Publications


Probing dynamic material strength using in situ x-ray diffraction

AIP Conference Proceedings 1426 (2012) 975-978

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

The lattice level strain measured using in situ x-ray diffraction during shock compression of rolled iron foils is used along with the pressure dependent elastic constants to estimate the dynamic strength of 1±1 GPa at 15 GPa. We examine these results in the context of the constant stress (Voigt) and constant strain (Ruess) limit of grain interaction, discussing the implications at the lattice level. © 2012 American Institute of Physics.


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.


Inelastic x-ray scattering from shocked liquid deuterium

Physical Review Letters 109 (2012)

SP Regan, K Falk, G Gregori, PB Radha, SX Hu, TR Boehly, BJB Crowley, SH Glenzer, OL Landen, DO Gericke, T Döppner, DD Meyerhofer, CD Murphy, TC Sangster, J Vorberger

The Fermi-degenerate plasma conditions created in liquid deuterium by a laser-ablation - driven shock wave were probed with noncollective, spectrally resolved, inelastic x-ray Thomson scattering employing Cl Lyα line emission at 2.96 keV. These first x-ray Thomson scattering measurements of the microscopic properties of shocked deuterium show an inferred spatially averaged electron temperature of 8±5 eV, an electron density of 2.2(±0.5)×1023 cm-3, and an ionization of 0.8 (-0.25, +0.15). Two-dimensional hydrodynamic simulations using equation-of-state models suited for the extreme parameters occurring in inertial confinement fusion research and planetary interiors are consistent with the experimental results. © 2012 American Physical Society.


Molecular Dynamics Simulations for the Shear Viscosity of the One-Component Plasma

CONTRIBUTIONS TO PLASMA PHYSICS 52 (2012) 58-61

JP Mithen, J Daligault, G Gregori


Transverse beam profile measurements of laser accelerated electrons using coherent optical radiation

AIP Conference Proceedings 1507 (2012) 258-261

N Bourgeois, M Heigoldt, W Rittershofer, A Popp, K Khrennikov, SI Bajlekov, S Karsh, SM Hooker

We use coherent optical transition radiation (COTR) to measure the transverse profile of laser-accelerated electron bunches. The retrieved electron beam profiles are compared to scintillator-based beam profile measurements. © 2012 American Institute of Physics.


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.


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.


Warm dense aluminum plasma generated by the free-electron-laser FLASH

AIP Conference Proceedings 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 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 from 10 13 to 10 17 W/cm 2. We investigate the absorption and temporal evolution of the sample under irradiation by use of XUV spectroscopy. We discuss the origin of saturable absorption, radiative decay, bremsstrahlung and ionic line emission. Our experimental results are in good agreement with hydrodynamic simulations. © 2012 American Institute of Physics.


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.


High Harmonic Optical Generator (Polarization Beating 2/2)

(2012) UK Patent Application GB1207963.8

LZ Liu, K O'Keeffe, SM Hooker

A high harmonic optical generator comprising a laser arrangement and an optical waveguide, wherein the laser arrangement is configured to couple a beam of linearly polarized radiation at a fundamental frequency into the optical waveguide to provide a beam of optical driving radiation that propagates along a propagation axis of the optical waveguide, the optical waveguide has a hollow core for a gaseous harmonic generation medium for generation of high harmonics of the fundamental frequency, and the generator is configured for the driving radiation to have a polarization ellipticity that periodically varies along the propagation axis of the optical waveguide.


High Harmonic Optical Generator (Optical Rotation)

(2012) UK Patent Application GB1208753.2

LZ Liu, K O'Keeffe, SM Hooker

A high harmonic optical generator comprising a laser arrangement for emitting a beam of polarized radiation at a fundamental frequency and an optical waveguide having a hollow core for a gaseous harmonic generation medium for the generation of high harmonics of the fundamental frequency, the optical waveguide having an optical propagation axis along the hollow core, the laser arrangement is configured to couple the beam of polarized radiation along the propagation axis of the hollow core optical waveguide to provide a beam of optical driving radiation for the high harmonic generation, the optical driving radiation having a plane of polarization that rotates about the propagation axis.


Quasi-phase-matching of high-order-harmonic generation using polarization beating in optical waveguides

Phys Rev A 85 (2012) 053823

LZ Liu, K O'Keeffe, SM Hooker

A scheme for quasi-phase-matching high-harmonic generation is proposed in which polarization beating within a hollow core birefringent waveguide modulates the generation of harmonics. The evolution of the polarization of a laser pulse propagating in a birefringent waveguide is calculated and is shown to periodically modulate the harmonic generation process. The optimum conditions for achieving quasi-phase-matching using this scheme are explored and the growth of the harmonic intensity as a function of experimental parameters is investigated.


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.


Laboratory investigations on the origins of cosmic rays

Plasma Physics and Controlled Fusion 54 (2012)

Y Kuramitsu, Y Sakawa, T Morita, T Ide, K Nishio, H Tanji, H Aoki, S Dono, CD Gregory, JN Waugh, N Woolsey, A Dizière, A Pelka, A Ravasio, B Loupias, M Koenig, SA Pikuz, YT Li, Y Zhang, X Liu, JY Zhong, J Zhang, G Gregori, N Nakanii, K Kondo, Y Mori, E Miura, R Kodama, Y Kitagawa, K Mima, KA Tanaka, H Azechi, T Moritaka, Y Matsumoto, T Sano, A Mizuta, N Ohnishi, M Hoshino, H Takabe

We report our recent efforts on the experimental investigations related to the origins of cosmic rays. The origins of cosmic rays are long standing open issues in astrophysics. The galactic and extragalactic cosmic rays are considered to be accelerated in non-relativistic and relativistic collisionless shocks in the universe, respectively. However, the acceleration and transport processes of the cosmic rays are not well understood, and how the collisionless shocks are created is still under investigation. Recent high-power and high-intensity laser technologies allow us to simulate astrophysical phenomena in laboratories. We present our experimental results of collisionless shock formations in laser-produced plasmas. © 2012 IOP Publishing Ltd.


Design considerations for unmagnetized collisionless-shock measurements in homologous flows

Astrophysical Journal 749 (2012)

RP Drake, G Gregori

The subject of this paper is the design of practical laser experiments that can produce collisionless shocks mediated by the Weibel instability. Such shocks may be important in a wide range of astrophysical systems. Three issues are considered. The first issue is the implications of the fact that such experiments will produce expanding flows that are approximately homologous. As a result, both the velocity and the density of the interpenetrating plasma streams will be time dependent. The second issue is the implications of the linear theory of the Weibel instability. For the experiments, the instability is in a regime where standard simplifications do not apply. It appears feasible but non-trivial to obtain adequate growth. The third issue is collisionality. The need to keep resistive magnetic-field dissipation small enough implies that the plasmas should not be allowed to cool substantially. © 2012. The American Astronomical Society. All rights reserved.


Measurement of radiative shock properties by X-ray Thomson scattering

Physical Review Letters 108 (2012)

AJ Visco, RP Drake, SH Glenzer, T Döppner, G Gregori, DH Froula, MJ Grosskopf

X-ray Thomson scattering has enabled us to measure the temperature of a shocked layer, produced in the laboratory, that is relevant to shocks emerging from supernovas. High energy lasers are used to create a shock in argon gas which is probed by x-ray scattering. The scattered, inelastic Compton feature allows inference of the electron temperature. It is measured to be 34 eV in the radiative precursor and ∼60eV near the shock. Comparison of energy fluxes implied by the data demonstrates that the shock wave is strongly radiative. © 2012 American Physical Society.


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.


Observation of inhibited electron-ion coupling in strongly heated graphite.

Sci Rep 2 (2012) 889-

TG White, J Vorberger, CR Brown, BJ Crowley, P Davis, SH Glenzer, JW Harris, DC Hochhaus, S Le Pape, T Ma, CD Murphy, P Neumayer, LK Pattison, S Richardson, DO Gericke, G Gregori

Creating non-equilibrium states of matter with highly unequal electron and lattice temperatures (T(ele)≠T(ion)) allows unsurpassed insight into the dynamic coupling between electrons and ions through time-resolved energy relaxation measurements. Recent studies on low-temperature laser-heated graphite suggest a complex energy exchange when compared to other materials. To avoid problems related to surface preparation, crystal quality and poor understanding of the energy deposition and transport mechanisms, we apply a different energy deposition mechanism, via laser-accelerated protons, to isochorically and non-radiatively heat macroscopic graphite samples up to temperatures close to the melting threshold. Using time-resolved x ray diffraction, we show clear evidence of a very small electron-ion energy transfer, yielding approximately three times longer relaxation times than previously reported. This is indicative of the existence of an energy transfer bottleneck in non-equilibrium warm dense matter.


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.


Optical rotation quasi-phase-matching for circularly polarized high harmonic generation

Optics Letters 37 (2012) 167066

LZ Liu, K O'Keeffe, SM Hooker

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