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We demonstrate a significantly simplified experimental approach for investigating liquid metallic hydrogen, which is crucial to understand the internal structure and evolution of giant planets. Plastic samples were shockcompressed and then probed by short pulses of X-rays generated by free electron lasers. By comparison with ab initio simulations, we provide indirect evidence for the creation of elemental hydrogen in shock-compressed plastics at ∼150GPa and ∼5,000K and thus in a regime where hydrogen is predicted to be metallic. Being the most common form of condensed matter in our solar system, and ostensibly the simplest of all elements, hydrogen is the model case for many theoretical studies and we provide a new possibility to benchmark models for conditions with extreme pressures and temperatures. Moreover, this approach will also allow to probe the chemical behavior of metallic hydrogen in mixture with other elements, which, besides its importance for planetary physics, may open up promising pathways for the synthesis of new materials.

Abstract We present structure and equation of state (EOS) measurements of biaxially orientated polyethylene terephthalate (PET, $$({\hbox {C}}_{10} {\hbox {H}}_8 {\hbox {O}}_4)_n$$ ( C 10 H 8 O 4 ) n , also called mylar) shock-compressed to ( $$155 \pm 20$$ 155 ± 20 ) GPa and ( $$6000 \pm 1000$$ 6000 ± 1000 ) K using in situ X-ray diffraction, Doppler velocimetry, and optical pyrometry. Comparing to density functional theory molecular dynamics (DFT-MD) simulations, we find a highly correlated liquid at conditions differing from predictions by some equations of state tables, which underlines the influence of complex chemical interactions in this regime. EOS calculations from ab initio DFT-MD simulations and shock Hugoniot measurements of density, pressure and temperature confirm the discrepancy to these tables and present an experimentally benchmarked correction to the description of PET as an exemplary material to represent the mixture of light elements at planetary interior conditions.

Intense ultrashort laser pulses can create highly excited matter with extraordinary properties. Experimental and theoretical investigations of these extreme conditions are very complex and usually intertwined. Here, we report on a theoretical approach for the electron scattering rates and the optical properties in gold at elevated temperatures. Our theory is based on the degree of occupancy of the conduction band as well as inputs from ab initio simulations and experimental data. After the electron system has reached a quasi-equilibrium, the occupancy is fully determined by the electron temperature. Thus, our approach covers the important relaxation stage after fast excitations when the two-temperature model can be applied. Being based on the electronic structure of solids, the model is valid for lattice temperatures up to melting but the electron temperature might exceed this limit by far. Our results agree well with recent experimental data for both the collision frequencies and the conductivity of highly excited gold. Scattering of sp-electrons by d-electrons is found to be the dominant damping mechanism at elevated electron temperatures and depends strongly on the number of conduction electrons, hence, revealing the microscopic origin of the conductivity change after heating. The supportive benchmarks with experiments are very valuable as the underlying scattering rates determine a number of other transport, optical and relaxation properties of laser-excited matter.

Angularly resolved X-ray scattering measurements from fs-laser heated hydrogen have been used to determine the equilibration of electron and ion temperatures in the warm dense matter regime. The relaxation of rapidly heated cryogenic hydrogen is visualized using 5.5 keV X-ray pulses from the Linac Coherent Light (LCLS) source in a 1 Hz repetition rate pump-probe setting. We demonstrate that the electron-ion energy transfer is faster than quasi-classical Landau-Spitzer models that use ad hoc cutoffs in the Coulomb logarithm.

It is challenging to reliably probe the miscibility behavior of elements in extreme conditions. Here, the authors use X-ray Thomson scattering and compare to the X-ray diffraction method in order to determine mixing of different atomic species in warm dense matter conditions.

The energy loss of ions in plasma is a challenging issue in inertial confinement fusion and many theoretical models exist on ion-stopping power. Here, the authors use laser-generated plasma probed by accelerator-produced ions in experiments to discriminate various ion stopping models near the Bragg peak.

Studying the properties of dense plasmas is challenging due to strong interactions between electrons and ions, and numerical methods overcome this difficulty using a static thermostat. Here the authors predict a strong diffusive ion mode at low energy by including dissipative processes in the model.

Shock synthesis of diamond and even harder carbon polymorphs from graphite is of great interest for science and technology. Here, the authors present unprecedented in situmeasurements of the structural changes, showing ultrafast formation of diamond and, at higher pressures, evidence for a pure lonsdaleite structure.

Using the Linac Coherent Light Source facility at the Stanford Linac Coherent Light Source National Accelerator Laboratory, we have observed x-ray scattering from iron compressed with laser-driven shocks to earth-core-like pressures above 400 GPa. The data show cases where melting is incomplete and we observe hexagonal-close-packed crystal structure at shock compressed densities up to 14.0 g cm^{−3} but no evidence of a double-hexagonal-close-packed crystal. The observation of a crystalline structure at these densities, where shock heating is expected to be in excess of the equilibrium melt temperature, may indicate superheating of the solid. These results are important for equation of state modeling at high strain rates relevant for impact scenarios and laser-driven shock-wave experiments.

We discuss the possibility of obtaining highly precise measurements of the ionization potential depression in dense plasmas with spectrally resolved x-ray scattering, while simultaneously determining the electron temperature and the free electron density. A proof-of-principle experiment at the Linac Coherent Light Source, probing isochorically heated carbon samples, demonstrates the capabilities of this method and motivates future experiments at x-ray free electron laser facilities. [ABSTRACT FROM AUTHOR]

The matter in extreme conditions end station at the Linac Coherent Light Source (LCLS) is a new tool enabling accurate pump–probe measurements for studying the physical properties of matter in the high-energy density (HED) physics regime. This instrument combines the world’s brightest x-ray source, the LCLS x-ray beam, with high-power lasers consisting of two nanosecond Nd:glass laser beams and one short-pulse Ti:sapphire laser. These lasers produce short-lived states of matter with high pressures, high temperatures or high densities with properties that are important for applications in nuclear fusion research, laboratory astrophysics and the development of intense radiation sources. In the first experiments, we have performed highly accurate x-ray diffraction and x-ray Thomson scattering measurements on shock-compressed matter resolving the transition from compressed solid matter to a co-existence regime and into the warm dense matter state. These complex charged-particle systems are dominated by strong correlations and quantum effects. They exist in planetary interiors and laboratory experiments, e.g., during high-power laser interactions with solids or the compression phase of inertial confinement fusion implosions. Applying record peak brightness x-rays resolves the ionic interactions at atomic (Ångstrom) scale lengths and measure the static structure factor, which is a key quantity for determining equation of state data and important transport coefficients. Simultaneously, spectrally resolved measurements of plasmon features provide dynamic structure factor information that yield temperature and density with unprecedented precision at micron-scale resolution in dynamic compression experiments. These studies have demonstrated our ability to measure fundamental thermodynamic properties that determine the state of matter in the HED physics regime. [ABSTRACT FROM AUTHOR]

We re-investigate some classical approaches for collisional absorption of laser radiation in dense plasmas and compare them to quantum theories. The typical break-down of the classical approaches can be avoided by using the quantum dielectric function in the seminal Dawson and Oberman formula which is equivalent to recently published quantum theories of collisional absorption. Strong electron-ion scattering can however be included more easily in classical approaches. [ABSTRACT FROM AUTHOR]

We present results for the ionic structure in dense, moderately to strongly coupled plasmas using two models: the mean spherical approximation (MSA) and the hypernetted chain (HNC) approach. While the first method allows for an analytical solution, the latter has to be solved iteratively. Independent of the coupling strength, the results show only small differences when the ions are considered to form an unscreened one-component plasma (OCP) system. If the electrons are treated as a polarizable background, the different ways to incorporate the screening yield, however, large discrepancies between the models, particularly for more strongly coupled plasmas. [ABSTRACT FROM AUTHOR]