Publications


Surface waves and electron acceleration from high-power, kilojoule-class laser interactions with underdense plasma

New Journal of Physics 15 (2013)

L Willingale, AGR Thomas, PM Nilson, H Chen, J Cobble, RS Craxton, A Maksimchuk, PA Norreys, TC Sangster, RHH Scott, C Stoeckl, C Zulick, K Krushelnick

Experiments were performed on the Omega EP laser facility to study laser pulse propagation, channeling phenomena and electron acceleration from high-intensity, high-power laser interactions with underdense plasma. A CH plasma plume was used as the underdense target and the interaction of the laser pulse channeling through the plasma was imaged using proton radiography. High-energy electron spectra were measured for different experimental laser parameters. Structures observed along the channel walls are interpreted as having developed from surface waves, which are likely to serve as an injection mechanism of electrons into the cavitated channel for acceleration via direct laser acceleration mechanisms. Two-dimensional particle-in-cell simulations give good agreement with these channeling and electron acceleration phenomena. © IOP Publishing and Deutsche Physikalische Gesellschaft.


Laboratory experiments on plasma jets in a magnetic field using high-power lasers

EPJ Web of Conferences 59 (2013)

K Nishio, Y Sakawa, Y Kuramitsu, T Morita, T Ide, M Kuwada, M Koga, T Kato, T Norimatsu, C Gregory, N Woolsey, C Murphy, G Gregori, K Schaar, A Diziere, M Koenig, A Pelka, S Wang, Q Dong, Y Li, H Takabe

The experiments to simulate astrophysical jet generation are performed using Gekko XII (GXII) HIPER laser system at the Institute of Laser Engineering. In the experiments a fast plasma flow generated by shooting a CH plane (10 μm thickness) is observed at the rear side of the plane. By separating the focal spot of the main beams, a non-uniform plasma is generated. The non-uniform plasma flow in an external magnetic field (0.2∼0.3 T) perpendicular to the plasma is more collimated than that without the external magnetic field. The plasma β, the ratio between the plasma and magnetic pressure, is ≠1, and the magnetic Reynolds number is ∼150 in the collimated plasma. It is considered that the magnetic field is distorted by the plasma flow and enhances the jet collimation. © Owned by the authors, published by EDP Sciences, 2013.


High Mach-number collisionless shock driven by a laser with an external magnetic field

EPJ Web of Conferences 59 (2013)

T Morita, Y Sakawa, Y Kuramitsu, T Ide, K Nishio, M Kuwada, H Ide, K Tsubouchi, H Yoneda, A Nishida, T Namiki, T Norimatsu, K Tomita, K Nakayama, K Inoue, K Uchino, M Nakatsutsumi, A Pelka, M Koenig, Q Dong, D Yuan, G Gregori, H Takabe

Collisionless shocks are produced in counter-streaming plasmas with an external magnetic field. The shocks are generated due to an electrostatic field generated in counter-streaming laser-irradiated plasmas, as reported previously in a series of experiments without an external magnetic field [T. Morita et al., Phys. Plasmas, 17, 122702 (2010), Kuramitsu et al., Phys. Rev. Lett., 106, 175002 (2011)] via laser-irradiation of a double-CH-foil target. A magnetic field is applied to the region between two foils by putting an electro-magnet (∼10 T) perpendicular to the direction of plasma expansion. The generated shocks show different characteristics later in time (t > 20ns). © Owned by the authors, published by EDP Sciences, 2013.


High-power laser experiments to study collisionless shock generation

EPJ Web of Conferences 59 (2013)

Y Sakawa, Y Kuramitsu, T Morita, T Kato, H Tanji, T Ide, K Nishio, M Kuwada, T Tsubouchi, H Ide, T Norimatsu, C Gregory, N Woolsey, K Schaar, C Murphy, G Gregori, A Diziere, A Pelka, M Koenig, S Wang, Q Dong, Y Li, HS Park, S Ross, N Kugland, D Ryutov, B Remington, A Spitkovsky, D Froula, H Takabe

A collisionless Weibel-instability mediated shock in a self-generated magnetic field is studied using two-dimensional particle-in-cell simulation [Kato and Takabe, Astophys. J. Lett. 681, L93 (2008)]. It is predicted that the generation of the Weibel shock requires to use NIF-class high-power laser system. Collisionless electrostatic shocks are produced in counter-streaming plasmas using Gekko XII laser system [Kuramitsu et al., Phys. Rev. Lett. 106, 175002 (2011)] . A NIF facility time proposal is approved to study the formation of the collisionless Weibel shock. OMEGA and OMEGA EP experiments have been started to study the plasma conditions of counter-streaming plasmas required for the NIF experiment using Thomson scattering and to develop proton radiography diagnostics. © Owned by the authors, published by EDP Sciences, 2013.


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.


Laminar shocks in high power laser interactions

40th EPS Conference on Plasma Physics, EPS 2013 2 (2013) 850-853

RA Cairns, R Bingham, PA Norreys, RMGM Trines


Ultrafast three-dimensional imaging of lattice dynamics in individual gold nanocrystals.

Science 341 (2013) 56-59

JN Clark, L Beitra, G Xiong, A Higginbotham, DM Fritz, HT Lemke, D Zhu, M Chollet, GJ Williams, M Messerschmidt, B Abbey, RJ Harder, AM Korsunsky, JS Wark, IK Robinson

Key insights into the behavior of materials can be gained by observing their structure as they undergo lattice distortion. Laser pulses on the femtosecond time scale can be used to induce disorder in a "pump-probe" experiment with the ensuing transients being probed stroboscopically with femtosecond pulses of visible light, x-rays, or electrons. Here we report three-dimensional imaging of the generation and subsequent evolution of coherent acoustic phonons on the picosecond time scale within a single gold nanocrystal by means of an x-ray free-electron laser, providing insights into the physics of this phenomenon. Our results allow comparison and confirmation of predictive models based on continuum elasticity theory and molecular dynamics simulations.


Probing the complex ion structure in liquid carbon at 100 GPa

Physical Review Letters 111 (2013)

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

We present the first direct experimental test of the complex ion structure in liquid carbon at pressures around 100 GPa, using spectrally resolved x-ray scattering from shock-compressed graphite samples. Our results confirm the structure predicted by ab initio quantum simulations and demonstrate the importance of chemical bonds at extreme conditions similar to those found in the interiors of giant planets. The evidence presented here thus provides a firmer ground for modeling the evolution and current structure of carbon-bearing icy giants like Neptune, Uranus, and a number of extrasolar planets. © 2013 American Physical Society.


FLASH hydrodynamic simulations of experiments to explore the generation of cosmological magnetic fields

High Energy Density Physics 9 (2013) 75-81

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

We report the results of FLASH hydrodynamic simulations of the experiments conducted by the University of Oxford High Energy Density Laboratory Astrophysics group and its collaborators at the Laboratoire pour l'Utilisation de 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. The simulations show that the result of the laser illuminating the target is a series of complex hydrodynamic phenomena. © 2012 Elsevier B.V.


The effects of ionization potential depression on the spectra emitted by hot dense aluminium plasmas

High Energy Density Physics 9 (2013) 258-263

TR Preston, SM Vinko, O Ciricosta, HK Chung, RW Lee, JS Wark

Recent experiments at the Linac Coherent Light Source (LCLS) X-ray Free-Electron-Laser (FEL) have demonstrated that the standard model used for simulating ionization potential depression (IPD) in a plasma (the Stewart-Pyatt (SP) model, J.C. Stewart and K.D. Pyatt Jr., Astrophysical Journal 144 (1966) 1203) considerably underestimates the degree of IPD in a solid density aluminium plasma at temperatures up to 200 eV. In contrast, good agreement with the experimental data was found by use of a modified Ecker-Kröll (mEK) model (G. Ecker and W. Kröll, Physics of Fluids 6 (1963) 62-69). We present here detailed simulations, using the FLYCHK code, of the predicted spectra from hot dense, hydrogenic and helium-like aluminium plasmas ranging in densities from 0.1 to 4 times solid density, and at temperatures up to 1000 eV. Importantly, we find that the greater IPDs predicted by the mEK model result in the loss of the n = 3 states for the hydrogenic ions for all densities above ≈0.8 times solid density, and for the helium-like ions above ≈0.65 solid density. Therefore, we posit that if the mEK model holds at these higher temperatures, the temperature of solid density highly-charged aluminium plasmas cannot be determined by using spectral features associated with the n = 3 principal quantum number, and propose a re-evaluation of previous experimental data where high densities have been inferred from the spectra, and the SP model has been used. © 2013 Elsevier B.V.


Numerical modeling of the sensitivity of x-ray driven implosions to low-mode flux asymmetries

Physical Review Letters 110 (2013)

RHH Scott, DS Clark, DK Bradley, DA Callahan, MJ Edwards, SW Haan, OS Jones, BK Spears, MM Marinak, RPJ Town, PA Norreys, LJ Suter

The sensitivity of inertial confinement fusion implosions, of the type performed on the National Ignition Facility (NIF), to low-mode flux asymmetries is investigated numerically. It is shown that large-amplitude, low-order mode shapes (Legendre polynomial P 4 ), resulting from low-order flux asymmetries, cause spatial variations in capsule and fuel momentum that prevent the deuterium and tritium (DT) "ice" layer from being decelerated uniformly by the hot spot pressure. This reduces the transfer of implosion kinetic energy to internal energy of the central hot spot, thus reducing the neutron yield. Furthermore, synthetic gated x-ray images of the hot spot self-emission indicate that P 4 shapes may be unquantifiable for DT layered capsules. Instead the positive P 4 asymmetry "aliases" itself as an oblate P 2 in the x-ray images. Correction of this apparent P 2 distortion can further distort the implosion while creating a round x-ray image. Long wavelength asymmetries may be playing a significant role in the observed yield reduction of NIF DT implosions relative to detailed postshot two-dimensional simulations. © 2013 American Physical Society.


Comparison between x-ray scattering and velocity-interferometry measurements from shocked liquid deuterium

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 87 (2013)

K Falk, SP Regan, J Vorberger, BJB Crowley, SH Glenzer, SX Hu, CD Murphy, PB Radha, AP Jephcoat, JS Wark, DO Gericke, G Gregori

The equation of state of light elements is essential to understand the structure of Jovian planets and inertial confinement fusion research. The Omega laser was used to drive a planar shock wave in the cryogenically cooled deuterium, creating warm dense matter conditions. X-ray scattering was used to determine the spectrum near the boundary of the collective and noncollective scattering regimes using a narrow band x-ray source in backscattering geometry. Our scattering spectra are thus sensitive to the individual electron motion as well as the collective plasma behavior and provide a measurement of the electron density, temperature, and ionization state. Our data are consistent with velocity-interferometry measurements previously taken on the same shocked deuterium conditions and presented by K. Falk. This work presents a comparison of the two diagnostic systems and offers a detailed discussion of challenges encountered. ©2013 American Physical Society.


X-ray scattering from warm dense iron

High Energy Density Physics 9 (2013) 573-577

S White, G Nersisyan, B Kettle, TWJ Dzelzainis, K McKeever, CLS Lewis, A Otten, K Siegenthaler, D Kraus, M Roth, T White, G Gregori, DO Gericke, R Baggott, DA Chapman, K Wünsch, J Vorberger, D Riley

We have carried out X-ray scattering experiments on iron foil samples that have been compressed and heated using laser-driven shocks created with the VULCAN laser system at the Rutherford-Appleton Laboratory. This is the highest Z element studied in such experiments so far and the first time scattering from warm dense iron has been reported. Because of the importance of iron in telluric planets, the work is relevant to studies of warm dense matter in planetary interiors. We report scattering results as well as shock breakout results that, in conjunction with hydrodynamic simulations, suggest the target has been compressed to a molten state at several 100GPa pressure. Initial comparison with modelling suggests more work is needed to understand the structure factor of warm dense iron. © 2013.


Comparison between x-ray scattering and velocity-interferometry measurements from shocked liquid deuterium.

Phys Rev E Stat Nonlin Soft Matter Phys 87 (2013) 043112-

K Falk, SP Regan, J Vorberger, BJ Crowley, SH Glenzer, SX Hu, CD Murphy, PB Radha, AP Jephcoat, JS Wark, DO Gericke, G Gregori

The equation of state of light elements is essential to understand the structure of Jovian planets and inertial confinement fusion research. The Omega laser was used to drive a planar shock wave in the cryogenically cooled deuterium, creating warm dense matter conditions. X-ray scattering was used to determine the spectrum near the boundary of the collective and noncollective scattering regimes using a narrow band x-ray source in backscattering geometry. Our scattering spectra are thus sensitive to the individual electron motion as well as the collective plasma behavior and provide a measurement of the electron density, temperature, and ionization state. Our data are consistent with velocity-interferometry measurements previously taken on the same shocked deuterium conditions and presented by K. Falk et al. [High Energy Density Phys. 8, 76 (2012)]. This work presents a comparison of the two diagnostic systems and offers a detailed discussion of challenges encountered.


Method of time resolved refractive index measurements of x-ray laser heated solids

PHYSICS OF PLASMAS 20 (2013) ARTN 042701

GO Williams, H-K Chung, SM Vinko, S Kuenzel, AB Sardinha, P Zeitoun, M Fajardo


A Monte Carlo algorithm for degenerate plasmas

Journal of Computational Physics 249 (2013) C

AE Turrell, M Sherlock, SJ Rose


Electron–positron pair creation in burning thermonuclear plasmas

High Energy Density Physics 9 (2013) 480-483

SJ Rose


Kinetic simulations of the heating of solid density plasma by femtosecond laser pulses

High Energy Density Physics 9 (2013) 38-41

M Sherlock, EG Hill, SJ Rose


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.


Orbital-free density-functional theory simulations of the dynamic structure factor of warm dense aluminum

Physical Review Letters 111 (2013)

TG White, S Richardson, BJB Crowley, LK Pattison, JWO Harris, G Gregori

Here, we report orbital-free density-functional theory (OF DFT) molecular dynamics simulations of the dynamic ion structure factor of warm solid density aluminum at T=0.5 eV and T=5 eV. We validate the OF DFT method in the warm dense matter regime through comparison of the static and thermodynamic properties with the more complete Kohn-Sham DFT. This extension of OF DFT to dynamic properties indicates that previously used models based on classical molecular dynamics may be inadequate to capture fully the low frequency dynamics of the response function. © 2013 American Physical Society.

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