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This creates an ablation plasma in which the atoms are ionized to a He-like state with two bound electrons and produce K-shell emission. The experimental set-up is shown in Figure 7. The sample is dynamically compressed with either shock or ramp loading using a subset of the beams from the laser. At the expected time of peak compression the laser-plasma X-rays are generated and impinge on the sample.


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A metallic foil with a pinhole positioned in the sample assembly is used to collimate the incident X-ray beam. Diffracted X-rays are recorded on image plates behind the sample Figure 7. Diffraction from the edges of the pinhole produces peaks that provide a reference to calibrate the diffraction geometry. At pressures above a few hundred GPa, the rapid increase of X-ray noise from the drive plasma makes detection of diffracted photons from the sample more challenging.

Although laser-plasma sources are very bright, the source is uncollimated and only a small fraction of the generated X-rays pass through the sample. Figure 7. Experimental set-up for laser ramp-compression experiments with X-ray diffraction. The target package is affixed to one side of a box and illuminated by laser-plasma X-rays from a Cu, Fe, or Ge foil. Measured X-ray emission spectra left demonstrate the quasi-monochromatic nature of the radiation. X-rays scattered by the sample are recorded on image plates that line the box interior, while a VISAR laser is focused onto the rear surface of the target package through an aperture in the back panel.

Adapted from Wicks et al.

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X-ray absorption fine-structure XAFS spectroscopy probes the local atomic environment around an absorption edge of a specific element. The method includes XANES X-ray absorption near edge structure which examines fine structure near an absorption edge and EXAFS extended X-ray absorption fine structure which examines the structure over a larger energy range above the edge. Both methods arise from interference effects that occur when a photoelectron ejected from an atom by incoming X-rays is scattered by its neighbors.

XAFS methods are sensitive to a variety of atomic-level properties including electronic structure, bond lengths, and coordination The decay of interference-produced modulations is controlled by the Debye-Waller factor from which constraints on temperature can be obtained. XAFS is a widely established tool for materials under static high pressures using synchrotron X-ray sources Shen and Mao, and is being adapted for use in ultra-high pressure dynamic compression.

Experiments are performed using a thin sample foil embedded between two diamond layers that serve to confine the sample. Compression is achieved using a series of 1-ns long square laser pulses stacked in time to drive multiple shocks into the sample. A key requirement for XAFS is a bright and smooth X-ray source of sufficient energy range and resolution to capture the absorption fine structure. The X-ray source pulses are delayed in time relative to the loading pulses to probe the sample at peak pressure.

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X-rays transmitted through the target are dispersed by a spectrometer and recorded on image plates. A reference spectrum of the X-ray source is recorded in a separate experiment under identical conditions. XAFS measurements have also been reported on laser-driven samples at pressure extending to the multimegabar range using other X-ray sources including laser-driven backlighter foils Denoeud et al.

Diamond is an important material for planetary science, high-pressure physics, and inertial confinement fusion. In ice-giant planets such as Uranus and Neptune, decomposition of hydrocarbons at high pressure and temperature may lead to the formation of diamond-containing layers in the interior Benedetti et al. Exoplanets that form around C-rich host stars or by local carbon enrichment of a protoplanetary disk may also have diamond and silicon carbide bearing interior layers Bond et al.

Carbon is stable in the diamond structure over a wide range of pressures and temperatures. A phase transformation to a BC8—type structure near 1 TPa followed by a further transition to a simple cubic structure near 3 TPa have been predicted theoretically Yin and Cohen, ; Correa et al. The coordination increases from fourfold to sixfold in the simple cubic structure. Theoretical studies have explored the melting behavior of diamond, predicting a maximum in the melting curve around GPa and 8,—9, K Grumbach and Martin, ; Correa et al. A number of ultra-high pressure shock-compression experiments on diamond have been carried out extending to as high as 4 TPa Bradley et al.

In decaying shock experiments, it has been found that diamond melts to a dense metallic fluid with a negative melting slope at —1, GPa Brygoo et al. Evidence for the existence of a new solid phase, possibly BC8, has also been reported in shock-compression experiments at 90— GPa Knudson et al. Figure 8. Ultra-high pressure phase diagram of carbon. Shock temperatures from decaying-shock experiments in diamond samples are shown as black lines.

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Blue and orange symbols are from theoretical calculations. See Eggert et al. A comparison of the Hugoniot behavior of single-crystal and nanocrystalline diamond has been reported up to 2. Diamond has also been explored under ramp compression.

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The pressure—density relationship and strength of diamond has been characterized up to GPa using the Omega laser Bradley et al. In experiments at the National Ignition Facility, measurement of the stress-density relationship of diamond was extended to 5 TPa, achieving 3. These are the highest pressure equation-of-state data recorded under ramp compression and represent the first experimental data in the high-pressure, modest-temperature regime for constraining condensed-matter theory and planetary evolution models at terapascal conditions.

MgO periclase is an endmember of the Mg,Fe O solid solution which is expected to be a major component of the deep mantles of terrestrial planets and exoplanets Figure 1. Its high-pressure behavior has long attracted widespread attention due to its simple rocksalt B1-type structure, wide stability field, and geophysical importance Duffy et al.

Recent interest in the behavior of MgO at ultra-high pressure and temperature has focused on its phase transformation to the B2 CsCl-type structure, its melting behavior, and possible metallization Boates and Bonev, ; Cebulla and Redmer, ; Taniuchi and Tsuchiya, Experimental studies have been conducted using both steady and decaying shocks but have reached conflicting conclusions about the solid-solid phase transition and melting.

In contrast, plate-impact experiments performed using the Z machine Figure 9 coupled with theoretical calculations indicate that the B1—B2 transition occurs at lower pressure GPa and melting initiates near GPa and is completed by GPa Root et al. More recent results using laser-driven steady Miyanishi et al. Figure 9. Optical reflectivity measurements have also been used to place constraints on the electrical conductivity of shocked liquid MgO. The initial decaying-shock measurements suggested metallization occurred upon melting McWilliams et al. The transformation of periclase to the B2 phase in shock-compression experiments is inferred only indirectly through temperature or density changes.

The first direct identification of the B2 phase was made using laser-driven ramp compression combined with X-ray diffraction Coppari et al. In these experiments, diffraction peaks were recorded for MgO compressed up to GPa. Measured d -spacings were consistent with the B1 phase up to GPa whereas diffraction from the B2 phase was observed from to GPa Figure Temperature is not measured in these ramp-compression experiments, but is expected to be significantly lower than achieved under shock compression.

The observation of a B2 peak at higher pressure in the ramp data compared with inferences from shock measurements is consistent with a negative Clapeyron slope for the transition, consistent with theoretical predications. However, the experimentally measured pressure of the transition GPa is substantially higher than predicted along an isentrope GPa by theory Cebulla and Redmer, This may reflect over-pressurization of the equilibrium phase boundary under the short timescales of dynamic compression.

Understanding possible kinetics factors associated with phase transformations under ultra-high pressure—temperature conditions is an important goal for future experiments. Figure A Interplanar d -spacing vs.

B Density of MgO in the B1 open and filled red and B2 blue structures determined from ramp X-ray diffraction compared with shock data yellow. See Coppari et al. The transformation to the B2 phase is expected to occur in large rocky exoplanets Wagner et al. Empirical systematics and theoretical studies have suggested that the MgO phase transformation may be accompanied by a strong change in rheological properties with the high-pressure B2 phase exhibiting a reduction in viscosity Karato, ; Ritterbex et al. The viscosity of the constituent minerals strongly influences dynamic flow in the mantle and hence is important for understanding the heat flow and the style of mantle convection Driscoll, The negative Clapeyron slope of the phase transition combined with the viscosity reduction may produce mantle layering in super Earths with strong differences in convective flow above and below the transition which may affect the long-term thermal evolution of these planets Shahnas et al.

Silica is the most abundant oxide component of terrestrial mantles and serves as an archetype for the dense highly coordinated silicates of planetary interiors. Based on theoretical calculations, it is expected that silicates such as post-perovskite will eventually dissociate at conditions of the deep interior of super-Earths Umemoto et al.

Consequently, SiO 2 phases are expected to be potentially important constituents of these exoplanets Figure 1.

The Hugoniot behavior of quartz at ultra-high pressure has been extensively studied due to its role as an impedance-matching standard for shock experiments Hicks et al. A significant degree of non-linearity was found in the shock velocity-particle velocity relationship and attributed to disorder and dissociation in the SiO 2 fluid. Temperatures, shock velocities, and reflectivities were reported using pyrometry and velocimetry measurements on fused silica and quartz starting materials in decaying-shock experiments up to 1 TPa Hicks et al.

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The specific heat derived from the temperature measurements was found to be substantially above the classical Dulong-Petit limit and attributed to complex polymerization and bond breaking in a melt that evolves from a regime dominated by chemical bonding of Si-O units bonded liquid to an atomic fluid consisting of separated Si and O atoms.

Electrical conductivity values derived from measured reflectivities assuming Drude behavior indicate the atomic fluid is highly conductive. More recent decaying-shock measurements on quartz, fused silica, and stishovite starting materials extend constraints on the melting curve of SiO 2 to GPa and 8, K Millot et al. The melting curve of SiO 2 and other silicates was found to be higher than that of iron at these extreme conditions. Comparison of these results to planetary adiabats suggests that silica and MgO are likely to be in a solid state in the cores of giant planets such as Neptune and Jupiter.