High Energy Density Physics 5 (2009) 124-131
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.
Proceedings of SPIE - The International Society for Optical Engineering 7451 (2009)
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-.
ATOMIC PROCESSES IN PLASMAS 1161 (2009) 253-253
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 80 (2009)
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.
High Energy Density Physics 5 (2009) 212-215
We have performed measurements of the radiation and the hot electron temperature in sub-millimetre size hohlraums driven by a high intensity short-pulse laser. The results indicate that radiation temperatures ∼80 eV can be obtained with ∼20 J of laser energy delivered on target. Radiation-hydrodynamics simulations indicate an absorption into thermal X-rays of ≲1-2%, with peak temperatures similar to those measured experimentally. Crown Copyright © 2009.
Measurements of ionic structure in shock compressed lithium hydride from ultrafast x-ray Thomson scattering.
Phys Rev Lett 103 (2009) 245004-
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.
Nature Physics 5 (2009) 693-696
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.
IEEE International Conference on Plasma Science (2009)
Nature Physics 4 (2008) 940-944
One of the grand challenges of contemporary physics is understanding strongly interacting quantum systems comprising such diverse examples as ultracold atoms in traps, electrons in high-temperature superconductors and nuclear matter. Warm dense matter, defined by temperatures of a few electron volts and densities comparable with solids, is a complex state of such interacting matter. Moreover, the study of warm dense matter states has practical applications for controlled thermonuclear fusion, where it is encountered during the implosion phase, and it also represents laboratory analogues of astrophysical environments found in the core of planets and the crusts of old stars. Here we demonstrate how warm dense matter states can be diagnosed and structural properties can be obtained by inelastic X-ray scattering measurements on a compressed lithium sample. Combining experiments and ab initio simulations enables us to determine its microscopic state and to evaluate more approximate theoretical models for the ionic structure. © 2008 Macmillan Publishers Limited. All rights reserved.
Simulating picosecond x-ray diffraction from shocked crystals using post-processing molecular dynamics calculations
Journal of Physics Condensed Matter 20 (2008)
Calculations of the patterns of x-ray diffraction from shocked crystals derived from the results of non-equilibrium molecular dynamics (NEMD) simulations are presented. The atomic coordinates predicted from the NEMD simulations combined with atomic form factors are used to generate a discrete distribution of electron density. A fast Fourier transform (FFT) of this distribution provides an image of the crystal in reciprocal space, which can be further processed to produce quantitative simulated data for direct comparison with experiments that employ picosecond x-ray diffraction from laser-irradiated crystalline targets. © 2008 IOP Publishing Ltd.
5TH INTERNATIONAL CONFERENCE ON INERTIAL FUSION SCIENCES AND APPLICATIONS (IFSA2007) 112 (2008)
Bremsstrahlung and line spectroscopy of warm dense aluminum plasma heated by xuv free-electron-laser radiation
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 78 (2008)
We report the creation of solid-density aluminum plasma using free-electron laser (FEL) radiation at 13.5 nm wavelength. Ultrashort pulses were focused on a bulk Al target, yielding an intensity of 2× 1014/cm2. The radiation emitted from the plasma was measured using an xuv spectrometer. Bremsstrahlung and line intensity ratios yield consistent electron temperatures of about 38 eV, supported by radiation hydrodynamics simulations. This shows that xuv FELs heat up plasmas volumetrically and homogeneously at warm-dense-matter conditions, which are accurately characterized by xuv spectroscopy. © 2008 The American Physical Society.
Phys Rev Lett 100 (2008) 165002-
The heating of solid targets irradiated by 5 x 10(20) W cm(-2), 0.8 ps, 1.05 microm wavelength laser light is studied by x-ray spectroscopy of the K-shell emission from thin layers of Ni, Mo, and V. A surface layer is heated to approximately 5 keV with an axial temperature gradient of 0.6 microm scale length. Images of Ni Ly(alpha) show the hot region has <or=25 microm diameter. These data are consistent with collisional particle-in-cell simulations using preformed plasma density profiles from hydrodynamic modeling which show that the >100 G bar light pressure compresses the preformed plasma and drives a shock into the solid, heating a thin layer.
Physical Review Letters 101 (2008)
We have investigated the angular variation in elastic x-ray scattering from a dense, laser-shock-compressed aluminum foil. A comparison of the experiment with simulations using an embedded atom potential in a molecular dynamics simulation shows a significantly better agreement than simulations based on an unscreened one-component plasma model. These data illustrate, experimentally, the importance of screening for the dense plasma static structure factor. © 2008 The American Physical Society.
SCIENTIFIC MODELING AND SIMULATIONS 15 (2008) 159-186
Measurement Science and Technology 19 (2008)
We have measured the absolute response and detective quantum efficiency of image plates (IPs) for 5.9 keV x-rays using a calibrated iron-55 source. The types of IPs considered in this study are now commonly used as x-ray detectors in high-intensity laser-plasma interaction experiments, where conventional CCD fails because of the intense electromagnetic pulse that follows a high-intensity shot. Since the plates are not read out immediately after each laser shot, a detailed fading analysis of the plates is also presented. This work is important for future implementation of IPs as absolute x-ray photon detectors in large-scale laser facilities. © 2008 IOP Publishing Ltd.
PHYSICAL REVIEW B 78 (2008) ARTN 220101
Phys Rev E Stat Nonlin Soft Matter Phys 77 (2008) 056403-
We have characterized the plasma produced by a picosecond laser pulse using x-ray spectroscopy. High-resolution high-sensitivity spectra of K -shell emission from a Ti plasma have been obtained, showing a strong contribution from multiply ionized ions. Hydrodynamic and collisional-radiative codes are used to extract the plasma temperature and density from these measurements. We show that our measurements can provide benchmarks for particle-in-cell (PIC) simulations of preplasma conditions in ultraintense laser-matter interactions.
Physical Review Letters 100 (2008)
The heating of solid targets irradiated by 5×1020Wcm-2, 0.8 ps, 1.05μm wavelength laser light is studied by x-ray spectroscopy of the K-shell emission from thin layers of Ni, Mo, and V. A surface layer is heated to ∼5keV with an axial temperature gradient of 0.6μm scale length. Images of Ni Lyα show the hot region has ≤25μm diameter. These data are consistent with collisional particle-in-cell simulations using preformed plasma density profiles from hydrodynamic modeling which show that the >100Gbar light pressure compresses the preformed plasma and drives a shock into the solid, heating a thin layer. © 2008 The American Physical Society.
Physical Review B - Condensed Matter and Materials Physics 77 (2008)
We have studied the picosecond lattice dynamics of optically pumped hexagonal LuMnO3 by using ultrafast x-ray diffraction. The results show a shift and broadening of the diffraction curve due to the stimulated lattice expansion. To understand the transient response of the lattice, the measured time- and angle-resolved diffraction curves are compared to a theoretical calculation based on the dynamical diffraction theory of coherent phonon propagation modified for the hexagonal crystal structure of LuMnO3. Our simulations reveal that a large coupling coefficient (c13) between the a-b plane and the c axis is required to fit the data. Though we interpret the transient response within the framework of thermal coherent phonons, we do not exclude the possibility of strong nonthermal coupling of the electronic excitation to the atomic framework. We compare this result to our previous coherent phonon studies of LuMnO3 in which we used optical pump-probe spectroscopy. © 2008 The American Physical Society.