Session T2: CM.4 Warm Dense Matter II

Development of quasi-isentropic drives to 500 GPa and beyond

The ability to reliably measure materials at energy densities exceeding 5x10$^{11}$ J/m$^{3}$ (500 GPa) requires an experimental platform that reaches such energy densities in a controlled manner and in a configuration that allows measurements to occur. We have developed a staged shock drive that will quasi-isentropically ramp materials such as Ta and Mo into such a high energy density state and simultaneously keep the materials substantially below their melting point, i.e., in their solid phase. Recent measurements of our platform on the National Ignition Facility have confirmed our ability to predict the resultant drive with a peak pressure of $\sim$500 GPa. Separate experiments at the same facility have recently demonstrated that a drive with peak pressures \textgreater 800 GPa is possible. We will show the experimental platform, the simulated and measured drives produced by the platform for $\sim$500 GPa and $\sim$800 GPa drive shots. We will also show an experimental platform which utilizes the drive and measurements of the samples to infer material strength. This work was performed under the auspices of the Lawrence Livermore National Security, LLC, (LLNS) under Contract No. DE-AC52-07NA27344. LLNL-ABS-620612   

Absolute equation of state and opacity measurements of CH plastic to 40 TPa using the National Ignition Facility

We have developed an experimental configuration using a hohlraum-driven spherically-convergent shock to induce pressures into the gigabar range, measuring the Hugoniot radiographically. The shock pressure increases with convergence, so a range of Hugoniot states is obtained from a single experiment. The opacity along the Hugoniot is also deduced, which is essential in gigabar experiments as it changes significantly from its initial value. We are focusing initially on plastics, as we can reliably obtain spherical samples with the desired design of ablator, and the radiographic signal is reasonable. Our initial measurements on NIF used a conservative timing of the x-ray backlighter to allow for uncertainty in the EOS, and probed only part of the pressure range. The shock speed and compression, obtained from radiographic analysis, gave absolute Hugoniot states from 12-41 TPa, which is an order of magnitude greater than previously measured in CH. The measured EOS locus was consistent with the previous measurements, and significantly stiffer than the theoretical EOS used for comparison. Our analysis also gave the variation of opacity along the Hugoniot, which showed a decrease of an order of magnitude, as expected from atomic physics calculations.   

Non equilibrium studies on FEL facilities

The recent development of Free Electron Lasers (FEL), giving ultrafast, high intensity pulses in the X-ray and XUV energy range is opening new opportunities for WDM studies. Development of X-ray diagnostics such as X-ray absorption spectroscopy and X-ray scattering, has received much attention for the in situ measurement of the structure and physical properties of matter at extreme conditions [1]. Coupled to ultrafast pump - probe schemas, such diagnostics are giving new insights into out-of-equilibrium processes and thus validate current models. We report recent developments to perform few fs time resolved pump - probe experiments [2], giving access to ultrafast transient WDM states. We also present collective Thomson Scattering with soft x-ray Free Electron Laser radiation (at FLASH) as a method to track the evolution of highly transient warm dense hydrogen with around 100 fs time resolution. In addition, recent experiments at LCLS are suggesting the possibility to perform X-ray absorption spectroscopy (XANES) on FEL facilities to provide simultaneously information on the valence electrons and on the atomic local arrangement within sub-ps time scales.\\[4pt] [1] R.R. F\"{a}ustlin et al., Phys. Rev. Lett. 104, 125002 (2010).\\[0pt] [2] M.Harmand et al., Nature photonics Doi~: 1010.1038/NPHOTON.2013.11   

Ultrafast XFEL diffraction measurements of femtosecond laser-driven shock-compressed iron

We measured lattice dynamics under femtosecond laser-driven shock-induced phase transition in iron using X-ray free electron laser with sub-picosecond temporal resolution. Sluggish behavior of iron under shock-induced bcc to hcp transition is the unsolved issue although the diffusionless type of structural transition should be completed within picoseconds. We performed in-situ XRD at SPring-8 Angstrom Compact free electron Laser (SACLA) facility to take a series of snapshots of diffraction patterns under the transition in iron using XFEL pulse with pulse duration of 10 fs varying the delay time from the pumping femtosecond laser pulse. The evolution of lattice defects and high-pressure phase will be addressed in the talk.