik, H Redlin, M Schulz, S Skruszewicz, F Tavella, T Tschentscher, T White, U Zastrau, S Toleikis

We have developed an easy-to-use and reliable timing tool to determine the arrival time of an optical laser and a free electron laser (FEL) pulses within the jitter limitation. This timing tool can be used from XUV to X-rays and exploits high FELs intensities. It uses a shadowgraph technique where we optically (at 800 nm) image a plasma created by an intense XUV or X-ray FEL pulse on a transparent sample (glass slide) directly placed at the pump - probe sample position. It is based on the physical principle that the optical properties of the material are drastically changed when its free electron density reaches the critical density. At this point the excited glass sample becomes opaque to the optical laser pulse. The ultra-short and intense XUV or X-ray FEL pulse ensures that a critical electron density can be reached via photoionization and subsequent collisional ionization within the XUV or X-ray FEL pulse duration or even faster. This technique allows to determine the relative arrival time between the optical laser and the FEL pulses in only few single shots with an accuracy mainly limited by the optical laser pulse duration and the jitter between the FEL and the optical laser. Considering the major interest in pump-probe experiments at FEL facilities in general, such a femtosecond resolution timing tool is of utmost importance. © 2012 IOP Publishing Ltd and Sissa Medialab srl.


Characterizing counter-streaming interpenetrating plasmas relevant to astrophysical collisionless shocks

Physics of Plasmas 19 (2012)

JS Ross, SH Glenzer, P Amendt, R Berger, L Divol, NL Kugland, OL Landen, C Plechaty, B Remington, D Ryutov, W Rozmus, DH Froula, G Fiksel, C Sorce, Y Kuramitsu, T Morita, Y Sakawa, H Takabe, RP Drake, M Grosskopf, C Kuranz, G Gregori, J Meinecke, CD Murphy, M Koenig, A Pelka, A Ravasio, T Vinci, E Liang, R Presura, A Spitkovsky, F Miniati, HS Park

A series of Omega experiments have produced and characterized high velocity counter-streaming plasma flows relevant for the creation of collisionless shocks. Single and double CH 2 foils have been irradiated with a laser intensity of ∼ 10 16 W/cm 2 . The laser ablated plasma was characterized 4 mm from the foil surface using Thomson scattering. A peak plasma flow velocity of 2000 km/s, an electron temperature of ∼ 110 eV, an ion temperature of ∼ 30 eV, and a density of ∼ 10 18 cm -3 were measured in the single foil configuration. Significant increases in electron and ion temperatures were seen in the double foil geometry. The measured single foil plasma conditions were used to calculate the ion skin depth, c/ω pi ∼ 0.16 mm, the interaction length, l int , of ∼ 8 mm, and the Coulomb mean free path, λ mfp ∼ 27 mm. With c/ω pi ≪ l int ≪λ mfp , we are in a regime where collisionless shock formation is possible. © 2012 American Institute of Physics.


Quasi-phase-matched high harmonic generation using trains of uniformly-spaced ultrafast pulses

High Intensity Lasers and High Field Phenomena, HILAS 2012 (2012)

K O'Keeffe, T Robinson, SM Hooker

We investigate quasi-phase-matching of high harmonic generation over a range of harmonic orders using trains of up to 8 uniformly-spaced counter-propagating pulses, produced using an array of birefringent crystals. © 2012 OSA.


Testing quantum mechanics in non-Minkowski space-time with high power lasers and 4 th generation light sources

Scientific Reports 2 (2012)

BJB Crowley, R Bingham, RG Evans, DO Gericke, OL Landen, CD Murphy, PA Norreys, SJ Rose, T Tschentscher, CHT Wang, JS Wark, G Gregori

A common misperception of quantum gravity is that it requires accessing energies up to the Planck scale of 10 19 GeV, which is unattainable from any conceivable particle collider. Thanks to the development of ultra-high intensity optical lasers, very large accelerations can be now the reached at their focal spot, thus mimicking, by virtue of the equivalence principle, a non Minkowski space-time. Here we derive a semiclassical extension of quantum mechanics that applies to different metrics, but under the assumption of weak gravity. We use our results to show that Thomson scattering of photons by uniformly accelerated electrons predicts an observable effect depending upon acceleration and local metric. In the laboratory frame, a broadening of the Thomson scattered x ray light from a fourth generation light source can be used to detect the modification of the metric associated to electrons accelerated in the field of a high power optical laser.

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