Session JM10: Mini-Conference: Frontier HED Science Enabled by Advanced Light Sources I

Transitions in matter triggered by intense ultrashort X-ray pulses

In my talk I will give an overview on the recent results of our theoretical investigation how the unique properties of X-ray free-electron laser (FEL) radiation can be employed to modify extended atomic or molecular assemblies, and to create new states of matter. I will discuss three topics that are related to various irradiation regimes that can be achieved, depending on the FEL pulse fluence: (i) atomic processes within laser-created plasmas, (ii) ultrafast electron kinetics in irradiated semiconductors, and (iii) radiation-induced structural changes in solids.   

EXAFS study of solid iron up to 560 GPa

Dynamic compression by multiple shocks is used to compress iron up to 560 GPa (5.6 Mbar), the highest solid-state pressure yet attained for iron in the laboratory. EXAFS (extended x-ray absorption fine structure) spectroscopy offers simultaneous density, temperature and local-structure measurements for compressed iron, providing highest-pressure data up to date for constraining solid state theory and evolution models for many newly discovered extra-solar terrestrial planets. The data show that the close-packed structure of iron is stable up to 560 GPa, the temperature at peak compression is significantly higher than expected from pure compressive work, and the strength of iron many times greater than expected from lower pressure data.   

Imaging High-Pressure Shock Waves by Magnified X-Ray Phase Contrast Imaging at the LCLS

The emergence of the new x-ray free-electron lasers (XFELs) comes along with completely new research opportunities in various scientific fields. The availability of short x-ray pulses of about 50fs time duration enables one to capture snapshots of the state of matter with ultrahigh temporal resolution. During the last year we developed an x-ray microscope based on beryllium compound refractive lenses (Be-CRLs), which provides focusing capabilities down to 100nm and even below. This new setup enables us to perform x-ray imaging experiments requiring additionally high spatial resolution. In a first experiment, carried out at the Matter in Extreme Conditions (MEC) endstation of the LCLS, the performance of the microscope was investigated by direct imaging of shock waves in different materials. The shock wave was induced by a 150ps infrared laser pulse. The evolution of this shock wave was then monitored with with an XFEL-pulse by magnified x-ray phase contrast imaging. In this contribution we report on the current status of the instrument and show first analysis results.   

Theory of Ultrafast Plastic Deformation of Metals at the LCLS

High-rate plastic deformation is the subject of increasing experimental activity. As high energy laser platforms such as those at the National Ignition Facility (NIF) are pushing the horizons plasticity at extremely high pressures and rates in shock and non-shock ramp-compression waves, fourth generation light sources like the Linac Coherent Light Source (LCLS) are opening the door to dynamic plasticity experiments with extremely high temporal resolution. Here we describe the theory of high-rate deformation of metals and how high energy lasers can be, and are, used to study the mechanical strength of materials under extreme conditions. Specifically, we describe predictions of LLNL's multiscale strength model and molecular dynamics (MD) simulations that can be directly, validated or refuted at these new facilities. This work focuses on the bcc metal tantalum at pressures up to 2 Mbar, but other metals such as vanadium and titanium will be considered briefly. In situ x-ray diffraction and scattering experiments provide a direct diagnostic of the lattice-level response of the material that can be compared to the MD and multiscale modeling predictions including deformation modes and plastic relaxation times.   

Lattice-level measurement of material strength with LCLS during ultrafast dynamic compression

An in-depth understanding of the stress-strain behavior of materials during ultrafast dynamic compression requires experiments that offer in-situ observation of the lattice at the pertinent temporal and spatial scales. To date, the lattice response under extreme strain-rate conditions (\textgreater~10$^{8}$ s$^{-1})$ has been inferred predominantly from continuum-level measurements and multi-million atom molecular dynamics simulations. Several time-resolved x-ray diffraction experiments have captured important information on plasticity kinetics, while limited to nanosecond timescales due to the lack of high brilliance ultrafast x-ray sources. Here we present experiments at LCLS combining ultrafast laser-shocks and serial femtosecond x-ray diffraction. The high spectral brightness ($\sim$ 10$^{12}$ photons per pulse, $\Delta $E/E$=$0.2{\%}) and subpicosecond temporal resolution (\textless~100 fs pulsewidth) of the LCLS x-ray free electron laser allow investigations that link simulations and experiments at the fundamental temporal and spatial scales for the first time. We present movies of the lattice undergoing rapid shock-compression, composed by a series of single femtosecond x-ray snapshots, demonstrating the transient behavior while successfully decoupling the elastic and plastic response in polycrystalline Cu.   

Frontier Science at XFELs

4$^{th}$ generation light sources have the potential to revolutionize our understanding of matter at extreme conditions, generating x-rays that are a billion times brighter than existing sources. Delivering hard x-rays with a typical pulse length of less than 100 fs, these sources can effectively freeze ionic motion, allowing us to enter a new regime of time-resolved measurements. These exceptionally bright x-rays allow, for example, diffraction, Thompson scattering or phase-contrast imaging in a single shot. At MEC at the LCLS, and at the HED instrument at the European XFEL coming online in 2015, both long and short pulse lasers play a key part in accessing extreme states of matter, previously inaccessible to laser plasma techniques. In this talk I will particularly focus on potential applications in the field of dynamic compression, utilising long pulse nanosecond lasers to drive materials to multi-megabar pressures. I will show pioneering results using XFEL beams to probe, \textit{in situ}, the often complex response of these materials over a wide variety of conditions. Furthermore, I will discuss the potential advances in this already exciting field with the advent of the HiBEF beamline at the European XFEL.