X-ray absorption spectroscopy of iron at multimegabar pressures in laser shock experiments

Abstract

International audienceTaking advantage of the new opportunities provided by x-ray free electron laser (FEL) sources when coupled to a long laser pulse as available at the Linear Coherent Light Source (LCLS), we have performed x-ray absorption near-edge spectroscopy (XANES) of laser shock compressed iron up to 420 GPa (±50) and 10 800 K (±1390). Visible diagnostics coupled with hydrodynamic simulations were used to infer the thermodynamical conditions along the Hugoniot and the release adiabat. A modification of the pre-edge feature at 7.12 keV in the XANES spectra is observed above pressures of 260 GPa along the Hugoniot. Comparing with ab initio calculations and with previous laser-heated diamond cell data, we propose that such changes in the XANES pre-edge could be a signature of molten iron. This interpretation then suggests that iron is molten at pressures and temperatures higher than 260 GPa (±29) and 5680 K (±700) along the principal Fe Hugoniot

Dynamic X-ray diffraction observation of shocked solid iron up to 170 GPa

“…Observation of solid hcp iron up to 170 GPa and 4,150 K provides a new useful constrain on the structure of iron along the Hugoniot. Regarding published experimental data of the melting curve of iron, we notice that all experimental studies agree with the observation of solid iron at 100 GPa and 2,000 K. Disregarding extrapolations, at 170 GPa and 4,150 K, the observation of solid iron is in agreement with conclusions from dynamic compression in this pressure range (5)(6)(7)(8)(9). It also agrees with ab initio calculations (37,38) (melting at 4,800 K, 170 GPa) and with recent static compression measurements (2) (melting at 4,600 K, 170 GPa) and is, by 650 K, higher than the melting point measured in ref.…”

“…2. A Velocity Interferometer System for Any Reflector (VISAR) system and hydrodynamic simulations (Methods) allowed us to estimate the pressure induced by the shock in iron to be 100 ± 10 GPa in the first case and 170 ± 17 GPa in the second case, by using a new equation of state (EOS) for iron (9). New diffraction maxima appear that are consistent with the (101) most intense diffraction peaks of hcp iron.…”

The Time-resolved and Extreme-conditions XAS (TEXAS) facility at the European Synchrotron Radiation Facility: the energy-dispersive X-ray absorption spectroscopy beamline ID24

“…Measuring the microscopic and atomic properties in the WDM regime is a challenging task. Recently, remarkable progress has been achieved using lasers to dynamically compress and heat the system and plasmagenerated X-ray pulses (Denoeud et al, 2014;Ping et al, 2013) or X-ray free-electron lasers (Harmand et al, 2015) to probe such extreme states. One of the future aims of the ESRF is to provide the dynamic compression user community the possibility to produce and probe samples in the WDM regime, with sufficient data quality allowing to validate theoretical models.…”

Direct experimental observation of the gas density depression effect using a two-bunch X-ray FEL beam

“…The high-repetition-rate operation of the European XFEL and LCLS-II is expected to provide additional capabilities to those already offered by the low-repetition-rate facilities that are either currently in user operation or being commissioned, including FLASH (Ackermann et al, 2007) and its upgrade FLASH-II, LCLS, SACLA (Ishikawa et al, 2012), FERMI (Allaria et al, 2012), PAL-XFEL (Kang et al, 2013) and SwissFEL (Ganter et al, 2010). At the same time, these new capabilities also bring about new challenges in conceiving, designing and implementing high-repetition-ratecompatible X-ray diagnostic, optics, beam regulation and safety devices, which have been proven to be very important to help fulfill FEL's great scientific potentials in the frontier research of physics, chemistry, life science, material, energy and earth sciences (Young et al, 2010;Glover et al, 2012;Fuchs et al, 2015;Minitti et al, 2015;Chapman et al, 2011;Seibert et al, 2011;Shwartz et al, 2014;Gerber et al, 2015;Harmand et al, 2015;Yoneda et al, 2015). The ever-soimportant X-ray diagnostics for a FEL stems from the stochastic nature of its lasing mechanism, i.e.…”