Equation of state of CH1.36: first-principles molecular dynamics simulations and shock-and-release wave speed measurements

We report the computation and measurement of the equation of state of a plastic with composition CH1.36. The computational scheme employed is density functional theory based molecular dynamics, at the conditions: 1.8 g/cm3 <ρ<10 g/cm3, and 4000 K <T< 100 000 K. Experimental measurements are of the shock speeds in a geometry in which the plastic is directly abutting a different material, liquid deuterium, from which release wave behavior in the plastic can be deduced. After fitting our computed pressure and internal energy with a Mie-Grüneisen free energy model, we predict the principal shock Hugoniot and various shock-and-release paths and show that they agree with both recently published laser-shock data and our new data regarding the shock speeds on release. We also establish that, at least in the particular (ρ,T) range considered, the equation of state of this complex two-component material is well described by an equal pressure and temperature mixture of pure C and H equations of state with a composition-weighted additive-volume assumption. This observation, together with our fit to the limited-range simulation data, can form the basis for the construction of an accurate wide-range equation of state model for this plastic. Implications for its use as an ablator in inertial confinement fusion capsules are discussed

Bremsstrahlung and line spectroscopy of warm dense aluminum plasma heated by xuv free-electron-laser radiation

We report the creation of solid-density aluminum plasma using free-electron laser (FEL) radiation at 13.5nm wavelength. Ultrashort pulses were focused on a bulk Al target, yielding an intensity of 2x10;{14}Wcm;{2} . The radiation emitted from the plasma was measured using an xuv spectrometer. Bremsstrahlung and line intensity ratios yield consistent electron temperatures of about 38eV , 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

Equilibration dynamics and conductivity of warm dense hydrogen

We investigate subpicosecond dynamics of warm dense hydrogen at the XUV free-electron laser facility(FLASH) at DESY (Hamburg). Ultrafast impulsive electron heating is initiated by a < 300-fs short x-rayburst of 92-eV photon energy. A second pulse probes the sample via x-ray scattering at jitter-free variabletime delay. We show that the initial molecular structure dissociates within (0.9 ± 0.2) ps, allowing us to inferthe energy transfer rate between electrons and ions. We evaluate Saha and Thomas-Fermi ionization modelsin radiation hydrodynamics simulations, predicting plasma parameters that are subsequently used to calculatethe static structure factor. A conductivity model for partially ionized plasma is validated by two-temperaturedensity-functional theory coupled to molecular dynamic simulations and agrees with the experimental data. Ourresults provide important insights and the needed experimental data on transport properties of dense plasmas

Probing near-solid density plasmas using soft x-ray scattering

X-ray scattering using highly brilliant x-ray free-electron laser (FEL) radiation provides new access to probe free-electron density, temperature and ionization in near-solid density plasmas. First experiments at the soft x-ray FEL FLASH at DESY, Hamburg, show the capabilities of this technique. The ultrashort FEL pulses in particular can probe equilibration phenomena occurring after excitation of the plasma using ultrashort optical laser pumping. We have investigated liquid hydrogen and find that the interaction of very intense soft x-ray FEL radiation alone heats the sample volume. As the plasma establishes, photons from the same pulse undergo scattering, thus probing the transient, warm dense matter state. We find a free-electron density of (2.6 ± 0.2) × 1020 cm-3 and an electron temperature of 14 ± 3.5 eV. In pump-probe experiments, using intense optical laser pulses to generate more extreme states of matter, this interaction of the probe pulse has to be considered in the interpretation of scattering data. In this paper, we present details of the experimental setup at FLASH and the diagnostic methods used to quantitatively analyse the data. © 2010 IOP Publishing Ltd

Turning solid aluminium transparent by intense soft X-ray photoionization

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

Hohlraum Designs for High Velocity Implosions on NIF Hohlraum Designs for High Velocity Implosions on NIF

Abstract. In this paper, we compare experimental shock and capsule trajectories to design calculations using the radiation-hydrodynamics code hydra. The measured trajectories from surrogate ignition targets are consistent with reducing the x-ray flux on the capsule by about 85 %. A new method of extracting the radiation temperature as seen by the capsule from x-ray intensity and image data shows that about half of the apparent 15 % flux deficit in the data with respect to the simulations can be explained by hydra overestimating the x-ray flux on the capsule