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.


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


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.


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.


Simulation of X-ray scattering diagnostics in multi-dimensional plasma

High Energy Density Physics 9 (2013) 510-515

I Golovkin, JJ MacFarlane, P Woodruff, I Hall, G Gregori, J Bailey, E Harding, T Ao, S Glenzer

X-ray scattering is a powerful diagnostic technique that has been used in a variety of experimental settings to determine the temperature, density, and ionization state of warm dense matter. In order to maximize the intensity of the scattered signal, the x-ray source is often placed in close proximity to the target plasma. Therefore, the interpretation of the experimental data can become complicated by the fact that the detector records photons scattered at different angles from points within the plasma volume. In addition, the target plasma that is scattering the x-rays can have significant temperature and density gradients. To address these issues, we have developed the capability to simulate x-ray scattering for realistic experimental configurations where the effects of plasma non-uniformities and a range of x-ray scattering angles are included. We will discuss the implementation details and show results relevant to previous and ongoing experimental investigations. © 2013 Elsevier B.V.


Radiative shocks produced from spherical cryogenic implosions at the National Ignition Facility

Physics of Plasmas 20 (2013)

A Pak, L Divol, G Gregori, S Weber, J Atherton, R Bennedetti, DK Bradley, D Callahan, DT Casey, E Dewald, T Döppner, MJ Edwards, JA Frenje, S Glenn, GP Grim, D Hicks, WW Hsing, N Izumi, OS Jones, MG Johnson, SF Khan, JD Kilkenny, JL Kline, GA Kyrala, J Lindl, OL Landen, S Le Pape, T Ma, A Macphee, BJ Macgowan, AJ Mackinnon, L Masse, NB Meezan, JD Moody, RE Olson, JE Ralph, HF Robey, HS Park, BA Remington, JS Ross, R Tommasini, RPJ Town, V Smalyuk, SH Glenzer, EI Moses

Spherically expanding radiative shock waves have been observed from inertially confined implosion experiments at the National Ignition Facility. In these experiments, a spherical fusion target, initially 2 mm in diameter, is compressed via the pressure induced from the ablation of the outer target surface. At the peak compression of the capsule, x-ray and nuclear diagnostics indicate the formation of a central core, with a radius and ion temperature of ∼20 μm and ∼ 2 keV, respectively. This central core is surrounded by a cooler compressed shell of deuterium-tritium fuel that has an outer radius of ∼40 μm and a density of >500 g/cm3. Using inputs from multiple diagnostics, the peak pressure of the compressed core has been inferred to be of order 100 Gbar for the implosions discussed here. The shock front, initially located at the interface between the high pressure compressed fuel shell and surrounding in-falling low pressure ablator plasma, begins to propagate outwards after peak compression has been reached. Approximately 200 ps after peak compression, a ring of x-ray emission created by the limb-brightening of a spherical shell of shock-heated matter is observed to appear at a radius of ∼100 μm. Hydrodynamic simulations, which model the experiment and include radiation transport, indicate that the sudden appearance of this emission occurs as the post-shock material temperature increases and upstream density decreases, over a scale length of ∼10 μm, as the shock propagates into the lower density (∼1 g/cc), hot (∼250 eV) plasma that exists at the ablation front. The expansion of the shock-heated matter is temporally and spatially resolved and indicates a shock expansion velocity of ∼300 km/s in the laboratory frame. The magnitude and temporal evolution of the luminosity produced from the shock-heated matter was measured at photon energies between 5.9 and 12.4 keV. The observed radial shock expansion, as well as the magnitude and temporal evolution of the luminosity from the shock-heated matter, is consistent with 1-D radiation hydrodynamic simulations. Analytic estimates indicate that the radiation energy flux from the shock-heated matter is of the same order as the in-flowing material energy flux, and suggests that this radiation energy flux modifies the shock front structure. Simulations support these estimates and show the formation of a radiative shock, with a precursor that raises the temperature ahead of the shock front, a sharp μ m-scale thick spike in temperature at the shock front, followed by a post-shock cooling layer. © 2013 AIP Publishing LLC.


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 P4), 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 P4 shapes may be unquantifiable for DT layered capsules. Instead the positive P4 asymmetry "aliases" itself as an oblate P2 in the x-ray images. Correction of this apparent P2 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.


Visualizing electromagnetic fields in laser-produced counter-streaming plasma experiments for collisionless shock laboratory astrophysics

Physics of Plasmas 20 (2013)

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

Collisionless shocks are often observed in fast-moving astrophysical plasmas, formed by non-classical viscosity that is believed to originate from collective electromagnetic fields driven by kinetic plasma instabilities. However, the development of small-scale plasma processes into large-scale structures, such as a collisionless shock, is not well understood. It is also unknown to what extent collisionless shocks contain macroscopic fields with a long coherence length. For these reasons, it is valuable to explore collisionless shock formation, including the growth and self-organization of fields, in laboratory plasmas. The experimental results presented here show at a glance with proton imaging how macroscopic fields can emerge from a system of supersonic counter-streaming plasmas produced at the OMEGA EP laser. Interpretation of these results, plans for additional measurements, and the difficulty of achieving truly collisionless conditions are discussed. Future experiments at the National Ignition Facility are expected to create fully formed collisionless shocks in plasmas with no pre-imposed magnetic field. © 2013 AIP Publishing LLC.


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


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.


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


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.


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.

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