Publications


Soft x-ray free electron laser microfocus for exploring matter under extreme conditions.

Opt Express 17 (2009) 18271-18278

AJ Nelson, S Toleikis, H Chapman, S Bajt, J Krzywinski, J Chalupsky, L Juha, J Cihelka, V Hajkova, L Vysin, T Burian, M Kozlova, RR Fäustlin, B Nagler, SM Vinko, T Whitcher, T Dzelzainis, O Renner, K Saksl, AR Khorsand, PA Heimann, R Sobierajski, D Klinger, M Jurek, J Pelka, B Iwan, J Andreasson, N Timneanu, M Fajardo, JS Wark, D Riley, T Tschentscher, J Hajdu, RW Lee

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.


Hybrid Simulations for the Ion Structure and Dynamics in Dense Plasmas

ATOMIC PROCESSES IN PLASMAS 1161 (2009) 280-285

G Gregori, J Vorberger, DO Gericke


Making relativistic positrons using ultraintense short pulse lasers

Physics of Plasmas 16 (2009)

H Chen, SC Wilks, JD Bonlie, SN Chen, KV Cone, LN Elberson, DF Price, MB Schneider, R Shepherd, DC Stafford, R Tommasini, R Van Maren, P Beiersdorfer, G Gregori, DD Meyerhofer, J Myatt

This paper describes a new positron source using ultraintense short pulse lasers. Although it has been theoretically studied since the 1970s, the use of lasers as a valuable new positron source was not demonstrated experimentally until recent years, when the petawatt-class short pulse lasers were developed. In 2008 and 2009, in a series of experiments performed at the Lawrence Livermore National Laboratory, a large number of positrons were observed after shooting a millimeter thick solid gold target. Up to 2× positrons/s ejected at the back of approximately millimeter thick gold targets were detected. The targets were illuminated with short (∼1 ps) ultraintense (∼1× 10 W/ cm) laser pulses. These positrons are produced predominantly by the Bethe-Heitler process and have an effective temperature of 2-4 MeV, with the distribution peaking at 4-7 MeV. The angular distribution of the positrons is anisotropic. For a wide range of applications, this new laser-based positron source with its unique characteristics may complement the existing sources based on radioactive isotopes and accelerators. © 2009 American Institute of Physics.


Predicting EXAFS signals from shock compressed iron by use of molecular dynamics simulations

High Energy Density Physics 5 (2009) 44-50

A Higginbotham, WJ Murphy, B Nagler, JS Wark, RC Albers, TC Germann, B Lee Holian, K Kadau, PS Lomdahl

Simulated EXAFS signals from ab initio models and configurational averaging of molecular dynamics (MD) data are compared for α-Fe, and configurationally averaged MD EXAFS signals are compared with experimental data for iron shock compressed to pressures above the α-ε{lunate} transition pressure. It is shown that molecular dynamics potentials and ab initio models capable of recreating similar vibrational density of states lead to EXAFS signals in good mutual agreement. The effects of the classical nature of the phonon distribution in the MD and the anharmonicity of the potential give rise to noticeable differences between ab initio models and configurational averaging of MD data. However, the greatest influence on the spectra is the form of the potential itself. We discuss the importance of these effects in simulating EXAFS spectra for shock compressed polycrystalline iron. It is shown that EXAFS is an insensitive probe for determining the nature of the close packed product phase in this system. © 2009 Elsevier B.V.


Measurement of short-range correlations in shock-compressed plastic by short-pulse x-ray scattering.

Phys Rev Lett 102 (2009) 165004-

B Barbrel, M Koenig, A Benuzzi-Mounaix, E Brambrink, CR Brown, DO Gericke, B Nagler, M Rabec le Gloahec, D Riley, C Spindloe, SM Vinko, J Vorberger, J Wark, K Wünsch, G Gregori

We have performed short-pulse x-ray scattering measurements on laser-driven shock-compressed plastic samples in the warm dense matter regime, providing instantaneous snapshots of the system evolution. Time-resolved and angularly resolved scattered spectra sensitive to the correlation effects in the plasma show the appearance of short-range order within a few interionic separations. Comparison with radiation-hydrodynamic simulations indicates that the shocked plastic is compressed with a temperature of a few electron volts. These results are important for the understanding of the thermodynamic behavior of strongly correlated matter for conditions relevant to both laboratory astrophysics and inertial confinement fusion research.


Measurements of ionic structure in shock compressed lithium hydride from ultrafast X-ray Thomson scattering

Physical Review Letters 103 (2009)

AL Kritcher, P Neumayer, P Davis, T Döppner, OL Landen, SH Glenzer, EC Morse, CRD Brown, RW Falcone, HJ Lee, DO Gericke, J Vorberger, K Wünsch, G Gregori, B Holst, R Redmer, A Pelka, M Roth

We present the first ultrafast temporally, spectrally, and angularly resolved x-ray scattering measurements from shock-compressed matter. The experimental spectra yield the absolute elastic and inelastic scattering intensities from the measured density of free electrons. Laser-compressed lithium-hydride samples are well characterized by inelastic Compton and plasmon scattering of a K-α x-ray probe providing independent measurements of temperature and density. The data show excellent agreement with the total intensity and structure when using the two-species form factor and accounting for the screening of ion-ion interactions. © 2009 The American Physical Society.


Evolution of elastic x-ray scattering in laser-shocked warm dense lithium

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 80 (2009)

NL Kugland, C Constantin, C Niemann, SH Glenzer, AL Kritcher, G Gregori, S Bandyopadhyay, CM Brenner, J Pasley, C Spindloe, CRD Brown, FY Khattak, A Otten, A Pelka, M Roth, D Riley

We have studied the dynamics of warm dense Li with near-elastic x-ray scattering. Li foils were heated and compressed using shock waves driven by 4-ns-long laser pulses. Separate 1-ns-long laser pulses were used to generate a bright source of 2.96 keV Cl Ly- α photons for x-ray scattering, and the spectrum of scattered photons was recorded at a scattering angle of 120° using a highly oriented pyrolytic graphite crystal operated in the von Hamos geometry. A variable delay between the heater and backlighter laser beams measured the scattering time evolution. Comparison with radiation-hydrodynamics simulations shows that the plasma is highly coupled during the first several nanoseconds, then relaxes to a moderate coupling state at later times. Near-elastic scattering amplitudes have been successfully simulated using the screened one-component plasma model. Our main finding is that the near-elastic scattering amplitudes are quite sensitive to the mean ionization state Z̄ and by extension to the choice of ionization model in the radiation- hydrodynamics simulations used to predict plasma properties within the shocked Li. © 2009 The American Physical Society.


Turning solid aluminium transparent by intense soft X-ray photoionization

Nature Physics 5 (2009) 693-696

B Nagler, SM Vinko, T Whitcher, G Gregori, WJ Murphy, JS Wark, U Zastrau, E Förster, I Uschmann, RR Fäustlin, S Bajt, S Düsterer, T Laarmann, S Toleikis, AJ Nelson, T Döppner, SH Glenzer, RW Lee, HJ Vollmer, R Sobierajski, M Jurek, D Klinger, AR Khorsand, J Krzywinski, J Chalupsky, T Burian, J Cihelka, V Hajkova, L Juha, E Abreu, M Fajardo, M Kozlova, T Bornath, C Fortmann, S Göde, K-H Meiwes-Broer, A Przystawik, R Redmer, H Reinholz, G Röpke, R Thiele, J Tiggesbäumker, H Chapman, D Riley, F Rosmej, R Schott, T Dzelzainis, E Galtier, P Heimann, FY Khattak, HJ Lee, P Mercere, K Saksl, T Tschentscher

Saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths. It has never been observed in core-electron transitions owing to the short lifetime of the excited states involved and the high intensities of the soft X-rays needed. We report saturable absorption of an L-shell transition in aluminium using record intensities over 10 16 W cm 2 at a photon energy of 92 eV. From a consideration of the relevant timescales, we infer that immediately after the X-rays have passed, the sample is in an exotic state where all of the aluminium atoms have an L-shell hole, and the valence band has approximately a 9 eV temperature, whereas the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to the warm dense matter regime, at around 25 eV temperatures. The method is an ideal candidate to study homogeneous warm dense matter, highly relevant to planetary science, astrophysics and inertial confinement fusion. © 2009 Macmillan Publishers Limited. All rights reserved.


Ion structure in dense plasmas: MSA versus HNC

Journal of Physics A: Mathematical and Theoretical 42 (2009)

K Wünsch, J Vorberger, DO Gericke, G Gregori

We present results for the ionic structure in dense, moderately to strongly coupled plasmas using two models: the mean spherical approximation (MSA) and the hypernetted chain (HNC) approach. While the first method allows for an analytical solution, the latter has to be solved iteratively. Independent of the coupling strength, the results show only small differences when the ions are considered to form an unscreened one-component plasma (OCP) system. If the electrons are treated as a polarizable background, the different ways to incorporate the screening yield, however, large discrepancies between the models, particularly for more strongly coupled plasmas. © 2009 IOP Publishing Ltd.


A dual-channel, curved-crystal spectrograph for petawatt laser, x-ray backlighter source studies

Review of Scientific Instruments 80 (2009)

W Theobald, C Stoeckl, PA Jaanimagi, PM Nilson, M Storm, DD Meyerhofer, TC Sangster, D Hey, AJ MacKinnon, H-S Park, PK Patel, R Shepherd, RA Snavely, MH Key, JA King, B Zhang, RB Stephens, KU Akli, K Highbarger, RL Daskalova, L Van Woerkom, RR Freeman, JS Green, G Gregori, K Lancaster, PA Norreys

A dual-channel, curved-crystal spectrograph was designed to measure time-integrated x-ray spectra in the ∼1.5 to 2 keV range (6.2-8.2 Å wavelength) from small-mass, thin-foil targets irradiated by the VULCAN petawatt laser focused up to 4× 10 W/ cm . The spectrograph consists of two cylindrically curved potassium-acid-phthalate crystals bent in the meridional plane to increase the spectral range by a factor of ∼10 compared to a flat crystal. The device acquires single-shot x-ray spectra with good signal-to-background ratios in the hard x-ray background environment of petawatt laser-plasma interactions. The peak spectral energies of the aluminum He and Ly resonance lines were ∼1.8 and ∼1.0 mJ/eV sr (∼0.4 and 0.25 J/Å sr), respectively, for 220 J, 10 ps laser irradiation. © 2009 American Institute of Physics.


Perspective for high energy density studies on X-ray FELs

Proceedings of SPIE - The International Society for Optical Engineering 7451 (2009)

RW Lee, SM Vinko, T Whitcher, AJ Nelson, K Budil, RC Cauble, T Döppner, SH Glenzer, R Shepherd, B Nagler, J Chalupsky, G Gregori, WJ Murphy, JS Wark, U Zastrau, E Förster, I Uschmann, R Fäustlin, S Bajt, H Chapman, S Düsterer, T Laarmann, S Toleikis, T Tschentscher, R Sobierajski, M Jurek, D Klinger, J Krzywinski, JB Hastings, WE White, L Juha, T Burian, J Cihelka, V Hajkova, T Bornath, C Fortmann, S Göde, K-H Meiwes-Broer, A Przystawik, R Redmer, H Reinholz, G Röpke, R Thiele, J Tiggesbäumker, T Dzelzainis, D Riley, M Ajardo, M Kozlova, P Heimann, RW Falcone, FY Khattak, AR Khorsand, H-J Lee, P Mercere, K Saksl

We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ∼15fs pulse of containing >10 photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 10 W/cm . Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given. © 2009 SPIE.


Free-free opacity in warm dense aluminum

High Energy Density Physics 5 (2009) 124-131

SM Vinko, G Gregori, B Nagler, TJ Whitcher, JS Wark, MP Desjarlais, RW Lee, P Audebert

We present calculations of the free-free opacity of warm, solid-density aluminum at photon energies between the plasma frequency at 15 eV and the L-edge at 73 eV, using both density functional theory combined with molecular dynamics and a semi-analytical model in the RPA framework which includes exciton contributions. As both the ion and electron temperature is increased from room temperature to 10 eV, we see a marked increase in the opacity. The effect is less pronounced if only the electron temperature is allowed to increase, while the lattice remains at room temperature. The physical significance of these increases is discussed in terms of intense light-matter interactions on both femtosecond and picosecond time scales. © 2009 Elsevier B.V. All rights reserved.


Perspective for high energy density studies using x-ray free electron lasers

IEEE International Conference on Plasma Science (2009)

RW Lee, S Vinko, T Whitcher, A Nelson, H Chapman, T Döppner, SH Glenzer, B Nagler, J Chalupsky, G Gregori, WJ Murphy, JS Wark, U Zastrau, E Förster, I Uschmann, R Fäustlin, S Bajt, S Düsterer, T Laarmann, S Toleikis, T Tschentscher, R Sobierajski, M Jurek, D Klinger, J Krzywinski, L Juha, T Burian, J Cihelka, V Hajkova, T Bornath, C Fortmann, S Göde, K Meiwes-Broer, A Przystawik, R Redmer, H Reinholz, G Röpke, R Thiele, J Tiggesbäumker, T Dzelzainis, D Riley, M Fajardo, M Kozlova, P Heimann, F Khattak, AR Khorsand, H Lee, P Mercere, K Saksl


Femtosecond X-ray Diffraction: Applications for Laser-Irradiated Materials

ATOMIC PROCESSES IN PLASMAS 1161 (2009) 253-253

JS Wark


Design of a sub 100-femtosecond X-ray streak camera

Optics InfoBase Conference Papers (2009)

B Li, PP Rajeev, G Gregori, M Benetou, B Dobson, L Pickworth, D Neely, A Cavalleri, P Lau, J Lynn, P Jaanimagi, F Read

The temporal resolution of existing streak cameras are limited by electron transit time dispersion. Here we present a state-of-art design compensating this to achieve a breakthrough of 100fs time resolution. © 2009 Optical Society of America.


Simulating sub-wavelength temporal effects in a seeded FEL driven by laser-accelerated electrons

FEL 2009 - 31st International Free Electron Laser Conference (2009) 119-122

SI Bajlekov, SM Hooker, R Bartolini

Ultrashort electron bunches from laser-driven plasma accelerators hold promise as drivers for short-wavelength free electron lasers. While FEL simulation techniques have been successful in simulating lasing at present-day facilities, the novel sources investigated here are likely to violate a number of widely-held assumptions. For instance the HHG seed radiation, as well as the radiation generated by the bunch, may not conform to the slowly-varying envelope approximation (SVEA) on which the majority of codes rely. Additionally, the longitudinal macroparticle binning precludes the modeling of the full physics of the system. In order to more completely simulate the sub-wavelength effects which arise, we have developed an unaveraged 1-D time-dependent code without the SVEA. We use this to perform numerical analyses and highlight some of the additional features that these new systems present. We conclude that while coherent spontaneous emission from ultra-short bunches may significantly affect start-up, the sub-wavelength structure of HHG seeds has little effect.


Soft X-Ray Thomson scattering in warm dense hydrogen at FLASH

Proceedings of SPIE - The International Society for Optical Engineering 7451 (2009)

RR Fäustlin, S Toleikis, S Düsterer, T Laarmann, H Redlin, F Tavella, T Tschentscher, T Bornath, C Fortmann, S Göde, R Irsig, K-H Meiwes-Broer, A Przystawik, R Redmer, H Reinholz, G Röpke, R Thiele, J Tiggesbäumker, E Förster, I Uschmann, U Zastrau, T Döppner, SH Glenzer, G Gregori, J Mithen, HJ Lee, B Li

We present collective Thomson scattering with soft x-ray free electron laser radiation as a method to track the evolution of warm dense matter plasmas with ∼200 fs time resolution. In a pump-probe scheme an 800 nm laser heats a 20 ∼m hydrogen droplet to the plasma state. After a variable time delay in the order of ps the plasma is probed by an x-ray ultra violet (XUV) pulse which scatters from the target and is recorded spectrally. Alternatively, in a self-Thomson scattering experiment, a single XUV pulse heats the target while a portion of its photons are being scattered probing the target. From such inelastic x-ray scattering spectra free electron temperature and density can be inferred giving insight on relaxation time scales in plasmas as well as the equation of state. We prove the feasibility of this method in the XUV range utilizing the free electron laser facility in Hamburg, FLASH. We recorded Thomson scattering spectra for hydrogen plasma, both in the self-scattering and in the pump-probe mode using optical laser heating. © 2009 SPIE-.


Proton acceleration experiments and warm dense matter research using high power lasers

Plasma Physics and Controlled Fusion 51 (2009)

M Roth, I Alber, M Günther, K Harres, F Nürnberg, A Otten, A Pelka, G Schaumann, M Schollmeier, J Schütrumpf, V Bagnoud, A Tauschwitz, CRD Brown, R Clarke, R Heathcote, B Li, H Daido, M Tampo, J Fernandez, K Flippo, S Gaillard, C Gauthier, D Offermann, M Geissel, S Glenzer, A Kritcher, N Kugland, S Lepape, G Gregori, J Mithen, M Makita, D Riley, C Niemann

The acceleration of intense proton and ion beams by ultra-intense lasers has matured to a point where applications in basic research and technology are being developed. Crucial for harvesting the unmatched beam parameters driven by the relativistic electron sheath is the precise control of the beam. In this paper we report on recent experiments using the PHELIX laser at GSI, the VULCAN laser at RAL and the TRIDENT laser at LANL to control and use laser accelerated proton beams for applications in high energy density research. We demonstrate efficient collimation of the proton beam using high field pulsed solenoid magnets, a prerequisite to capture and transport the beam for applications. Furthermore, we report on two campaigns to use intense, short proton bunches to isochorically heat solid targets up to the warm dense matter state. The temporal profile of the proton beam allows for rapid heating of the target, much faster than the hydrodynamic response time thereby creating a strongly coupled plasma at solid density. The target parameters are then probed by x-ray Thomson scattering to reveal the density and temperature of the heated volume. This combination of two powerful techniques developed during the past few years allows for the generation and investigation of macroscopic samples of matter in states present in giant planets or the interior of the earth. © 2009 IOP Publishing Ltd.


Low frequency structural dynamics of warm dense matter

Physics of Plasmas 16 (2009)

G Gregori, DO Gericke

Measurements of the microscopic response of warm dense matter have been demonstrated by multi-keV inelastic x-ray scattering using laser-based sources. These techniques have been used to study the high frequency electron correlations (plasmons) in low to mid- Z plasmas. The advent of fourth generation light sources will provide high fluxes of narrowband and coherent x rays that will allow to look at the low frequency correlations (the ion-acoustic waves). In this paper we present an analysis of such low frequency modes by calculating the frequency dependent ion-ion structure factor. Our model includes all the relevant multibody contributions arising from strong coupling and nonideal plasma effects. In particular, the ion-ion structure factor is obtained within the memory function formalism by satisfying a finite number of sum rules. This work could be used as a basis to a direct experimental test of dense plasma model as soon as keV free electron laser sources will become available. © 2009 American Institute of Physics.


Measurements of ionic structure in shock compressed lithium hydride from ultrafast x-ray Thomson scattering.

Phys Rev Lett 103 (2009) 245004-

AL Kritcher, P Neumayer, CR Brown, P Davis, T Döppner, RW Falcone, DO Gericke, G Gregori, B Holst, OL Landen, HJ Lee, EC Morse, A Pelka, R Redmer, M Roth, J Vorberger, K Wünsch, SH Glenzer

We present the first ultrafast temporally, spectrally, and angularly resolved x-ray scattering measurements from shock-compressed matter. The experimental spectra yield the absolute elastic and inelastic scattering intensities from the measured density of free electrons. Laser-compressed lithium-hydride samples are well characterized by inelastic Compton and plasmon scattering of a K-alpha x-ray probe providing independent measurements of temperature and density. The data show excellent agreement with the total intensity and structure when using the two-species form factor and accounting for the screening of ion-ion interactions.