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Investigating how solid matter behaves at enormous pressures, such as those found in the deep interiors of giant planets, is a great experimental challenge. Over the past decade, computational predictions have revealed that compression to terapascal pressures may bring about counter-intuitive changes in the structure and bonding of solids as quantum mechanical forces grow in influence1–6. Although this behaviour has been observed at modest pressures in the highly compressible light alkali metals7,8, it has not been established whether it is commonplace among high-pressure solids more broadly. We used shaped laser pulses at the National Ignition Facility to compress elemental Mg up to 1.3 TPa, which is approximately four times the pressure at the Earth’s core. By directly probing the crystal structure using nanosecond-duration X-ray diffraction, we found that Mg changes its crystal structure several times with non-close-packed phases emerging at the highest pressures. Our results demonstrate that phase transformations of extremely condensed matter, previously only accessible through theoretical calculations, can now be experimentally explored.

The rapid irradiation and heating of matter can lead to material removal via a process known as ablation. While previous investigations have focused on ablation with optical and soft x-ray pulses, the process is not well understood for the high energy x-rays delivered at current x-ray free electron (XFEL) facilities. In this paper, we use hybrid two-temperature model molecular dynamics simulations to determine the damage threshold and dynamics fortantalum foils under irradiation with x-rays in the range 1-5 keV. We report that damage occurs for foils with thickness ≥300 nm when heated to around 1.25 eV / atom. This damage results from the combined processes of melting and cavitation, finally resulting in the removal of material layers. The predictions of this study, in terms of the cavitation threshold and underlying dynamics, could guide interpretation of experiments as well as applications including development of beamline optics for free-electron lasers. We reportconsistency between cavitation and ablation behavior in isochoric heating experiments and spall processes in hydrodynamic compression and release experiments, confirming the primary modes of damage are mechanical in nature for the x-ray energies investigated.

X-ray diffraction is a widely used technique for measuring the crystal structure of a compressed material. Recently, short pulse x-ray sources have been used to measure the crystal structure in-situ while a sample is being dynamically loaded. To reach the ultra high pressures that are unattainable in static experiments at temperatures lower than using shock techniques, shockless quasi-isentropic compression is required. Shockless compression has been demonstrated as a successful means of accessing high pressures. The National Ignition Facility (NIF), which will begin doing high pressure material science in 2010, it should be possible to reach over 2 TPa quasi-isentropically. This paper outlines how x-ray diffraction could be used to study the crystal structure in laser driven, shocklessly compressed targets the same way it has been used in shock compressed samples. A simulation of a shockless laser driven iron is used to generate simulated diffraction signals, and recent experimental results are presented.

In situ x-ray diffraction studies of iron under shock conditions confirm unambiguously a phase change from the bcc (alpha) to hcp (epsilon) structure. Previous identification of this transition in shock-loaded iron has been inferred from the correlation between shock-wave-profile analyses and static high-pressure x-ray measurements. This correlation is intrinsically limited because dynamic loading can markedly affect the structural modifications of solids. The in situ measurements are consistent with a uniaxial collapse along the [001] direction and shuffling of alternate (110) planes of atoms, and are in good agreement with large-scale nonequilibrium molecular dynamics simulations.

A 3-year LDRD-ER project to study the response of shocked materials at high pressure and high strain rate has concluded. This project involved a coordinated effort to study single crystal samples that were shock loaded by direct laser irradiation, in-situ and post-recovery measurements, and molecular dynamics and continuum modeling. Laser-based shock experiments have been conducted to study the dynamic response of materials under shock loading materials at a high strain-rate. Experiments were conducted at pressures above the published Hugoniot Elastic Limit (HEL). The residual deformation present in recovered samples was characterized by transmission electron microscopy, and the response of the shocked lattice during shock loading was measured by in-situ x-ray diffraction. Static film and x-ray streak cameras recorded x-rays diffracted from lattice planes of Cu and Si both parallel and perpendicular to the shock direction. Experiments were also conducted using a wide-angle detector to record x-rays diffracted from multiple lattice planes simultaneously. This data showed uniaxial compression of Si (100) along the shock direction and 3-dimensional compression of Cu (100). In the case of the Si diffraction, there was a multiple wave structure observed. We present results of shocked Si and Cu obtained with a new large angle diffraction diagnostic,more » and discuss the results in the context of detailed molecular dynamics simulations and post-processing.« less

A rigorous derivation of the dynamical theory of X-ray diffraction from crystals with “optical phonon”-type distortions is presented. The result is a compact, surprisingly simple equation with a strong formal similarity to the well known Takagi-Taupin equation, with the latter included as a special case. Time-dependence is explicitly retained, and thus the analysis is applicable to situations where the crystal is modified on time-scales comparable with that for the X-rays to traverse an extinction depth, as well as being valid for the analysis of diffraction from disturbances that travel at close to luminal velocities, such as polaritons. A comparison is made between the influence of coherent acoustic and optical phonons on the diffraction of X-rays. Numerical and perturbative analytical solutions of the generalised Takagi-Taupin equation are presented in the presence of such phonons.

Laser‐based experiments are being developed to study the response of solids under high pressure loading. Diagnostic techniques that have been applied include dynamic x‐ray diffraction, VISAR wave profile measurements, and post‐shock recovery and analysis. These techniques are presented with some results from shocked Si, Al, and Cu experiments.

In the transient diffraction NLYF proposal we set forward a program of work to investigate the response of crystals to shock compression in regions of strain rates previously unexplored, in a coordinated experimental, computational, and analytical program. Time resolved x-ray diffraction was used to directly determine the lattice parameters of crystals during shock loading previously on the Nova and Trident laser facilities. Under this proposal we extended this work to exploit the multi-beam direct drive capability of the Omega laser facility to allow more extensive diagnostic access for measuring the lattice parameters both parallel and perpendicular to the shock front. Under the NLUF Program in FY 99, we transitioned the dynamic diffraction experiments to the OMEGA facility. We developed a direct drive target configuration that uses a single beam to direct irradiate the surface of a thin crystal and 4 beams to irradiate a separate metal backlighter foil. Experiments were done with single crystal Si to demonstrate that the target design worked and that simultaneous measurements of compression both parallel and perpendicular to the shock propagation direction could be performed. We obtained simultaneous measurements of the (400) and (040) lattice planes during the period when a shock traveled through themore » crystal in the (100) direction. Follow-up experiments were done to demonstrate that this technique would work with thin metal crystals such as Cu. Simultaneous measurements were made of the (200) and (020) lattice planes of Cu shocked along the (100) direction. Future experiments (FY 00 and beyond) will be focused on further studying the time resolved lattice response in Cu at a range of shock pressures. Additional techniques such as introducing a knife edge near the x-ray source are expected to provide information about the density of lattice dislocations created by the shock front. Results from these experiments will enable the separation of elastic and plastic components of strain in directions both parallel and perpendicular to the front at the strain rates of 10{sup 6}-10{sup 10} s{sup -1}. Current constitutive equations are highly speculative in this strain rate region. Time resolved measurements of lattice spacing at the shock front will establish plastic deformation rates independent of any model assumptions. During FY 99, we performed a total of 24 laser experiments to demonstrate this technique on OMEGA and resolve diagnostic and timing issues. The results from this work were presented at three conferences and several seminar presentations, and will also be described in future conference presentations.« less

XUV lasers are being exploited for radiography with high space, time and amplitude resolution of perturbations in laser driven targets. Information of importance for research into the possibility of generating energy by inertially confined fusion has been obtained using a saturated 15.4 nm neon-like Yttrium laser [1,2,3]. More recently further work has been carried out with a neon-like Ge laser also operating at saturation with prepulse drive on the J=0-1 transition at 19.6 nm. This has enabled new avenues to be explored including the effect on the laser driven targets of single mode perturbations in the drive intensity and the transient behavior of thermal smoothing of the perturbations as illustrated by figures 1 and 2. The change in XUV opacity of the dense shock compressed target material has also been studied and is of basic science interest as it gives the first direct measurement of the scaling with density of inverse Bremsstrahlung absorption in a strongly degenerate plasma [4].

We have constructed an ultra-fast time-dependant x-ray diffraction experiment in order to probe laser-induced phase transitions in solids. Such studies have previously been performed using laser probes in the infra-red, visible or ultraviolet spectral range. However, in order to directly probe structures in solids, it is necessary to use x-ray radiation. A laser system producing pulses with a duration of 100 fs has been set-up adjacent to a bending magnet at the Advanced Light Source (ALS). In addition an averaging streak camera with a temporal jitter smaller than 1.5 ps has been incorporated [1]. This diagnostic will permit us to study lattice disordering in materials such as Si. Single-shot measurements were also made using a Kentech streak camera.

An experiment to create highly uniform shocks in crystalline solids suitable for studying the thickness of shock fronts by in situ x-ray diffraction is described. A laser is used to create a soft x-ray radiation source. This radiation source heats a thin layer of the silicon crystal; the ablation of this layer produces a uniform near-one-megabar shock. We discuss how x-ray diffraction can be used to measure the shock front thickness from a uniformly shock compressed solid. The x-ray diffraction technique is described through previously measured experimental data and by comparison to simulated diffraction records.