Session F4: High Energy Density Physics/Warm Dense Matter II

X-ray Thomson scattering of shocked carbon foam on the Z accelerator

Experiments on the Sandia Z pulsed-power accelerator have demonstrated the ability to produce warm dense matter (WDM) states with unprecedented uniformity, duration, and size, which are ideal for investigations of fundamental WDM properties. For the first time, a space-resolved x-ray Thomson scattering (XRTS) spectra from shocked carbon foam was recorded on Z. The large electrical current produced by Z was used to launch Al flyer plates up to 25 km/s. The impact of the flyer plate on a CH$_2$ foam target produced a shocked state with an estimated pressure of 0.75 Mbar, density of 0.52 g/cc, and temperature of 4.3 eV. Both unshocked and shocked portions of the foam target were probed with 6.2 keV x-rays produced by focusing the Z-Beamlet laser onto a nearby Mn foil. The data is comprised of three spatially distinct spectra that were simultaneously captured with a single spectrometer. These three spectra provide detailed information on the following target locations: the laser spot, the unshocked foam, and the shocked foam.   

Shockwave compression of Argon gas at several initial densities

Experimental data of the principal Hugoniot locus of \textit{gas-phase} noble gases are rare. The majority of Hugoniot data is either from shock tube experiments on low-pressure gases or from plate impact experiments on cryogenic, liquefied gases. In both cases, physics regarding shock compressibility, thresholds for the on-set of ionization, and dissociation chemistry are difficult to infer for gases at intermediate densities. We have developed an experimental target for gas gun-driven plate impact experiments on gases at initial pressures between 200-1000 psi. Using optical velocimetry, we directly determine shock and particle velocities on the principal Hugoniot locus, as well as clearly differentiate ionization thresholds. The target design also results in multiply shocking the gas in a quasi-isentropic fashion yielding off-Hugoniot compression data. Using an impactor with higher impedance than the drive plate, we are able to achieve initial particle velocities well in excess of the impactor velocities. We will describe the results of plate impact experiments on Ar with initial densities between 0.02-0.05 g/cm$^{3}$. By coupling optical fibers to the targets, we have measured the time-resolved optical emission, spectrally-resolved with a spectrometer coupled to an optical streak camera, and with a 5-color optical pyrometer for temperature determination. The experimental results are compared with hydrodynamic simulations using ideal gas and Sesame tabular equations of state.   

Investigating strong shocks in low density Aerogel foams using x-ray phase contrast imaging

Understanding the performance of heterogeneous materials and the impact of changes in composition and morphology are complex to understand at a fundamental level because the relevant physical phenomena occur over a wide range of time and length scales. Low density foams are a unique subset of heterogeneous materials where morphology not composition is varied to achieve the desired physical properties. Assumptions about bulk behavior of these foams ignore that foams are fundamentally a heterogeneous material at some length scale. We use the coherent free electron laser of the Linac Coherent Light Source at the Materials in Extreme Conditions end station to probe the shock front generated by a tight laser focal spot in low density silica aerogels. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences   

Onset of ice VII phase during ps laser pulse propagation through liquid water

Water dominantly present in liquid state on earth gets transformed to crystalline polymorphs under different dynamic loading conditions. Out of 15 different crystalline phases discovered till date, ice VII is observed to be stable over wide pressure (2-63 GPa) and temperature (\textgreater 273 K) ranges. We present the onset of ice VII phase at low threshold of 2 mJ/pulse during 30 ps (532 nm, 10 Hz) laser pulse induced shock propagating through liquid water. Role of input pulse energy on the evolution of Stoke's and anti-Stoke's Raman shift of the dominant A$_{\mathrm{1g}}$ mode of ice VII, filamentation, free-electrons, plasma shielding is presented. The H-bond network rearrangement, electron ion energy transfer time coinciding with the excitation pulse duration supported by the filamentation and plasma shielding of the ps laser pulses reduced the threshold of ice VII structure formation. Filamentation and the plasma shielding have shown the localized creation and sustenance of ice VII structure in liquid water over 3 mm length and 50 $\mu $m area of cross-section.