Publications


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.


Quantum hydrodynamics of strongly coupled electron fluids

PHYSICAL REVIEW E 85 (2012) ARTN 046408

R Schmidt, BJB Crowley, J Mithen, G Gregori


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.


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.


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 × 1018 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 × 1017 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.


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.


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.


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.


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

2012 Conference on Lasers and Electro-Optics, CLEO 2012 (2012)

BI Cho, K Engelhorn, SM Vinko, JS Wark, RW Falcone, PA Heimann

Interaction of intense x-ray and solid density Al plasma is studied via K-shell emission spectroscopy. A high fluence, high-intensity x-ray pulse from an x-ray free-electron laser unveils multiphoton ionization pathway and drives hidden resonances. © 2012 OSA.


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 CH2 foils have been irradiated with a laser intensity of ∼ 1016 W/cm2. 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 ∼ 1018 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/ωpi ∼ 0.16 mm, the interaction length, lint, of ∼ 8 mm, and the Coulomb mean free path, λmfp ∼ 27 mm. With c/ωpi ≪ l int ≪λmfp, we are in a regime where collisionless shock formation is possible. © 2012 American Institute of Physics.


Nanosecond white-light Laue diffraction measurements of dislocation microstructure in shock-compressed single-crystal copper.

Nat Commun 3 (2012) 1224-

MJ Suggit, A Higginbotham, JA Hawreliak, G Mogni, G Kimminau, P Dunne, AJ Comley, N Park, BA Remington, JS Wark

Under uniaxial high-stress shock compression it is believed that crystalline materials undergo complex, rapid, micro-structural changes to relieve the large applied shear stresses. Diagnosing the underlying mechanisms involved remains a significant challenge in the field of shock physics, and is critical for furthering our understanding of the fundamental lattice-level physics, and for the validation of multi-scale models of shock compression. Here we employ white-light X-ray Laue diffraction on a nanosecond timescale to make the first in situ observations of the stress relaxation mechanism in a laser-shocked crystal. The measurements were made on single-crystal copper, shocked along the [001] axis to peak stresses of order 50 GPa. The results demonstrate the presence of stress-dependent lattice rotations along specific crystallographic directions. The orientation of the rotations suggests that there is double slip on conjugate systems. In this model, the rotation magnitudes are consistent with defect densities of order 10(12) cm(-2).


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.


Self-consistent measurement of the equation of state of liquid deuterium

High Energy Density Physics 8 (2012) 76-80

K Falk, SP Regan, J Vorberger, MA Barrios, TR Boehly, DE Fratanduono, SH Glenzer, DG Hicks, SX Hu, CD Murphy, PB Radha, S Rothman, AP Jephcoat, JS Wark, DO Gericke, G Gregori

We combine experiments and theoretical models to characterize warm dense deuterium. A shockwave was driven in a planar target by the OMEGA laser without a standard pusher making the analysis independent of a quartz or aluminium pressure standard. The conditions of the shocked material were diagnosed with VISAR and optical pyrometry which yields the shock velocity (16.9 ± 0.9 km/s) and the temperature (0.57 ± 0.05 eV). We find a self-consistent description of the data when using ab initio simulations (DFT-MD), but not for other equation of state (EOS) models tested. © 2011 Elsevier B.V.


Comparative merits of the memory function and dynamic local-field correction of the classical one-component plasma

PHYSICAL REVIEW E 85 (2012) ARTN 056407

JP Mithen, J Daligault, G Gregori


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.


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


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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