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The shock-induced transition from graphite to diamond has been of great scientific and technological interest since the discovery of microscopic diamonds in remnants of explosively driven graphite. Furthermore, shock synthesis of diamond and lonsdaleite, a speculative hexagonal carbon polymorph with unique hardness, is expected to happen during violent meteor impacts. Here, we show unprecedented in situ X-ray diffraction measurements of diamond formation on nanosecond timescales by shock compression of pyrolytic as well as polycrystalline graphite to pressures from 19 GPa up to 228 GPa. While we observe the transition to diamond starting at 50 GPa for both pyrolytic and polycrystalline graphite, we also record the direct formation of lonsdaleite above 170 GPa for pyrolytic samples only. Our experiment provides new insights into the processes of the shock-induced transition from graphite to diamond and uniquely resolves the dynamics that explain the main natural occurrence of the lonsdaleite crystal structure being close to meteor impact sites. 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 situ measurements of the structural changes, showing ultrafast formation of diamond and, at higher pressures, evidence for a pure lonsdaleite structure.

In megabar shock waves, materials compress and undergo a phase transition to a dense charged-particle system that is dominated by strong correlations and quantum effects. This complex state, known as warm dense matter, exists in planetary interiors and many laboratory experiments (for example, during high-power laser interactions with solids or the compression phase of inertial confinement fusion implosions). Here, we apply record peak brightness X-rays at the Linac Coherent Light Source to resolve ionic interactions at atomic (ångström) scale lengths and to determine their physical properties. Our in situ measurements characterize the compressed lattice and resolve the transition to warm dense matter, demonstrating that short-range repulsion between ions must be accounted for to obtain accurate structure factor and equation of state data. In addition, the unique properties of the X-ray laser provide plasmon spectra that yield the temperature and density with unprecedented precision at micrometre-scale resolution in dynamic compression experiments. Warm dense matter (WDM), which falls in the category between plasmas and condensed matter, is expected to exist in planetary interiors. Now, researchers use an X-ray laser to observe the transition to WDM.

Ion stopping in warm dense matter is a process of fundamental importance for the understanding of the properties of dense plasmas, the realization and the interpretation of experiments involving ion-beam-heated warm dense matter samples, and for inertial confinement fusion research. The theoretical description of the ion stopping power in warm dense matter is difficult notably due to electron coupling and degeneracy, and measurements are still largely missing. In particular, the low-velocity stopping range, that features the largest modelling uncertainties, remains virtually unexplored. Here, we report proton energy-loss measurements in warm dense plasma at unprecedented low projectile velocities. Our energy-loss data, combined with a precise target characterization based on plasma-emission measurements using two independent spectroscopy diagnostics, demonstrate a significant deviation of the stopping power from classical models in this regime. In particular, we show that our results are in closest agreement with recent first-principles simulations based on time-dependent density functional theory. Charged particle interaction and energy dissipation in plasma is fundamentally interesting. Here the authors study proton stopping in laser-produced plasma for the moderate to strong coupling with electrons.

The gravitational pressure in many astrophysical objects exceeds one gigabar (one billion atmospheres) 1 – 3 , creating extreme conditions where the distance between nuclei approaches the size of the K shell. This close proximity modifies these tightly bound states and, above a certain pressure, drives them into a delocalized state 4 . Both processes substantially affect the equation of state and radiation transport and, therefore, the structure and evolution of these objects. Still, our understanding of this transition is far from satisfactory and experimental data are sparse. Here we report on experiments that create and diagnose matter at pressures exceeding three gigabars at the National Ignition Facility 5 where 184 laser beams imploded a beryllium shell. Bright X-ray flashes enable precision radiography and X-ray Thomson scattering that reveal both the macroscopic conditions and the microscopic states. The data show clear signs of quantum-degenerate electrons in states reaching 30 times compression, and a temperature of around two million kelvins. At the most extreme conditions, we observe strongly reduced elastic scattering, which mainly originates from K-shell electrons. We attribute this reduction to the onset of delocalization of the remaining K-shell electron. With this interpretation, the ion charge inferred from the scattering data agrees well with ab initio simulations, but it is significantly higher than widely used analytical models predict 6 . Experiments at the National Ignition Facility show how delocalization of K-shell electrons is driven by extreme pressure and temperature.

The gas and ice giants in our solar system can be seen as a natural laboratory for the physics of highly compressed matter at temperatures up to thousands of kelvins. In turn, our understanding of their structure and evolution depends critically on our ability to model such matter. One key aspect is the miscibility of the elements in their interiors. Here, we demonstrate the feasibility of X-ray Thomson scattering to quantify the degree of species separation in a 1:1 carbon–hydrogen mixture at a pressure of ~150 GPa and a temperature of ~5000 K. Our measurements provide absolute values of the structure factor that encodes the microscopic arrangement of the particles. From these data, we find a lower limit of 2 4 − 7 + 6 % of the carbon atoms forming isolated carbon clusters. In principle, this procedure can be employed for investigating the miscibility behaviour of any binary mixture at the high-pressure environment of planetary interiors, in particular, for non-crystalline samples where it is difficult to obtain conclusive results from X-ray diffraction. Moreover, this method will enable unprecedented measurements of mixing/demixing kinetics in dense plasma environments, e.g., induced by chemistry or hydrodynamic instabilities. 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 effects of hydrocarbon reactions and diamond precipitation on the internal structure and evolution of icy giant planets such as Neptune and Uranus have been discussed for more than three decades 1 . Inside these celestial bodies, simple hydrocarbons such as methane, which are highly abundant in the atmospheres 2 , are believed to undergo structural transitions 3 , 4 that release hydrogen from deeper layers and may lead to compact stratified cores 5 – 7 . Indeed, from the surface towards the core, the isentropes of Uranus and Neptune intersect a temperature–pressure regime in which methane first transforms into a mixture of hydrocarbon polymers 8 , whereas, in deeper layers, a phase separation into diamond and hydrogen may be possible. Here we show experimental evidence for this phase separation process obtained by in situ X-ray diffraction from polystyrene (C 8 H 8 ) n samples dynamically compressed to conditions around 150 GPa and 5,000 K; these conditions resemble the environment around 10,000 km below the surfaces of Neptune and Uranus 9 . Our findings demonstrate the necessity of high pressures for initiating carbon–hydrogen separation 3 and imply that diamond precipitation may require pressures about ten times as high as previously indicated by static compression experiments 4 , 8 , 10 . Our results will inform mass–radius relationships of carbon-bearing exoplanets 11 , provide constraints for their internal layer structure and improve evolutionary models of Uranus and Neptune, in which carbon–hydrogen separation could influence the convective heat transport 7 . Diamonds precipitate from methane under the intense pressures of the atmospheres of Neptune and Uranus. Here, a laser shock experiment on a hydrocarbon sample shows that diamonds may require ten times as much pressure to precipitate as was previously thought.

Here we present results from an experiment performed at the GSI Helmholtz Center for Heavy Ion Research. A mono-energetic beam of chromium ions with initial energies of  ~ 450 MeV was fired through a magnetized interaction region formed by the collision of two counter-propagating laser-ablated plasma jets. While laser interferometry revealed the absence of strong fluid-scale turbulence, acceleration and diffusion of the beam ions was driven by wave-particle interactions. A possible mechanism is particle acceleration by electrostatic, short scale length kinetic turbulence, such as the lower-hybrid drift instability. Laboratory astrophysics experiments can provide useful insight into processes such as cosmic ray acceleration, yet the understanding of these phenomena is still incomplete. Here, the authors provide an experimental study of ion acceleration and diffusion at the GSI facilities, providing insight into wave-particle interactions and lower-hybrid drift instabilities.

Multi-wavelength pyrometry is an efficient tool for measuring high temperatures in dynamic experiments. A fast 5-channel pyrometer was built and successfully employed in ion-beam heating experiments at the GSI Centre for Heavy Ion Research (Darmstadt, Germany). Temperatures of metallic samples heated by an intense focused heavy ion beam up to their melting points and beyond were measured with nanosecond time resolution and a spatial resolution of about 200 μm. The modular instrument has demonstrated its high versatility also for temperature measurements of exothermic reactions with millisecond temporal resolution.

X-ray phase contrast imaging (XPCI) is more sensitive to density variations than X-ray absorption radiography, which is a crucial advantage when imaging weakly-absorbing, low-Z materials, or steep density gradients in matter under extreme conditions. Here, we describe the application of a polychromatic X-ray laser-plasma source (duration ~0.5 ps, photon energy >1 keV) to the study of a laser-driven shock travelling in plastic material. The XPCI technique allows for a clear identification of the shock front as well as of small-scale features present during the interaction. Quantitative analysis of the compressed object is achieved using a density map reconstructed from the experimental data.

The high-energy/high-intensity laser facility PHELIX of the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany, has been in operation since 2008. Here, we review the current system performance, which is the result of continuous development and further improvement. Through its versatile frontend architecture, PHELIX can be operated in both long- and short-pulse modes, corresponding to ns-pulses with up to 1 kJ pulse energy and sub-ps, 200 J pulses, respectively. In the short-pulse mode, the excellent temporal contrast and the control over the wavefront make PHELIX an ideal driver for secondary sources of high-energy ions, neutrons, electrons and X-rays. The long-pulse mode is mainly used for plasma heating, which can then be probed by the heavy-ion beam of the linear accelerator of GSI. In addition, PHELIX can now be used to generate X-rays for studying exotic states of matter created by heavy-ion heating using the ion beam of the heavy-ion synchrotron of GSI.